Biomechanics of lateral lumbar interbody fusion constructs with lateral and posterior plate fixation: laboratory investigation

Single-Position Prone Lateral Lumbar Interbody Fusion Increases Operative Efficiency and Maintains Safety in Revision Lumbar Spinal Fusion

STUDY DESIGN

Multi-centre retrospective cohort study.

OBJECTIVE

To evaluate the feasibility and safety of the single-position prone lateral lumbar interbody fusion (LLIF) technique for revision lumbar fusion surgery.

BACKGROUND CONTEXT

Prone LLIF (P-LLIF) is a novel technique allowing for placement of a lateral interbody in the prone position and allowing posterior decompression and revision of posterior instrumentation without patient repositioning. This study examines perioperative outcomes and complications of single position P-LLIF against traditional Lateral LLIF (L-LLIF) technique with patient repositioning.

METHOD

A multi-centre retrospective cohort study involving patients undergoing 1 to 4 level LLIF surgery was performed at 4 institutions in the US and Australia. Patients were included if their surgery was performed via either: P-LLIF with revision posterior fusion; or L-LLIF with repositioning to prone. Demographics, perioperative outcomes, complications, and radiological outcomes were compared using independent samples t-tests and chi-squared analyses as appropriate with significance set at P <0.05.

RESULTS

101 patients undergoing revision LLIF surgery were included, of which 43 had P-LLIF and 58 had L-LLIF. Age, BMI and CCI were similar between groups. The number of posterior levels fused (2.21 P-LLIF vs. 2.66 L-LLIF, P =0.469) and number of LLIF levels (1.35 vs. 1.39, P =0.668) was similar between groups.Operative time was significantly less in the P-LLIF group (151 vs. 206 min, P =0.004). EBL was similar between groups (150mL P-LLIF vs. 182mL L-LLIF, P =0.31) and there was a trend toward reduced length of stay in the P-LLIF group (2.7 vs. 3.3d, P =0.09). No significant difference was demonstrated in complications between groups. Radiographic analysis demonstrated no significant differences in preoperative or postoperative sagittal alignment measurements.

CONCLUSION

P-LLIF significantly improves operative efficiency when compared to L-LLIF for revision lumbar fusion. No increase in complications was demonstrated by P-LLIF or trade-offs in sagittal alignment restoration.

LEVEL OF EVIDENCE

Level 4.

Interspinous-Interbody Fusion via a Strictly Lateral Surgical Approach: A Biomechanical Stabilization Comparison to Constructs Requiring Both Lateral and Posterior Approaches

Objective Lumbar fusion performed through lateral approaches is becoming more common. The interbody devices are generally supported by supplemental posterior fixation implanted through a posterior approach, potentially requiring a second incision and intraoperative repositioning of the patient. A minimally invasive lateral interspinous fixation device may eliminate the need for intraoperative repositioning and avoid disruption of the supraspinous ligament. The objective of this in vitrobiomechanical study was to investigate segmental multidirectional stability and maintenance of foraminal distraction of a lateral interspinous fixation device compared to commonly used pedicle screw and facet screw posterior fixation constructs when combined with lumbar interbody cages. Methods Six human cadaver lumbar spine specimens were subjected to nondestructive quasistatic loading in the following states: (1) intact; (2) interspinous fixation device alone and (3) with lateral interbody cage; (4) lateral lumbar interbody cage with bilateral pedicle screws; (5) lateral lumbar interbody cage with unilateral pedicle screws; and (6) lateral lumbar interbody cage with facet screws. Multidirectional pure bending in 1.5 Nm increments to 7.5 Nm, and 7.5 Nm flexion-extension bending with a 700 N compressive follower load were performed separately with optoelectronic segmental motion measurement. Relative angular motions of L2-L3, L3-L4, and L4-L5 functional spinal units were evaluated, and the mean instantaneous axis of rotation in the sagittal plane was calculated for the index level. Foraminal height was assessed during combined flexion-extension and compression loading for each test construct. Results All implant configurations significantly restricted flexion-extension motion compared with intact (p < 0.05). No significant differences were found in flexion-extension when comparing the different posterior implants combined with lateral lumbar interbody cages. All posterior fixation devices provided comparable neuroforaminal distraction and maintained distraction during flexion and extension. Conclusions When combinedwith lateral lumbar interbody cages, the minimally invasive lateral interspinous fixation device effectively stabilized the spine and maintained neuroforaminal distraction comparable to pedicle screw constructs or facet screws. These results suggest the lateral interspinous fixation device may provide a favorable alternative to other posterior systems that require patient repositioning during surgery and involve a greater disruption of native tissues.

Biomechanical Evaluation of Lateral Lumbar Interbody Fusion with Various Fixation Options for Adjacent Segment Degeneration: A Finite Element Analysis

OBJECTIVE

Adjacent segment degeneration (ASD) is a common phenomenon after lumbar fusion. Lateral lumbar interbody fusion (LLIF) may provide an alternative treatment method for ASD. This study used finite element analysis to evaluate the biomechanical effects of LLIF with various fixation options and identify an optimal surgical strategy for ASD.

METHODS

A validated L1-S1 finite element model was modified for simulation. Six finite element models of the lumbar spine were created and were divided into group 1 (L4-5 posterior lumbar interbody fusion [PLIF] + L3-4 LLIF) and group 2 (L5-S1 PLIF + L4-5 LLIF). Each group consisted of 1) cage-alone, 2) cage + lateral screw fixation (LSF), and 3) cage + bilateral pedicle screw fixation (BPSF) models. The range of motion, intradiscal pressure, and facet loads of adjacent segments as well as interbody cage stress were analyzed.

RESULTS

The stress on the LLIF cage-superior endplate interface was highest in the cage-alone model followed by the cage + LSF model and cage + BPSF model. The increase in range of motion, intradiscal pressure, and facet loads at the adjacent segment was highest in the cage + BPSF model followed by the cage + LSF model and cage-alone model. However, the biomechanical effect on the adjacent segment seemed similar in the cage-alone and cage + LSF models.

CONCLUSIONS

LLIF with BPSF is recommended when performing LLIF surgery for ASD after L4-5 and L5-S1 PLIF. Considering cage subsidence and biomechanical effects on the adjacent segment, LLIF with LSF may be a suboptimal option for ASD surgery.

Biomechanical evaluation of position and bicortical fixation of anterior lateral vertebral screws in a porcine model

Although an anterior approach with anterior lateral screw fixation has been developed for stabilizing the thoracolumbar spine clinically, screw loosening still occurs. In this novel in vitro study, we attempted to elucidate the optimal screw position in the lateral lumbar vertebra and the effect of bicortical fixation. A total of 72 fresh-frozen lumbar vertebrae from L1-6 were harvested from 12 mature pigs and randomly assigned to two modalities: bicortical fixation (n = 36) and unicortical fixation (n = 36). Six groups of screw positions in the lateral vertebral body in each modality were designated as central-anterior, central-middle, central-posterior, lower-anterior, lower-middle, and lower- posterior; 6 specimens were used in each group. The correlations between screw fixation modalities, screw positions and axial pullout strength were analyzed. An appropriate screw trajectory and insertional depth were confirmed using axial and sagittal X-ray imaging prior to pullout testing. In both bicortical and unicortical fixation modalities, the screw pullout force was significantly higher in the posterior or middle position than in the anterior position (p < 0.05), and there was no significant differences between the central and lower positions. The maximal pullout forces from the same screw positions in unicortical fixation modalities were all significantly lower, decreases that ranged from 32.7 to 74%, than those in bicortical fixation modalities. Our study using porcine vertebrae showed that screws in the middle or posterior position of the lateral vertebral body had a higher pullout performance than those in the anterior position. Posteriorly positioned lateral vertebral screws with unicortical fixation provided better stability than anteriorly positioned screws with bicortical fixation.

Computational comparison of anterior lumbar interbody fusion and oblique lumbar interbody fusion with various supplementary fixation systems: a finite element analysis

BACKGROUND AND OBJECTIVE

Anterior lumbar interbody fusion (ALIF) and oblique lumbar interbody fusion (OLIF) have shown a great surgical potential, while it has always been controversial which surgical approach and which type of fixation system should be selected. This study investigated the biomechanical response of ALIF and OLIF with various supplementary fixation systems using the finite element method.

MATERIALS AND METHODS

Lumbar L4-L5 ALIF and OLIF models stabilized by different supplementary fixation systems (stand-alone cage, integrated stand-alone cage, anterior plate, and bilateral pedicle screw) were developed to assess the segmental range of motion (ROM), endplate stress (EPS), and screw-bone interface stress (SBIS).

EXPERIMENTAL RESULTS

ALIF showed lower ROM and EPS than OLIF in all motion planes and less SBIS in the most of motion planes compared with OLIF when the anterior plate or pedicle screw was used. ALIF induced higher ROM, while lower EPS and SBIS than OLIF in the majority of motion planes when integrated stand-alone cage was utilized. Using a stand-alone cage in ALIF and OLIF led to cage migration. Integrated stand-alone cage prevented the cage migration, whereas caused significantly larger ROM, EPS, and SBIS than other fixation systems except for the rotation plane. In the most of motion planes, the pedicle screw had the lowest ROM, EPS, and SBIS. The anterior plate induced a slightly larger ROM, EPS, and SBIS than the pedicle screw, while the differences were not significant.

CONCLUSION

ALIF exhibited a better performance in postoperative segmental stability, endplate stress, and screw-bone interface stress than OLIF when the anterior plate or the pedicle screw was used. The pedicle screw could provide the greatest postoperative segmental stability, less cage subsidence incidence, and lower risk of fixation system loosening in ALIF and OLIF. The anterior plate could also contribute to the stability required and fewer complications, while not as effectively as the pedicle screw. Extreme caution should be regarded when the stand-alone cage is used due to the risk of cage migration. The integrated stand-alone cage may be an alternative method; however, further optimization is needed to reduce complications and improve postoperative segmental stability.

The preoperative Hounsfield unit value at the position of the future screw insertion is a better predictor of screw loosening than other methods

OBJECTIVE

Screw loosening is a widely reported issue after spinal screw fixation and triggers several complications after lumbar interbody fusion. Osteoporosis is an essential risk factor for screw loosening. Hounsfield units (HU) value is a credible indicator during bone mineral density (BMD) evaluation. As compared with the general evaluation of BMD, we hypothesized that specific measurements of HU at the precise location of the future screw insertion may be a better predictor of screw loosening.

METHODS

Clinical data of 56 patients treated by oblique lumbar interbody fusion (OLIF) of the L4-L5 segments with an anterior lateral single rod (ALSR) screw fixation were reviewed in this study. Vertebral bodies with ≥ 1 mm width radiolucent zones around the screw were defined as screw loosening. HU in the insertional screw positions, the central transverse plane, and the average values of three and four planes were measured. Regression analyses identified independent risk factors for screw loosening separately. The area under the receiver operating characteristic curve (AUC) was computed to evaluate predictive performance.

RESULTS

The local HU values were significantly lower in the loosening group, regardless of the selected measuring methods. The AUC of screw loosening prediction was higher in the insertional screw positions' HU than other frequently used methods.

CONCLUSIONS

The HU value measured in the insertional screw position is a better predictor of ALSR screw loosening than other methods. The risk of screw loosening should be reduced by optimizing the trajectory of the screw based on the measurement of HU in preoperative CT.

KEY POINTS

• Osteoporosis is an essential risk factor for screw loosening, and Hounsfield units (HU) are a credible predictor during bone mineral density (BMD) evaluation. • The HU value measured in the insertional screw position is a better predictor of screw loosening than other frequently used HU measurement methods. • The risk of screw loosening might potentially be reduced by optimizing the trajectory of the screw based on the measurement of HU in preoperative CT.

Biomechanical studies of different numbers and positions of cage implantation on minimally invasive transforaminal interbody fusion: A finite element analysis

BACKGROUND

The position and number of cages in minimally invasive transforaminal interbody fusion (MIS-TLIF) are mainly determined by surgeons based on their individual experience. Therefore, it is important to investigate the optimal number and position of cages in MIS-TLIF.

METHODS

The lumbar model was created based on a 24-year-old volunteer's computed tomography data and then tested using three different cage implantation methods: single transverse cage implantation (model A), single oblique 45° cage implantation (model B), and double vertical cage implantation (model C). A preload of 500 N and a moment of 10 Nm were applied to the models to simulate lumbar motion, and the models' range of motion (ROM), ROM ratio, peak stress of the internal fixation system, and cage were assessed.

RESULTS

The ROM ratios of models A, B, and C were significantly reduced by >71% compared with the intact model under all motions. Although there were subtle differences in the ROM ratio for models A, B, and C, the trends were similar. The peak stress of the internal fixation system appeared in model B of 136.05 MPa (right lateral bending), which was 2.07 times that of model A and 1.62 times that of model C under the same condition. Model C had the lowest cage stress, which was superior to that of the single-cage model.

CONCLUSION

In MIS-TLIF, single long-cage transversal implantation is a promising standard implantation method, and double short-cage implantation is recommended for patients with severe osteoporosis.

Instrumentation choice and early radiographic outcome following lateral lumbar interbody fusion (LLIF): Lateral instrumentation versus posterior pedicle screw fixation

Background

Lateral lumbar interbody fusion (LLIF) is a minimally invasive fusion procedure that may be performed with or without supplemental instrumentation. However, there is a paucity of evidence on the effect of supplemental instrumentation technique on perioperative morbidity and fusion rate in LLIF.

Methods

A single-institutional retrospective review of patients who underwent LLIF for lumbar spondylosis was conducted. Patients were grouped according to supplemental instrumentation technique: stand-alone LLIF, LLIF with laterally placed instrumentation, or LLIF with posterior percutaneous pedicle screw fixation (PPSF). Outcomes included fusion rates, peri-operative complication, and reoperation; estimated blood loss (EBL); surgery duration; length of stay; and length of follow-up.

Results

82 patients underwent LLIF at 114 levels. 35 patients (42.7%) received supplemental lateral instrumentation, 30 (36.6%) received supplemental PPSF, and 17 (20.7%) underwent stand-alone LLIF. More patients in the lateral instrumentation group had prior lumbar fusion at adjacent levels (23/35, 65.71%) versus stand-alone (3/17, 17.6%) or PPSF (2/30, 6.67%) groups (p = 0.003). 4/17 patients (23.5%) with stand-alone LLIF and 4/35 patients (11.42%) with lateral instrumentation underwent reoperation, versus 0/30 with PPSF (p = 0.030). There was no difference in fusion rates between groups (p = 0.717). Operation duration was longer in patients with PPSF (p < 0.005) and length of follow-up was longer for PPSF than lateral instrumentation (p = 0.001). Choice of instrumentation group was a statistically significant predictor of reoperation.

Conclusions

While rates of complete radiographic fusion on imaging follow-up didn't differ, patients receiving PPSF were less likely than stand-alone or lateral instrumentation groups to require reoperation, though operative time was significantly longer. Further study of choice of supplemental instrumentation with LLIF is indicated.

Biomechanical effects of an oblique lumbar interbody fusion combined with posterior augmentation: a finite element analysis

Background

Oblique lateral interbody fusion (OLIF) is widely used to treat lumbar degenerative disc disease. This study aimed to evaluate the biomechanical stability of OLIF, OLIF including posterior pedicle screw and rod (PSR), and OLIF including cortical screw and rod (CSR) instrumentation through finite element analysis.

Methods

A complete L2-L5 finite element model of the lumbar spine was constructed. Surgical models of OLIF, such as stand-alone, OLIF combined with PSR, and OLIF combined with CSR were created in the L3-L4 surgical segments. Range of motion (ROM), end plate stress, and internal fixation peak stress were compared between different models under the same loading conditions.

Results

Compared to the intact model, ROM was reduced in the OLIF model under all loading conditions. The surgical models in order of increasing ROM were PSR, CSR, and stand-alone; however, the difference in ROM between BPS and CSR was less than 0.4° and was not significant under any loading conditions. The stand-alone model had the highest stress on the superior L4 vertebral body endplate under all loading conditions, whereas the end plate stress was relatively low in the BPS and CSR models. The CSR model had the highest internal fixation stress, concentrated primarily at the end of the screw.

Conclusions

OLIF alone significantly reduces ROM but does not provide sufficient stability. Addition of posterior PSR or CSR internal fixation instrumentation to OLIF surgery can significantly improve biomechanical stability of the segment undergoing surgery.

Effect of the In Situ Screw Implantation Region and Angle on the Stability of Lateral Lumbar Interbody Fusion: A Finite Element Study

Objective

To investigate the effect of the in situ screw implantation region and angle on the stability of lateral lumbar interbody fusion (LLIF) from a biomechanical perspective.

Methods

A validated L2‐4 finite element (FE) model was modified for simulation. The L3‐4 fused segment undergoing LLIF surgery was modeled. The area between the superior and inferior edges and the anterior and posterior edges of the vertebral body (VB) is divided into four zones by three parallel lines in coronal and horizontal planes. In situ screw implantation methods with different angles based on the three parallel lines in coronal plane were applied in Models A, B, and C (A: parallel to inferior line; B: from inferior line to midline; C: from inferior line to superior line). In addition, four implantation methods with different regions based on the three parallel lines in horizontal plane were simulated as types 1–2, 1–3, 2–2, and 2–3 (1–2: from anterior line to midline; 1–3: from anterior line to posterior line; 2–2: parallel to midline; 2–3: from midline to posterior line). L3‐4 ROM, interbody cage stress, screw‐bone interface stress, and L4 superior endplate stress were tracked and calculated for comparisons among these models.

Results

The L3‐4 ROM of Models A, B, and C decreased with the extent ranging from 47.9% (flexion‐extension) to 62.4% (lateral bending) with no significant differences under any loading condition. Types 2–2 and 2–3 had 45% restriction, while types 1–2 and 1–3 had 51% restriction in ROM under flexion‐extension conditions. Under lateral bending, types 2–2 and 2–3 had 70.6% restriction, while types 1–2 and 1–3 had 61.2% restriction in ROM. Under axial rotation, types 2–2 and 2–3 had 65.2% restriction, while types 1–2 and 1–3 had 59.3% restriction in ROM. The stress of the cage in types 2–2 and 2–3 was approximately 20% lower than that in types 1–2 and 1–3 under all loading conditions in all models. The peak stresses at the screw‐bone interface in types 2–2 and 2–3 were much lower (approximately 35%) than those in types 1–2 and 1–3 under lateral bending, while no significant differences were observed under flexion‐extension and axial rotation. The peak stress on the L4 superior endplate was approximately 30 MPa and was not significantly different in all models under any loading condition.

Conclusions

Different regions of entry‐exit screws induced multiple screw trajectories and influenced the stability and mechanical responses. However, different implantation angles did not. Considering the difficulty of implantation, the ipsilateral‐contralateral trajectory in the lateral middle region of the VB can be optimal for in situ screw implantation in LLIF surgery.

Comparing the medium-term outcomes of lumbar interbody fusion via transforaminal and oblique approach in treating lumbar degenerative disc diseases

BACKGROUND CONTEXT

Oblique lumbar interbody fusion (OLIF) has been proven to be effective in treating lumbar degenerative disorders (LDDs) via indirect decompression. However, its superiority over transforaminal lumbar interbody fusion (TLIF) remains questionable, especially in terms of medium-term follow-up.

PURPOSE

To compare the medium-term clinical and radiological outcomes of TLIF and OLIF in treating patients with LDDs.

STUDY DESIGN

Retrospective comparative study.

PATIENT SAMPLE

Fifty-two patients treated by TLIF and forty-six patients treated by OLIF.

OUTCOME MEASURES

Clinical records including the visual analog scale (VAS) score of the lower back and leg and the Oswestry Disability Index (ODI). Radiological records including disk height (DH), lumbar lordosis (LL), segmental lordosis (SL), the cross-sectional area (CSA) of the spinal canal, and fusion rate. Surgical-related information and complications were also recorded.

METHODS

A retrospective review was performed on patients who were surgically managed for LDDs at L4-5 between 2015 and 2017 and completed at least 4 years of follow-up. A total of 98 patients were analyzed, with 46 patients treated by OLIF combined with anterolateral single screw-rod fixation (OLIF-AF group), and 52 patients treated by TLIF (TLIF group). Parameters including postoperative outcomes and perioperative complications were compared with evaluate the efficacy of the two approaches.

RESULTS

There was significantly less bleeding, surgical duration, and hospitalization in the OLIF-AF group than in the TLIF group. Significant improvements in the clinical score were achieved in both groups. However, the VAS score of the lower back was significantly higher in the TLIF group than in the OLIF-AF group throughout the whole follow-up period. Significantly higher expansion of the CSA was found in the TLIF group than in the OLIF-AF group. However, the improvements in DH, LL, and SL were significantly lower in the TLIF group. The fusion rate was significantly higher in the OLIF-AF group than in the TLIF group within 6 months postoperatively, and there was no significant difference between the two groups at the final record. No significant difference was found in the rate of overall complications between the two groups (25.0% vs. 23.9%, p=.545). The intraoperative complication rate in the TLIF group (13.5%) was slightly higher than that in the OLIF-AF group (6.5%) (p=.257). There was no significant difference in the incidence of adjacent segment disorder (ASD) between the two groups (7.7% vs. 10.9%, p=.422). Cage subsidence was slightly lower in the TLIF group (5.8%) than in the OLIF-AF group (13.0%) (p=.298).

CONCLUSIONS

Both the TLIF and OLIF-AF approaches demonstrated good medium-term outcomes in treating LDDs. Compared with TLIF, OLIF-AF showed advantages in postoperative recovery, improvement of intervertebral space and lumbar sagittal balance, and early intervertebral fusion but was associated with inferior spinal canal decompression efficacy. The two approaches shared comparable overall complication rates. However, OLIF-AF tended to have fewer intraoperative complications, and a higher incidence of subsidence.

Finite Element Analysis of a Novel Fusion Strategy in Minimally Invasive Transforaminal Lumbar Interbody Fusion

Purpose

To evaluate the biomechanics of a novel fusion strategy (hybrid internal fixation+horizontal cage position) in minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF).

Methods

MIS-TLIF finite element models for three fusion strategies were created based on computed tomography images, namely, Model-A, hybrid internal fixation (ipsilateral pedicle screw and contralateral translaminar facet screw fixation)+horizontal cage position; Model-B, bilateral pedicle screw (BPS) fixation+horizontal cage position; and Model-C, BPS fixation+oblique 45° cage position. A preload of 500 N and a moment of 10 Nm were applied to the models to simulate lumbar motion, and the models' range of motion (ROM), peak stress of the internal fixation system, and cage were assessed.

Results

The ROM for Models A, B, and C were not different (P > 0.05) but were significantly lower than the ROM of Model-INT (P < 0.0001). Although there were subtle differences in the ROM ratio for Models A, B, and C, the trend was similar. The peak stress of the internal fixation system was significantly higher in Model-A than that of Models B and C, but only the difference between Models A and B was significant (P < 0.05). The peak stress of the cage in Model-A was significantly lower than that of Models B and C (P < 0.01).

Conclusion

Hybrid internal fixation with horizontal single cage implantation can provide the same biomechanical stability as traditional fixation while reducing peak stress on the cage and vertebral endplate.

Whether Anterolateral Single Rod Can Maintain the Surgical Outcomes Following Oblique Lumbar Interbody Fusion for Double-Segment Disc Disease

Objective

To evaluate the outcomes of oblique lumbar interbody fusion (OLIF) combined with anterolateral single‐rod screw fixation (AF) in treating two‐segment lumbar degenerative disc disease (LDDD) and to determine whether AF can maintain the surgical results.

Methods

A retrospective analysis was performed on patients who underwent OLIF combined with AF (OLIF‐AF) for LDDD at the L3‐5 levels between October 2017 and May 2018. A total of 84 patients, including 44 males and 40 females, with a mean age of 62.8 ± 6.8 years, who completed the 12‐month follow‐up were eventually enrolled. Clinical outcomes, including the Oswestry Disability Index (ODI), visual analog scale (VAS) score for the low back and leg, and radiographic parameters, including the cross‐sectional area (CSA) of the spinal canal, disc height (DH), foraminal height (FH), degree of upper vertebral slippage (DUVS), segmental lumbar lordosis (SL), fusion rate, and lumbar lordosis (LL), were recorded before surgery and 1 and 12 months after surgery. Surgical‐related complications, including cage subsidence (CS), were also evaluated. The local radiographic parameters were compared between L3‐4 and L4‐5. The clinical results and all radiographic parameters were compared between patients with and without CS.

Results

Significant improvements were observed in radiographic parameters 1 day postoperatively (p < 0.05). Local radiological parameters in L4‐5 had a significant decrease at 12 months postoperatively (p < 0.05), while they were well‐maintained at L3‐4 throughout the follow‐up period (p > 0.05). CS was observed in 26 segments (15.5%). Endplate injury was observed in four segments (2.4%). There was no significant difference in the fusion rate between the segments with and without CS (p = 0.355). The clinical results improved significantly after surgery (p < 0.05), and no significant difference was observed between the groups with and without CS (p > 0.05).

Conclusions

Anterolateral fixation combined with OLIF provides sufficient stability to sustain most radiological improvements in treating double‐segment LDDD. Subsidence was the most common complication, which was prone to occur in L4‐5 compared to L3‐4, but did not impede the fusion process or diminish the surgical results.

Oblique lateral interbody fusion combined with lateral plate fixation for the treatment of degenerative diseases of the lumbar spine: A retrospective study

Oblique lateral interbody fusion (OLIF) is a minimally invasive decompression technique used in the treatment of lumbar degenerative diseases (LDDs). It is usually combined with posterior pedicle screw fixation to decrease perioperative complications. Few studies have reported the efficacy of OLIF combined with lateral plate instrumentation (OLIF-LP) for the treatment of LDDs.The purpose of this retrospective study was to evaluate the clinical efficacy of OLIF combined with lateral plate instrumentation for the treatment of LDDs.From May 2020 to September 2020, the clinical data of 52 patients who underwent OLIF-LP were analyzed. The operation time, blood loss, and complications were recorded. The radiological parameters, visual analog scale score, and Oswestry Disability Index were evaluated.The average operation time, blood loss, and length of hospital stay were 75.41 ± 11.53 minutes, 39.57 ± 9.22 mL, and 7.22 ± 1.85 days, respectively. The visual analog scale score and Oswestry Disability Index both improved significantly after surgery (7.23 ± 1.26 vs 2.15 ± 0.87; 60.27 ± 7.91 vs 21.80 ± 6.32, P < .01). The postoperative disk height was 13.02 ± 8.83 mm, which was much greater than the preoperative value. The postoperative foraminal height improved significantly (16.18 ± 3.49 vs 21.54 ± 2.12 mm, P < .01), and the cross-sectional area improved from 88.95 ± 14.79 to 126.53 ± 8.83 mm2 (P < .001). The radiological fusion rate was 88% at the last follow-up. No major complications, such as ureteral injury, vascular injury, or vertebral body fracture, occurred.Use of the OLIF-LP technique can help avoid lumbar posterior surgery and minimize the operative time and blood loss. OLIF-LP can achieve 1-stage intervertebral fusion and instrumentation through a single small incision.

Biomechanical Effects of Pedicle Screw Positioning on the Surgical Segment in Models After Oblique Lumbar Interbody Fusion: An in-silico Study

Background

Bilateral pedicle screw (BPS) is the “gold standard” of fixation methods for patients with lumbar interbody fusion. Biomechanical deterioration initially triggers complications in the surgical segment. Studies proved that BPS positions and trajectory changes affect the local biomechanical environment. However, no study illustrates the biomechanical effect of insertional screw positions’ change on the surgical segment.

Methods

Oblique lumbar interbody fusion (OLIF) with different BPS insertional positions has been simulated in a well-validated lumbo-sacral model. Fixation stability and stress responses on the surgical segment were evaluated under identical loading conditions.

Results

There is no clear variation tendency for the risk of BPS failure and the change of strain energy density of the grafted bone. However, shifting the insertional screw position close to the surgical segment will increase the range of motions (ROM) in the surgical segment and lead to stress concentration of bony structures, especially in the caudal side of the surgical segment.

Conclusion

Adjusting the insertional position of BPS close to the surgical segment in OLIF models will lead to stress concentration of bony structures and surgical segmental instability. Therefore, reducing BPS’s fixation length was not recommended, which may increase the risk of segmental instability, non-union, and cage subsidence.

Evaluation of the Stability of a Novel Lateral Plate Internal Fixation: An In Vitro Biomechanical Study

BACKGROUND

This study aims to evaluate the biomechanical stability of a novel lateral plate (NLP) that can be used in oblique lateral lumbar fusion (OLIF).

METHODS

In vitro biomechanical tests were performed on 6 fresh calf lumbar vertebrae specimens. The surgical segment was set at L3-L4. Each specimen was tested in the following order: intact state (INT); OLIF cage only/stand-alone (SA); cage supplemented with lateral screw-rod (LSR); cage supplemented with novel lateral plate (NLP); and cage supplemented with unilateral or bilateral pedicle screw-rod (UPS or BPS). A pure moment of ±7.5 Nm was applied to the specimen to produce 6 different motion directions, including flexion and extension, lateral bending, and axial rotation, and the range of motion (ROM) of L3-L4 in each direction was recorded.

RESULTS

In addition to flexion-extension, NLP reduced the ROM of SA (P < 0.05). In flexion-extension, the ROM of NLP was similar to those of SA and LSR (P > 0.05); compared to pedicle screw-rod (PSD), the ROM of NLP was higher (P < 0.05). In lateral bending, the ROM of NLP was close to that of LSR and PSD (P > 0.05). In axial rotation, the ROM of NLP was higher than that of PSD (P < 0.05), and close to that of LSR (P > 0.05).

CONCLUSIONS

NLP can enhance surgical segment stability in all directions of motion, similar to LSR, but weaker than UPS and BPS in flexion-extension and rotation.

Development of a decision-making pathway for utilizing standalone lateral lumbar interbody fusion

PURPOSE

To develop a decision-making pathway for primary SA-LLIF. Furthermore, we analyzed the agreement of this pathway and compared outcomes of patients undergoing either SA-LLIF or 360-LLIF.

METHOD

A decision-making pathway for SA-LLIF was created based on the results of interviews/surveys of senior spine surgeons with over 10 years of experience. Internal validity was retrospectively evaluated using consecutive patients undergoing either SA-LLIF or 360-LLIF between 01/2018 and 07/2020 with 3D-printed Titanium cages. An outcome assessment looking primarily at revision surgery and secondary at cage subsidence, changes in disk and foraminal height, global and segmental lumbar lordosis, duration of surgery, estimated blood loss, and length of stay was carried out.

RESULTS

78 patients with 124 treated levels (37 SA-LLIF, 41 360-LLIF) were retrospectively analyzed. The pathway showed a direct agreement (SA-LLIF) of 100.0% and an indirect agreement (360-LLIF) of 95.1%. Clinical follow-up averaged 13.5 ± 6.5 months including 4 revision surgeries in the 360-LLIF group and none in the SA-LLIF group (p = 0.117). Radiographic follow-up averaged 9.5 ± 4.3 months, with no statistically significant difference in cage subsidence rate between the groups (p = 0.440). Compared to preoperative images, patients in both groups showed statistically significant changes in disk height (p < 0.001), foraminal height (p < 0.001), as well as restoration of segmental lordosis (p < 0.001 and p = 0.018). The SA-LLIF group showed shorter duration of surgery, less estimated blood loss and shorter LOS (p < 0.001).

CONCLUSION

The proposed decision-making pathway provides a guide to adequately select patients for SA-LLIF. Further studies are needed to assess the external applicability and validity.

LEVEL OF EVIDENCE III

Diagnostic: individual cross-sectional studies with consistently applied reference standard and blinding.

Additional lateral plate fixation has no effect to prevent cage subsidence in oblique lumbar interbody fusion

Background

For lumbar degenerative diseases, cage subsidence is a serious complication and can result in the failure of indirect decompression in the oblique lumbar interbody fusion (OLIF) procedure. Whether additional lateral plate fixation was effective to improve clinical outcomes and prevent cage subsidence was still unknown. This study aimed to compare the incidence and degree of cage subsidence between stand-alone oblique lumbar interbody fusion (SA-OLIF) and OLIF combined with lateral plate fixation (OLIF + LP) for the treatment of lumbar degenerative diseases and to evaluate the effect of the lateral plate fixation.

Methods

This was a retrospective comparative study. 20 patients with 21 levels underwent SA-OLIF and 21 patients with 26 levels underwent OLIF + LP. We compared clinical and radiographic outcomes between two groups. Clinical evaluation included Visual Analog Scale (VAS) for back pain and leg pain, Japanese Orthopaedic Association (JOA) scores and Oswestry Disability Index (ODI). Radiographical evaluation included disc height (DH), segmental lordosis angle (SL), and subsidence rate on standing lateral radiographs. Cage subsidence was classified using Marchi’s criteria.

Results

The mean follow-up duration was 6.3 ± 2.4 months. There were no significant differences among perioperative data (operation time, estimated intraoperative blood loss, and complication), clinical outcome (VAS, ODI, and JOA) and radiological outcome (SH and SL). The subsidence rate was 19.0% (4/21) in SA-OLIF group and 19.2% (5/26) in OLIF + LP group. 81.0% in SA-OLIF group and 80.8% in OLIF + LP group had Grade 0 subsidence, 14.3% in SA-OLIF group and 15.4% in OLIF + LP group had Grade I subsidence, and 4.8% in SA-OLIF group and 3.8% in OLIF + LP group had Grade II subsidence (P = 0.984). One patient with severe cage subsidence and lateral plate migration underwent revision surgery.

Conclusions

The additional lateral plate fixation does not appear to be more effective to prevent cage subsidence in the oblique lumbar interbody fusion, compared with stand-alone technique. If severe cage subsidence occurs, it may result in lateral plate migration in OLIF combined with lateral plate fixation.

An In Vitro Biomechanical Evaluation of a Lateral Lumbar Interbody Fusion Device With Integrated Lateral Modular Plate Fixation

STUDY DESIGN

In vitro cadaveric biomechanical study.

OBJECTIVE

Biomechanically characterize a novel lateral lumbar interbody fusion (LLIF) implant possessing integrated lateral modular plate fixation (MPF).

METHODS

A human lumbar cadaveric (n = 7, L1-L4) biomechanical study of segmental range-of-motion stiffness was performed. A ±7.5 Nċm moment was applied in flexion/extension, lateral bending, and axial rotation using a 6 degree-of-freedom kinematics system. Specimens were tested first in an intact state and then following iterative instrumentation (L2/3): (1) LLIF cage only, (2) LLIF + 2-screw MPF, (3) LLIF + 4-screw MPF, (4) LLIF + 4-screw MPF + interspinous process fixation, and (5) LLIF + bilateral pedicle screw fixation. Comparative analysis of range-of-motion outcomes was performed between iterations.

RESULTS

Key biomechanical findings: (1) Flexion/extension range-of-motion reduction with LLIF + 4-screw MPF was significantly greater than LLIF + 2-screw MPF (P < .01). (2) LLIF with 2-screw and 4-screw MPF were comparable to LLIF with bilateral pedicle screw fixation in lateral bending and axial rotation range-of-motion reduction (P = 1.0). (3) LLIF + 4-screw MPF and supplemental interspinous process fixation range-of-motion reduction was comparable to LLIF + bilateral pedicle screw fixation in all directions (P ≥ .6).

CONCLUSIONS

LLIF with 4-screw MPF may provide inherent advantages over traditional 2-screw plating modalities. Furthermore, when coupled with interspinous process fixation, LLIF with MPF is a stable circumferential construct that provides biomechanical utility in all principal motions.

Biomechanical evaluation of anterior lumbar interbody fusion with various fixation options: Finite element analysis of static and vibration conditions

BACKGROUND

Anterior lumbar interbody fusion combined with supplementary fixation has been widely used to treat lumbar diseases. However, few studies have investigated the influence of fixation options on facet joint force and cage subsidence. The aim of this study was to explore the biomechanical performance of anterior lumbar interbody fusion with various fixation options under both static and vertical vibration loading conditions.

METHODS

A previously validated finite element model of the intact L1-5 lumbar spine was employed to compare five conditions: (1) Intact; (2) Fusion alone; (3) Fusion combined with anterior lumbar plate; (4) Fusion combined with Coflex-F fixation; (5) Fusion combined with bilateral pedicle screw fixation. The models were analyzed under static and vertical vibration loading conditions respectively.

FINDINGS

Bilateral pedicle screws provided highest stability at surgical level. Applying supplementary fixation diminished the dynamic responses of lumbar spine. Compared with anterior lumbar plate and Coflex-F device, bilateral pedicle screws decreased the stress responses of the endplates and cage under both static and vibration conditions, while increased the facet joint force at adjacent levels. As for comparison between Coflex-F device and anterior lumbar plate, results showed a similarity in biomechanical performance under static loading, and a slightly higher dynamic response of the latter under vertical vibration.

INTERPRETATION

The biomechanical performance of lumbar spine was significantly influenced by the variation of fixations under both static and vibration conditions. Bilateral pedicle screws showed advantages in stabilizing surgical segment and relieving cage subsidence, but may increase the facet joint force at adjacent levels.

Cadaveric biomechanical analysis of multilevel lateral lumbar interbody fusion with and without supplemental instrumentation

BACKGROUND

This study was to evaluate and compare the biomechanical features of multilevel lateral lumbar interbody fusion (LLIF) with or without supplemental instrumentations.

METHODS

Six human lumbar specimens were tested under multidirectional nondestructive moments (7.5 N·m), with a 6 degree-of-freedom spine simulator. The overall and intervertebral range of motion (ROM) were measured optoelectronically. Each specimen was tested under the following conditions at L2-5 levels: intact; stand-alone; cage supplemented with lateral plate (LP); cage supplemented with unilateral or bilateral pedicle screw/rod (UPS or BPS).

RESULTS

Compared with intact condition, the overall and intersegmental ROM were significantly reduced after multilevel stand-alone LLIF. The ROM was further reduced after using LP instrumentation. In flexion-extension (FE) and axial rotation (AR), pedicle screw/rod demonstrated greater overall ROM reduction compared to LP (P < 0.01), and bilateral greater than unilateral (P < 0.01). In lateral bending (LB), BPS demonstrated greater overall ROM reduction compared to UPS and LP (P < 0.01), however, UPS and LP showed similar reduction (P = 0.245). Intervertebral ROM reductions showed similar trend as the overall ones after using different types of instrumentation. However, at L2/3 (P = 0.57) and L3/4 (P = 0.097) levels, the intervertebral ROM reductions in AR were similar between UPS and LP.

CONCLUSIONS

The overall and intervertebral stability increased significantly after multilevel LLIF with or without supplemental instrumentation. BPS provided the greatest stability, followed by UPS and LP. However, in clinical practice, less invasive adjunctive fixation methods including UPS and LP may provide sufficient biomechanical stability for multilevel LLIF.

Biomechanical Evaluation of Oblique Lumbar Interbody Fusion with Various Fixation Options: A Finite Element Analysis

Objective

The aim of the present study was to clarify the biomechanical properties of oblique lumbar interbody fusion (OLIF) using different fixation methods in normal and osteoporosis spines.

Methods

Normal and osteoporosis intact finite element models of L₁–S₁ were established based on CT images of a healthy male volunteer. Group A was the normal models and group B was the osteoporosis model. Each group included four subgroups: (i) intact; (ii) stand‐alone cage (Cage); (iii) cage with lateral plate and two lateral screws (LP); and (iv) cage with bilateral pedicle screws and rods (BPSR). The L₃–L₄ level was defined as the surgical segment. After validating the normal intact model, compressive load of 400 N and torsional moment of 10 Nm were applied to the superior surface of L₂ to simulate flexion, extension, left bending, right bending, left rotation, and right rotation motions. Surgical segmental range of motion (ROM), cage stress, endplate stress, supplemental fixation stress, and stress distribution were analyzed in each group.

Results

Cage provided the minimal reduction of ROM among all motions (normal, 82.30%–98.81%; osteoporosis, 92.04%–97.29% of intact model). BPSR demonstrated the maximum reduction of ROM (normal, 43.94%–61.13%; osteoporosis, 45.61%–62.27% of intact model). The ROM of LP was between that of Cage and BPSR (normal, 63.25%–79.72%; osteoporosis, 70%–87.15% of intact model). Cage had the minimal cage stress and endplate stress. With the help of LP and BPSR fixation, cage stress and endplate stress were significantly reduced in all motions, both in normal and osteoporosis finite element models. However, BPSR had more advantages. For cage stress, BPSR was at least 75.73% less than that of Cage in the normal model, and it was at least 80.10% less than that of Cage in the osteoporosis model. For endplate stress, BPSR was at least 75.98% less than that of Cage in the normal model, and it was at least 78.06% less than that of Cage in the osteoporosis model. For supplemental fixation stress, BPSR and LP were much less than the yield strength in all motions in the two groups. In addition, the comparison between the two groups showed that the ROM, cage stress, endplate stress, and supplemental fixation stress in the normal model were less than in the osteoporosis model when using the same fixation option of OLIF.

Conclusion

Oblique lumbar interbody fusion with BPSR provided the best biomechanical stability both in normal and osteoporosis spines. The biomechanical properties of the normal spine were better than those of the osteoporosis spine when using the same fixation option of OLIF.

The anterior and traverse cage can provide optimal biomechanical performance for both traditional and percutaneous endoscopic transforaminal lumbar interbody fusion

BACKGROUND

Transforaminal lumbar interbody fusion (TLIF) is a well-established surgical treatment for patients with lumbar degenerative disc disease; however, the optimal position for the interbody fusion cage in TLIF procedures for reducing cage-related complications remains uncertain. The present study aims to compare the biomechanical effects between different cage positions in TLIF and percutaneous endoscopic-TLIF (PE-TLIF).

METHOD

An intact finite element model of L3-L5 from computed tomography images of a 25-year-old healthy male without any lumbar disease was reconstructed and validated. TLIF and PE-TLIF were performed on L4-L5 with bilateral pedicle screws fixation. Two surgical finite element models were subjected to loads with six degrees of freedom. The range of motion (ROM) and von Mises stress of the implantations and endplates were measured for the anterior, middle, and posterior district and the traverse or oblique direction of the cage respectively.

RESULTS

As the cage was implanted forward, the ROMs in the fused L4-L5 segments and the von Mises stress of the cage and endplates decreased while the von Mises stress of the screws increased; this was also shown in the traverse cage when compared with the oblique cage (A-90-compared with A-45- had a 31.3%, 1.7%, 12.6%, and 5.7% decrease in FL, EX, LB and AR). The ROMs (TLIF A-45 increase of 80.8%, 23.8%, and 12.2% in FL, EX, and LB when compared with PE-TLIF), cage stress, and endplate stress of PE-TLIF were lower than those of TLIF.

CONCLUSIONS

Considering the ROM of the fusion segments, implanting the cage in the anterior district in the traverse direction can effectively enhance the fusion segment stiffness, thus contributing to the stability of the lumbar spine after fusion. It can also cause less cage stress and endplate stress, which indicates its beneficial effect in avoiding cage injury or subsidence. However, the higher stress of the pedicle screws and rods indicates higher failure risk. PE-TLIF had better biomechanical performance than TLIF. Therefore, it is recommended that the surgeon implant the cage in the anterior district of the L5 vertebra's upper endplate in the traverse direction using the PE-TLIF technique.

Clinical and Radiographic Outcomes of Lateral Interbody Fusion for Adjacent Segment Degeneration

STUDY DESIGN

Retrospective cohort study.

OBJECTIVE

Assessment of outcomes in patients undergoing lateral interbody fusion as part of the surgical treatment of adjacent segment deterioration after previous lumbar spine fusion.

METHODS

Adult patients with previous lumbar posterior spinal fusion who presented with adjacent segment degeneration and stenosis refractory to nonoperative treatment and who underwent lateral lumbar interbody fusion were retrospectively analyzed. Clinical and radiographic outcomes were assessed and comparisons made between preoperative baseline and postoperative values.

RESULTS

Thirty-six patients with symptomatic adjacent segment degeneration at 46 motion segments were included. Thirty (83.3%) of the 36 patients had complete relief of both preoperative lower extremity pain and back pain at the time of final follow-up. Six (16.7%) of the 36 patients had persistent pain, though in all 6 cases, the pain was less postoperatively than preoperatively. Oswestry Disability Index scores were improved significantly at final follow-up (P = .001). Compared with preoperative baseline parameters, initial and final postoperative radiographs had an increase in segmental lordosis (P < .001 and P < .001, respectively), increase in overall lumbar lordosis (P < .05 and P = .094, respectively), decrease in segmental coronal angulation (P = .63 and P < .01, respectively), decrease in overall coronal angulation (P = .063 and P = .009, respectively), and increase in intervertebral height (P < .001 and P < .001, respectively).

CONCLUSION

Lateral lumbar interbody fusion achieves favorable clinical and radiographic outcomes for the treatment of adjacent segment degeneration after previous lumbar fusion.

LEVEL OF EVIDENCE

4.

Biomechanical Analysis of an Interspinous Process Fixation Device with In Situ Shortening Capabilities: Does Spinous Process Compression Improve Segmental Stability?

OBJECTIVE

The objective of this study was to characterize the biomechanical implications of spinous process compression, via in situ shortening of a next-generation interspinous process fixation (ISPF) device, in the context of segmental fusion.

METHODS

Seven lumbar cadaveric spines (L1-L4) were tested. Specimens were first tested in an intact state, followed by iterative instrumentation at L2-3 and subsequent testing. The order followed was 1) stand-alone ISPF (neutral height); 2) stand-alone ISPF (shortened in situ from neutral height; shortened); 3) lateral lumbar interbody fusion (LLIF) + ISPF (neutral); 4) LLIF + ISPF (shortened); 5) LLIF + unilateral pedicle screw fixation; 6) LLIF + bilateral pedicle screw fixation. A 7.5-Nm moment was applied in flexion/extension, lateral bending, and axial rotation via a kinematic test frame. Segmental range of motion (ROM) and lordosis were measured for all constructs. Comparative analysis was performed.

RESULTS

Statistically significant flexion/extension ROM reductions: all constructs versus intact condition (P < 0.01); LLIF + ISPF (neutral and shortened) versus stand-alone ISPF (neutral and shortened) (P < 0.01); LLIF + USPF versus ISPF (neutral) (P = 0.049); bilateral pedicle screw fixation (BPSF) versus stand-alone ISPF (neutral and shortened) (P < 0.01); LLIF + BPSF versus LLIF + unilateral pedicle screw fixation (UPSF) (P < 0.01). Significant lateral bending ROM reductions: LLIF + ISPF (neutral and shortened) versus intact condition and stand-alone ISPF (neutral) (P < 0.01); LLIF + UPSF versus intact condition and stand-alone ISPF (neutral and shortened) (P < 0.01); LLIF + BPSF versus intact condition and all constructs (P < 0.01). Significant axial rotation ROM reductions: LLIF + ISPF (shortened) and LLIF + UPSF versus intact condition and stand-alone ISPF (neutral) (P ≤ 0.01); LLIF + BPSF versus intact condition and all constructs (P ≤ 0.04).

CONCLUSIONS

In situ shortening of an adjustable ISPF device may support increased segmental stabilization compared with static ISPF.

Biomechanical evaluation of autologous bone-cage in posterior lumbar interbody fusion: a finite element analysis

BACKGROUND

An autologous bone-cage made from the spinous process and laminae might provide a stability in posterior lumbar interbody fusion (PLIF) close that of the traditional-cage made of polyetheretherketone (PEEK) or titanium. The biomechanical effect of autologous bone-cages on cage stability, stress, and strains, and on the facet contact force has not been fully described. This study aimed to verify whether autologous bone-cages can achieve similar performance as that of PEEK cages in PLIF by using a finite element analysis.

METHODS

The finite element models of PLIF with an autologous bone-cage, a titanium cage, and a PEEK cage were constructed. The autologous bone-cage was compared with the titanium and PEEK cages. The mechanical properties of the autologous bone-cage were obtained through mechanical tests. The four motion modes were simulated. The range of motion (ROM), the stress in the cage-end plate interface, and the facet joint force (FJF) were compared.

RESULTS

The ROM was increased at adjacent levels but decreased over 97% at the treated levels, and the intradiscal pressure at adjacent levels was increased under all conditions in all models. The FJF disappeared at treated levels and increased under extension, lateral bending, and lateral rotation in all models. The maximum stress of the cage-endplate interface was much lower in the autologous bone-cage model than those in the PEEK and titanium cage models.

CONCLUSIONS

In a finite model of PLIF, the autologous bone-cage model could achieve stability close that of traditional titanium or PEEK cages, reducing the risk of subsidence.

Biomechanical evaluation of strategies for adjacent segment disease after lateral lumbar interbody fusion: is the extension of pedicle screws necessary?

Background

Adjacent segment disease (ASD) is a well-known complication after interbody fusion. Pedicle screw-rod revision possesses sufficient strength and rigidity. However, is a surgical segment with rigid fixation necessary for ASD reoperation? This study aimed to investigate the biomechanical effect of different instrumentation on lateral lumbar interbody fusion (LLIF) for ASD treatment.

Methods

A validated L2~5 finite element (FE) model was modified for simulation. ASD was considered the level cranial to the upper-instrumented segment (L3/4). Bone graft fusion in LLIF with bilateral pedicle screw (BPS) fixation occurred at L4/5. The ASD segment for each group underwent a) LLIF + posterior extension of BPS, b) PLIF + posterior extension of BPS, c) LLIF + lateral screw, and d) stand-alone LLIF. The L3/4 range of motion (ROM), interbody cage stress and strain, screw-bone interface stress, cage-endplate interface stress, and L2/3 nucleus pulposus of intradiscal pressure (NP-IDP) analysis were calculated for comparisons among the four models.

Results

All reconstructive models displayed decreased motion at L3/4. Under each loading condition, the difference was not significant between models a and b, which provided the maximum ROM reduction (73.8 to 97.7% and 68.3 to 98.4%, respectively). Model c also provided a significant ROM reduction (64.9 to 77.5%). Model d provided a minimal restriction of the ROM (18.3 to 90.1%), which exceeded that of model a by 13.1 times for flexion-extension, 10.3 times for lateral bending and 4.8 times for rotation. Model b generated greater cage stress than other models, particularly for flexion. The maximum displacement of the cage and the peak stress of the cage-endplate interface were found to be the highest in model d under all loading conditions. For the screw-bone interface, the stress was much greater with lateral instrumentation than with posterior instrumentation.

Conclusions

Stand-alone LLIF is likely to have limited stability, particularly for lateral bending and axial rotation. Posterior extension of BPS can provide reliable stability and excellent protective effects on instrumentation and endplates. However, LLIF with the use of an in situ screw may be an alternative for ASD reoperation.

Anterior lumbar interbody fusion in a lateral decubitus position: technique and outcomes in obese patients

Background

Multilevel lumbar interbody fusion (LIF) surgery in obese patients is problematic, with positioning and anaesthetic risks during posterior approaches, vascular and visceral complications during anterior approaches, and lack of access to L5/S1 during lateral approaches. Modified anterior LIF (ALIF) via an anterolateral retroperitoneal approach in the lateral decubitus position permits access to L3/4, L4/5, and L5/S1 levels without patient repositioning. This study reports our initial experience with this lateral ALIF in obese patients and describes modifications of existing lateral and anterior techniques.

Methods

We retrospectively analysed a prospectively maintained registry including the first 30 consecutive patients who underwent lateral ALIF. In all patients, supine ALIF was relatively contraindicated because of obesity or previous abdominal surgery. All patients had a body mass index (BMI) ≥30 kg/m². Fusion was assessed by high-definition computed tomography. Patient-reported outcomes included visual analogue scale pain scores, Oswestry Disability Index (ODI), and 36-Item Short-Form Survey (SF-36) physical and mental component scores (PCS and MCS). All patients underwent ≥2 years follow-up.

Results

At last follow-up (mean, 35.0 months) mean back pain improved 64%, leg pain improved 67%, ODI improved 54%, and PCS and MCS both improved 37% (P<0.05 versus preoperative for all). Mean BMI was unchanged postoperatively (P=0.83). Complications occurred in 7 (23%) patients: dysesthesia [2], retroperitoneal hematoma [2], radiculopathy [1], and subsidence [2]. Solid interbody fusion occurred in 19 (63%) patients at 12 months postoperatively and in 26 (87%) patients at 24 months.

Conclusions

Lateral ALIF enables L5/S1 anterior fusion in obese patients and permits multilevel fusion using a single position. Satisfactory clinical outcomes and complication rates are achieved despite unchanged BMI and 87% radiological fusion rates. Lateral ALIF appears to be a reasonable alternative to posterior, lateral, and supine-position anterior approaches for L3/4, L4/5, and L5/S1 interbody fusions.

Biomechanical Analysis of Stand-alone Lateral Lumbar Interbody Fusion for Lumbar Adjacent Segment Disease

Study design

Biomechanical cadaveric study 

Objective

To compare biomechanical properties of a single stand-alone interbody fusion and a single-level pedicle screw construct above a previous lumbar pedicle fusion.

Summary of background data

Adjacent segment disease (ASD) is spondylosis of adjacent vertebral segments after previous spinal fusion. Despite the consensus that ASD is clinically significant, the surgical treatment of ASD is controversial.

Methods

Lateral lumbar interbody fusion (LLIF) and posterior spinal fusion (PSF) with pedicle screws were analyzed within a validated cadaveric lumbar fusion model. L3-4 vertebral segment motion was analyzed within the following simulations: without implants (intact), L3-4 LLIF-only, L3-4 LLIF with previous L4-S1 PSF, L3-4 PSF with previous L4-S1 PSF, and L4-S1 PSF alone. L3-4 motion values were measured during flexion/extension with and without axial load, side bending, and axial rotation.

Results

L3-4 motion in the intact model was found to be 4.7 ± 1.2 degrees. L3-4 LLIF-only decreased motion to 1.9 ± 1.1 degrees. L3-4 LLIF with previous L4-S1 fusion demonstrated less motion in all planes with and without loading (p < 0.05) compared to an intact spine. However, L3-4 motion with flexion/extension and lateral bending was noted to be greater compared to the L3-S1 construct (p < 0.5). The L3-S1 PSF construct decreased motion to less than 1° in all planes of motion with or without loading (p < 0.05). The L3-4 PSF with previous L4-S1 PSF constructs decreased the flexion/extension motion by 92.4% compared to the intact spine, whereas the L3-4 LLIF with previous L4-S1 PSF constructs decreased motion by 61.2%.

Conclusions

Stand-alone LLIF above a previous posterolateral fusion significantly decreases motion at the adjacent segment, demonstrating its utility in treating ASD without necessitating revision. The stand-alone LLIF is a biomechanically sound option in the treatment of ASD and is advantageous in patient populations who may benefit from less invasive surgical options.

Adjustable Rigid Interspinous Process Fixation: A Biomechanical Study of Segmental Lordosis and Interbody Loading in the Lumbar Spine

Background

Rigid interspinous process fixation (ISPF) may serve as a minimally disruptive adjunct to lumbar interbody fusion. Previous biomechanical assessments of ISPF have demonstrated particularly advantageous outcomes in stabilizing the sagittal plane. However, ISPF has not been well characterized in regard to its impact on interbody load, which has implications for the risk of cage migration or subsidence, and sagittal alignment. The purpose of this study was to biomechanically assess in vitro the interbody load (IBL), focal lordosis (FL), and spinous process loading generated by in situ compression/distraction with a novel ISPF device capable of incremental in situ shortening/extension. Bilateral pedicle screw fixation (BPSF) was used as a control.

Methods

Two fresh frozen human lumbar spines were thawed and musculature was removed, leaving ligaments intact. Seven functional spinal units were iteratively tested, which involved a standard lateral discectomy, placement of a modified lateral cage possessing two load cells, and posterior fixation. BPSF and ISPF were performed at each level, with order of fixation was randomized. BPSF was first performed with maximum compressive exertion followed by 75% exertion to represent clinical application. The ISPF device was implanted at a neutral height and incrementally shortened/extended in situ in 1-mm increments. IBL and FL were measured under each condition. Loads on the spinous processes were estimated through bench-top mechanical calibration.

Results

No significant differences in IBL were observed, but the ISPF device produced a significantly greater change in FL compared to the clinically relevant BPSF compression. IBL, as a function of ISPF device height, expressed linear behavior during compression and exponential behavior during distraction.

Conclusions

The novel ISPF device produced clinically effective IBL and FL, performing well in comparison to BPSF. Additionally, incremental ISPF device manipulation demonstrated predictable and clinically safe trends regarding loading of the interbody space and spinous processes.

Biomechanical Effects of an Oblique Lumbar PEEK Cage and Posterior Augmentation

OBJECTIVE

Lumbar interbody spacers are widely used in lumbar spinal fusion. The goal of this study is to analyze the biomechanics of a lumbar interbody spacer (Clydesdale Spinal System, Medtronic Sofamor Danek, Memphis, Tennessee, USA) inserted via oblique lumbar interbody fusion (OLIF) or direct lateral interbody fusion (DLIF) approaches, with and without posterior cortical screw and rod (CSR) or pedicle screw and rod (PSR) instrumentation.

METHODS

Lumbar human cadaveric specimens (L2-L5) underwent nondestructive flexibility testing in intact and instrumented conditions at L3-L4, including OLIF or DLIF, with and without CSR or PSR.

RESULTS

OLIF alone significantly reduced range of motion (ROM) in flexion-extension (P = 0.005) but not during lateral bending or axial rotation (P ≥ 0.63). OLIF alone reduced laxity in the lax zone (LZ) during flexion-extension (P < 0.001) but did not affect the LZ during lateral bending or axial rotation (P ≥ 0.14). The stiff zone (SZ) was unaffected in all directions (P ≥ 0.88). OLIF plus posterior instrumentation (cortical, pedicle, or hybrid) reduced the mean ROM in all directions of loading but only significantly so with PSR during lateral bending (P = 0.004), without affecting the compressive stiffness (P > 0.20). The compressive stiffness with the OLIF device without any posterior instrumentation did not differ from that of the intact condition (P = 0.97). In terms of ROM, LZ, or SZ, there were no differences between OLIF and DLIF as standalone devices or OLIF and DLIF with posterior instrumentation (CSR or PSR) (P > 0.5).

CONCLUSIONS

OLIF alone significantly reduced mobility during flexion-extension while maintaining axial compressive stiffness compared with the intact condition. Adding posterior instrumentation to the interbody spacer increased the construct stability significantly, regardless of cage insertion trajectory or screw type.

Biomechanical evaluation of lumbar lateral interbody fusion for the treatment of adjacent segment disease

BACKGROUND CONTEXT

Adjacent segment disease (ASD) is a well-known complication after lumbar fusion. Lumbar lateral interbody fusion (LLIF) may provide an alternative method of treatment for ASD while avoiding the morbidity associated with revision surgery through a traditional posterior approach. This is the first biomechanical study to evaluate the stability of lateral-based constructs for treating ASD in existing multilevel fusion model.

PURPOSE

We aimed to evaluate the biomechanical stability of anterior column reconstruction through the less invasive lateral-based interbody techniques compared with traditional posterior spinal fusion for the treatment of ASD in existing multilevel fusion.

STUDY DESIGN/SETTING

Cadaveric biomechanical study of laterally based interbody strategies for treating ASD.

METHODS

Eighteen fresh-frozen cadaveric specimens were nondestructively loaded in flexion, extension, and lateral bending. The specimens were randomized into three different groups according to planned posterior spinal instrumented fusion (PSF): group 1: L5-S1, group 2: L4-S1, and group 3: L3-S1. In each group, ASD was considered the level cranial to the upper-instrumented vertebrae (UIV). After testing the intact spine, each specimen underwent PSF representing prior fusion in the ASD model. The adjacent segment for each specimen then underwent (1) Stand-alone LLIF, (2) LLIF + plate, (3) LLIF + single screw rod (SSR) anterior instrumentation, and (4) LLIF + traditional posterior extension of PSF. In all conditions, three-dimensional kinematics were tracked, and range of motion (ROM) was calculated for the comparisons.

RESULTS

ROM results were expressed as a percentage of the intact spine ROM. LLIF effectively reduces ROM in all planes of ROM. Supplementation of LLIF with plate or SSR provides further stability as compared with stand-alone LLIF. Expansion of posterior instrumentation provides the most substantial stability in all planes of ROM (p <.05). All constructs demonstrated a consistent trend of reduction in ROM between all the groups in all bending motions.

CONCLUSIONS

This biomechanical study suggests potential promise in exploring LLIF as an alternative treatment of ASD but reinforces previous studies' findings that traditional expansion of posterior instrumentation provides the most biomechanically stable construct.

Anterior and Lateral Lumbar Interbody Fusion With Supplemental Interspinous Process Fixation: Outcomes from a Multicenter, Prospective, Randomized, Controlled Study

Background

Rigid interspinous process fixation (ISPF) has received consideration as an efficient, minimally disruptive technique in supporting lumbar interbody fusion. However, despite advantageous intraoperative utility, limited evidence exists characterizing midterm to long-term clinical outcomes with ISPF. The objective of this multicenter study was to prospectively assess patients receiving single-level anterior (ALIF) or lateral (LLIF) lumbar interbody fusion with adjunctive ISPF.

Methods

This was a prospective, randomized, multicenter (11 investigators), noninferiority trial. All patients received single-level ALIF or LLIF with supplemental ISPF (n = 66) or pedicle screw fixation (PSF; n = 37) for degenerative disc disease and/or spondylolisthesis (grade ≤2). The randomization patient ratio was 2:1, ISPF/PSF. Perioperative and follow-up outcomes were collected (6 weeks, 3 months, 6 months, and 12 months).

Results

For ISPF patients, mean posterior intraoperative outcomes were: blood loss, 70.9 mL; operating time, 52.2 minutes; incision length, 5.5 cm; and fluoroscopic imaging time, 10.4 seconds. Statistically significant improvement in patient Oswestry Disability Index scores were achieved by just 6 weeks after operation (P < .01) and improved out to 12 months for the ISPF cohort. Patient-reported 36-Item Short Form Health Survey and Zurich Claudication Questionnaire scores were also significantly improved from baseline to 12 months in the ISPF cohort (P < .01). A total of 92.7% of ISPF patients exhibited interspinous fusion at 12 months. One ISPF patient (1.5%) required a secondary surgical intervention of possible relation to the posterior instrumentation/procedure.

Conclusion

ISPF can be achieved quickly, with minimal tissue disruption and complication. In supplementing ALIF and LLIF, ISPF supported significant improvement in early postoperative (≤12 months) patient-reported outcomes, while facilitating robust posterior fusion.

Biomechanical evaluation of interbody fixation with secondary augmentation: lateral lumbar interbody fusion versus posterior lumbar interbody fusion

Background

Many approaches to the lumbar spine have been developed for interbody fusion. The biomechanical profile of each interbody fusion device is determined by the anatomical approach and the type of supplemental internal fixation. Lateral lumbar interbody fusion (LLIF) was developed as a minimally invasive technique for introducing hardware with higher profiles and wider widths, compared with that for the posterior lumbar interbody fusion (PLIF) approach. However, the biomechanics of the interbody fusion construct used in the LLIF approach have not been rigorously evaluated, especially in the presence of secondary augmentation.

Methods

Spinal stability of 21 cadaveric lumbar specimens was compared using standard nondestructive flexibility studies [mean range of motion (ROM), lax zone (LZ), stiff zone (SZ) in flexion-extension, lateral bending, and axial rotation]. Non-paired comparisons were made among four conditions: (I) intact; (II) with unilateral interbody + bilateral pedicle screws (BPS) using the LLIF approach (referred to as the LLIF construct); (III) with bilateral interbody + BPS using the PLIF approach (referred to as the PLIF construct); and (IV) with no lumbar interbody fusion (LIF) + BPS (referred to as the no-LIF construct).

Results

With bilateral pedicle screw-rod fixation, stability was equivalent between PLIF and LLIF constructs in lateral bending and flexion-extension. PLIF and LLIF constructs had similar biomechanical profiles, with a trend toward less ROM in axial rotation for the LLIF construct.

Conclusions

LLIF and PLIF constructs had similar stabilizing effects.

Clinical and Radiographic Results of a Minimally Invasive Lateral Transpsoas Approach for Treatment of Septic Spondylodiscitis of the Thoracolumbar and Lumbar Spine

BACKGROUND

The minimally invasive lateral transpsoas approach allows retroperitoneal access for discectomy and graft placement. However, the procedure has rarely been used for the treatment of septic spondylodiscitis. The purposes of this study were to evaluate the clinical and radiographic outcomes from this minimally invasive procedure for septic spondylodiscitis.

METHODS

Thirty-one consecutive patients (17 males and 14 females) were included in this study from July 2013 to January 2016. Clinical outcomes were assessed by Oswestry Disability Index, visual analog scale, modified Macnab criteria, and inflammatory parameters. Radiographic results were analyzed by studying the changes in diseased disc height, lordosis, and fusion status.

RESULTS

The Oswestry Disability Index and visual analog scale score improved by 58% and 69% at the last follow-up. The modified Macnab criteria were found to be excellent in 21 patients (68%) and good in 10 (32%). Inflammatory parameters normalized over the average 24 months follow-up. There were no major complications that might have influenced the outcomes in this cohort. A complete fusion after 12 months was achieved in 87% of patients. A mean 7.5 mm restoration in disc height and 6.4° restoration in lumbar lordosis were observed in all patients, whereas an average 4.5 mm loss in restored height resulting from graft subsidence was observed in 24 patients during the follow-up. However, graft subsidence did not influence clinical outcomes significantly.

CONCLUSIONS

A minimally invasive lateral transpsoas approach in combination with instrumentation provides a novel treatment for patients with septic spondylodiscitis without severe kyphosis and neurologic impairment.

Biomechanical Analysis of Lateral Lumbar Interbody Fusion Constructs with Various Fixation Options: Based on a Validated Finite Element Model

BACKGROUND

Lateral lumbar interbody fusion using cage supplemented with fixation has been used widely in the treatment of lumbar disease. A combined fixation (CF) of lateral plate and spinous process plate may provide multiplanar stability similar to that of bilateral pedicle screws (BPS) and may reduce morbidity. The biomechanical influence of the CF on cage subsidence and facet joint stress has not been well described. The aim of this study was to compare biomechanics of various fixation options and to verify biomechanical effects of the CF.

METHODS

The surgical finite element models with various fixation options were constructed based on computed tomography images. The lateral plate and posterior spinous process plate were applied (CF). The 6 motion modes were simulated. Range of motion (ROM), cage stress, endplate stress, and facet joint stress were compared.

RESULTS

For the CF model, ROM, cage stress, and endplate stress were the minimum in almost all motion modes. Compared with BPS, the CF reduced ROM, cage stress, and endplate stress in all motion modes. The ROM was reduced by more than 10% in all motion modes except for flexion; cage stress and endplate stress were reduced more than 10% in all motion modes except for rotation-left. After interbody fusion, facet joint stress was reduced substantially compared with the intact conditions in all motion modes except for flexion.

CONCLUSIONS

The combined plate fixation may offer an alternative to BPS fixation in lateral lumbar interbody fusion.

Biomechanical Analysis of Porous Additive Manufactured Cages for Lateral Lumbar Interbody Fusion: A Finite Element Analysis

BACKGROUND

A porous additive manufactured (AM) cage may provide stability similar to that of traditional solid cages and may be beneficial to bone ingrowth. The biomechanical influence of various porous cages on stability, subsidence, stresses in cage, and facet contact force has not been fully described. The purpose of this study was to verify biomechanical effects of porous AM cages.

METHODS

The surgical finite element models with various cages were constructed. The partially porous titanium (PPT) cages and fully porous titanium (FPT) cages were applied. The mechanical parameters of porous materials were obtained by mechanical test. Then the porous AM cages were compared with solid titanium (TI) cage and solid polyetheretherketone (PEEK) cage. The 4 motion modes were simulated. Range of motion (ROM), cage stress, end plate stress, and facet joint force (FJF) were compared.

RESULTS

For all the surgical models, ROM decreased by >90%. Compared with TI and PPT cages, PEEK and FPT cages substantially reduced the maximum stresses in cage and end plate in all motion modes. Compared with PEEK cages, the stresses in cage and end plate for FPT cages decreased, whereas the ROM increased. Comparing FPT cages, the stresses in cage and end plate decreased with increasing porosity, whereas ROM increased with increasing porosity. After interbody fusion, FJF was substantially reduced in all motion modes except for flexion.

CONCLUSIONS

Fully porous cages may offer an alternative to solid PEEK cages in lateral lumbar interbody fusion. However, it may be prudent to further increase the porosity of the cage.

Custom-Made Titanium 3-Dimensional Printed Interbody Cages for Treatment of Osteoporotic Fracture-Related Spinal Deformity

BACKGROUND

Advances in minimally invasive interbody fusion have greatly enhanced surgeons' capability to correct adult spinal deformity with reduced morbidity. However, the feasibility of such approaches is limited in patients with previous osteoporotic fractures as the resultant vertebral deformity renders the end plate geometry incongruous with conventional interbody implants. Current 3-dimensional (3D) printing technology offers a novel solution by fabricating custom-made implants tailored to individual anatomy. We present the results of a patient with osteoporotic lumbar fractures treated by such technology.

CASE DESCRIPTION

A 74-year-old woman, with previous osteoporotic fractures at L2 and L3 resulting in concave deformity of the end plates, presented with intractable radiculopathy secondary to lateral recess and foraminal stenosis (L2-3 and L3-4). A minimally invasive lateral lumbar interbody fusion at L2-3 and L3-4 was considered favorable, but due to the associated vertebral collapse, off-the-shelf implants were not compatible with patient anatomy. In silico simulation based on preoperative computed tomography (CT) imaging was thus conducted to design customized cages to cater for the depressed recipient end plates and vertebral loss. The design was converted to implantable titanium cages through 3D additive manufacturing. At surgery, a tight fit between the implants and the targeted disk space was achieved. Postoperative CT scan confirmed excellent implant-end plate matching and restoration of lost disk space. The patient began to ambulate from postoperative day 1 and at 6-month follow-up resolution of radicular symptoms and CT evidence of interbody fusion were recorded.

CONCLUSIONS

3D-printed custom-made interbody cages can help overcome the difficulties in deformity correction secondary to osteoporotic fractures.

Biomechanical Evaluation of Lumbar Decompression Adjacent to Instrumented Segments

BACKGROUND

Multilevel lumbar stenosis, in which 1 level requires stabilization due to spondylolisthesis, is routinely treated with multilevel open laminectomy and fusion. We hypothesized that a minimally invasive (MI) decompression is biomechanically superior to open laminectomy and may allow decompression of the level adjacent the spondylolisthesis without additional fusion.

OBJECTIVE

To study the mechanical effect of various decompression procedures adjacent to instrumented segments in cadaver lumbar spines.

METHODS

Conditions tested were (1) L4-L5 instrumentation, (2) L3-L4 MI decompression, (3) addition of partial facetectomy at L3-L4, and (4) addition of laminectomy at L3-L4. Flexibility tests were performed for range of motion (ROM) analysis by applying nonconstraining, pure moment loading during flexion-extension, lateral bending, and axial rotation. Compression flexion tests were performed for motion distribution analysis.

RESULTS

After instrumentation, MI decompression increased flexion-extension ROM at L3-L4 by 13% (P = .03) and axial rotation by 23% (P = .003). Partial facetectomy further increased axial rotation by 15% (P = .03). After laminectomy, flexion-extension ROM further increased by 12% (P = .05), a 38% increase from baseline, and axial rotation by 17% (P = .02), a 58% increase from baseline. MI decompression yielded no significant increase in segmental contribution of motion at L3-L4, in contrast to partial facetectomy and laminectomy (<.05).

CONCLUSION

MI tubular decompression is biomechanically superior to open laminectomy adjacent to instrumented segments. These results lend support to the concept that in patients in whom a multilevel MI decompression is performed, the fusion might be limited to the segments with actual instability.

ABBREVIATION

MI, minimally invasive.

Lumbar Spinous Process Fixation and Fusion: A Systematic Review and Critical Analysis of an Emerging Spinal Technology

STUDY DESIGN

A systematic review.

OBJECTIVE

The available literature on interspinous rigid fixation/fusion devices (IFD) was systematically reviewed to explore the devices' efficacy and complication profile.

SUMMARY OF BACKGROUND DATA

The clinical application of new spinal technologies may proceed without well-established evidence, as is the case with IFDs. IFDs are plate-like devices that are attached to the lateral aspects of 2 adjacent spinous processes to promote rigidity at that segment. Despite almost a decade since the devices' introduction, the literature regarding efficacy and safety is sparse. Complications have been reported but no definitive study is known to the authors.

METHODS

A systematic review of the past 10 years of English literature was conducted according to PRISMA guidelines. The timeframe was chosen based on publication of the first study containing a modern IFD, the SPIRE, in 2006. All PubMed publications containing MeSH headings or with title or abstract containing any combination of the words "interspinous," "spinous process," "fusion," "fixation," "plate," or "plating" were included. Exclusion criteria consisted of dynamic stabilization devices (X-Stop, DIAM, etc.), cervical spine, pediatrics, and animal models. The articles were blinded to author and journal, assigned a level of evidence by Oxford Centre of Evidence-Based Medicine (OCEBM) criteria, and summarized in an evidentiary table.

RESULTS

A total of 293 articles were found in the initial search, of which 15 remained after examination for exclusion criteria. No class I or class II evidence regarding IFDs was found. IFDs have been shown by methodologically flawed and highly biased class III evidence to reduce instability at 1 year, without statistical comparison of complication rates against other treatment modalities.

CONCLUSIONS

Although IFDs are heavily marketed and commonly applied in modern practice, data on safety and efficacy are inadequate. The paucity of evidence warrants reexamination of these devices' value and indications by the spine surgery community.

Biomechanical analysis of lateral interbody fusion strategies for adjacent segment degeneration in the lumbar spine

BACKGROUND CONTEXT

Surgical treatment of symptomatic adjacent segment disease (ASD) typically involves extension of previous instrumentation to include the newly affected level(s). Disruption of the incision site can present challenges and increases the risk of complication. Lateral-based interbody fusion techniques may provide a viable surgical alternative that avoids these risks. This study is the first to analyze the biomechanical effect of adding a lateral-based construct to an existing fusion.

PURPOSE

The study aimed to determine whether a minimally invasive lateral interbody device, with and without supplemental instrumentation, can effectively stabilize the rostral segment adjacent to a two-level fusion when compared with a traditional posterior revision approach.

STUDY DESIGN/SETTING

This is a cadaveric biomechanical study of lateral-based interbody strategies as add-on techniques to an existing fusion for the treatment of ASD.

METHODS

Twelve lumbosacral specimens were non-destructively loaded in flexion, extension, lateral bending, and torsion. Sequentially, the tested conditions were intact, two-level transforaminal lumbar interbody fusion (TLIF) (L3-L5), followed by lateral lumbar interbody fusion procedures at L2-L3 including interbody alone, a supplemental lateral plate, a supplemental spinous process plate, and then either cortical screw or pedicle screw fixation. A three-level TLIF was the final instrumented condition. In all conditions, three-dimensional kinematics were tracked and range of motion (ROM) was calculated for comparisons. Institutional funds (<$50,000) in support of this work were provided by Medtronic Spine.

RESULTS

The addition of a lateral interbody device superadjacent to a two-level fusion significantly reduced motion in flexion, extension, and lateral bending (p<.05). Supplementing with a lateral plate further reduced ROM during lateral bending and torsion, whereas a spinous process plate further reduced ROM during flexion and extension. The addition of posterior cortical screws provided the most stable lateral lumbar interbody fusion construct, demonstrating ROM comparable with a traditional three-level TLIF.

CONCLUSIONS

The data presented suggest that a lateral-based interbody fusion supplemented with additional minimally invasive instrumentation may provide comparable stability with a traditional posterior revision approach without removal of the existing two-level rod in an ASD revision scenario.

Ex vivo loading of trussed implants for spine fusion induces heterogeneous strains consistent with homeostatic bone mechanobiology

A truss structure was recently introduced as an interbody fusion cage. As a truss system, some of the connected elements may be in a state of compression and others in tension. This study aimed to quantify both the mean and variance of strut strains in such an implant when loaded in a simulated fusion condition with vertebral body or contoured plastic loading platens ex vivo. Cages were each instrumented with 78 fiducial spheres, loaded between platens (vertebral body or contoured plastic), imaged using high resolution micro-CT, and analyzed for deformation and strain of each of the 221 struts. With repeated loading of a cage by vertebral platens, the distribution (variance, indicated by SD) of strut strains widened from 50N control (4±114με, mean±SD) to 1000N (-23±273με) and 2000N (-48±414με), and between 1000N and 2000N. With similar loading of multiple cages, the strain distribution at 2000N (23±389με) increased from 50N control. With repeated loading by contoured plastic platens, induced strains at 2000N had a distribution similar to that induced by vertebral platens (84±426με). In all studies, cages exhibited increases in strut strain amplitude when loaded from 50N to 1000N or 2000N. Correspondingly, at 2000N, 59-64% of struts exhibited strain amplitudes consistent with mechanobiologically-regulated bone homeostasis. At 2000N, vertically-oriented struts exhibited deformation of -2.87±2.04μm and strain of -199±133με, indicating overall cage compression. Thus, using an ex vivo 3-D experimental biomechanical analysis method, a truss implant can have strains induced by physiological loading that are heterogeneous and of amplitudes consistent with mechanobiological bone homeostasis.

Biomechanical evaluation of lateral lumbar interbody fusion with secondary augmentation

OBJECTIVE Lateral lumbar interbody fusion (LLIF) has emerged as a popular method for lumbar fusion. In this study the authors aimed to quantify the biomechanical stability of an interbody implant inserted using the LLIF approach with and without various supplemental fixation methods, including an interspinous plate (IP). METHODS Seven human cadaveric L2-5 specimens were tested intact and in 6 instrumented conditions. The interbody implant was intended to be used with supplemental fixation. In this study, however, the interbody was also tested without supplemental fixation for a relative comparison of these conditions. The instrumented conditions were as follows: 1) interbody implant without supplemental fixation (LLIF construct); and interbody implant with supplemental fixation performed using 2) unilateral pedicle screws (UPS) and rod (LLIF + UPS construct); 3) bilateral pedicle screws (BPS) and rods (LLIF + BPS construct); 4) lateral screws and lateral plate (LP) (LLIF + LP construct); 5) interbody LP and IP (LLIF + LP + IP construct); and 6) IP (LLIF + IP construct). Nondestructive, nonconstraining torque (7.5 Nm maximum) induced flexion, extension, lateral bending, and axial rotation, whereas 3D specimen range of motion (ROM) was determined optoelectronically. RESULTS The LLIF construct reduced ROM by 67% in flexion, 52% in extension, 51% in lateral bending, and 44% in axial rotation relative to intact specimens (p < 0.001). Adding BPS to the LLIF construct caused ROM to decrease by 91% in flexion, 82% in extension and lateral bending, and 74% in axial rotation compared with intact specimens (p < 0.001), providing the greatest stability among the constructs. Adding UPS to the LLIF construct imparted approximately one-half the stability provided by LLIF + BPS constructs, demonstrating significantly smaller ROM than the LLIF construct in all directions (flexion, p = 0.037; extension, p < 0.001; lateral bending, p = 0.012) except axial rotation (p = 0.07). Compared with the LLIF construct, the LLIF + LP had a significant reduction in lateral bending (p = 0.012), a moderate reduction in axial rotation (p = 0.18), and almost no benefit to stability in flexion-extension (p = 0.86). The LLIF + LP + IP construct provided stability comparable to that of the LLIF + BPS. The LLIF + IP construct provided a significant decrease in ROM compared with that of the LLIF construct alone in flexion and extension (p = 0.002), but not in lateral bending (p = 0.80) and axial rotation (p = 0.24). No significant difference was seen in flexion, extension, or axial rotation between LLIF + BPS and LLIF + IP constructs. CONCLUSIONS The LLIF construct that was tested significantly decreased ROM in all directions of loading, which indicated a measure of inherent stability. The LP significantly improved the stability of the LLIF construct in lateral bending only. Adding an IP device to the LLIF construct significantly improves stability in sagittal plane rotation. The LLIF + LP + IP construct demonstrated stability comparable to that of the gold standard 360° fixation (LLIF + BPS).

Prospective evaluation of 1-year outcomes in single-level percutaneous lumbar transfacet screw fixation in the lateral decubitus position following lateral transpsoas interbody fusion

PURPOSE

Lateral transpsoas lumbar interbody fusion (LTIF) is an accepted treatment for degenerative lumbar disc disease. Bilateral percutaneous transfacet (TF) fixation is a promising option for stabilization following LTIF. Here, we describe our experience with this technique and assess the clinical outcomes and efficacy.

METHODS

Thirty-eight consecutive patients were identified who underwent LTIF followed by bilateral percutaneous transfacet fixation in the lateral position. Preoperative and 1-year postoperative VAS scores, and operative data were prospectively recorded. One-year outcomes were also assessed according to the MacNab criteria. Fusion was assessed at 1 year via computed tomography and dynamic radiography. Two-tailed Student's t test was used to compare VAS scores.

RESULTS

Twenty-six patients underwent fusion at L4-5, 11 at L3-4, and one at L2-3; two patients were lost to follow-up. Mean operative time was 148.0 ± 47.9 min; mean blood loss was 33.0 ± 26.1 ml; mean hospital stay was 53.5 ± 51.2 h. Mean preoperative VAS scores for back and leg pain were 7.4 ± 3.0 and 7.0 ± 2.9, respectively; mean postoperative VAS scores for back and leg pain were 1.9 ± 2.4 (p < 0.0001) and 2.0 ± 3.0 (p < 0.0001), respectively. Most (89 %) patients had some relief, 72 % good to excellent and 17 % fair outcomes; eleven percent had little to no relief. There was one postoperative complication (pulmonary embolus). All patients had evidence of solid bony fusion.

CONCLUSIONS

Percutaneous transfacet fixation in the lateral position is a safe and effective alternative for fixation after LTIF and may be associated with shorter operative time and less blood loss than other posterior fixation techniques.

Biomechanical comparison of an interspinous fusion device and bilateral pedicle screw system as additional fixation for lateral lumbar interbody fusion

BACKGROUND

This investigation compares an interspinous fusion device with posterior pedicle screw system in a lateral lumbar interbody lumbar fusion.

METHODS

We biomechanically tested six cadaveric lumbar segments (L1-L2) under an axial preload of 50N and torque of 5Nm in flexion-extension, lateral bending and axial rotation directions. We quantified range of motion, neutral zone/elastic zone stiffness in the following conditions: intact, lateral discectomy, lateral cage, cage with interspinous fusion, and cage with pedicle screws.

FINDINGS

A complete lateral discectomy and annulectomy increased motion in all directions compared to all other conditions. The lateral cage reduced motion in lateral bending and flexion/extension with respect to the intact and discectomy conditions, but had minimal effect on extension stiffness. Posterior instrumentation reduced motion, excluding interspinous augmentation in axial rotation with respect to the cage condition. Interspinous fusion significantly increased flexion and extension stiffness, while pedicle screws increased flexion/extension and lateral bending stiffness, with respect to the cage condition. Both posterior augmentations performed equivalently throughout the tests except in lateral bending stiffness where pedicle screws were stiffer in the neutral zone.

INTERPRETATION

A lateral discectomy and annulectomy generates immediate instability. Stand-alone lateral cages restore a limited amount of immediate stability, but posterior supplemental fixation increases stability. Both augmentations are similar in a single level lateral fusion in-vitro model, but pedicle screws are more equipped for coronal stability. An interspinous fusion is a less invasive alternative than pedicle screws and is potentially a conservative option for various interbody cage scenarios.