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

Biomechanics after spinal decompression and posterior instrumentation

PURPOSE

The aim of this study was to elucidate segmental range of motion (ROM) before and after common decompression and fusion procedures on the lumbar spine.

METHODS

ROM of fourteen fresh-frozen human cadaver lumbar segments (L1/2: 4, L3/4: 5, L5/S1: 5) was evaluated in six loading directions: flexion/extension (FE), lateral bending (LB), lateral shear (LS), anterior shear (AS), axial rotation (AR), and axial compression/distraction (AC). ROM was tested with and without posterior instrumentation under the following conditions: 1) native 2) after unilateral laminotomy, 3) after midline decompression, and 4) after nucleotomy.

RESULTS

Median native ROM was FE 6.8°, LB 5.6°, and AR 1.7°, AS 1.8 mm, LS 1.4 mm, AC 0.3 mm. Unilateral laminotomy significantly increased ROM by 6% (FE), 3% (LB), 12% (AR), 11% (AS), and 8% (LS). Midline decompression significantly increased these numbers to 15%, 5%, 21%, 20%, and 19%, respectively. Nucleotomy further increased ROM in all directions, most substantially in AC of 153%. Pedicle screw fixation led to ROM decreases of 82% in FE, 72% in LB, 42% in AR, 31% in AS, and 17% in LS. In instrumented segments, decompression only irrelevantly affected ROM.

CONCLUSIONS

The amount of posterior decompression significantly impacts ROM of the lumbar spine. The here performed biomechanical study allows creation of a simplified rule of thumb: Increases in segmental ROM of approximately 10%, 20%, and 50% can be expected after unilateral laminotomy, midline decompression, and nucleotomy, respectively. Instrumentation decreases ROM by approximately 80% in bending moments and accompanied decompression procedures only minorly destabilize the instrumentation construct.

Residual motion of different posterior instrumentation and interbody fusion constructs

PURPOSE

To elucidate residual motion of cortical screw (CS) and pedicle screw (PS) constructs with unilateral posterior lumbar interbody fusion (ul-PLIF), bilateral PLIF (bl-PLIF), facet-sparing transforaminal lumbar interbody fusion (fs-TLIF), and facet-resecting TLIF (fr-TLIF).

METHODS

A total of 35 human cadaver lumbar segments were instrumented with PS (n = 18) and CS (n = 17). Range of motion (ROM) and relative ROM changes were recorded in flexion/extension (FE), lateral bending (LB), axial rotation (AR), lateral shear (LS), anterior shear (AS), and axial compression (AC) in five instrumentational states: without interbody fusion (wo-IF), ul-PLIF, bl-PLIF, fs-TLIF, and fr-TLIF.

RESULTS

Whereas FE, LB, AR, and AC noticeably differed between the instrumentational states, AS and LS were less prominently affected. Compared to wo-IF, ul-PLIF caused a significant increase in ROM with PS (FE + 42%, LB + 24%, AR + 34%, and AC + 77%), however, such changes were non-significant with CS. ROM was similar between wo-IF and all other interbody fusion techniques. Insertion of a second PLIF (bl-PLIF) significantly decreased ROM with CS (FE -17%, LB -26%, AR -20%, AC -51%) and PS (FE - 23%, LB - 14%, AR - 20%, AC - 45%,). Facet removal in TLIF significantly increased ROM with CS (FE + 6%, LB + 9%, AR + 17%, AC of + 23%) and PS (FE + 7%, AR + 12%, AC + 13%).

CONCLUSION

bl-PLIF and TLIF show similarly low residual motion in both PS and CS constructs, but ul-PLIF results in increased motion. The fs-TLIF technique is able to further decrease motion compared to fr-TLIF in both the CS and PS constructs.

Effects of osteoporosis on the biomechanics of various supplemental fixations co-applied with oblique lumbar interbody fusion (OLIF): a finite element analysis

Background

Oblique lumbar interbody fusion (OLIF) is an important surgical modality for the treatment of degenerative lumbar spine disease. Various supplemental fixations can be co-applied with OLIF, increasing OLIF stability and reducing complications. However, it is unclear whether osteoporosis affects the success of supplemental fixations; therefore, this study analyzed the effects of osteoporosis on various supplemental fixations co-applied with OLIF.

Methods

We developed and validated an L3-S1 finite element (FE) model; we assigned different material properties to each component and established models of the osteoporotic and normal bone lumbar spine. We explored the outcomes of OLIF combined with each of five supplemental fixations: standalone OLIF; OLIF with lateral plate fixation (OLIF + LPF); OLIF with translaminar facet joint fixation and unilateral pedicle screw fixation (OLIF + TFJF + UPSF); OLIF with unilateral pedicle screw fixation (OLIF + UPSF); and OLIF with bilateral pedicle screw fixation (OLIF + BPSF). Under the various working conditions, we calculated the ranges of motion (ROMs) of the normal bone and osteoporosis models, the maximum Mises stresses of the fixation instruments (MMSFIs), and the average Mises stresses on cancellous bone (AMSCBs).

Results

Compared with the normal bone OLIF model, no demonstrable change in any segmental ROM was apparent. The MMSFIs increased in all five osteoporotic OLIF models. In the OLIF + TFJF + UPSF model, the MMSFIs increased sharply in forward flexion and extension. The stress changes of the OLIF + UPSF, OLIF + BPSF, and OLIF + TFJF + UPSF models were similar; all stresses trended upward. The AMSCBs decreased in all five osteoporotic OLIF models during flexion, extension, lateral bending, and axial rotation. The average stress change of cancellous bone was most obvious under extension. The AMSCBs of the five OLIF models decreased by 14%, 23.44%, 21.97%, 40.56%, and 22.44% respectively.

Conclusions

For some supplemental fixations, the AMSCBs were all reduced and the MMSFIs were all increased in the osteoporotic model, compared with the OLIF model of normal bone. Therefore, the biomechanical performance of an osteoporotic model may be inferior to the biomechanical performance of a normal model for the same fixation method; in some instances, it may increase the risks of fracture and internal fixation failure.

Expandable vs Static Interbody Devices for Lateral Lumbar Interbody Fusion

Lateral lumbar interbody fusion (LLIF) has paved a way for minimally invasive surgical treatment of a wide variety of spine pathologies. Interbody devices are used to stabilize painful disc levels, provide indirect decompression of neural elements, correct deformity, restore lordosis, and provide a sound durable fusion. Through the years, new static and expandable interbody devices have been developed in an attempt to improve radiographic and clinical outcomes in lumbar spine surgery. The purpose of this article is to explore the advantages and disadvantages between static and expandable interbody devices when used in LLIF. Specifically, this article addresses the differences in subsidence, indirect decompression, restoration of lumbar lordosis, complications, patient-reported outcomes, and cost between static and expandable interbody devices.

Biomechanical analysis of stand-alone lumbar interbody cages versus 360° constructs: an in vitro and finite element investigation

OBJECTIVE

Low fusion rates and cage subsidence are limitations of lumbar fixation with stand-alone interbody cages. Various approaches to interbody cage placement exist, yet the need for supplemental posterior fixation is not clear from clinical studies. Therefore, as prospective clinical studies are lacking, a comparison of segmental kinematics, cage properties, and load sharing on vertebral endplates is needed. This laboratory investigation evaluates the mechanical stability and biomechanical properties of various interbody fixation techniques by performing cadaveric and finite element (FE) modeling studies.

METHODS

An in vitro experiment using 7 fresh-frozen human cadavers was designed to test intact spines with 1) stand-alone lateral interbody cage constructs (lateral interbody fusion, LIF) and 2) LIF supplemented with posterior pedicle screw-rod fixation (360° constructs). FE and kinematic data were used to validate a ligamentous FE model of the lumbopelvic spine. The validated model was then used to evaluate the stability of stand-alone LIF, transforaminal lumbar interbody fusion (TLIF), and anterior lumbar interbody fusion (ALIF) cages with and without supplemental posterior fixation at the L4-5 level. The FE models of intact and instrumented cases were subjected to a 400-N compressive preload followed by an 8-Nm bending moment to simulate physiological flexion, extension, bending, and axial rotation. Segmental kinematics and load sharing at the inferior endplate were compared.

RESULTS

The FE kinematic predictions were consistent with cadaveric data. The range of motion (ROM) in LIF was significantly lower than intact spines for both stand-alone and 360° constructs. The calculated reduction in motion with respect to intact spines for stand-alone constructs ranged from 43% to 66% for TLIF, 67%-82% for LIF, and 69%-86% for ALIF in flexion, extension, lateral bending, and axial rotation. In flexion and extension, the maximum reduction in motion was 70% for ALIF versus 81% in LIF for stand-alone cases. When supplemented with posterior fixation, the corresponding reduction in ROM was 76%-87% for TLIF, 86%-91% for LIF, and 90%-92% for ALIF. The addition of posterior instrumentation resulted in a significant reduction in peak stress at the superior endplate of the inferior segment in all scenarios.

CONCLUSIONS

Stand-alone ALIF and LIF cages are most effective in providing stability in lateral bending and axial rotation and less so in flexion and extension. Supplemental posterior instrumentation improves stability for all interbody techniques. Comparative clinical data are needed to further define the indications for stand-alone cages in lumbar fusion surgery.

Does the Choice of Spinal Interbody Fusion Approach Significantly Affect Adjacent Segment Mobility?

STUDY DESIGN

Biomechanical study of range of motion (ROM) at the vertebral levels adjacent to the construct of posterior pedicle screw-rod fixation with different types of lumbar interbody fusion techniques (LIF).

OBJECTIVE

To investigate the differences in adjacent segment mobility among three types of LIF: lateral lumbar interbody fusion (LLIF), transforaminal lumbar interbody fusion (TLIF), and posterior lumbar interbody fusion (PLIF).

SUMMARY OF BACKGROUND DATA

Previous studies have concluded that LLIF, TLIF, and PLIF with posterior pedicle screw-rod fixation (PSR) provide equivalent stability in cadaveric specimens and are comparable in fusion rate and functional outcome. However, long-term complications, such as adjacent segment degeneration associated with each type of interbody device, are currently unclear. Little is known about the biomechanical effects of interbody fusion technique on the mobility of adjacent segments.

METHODS

Normalized ROM data at the levels adjacent to L3-L4 PSR fixation with three different types of lumbar interbody fusion approaches (LLIF, TLIF, and PLIF) were analyzed. Intact (n = 21) and instrumented (n = 7 per group) L2-L5 cadaveric specimens were tested multidirectionally under pure moment loading (7.5 Nm). Analysis of variance of adjacent segment ROM among the groups was performed. Statistical significance was set at P < 0.05.

RESULTS

Normalized ROM was significantly greater with PLIF than with LLIF in all directions at both proximal and distal adjacent segments (P ≤ 0.02) except for axial rotation at the distal adjacent segment (P = 0.07). TLIF also had greater normalized ROM than LLIF during lateral bending at the proximal adjacent segment (P = 0.008) and during flexion, extension, and lateral bending at the distal adjacent segment (P ≤ 0.03). Normalized ROM was not significantly different between PLIF and TLIF.

CONCLUSION

The choice of lumbar interbody fusion approach influences adjacent segment motion in a cadaveric model. LLIF had the least adjacent segment motion.Level of Evidence: 3.

Does Interbody Support at L5-S1 Matter in Long Fusions to the Pelvis?: A 5-year Analysis

STUDY DESIGN

Retrospective review of prospectively collected data.

OBJECTIVE

To determine if the addition of L5-S1 interbody support in long fusion deformity constructs is associated with superior long-term clinical and radiographic outcomes. To compare the 5-year clinical and radiographic outcomes and complications between long fusion constructs with L5-S1 interbody support versus posterolateral fusion (PLF) alone.

SUMMARY OF BACKGROUND DATA

Cadaveric biomechanical studies have suggested that an interbody fusion at L5-S1 is beneficial in long fusion constructs with sacropelvic fixation. However, there is limited data reflecting the superiority of interbody support augmentation in optimizing arthrodesis and deformity correction relative to PLF alone.

METHODS

Eighty-eight consecutive adults with spinal deformity who underwent at minimum T11-pelvis posterior pedicle screw instrumentation with 5-year follow-up were included. Two cohorts were compared based on technique used at the lumbosacral junction (L5-S1): (A) no interbody (PLF; n = 23), or (B) interbody support at L5-S1 (IB; n = 65). Radiographic measurements and clinical outcome measures were compared at multiple time points. Complications were recorded and compared.

RESULTS

No differences in baseline patient characteristics between cohorts. One nonunion occurred at L5-S1 in the PLF group (P = 0.091). Initial postop sagittal alignment was better in the IB group (PLF: 6.46 cm, IB: 2.48 cm, P = 0.007); however, this was not maintained over long-term follow-up. No significant differences in proximal junctional kyphosis (PLF: 7/23, IB: 9/65, P = 0.076). Proximal junctional failure was more frequent in the PLF group (PLF: 6/23, IB: 6/65, P = 0.043). No significant differences in complications were found. Both cohorts had improvement from baseline pain and functional scores.

CONCLUSION

There is no absolute long-term advantage for lumbar interbody support in adult spinal deformity patients undergoing spinal arthrodesis to the pelvis.Level of Evidence: 3.

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.

Comparative Effectiveness of Expandable Versus Static Interbody Spacers via MIS LLIF: A 2-Year Radiographic and Clinical Outcomes Study

STUDY DESIGN

Retrospective cohort study.

OBJECTIVE

The purpose of this study is to compare the radiographic and clinical outcomes of expandable interbody spacers to static interbody spacers.

METHODS

This is a retrospective, institutional review board-exempt chart review of 62 consecutive patients diagnosed with degenerative disc disease who underwent minimally invasive spine surgery lateral lumbar interbody fusion (MIS LLIF) using static or expandable spacers. There were 27 patients treated with static spacers, and 35 with expandable spacers. Radiographic and clinical functional outcomes were collected. Statistical results were significant if P < .05.

RESULTS

Mean improvement in visual analogue scale back and leg pain scores was significantly greater in the expandable group compared to the static group at 6 and 24 months by 42.3% and 63.8%, respectively (P < .05). Average improvement in Oswestry Disability Index scores was significantly greater in the expandable group than the static group at 3, 6, 12, and 24 months by 28%, 44%, 59%, 53%, and 89%, respectively (P < .05). For disc height, the mean improvement from baseline to 24 months was greater in the static group compared to the expandable group (P < .05). Implant subsidence was significantly greater in the static group (16.1%, 5/31 levels) compared with the expandable group (6.7%, 3/45 levels; P < .05).

CONCLUSIONS

This study showed positive clinical and radiographic outcomes for patients who underwent MIS LLIF with expandable spacers compared to those with static spacers. Sagittal correction and pain relief was achieved and maintained through 24-month follow-up. The expandable group had a lower subsidence rate than the static group.