A biomechanical comparison between cortical bone trajectory fixation and pedicle screw fixation

Comparing Cortical Bone Trajectory and Traditional Pedicle Screws in Transforaminal Lumbar Interbody Fusion: A Retrospective Cohort Study of One-Year Outcomes

INTRODUCTION

This is a retrospective study of consecutive patients undergoing transforaminal lumbar interbody fusion (TLIF) at a single institution. The objective of this study was to compare the long-term results associated with cortical bone trajectory (CBT) and traditional pedicle screw (TPS) via posterolateral approach in TLIF.

METHODS

Consecutive patients treated from November 2014 to March 2019 were included in the CBT TLIF group, while consecutive patients treated from October 2010 to August 2017 were included in the TPS TLIF group. Inclusion criteria comprised single-level or two-level TLIF for degenerative spondylolisthesis with stenosis and at least one year of clinical and radiographic follow-up. Variables of interest included pertinent preoperative, perioperative, and postoperative data. Non-parametric evaluation was performed using the Wilcoxon test. Fisher's exact test was used to assess group differences for nominal data.

RESULTS

Overall, 140 patients met the inclusion criteria; 69 patients had CBT instrumentation (mean follow-up 526 days) and 71 patients underwent instrumentation placement via TPS (mean follow-up 825 days). Examination of perioperative and postoperative outcomes demonstrate comparable results between the groups with perioperative complications, length of stay, discharge destination, surgical revision rate, and fusion rates all being similar between groups (p = 0.1; p = 0.53; p = 0.091; p = 0.61; p = 0.665, respectively).

CONCLUSIONS

CBT in the setting of TLIF offer equivalent outcomes to TPS with TLIF at both short- and long-term intervals of care.

Biomechanical Analysis of Cortical Bone Trajectory Screw Versus Bone Cement Screw for Fixation in Porcine Spinal Low Bone Mass Model

STUDY DESIGN

A prospective study of in vitro animal.

OBJECTION

To compare the biomechanics of cortical bone trajectory screw (CBT) and bone cement screw (BC) in an isolated porcine spinal low bone mass model.

SUMMARY OF BACKGROUND DATA

The choice of spinal fixation in patients with osteoporosis remains controversial. Is CBT better than BC? Research on this issue is lacking.

METHODS

Ten porcine spines with 3 segments were treated with EDTA decalcification. After 8 weeks, all the models met the criteria of low bone mass. Ten specimens were randomly divided into groups, group was implanted with CBT screw (CBT group) and the other group was implanted with bone cement screw (BC group). The biomechanical material testing machine was used to compare the porcine spine activities of the two groups in flexion, extension, bending, and axial rotation, and then insertional torque, pull-out force, and anti-compression force of the 2 groups were compared. Independent sample t test was used for comparison between groups.

RESULTS

Ten 3 segments of porcine spine models with low bone mass were established, and the bone mineral density of all models was lower than 0.75 g/cm 2 . There is no difference between the CBT and BC groups in flexion, extension, bending, and axial rotation angle, P >0.05. However, there were significant differences between the 2 groups and the control group, with P ＜0.01. The 2 groups significantly differed between the insertional torque ( P =0.03) and the screw pull-out force ( P =0.021). The anti-compression forces between the 2 groups have no significant difference between the two groups ( P =0.946).

CONCLUSIONS

The insertional torque and pull-out force of the CBT were higher than those of the BC in the isolated low bone porcine spine model. The range of motion and anti-compression ability of the model was similar between the 2 fixation methods.

Three-Dimensional Patient-Matched Template Guides Are Able to Increase Mean Diameter and Length and to Improve Accuracy of Cortical Bone Trajectory Screws: A 5-Year International Experience

OBJECTIVE

To analyze whether significant differences exist between free-hand three-dimensional (3D) planning-guided cortical bone trajectory (CBT) screw placement and 3D-printed template-guided CBT screw positioning in terms of accuracy, size of screws, and potential complications.

METHODS

In this retrospective study, data of adult patients in whom CBT screws were placed for lumbar degenerative pathologies were extracted from a prospectively collected database and analyzed. Patients in whom screws were placed using free-hand 3D planning-guided technique were compared with patients in whom screws were positioned using customized 3D-printed templates. Size of the screws, accuracy, clinical outcomes, and complications were analyzed.

RESULTS

The study evaluated 251 patients (1004 screws). The free-hand 3D planning-guided group included 158 patients (632 screws), and the 3D-printed template-guided group included 93 patients (372 screws). The 3D-printed template-guided group involved screws of larger size from L3 to S1. Differences between the 2 groups in terms of accuracy parameters reached statistical significance (P ≤ 0.05).

CONCLUSIONS

With the use of 3D patient-matched template guides, mean diameter and length of CBT screws could be safely increased due to improved accuracy of screw placement. Based on previous evidence regarding CBT biomechanical properties, these advantages could allow increased fixation strength over traditional convergent pedicle screw trajectories. Further biomechanics studies are needed.

Cortical Bone Trajectory Pedicle Screw Fixation in Surgical Treatment of Monosegmental Pyogenic Lumbar Spondylodiskitis

BACKGROUND AND STUDY OBJECT

 Pedicle screw fixation has been widely used in surgical treatment for infective lumbar spondylodiskitis to prevent instability and deformity. The cortical bone trajectory pedicle screw (CBTPS) fixation is a minimally invasive posterior spinal fixation system that runs from the pedicle's entry point of the caudiomedial region toward the cephalad-divergent direction. Successful results with CBTPS fixation have been reported to treat degenerative and osteoporotic spinal diseases. This study aims to investigate the clinical feasibility of CBTPS in the surgical treatment of pyogenic lumbar spondylodiskitis.

PATIENT AND METHODS

 We retrospectively retrieved 20 consecutive patients from two academic centers who were surgically treated for monosegmental lumbar pyogenic spondylodiskitis. The primary surgical treatment was the anterior lumbar interbody fusion with decompression, debridement, and reconstruction using an autogenous iliac strut bone graft. One to 2 weeks after the primary surgery, patients underwent a second surgery for posterior instrumentation using conventional pedicle screws (CPS; group I) and CBTPS (group II). Radiographic parameters of the deformity angle at the fusion segment and clinical parameter of visual analog scale (VAS) scores were assessed preoperatively, postoperatively, and at the last follow-up.

RESULTS

 There were 10 patients in each group. The mean follow-up periods of groups I and II were 51.10 ± 6.95 and 28.60 ± 9.31 months, respectively. Intergroup analysis indicated the two groups area age-matched (p = 0.38), but initial C-reactive protein (CRP; mg/dL, p = 0.04), CRP normalization (months, p = 0.00), and follow-up duration (months, p = 0.00) were heterogeneous. Meanwhile, deformity angles (segmental lordosis) between the two groups were not significantly different preoperatively (p = 0.25), postoperatively (p = 0.13), and at last follow-up (p = 0.38). The intragroup analysis indicated a significant postoperative increase of lordosis in both group I and II (p = 0.00 and 0.04, respectively) with subsequent subsidence. Lordosis remained increased at the last follow-up with or without significance (group I, p = 0.02; group II, p = 0.62). Both groups showed significant improvement in VAS scores (group I, p = 0.00; group II, p = 0.00).

CONCLUSION

 In monosegmental lumbar spondylodiskitis, posterior stabilization of the anterior strut bone graft by CBTPS and CPS was comparable via the radiographic parameter of segmental lordosis or deformity angle. Our observation suggests the clinical feasibility of CBTPS in the treatment of relatively mild monosegmental pyogenic lumbar spondylodiskitis.

Biomechanical comparison of pedicle screw fixation strength among three different screw trajectories using single vertebrae and one-level functional spinal unit

Three key factors are responsible for the biomechanical performance of pedicle screw fixation: screw mechanical characteristics, bone quality and insertion techniques. To the best of the authors' knowledge, no study has directly compared the biomechanical performance among three trajectories, i.e., the traditional trajectory (TT), modified trajectory (MT) and cortical bone trajectory (CBT), in a porcine model. This study compared the pullout strength and insertion torque of three trajectory methods in single vertebrae, the pullout strength and fixation stiffness including flexion, extension, and lateral bending in a one-level instrumented functional spinal unit (FSU) that mimics the in vivo configuration were clarified. A total of 18 single vertebrae and 18 FSUs were randomly assigned into three screw insertion methods (n = 6 in each trajectory group). In the TT group, the screw converged from its entry point, passed completely inside the pedicle, was parallel to the superior endplate, was located in the superior third of the vertebral body and reached to at least the anterior third of the vertebral body. In the MT group, the convergent angle was similar to that of the TT method but directed caudally to the anterior inferior margin of the vertebral body. The results of insertion torque and pullout strength in single vertebrae were analyzed; in addition, the stiffness and pullout strength in the one-level FSU were also investigated. This study demonstrated that, in single vertebrae, the insertion torque was significantly higher in CBT groups than in TT and MT groups (p < 0.05). The maximal pullout strength was significantly higher in MT groups than in TT and CBT groups (p < 0.05). There was no significant difference in stiffness in the three motions among all groups. The maximal pullout strength in FSUs of MT and CBT groups were significantly higher than the TT groups (p < 0.05). We concluded that either MT or CBT provides better biomechanical performance than TT in single vertebrae or FSUs. The lack of significance of stiffness in FSUs among three methods suggested that MT or CBT could be a reasonable alternative to TT if the traditional trajectory was not feasible.

A parametric investigation on traditional and cortical bone trajectory screws for transpedicular fixation

Background

Many studies have been conducted to compare traditional trajectory (TT) and cortical bone trajectory (CBT) screws; however, how screw parameters affect the biomechanical properties of TT and CBT screws, and so their efficacy remains to be investigated.

Methods

A finite element model was used to simulate screws with different trajectories, diameters, and lengths. Responses for implant and tissues at the adjacent and fixed segments were used as the comparison indices. The contact lengths and spanning areas of the inserted screws were defined and compared across the varieties.

Results

The trajectory and diameter had a greater impact on the responses from the implant and tissues than the length. The CBT has shorter length than the TT; however, the contact length and supporting area of the CBT within the cortical bone were 19.6%. and 14.5% higher than those of the TT, respectively. Overall, the TT and CBT were equally effective at stabilizing the instrumented segment, except for bending and rotation. The CBT experienced less adjacent segment compensations than the TT. With the same diameter and length, the TT was considerably less stressed than the CBT, especially for flexion and extension.

Conclusions

The CBT may provide less stress at adjacent segments compared with the TT. The CBT may provide more stiffer in osteoporotic segments than the TT due to greater contact with cortical bone and a wider supporting base between the paired screws. However, both entry point and insertion trajectory of the CBT should be carefully executed to avoid vertebral breach and ensure a stable cone-screw purchase.

Comparative outcomes of cortical bone trajectory screw fixation and traditional pedicle screws in lumbar fusion: A meta-analysis

BACKGROUND

Pedicular screws (PS) is often used in lumbar fusion. Cortical bone trajectory (CBT) is a novel technology in lumbar fusion with less clinical outcomes evidence. So we conduct a meta-analysis to compare the efficacy and safety between cortical bone trajectory screw fixation and traditional pedicle screws in lumbar fusion surgery.

METHODS

Multiple databases were searched for the articles about comparison of cortical bone trajectory (CBT) and traditional pedicle screws (PS) in lumbar fusion surgeries. The Meta-analysis was conducted by Revman 5.3 software. The following indicators were abstracted: visual analog scale (VAS) scores for back and leg pain, Oswestry Disability Index (ODI), Japanese Orthopedic Association (JOA), surgical duration, complications, and blood loss. The quality of the articles was assessed by the Newcastle-Ottawa Scale or Cochrane Handbook.

RESULTS

25 studies were included involving a total of 1735 patients. There is no difference in preoperative VAS scores, JOA, ODI, postoperative VAS scores and fusion rates. Besides, postoperative JOA(MD = 0.78, P = 0.02), ODI (MD = -2.09, P＝0.03), surgical duration(MD = -26.90, P = 0.02), complications(MD = 0.70, P = 0.03), and blood loss(MD = -85.27, P＝0.0009) showed greater improvement trends in CBT group than PS group with significant difference.

CONCLUSION

CBT reduced the rate of complications, surgical duration, blood loss, postoperative ODI and JOA scores. CBT technique with better postoperative outcomes achieved similar fusion rates compared with PS technique.

Is a cross-connector beneficial for single level traditional or cortical bone trajectory pedicle screw instrumentation?

The cortical bone trajectory (CBT) has been introduced with the aim of better screw hold, however, screw-rod constructs with this trajectory might provide less rigidity in lateral bending (LB) and axial rotation (AR) compared to the constructs with the traditional trajectory (TT). Therefore, the addition of a horizontal cross-connector could be beneficial in counteracting this possible inferiority. The aim of this study was to compare the primary rigidity of TT with CBT screw-rod constructs and to quantify the effect of cross-connector-augmentation in both. Spines of four human cadavers (T9 –L5) were cropped into 15 functional spine units (FSU). Eight FSUs were instrumented with TT and seven FSUs with CBT pedicle screws. The segments were tested in six loading directions in three configurations: uninstrumented, instrumented with and without cross-connector. The motion between the cranial and caudal vertebra was recorded. The range of motion (ROM) between the CBT and the TT group did not differ significantly in either configuration. Cross-connector -augmentation did reduce the ROM in AR (16.3%, 0.27°, p = 0.02), LB (2.9%, 0.07°, p = 0.03) and flexion-extension FE (2.3%, 0.04°, p = 0.02) for the TT group and in AR (20.6%, 0.31°, p = 0.01) for the CBT-group. The primary rigidity of TT and CBT single level screw-rod constructs did not show significant difference. The minimal reduction of ROM due to cross-connector-augmentation seems clinically not relevant. Based on the findings of these study there is no increased necessity to use a cross-connector in a CBT-construct.

Cortical Trajectory Screw Fixation in Lumbar Spine Surgery: A Review of the Existing Literature

Posterior lumbar fusion is a safe and effective surgical method for diseases, such as lumbar stenosis, spondylolisthesis, lumbar instability, spinal deformity, and tumor. Pedicle screw (PS) fixation was first introduced by Bouche and has been adopted as the gold standard for posterior lumbar fusion. Santoni and colleagues introduced a new methodological screw insertion technique that uses a cortical bone trajectory (CBT), described as that from a medial to lateral path in the transverse axial plane and caudal to the cephalad path in the sagittal plane through the pedicle for maximum contact of the screw with the cortical bone. Owing to the lower invasiveness, superior cortical bone contact, and reduced neurovascular injury incidence, the CBT technique has been widely used in posterior lumbar fusion; however, these advantages have not been proven in clinical/radiological and biomechanical studies. We designed the present study to review the existing evidence and evaluate the merit of CBT screw fixation. Six electronic databases were searched for relevant articles published in August 2020 using the search terms "cortical bone trajectory," "CBT spine," "CBT fixation," "cortical pedicle screws," and "cortical screws." Studies were analyzed and divided into the following groups: "biomechanics investigation," "surgical technique," and "clinical/radiological studies." Most studies compared CBT and PS fixation, and the CBT screw fixation method showed better or similar outcomes.

Guidelines for cortical screw versus pedicle screw selection from a fatigued decompressive lumbar laminectomy model show similar stability and less bone mineral density dependency

BACKGROUND

Traditional pedicle screws are the gold standard for lumbar spine fixation; however, cortical screws along the midline cortical bone trajectory may be advantageous when lumbar decompression is required. While biomechanic investigation of both techniques exists, cortical screw performance in a multi-level lumbar laminectomy and fusion model is unknown. Furthermore, longer-term viability of cortical screws following cyclic fatigue has not been investigated.

METHODS

Fourteen human specimens (L1-S1) were divided into cortical and pedicle screw treatment groups. Motion was captured for the following conditions: intact, bilateral posterior fixation (L3-L5), fixation with laminectomy at L3-L5, fixation with laminectomy and transforaminal lumbar interbody fusion at L3-L5 both prior to, and following, simulated in vivo fatigue. Following fatigue, screw pullout force was collected and "effective shear stress" [pullout force/screw surface area] (N/mm²) was calculated; comparisons and correlations were performed.

FINDINGS

In flexion-extension and lateral bending, all operative constructs significantly reduced motion compared to intact (P < 0.05), regardless of pedicle or cortical screws; only posterior fixation with and without laminectomy significantly reduced motion in axial rotation (P < 0.05). Pedicle screws significantly increased average pullout strength (944.2 N vs. 690.2 N, P < 0.05), but not the "effective shear stress" (1.01 N/mm² vs. 1.1 N/mm², P > 0.05).

INTERPRETATION

In a posterior laminectomy and fusion model, cortical screws provided equivalent stability to pedicle screw fixation, yet had significantly lower screw pullout force. No differences in "effective shear stress" warrant further investigation of the effect of screw length/diameter in the aforementioned screw trajectories.

Early Clinical Outcome of Lumbar Spinal Fixation With Cortical Bone Trajectory Pedicle Screws in Patients With Osteoporosis With Degenerative Disease

This cohort study aimed to elucidate early surgical outcomes after midline lumbar fusion (MidLIF) with cortical bone trajectory (CBT) screw fixation compared with transforaminal lumbar interbody fusion (TLIF) using traditional pedicle screw (TPS) fixation for lumbar degenerative disease (LDD) in patients with osteoporosis. The authors randomly assigned 70 patients with osteoporosis who had LDD at 1 or 2 adjacent vertebral levels to undergo either MidLIF with CBT (CBT group) or TLIF with TPS fixation (TPS group) from February 2015 to March 2016. Pre- and postoperative lumbar Japanese Orthopaedic Association (JOA) scale scores were assessed and radiographic measurements on dynamic plain radiographs and computed tomography images were analyzed. The final data analysis included 31 patients in the CBT group (mean age, 73.42±6.74 years; t-score, -2.94±0.75) and 32 patients in the TPS group (mean age, 74.84±5.37 years; t-score, -2.92±0.66). Mean JOA score improved significantly in both groups, although no intragroup differences of JOA score improvement were found at the latest follow-up evaluation (P>.05). In addition, significantly higher rates of screw loosening (28.13% vs 6.5%, P=.03) and the amount of subsidence (3.01±0.52 vs 2.49±0.45 mm, P=.02) were found in the TPS group. Rate of radiographic fusion of both groups showed no statistical difference. Both groups of patients achieved a similar rate of radiographic fusion at the 1.5-year follow-up and experienced similar intra- or postoperative complications and postoperative recovery. The MidLIF with CBT screw fixation for short-level lumbar fusion in patients with osteoporosis provided improvement of clinical symptoms comparable to that of TLIF using traditional TPS fixation. In addition, statistically significant lumbar stability was found in the CBT group compared with the TPS group. [Orthopedics. 2019; 42(5):e465-e471.].

Retrospective Analysis of EMG-evoked Potentials in Cortical Bone Trajectory Pedicle Screws

STUDY DESIGN

This is a retrospective analysis of electromyographic (EMG) stimulation thresholds of 64 cortical bone trajectory (CBT) screws.

OBJECTIVE

The authors seek to determine whether recordings below stimulation threshold correlate with CBT screw pedicle breach on computed tomographic imaging, and to explore which specific nerve roots are most at risk with this new trajectory.

SUMMARY OF BACKGROUND DATA

Intraoperative EMG monitoring has been utilized to verify accurate placement of pedicle screws. Although CBT screws are becoming increasingly popular, to the authors' knowledge there are no existing evaluations of the accuracy of intraoperative triggered EMG (tEMG) monitoring in this trajectory.

MATERIALS AND METHODS

Retrospective analysis of EMG stimulation thresholds of 64 CBT screws placed in patients at NYU Langone Medical Center from 2015-2017. EMG results including threshold values and muscle group stimulated were correlated with screw positioning determined on postoperative or intraoperative computed tomographic imaging.

RESULTS

In total, 4.7% of EMG threshold values indicated true breach, 1.6% were falsely positive for breach, 76.5% showed true absence of breach, 17.1% failed to reveal a present breach though 0% of medial breaches were undetected. L4 screws showed tEMG responses from adductor longus in 22%, L5 screws, from rectus femoris in 16.7%, and S1 screws from tibialis anterior in 50%.

CONCLUSIONS

tEMG testing is effective for medial breaches in CBT screws. In addition, there is evidence that bicortical placement of these screws causes lower stimulation values due to distal breach. Importantly, it seems that this is due in part to stimulation of the exiting nerve root at the level above.

Computer-assisted Patient-specific Prototype Template for Thoracolumbar Cortical Bone Trajectory Screw Placement: A Cadaveric Study

The cortical bone trajectory (CBT) screw technique yields effective mechanical and clinical results, improving the holding screw strength with a less invasive exposure. Accurate and safe screw placement is crucial. A patient-specific drill template with a preplanned trajectory was considered a promising solution; however, it is critical to assess the efficacy and safety of this technique. This study aims to evaluate the accuracy of patient-specific computed tomography (CT)-based rapid prototype drill guide templates for the CBT technique. CT scanning was performed in 7 cadaveric thoracolumbar spines, and a 3-dimensional reconstruction model was generated. By using computer software, we constructed drill templates that fit onto the posterior surface of thoracolumbar vertebrae with drill guides to match the CBT. In total, 80 guide templates from T11 to L5 were created from the computer models by using rapid prototyping. The drill templates were used to guide the drilling of CBT screws without any fluoroscopic control, and CT images were obtained after fixation. The entry point and direction of the planned and inserted screws were measured and compared. In total, 80 screws were inserted from T11 to L5. No misplacement or bony perforation was observed on postoperative CT scan. The patient-specific prototype template system showed the advantage of safe and accurate cortical screw placement in the thoracolumbar spine. This method showed its ability to customize the patient-specific trajectory of the spine, based on the unique morphology of the spine. The potential use of drill templates to place CBT screws is promising.

Pedicle screws with modular head vs. preassembled head used in cortical bone trajectory: Can pars and pedicle fractures be prevented in osteoporotic bone?

Pars and pedicle fractures as a result of CBT (cortical bone trajectory) during pedicle screw placement have been reported. The primary aim of the study is to compare the fracture rate between screws with modular heads to screws with standard pre-assembled tulip heads. The secondary aim of the study is to determine the potential variables that can be identified prior to instrumentation in order to predict risk of fractures. Twenty-four fresh frozen lumbar vertebrae were obtained from five different cadavers. Anatomical landmark measurements were obtained. Right and left pedicles of each vertebra were randomly instrumented with the preassembled head screws (n=24) and modular head screws (n=24) under video recording. X-ray images were obtained for measuring relative angle deviations between tapped and final screw trajectories. Finally, pullout tests were performed. Seventeen out of twenty-four (70.8%) of the spinous processes had to be excised in order to obtain proper trajectories. Six fractures occurred with pre-assembled head screws versus one in the modular head screws (p=0.04). Distances from the midline to the medial wall of the pedicle were marginally significant as a predictor for fracture (p=0.08). The pullout loads between both types of screws were not statistically different (p=0.38). Age was better correlated with pullout load than absolute bone density value (p<0.001). In conclusion, modular head screws had a significantly lower fracture rate than pre-assembled head screws for cortical bone trajectory in osteoporotic bone. There was no clear anatomic variable that could be measured pre-operatively to predict potential fracture risk in CBT.

Review of Cortical Bone Trajectory: Evidence of a New Technique

This article summarizes recent evidence on the cortical bone trajectory (CBT) obtained from published anatomical, biomechanical, and clinical studies. CBT was proposed by Santoni in 2009 as a new trajectory that can improve the fixation of pedicle screws in response to screw loosening in osteoporotic patients. Recently, research interest has been growing with increasing numbers of published series and frequent reports of new applications. We performed an online database search using the terms “cortical bone trajectory,” “pedicle screw,” “CBT spine,” “CBT fixation,” “MISS CBT,” and “traditional trajectory.” The search included the PubMed, Ovid MEDLINE, Cochrane, and Google Scholar databases, resulting in an analysis of 42 articles in total. These covered three aspects of CBT research: anatomical studies, biomechanical parameters, and clinical cases or series. Compared to the traditional trajectory, CBT improves pullout strength, provides greater stiffness in cephalocaudal and mediolateral loading, and shows superior resistance to flexion/extension; however, it is inferior in lateral bending and axial rotation. CBT seems to provide better immediate implant stability. In clinical studies, CBT has shown better perioperative results for blood loss, length of stay in hospital, and surgery time; similar or better clinical postoperative scores; and similar comorbidity, without any major fixation system complications due to instrumentation failure or screw misplacement. In addition, advantages such as less lateral exposure allow it to be used as a minimally invasive technique. However, most of the clinical studies were retrospective case series or case-control studies; prospective evidence on this technique is scarce, making a definitive comparison with the traditional trajectory difficult. Nevertheless, we can conclude that CBT is a safe technique that offers good clinical results with similar biomechanical and perioperative parameters to those of the traditional trajectory. In addition, new applications can improve its results and make it useful for additional pathologies.