Minimizing Pedicle Screw Pullout Risks: A Detailed Biomechanical Analysis of Screw Design and Placement

Optimization of S2-alar-iliac screw (S2AI) fixation in adult spine deformity using a comprehensive genetic algorithm and finite element model personalized to patient geometry and bone mechanical properties

PURPOSE

To optimize the biomechanical performance of S2AI screw fixation using a genetic algorithm (GA) and patient-specific finite element analysis integrating bone mechanical properties.

METHODS

Patient-specific pelvic finite element models (FEM), including one normal and one osteoporotic model, were created from bi-planar multi-energy X-rays (BMEXs). The genetic algorithm (GA) optimized screw parameters based on bone mass quality (BM method) while a comparative optimization method maximized the screw corridor radius (GEO method). Biomechanical performance was evaluated through simulations, comparing both methods using pullout and toggle tests.

RESULTS

The optimal screw trajectory using the BM method was more lateral and caudal with insertion angles ranging from 49° to 66° (sagittal plane) and 29° to 35° (transverse plane). In comparison, the GEO method had ranges of 44° to 54° and 24° to 30° respectively. Pullout forces (PF) using the BM method ranged from 5 to 18.4 kN, which were 2.4 times higher than the GEO method (2.1-7.7 kN). Toggle loading generated failure forces between 0.8 and 10.1 kN (BM method) and 0.9-2.9 kN (GEO method). The bone mass surrounding the screw representing the fitness score and PF of the osteoporotic case were correlated (R2 > 0.8).

CONCLUSION

Our study proposed a patient-specific FEM to optimize the S2AI screw size and trajectory using a robust BM approach with GA. This approach considers surgical constraints and consistently improves fixation performance.

Comparison of facet fusion rates and clinical outcomes between cortical bone trajectory screw and percutaneous pedicle screw fixation for degenerative lumbar spondylolisthesis

BACKGROUND CONTEXT

Cortical bone trajectory (CBT) screws have been introduced as an alternative technique for pedicle screw (PS) insertion because they have greater contact with the cortex and a greater uniaxial pullout load than traditional PS. CBT screwing can also minimize muscle dissection. However, CBT screws and traditional PSs have not yet been compared in terms of fusion rates and clinical outcomes for particular operative procedures.

PURPOSE

This study aimed to assess the fusion rate and clinical outcomes of facet fusion (FF) fixed with CBT screws (CBT-FF) and to compare them with those of FF fixed with percutaneous PS (PPS-FF).

STUDY DESIGN

Retrospective study.

PATIENT SAMPLE

Records of 68 patients who underwent CBT-FF for single-level degenerative lumbar spondylolisthesis (DLS) with at least 1 year of follow-up were retrospectively reviewed. The control group comprised 143 patients who underwent PPS-FF under the same conditions.

OUTCOME MEASURES

Computed tomography was performed to confirm fusion. Therapeutic effectiveness was assessed as a clinical outcome using the Japanese Orthopaedic Association Back Pain Evaluation Questionnaire (JOABPEQ), Roland-Morris Disability Questionnaire (RMDQ), and visual analog scale (VAS) preoperatively and 1 year postoperatively. The rate of revision surgery was also calculated. Intraoperative blood loss was measured.

METHODS

Fusion rate, clinical outcomes, revision surgery rate, and intraoperative blood loss of CBT-FF and PPS-FF were compared.

RESULTS

The CBT-FF and PPS-FF fusion rates were 91.2% and 90.1%, respectively. The JOABPEQ category scores demonstrated therapeutic effectiveness in 74.5% and 77.1% of the patients for low back pain; the corresponding proportions for walking ability were 84.7% and 89.3%, respectively. No significant differences in therapeutic effectiveness were observed for any category, including the RMDQ and VAS scores for buttock and lower limb pain. Three patients required revision surgery for adjacent segment disease between 6 months and 3.5 years after CBT-FF (revision surgery rate, 4.4%), whereas the revision surgery rate for PPS-FF was 6.3% (9/143 cases). Average intraoperative blood loss was significantly less in the CBT-FF group than in the PPS-FF group.

CONCLUSIONS

Both procedures were equally useful in terms of fusion rate and clinical outcomes for DLS management.

Influence of Pedicle Screw Insertion Depth on Posterior Lumbar Interbody Fusion: Radiological Significance of Deeper Screw Placement

STUDY DESIGN

Retrospective case series.

OBJECTIVES

To investigate the influence of screw size on achieving bone fusion in posterior lumbar interbody fusion (PLIF).

METHODS

In total, 137 consecutive patients with L4 degenerative spondylolisthesis who underwent single-level PLIF at L4-L5 were evaluated. Factors investigated for their contribution to bone fusion included: 1) age, 2) sex, 3) body mass index, 4) bone mineral density, 5) intervertebral mobility, 6) screw diameter, 7) screw length, 8) screw fitness in the pedicle (%fill), 9) screw depth in the vertebra (%depth), 10) screw angle, 11) facetectomy, 12) crosslink connector, and 13) cage material.

RESULTS

Bone fusion was confirmed in 88.2% of patients. The comparison between fusion (+) and fusion (-) groups showed no significant differences in screw size. The %fill and %length were significantly greater in the fusion (+) group than in the fusion (-) group (%fill: 58.5% ± 7.5% vs 52.3% ± 7.3%, respectively, P = .005; %depth: 59.8% ± 9.7% vs 50.3% ± 13.8%, respectively, P = .025). Multivariate logistic regression analysis revealed that %fill (odds ratio [OR]= 1.11, P = .025) and %depth (OR = 1.09, P = .003) were significant independent factors affecting bone fusion. Receiver operating characteristic curve analyses identified a %fill of 60.0% and a %depth of 54.2% as optimal cutoff values for achieving bone fusion.

CONCLUSIONS

Screw size should be determined based on the screw fitness in the pedicle (%fill > 60%) and screw insertion depth in the vertebral body (%depth > 54.2%) according to individual vertebral anatomy in L4-L5 PLIF.

A Propensity Score-Matched Cohort Study Comparing 3 Different Spine Pedicle Screw Fixation Methods: Freehand, Fluoroscopy-Guided, and Robot-Assisted Techniques

OBJECTIVE

This study aimed to compare the accuracy of robotic spine surgery and conventional pedicle screw fixation in lumbar degenerative disease. We evaluated clinical and radiological outcomes to demonstrate the noninferiority of robotic surgery.

METHODS

This study employed propensity score matching and included 3 groups: robot-assisted mini-open posterior lumbar interbody fusion (PLIF) (robotic surgery, RS), c-arm guided minimally invasive surgery transforaminal lumbar interbody fusion (C-arm guidance, CG), and freehand open PLIF (free of guidance, FG) (54 patients each). The mean follow-up period was 2.2 years. The preoperative spine condition was considered. Accuracy was evaluated using the Gertzbein-Robbins scale (GRS score) and Babu classification (Babu score). Radiological outcomes included adjacent segmental disease (ASD) and mechanical failure. Clinical outcomes were assessed based on the visual analogue scale, Oswestry Disability Index, 36-item Short Form health survey, and clinical ASD rate.

RESULTS

Accuracy was higher in the RS group (p < 0.01) than in other groups. The GRS score was lower in the CG group, whereas the Babu score was lower in the FG group compared with the RS group. No significant differences were observed in radiological and clinical outcomes among the 3 groups. Regression analysis identified preoperative facet degeneration, GRS and Babu scores as significant variables for radiological and clinical ASD. Mechanical failure was influenced by the GRS score and patients' age.

CONCLUSION

This study showed the superior accuracy of robotic spine surgery compared with conventional techniques. When combined with minimally invasive surgery, robotic surgery is advantageous with reduced ligament and muscle damage associated with traditional open procedures.

Incomplete insertion of pedicle screws triggers a higher biomechanical risk of screw loosening: mechanical tests and corresponding numerical simulations

Screw loosening is a widely reported issue after spinal screw fixation and triggers several complications. Biomechanical deterioration initially causes screw loosening. Studies have shown that incomplete insertion of pedicle screws increases the risk of screw breakage by deteriorating the local mechanical environment. However, whether this change has a biomechanical effect on the risk of screw loosening has not been determined. This study conducted comprehensive biomechanical research using polyurethane foam mechanical tests and corresponding numerical simulations to verify this topic. Pedicle screw-fixed polyurethane foam models with screws with four different insertion depths were constructed, and the screw anchoring ability of different models was verified by toggle tests with alternating and constant loads. Moreover, the stress distribution of screw and bone-screw interfaces in different models was computed in corresponding numerical mechanical models. Mechanical tests presented better screw anchoring ability with deeper screw insertion, but parameters presented no significant difference between groups with complete thread insertion. Correspondingly, higher stress values can be recorded in the model without complete thread insertion; the difference in stress values between models with complete thread insertion was relatively slight. Therefore, incomplete thread insertion triggers local stress concentration and the corresponding risk of screw loosening; completely inserting threads could effectively alleviate local stress concentration and result in the prevention of screw loosening.

How to Optimize Pedicle Screw Parameters for the Thoracic Spine? A Biomechanical and Finite Element Method Study

STUDY DESIGN

Pedicle screw study.

OBJECTIVE

The selection of pedicle screw parameters usually involves the surgeon's analysis of preoperative CT imaging along with anatomical landmarks and tactile examination. However, there is minimal consensus on a standardized guideline for selection methods on pedicle screws. We aimed to determine the effects of thoracic screw diameter to pedicle width on pullout strength determined by cortical bone purchase.

METHODS

Biomechanical study performed with human cadaveric thoracic vertebrae and experimentally validated three-dimensional finite element model instrumented with pedicle screws of various diameters. We used a variable (SD/PW) ratio to express the screw selection. We hypothesized a positive correlation between the pullout load determined by the bone purchase and the SD/PW. This relationship was first investigated in a validated finite element model considering bone purchase related to the strength of an upper thoracic vertebra. Then, the correlation to the entire spine is evaluated.

RESULTS

The failure load ranged from 371.3 to 1601.0 N, respectively, for 3 and 6 mm screws. The determinant coefficient was increased to R2=.421 when a linear relationship between pullout load and the SD/PW ratio was used. The peak loads of 1216 and 1288N were found for an SD/PW ratio of .83.

CONCLUSION

We have found that the screw pullout load is more correlated to SD/PW than other pedicle measures for a maximized SD/PW ratio of .83. This particular value should be considered the upper limit of the indicated SD/PW ratio and a means to determine the optimal screw diameter to enhance pullout strength.

Sliding on cortical shell: Biomechanical characterization of the vertebral cannulation for pedicle screw insertion

BACKGROUND

Pedicular screws pull-out has been well studied unlike their insertion. A need for characterizing cannulation before pedicle screw implantation is highlighted in literature and offers promising prospects for future intra-operation instrumentation. A reliable cannulation protocol for ex-vivo testing in swine and cadaver vertebrae is presented in this work to predict extra pedicular perforation.

METHODS

An MTS Acumen 3 A/T electrodynamic device, with a tri-axis 3 kN Kistler load cell mounted on a surgical tool was used to reproduce surgeon's gesture by moving at a constant rotational speed of 10°/mm and performing a three-section test. Perforation of the pedicle's cortical shell was planned through a design of experiment on the surgical tool angle at the entry point. Samples were scanned before and after mechanical tests and reproducibility of the protocol was tested on synthetic foam. Computation of the angle between cannulation tool and pedicle cortical shell was performed as well as cannulation coefficient of each perforation section.

FINDINGS

A total of 68 pedicles were tested: 19 perforated and 21 non-perforated human pedicles, 17 perforated and 16 non-perforated swine pedicles. The reproducibility of the protocol for cannulation coefficient computation resulted in an intraclass correlation coefficient of 0.979. Cannulation coefficients results presented variability within spinal levels as well as between swine and human model. Correlation between bone density and cannulation coefficient was found significant (p < 0.005). Torque measurement was found to be the best predictor of perforation. Threshold of angle for prediction of perforation was found to be 21.7°.

INTERPRETATION

Characterizing pedicle cannulation enables to predict extra pedicular perforation. Influence of bone mineral density and patient-specific morphology on pedicle cannulation has been highlighted together with a comparison of swine and cadaver models.

A novel method for assigning bone material properties to a comprehensive patient-specific pelvic finite element model using biplanar multi-energy radiographs

The increasing prevalence of adult spinal deformity requires long spino-pelvic instrumentation, but pelvic fixation faces challenges due to distal forces and reduced bone quality. Bi-planar multi-energy X-rays (BMEX) were used to develop a patient-specific finite element model (FEM) for evaluating pelvic fixation. Calibration involved 10 patients, and an 81-year-old female test case was used for FEM customization and pullout simulation validation. Calibration yielded a root mean square error of 74.7 mg/cm3 for HU. The simulation accurately replicated the experimental pullout test with a force of 565 N, highlighting the method's potential for optimizing biomechanical performance for pelvic fixation.

Screw Pullout Strength After Pedicle Screw Reposition: A Finite Element Analysis

RESEARCH DESIGN

Finite element analysis based on computed tomography images from the lumbar spine.

OBJECTIVE

Determined the pullout strength of unsatisfactorily placed screws and repositioned screws after unsatisfactory place in lumbar spine surgery.

BACKGROUND

Pedicle screws are widely used to stabilize the spinal vertebral body. Unsatisfactory screws could lead to surgical complications, and may need to be repositioned. Screw removal and reposition, however, may decrease pullout strength.

METHODS

We conducted a three-dimensional finite element analysis based on high-resolution computed tomography images from a 39-year-old healthy woman. Pullout strength was determined with the screw placed in different orientations at the same entry point (as selected by the Magerl method), as well as after removal and reposition. The material properties of the vertebral body and the screw were simulated by using grayscale values and verified data, respectively. A load along the screw axis was applied to the end of the screw to simulate the pullout.

RESULTS

The pullout strength was 1840.0 N with the Magerl method. For unsatisfactorily placed screws, the pullout strength was 1500.8 N at 20% overlap, 1609.6 N at 40% overlap, 1628.9 N at 60% overlap, and 1734.7 N at 80% overlap with the hypothetical screw path of the Magerl method. For repositioned screws, the pullout strength was 1763.6 N, with 20% overlap, 1728.3 N at 40% overlap, 1544.0 N at 60% overlap, and 1491.1 N at 80% overlap, with the original path. Comparison of repositioned screw with unsatisfactorily placed screw showed 14.04% decrease in pullout strength at 80% overlap, 5.21% decrease at 60% overlap, 7.37% increase at 40% overlap, and 17.51% increase at 20% overlap, with the screw path of the Magerl method.

CONCLUSIONS

Removal and reposition increased the pullout strength at 20% and 40% overlap, but decreased the pullout strength at 60% and 80% overlap. For clinical translation, we recommend removal and reposition of the screw when the overlap is in the range of 20% to 40% or less. In vitro specimen studies are needed to verify these preliminary findings.

Biomechanical Comparison of Facet Versus Laminar C2 Screws

BACKGROUND

Transpedicular or transisthmic screws for C2 instrumentation represent the gold standard; however, the anatomy is not always compatible (hypoplastic pedicles, procidentia of the vertebral artery). Laminar screws (LS) have been proposed as a rescue technique and recently, bicortical facet screws (FS). To date, the biomechanical property of FS remains unknown.

OBJECTIVE

To compare the pull-out resistance of bicortical facet (FS) vs laminar (LS) C2 screws.

METHODS

Thirty-two human cadaveric C2 vertebrae were screened by CT scan imaging and dual x-ray absorptiometry before receiving both techniques and were randomized according to side and sequence (FS or LS first). Screw positioning was validated using 2-dimensional x-rays. Sixty-four mechanical tests were performed using pure tensile loading along the axis of the screws until pull-out. Mean pull-out strengths were compared using paired tests, multivariate and survival analysis (Kaplan-Meier curves).

RESULTS

The morphometric data were consistent with previous studies. Over 64 tests, the mean pull-out strength of LS (707 ± 467 N) was significantly higher than that of FS (390 ± 230 N) ( P = .0004). Bone mineral density was weakly correlated with pull-out strength (r = 0.42 for FS and r = 0.3 for LS). Both techniques were mechanically equivalent for vertebrae in which intralaminar cortical grip was not achievable for LS. The mean pull-out strength for LS with laminar cortical grip (1071 ± 395 N) was significantly higher than that of LS without (423 ± 291 N) ( P < .0001).

CONCLUSION

Our results suggest that bicortical FS of C2 offer less mechanical resistance than LS.

Biomechanical evaluation and optimal design of a pedicle screw with double bent rods internal fixation system based on PE-PLIF fusion

Problems, such as broken screws, broken rods, and cage subsidence after clinical spinal fusion surgery affect the success rate of fusion surgery and the fixation effect of fusion segments, and these problems still affect the treatment and postoperative recovery of patients. In this study, we used the biomechanical finite element analysis method to analyze and study the fixation effect of three kinds of spinal internal fixation systems on L4-L5 lumbar spine segments in percutaneous endoscopic posterior lumbar interbody fusion (PE-PLIF). The three different fixation systems compared in this study include bilateral pedicle screw fixation (M1); bilateral pedicle screw with cross-link fixation (M2); bilateral pedicle screws with double bent rods fixation (M3). The internal fixation systems with different structures were analyzed with the help of Hypermesh, and Abaqus. It was found that the internal fixation system with double bent rods reduced screw stresses by 23.8 and 22.2% in right and left axial rotation than the traditional bilateral pedicle screw system, while titanium rod stresses were reduced by 9.6, 3.7, 9.6, and 2.9% in flexion, left and right lateral bending, and right axial rotation, respectively, and L5 upper endplate stresses were reduced by 35.5, 18.9, 38.4, 10.2, and 48.3% in flexion, left and right lateral bending, and left and right axial rotation, respectively. The spinal range of motion (ROM) of the M3 internal fixation system was less than that of the M1 and M2 internal fixation systems in left lateral bending, left lateral rotation, and right axial rotation, and the intact vertebral ROM was reduced by 93.7, 94.9, and 90.9%, respectively. The double bent rod structure of the spinal internal fixation system has better biomechanical properties, which can effectively reduce the risk of screw breakage, loosening, cage subsidence, and endplate collapse after fusion surgery.

Screw Osteointegration-Increasing Biomechanical Resistance to Pull-Out Effect

Spinal disorders cover a broad spectrum of pathologies and are among the most prevalent medical conditions. The management of these health issues was noted to be increasingly based on surgical interventions. Spinal fixation devices are often employed to improve surgery outcomes, increasing spinal stability, restoring structural integrity, and ensuring functionality. However, most of the currently used fixation tools are fabricated from materials with very different mechanical properties to native bone that are prone to pull-out effects or fail over time, requiring revision procedures. Solutions to these problems presently exploited in practice include the optimal selection of screw shape and size, modification of insertion trajectory, and utilization of bone cement to reinforce fixation constructs. Nevertheless, none of these methods are without risks and limitations. An alternative option to increasing biomechanical resistance to the pull-out effect is to tackle bone regenerative capacity and focus on screw osteointegration properties. Osteointegration was reportedly enhanced through various optimization strategies, including use of novel materials, surface modification techniques (e.g., application of coatings and topological optimization), and utilization of composites that allow synergistic effects between constituents. In this context, this paper takes a comprehensive path, starting with a brief presentation of spinal fixation devices, moving further to observations on how the pull-out strength can be enhanced with existing methods, and further focusing on techniques for implant osteointegration improvement.

Implications of navigation in thoracolumbar pedicle screw placement on screw accuracy and screw diameter/pedicle width ratio

Introduction

There is ample evidence that higher accuracy can be achieved in thoracolumbar pedicle screw placement by using spinal navigation. Still, to date, the evidence regarding the influence of the use of navigation on the screw diameter to pedicle width ratio remains limited.

Research question

The aim of this study was to investigate the implications of navigation in thoracolumbar pedicle screw placement not only on screw accuracy, but on the screw diameter to pedicle width ratio as well.

Material and methods

In this single-center single-surgeon study, 45 Patients undergoing navigated thoracolumbar pedicle screw placement were prospectively included. The results were compared with a matched comparison group of patients in which screw placement was performed under fluoroscopic guidance. The screw accuracy and the screw diameter to pedicle width ratio of every screw were compared between the groups.

Results

Screw accuracy was significantly higher in the navigation group compared to the fluoroscopic guidance group, alongside with a significant increase of the screw diameter to pedicle width ratio by approximately 10%. In addition, both the intraoperative radiation dose and the operating time tended to be lower in the study group.

Conclusion

This study was able to show that navigated thoracolumbar pedicle screw placement not only increases the accuracy of screw placement but also facilitates the selection of the adequate screw sizes, which according to the literature has positive effects on fixation strength. Meanwhile, the use of navigation did not negatively affect the time needed for surgery or the patient's intraoperative exposure to radiation.

Craniocaudal toggling increases the risk of screw loosening in osteoporotic vertebrae

BACKGROUND AND OBJECTIVE

Screw loosening remains a prominent problem for osteoporotic patients undergoing pedicle screw fixation surgeries but its underlying mechanisms are not fully understood. This study sought to examine the interactive effect of craniocaudal or axial cyclic loading (toggling) and osteoporosis on screw fixation.

METHODS

QCT-based finite element models of normal (n = 7; vBMD = 156 ± 13 mg/cm³) and osteoporotic vertebrae (n = 7; vBMD = 72 ± 6 mg/cm³) were inserted with pedicle screws and loaded with or without craniocaudal toggling. Among them, a representative normal vertebra (age: 55; BMD: 140 mg/cm³) and an osteoporotic vertebra (age: 64; BMD: 79 mg/cm³) were also loaded with or without axial toggling. The individual and interactive effects of craniocaudal toggling and osteoporosis on screw fixation strength (the force when the pull-up displacement of the screw head reached 1 mm) and bone tissue failure (characterized by equivalent plastic strain) were examined by repeated measure ANOVA.

RESULTS

A significant interactive effect between craniocaudal toggling and osteoporosis on screw fixation strength was detected (p = 0.008). Specifically, craniocaudal toggling led to a marked decrease in the fixation strength (68%, p < 0.05) and stiffness (83%, p < 0.05) only in the osteoporotic vertebrae but had no effect on screw fixation strength and stiffness of the normal vertebrae (p > 0.05). Likewise, most of the bone tissues around the screw in the osteoporotic vertebrae yielded following craniocaudal toggling whereas this result was not seen in the normal vertebrae. The axial toggling had no significant effect on bone tissue failure as well as pedicle screw fixation in normal or osteoporotic vertebrae.

CONCLUSIONS

Craniocaudal toggling substantially reduces the screw fixation strength of the osteoporotic vertebrae by progressively increasing tissue failure around the screw, and therefore may contribute to the higher rates of screw loosening in osteoporotic compared to normal patients, whereas axial toggling is not a risk factor for pedicle screw loosening in normal or osteoporotic patients.

Second-generation bone cement-injectable cannulated pedicle screws for osteoporosis: biomechanical and finite element analyses

BACKGROUND

Biomechanical and finite element analyses were performed to investigate the efficacy of second-generation bone cement-injectable cannulated pedicle screws (CICPS) in osteoporosis.

METHODS

This study used the biomechanical test module of polyurethane to simulate osteoporotic cancellous bone. Polymethylmethacrylate (PMMA) bone cement was used to anchor the pedicle screws in the module. The specimens were divided into two groups for the mechanical tests: the experimental group (second-generation CICPS) and control group (first-generation CICPS). Safety was evaluated using maximum shear force, static bending, and dynamic bending tests. Biomechanical stability evaluations included the maximum axial pullout force and rotary torque tests. X-ray imaging and computed tomography were used to evaluate the distribution of bone cement 24 h after PMMA injection, and stress distribution at the screw fracture and screw-cement-bone interface was assessed using finite element analysis.

RESULTS

Mechanical testing revealed that the experimental group (349.8 ± 28.6 N) had a higher maximum axial pullout force than the control group (277.3 ± 8.6 N; P < 0.05). The bending moments of the experimental group (128.5 ± 9.08 N) were comparable to those of the control group (113.4 ± 20.9 N; P > 0.05). The screw-in and spin-out torques of the experimental group were higher than those of the control group (spin-in, 0.793 ± 0.015 vs. 0.577 ± 0.062 N, P < 0.01; spin-out, 0.764 ± 0.027 vs. 0.612 ± 0.049 N, P < 0.01). Bone cement was mainly distributed at the front three-fifths of the screw in both groups, but the distribution was more uniform in the experimental group than in the control group. After pullout, the bone cement was closely connected to the screw, without loosening or fragmentation. In the finite element analysis, stress on the second-generation CICPS was concentrated at the proximal screw outlet, whereas stress on the first-generation CICPS was concentrated at the screw neck, and the screw-bone cement-bone interface stress of the experimental group was smaller than that of the control group.

CONCLUSION

These findings suggest that second-generation CICPS have higher safety and stability than first-generation CICPS and may be a superior choice for the treatment of osteoporosis.

Avoiding Spinal Implant Failures in Osteoporotic Patients: A Narrative Review

STUDY DESIGN

Narrative review.

OBJECTIVES

With an aging population, the prevalence of osteoporosis is continuously rising. As osseous integrity is crucial for bony fusion and implant stability, previous studies have shown osteoporosis to be associated with an increased risk for implant failure and higher reoperation rates after spine surgery. Thus, our review's purpose was to provide an update of evidence-based solutions in the surgical treatment of osteoporosis patients.

METHODS

We summarize the existing literature regarding changes associated with decreased bone mineral density (BMD) and resulting biomechanical implications for the spine as well as multidisciplinary treatment strategies to avoid implant failures in osteoporotic patients.

RESULTS

Osteoporosis is caused by an uncoupling of the bone remodeling cycle based on an unbalancing of bone resorption and formation and resulting reduced BMD. The reduction in trabecular structure, increased porosity of cancellous bone and decreased cross-linking between trabeculae cause a higher risk of complications after spinal implant-based surgeries. Thus, patients with osteoporosis require special planning considerations, including adequate preoperative evaluation and optimization. Surgical strategies aim towards maximizing screw pull-out strength, toggle resistance, as well as primary and secondary construct stability.

CONCLUSIONS

As osteoporosis plays a crucial role in the fate of patients undergoing spine surgery, surgeons need to be aware of the specific implications of low BMD. While there still is no consensus on the best course of treatment, multidisciplinary preoperative assessment and adherence to specific surgical principles help reduce the rate of implant-related complications.

Computed Tomography-Based Prediction of Lumbar Pedicle Screw Loosening

Objective

Pedicle screw loosening is one of the main complications after pedicle screw fixation. However, there are few reliable measures for prediction of screw loosening. The current study was carried out to find an effective method to use preoperative CT scanning as a predictor of screw loosening in the elderly patients and provide guidance for preoperative surgical planning.

Methods

Patients who were treated with lumbar pedicle screw fixation procedure in our department for degenerative lumbar disorders between January 2015 and January 2021 were retrospectively included in the current study. CT scan attenuation of each vertebra was measured with Hounsfield units (HU). Screw loosening was determined in postoperatively X-ray tests. One-way analysis of variance (ANOVA) and receiver operating characteristic (ROC) curve analysis were carried out with IBMSPSS 24.00 software.

Results

Screw loosening was observed in 44 of 215 patients (124 male, 91 female, average age 58.4 ± 7.6 years) during a mean follow-up time of 19.0 ± 11.2 months (range 12-32 months). No significant differences were found among the patients concerning patient gender, BMI, habit of smoking, and whether or not the patient had diabetes or suffered from spondylolisthesis (P > 0.05). The average HU value of lumbar vertebra was 122.4 ± 32.8 HU in the screw loosening group and 142.4 ± 38.2 HU in the control group, and the difference was significant (P < 0.01). ROC curve analysis revealed that the average HU value of L1-L5 has a relatively larger area under the curve (AUC) of 0.689 (95% CI: 0.605-0.773). With the sensitivity of 68% and specificity of 57%, a HU cut-off value of ≤124 HU is a plausible cut-off point to predict screw loosening.

Conclusions

A prospective CT scan HU value-based prediction can be used to decide whether or not to use screw augmentation methods. A cut-off L1-L5 average HU value of 124 HU can be used as an independent risk factor for screw loosening in instrumented lumbar vertebra. More predictive indexes should be involved to achieve higher sensitivity and specificity in future clinical practice.

Patient-specific finite element modeling of scoliotic curve progression using region-specific stress-modulated vertebral growth

PURPOSE

This study describes the creation of patient-specific (PS) osteo-ligamentous finite element (FE) models of the spine, ribcage, and pelvis, simulation of up to three years of region-specific, stress-modulated growth, and validation of simulated curve progression with patient clinical angle measurements.

RESEARCH QUESTION

Does the inclusion of region-specific, stress-modulated vertebral growth, in addition to scaling based on age, weight, skeletal maturity, and spine flexibility allow for clinically accurate scoliotic curve progression prediction in patient-specific FE models of the spine, ribcage, and pelvis?

METHODS

Frontal, lateral, and lateral bending X-Rays of five AIS patients were obtained for approximately three-year timespans. PS-FE models were generated by morphing a normative template FE model with landmark points obtained from patient X-rays at the initial X-ray timepoint. Vertebral growth behavior and response to stress, as well as model material properties were made patient-specific based on several prognostic factors. Spine curvature angles from the PS-FE models were compared to the corresponding X-ray measurements.

RESULTS

Average FE model errors were 6.3 ± 4.6°, 12.2 ± 6.6°, 8.9 ± 7.7°, and 5.3 ± 3.4° for thoracic Cobb, lumbar Cobb, kyphosis, and lordosis angles, respectively. Average error in prediction of vertebral wedging at the apex and adjacent levels was 3.2 ± 2.2°. Vertebral column stress ranged from 0.11 MPa in tension to 0.79 MPa in compression.

CONCLUSION

Integration of region-specific stress-modulated growth, as well as adjustment of growth and material properties based on patient-specific data yielded clinically useful prediction accuracy while maintaining physiological stress magnitudes. This framework can be further developed for PS surgical simulation.

Brachial artery thrombosis secondary to fixation screw pullout: Case report

The vascular supply of the shoulder and forearm are principal derivates of the ipsilateral subclavian artery. The trajectory of this arterial supply predisposes it to concomitant injuries in the shoulder and clavicular fractures proximally and elbow dislocation distally. Distal bicep tendon tears often occur most commonly in middle-aged men due to trauma to the elbow, typically in weight-bearing situations [1]. To our knowledge, this is the first case of distal biceps tendon tear repair resulting in distal brachial artery injury from displaced hardware due to postoperative re-injury. We present a case of a 41-year-old male who developed a vaso-occluding hematoma at the distal biceps secondary to a displaced fixation screw. The patient required emergency vascular surgery with embolectomy and arterial bypass. Although this patient fully recovered, the clinical course the patient experienced could have been minimized with appropriate postoperative care. This report aims to alert clinicians to the relevant local anatomy and relate it to the proposed mechanism of injury, thereby bringing attention to the importance of postoperative limb protection in at-risk patients. The timing of the injury, and the protracted rate of thrombus formation suggest that the brachial artery's thrombosis was associated with the screw pullout during reinjury of the area. Screw pullout in orthopedics is a rare phenomenon that can lead to significant complications. The risk of reinjury, screw pullout, and other complications such as thrombosis is evidence to support the careful treatment of the area postoperatively.

THE FIXATION EFFECT OF DIFFERENT TYPES OF SCREWS IN THE WHOLE OSTEOPOROTIC LUMBAR VERTEBRAE: AN FEA STUDY

Purpose: The aim of this study is to explore how pedicle screws (PSs) and cortical bone trajectory (CBT) screws differ in fixation strength when implanted in L1–L5 with osteoporosis, providing support for choosing implants and trajectories in spine internal fixation surgeries. Methods: We filtered 30 lumbar segments out from CT images of eight osteoporotic participants and simulated PS or CBT screw implantation in each segment, generating 60 vertebra-screw assembly FE models. To evaluate the fixation effect, we performed a pull-out force test simulation in each model and analyzed the maximal pull-out force, pull-out stiffness, and equivalent stress of vertebrae and screws. Results: The maximal pull-out force of PS and CBT screws in L1–L5 was in the range of 905–1552 (N) and 587–1012 (N), while the pull-out stiffness was in the range of 1990–2617 (N/mm) and 1007–1681 (N/mm). The fixation strength of PS in L4 and L5 was higher ([Formula: see text]), while in L1–L3 PS and CBT screws are similar ([Formula: see text]). The maximal stress of vertebrae and screws when PS was pulled at 0.25[Formula: see text]mm was larger than that of CBT screws. Conclusions: For patients with moderate osteoporosis, it is recommended to insert PS into L4 and L5 to attain better fixation strength, but vertebrae are more prone to fracture. Consequently, under severe osteoporosis, the implantation of CBT screws should be considered first. Bone cement injection may be necessary to consolidate the screw-vertebrae interface with osteoporosis.

A Novel Screw Modeling Approach to Study the Effects of Screw Parameters on Pullout Strength

Screw loosening remains a prominent problem for osteoporotic patients undergoing pedicle screw fixation surgeries and is affected by screw parameters (e.g., diameter, pitch and thread angle). However, the individual and interactive effects of these parameters on screw fixation are not fully understood. Furthermore, current finite element modeling of an threaded screw is less computationally efficient. To address these issues, we (1) explored a novel "simulated threaded screw" approach (virtual threads assigned to the contact elements of a simplified screw) and compared its performance with threaded and simplified screws, and (2) examined with this approach the individual and interactive effects of altering screw diameter (5.5-6.5 mm), pitch (1-2 mm) and half-thread angle (20-30°) on pullout strength of normal vertebrae. Results demonstrated that the "simulated threaded screw" approach equivalently predicted pullout strength compared to the "threaded screw" approach (R2 = 0.99, slope = 1). We further found that the pullout strength was most sensitive to the change in screw diameter, followed by thread angle, pitch and interactions of diameter*pitch or diameter*angle. In conclusion, the "simulated threaded screw" approach can achieve the same predictive capability compared to threaded modeling of the screw. The current findings may serve as useful references for planning of screw parameters, so as to improve the complication of screw loosening.

Pedicle Screw Plowing in Adolescent Idiopathic Scoliosis: How Common Is It and Is It a Problem?

STUDY DESIGN

Multicenter retrospective review.

OBJECTIVE

To calculate overall incidence of pedicle screw "plowing" in adolescent idiopathic scoliosis (AIS) patients who underwent posterior spinal fusion (PSF). To identify risk factors for pedicle screw plowing and associated postoperative outcomes, including loss of correction and revision rate.

SUMMARY OF BACKGROUND DATA

Curve correction of AIS generates perpendicular stresses that can cause pedicle screws to lose alignment and "plow" through pedicles craniocaudally.

METHODS

We reviewed records of 1057 patients who underwent PSF for AIS from 2002 to 2015. Preoperative and first postoperative erect radiographs were evaluated by two observers to determine (1) presence of plowing and (2) subsequent loss of correction (LOC). Plowing was defined as more than 25° sagittal angulation compared with pedicle axis or entry of the most dorsal part of the screw outside the pedicle projection. LOC was defined as postoperative change in focal angulation of an instrumented spinal level, when in consensus of both reviewers. Bivariate analyses were performed (alpha = 0.05).

RESULTS

Nineteen thousand five hundred sixty nine screws were assessed across our cohort of 1057 patients. Both observers agreed that 48 patients (4.5%) demonstrated plowing of more than or equal to one pedicle screw. For 72 screws (0.4%), both observers noted plowing, most commonly through the cranial cortex of the pedicle (65/72 screws) and at the lowest instrument vertebra (LIV) (17/72 screws). Factors associated with plowing included larger curves (P = 0.02); lower mean pedicle screw density (P = 0.0003); skeletal immaturity as measured by open triradiate cartilage (P = 0.04); and younger chronological age at time of surgery (P = 0.04). LOC occurred in 13 patients, most commonly at LIV (P < 0.0001). Revision rate for loss of screw fixation was higher in the plowing group (P = 0.003).

CONCLUSION

Pedicle screw plowing occurred in 4.5% of AIS patients, especially in those skeletally immature and with decreased implant density. Plowing commonly occurred in the cranial direction and was associated with LOC, particularly at the LIV.Level of Evidence: 3.

Influence of thread design on anchorage of pedicle screws in cancellous bone: an experimental and analytical analysis

Threads of modern pedicle screws can vary greatly in design. It is difficult to assess which interplay of design features is particularly advantageous for screw anchorage. This study aims to increase the understanding of the anchorage behaviour between screw and cancellous bone. Pull-out tests of six pedicle screws in two sizes each were performed on three densities of biomechanical test material. More general screw characteristics were derived from the screw design and evaluated using the test data. Selected screws were tested on body donor material. Some screw characteristics, such as compacting, are well suited to compare the different thread designs of screws with tapered core. The combination of two characteristics, one representing bone compacting and one representing thread flank area, appears to be particularly advantageous for assessing anchorage behaviour. With an equation derived from these characteristics, the pull-out strength could be calculated very accurately (mean deviation 1%). Furthermore, findings are corroborated by tests on donor material. For screws with tapered core, the design demands for good anchorage against pull-out from cancellous bone change with material density. With sufficient bone quality, screws with a high compacting effect are advantageous, while with low bone density a high thread flank area also appears necessary for better screw anchorage.

Interactive effects of various loading parameters on the fluid dynamics within the lacunar-canalicular system for a single osteocyte

The osteocyte lacunar-canalicular system (LCS) serves as a mechanotransductive core where external loading applied to the skeleton is transduced into mechanical signals (e.g., fluid shear) that can be sensed by mechanosensors (osteocytes). The fluid velocity and shear stress within the LCS are affected by various loading parameters. However, the interactive effect of distinct loading parameters on the velocity and shear stress in the LCS remains unclear. To address this issue, we developed a multiscale modeling approach, combining a poroelastic finite element (FE) model with a single osteocytic LCS unit model to calculate the flow velocity and shear stress within the LCS. Next, a sensitivity analysis was performed to investigate individual and interactive effects of strain magnitude, strain rate, number of cycles, and intervening short rests between loading cycles on the velocity and shear stress around the osteocyte. Lastly, we developed a relatively simple regression model to predict those outcomes. Our results demonstrated that the strain magnitude or rate alone were the main factors affecting the velocity and shear stress; however, the combination of these two was not directly additive, and addition of a short rest between cycles could enhance the combination of these two related factors. These results show highly interactive effects of distinct loading parameters on fluid velocity and shear stress in the LCS. Specifically, our results suggest that an enhanced fluid dynamics environment in the LCS can be achieved with a brief number of load cycles combined with short rest insertion and high strain magnitude and rate.

Anterior Transarticular Crossing Screw Placement for Atlantoaxial Instability in Children: Computed Tomography-Based Study

OBJECTIVE

The feasibility of anterior transarticular crossing screw (ATCS) fixation for atlantoaxial instability was confirmed in adults. However, atlantoaxial instability is more common in children. Therefore this study was aimed to ascertain the pediatric morphometric characteristics of ATCS in C1-2.

METHODS

Morphometric analysis was conducted on computed tomography scan in 87 pediatric patients who were divided into groups based on ages (1-6 years, 7-10 years, and 11-16 years). Measurements were taken in sagittal and axial planes of computed tomography imaging to determine the range of screw lateral angles, incline angles, and screw lengths.

RESULTS

The overall screw lengths were relatively longer in males than females. For those aged 1-6 years, the screw lengths were 25.5-32.8 mm in males and 24.2-31.3 mm in females, respectively. The screw lengths showed no difference in the 7- to 10-year group between sexes, while the incline angle was larger in females than males. And the screw lengths were 33.5-43.2 mm in males and 31.2-40.4 mm in females in the 11- to 16-year group. The screw lengths were increased with age, yet the lateral angles were decreased. We also found that the epiphyseal closure of odontoid reached 93.6% when the age was older than 7 years old. Therefore ATCS was recommended for children older than 7 years.

CONCLUSIONS

The overall screw lengths and lateral angles of ATCS were larger in male children than those in females, but the incline angles were larger in females. ATCS is feasible in children, particularly those aged 7 years or older.

Development of a Finite Element Model of the Pediatric Thoracic and Lumbar Spine, Ribcage, and Pelvis with Orthotropic Region-Specific Vertebral Growth

Finite element (FE) modeling of the spine has increasingly been applied in orthopedic precision-medicine approaches. Previously published FE models of the pediatric spine growth have made simplifications in geometry of anatomical structures, material properties, and representation of vertebral growth. To address those limitations, a comprehensive FE model of a pediatric (10-year-old) osteo-ligamentous thoracic and lumbar spine (T1-L5 with intervertebral discs (IVDs) and ligaments), ribcage, and pelvis with age- and level-specific ligament properties and orthotropic region-specific vertebral growth was developed and validated. Range of motion (ROM) measures, namely lateral bending, flexion-extension, and axial rotation, of the current 10 YO FE model were generally within reported ranges of scaled in vitro adult ROM data. Changes in T1-L5 spine height, as well as kyphosis (T2-T12) and lordosis (L1-L5) angles in the current FE model for two years of growth (from ages 10 to 12 years) were within ranges reported from corresponding pediatric clinical data. The use of such comprehensive pediatric FE models can provide clinically relevant insights into normative and pathological biomechanical responses of the spine, and also contribute to the development and optimization of clinical interventions for spine deformities.

Finite element analysis of human lumbar vertebrae in internal fixation system model with different bone density trajectories

PURPOSE

To evaluate the biomechanics effect of modified cortical bone screw technique (MCBT) with other traditional internal fixation systems on lumbar osteoporotic wet specimen.

METHODS

Four different finite element models were established using CT data: (1) lumbar osteoporosis model without internal fixation system; (2) traditional pedicle screw technology (TT) model; (3) traditional cortical bone screw technology (CBT) model; (4) MCBT model. The changes of global displacement, intervertebral disc displacement of all models and internal fixation system Von Mises stress among the three models were compared under the same physiological load.

RESULTS

Compared with the other three models, the total displacement of the modified CBT screw model was the smallest, with the maximum displacement of 0.216 mm; The intervertebral disc displacement of the modified CBT screw model was the smallest, with the maximum displacement of 0.149 mm; the internal fixation system Von Mises stress of the modified CBT screw technique model was the largest compared with the other three models, The maximum Von Mises stress is 232.73 MPa.

CONCLUSION

Compared to traditional pedicle screw and traditional CBT, MCBT has better mechanical stability, and it is of certain clinical application value.

Instrumentation of the sacroiliac joint with cylindrical threaded implants: A detailed finite element study of patient characteristics affecting fixation performance

The sacroiliac joint (SIJ) is a known pain generator that, in severe cases, may require surgical fixation to reduce intra-articular displacements and allow for arthrodesis. The objective of this computational study was to analyze how the number of implants affected SIJ stabilization with patient-specific characteristics such as the pelvic geometry and bone quality. Detailed finite element models were developed to account for three pelvises of differing anatomy. Each model was tested with a normal and low bone density (LD) under two types of loading: compression only and compression with flexion and extension moments. These models were instrumented with one to three cylindrical, threaded and fenestrated implants through a posterior oblique trajectory, requiring less muscle dissection than the more common lateral trajectory used with triangular implants. Compared with the noninstrumented pelvis, the change in range of motion (ROM) and stress distribution were used to characterize joint stabilization. Noninstrumented mobility ranged from 0.86 to 2.55 mm and from 1.37° to 6.11°. Across patient-specific characteristics, the ROM reduction with one implant varied from 3% to 21% for vertical and 15% to 47% for angular displacements. With two implants, the ROM reduction ranged from 12% to 41% for vertical and from 28% to 61% for angular displacements. Three implants, however, did not further improve the joint stability (14% to 42% for vertical and 32% to 63% for angular displacements). With respect to patient characteristics, an LD led to a decreased stabilization and a higher volume of stressed bone (>75% of yield stress). A better understanding of how patient characteristics affect the implant performance could help improve surgical planning of sacroiliac arthrodesis.

Autonomous lumbar spine pedicle screw planning using machine learning: A validation study

Introduction:

Several techniques for pedicle screw placement have been described including freehand techniques, fluoroscopy assisted, computed tomography (CT) guidance, and robotics. Image-guided surgery offers the potential to combine the benefits of CT guidance without the added radiation. This study investigated the ability of a neural network to place lumbar pedicle screws with the correct length, diameter, and angulation autonomously within radiographs without the need for human involvement.

Materials and Methods:

The neural network was trained using a machine learning process. The method combines the previously reported autonomous spine segmentation solution with a landmark localization solution. The pedicle screw placement was evaluated using the Zdichavsky, Ravi, and Gertzbein grading systems.

Results:

In total, the program placed 208 pedicle screws between the L1 and S1 spinal levels. Of the 208 placed pedicle screws, 208 (100%) had a Zdichavsky Score 1A, 206 (99.0%) of all screws were Ravi Grade 1, and Gertzbein Grade A indicating no breech. The final two screws (1.0%) had a Ravi score of 2 (<2 mm breech) and a Gertzbein grade of B (<2 mm breech).

Conclusion:

The results of this experiment can be combined with an image-guided platform to provide an efficient and highly effective method of placing pedicle screws during spinal stabilization surgery.

3D pull-out finite element simulation of the pedicle screw-trabecular bone interface at strain rates

Biomedical experimental studies such as pull-out (PO), screw loosening experience variability mechanical properties of fresh bone, legal procedures of cadaver bone samples and time-consuming problems. Finite Element Method (FEM) could overcome experimental problems in biomechanics. However, material modelling of bone is quite difficult, which has viscoelastic and viscoplastic properties. The study presents a bone material model which is constructed at the strain rates with the Johnson-Cook (JC) material model, one of the robust constitutive material models. The JC material constants of trabecular bone are determined by the curve fitting method at strain rates for the 3D PO finite element simulation, which defines the screw-bone interface relationship. The PO simulation is performed using the Abaqus/CAE software program. Bone fracture mechanisms are simulated with dynamic/explicit solutions during the PO phenomenon. The paper exposes whether the strain rate has effects on the PO performance. Moreover, simulation reveals the relationship between pedicle screw diameter and PO performance. The results obtained that the maximum pull-out force (POF) improves as both the screw diameter and the strain rate increase. For 5.5 mm diameter pedicle screw POFs were 487, 517 and 1708 N at strain rate 0.00015, 0.015 and 0.015 s^-1, respectively. The FOFs obtained from the simulation of the other screw were 730, 802 and 2008 N at strain rates 0.00015, 0.0015 and 0.015, respectively. PO phenomenon was also simulated realistically in the finite element analysis (FEA).

Bone density optimized pedicle screw instrumentation improves screw pull-out force in lumbar vertebrae

Pedicle screw instrumentation is performed in the surgical treatment of a wide variety of spinal pathologies. A common postoperative complication associated with this procedure is screw loosening. It has been shown that patient-specific screw fixation can be automated to match standard clinical practice and that failure can be estimated preoperatively using computed tomography images. Hence, we set out to optimize three-dimensional preoperative planning to achieve more mechanically robust screw purchase allowing deviation from intuitive, standard screw parameters. Toward this purpose, we employed a genetic algorithm optimization to find optimal screw sizes and trajectories by maximizing the CT derived bone mechanical properties. The method was tested on cadaveric lumbar vertebrae (L1 to L5) of four human spines (2 female/2 male; age range 60-78 years). The main boundary conditions were the predefined, level-dependent areas of possible screw entry points, as well as the automatically located pedicle structures. Finite element analysis was used to compare the genetic algorithm output to standard clinical planning of screw positioning in terms of the simulated pull-out strength. The genetic algorithm optimization successfully found screw sizes and trajectories that maximize the sum of the Young's modulus within the screw's volume for all 40 pedicle screws included in this study. Overall, there was a 26% increase in simulated pull-out strength for optimized compared to traditional screw trajectories and sizes. Our results indicate that optimizing pedicle screw instrumentation in lumbar vertebrae based on bone quality measures improves screw purchase as compared to traditional instrumentation.

Caudally directed upper-instrumented vertebra pedicle screws associated with minimized risk of proximal junctional failure in patients with long posterior spinal fusion for adult spinal deformity

BACKGROUND CONTEXT

It is unknown whether upper instrumented vertebra (UIV) pedicle screw trajectory and UIV screw-rod angle are associated with development of proximal junctional kyphosis (PJK) and/or proximal junctional failure (PJF).

PURPOSE

To determine whether (1) the cranial-caudal trajectory of UIV pedicle screws and (2) UIV screw-vertebra angle are associated with PJK and/or PJF after long posterior spinal fusion in patients with adult spinal deformity (ASD).

STUDY DESIGN/SETTING

Retrospective review.

PATIENT SAMPLE

We included 96 patients with ASD who underwent fusion from T9-T12 to the pelvis (>5 vertebrae fused) between 2008 and 2015.

OUTCOME MEASURES

Pedicle screw trajectory was measured as the UIV pedicle screw-vertebra angle (UIV-PVA), which is the mean of the two angles between the UIV superior endplate and both UIV pedicle screws. (Positive values indicate screws angled cranially; negative values indicate screws angled caudally.) We measured UIV rod-vertebra angle (UIV-RVA) between the rod at the point of screw attachment and the UIV superior endplate.

METHODS

During ≥2-year follow-up, 38 patients developed PJK, and 28 developed PJF. Mean (± standard deviation) UIV-PVA was -0.9° ± 6.0°. Mean UIV-RVA was 87° ± 5.2°. We examined the development of PJK and PJF using a UIV-PVA/UIV-RVA cutoff of 3° identified by a receiver operating characteristic curve, while controlling for osteoporosis, age, sex, and preoperative thoracic kyphosis.

RESULTS

Patients with UIV-PVA ≥3° had significantly greater odds of developing PJK (odds ratio 2.7; 95% confidence interval: 1.0-7.1) and PJF (odds ratio 3.6; 95% confidence interval: 1.3-10) compared with patients with UIV-PVA <3°. UIV-RVA was not significantly associated with development of PJK or PJF.

CONCLUSIONS

In long thoracic fusion to the pelvis for ASD, UIV-PVA ≥3° was associated with 2.7-fold greater odds of PJK and 3.6-fold greater odds of PJF compared with UIV-PVA <3°. UIV-RVA was not associated with PJK or PJF.

LEVEL OF EVIDENCE

III.

Potential contribution of pedicle screw design to loosening rate in patients with degenerative diseases of the lumbar spine: An observational study

BACKGROUND

The majority of published data report the results of biomechanical tests of various design pedicle screw performance. The clinical relevance and relative contribution of screw design to instrumentation stability have been insufficiently studied.

AIM

To estimate the contribution of screw design to rate of pedicle screw loosening in patients with degenerative diseases of the lumbar spine.

METHODS

This study is a prospective evaluation of 175 patients with degenerative diseases and instability of the lumbar spine segments. Participants underwent spinal instrumentation employing pedicle screws with posterior only or transforaminal interbody fusion. Follow-up was for 18 mo. Patients with signs of pedicle screw loosening on computed tomography were registered; logistic regression analysis was used to identify the factors that influenced the rate of loosening.

RESULTS

Parameters included in the analysis were screw geometry, type of thread, external and internal screw diameter and helical pitch, bone density in Hounsfield units, number of levels fused, instrumentation without anterior support, laminectomy, and unilateral and bilateral total facet joint resection. The rate of screw loosening decreased with the increment in outer diameter, decrease in core diameter and helical pitch. The rate of screw loosening correlated positively with the number of fused levels and decreasing bone density. Bilateral facet joint removal significantly favored pedicle screw loosening. The influence of other factors was insignificant.

CONCLUSION

Screw parameters had a significant impact on the loosening rate along with bone quality characteristics, the number of levels fused and the extensiveness of decompression. The significance of the influence of screw parameters was comparable to those of patient- and surgery-related factors. Pedicle screw loosening was influenced by helical pitch, inner and outer diameter, but screw geometry and thread type were insignificant factors.

Analyzing parsicle screws as a viable alternative to pars screws and pedicle screws for C2 posterior instrumentation fixation

Introduction

This study examined the possible clinical utility of "parsicle screws" in securing C2 instrumentation.

Methods

Ten patients' C2 vertebrae were virtually reconstructed using computer-aided design software. Pedicle, pars, and parsicle screws were virtually placed in the vertebrae.

Results

In addition to establishing the trajectory and theoretical safety of parsicle screws, this study determined that parsicle screws were significantly longer than pars screws (p = 0.005).

Conclusion

The additional length of parsicle screws may improve construct stability. As such, parsicle screws should be examined as an alternative to pars screws in patients unable to receive C2 pedicle screws.

Demographic Analysis of Pedicle Diameter, and Estimated Pedicle Screw Length of the Lumbar Spine in a Diverse Population

BACKGROUND

Recent literature confirms the importance of understanding the variability in pedicle morphology among races. These studies suggest that more detailed and reliable measurements of pedicles should be undertaken. However, there is limited data on average pedicle diameters (PDs) or estimated pedicle screw lengths (EPSLs) between diverse racial populations. We sought to determine the differences in PD and EPSL in the lumbar spine between various races: "Asian," "Black," "White," and "Hispanic" to aid in perioperative planning during instrumented spinal fusion.

METHODS

Axial cuts of 404 patients were inspected to obtain their transverse outer cortical PD as measured through the isthmus, and EPSL by measuring the posterior entry point at the longest distance, which perpendicularly transected the measured isthmic diameter, to the anterior vertebral cortex from L1 to L5. We examined the average PD and PD range at each level for each race. To determine the significance, we used a mixed analysis of variance and a post hoc analysis.

RESULTS

In this retrospective chart review the races were found to be significantly different in PD and EPSL (P < .001). Post hoc analysis using Dunn-Bonferroni correction showed that Asians had significantly smaller PDs than Blacks and Whites (P < .002 and P < .014, respectively). The White and Hispanic population had significantly longer EPSLs when compared to Blacks and Asians from L1 to L5 (P < .01).

CONCLUSIONS

This study demonstrates that there are significant differences in pedicle morphology among races that must be taken into consideration when inserting pedicle screws during lumbar spinal fusion. Knowledge of these differences is of the utmost importance in order to limit complications while improving fixation.

LEVEL OF EVIDENCE

3.

CLINICAL RELEVANCE

Pedicle morphology is variable between races and understanding these differences is important for the safe placement of pedicle screws.

Patient-Specific Finite Element Models of Posterior Pedicle Screw Fixation: Effect of Screw's Size and Geometry

Pedicle screw fixation is extensively performed to treat spine injuries or diseases and it is common for thoracolumbar fractures. Post-operative complications may arise from this surgery leading to back pain or revisions. Finite element (FE) models could be used to predict the outcomes of surgeries but should be verified when both simplified and realistic designs of screws are used. The aim of this study was to generate patient-specific Computed Tomography (CT)-based FE models of human vertebrae with two pedicle screws, verify the models, and use them to evaluate the effect of the screws' size and geometry on the mechanical properties of the screws-vertebra structure. FE models of the lumbar vertebra implanted with two pedicle screws were created from anonymized CT-scans of three patients. Compressive loads were applied to the head of the screws. The mesh size was optimized for realistic and simplified geometry of the screws with a mesh refinement study. Finally, the optimal mesh size was used to evaluate the sensitivity of the model to changes in screw's size (diameter and length) and geometry (realistic or simplified). For both simplified and realistic models, element sizes of 0.6 mm in the screw and 1.0 mm in the bone allowed to obtain relative differences of approximately 5% or lower. Changes in screw's length resulted in 4-10% differences in maximum deflection, 1-6% differences in peak stress in the screws, 10-22% differences in mean strain in the bone around the screw; changes in screw's diameter resulted in 28-36% differences in maximum deflection, 6-27% differences in peak stress in the screws, and 30-47% differences in mean strain in the bone around the screw. The maximum deflection predicted with realistic or simplified screws correlated very well (R ² = 0.99). The peak stress in screws with realistic or simplified design correlated well (R ² = 0.82) but simplified models underestimated the peak stress. In conclusion, the results showed that the diameter of the screw has a major role on the mechanics of the screw-vertebral structure for each patient. Simplified screws can be used to estimate the mechanical properties of the implanted vertebrae, but the systematic underestimation of the peak stress should be considered when interpreting the results from the FE analyses.

Unilateral Posterior Surgery for Severe Osteoporotic Vertebrae Fractures' Sequelae in Geriatric Population: Minimum 5-Year Results of 109 Patients

Objective

This study aimed to evaluate the efficacy and safety of modified posterior vertebral column resection (PVCR) combined with anterior column restoration in elderly patients presenting with thoracic or thoracolumbar osteoporotic fractures with spinal cord compression and severe pain.

Methods

One hundred nine patients with one level thoracolumbar osteoporotic fracture and at least 5 years of follow-up were included. They underwent posterior instrumentation performed with polymethymetachrylate augmented pedicle screws. A modified PVCR (unilateral costotransversectomy+hemilaminectomy) combined with the insertion of an expandable titanium cage for anterior column restoration was undertaken. Patients were evaluated clinically and radiographically.

Results

Patients had a mean age of 74.1 and a follow-up duration of 92.3 months. Mean duration of operations, hospital stays, and mean loss of blood were 172.3 minutes, 4.3 days, and 205.4 mL. All of the patients were mobilized immediately after surgery. The mean preoperative local kyphosis angle improved from 39.3° to 4.7° at the last follow-up (p = 0.003). Patients preoperative mean visual analogue score, Japanese Orthopaedic Association, and Oswestry Disability Index scores improved from 7.7/8.6/76.3 to 1.6/26.1/17.4 (p < 0.001 for all), respectively. The average 36-item Short-Form survey physical component summary/mental component summary scores at the last follow-up were 55.1/56.8. A dural tear was detected intraoperatively in 1 patient and repaired immediately.

Conclusion

Subtotal PVCR combined with the insertion of an expandable titanium cage was detected as a safe and effective method for osteoporotic vertebrae fractures’ sequelae in the older population involving spinal cord compression by enabling the decompression of the spinal canal and reconstruction of the resected segment, resulting in significant improvement in clinical and radiographic outcomes.

Application of Inverse Neural Networks for Optimal Pretension of Absorbable Mini Plate and Screw System

Mandibular fractures are common facial lesions typically treated with titanium plate and screw systems; nevertheless, this material is associated with secondary effects. Absorbable material for implants is an alternative to titanium, but there are also problems such as incomplete screw insertion and screw breakage due to high pretension in the screw caused by the insertion torque. The purpose of this paper is to find the optimal screw pretension (SP) in absorbable plate and screw systems by means of artificial neural network (ANN) and its inverse (ANNi). This optimal SP must satisfy a desired maximum von Mises strain (MVMS). For training the ANN, a database was generated by means of a design of experiments (DOE). Each DOE configuration was solved by means of finite element method (FEM) calculations. To obtain the optimal value for (SP) in the mini absorbable screw for fracture fixation, a strategy to invert the ANN is developed. Using the ANN coefficients, a sensitive study was performed to identify the influence of the design parameters in the MVMS. The optimal SP obtained was 14.9742 N. The MVMS condition was satisfied with an error less than 1.1% in comparison with FEM and ANN results. The screw shaft length is the most influencing MVMS parameter.

Resumption of sport after spinal fusion for adolescent idiopathic scoliosis: a review of the current literature

BACKGROUND

Adolescent idiopathic scoliosis (AIS) is a frequent disorder. Since patients with AIS are typically as active as age-matched controls and post-operative reduction in physical activity has detrimental effects on their well-being, return to sport (RTS) is an important perioperative concern. Aim of the present study is to review the literature concerning return to sport after spinal fusion for AIS.

METHODS

This work was carried out in accordance with Preferential Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) guidelines. The search was carried out in December 2020. Only peer-reviewed randomized controlled trials (RCTs), retrospective studies (RS), retrospective case series (RCS) and perspective cohort studies (PCS) were considered for inclusion.

RESULTS

Six studies were included; only one of them was prospective. All the authors reported a time to RTS ranging between 6 and 18 months. Between 28 and 36.6% of all patients changed sport, choosing lower impact activities, mostly due to loss of flexibility of the spine. No complications due to return to play were noted.

CONCLUSION

According to current evidence, patients who received spinal arthrodesis for AIS can safely return to any sport, even those that require extreme levels of spinal and pelvic movements such as gymnastics and golf. As there is little evidence, however, of the spinal loading that occurs during such movements, there is a lack of scientific evidence-based recommendations or guidelines surgeons and other health care providers can follow. Prospective comparative studies are needed to investigate these biomechanical and clinical issues.

LEVEL OF EVIDENCE

Level III.

Impact of Screw Diameter and Length on Pedicle Screw Fixation Strength in Osteoporotic Vertebrae: A Finite Element Analysis

Study Design

Biomechanical study.

Purpose

To quantitatively investigate the effect of screw size on screw fixation in osteoporotic vertebrae with finite element analysis (FEA)

Overview of Literature

Osteoporosis poses a challenge in spinal instrumentation; however, the selection of screw size is directly related to fixation and is closely dependent on each surgeon’s experience and preference.

Methods

Total 1,200 nonlinear FEA with various screw diameters (4.5–7.5 mm) and lengths (30–50 mm) were performed on 25 patients (seven men and 18 women; mean age, 75.2±10.8 years) with osteoporosis. The axial pullout strength, and the vertebral fixation strength of a paired-screw construct against flexion, extension, lateral bending, and axial rotation were examined. Thereafter, we calculated the equivalent stress of the bone-screw interface during nondestructive loading. Then, using diameter parameters (screw diameter or screw fitness in the pedicle [%fill]), and length parameters (screw length or screw depth in the vertebral body [%length]), multiple regression analyses were performed in order to evaluate the factors affecting various fixations.

Results

Larger diameter and longer screws significantly increased the pullout strength and vertebral fixation strength; further, they decreased the equivalent stress around the screws. Multiple regression analyses showed that the actual screw diameter and %length were factors that had a stronger effect on the fixation strength than %fill and the actual screw length. Screw diameter had a greater effect on the resistance to screw pullout and flexion and extension loading (β=0.38–0.43, p<0.01); while the %length had a greater effect on resistance to lateral bending and axial rotation loading (β=0.25–0.36, p<0.01) as well as mechanical stress of the bone-screw interface (β=−0.42, p<0.01).

Conclusions

The screw size should be determined based on the biomechanical behavior of the screws, type of mechanical force applied on the corresponding vertebra, and anatomical limitations.

Comparison of Preoperative Pedicle Screw Measurement Between Computed Tomography and Magnet Resonance Imaging

BACKGROUND

Pedicle screw fixation is commonly used in the treatment of spinal pathologies. While the biomechanical factors that affect bone fixation have been frequently described, questions remain as to which imaging modality is the ideal medium for preoperative planning. Due to its perceived superiority in assessing bony changes, computed tomography (CT) scan is assumed to be the gold standard for preparative planning, and we hypothesize that magnetic resonance imaging (MRI) is sufficiently accurate to predict screw length and diameter compared to CT.

METHODS

We retrospectively measured the length and diameter of vertebral bodies in the lumbar region in both MRI and CT and tested for differences between the modalities as well as for confounding effects of age, sex, and the presence of spondyloarthrosis.

RESULTS

We found a significant difference in pedicle screw length between CT and MRI measurements for both sides. For the left pedicle, the mean difference was 1.89 mm (95% confidence interval [CI] -3.03 to -0.75; P < .002), while for the right pedicle, the mean difference was 2.05 mm (95% CI -3.27 to -0.84; P = .001). We also found a significant difference in diameter measurements between CT and MRI for the left pedicle (0.53 mm; 95% CI 0.13 to 0.93; P = .011) but not for the right pedicle (0.36 mm; 95% CI -0.06 to 0.78; P = .094). We identified no significant effect of sex, age or spondyloarthrosis on the results (P > .05).

CONCLUSIONS

Pedicle screw planning measurements were more accurate using CT images compared to MRI images. CT scan remains the gold standard for pedicle screw planning in trauma surgery. When using MRI images, the surgeon should be aware of the differences in screw length and diameter compared to CT in order to avoid intra- and postoperative risks.

Biomechanical comparison of sacral and transarticular sacroiliac screw fixation

STUDY DESIGN

A detailed finite element analysis of screw fixation in the sacrum and pelvis.

OBJECTIVE

To biomechanically assess and compare the fixation performance of sacral and transarticular sacroiliac screws. Instrumentation constructs are used to achieve fixation and stabilization for the treatment of spinopelvic pathologies. The optimal screw trajectory and type of bone engagement to caudally anchor long fusion constructs are not yet known.

METHODS

A detailed finite element model of the sacroiliac articulation with two different bone densities was developed. Two sacral and one transarticular sacroiliac screw trajectories were modeled with different diameters (5.5 and 6.5 mm) and lengths (uni-cortical, bi-cortical and quad-cortical purchase). Axial pullout and flexion/extension toggle forces were applied on the screws representing intra and post-operative loads. The force-displacement results and von Mises stresses were used to characterize the failure pattern.

RESULTS

Overall, sacroiliac screws provided forces to failure 2.75 times higher than sacral fixation screws. On the contrary, the initial stiffness was approximately half as much for sacroiliac screws. High stresses were located at screw tips for the sacral trajectories and near the cortical bone screw entry points for the sacroiliac trajectory. Overall, the diameter and length of the screws had significant effects on the screw fixation (33% increase in force to failure; 5% increase in initial stiffness). A 20% drop in bone mineral density (lower bone quality) decreased the initial stiffness by 25% and the force to failure by 5-10%. High stresses and failure occurred at the screw tip for uni- and tri-cortical screws and were close to trabecular-cortical bone interface for bi-cortical and quad-cortical screws.

CONCLUSIONS

Sacroiliac fixation provided better anchorage than sacral fixation. The transarticular purchase of the sacroiliac trajectory resulted in differences in failure pattern and fixation performance.

Alternative Pedicle Screw Design via Biomechanical Evaluation

With the recent increase in the elderly population, many people suffer from spinal diseases, and, accordingly, spinal fusion surgery using pedicle screws has been widely applied to treat them. However, most research on pedicle screw design has been focused on the test results rather than the behavior of the screws and vertebrae. In this study, a design platform with a series of biomechanical tests and analyses were presented for pedicle screw improvement and evaluation. The platform was then applied to an alternative hybrid screw design with quadruple and double threads. An experimental apparatus was developed to investigate the bending strength of the screw, and several tests were performed based on the ASTM F1717 standard. In the experiments, it was confirmed that the alternative pedicle screw has the highest bending strength. To examine the stress distribution of pedicle screws, finite element models were established, through which it was found that the proposed pedicle screw has sufficient mechanical safety to make it acceptable for spinal fusion treatment. Finally, we conclude that the platform has good potential for the design and evaluation of pedicle screws, and the alternative dual screw design is one of the best options for spinal fusion surgery.

Biomechanical comparison of pedicle screw fixation strength in synthetic bones: Effects of screw shape, core/thread profile and cement augmentation

Pedicle screw loosening resulting from insufficient bone-screw interfacial holding power is not uncommon. The screw shape and thread profile are considered important factors of the screw fixation strength. This work investigated the difference in pullout strength between conical and cylindrical screws with three different thread designs. The effects of the thread profiles on the screw fixation strength of cannulated screws with or without cement augmentation in osteoporotic bone were also evaluated. Commercially available artificial standard L4 vertebrae and low-density polyurethane foam blocks were used as substitutes for healthy vertebrae and osteoporotic bones, respectively. The screw pullout strengths of nine screw systems were investigated (six in each). These systems included the combination of three different screw shapes (solid/cylindrical, solid/conical and cannulated/cylindrical) with three different thread profiles (fine-thread, coarse-thread and dual-core/dual-thread). Solid screws were designed for the cementless screw fixation of vertebrae using the standard samples, whereas cannulated screws were designed for the cemented screw fixation of osteoporotic bone using low-density test blocks. Following specimen preparation, a screw pullout test was conducted using a material test machine, and the maximal screw pullout strength was compared among the groups. This study demonstrated that, in healthy vertebrae, both the conical and dual-core/dual-thread designs can improve pullout strength. A combination of the conical and dual-core/dual-thread designs may achieve optimal postoperative screw stability. However, in osteoporotic bone, the thread profile have little impact on the screw fixation strength when pedicle screws are fixed with cement augmentation. Cement augmentation is the most important factor contributing to screw pullout fixation strength as compared to screw designs.

Individualized prediction of pedicle screw fixation strength with a finite element model

Pedicle screws are used for the treatment of a wide variety of spinal pathologies. A good screw holding power in bone is required for treatment success, but has so far not been predictable computationally. The goal of this study was to develop an automated tool able to predict patient-specific screw fixation strength through finite element simulation. We compared the simulation results with results from biomechanical pull-out tests performed on animal lumbar specimens. Experimental and simulation pull-out strengths were highly correlated [Formula: see text] and the mean error was 20.25%. The fixation strength was also associated to great extent with pull-out stiffness and strain energy, as well as the screw size and mean vertebral density.

A simple formula for predicting diameter of safely inserted cortical bone trajectory screws for fixation of the lower lumbar spine

BACKGROUND

Cortical bone trajectory (CBT) screws are popular for spinal fixation, but their ideal diameter has not been determined. Studies using postoperative computed tomography (CT) have revealed ample bone marrow space around 5.5-mm screws, which are commonly used. However, evidence indicates that a larger screw diameter provides a greater fixation strength. This study aimed to develop a generalizable formula for computing the diameter of CBT screws that could be inserted safely for fixation of the lower lumbar spine.

METHODS

Records of 44 consecutive patients who had undergone posterior fusion with CBT screws for single-level degenerative lumbar spondylolisthesis were retrospectively reviewed. We estimated the maximum diameter for conventional pedicle screws by the minimum diameter of the pedicle using preoperative CT (PSD). We measured the minimum endosteal diameter of the pedicle on the reconstructed plane of the postoperative CT which passed through the cannula used for the screw and estimated the maximum diameter for the CBT screws that could be inserted within the bone marrow space of the pedicle (CBTD).

RESULTS

Among the 176 pedicles measured, there were 151 (85.8%) with a PSD of 8.5 mm and 13, 7.5 mm. Because of a slight pedicle wall breach, 13 screws were excluded from the sample. There were 64 (39.3%) screws with a CBTD of 8.5 mm; 45, 7.5 mm; and 40, 6.5 mm. Of 163 screws, 156 (95.7%) had PSD minus CBTD ≤2 mm for each pedicle. PSD minus the minimum outer cortical diameter was ≤1 mm for each pedicle in 155 (95.1%) screws.

CONCLUSION

Our results show that CBT screws with a diameter 1 mm smaller than the endosteal diameter of the pedicle were inserted safely.

STUDY

Design: Clinical study.

Usefulness of a New Electronic Conductivity Device with a Pedicle Probe and a Multi-axis Angiography Unit for Inserting a C1 Lateral Mass Screw Safely and Tightly: A Technical Note

The C1 lateral mass screw (LMS) is widely used as one of the screws for atlantoaxial fixation. Tight bicortical screwing from the posterior to anterior cortical margin of the atlas is recommended. However, important structures, such as the internal carotid artery, are located around this area so precision is required to avoid injuring them. We describe the usefulness of a new electronic conductivity device (ECD) with a pedicle probe and a multi-axis angiography unit for inserting the C1 LMS. Four consecutive patients who were treated with C1 and C2 posterior fixation using an ECD and a multi-axis angiography unit in the hybrid operating room were included. All patients were treated successfully. Seven of eight bicortical screws could be inserted into the perfectly ideal location. The median (interquartile range) distance from the anterior margin of the atlas to the tip of the screw was 0.81 mm (0.43, 1.21 mm). This study suggested that the ECD and multi-axis angiography unit are useful for inserting the C1 LMS safely and tightly.

Thoracic pedicle screw fixation under axial and perpendicular loadings: A comprehensive numerical analysis

BACKGROUND

Many studies have assessed the pullout fixation strength of pedicle screws, but only a few investigated the fixation strength under non-axial forces such as the ones applied with modern instrumentation techniques. The purpose is to biomechanically compare the fixation strength of different pedicle screw dimensions, bone engagement, entry point preparation and vertebra dimensions under axial pull-out and perpendicular loadings.

METHODS

A finite element model of two thoracic vertebrae (T3, T8) with three different cortical bone thickness configurations (5th, 50th and 95th percentile) was used. Two bone engagements, two screw diameters and three entry point enlargement scenarios were numerically tested under an axial and four perpendicular forces (cranial, caudal, medial and lateral) until failure for a total of 180 simulations. Force-displacement responses were analyzed using ANOVA and Pareto charts to determine the individual effects of each parameter.

FINDINGS

The screw diameter was the predominant parameter affecting the screw anchorage in all loading directions. The larger screw diameter increased by 35% the initial stiffness and force to failure. Cortical bone removal around the entry point reduced the axial and perpendicular initial stiffness (27% and 17% respectively) and force to failure (20% and 13%). Better screw anchorage was obtained with bicortical bone engagement.

INTERPRETATION

The screw diameter and amount of cortical bone left around the entry point are essential for pedicle screw fixation in all loading scenarios. The proximity of the screw threads to the cortical bone (pedicle fill) has a major role in pedicle screw fixation.