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

Preventing Iatrogenic Fibula Fractures Using the Push-Pull Technique: A Biomechanical Comparison of Unicortical Versus Bicortical Post Screws

BACKGROUND

Displaced diaphyseal fractures can be reduced using the push-pull technique, wherein a plate is affixed to the distal fragment of the fracture, a post screw is placed proximal to the plate, and a lamina spreader creates distraction. This study evaluated the load to failure and mechanism of failure of bicortical and unicortical post screws during reduction.

MATERIALS AND METHODS

Four matched pairs of cadaver legs were subjected to a 2-cm oblique osteotomy simulating a displaced, oblique diaphyseal fracture. A 6-hole compression plate was affixed to the distal fragment with 2 unicortical locking screws, and a 12-mm uni-cortical or 20-mm bicortical screw was inserted as a post screw proximal to the plate. A lamina bone spreader was used to exert a distraction force between the plate and the post screw. A mechanical actuator simulated the distraction procedure until failure. Maximum applied load, displacement, and absorbed energy were recorded and compared across unicortical and bicortical groups by paired t tests.

RESULTS

At maximum load, we found statistically significant differences in displacement (P=.003) and energy absorbed (P=.022) between the two groups. All unicortical screws failed through screw toggle and bone cut-out. Bicortical screws failed through bending, with no visible damage to the bone at the screw site.

CONCLUSION

When diaphyseal fractures are significantly shortened and require a greater distraction force to achieve reduction, bicortical screws demonstrate a higher mechanical load to failure and increased bone loss from the screw-removal site. A unicortical post screw may be used if minimal distraction is needed. [Orthopedics. 202x;4x(x):xx-xx.].

Biomechanical comparison of a new undercut thread design vs. the V-shape thread design for pedicle screws

BACKGROUND CONTEXT

Thread shape is regarded as an important factor influencing the fixation strength and osseointegration of bone screws. However, commercial pedicle screws with a V-shaped thread are prone to generating stress concentration at the bone-screw interface, thereby increasing the risk of screw loosening. Thus, modification of the pedicle-screw thread is imperative.

PURPOSE

This study aimed to investigate the fixation stability of pedicle screws with the new undercut thread design in comparison to pedicle screws with a V-shaped thread.

STUDY DESIGN

In vitro cadaveric biomechanical test and finite element analysis (FEA).

METHODS

Pedicle screws with the undercut thread (characterized by a flat crest feature and a tip-facing undercut feature) were custom-manufactured, whereas those with the V-shaped thread were procured from a commercial supplier. Fixation stability was assessed by the cyclic nonpullout compressive biomechanical testing on cadaveric female osteoporotic vertebrae. The vertical displacement and rotation angle of the 2 types of pedicle screws were calculated every 100 cycles to evaluate their resistance to migration and rotation. FEA was conducted to investigate the stress distribution and bone damage at the bone-screw interface for both types of pedicle screws.

RESULTS

Biomechanical testing revealed that the pedicle screws with the undercut thread exhibited significantly lower vertical displacement and rotation angles than the pedicle screws with the V-shape thread (P < 0.05). FEA results demonstrated a more uniform stress distribution in the bone surrounding the thread in the undercut design than in the V-shape design. Additionally, bone damage resulting from the pedicle screw was lower in the undercut design than in the V-shape design.

CONCLUSIONS

Pedicle screws with an undercut thread are less prone to migration and rotation and thus more stable in the bone than those with a V-shape thread.

CLINICAL SIGNIFICANCE

The undercut thread design may reduce the incidence of pedicle-screw loosening.

An Outline on the Advancements in Surgical Management of Osteoporosis-Associated Fractures

Osteoporosis significantly impairs bone density and increases fracture risk, representing a substantial global health challenge. The effectiveness of traditional treatments such as calcium supplementation and exercise in completely preventing fractures is limited. This review explores recent advancements in surgical techniques and treatment modalities to manage osteoporotic fractures better and improve patient outcomes. Osteoporotic fractures demand specialized surgical techniques due to compromised bone quality. Vertebroplasty and kyphoplasty are minimally invasive procedures that provide rapid pain relief and structural support using bone cement. While vertebroplasty is effective, it carries risks of cement leakage and new fractures. Kyphoplasty, with added balloon inflation, reduces leakage risk and improves vertebral height restoration but is costlier. Cement-augmented screws enhance fixation but can increase adjacent fracture risk and pose long-term complications. Surgical advancements encompass robotic-assisted surgery, offering precision and accelerated recovery, alongside biologic agents like bone morphogenetic proteins (BMPs), which enhance bone healing while reducing secondary interventions and eliminating donor site morbidity. Bone graft substitutes such as calcium phosphate cements enhance biomechanical compatibility, decrease morbidity, and reduce fracture loss and pain. Balloon kyphoplasty aids in height restoration and pain relief and diminishes the risk of subsequent vertebral fractures. Bioglass scaffolds promote bone regeneration by improving bone mineral density and lowering the incidence of new fractures. Optimal perioperative care, including patient selection, nutritional management, and early mobilization strategies, is crucial for mitigating risks in vulnerable populations. While current surgical interventions provide significant pain relief and functional benefits, ongoing research and multidisciplinary collaboration are crucial to prospectively refine these techniques and reduce the burden of osteoporosis. New technologies, such as tissue engineering and gene editing, hold potential for future treatment paradigms.

Biomechanical Comparison of Unilateral and Bilateral Pedicle Screw Fixation after Multilevel Lumbar Lateral Interbody Fusion

STUDY DESIGN

Human Cadaveric Biomechanical Study.

OBJECTIVES

Lumbar Lateral Interbody Fusion (LLIF) utilizing a wide cage has been reported as having favorable biomechanical characteristics. We examine the biomechanical stability of unilateral pedicle screw and rod fixation after multilevel LLIF utilizing 26 mm wide cages compared to bilateral fixation.

METHODS

Eight human cadaveric specimens of L1-L5 were included. Specimens were attached to a universal testing machine (MTS 30/G). Three-dimensional specimen range of motion (ROM) was recorded using an optical motion-tracking device. Specimens were tested in 3 conditions: 1) intact, 2) L1-L5 LLIF (4 levels) with unilateral rod, 3) L1-L5 LLIF with bilateral rods.

RESULTS

From the intact condition, LLIF with unilateral rod decreased flexion-extension by 77%, lateral bending by 53%, and axial rotation by 26%. In LLIF with bilateral rods, flexion-extension decreased by 83%, lateral bending by 64%, and axial rotation by 34%. Comparing unilateral and bilateral fixation, LLIF with bilateral rods reduced ROM by a further 23% in flexion-extension, 25% in lateral bending, and 11% in axial rotation. The difference was statistically significant in flexion-extension and lateral bending (P < .005).

CONCLUSIONS

Considerable decreases in ROM were observed after multilevel (4-level) LLIF utilizing 26 mm cages supplemented with both unilateral and bilateral pedicle screws and rods. The addition of bilateral fixation provides a 10-25% additional decrease in ROM. These results can inform surgeons of the incremental biomechanical benefit when considering unilateral or bilateral posterior fixation after multilevel LLIF.

Bone Quality as Measured by Hounsfield Units More Accurately Predicts Proximal Junctional Kyphosis than Vertebral Bone Quality Following Long-Segment Thoracolumbar Fusion

OBJECTIVE

To compare the prognostic power of Hounsfield units (HU) and Vertebral Bone Quality (VBQ) score for predicting proximal junctional kyphosis (PJK) following long-segment thoracolumbar fusion to the upper thoracic spine (T1-T6).

METHODS

Vertebral bone quality around the upper instrumented vertebrae (UIV) was measured using HU on preoperative CT and VBQ on preoperative MRI. Spinopelvic parameters were also categorized according to the Scoliosis Research Society-Schwab classification. Univariable analysis to identify predictors of the occurrence of PJK and survival analyses with Kaplan-Meier method and Cox regression were performed to identify predictors of time to PJK (defined as ≥10° change in Cobb angle of UIV+2 and UIV). Sensitivity analyses showed thresholds of HU < 164 and VBQ > 2.7 to be most predictive for PJK.

RESULTS

Seventy-six patients (mean age 66.0 ± 7.0 years; 27.6% male) were identified, of whom 15 suffered PJK. Significant predictors of PJK were high postoperative pelvic tilt (P = 0.038), high postoperative T1-pelvic angle (P = 0.041), and high postoperative PI-LL mismatch (P = 0.028). On survival analyses, bone quality, as assessed by the average HU of the UIV and UIV+1 was the only significant predictor of time to PJK (odds ratio [OR] 3.053; 95% CI 1.032-9.032; P = 0.044). VBQ measured using the UIV, UIV+1, UIV+2, and UIV-1 vertebrae approached, but did not reach significance (OR 2.913; 95% CI 0.797-10.646; P = 0.106).

CONCLUSIONS

In larger cohorts, VBQ may prove to be a significant predictor of PJK following long-segment thoracolumbar fusion. However, Hounsfield units on CT have greater predictive power, suggesting preoperative workup for long-segment thoracolumbar fusion benefits from computed tomography versus magnetic resonance imaging alone to identify those at increased risk of PJK.

Effect of Insertional Direction of Pedicle Screw on Screw Loosening: A Biomechanical Study on Synthetic Bone Vertebra under a Physiology-like Load

OBJECTIVES

It is now understood that pedicle screw loosening at the implant-bone interface can lead to poor screw-bone interface purchase and decreased fixation stability. Previous biomechanical tests used cadaveric vertebrae and pull-out or torque loads to assess the effect of the insertional direction of pedicle screws on screw loosening. However, these tests faced challenges in matching biomechanical differences among specimens and simulating in vivo loads applied on pedicle screws. This study aimed to evaluate the effect of the insertional direction of pedicle screws on screw loosening using tension-compression-bending loads and synthetic bone vertebrae.

METHODS

Polyaxial pedicle screws were inserted into nine synthetic bone vertebrae in three directions (three samples per group): cranial, parallel, and caudad (-10°, 0°, +10° of the pedicle screw rod to the upper plane of the vertebra, respectively). Pedicle screws in the vertebrae were loaded using a polyethylene block connected to a material testing machine. Tension-compression-bending loads (100N-250N) with 30,000 cycles were applied to the pedicle screws, and displacements were recorded and then cycle-displacement curve was drawn based on cycle number. Micro-CT scans were performed on the vertebrae after removing the pedicle screws to obtain images of the screw hole, and the screw hole volume was measured using imaging analysis software. Direct comparison of displacements was conducted via cycle-displacement curve. Screw hole volume was analyzed using analysis of variance. The correlation between the displacement, screw hole volume and the direction of pedicle screw was assessed by Spearman correlation analysis.

RESULTS

The smallest displacements were observed in the caudad group, followed by the parallel and cranial groups. The caudad group had the smallest screw hole volume (p < 0.001 and p = 0.009 compared to the cranial and parallel groups, respectively), while the volume in the parallel group was greater than that in the cranial group (p = 0.003). Correlation analysis revealed that the insertional direction of the pedicle screw was associated with the displacement (p = -0.949, p < 0.001) and screw hole volume (p = -0.944, p < 0.001).

CONCLUSION

Strong correlations were found between the insertional direction of the pedicle screw and relevant parameters, including displacement and screw hole volume. Pedicle screw insertion in the caudad direction resulted in the least pedicle screw loosening.

The Influence of Increased Pedicle Screw Diameter and Thicker Rods on Surgical Results in Adolescents Undergoing Posterior Spinal Fusion for Idiopathic Scoliosis

Background: Modern surgical techniques allow for the correction of spinal deformity, stopping its progression and improving pain relief and social and physical functioning. These instruments have different implant designs, screws, and rod diameters and can be composed of different metal alloys with different hardnesses, which can have a significant impact on the effect of correcting spinal deformities. We designed a retrospective cohort study based on the same surgical technique and spine system using different implant sizes, and compared the results across them. Methods: This is a retrospective review of adolescent idiopathic scoliosis (AIS) patients who underwent posterior spinal fusion (PSF) between 2016 and 2022 with a minimum two-year follow-up (FU) using two spinal implant systems: 5.5 and 6.0 mm diameter screws with double 5.5 mm titanium rods (Group 1 (G1)), and 6.0 and 6.5 mm diameter pedicle screws with double 6.0 mm cobalt-chromium rods (Group 2 (G2)). The evaluated data were as follows: preoperative personal data, radiographic outcomes, complications, and health-related quality of life questionnaire (HRQoL). The parameters were reviewed preoperatively, after the final fusion, and during the FU. Results: The mean age of all 260 patients at surgery was 14.8 years. The average BMI was also similar in both groups and was noted as 21. The mean levels of fusion and screw density were similar in both groups. The mean preoperative major curves (MCs) were 57.6° and 62.5° in G1 and G2, respectively. The mean flexibility of the curves was noted as 35% in G1 and 33% in G2. After definitive surgery, the mean percentage correction of the MC was better in G2 vs. G1, with 74.5% vs. 69.8%, respectively (p < 0.001). At the final FU, the average loss of correction was 5.9° for G1 and 3.2° for G2 (p < 0.001). The mean preoperative (TK) thoracic kyphosis (T2-T5) was 12.2° in G1 and 10.8° in G2. It was corrected to 15.2° in G1 and to 13° in G2. At the FFU, we noted a significant difference in the TK (T2-T5) between the groups, with 16.7° vs. 9.6° for G1 vs. G2, respectively (p < 0.001). Statistical significance was observed between the preoperative sagittal balance and the final follow-up for both groups (p < 0.001). Conclusions: AIS patients surgically treated with screws with a larger diameter and thicker and stiffer rods showed greater correction and postoperative thoracic kyphosis without implant failure. The complication rates, implant density, and clinical outcomes remained similar. The radiographic benefits reported in this cohort study suggest that large-sized screws and stiffer rods for the correction of pediatric spinal deformities are safe and very effective.

Design of a new detachable pedicle screw based on medical optical imaging inspection to improve osteoporosis and enhance vertebral body revision effect

Osteoporosis is a common bone disease that often leads to difficulty in vertebrae revision. Traditional pedicle screws are often complicated to operate and have poor visibility during implantation. A new detachable pedicle screw is needed to improve the revision effect. The aim of this study was to design a new detachable pedicle screw based on medical optical imaging to improve the outcome of vertebral revision in osteoporosis, and to improve operational feasibility and visibility. In this study, the parameters related to the degree of osteoporosis were obtained by optical imaging detection of the osteoporotic vertebral body. Then a new detachable pedicle screw was designed according to the test results to improve the effect of vertebral body revision. By preparing and optimizing the material and structure of the screw, it is ensured that it has sufficient mechanical strength and stability. Finally, the visibility and operability of the improved screw during implantation were verified by medical optical imaging. Compared with traditional screws, the new detachable pedicle screw can improve the vertebral body revision in the case of osteoporosis. The optical imaging test results show that the new screw has good visibility and maneuverability, providing more accurate guidance and positioning for the vertebral body revision operation.

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.

Increased stability due to symmetric cement volume in augmented pedicle screws? A biomechanical study

Pedicle screw instrumentation has become "state of the art" in surgical treatment of many spinal disorders. Loosening of pedicle screws due to poor bone mineral density is a frequent complication in osteoporotic patients. As prevalence of osteoporosis and spinal disorders are increasing with an aging demographic, optimizing the biomechanical properties of pedicle screw constructions and therefore outcome after spinal surgery in osteoporotic patients is a key factor in future surgical therapy. Therefore, this biomechanical study investigated the stability of polymethylmethacrylate (PMMA)-augmented pedicle screw-rod constructions under a deviating distribution of PMMA applied to the instrumentation in osteoporotic human cadaveric vertebrae. We showed that PMMA-augmented pedicle screw-rod constructions tend to be more stable than those with non-augmented pedicle screws. Further, there appears to be a larger risk of screw loosening in unilateral augmented pedicle screws than in non-augmented, therefore a highly asymmetrically distributed PMMA should be avoided.

Biomechanical effects of osteoporosis severity on the occurrence of proximal junctional kyphosis following long-segment posterior thoracolumbar fusion

BACKGROUND

Proximal junctional kyphosis is a common long-term complication in adult spinal deformity surgery that involves long-segment posterior spinal fusion. However, the underlying biomechanical mechanisms of the impact of osteoporosis on proximal junctional kyphosis remain unclear. The present study was to evaluate adjacent segment degeneration and spine mechanical instability in osteoporotic patients who underwent long-segment posterior thoracolumbar fusion.

METHODS

Finite element models of the thoracolumbar spine T1-L5 with posterior long-segment T8-L5 fusion under different degrees of osteoporosis were constructed to analyze intervertebral disc stress characterization, vertebrae mechanical transfer, and pedicle screw system loads during various motions.

FINDINGS

Compared with normal bone mass, the maximum von Mises stresses of T7 and T8 were increased by 20.32%, 22.38%, 44.69%, 4.49% and 29.48%, 17.84%, 40.95%, 3.20% during flexion, extension, lateral bending, and axial rotation in the mild osteoporosis model, and by 21.21%, 18.32%, 88.28%, 2.94% and 37.76%, 15.09%, 61.47%, -0.04% in severe osteoporosis model. The peak stresses among T6/T7, T7/T8, and T8/T9 discs were 14.77 MPa, 11.55 MPa, and 2.39 MPa under lateral bending conditions for the severe osteoporosis model, respectively. As the severity of osteoporosis increased, stress levels on SCR8 and SCR9 intensified during various movements.

INTERPRETATION

Osteoporosis had an adverse effect on proximal junctional kyphosis. The stress levels in cortical bone, intervertebral discs and screws were increased with bone mass loss, which can easily lead to intervertebral disc degeneration, bone destruction as well as screw pullout. These factors have significantly affected or accelerated the occurrence of proximal junctional kyphosis.

Effect of Pedicle Screw Size on Surgical Outcomes Following Surgery for 412 Adolescent Idiopathic Scoliosis Patients

STUDY DESIGN

Retrospective Review.

OBJECTIVE

The objective of this study was to determine differences in surgical and post-operative outcomes in AIS patients undergoing spinal deformity correction surgery using standard or large pedicle screw size.

SUMMARY OF BACKGROUND

Use of pedicle screw fixation in spinal deformity correction surgery is considered safe and effective. Still, the small size of the pedicle and the complex 3D anatomy of the thoracic spine makes screw placement challenging, with improper pedicle screw fixation leading to catastrophic complications including injuries to nerve roots, spinal cord, and major vessels. Thus, insertion of larger diameter screw sizes has raised concerns amongst surgeons, especially in the pediatric population.

MATERIALS AND METHODS

AIS patients undergoing PSF between 2013 and 2019 were included. Demographic, radiographic, and operative outcomes collected. Patients in the large screw size group (GpI) received 6.5 mm diameter screw sizes at all levels while standard screw size group (GpII) received 5.0 to 5.5 mm diameter screw sizes at all levels. Kruskall-Wallis and Fisher's exact test performed for continuous and categorical variables respectively.Subanalyses included (1) screw accuracy in patients with available CT scans, (2) stratified analysis of large- and standard-screw patients with ≥60% flexibility rate, (3) stratified analysis of large- and standard-screw patients with <60% flexibility rate, and (4) matched analysis of large- and standard-screw patients by surgeon and year of surgery.

RESULTS

GpI patients experienced significantly higher overall curve correction ( P <0.001), with 87.6% experiencing at least one grade reduction of apical vertebral rotation from preoperative to postoperative visit( P =0.008).Patients with larger screws displayed higher postoperative kyphosis. No patient experienced medial breaching.

CONCLUSION

Large screw sizes have similar safety profiles to standard screws without negatively impacting surgical and perioperative outcomes in AIS patients undergoing PSF. Additionally, coronal, sagittal, and rotational correction is superior for larger-diameter screws in AIS patients.

A finite element study and mathematical modeling of lumbar pedicle screw along with various design parameters

BACKGROUND

Lumbar pedicle screw is one of the most common and important elements in the field of lumbar surgery. It plays a great role in rectifying the spinal alignment and stabilization providing strength and stability to the affected area of spine. In spinal surgery, minimally invasive techniques and minor incisions are made which makes it less painful for the patients than the traditional methods. Moreover, the screws are not needed to be removed after the surgery which is yet another great advantage of the pedicle screw.

METHOD

In this study, 3D Finite Element (FE) model of human L4 vertebrae is taken for analysis using image processing tool. Pedicle screw design with varying mechanical and geometrical properties has been carried out at different applied loads on it along with considering the effect of frictional forces between all contact surfaces.

RESULT

Mathematical relationship among stress, strain, pitch of the screw and diameter have been developed for different thread profiles which will be beneficial for researchers for further development of pedicle screw implants.

CONCLUSION

Results from the different analysis shows that bending stress on the screw for different loads at triangular pitch is higher than the trapezoidal. Hence, trapezoidal thread is efficacious than triangular thread. In case of vertebral bone, the magnitude of stress is less for trapezoidal screw than triangular and stress has a linear relationship with pitch length. In term of strain, triangular thread develops more strain than trapezoidal thread. A set of mathematical relation has been developed for different thread profile based on pitch length, stress and strain which gives the idea about von Mises stress and strain.

Influences of Increasing Pedicle Screw Diameter on Widening Vertebral Pedicle Size during Surgery in Spinal Deformities in Children and Adolescents without Higher Risk of Pedicle and Vertebral Breaches

BACKGROUND

A very common technique for treating spinal deformities in children and adolescents is the use of segmental screws. In order to obtain proper stability and the best possible correction, the screws must first be precisely inserted. Additional factors influencing the quality and success of the operation are the size and quality of the bone, the skills of the surgeon, and biomechanical factors, i.e., the width and length of the screws used during surgery. Our study was focused on evaluating the effect of increasing the diameter of the instrumented pedicles by pedicle screws and assessing the safety of expanding these pedicles with screws of various sizes in children with spinal deformities during the growth period, using preoperative magnetic resonance imaging and postoperative computed tomography (CT) to assess and compare preoperative size measurements from MRI to postoperative CT measurements.

METHODS

We obtained data for evaluation from the available medical records and treatment histories of patients aged 2 to 18 who underwent surgical treatment of spinal deformities in the years 2016-2023. In 230 patients (28 male and 202 female), 7954 vertebral bodies were scanned by preoperative MRI, and 5080 pedicle screws were inserted during surgery, which were then assessed by postoperative CT scan. For the most accurate assessment, patients were classified into three age groups: 2-5 years (Group 1), 6-10 years (Group 2), and 11-18 years (Group 3). In addition, we studied implant subgroups: vertebral bodies with inserted pedicles of screw sizes 5.0 mm and 5.5 mm (Group S), and pedicles of screw sizes 6.0 mm, 6.5 mm, and 7.0 mm (Group L).

RESULTS

The morphology of pedicles (Lenke classification) analyzed before surgery using MRI was 55.2% type A, 33.8% type B, 4.7% type C, and 6.3% type D. The postoperative lateral and medial breaches were noted, and these did not cause any complications requiring revision surgery. The mean pedicle diameter before surgery for T1-L5 vertebral pedicles was between 3.79 (1.44) mm and 5.68 (1.64) mm. The mean expanding diameter of pedicles after surgery for T1-L5 vertebral pedicles ranged from 1.90 (0.39) mm to 2.92 (0.28) mm, which corresponds to the extension of the pedicle diameter in the mean range of 47% (4.1)-71% (3.0). We noted that the mean vertebral pedicle expansion was 49% in Group 1, 52% in Group 2, and 62% in Group 3 (N.S.), and the mean expansion for 7.0 mm screw pedicles was 78%.

CONCLUSIONS

Our study confirms that there is a wide range of expansion of the vertebral pedicle during screw insertion (up to 78%) with a low risk of lateral or medial breaches and without an increased risk of complications. The larger the diameter of the screw inserted into the pedicle, the more the pedicle expands. Pedicle measurements by preoperative MRI may be helpful for sufficient reliability in preoperative planning.

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.

Let's think beyond the pedicle: A biomechanical study of a new conceptual extra pedicular screw and hook construct

Background

Transpedicular screws have proven the test of time, yet they are not devoid of complications. Many newer techniques such as 2 D and 3D fluoroscopy,O arm Navigation assisted surgery, robotic assisted surgery have come into existence to the increase precision in pedicle screw insertion. But, complications do occur in their presence. We propose an Extra pedicular screw and hook system (EPSH) system with similar biomechanical property, better safety profile and short learning curve compared to traditional pedicle screw.

Purpose

To Compare the pull out strength of Traditional Pedicle screw Vs Extra pedicular screw and hook system(EPSH).

Methods

Biomechanical testing was conducted according ASTM F543 guidelines to compare the pull-out strength of EPSH based construct and traditional pedicle screw construct. Six saw bone samples in each group considered. Screw of 5.5 mm diameter and length of 35 mm was used in both the groups. Pull out strength assessed by giving 5 mm/min axial load. The axial load Vs displacement of the screw were recorded and plotted. The maximum load required for screw failure is noted in both the group. Statistical analysis was done.

Results

The mean peak load of pedicle screw group was found to be 1670.9 ± 393.2 N with mean displacement at peak load was found to be 13.44 ± 1.7 mm and in EPSH group it was 1416.4 ± 341.4 N and 15.78 ± 3.9 mm respectively. A paired t-test showed no statistical difference(p < 0.05) between 2 groups.

Conclusion

EPSH has shown to have almost similar biomechanical property as that pedicle screw construct. With Addition of the hook, it provides an extra rotational stability as well. Being an extra-pedicular screw it has high safety profile and needs less expertise for insertion.

Fast and versatile platform for pedicle screw insertion planning

PURPOSE

Computer-assisted surgical planning methods help to reduce the risks and costs in transpedicular fixation surgeries. However, most methods do not consider the speed and versatility of the planning as factors that improve its overall performance. In this work, we propose a method able to generate surgical plans in minimal time, within the required safety margins and accounting for the surgeon's personal preferences.

METHODS

The proposed planning module takes as input a CT image of the patient, initial-guess insertion trajectories provided by the surgeon and a reduced set of parameters, delivering optimal screw sizes and trajectories in a very reduced time frame.

RESULTS

The planning results were validated with quantitative metrics and feedback from surgeons. The whole planning pipeline can be executed at an estimated time of less than 1 min per vertebra. The surgeons remarked that the proposed trajectories remained in the safe area of the vertebra, and a Gertzbein-Robbins ranking of A or B was obtained for 95 % of them.

CONCLUSIONS

The planning algorithm is safe and fast enough to perform in both pre-operative and intra-operative scenarios. Future steps will include the improvement of the preprocessing efficiency, as well as consideration of the spine's biomechanics and intervertebral rod constraints to improve the performance of the optimisation algorithm.

Automatic Planning Tools for Lumbar Pedicle Screws: Comparison and Validation of Planning Accuracy for Self-Derived Deep-Learning-Based and Commercial Atlas-Based Approaches

Background: This ex vivo experimental study sought to compare screw planning accuracy of a self-derived deep-learning-based (DL) and a commercial atlas-based (ATL) tool and to assess robustness towards pathologic spinal anatomy. Methods: From a consecutive registry, 50 cases (256 screws in L1-L5) were randomly selected for experimental planning. Reference screws were manually planned by two independent raters. Additional planning sets were created using the automatic DL and ATL tools. Using Python, automatic planning was compared to the reference in 3D space by calculating minimal absolute distances (MAD) for screw head and tip points (mm) and angular deviation (degree). Results were evaluated for interrater variability of reference screws. Robustness was evaluated in subgroups stratified for alteration of spinal anatomy. Results: Planning was successful in all 256 screws using DL and in 208/256 (81%) using ATL. MAD to the reference for head and tip points and angular deviation was 3.93 ± 2.08 mm, 3.49 ± 1.80 mm and 4.46 ± 2.86° for DL and 7.77 ± 3.65 mm, 7.81 ± 4.75 mm and 6.70 ± 3.53° for ATL, respectively. Corresponding interrater variance for reference screws was 4.89 ± 2.04 mm, 4.36 ± 2.25 mm and 5.27 ± 3.20°, respectively. Planning accuracy was comparable to the manual reference for DL, while ATL produced significantly inferior results (p < 0.0001). DL was robust to altered spinal anatomy while planning failure was pronounced for ATL in 28/82 screws (34%) in the subgroup with severely altered spinal anatomy and alignment (p < 0.0001). Conclusions: Deep learning appears to be a promising approach to reliable automated screw planning, coping well with anatomic variations of the spine that severely limit the accuracy of ATL systems.

The modified occipital condyle screw: A quantitative anatomic study investigating the feasibility of a novel instrumented fixation technique for craniocervical fusion

Study Design

Prospective human anatomical study.

Objective

Occipitocervical fusion with occipital plate or condyle screws has shown higher failure rates in those with skeletal dysplasia. The modified occipital condyle screw connects the occipital condyle to the pars basilaris of the occipital bone that may achieve fortified bony purchase and serve as a more rigid fixation point. We evaluate anatomical feasibility of a novel cranial fixation technique designed to decrease risk of pseudarthrosis.

Materials and Methods

Occipital condyles were analyzed morphologically using multiplanar three-dimensional reconstructed, ultra-thin section computed tomography. The following parameters were obtained: occipital condyle length, maximal cross section, location of hypoglossal canal, axial and sagittal orientation of the long axis, occipital condyle pedicle (OCP) diameter, maximal length of OCP screw, and entry point.

Results

Forty patients with total of 80 occipital condyles were analyzed and the following measurements were obtained: occipital condyle length 24.1 mm (20.5-27.7, standard deviation [SD]: 2.2); condyle maximum axial cross-section 12.6 mm (9-15.8, SD: 1.9); length of OCP screw 38.9 mm (29.3-44, SD: 5.7); diameter of OCP 3.4 mm (3.2-3.6, SD: 0.2); clearance below hypoglossal canal 4.5 mm (3.4-7, SD: 1.1); and distance of screw entry point from condylar foramen 2 mm (range 0-4, SD 1.6).

Conclusion

The modified occipital condyle screw connects the condyle with the clivus through the pars basilaris and represents a safe and technically feasible approach to achieve craniocervical fusion in skeletally mature individuals. This cephalad anchor point serves as an alternate fixation point of the occipitocervical junction with increased strength of construct and decreased risk of hardware failure or pseudarthrosis given cortical bone purchase and longer screw instrumentation.

Analysis of stress and stabilization in adolescent with osteoporotic idiopathic scoliosis: finite element method

Objective: To explore the effect of osteoporosis on the stress, stability, and lumbar intervertebral disc of AIS lumbar vertebrae by finite element method. Better understand the biomechanical characteristics of osteoporotic scoliosis.Methods: Based on the CT images of normal lumbar vertebrae and lumbar vertebrae with AIS, the finite element models were established to simulate the estimated osteoporosis by changing the Young's modulus of cortical bone, cancellous bone, and endplate. Four finite element models of normal lumbar, osteoporotic lumbar, normal AIS lumbar and osteoporotic AIS lumbar were established, and the same load and boundary conditions were applied respectively. The displacement, stress, and intervertebral disc strain of the four models were compared to explore the effect of osteoporosis on the stability and injury risk of AIS.Results: After suffering from osteoporosis, under the same load, the displacement of lumbar spine increases, the stability decreases, and the stability of AIS lumbar spine decrease more obviously, especially under extension load. Suffering from osteoporosis will increase the stress of lumbar spine, AIS lumbar spine increases more obviously, and the stress is more concentrated, Osteoporotic lumbar spine only affects the strain of intervertebral disc when AIS lumbar spine bends on the concave side, resulting in greater strain behind the concave side of intervertebral disc.Conclusions: AIS patients with OP have lower lumbar stability, a higher risk of fracture of lumbar vertebrae, and spinal nerves are more likely to be compressed by intervertebral discs. OP can aggravate the scoliosis of lumbar vertebrae.

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.

Effect of the intermediate pedicle screws and their insertion depth on sagittal balance and functional outcomes of lumbar fracture

OBJECTIVE

This study aimed to examine the effect of the intermediate pedicle screws and their insertion depth on sagittal balance and functional outcomes of lumbar fracture.

METHODS

This study reviewed 1,123 patients with lumbar fractures between January 2015 and June 2019, and 97 patients were ultimately enrolled in this study: Group A: 32 patients in the four-pedicle screws fixation group; Group B: 28 patients in the six-pedicle screws fixation with long intermediate pedicle screws group; Group C: 37 patients in the six-pedicle screws fixation with short intermediate pedicle screws group. The radiographic outcomes were assessed with lumbar lordosis (LL), segmental lordosis (SL), fractured vertebral lordosis (FL), sacral slope (SS), pelvic incidence (PI), and pelvic tilt (PT). The visual analog scale (VAS) and the Oswestry disability index (ODI) scores were used for assessing functional outcomes.

RESULTS

The PI, PT, and SS showed no significant differences between the three groups (P > 0.05). Compared with Group A, Groups B and C showed better FL, SL, and LL 1 month after operation (5.96 ± 1.67/4.81 ± 1.49 vs. 8.78 ± 2.90, 24.39 ± 3.80/23.70 ± 4.10 vs. 20.09 ± 3.33, 39.07 ± 3.61/39.51 ± 3.23 vs. 36.41 ± 3.11, P < 0.05) and at final follow-up (8.75 ± 1.40/6.78 ± 1.70 vs. 11.31 ± 2.61, 22.11 ± 3.39/23.70 ± 4.10 vs. 17.66 ± 2.60, 38.04 ± 3.49/39.51 ± 3.23 vs. 35.41 ± 3.11, P < 0.05). The FL of Group C were significantly better than those of Group B 1 month after operation (4.81 ± 1.49 vs. 5.96 ± 1.67, P < 0.05) and at final follow-up (6.78 ± 1.70 vs. 8.75 ± 1.40, P < 0.05). No significant differences in VAS and ODI were found between Group A and Group B (P > 0.05). There were also no significant differences in VAS and ODI between Group A and Group C (P > 0.05). However, The VAS and ODI of Group C showed better than Group B 1 month after operation (3.05 ± 0.70 vs. 3.54 ± 0.79, 17.65 ± 3.41 vs. 19.71 ± 2.35, P < 0.05) and at final follow-up (2.19 ± 0.46 vs. 2.57 ± 0.57, 13.81 ± 2.20 vs. 15.57 ± 1.73, P < 0.05).

CONCLUSIONS

Both four-pedicle screw fixation and six-pedicle screw fixation were effective in treating lumbar fracture. However, six-pedicle screw fixation with short intermediate pedicle screws showed better radiographic and functional outcomes after surgery. Therefore, we recommend six-pedicle screws fixation with short intermediate pedicle screws for the long-term recovery of sagittal balance and function.

Comparison of Perpendicular to the Coronal Plane versus Medial Inclination for C2 Pedicle Screw Insertion Assisted by 3D Printed Navigation Template

OBJECTIVE

C2 pedicle screw insertion is very important in posterior upper cervical surgery. The traditional screw placement technique requires us to consider both medial inclination and cephalad angle, it is difficult to operate intraoperatively. This paper is to explore a novel method of C2 pedicle screw placement compared with traditional C2 pedicle screw.

METHODS

A total of 44 patients diagnosed with atlantoaxial fracture or instability from May 2018 to November 2020 were involved in this retrospective study, and they were divided into C2-PPS group (perpendicular to the coronal plane C2 screw, 24 patients) and C2-TPS group (traditional C2 pedicle screw, 20 patients). The diameter of the maximum tangential circle, distance between geometric center and median sagittal plane and screw length of PPS and TPS were measured based on the 3D model of C2, respectively. Then the 3D printed navigation templated were designed and manufactured by 3D printing to assisted the PPS and TPS placement, respectively. The surgical time and radiation exposure times during operation were recorded; the post-operative grading criteria, deviation of screw entry point and deviation of screw angle of two groups were evaluated, respectively.

RESULTS

A total of 48 screws were inserted in the C2-PPS group, and 40 screws were inserted in the C2-TPS group. There were 46 screws with grade 0 (95.8%) in the PPS group and 31 screws with grade 0 (77.5%) in the TPS group, (P = 0.03). The radiation exposure times in the C2-PPS group and C2-TPS group were 4.7 ± 1.5 and 7.8 ± 3.8, respectively, (P = 0.045). The deviations of screw entry point in the C2-PPS group and C2-TPS group were 1.2 ± 0.8 mm and 3.2 ± 1.3 mm, respectively; the deviations of screw angle in the C2-PPS group and C2-TPS group were 2.1 ± 1.6° and 4.8 ± 2.0°, respectively, (P = 0.000). The diameters of the maximum tangential circle in the C2-PPS group and C2-TPS group were 5.5 ± 1.0 mm and 5.3 ± 0.9 mm, respectively. The distances between the geometric center and median sagittal plane in the C2-PPS group and C2-TPS group were 15.4 ± 2.3 mm and 18.0 ± 3.3 mm, respectively; The screw lengths in the C2-PPS group and C2-TPS group were 25.9 ± 3.2 mm and 27.6 ± 3.7 mm, respectively, (P = 0.000).

CONCLUSION

Eighty percent of C2-PPS corridor can accommodate a 3.5 mm diameter screw, and with an average screw length of 26 mm. Navigation templates assisted the C2-PPS placement is less surgical time, less radiation exposure times, more safe and more accurate than C2-TPS.

A Review of Strategies to Improve Biomechanical Fixation in the Cervical Spine

STUDY DESIGN

Systematic review.

OBJECTIVES

Review the surgical techniques and construct options aimed at improving the biomechanical strength of cervical constructs.

METHODS

A systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. A search of the MEDLINE, Embase, and Cochrane Library databases was performed to identify all studies examining biomechanical strategies utilized in the osteoporotic cervical spine. Screening was performed in duplicate for all stages of the review process.

RESULTS

An initial search returned 3887 articles. After deletion of duplications and review of abstracts and full text, 39 articles met inclusion criteria. Overall, the surgical techniques reviewed aimed at obtaining rigid fixation in the setting of poor bone quality, or dispersing the forces at the bone-implant interface. We identified 6 key techniques to improve biomechanical fixation. These include bicortical fixation, appropriate screw selection (size and trajectory), PMMA augmentation, load sharing techniques, consideration of ancillary fixation around the occipitocervical junction, and supplementing the construct with post-operative collar or halo.

CONCLUSION

The summation of the literature highlights a framework of modalities available to surgeons to improve biomechanical fixation in the cervical spine. While these may improve construct strength in the setting of osteoporosis, there is a paucity of evidence available to make recommendations in this patient population.

Novel Pedicle Navigator Based on Micro Inertial Navigation System (MINS) and Bioelectric Impedance Analysis (BIA) to Facilitate Pedicle Screw Placement in Spine Surgery: Study in a Porcine Model

STUDY DESIGN

A porcine model.

OBJECTIVE

The study aims to design a novel pedicle navigator based on micro-inertial navigation system (MINS) and bioelectrical impedance analysis (BIA) to assist place pedicle screw placement and validate the utility of the system in enhancing pedicle screw placement.

SUMMARY OF BACKGROUND DATA

The incidence of pedicle screw malpositioning in complicated spinal surgery is still high.Procedures such as computed tomography image-guided navigation, and robot-assisted surgery have been used to improve the precision of pedicle screw placement, but it remains an unmet clinical need.

METHODS

The miniaturized integrated framework containing MINS was mounted inside the hollow handle of the pedicle finder. The inner core was complemented by a high-intensity electrode for measuring bioelectric impedance. Twelve healthy male Wuzhishan minipigs of similar age and weight were used in this experiment and randomized to the MINS-BIA or freehand (FH) group. Pedicle screw placement was determined according to the modified Gertzbein-Robbins grading system on computed tomography images. An impedance detected by probe equal to the baseline value for soft tissue was defined as cortical bone perforation.

RESULTS

A total of 216 screws were placed in 12 minipigs. There were 15 pedicle breaches in the navigator group and 31 in the FH group; the detection rates of these breaches were 14 of 15 (93.3%) and 25 of 31 (80.6%), respectively, with a statistically significant difference between groups. The mean offsets between the planned and postoperatively measured tilt angles of the screw trajectory were 4.5° ± 5.5° in the axial plane and 4.8° ± 3.3° in the sagittal plane with the navigator system and 7.0° ± 5.1° and 7.7° ± 4.7°, respectively, with the FH technique; the differences were statistically significant.

CONCLUSION

A novel and portable navigator based on MINS and BIA could be beneficial for improving or maintaining accuracy while reducing overall radiation exposure.

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.

Biomechanical investigation of the hybrid modified cortical bone screw–pedicle screw fixation technique: Finite-element analysis

Background

Hybrid fixation techniques including the both modified cortical bone trajectory (MCBT) and traditional trajectory (TT) at the L4 and L5 lumbar segment are firstly proposed by our team. Therefore, the purpose of this study is to evaluate and provide specific biomechanical data of the hybrid fixation techniques including the MCBT and TT.

Methods

Four human cadaveric specimens were from the anatomy laboratory of Xinjiang Medical University. Four finite-element (FE) models of the L4–L5 lumbar spine were generated. For each of them, four implanted models with the following fixations were established: TT-TT (TT screw at the cranial and caudal level), MCBT-MCBT (MCBT screw at the cranial and caudal level), hybrid MCBT-TT (MCBT screw at the cranial level and TT screw at the caudal level), and TT-MCBT (TT screw at the cranial level and MCBT screw at the caudal level). A 400-N compressive load with 7.5 N/m moments was applied to simulate flexion, extension, lateral bending, and rotation, respectively. The range of motion (ROM) of the L4–L5 segment and the posterior fixation, the von Mises stress of the intervertebral disc, and the posterior fixation were compared.

Results

Compared to the TT-TT group, the MCBT-TT showed a significant lower ROM of the L4–L5 segment (p ≤ 0.009), lower ROM of the posterior fixation (p &lt; 0.001), lower intervertebral disc stress (p &lt; 0.001), and lower posterior fixation stress (p ≤ 0.041). TT-MCBT groups showed a significant lower ROM of the L4–L5 segment (p ≤ 0.012), lower ROM of the posterior fixation (p &lt; 0.001), lower intervertebral disc stress (p &lt; 0.001), and lower posterior fixation stress (p ≤ 0.038).

Conclusions

The biomechanical properties of the hybrid MCBT-TT and TT-MCBT techniques at the L4–L5 segment are superior to that of stability MCBT-MCBT and TT-TT techniques, and feasibility needs further cadaveric study to verify.

Optimal Anchor at the Uppermost Instrumented Vertebra in Long Fusion From the Pelvis to the Lower Thoracic Spine in Elderly Patients With Degenerative Spinal Deformity: Hook Versus Pedicle Screw

STUDY DESIGN

This was a retrospective cohort study.

OBJECTIVE

The objective of this study was to compare pedicle screws (PSs) and transverse process hooks (TPHs) as anchors at the uppermost instrumented vertebra (UIV) in the lower thoracic spine in elderly patients with adult spinal deformity.

SUMMARY OF BACKGROUND DATA

Less-rigid fixation using hooks at the UIV are thought to best prevent proximal junctional kyphosis (PJK) in long spinal fusion surgery. Although adult spinal deformity is commonly treated via spinal fusion from the pelvis to the lower thoracic spine, few studies have focused on UIV anchors in the lower thoracic spine.

MATERIALS AND METHODS

We retrospectively reviewed 53 patients aged 65 years and above who underwent spinal fusion from the pelvis to T9 or T10, with a minimum follow-up of 1 year. Radiographic outcomes including the incidence of PJK and implant failure were compared between 28 patients with TPHs and 25 patients PSs at the UIV.

RESULTS

The TPH and PS groups had similar radiographic values for pelvic incidence-lumbar lordosis (preoperative: 42.8 vs. 49.0 degrees, postoperative: 9.9 vs. 7.3 degrees) and the sagittal vertical axis (preoperative: 109.3 vs. 106.8 mm; postoperative: 21.9 vs. 11.2 mm). However, the incidence of PJK was significantly higher in the TPH group (35.7%) than that in the PS group (8.0%) at the 1-year follow-up (P=0.012). PJK in the TPH group was associated with UIV or UIV±1 fracture accompanied by posterior dislodgement of the TPH.

CONCLUSION

Rigid fixation using PSs at the UIV in the lower thoracic spine produced better radiographic outcomes than did TPHs in elderly patients undergoing spinopelvic fusion.

LEVEL OF EVIDENCE

Level III.

Effects of different pedicle screw insertion depths on sagittal balance of lumbar degenerative spondylolisthesis, a retrospective comparative study

BACKGROUND

Few reports to date have evaluated the effects of different pedicle screw insertion depths on sagittal balance and prognosis after posterior lumbar interbody and fusion (PLIF) in patients with lumbar degenerative spondylolisthesis (LDS).

METHODS

A total of 88 patients with single-level PLIF for LDS from January 2018 to December 2019 were enrolled. Long screw group (Group L): 52 patients underwent long pedicle screw fixation (the leading edge of the screw exceeded 80% of the anteroposterior diameter of vertebral body). Short screw group (Group S): 36 patients underwent short pedicle screw fixation (the leading edge of the screw was less than 60% of the anteroposterior diameter of vertebral body). Local deformity parameters of spondylolisthesis including slip degree (SD) and segment lordosis (SL), spino-pelvic sagittal plane parameters including pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS) and lumbar lordosis (LL), Oswestry Disability Index (ODI), and Visual Analog Scale (VAS) for back pain of both groups were compared. Postoperative complications, including vertebral fusion rate and screw loosening rate, were recorded.

RESULTS

Except that PI in Group S at the final follow-up was not statistically different from the preoperative value (P > 0.05), other parameters were significantly improved compared with preoperative values one month after surgery and at the final follow-up (P < 0.05). There was no significant difference in parameters between Group L and Group S before and one month after surgery (P > 0.05). At the final follow-up, SD, SL, LL, PT and PI-LL differed significantly between the two groups (P < 0.05). Compared with the preoperative results, ODI and VAS in both groups decreased significantly one month after surgery and at the final follow-up (P < 0.05). Significant differences of ODI and VAS were found between the two groups at the final follow-up (P < 0.05). Postoperative complications were not statistically significant between the two groups (P > 0.05).

CONCLUSIONS

PLIF can significantly improve the prognosis of patients with LDS. In terms of outcomes with an average follow-up time of 2 years, the deeper the screw depth is within the safe range, the better the spino-pelvic sagittal balance may be restored and the better the quality of life may be.

Finite Element Method for the Evaluation of the Human Spine: A Literature Overview

The finite element method (FEM) represents a computer simulation method, originally used in civil engineering, which dates back to the early 1940s. Applications of FEM have also been used in numerous medical areas and in orthopedic surgery. Computing technology has improved over the years and as a result, more complex problems, such as those involving the spine, can be analyzed. The spine is a complex anatomical structure that maintains the erect posture and supports considerable loads. Applications of FEM in the spine have contributed to the understanding of bone biomechanics, both in healthy and abnormal conditions, such as scoliosis, fractures (trauma), degenerative disc disease and osteoporosis. However, since FEM is only a digital simulation of the real condition, it will never exactly simulate in vivo results. In particular, when it concerns biomechanics, there are many features that are difficult to represent in a FEM. More FEM studies and spine research are required in order to examine interpersonal spine stiffness, young spine biomechanics and model accuracy. In the future, patient-specific models will be used for better patient evaluations as well as for better pre- and inter-operative planning.

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.