Biomechanical analysis of corrective forces in spinal instrumentation for scoliosis treatment

Impact of Rod Placement and Tulip Design on Screw-Rod Gripping Capacity in Spinopelvic Fixation: Evaluation Across a Spectrum of Recessed to Extended Lengths

BACKGROUND CONTEXT

High rates of pelvic instrumentation failure (4.5-38%) have been reported, often attributed to issues within the screw-tulip-rod connection. While previous research has explored various aspects of this connection, the influence of tulip design and relative rod placement on mechanical failure remains unclear.

PURPOSE

This study aims to investigate how screw-tulip design and variations in rod placement relative to the tulip affect the integrity of the screw-tulip-rod connection, utilizing axial and torsional gripping capacity tests to evaluate mechanical stability.

STUDY DESIGN/SETTING

Biomechanical METHODS: Mechanical testing was conducted following ASTM F1798-21 to assess the interconnection mechanisms in pelvic fixation constructs. Using 5.5mm Cobalt Chromium rods with porous fusion/fixation (PFFS) screws, axial gripping capacity (AGC) tests measured the axial load before translatory slippage of the rod, while torsional gripping capacity (TGC) tests assessed the torque required to induce rotational slippage. Variations in rod placement at the tulip head were tested in recessed (-2mm, -1mm), flush (0mm), and extended positions (+1mm, +10mm), simulating failure during flexion, extension, and rotation for both open and closed tulip-head designs. ANOVA was used to evaluate the effects of rod placement on connection failure, with significance set at p<0.05.

RESULTS

AGC and TGC tests revealed significant reductions for recessed rod placements, indicating suboptimal placement. At -1mm and -2mm, AGC for simulated flexion decreased by 28.8% (p<0.010) and 45.6% (p<0.001) for the open-head design and 30.5% (p<0.018) and 57.5% (p<0.001) for the closed-head design, respectively, compared to the non-recessed rod placement. TGC also showed a significant decline at -2mm, with a 25.4% reduction compared to the +1mm extended length (p<0.001) and a 20.3% reduction compared to the -1mm recessed length (p=0.005), irrespective of head design. The open and closed-head designs exhibited similar trends; however, the closed-head design was shown to better resist structural failure at recessed lengths. At -2mm simulating extension, the closed-head design was 54.8% greater than the open-head design for AGC (p<.001) and 28.3% greater for TGC.

CONCLUSION

Our findings underscore that both flush (0mm) and extended (+1, +10mm) rod placements relative to the screw-tulip offer sufficient gripping capacity whereas recessed placements (-1, -2mm) have substantial reductions. The closed-head design was shown to better resist structural failure at recessed placements.

CLINICAL SIGNIFICANCE

Rod placement relative to the most distal pelvic screw during spinopelvic fixation varries in surgical practice - whether flush to, extended past, or recessed into the screw-head. Biomechanical evaluating of the axial and torsion gripping capacities at these positions provies a foundation for clinical decision-making.

Corrective Mechanism Aftermath Surgical Treatment of Spine Deformity due to Scoliosis: A Systematic Review of Finite Element Studies

This paper presents a systematic study in reviewing the application of finite element method for the analysis of correction mechanism of spine deformity due to scoliosis. The study is aimed at systematically (1) reviewing the use of finite element analysis in spine deformity case, (2) reviewing the modelling of pedicle screw and rod system in scoliosis surgery, and (3) analysing and discussing gap between the studies. Using the restricted key phrases, the review gathered studies from 2001 to 2021 from various electronic databases (Scopus, ScienceDirect, PubMed, Medline, and WorldCAT). Studies were included if they reported a finite element study on spine deformity. Studies that did not fully disclose their methodology and results had significant discrepancies, not published in English or not yet published were all disqualified. Regardless of inconsistencies in the methodological design of the studies, the quality of all papers was above the acceptable level. A total of fifteen manuscripts were considered for inclusion and were given a comprehensive review. This study indicates that analysing the forces acting on the spine, as well as the interrelationship between the force, stress, and degree of correction (which measured as the Cobb angle), could help to improve the corrective mechanism procedure of spine deformity. Pedicle screws and its placement strategies are also important as it influence the corrective forces for scoliosis treatment. Hence, the findings of this study could potentially be used as a guidance to develop a reliable finite element analysis that can predict the biomechanics responses during the corrective spine deformity treatment.

How do additional rods reduce loads on the primary rods in adult spinal instrumentation with pedicle subtraction osteotomy?

BACKGROUND

Additional auxiliary rods have been used in spinal instrumentation across pedicle subtraction osteotomy to reduce stresses in the primary rods. The auxiliary rods can be connected through dual-rod-screws, fixed-angle multi-rod connectors or variable-angle multi-rod connectors. The objective was to assess rod bending in conventional bilateral-rod construct vs. constructs with auxiliary rods.

METHODS

Computer models of two adult patients were developed to evaluate bending loads across a pedicle subtraction site in a control construct with bilateral rods vs. constructs with auxiliary rods bilaterally or unilaterally connected to the primary rods through either dual-rod-screws, fixed-angle multi-rod connectors, or variable-angle multi-rod connectors. Postoperative rod bending loads were computed and compared.

FINDINGS

Normalizing loads on the primary rods in the multi-rod constructs to the control construct, primary rod loads in multi-rod constructs were 17% to 48% lower than the control construct. Constructs with bilateral auxiliary rods through dual-rod-screws, fixed-angle multi-rod connectors, or variable-angle multi-rod connectors could result in similar primary rod bending loads. Bending loads on the auxiliary rods were higher or lower than those on the primary rods depending on how their curvatures matched the primary rods, and how they were locked onto the primary rods.

INTERPRETATION

Auxiliary rods noticeably reduced the bending loads on the primary rods compared with a standard bilateral-rod construct. Loads in the auxiliary rods were higher or lower than those in the primary rods depending on how their curvatures matched the primary rods, and how they were locked onto the primary rods.

The importance of curve severity, type and instrumentation strategy in the surgical correction of adolescent idiopathic scoliosis: an in silico clinical trial on 64 cases

Adolescent idiopathic scoliosis is a three-dimensional deformity of the spine which is frequently corrected with the implantation of instrumentation with generally good or excellent clinical results; mechanical post-operative complications such as implant loosening and breakage are however relatively frequent. The rate of complications is associated with a lack of consensus about the surgical decision-making process; choices about the instrumentation length, the anchoring implants and the degree of correction are indeed mostly based on personal views and previous experience of the surgeon. In this work, we performed an in silico clinical trial on a large number of subjects in order to clarify which factors have the highest importance in determining the risk of complications by quantitatively analysing the mechanical stresses and loads in the instrumentation after the correction maneuvers. The results of the simulations highlighted the fundamental role of the curve severity, also in its three-dimensional aspect, and of the instrumentation strategy, whereas the length of the fixation had a lower importance.

Spinal rod gripping capacity: how do 5.5/6.0-mm dual-diameter screws compare?

STUDY DESIGN

Biomechanical comparative study.

OBJECTIVE

To evaluate pedicle screw gripping capacity from five suppliers, comparing single-diameter (S-D) systems using 5.5-mm-diameter rods to dual-diameter (D-D) systems accepting 5.5- and 6.0-mm-diameter rods with both cobalt chromium (CoCr) and titanium alloy (Ti) rods. D-D systems have become increasingly prevalent; however, these systems theoretically may compromise spinal rod gripping, particularly when a smaller-diameter rod is used within a D-D pedicle screw.

METHODS

D-D pedicle screw systems from three suppliers (accepting 5.5- and 6.0-mm-diameter, Ti and CoCr rods), and S-D systems from two suppliers (accepting 5.5-mm-diameter, Ti and CoCr rods) were tested on an MTS MiniBionix machine. Axial load was applied in line with the rod to measure axial gripping capacity (AGC), and torsional load was applied to measure torsional gripping capacity (TGC) for each rod material and diameter. AGC and TGC were compared between D-D and S-D constructs, suppliers, rod diameters, and materials with subsequent classification and regression tree (CART) analysis.

RESULTS

5.5-mm rods within D-D screws were no weaker than 5.5-mm rods in S-D systems for AGC (dual > single, p = 0.043) and TGC (p = 0.066). As a whole, D-D systems had greater AGC than S-D systems (p = 0.01). AGC differed between suppliers (p < 0.001). No rod diameter (p = 0.227) or material (p = 0.131) effect emerged. With CART analysis, Supplier was the most significant predictor for greater AGC. As a whole, D-D systems had greater TGC than S-D systems (p = 0.008). TGC differed between suppliers (p < 0.001). Rod diameter was a significant predictor of higher TGC (6.0 > 5.5 mm, p = 0.002). CoCr rods had greater TGC than Ti (p < 0.001). CART analysis revealed that Supplier and CoCr material were significant predictors for increased TGC.

CONCLUSIONS

Despite 30%-70% variability in gripping capacity due to rod supplier and material, overall D-D pedicle screw systems had similar AGC and TGC as S-D systems.

LEVEL OF EVIDENCE

N/A.

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.

Preservation of Spine Motion in the Surgical Treatment of Adolescent Idiopathic Scoliosis Using an Innovative Apical Fusion Technique: A 2-Year Follow-Up Pilot Study

Background

This trial reports the 2-year and immediate postremoval clinical outcomes of a novel posterior apical short-segment (PASS) correction technique allowing for correction and stabilization of adolescent idiopathic scoliosis (AIS) with limited fusion.

Methods

Twenty-one consecutive female AIS patients were treated at 4 institutions with this novel technique. Arthrodesis was limited to the short apical curve after correction with translational and derotational forces applied to upper and lower instrumented levels. Instrumentation spanned fused and unfused segments with motion and flexibility of unfused segments maintained. The long concave rods were removed at maturity. Radiographic data collected included preoperative and postoperative data for up to 2 years as well as after long rod removal.

Results

All 21 patients are beyond 2 years postsurgery. Average age at surgery was 14.2 years (11-17 years). A mean of 10.5 ± 1 levels per patient were stabilized and 5.0 ± 0.5 levels (48%) were fused. Cobb angle improved from 56.1° ± 8.0° to 20.8° ± 7.8° (62.2% improvement) at 1 year and 20.9° ± 8.4°, (62.0% improvement) at 2 years postsurgery. In levels instrumented but not fused, motion was 26° ± 6° preoperatively compared to 10° ± 4° at 1 year postsurgery, demonstrating 38% maintenance of mobility in nonfused segments. There was no report of implant-related complications.

Conclusions

PASS correction technique corrected the deformity profile in AIS patients with a lower implant density while sparing 52% of the instrumented levels from fusion through the 2-year follow-up.

Choice of Rods in Surgical Treatment of Adolescent Idiopathic Scoliosis: What Are the Clinical Implications of Biomechanical Properties? - A Review of the Literature

The surgical treatment of adolescent idiopathic scoliosis (AIS) involves 3-dimensional curve correction with multisegmental pedicle screws attached to contoured bilateral rods. The substantial corrective forces exert a high level of stress on the rods, and the ability of the rod to withstand these forces without undergoing permanent deformation relies on its biomechanical properties. These properties, in turn, are dependent on the material, diameter, and shape of the rod. The surgical treatment of AIS is characterized by the requirement for a special biomechanical profile that may differ substantially from what is needed for adult deformity surgery. This overview summarizes the current knowledge of rod biomechanics in frequently used rod constructs, with a particular focus on translational research between biomechanical studies and clinical applicability in AIS patients.

Biomechanical Simulation of Stresses and Strains Exerted on the Spinal Cord and Nerves During Scoliosis Correction Maneuvers

STUDY DESIGN

Biomechanical analysis of the spinal cord and nerves during scoliosis correction maneuvers through numerical simulations.

OBJECTIVE

To assess the biomechanical effects of scoliosis correction maneuvers and stresses generated on the spinal nervous structures.

BACKGROUND DATA

Important forces are applied during scoliosis correction surgery, which could potentially lead to neurologic complications due to stresses exerted on the nervous structures. The biomechanical impact of the different types of stresses applied on the nervous structures during correction maneuvers is not well understood.

METHODS

Three correction techniques were simulated using a hybrid computer modeling approach, personalized to a right thoracic adolescent idiopathic scoliotic case (Cobb angle: 63°): (1) Harrington-type distraction; (2) segmental translation technique; and a (3) segmental rotation-based procedure. A multibody model was used to simulate the kinematics of the instrumentation maneuvers; a second comprehensive finite element model was used to analyze the local stresses and strains on the spinal cord and nerves. Average values of the internal medullar pressure (IMP), shear stresses, nerve compression, and strain were computed over three regions and compared between techniques.

RESULTS

Harrington distraction maneuver generated high stresses and strains over the thoracolumbar region. In the main thoracic region, the segmental translation maneuver technique induced 15% more shear stress, 25% more strain, and 62% lower nerve compression than Harrington distraction maneuver. The segmental rotation-based procedure induced 25% lower shear stresses and 18% more strain, respectively, at the apical level, as well as 72%, 57%, and 7% lower IMP, nerve compression, and strain in the upper thoracic region, compared with Harrington distraction maneuver.

CONCLUSION

This study quantified the relative stress induced on the spinal cord and spinal nerves for different correction maneuvers using a novel hybrid patient-specific model. Of the three maneuvers studied, the Harrington distraction maneuver induced the most important stresses over the thoracolumbar region.

Biomechanical effect of pedicle screw distribution in AIS instrumentation using a segmental translation technique: computer modeling and simulation

BACKGROUND

Efforts to select the appropriate number of implants in adolescent idiopathic scoliosis (AIS) instrumentation are hampered by a lack of biomechanical studies. The objective was to biomechanically evaluate screw density at different regions in the curve for AIS correction to test the hypothesis that alternative screw patterns do not compromise anticipated correction in AIS when using a segmental translation technique.

METHODS

Instrumentation simulations were computationally performed for 10 AIS cases. We simulated simultaneous concave and convex segmental translation for a reference screw pattern (bilateral polyaxial pedicle screws with dorsal height adjustability at every level fused) and four alternative patterns; screws were dropped respectively on convex or concave side at alternate levels or at the periapical levels (21 to 25% fewer screws). Predicted deformity correction and screw forces were compared.

RESULTS

Final simulated Cobb angle differences with the alternative screw patterns varied between 1° to 5° (39 simulations) and 8° (1 simulation) compared to the reference maximal density screw pattern. Thoracic kyphosis and apical vertebral rotation were within 2° of the reference screw pattern. Screw forces were 76 ± 43 N, 96 ± 58 N, 90 ± 54 N, 82 ± 33 N, and 79 ± 42 N, respectively, for the reference screw pattern and screw dropouts at convex alternate levels, concave alternate levels, convex periapical levels, and concave periapical levels. Bone-screw forces for the alternative patterns were higher than the reference pattern (p < 0.0003). There was no statistical bone-screw force difference between convex and concave alternate dropouts and between convex and concave periapical dropouts (p > 0.28). Alternate dropout screw forces were higher than periapical dropouts (p < 0.05).

CONCLUSIONS

Using a simultaneous segmental translation technique, deformity correction can be achieved with 23% fewer screws than maximal density screw pattern, but resulted in 25% higher bone-screw forces. Screw dropouts could be either on the convex side or on the concave side at alternate levels or at periapical levels. Periapical screw dropouts may more likely result in lower bone-screw force increase than alternate level screw dropouts.

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

STUDY DESIGN

Detailed biomechanical analysis of the anchorage performance provided by different pedicle screw designs and placement strategies under pullout loading.

OBJECTIVE

To biomechanically characterize the specific effects of surgeon-specific pedicle screw design parameters on anchorage performance using a finite element model.

SUMMARY OF BACKGROUND DATA

Pedicle screw fixation is commonly used in the treatment of spinal pathologies. However, there is little consensus on the selection of an optimal screw type, size, and insertion trajectory depending on vertebra dimension and shape.

METHODS

Different screw diameters and lengths, threads, and insertion trajectories were computationally tested using a design of experiment approach. A detailed finite element model of an L3 vertebra was created including elastoplastic bone properties and contact interactions with the screws. Loads and boundary conditions were applied to the screws to simulate axial pullout tests. Force-displacement responses and internal stresses were analyzed to determine the specific effects of each parameter.

RESULTS

The design of experiment analysis revealed significant effects (P<0.01) for all tested principal parameters along with the interactions between diameter and trajectory. Screw diameter had the greatest impact on anchorage performance. The best insertion trajectory to resist pullout involved placing the screw threads closer to the pedicle walls using the straightforward insertion technique, which showed the importance of the cortical layer grip. The simulated cylindrical single-lead thread screws presented better biomechanical anchorage than the conical dual-lead thread screws in axial loading conditions.

CONCLUSIONS

The model made it possible to quantitatively measure the effects of both screw design characteristics and surgical choices, enabling to recommend strategies to improve single pedicle screw performance under axial loading.

Biomechanical analysis of proximal junctional failure following adult spinal instrumentation using a comprehensive hybrid modeling approach

BACKGROUND

Proximal junctional failure is a severe proximal junctional complication following adult spinal instrumentation and involving acute proximal junctional kyphotic deformity, mechanical failure at the upper instrumented vertebra or just above, and/or proximal junctional osseoligamentous disruption. Clinical studies have identified potential risk factors, but knowledge on their biomechanics is still lacking for addressing the proximal junctional failure issues. The objective of this study was to develop comprehensive computational modeling and simulation techniques to investigate proximal junctional failure.

METHODS

A 3D multibody biomechanical model based on a 47year old lumbar scoliosis surgical case that subsequently had traumatic proximal junctional failure was first developed to simulate patient-specific spinal instrumentation (from T11 to S1), compute the postoperative geometry of the instrumented spine, simulate different physiological loads and movements. Then, a highly detailed finite element model of the proximal junctional spinal segment was created using as input the geometry and displacements from the multibody model. It enabled to perform detailed stress and failure analysis across the anatomical structures.

FINDINGS

The simulated postoperative correction and traumatic failure (wedge fracture at upper instrumented vertebra) agreed well with the clinical report (within 2° difference). Simulated stresses around the screw threads (up to 4.7MPa) generated during the instrumentation and the buckling effect of post-operative functional loads on the proximal junctional spinal segment, were identified as potential mechanical proximal junctional failure risk factors.

INTERPRETATION

Overall, we demonstrated the feasibility of the developed hybrid modeling technique, which realistically allowed the simulation of the spinal instrumentation and postoperative loads, which constitutes an effective tool to further investigate proximal junctional failure pathomechanisms.

How does implant distribution affect 3D correction and bone-screw forces in thoracic adolescent idiopathic scoliosis spinal instrumentation?

BACKGROUND

Optimal implant densities and configurations for thoracic spine instrumentation to treat adolescent idiopathic scoliosis remain unknown. The objective was to computationally assess the biomechanical effects of implant distribution on 3D curve correction and bone-implant forces.

METHODS

3D patient-specific biomechanical spine models based on a multibody dynamic approach were created for 9 Lenke 1 patients who underwent posterior instrumentation (main thoracic Cobb: 43°-70°). For each case, a factorial design of experiments was used to generate 128 virtual implant configurations representative of existing implant patterns used in clinical practice. All instances except implant configuration were the same for each surgical scenario simulation.

FINDINGS

Simulation of the 128 implant configurations scenarios (mean implant density=1.32, range: 0.73-2) revealed differences of 2° to 10° in Cobb angle correction, 2° to 7° in thoracic kyphosis and 2° to 7° in apical vertebral rotation. The use of more implants, at the concave side only, was associated with higher Cobb angle correction (r=-0.41 to -0.90). Increased implant density was associated with higher apical vertebral rotation correction for seven cases (r=-0.20 to -0.48). It was also associated with higher bone-screw forces (r=0.22 to 0.64), with an average difference between the least and most constrained instrumentation constructs of 107N per implant at the end of simulated instrumentation.

INTERPRETATION

Low-density constructs, with implants mainly placed on the concave side, resulted in similar simulated curve correction as the higher-density patterns. Increasing the number of implants allows for only limited improvement of 3D correction and overconstrains the instrumentation construct, resulting in increased forces on the implants.

Pedicle Screw Fixation Under Nonaxial Loads: A Cadaveric Study

STUDY DESIGN

An experimental study of pedicle screw fixation in human cadaveric vertebrae.

OBJECTIVE

The aim of this study was to experimentally characterize pedicle screw fixation under nonaxial loading and to analyze the effect of the surgeons' screw and placement choices on the fixation risk of failure.

SUMMARY OF BACKGROUND DATA

Pedicle screw fixation performance is traditionally characterized with axial pullout tests, which do not fully represent the various tridimensional loads sustained by the screws during correction maneuvers of severe spinal deformities. Previous studies have analyzed the biomechanics of nonaxial loads on pedicle screws, but their effects on the screw loosening mechanisms are still not well understood.

METHODS

A design of experiment (DOE) approach was used to evaluate 2 screw thread designs (single-lead and dual-lead threads), 2 insertion trajectories in the transverse and sagittal planes, and 2 loading directions (lateral and cranial). Pedicle screws were inserted in both pedicles of 12 cadaveric lumbar vertebrae for a total of 24 tests. Four sinewave loading cycles (0-400 N) were applied, orthogonally to the screw axis, at the screw head. The resulting forces, displacements, and rotations of the screws were recorded.

RESULTS

In comparison to the other cycles, the first loading cycle revealed important permanent deformation of the bone (mean permanent displacement of the screw head of 0.79 mm), which gradually accumulated over the following cycles to 1.75 mm on average (plowing effect). The cranial loading direction caused significantly lower (P < 0.05) bone deformation than lateral loading. The dual-lead screw had a significantly higher (P < 0.05) initial stiffness than the single-lead thread screw.

CONCLUSIONS

Nonaxial loads induce screw plowing that lead to bone compacting and subsequent screw loosening or even bone failure, thus reducing the pedicle screw fixation strength. Lateral loads induce greater bone deformation and risks of failure than cranial loads.

LEVEL OF EVIDENCE

N/A.

A BIOMECHANICAL STUDY OF SHEAR LOAD ON BONE–SCREW INTERFACE OF THORACOLUMBAR VERTEBRAE

Vertebral screw failure by cutting through or pulling out of the vertebral body is common in intraoperative and postoperative stages of anterior thoracolumbar surgery, especially in osteoporotic patients. This biomechanical study was conducted to investigate the maximum shear force thoracolumbar vertebrae can withstand and the corresponding displacement of screws and analyze their correlation with vertebral bone mineral density. Forty individual vertebra specimens (T11-L3) were obtained from eight fresh adult cadaveric thoracolumbar spine specimens and randomly divided into an experimental group and a control group. Screws were placed in the center point of vertebrae and penetrated the contralateral cortex. Shear loading and axial pullout tests were conducted on the experimental group while only axial pullout test was conducted on the control group. The maximum shear forces and maximum axial pullout forces were recorded. The conditions of vertebral body destruction and screw channel were observed and the maximum axial pullout forces were recorded and analyzed. A large amount of thread bone debris was observed in the control group. In the experimental group, however, only a small amount of thread bone debris was observed; the widths of screw paths were larger than those in the control group and gradually increased from screw tips in the direction of loading. The vertebral bodies had an average shear strength of [Formula: see text][Formula: see text]N, and the corresponding average screw displacement was [Formula: see text][Formula: see text]mm. Linear regression analysis showed that the shear strength had a significant positive correlation with vertebral bone mineral density (BMD) (r[Formula: see text][Formula: see text][Formula: see text]0.958, P[Formula: see text][Formula: see text][Formula: see text]0.01), while the screw displacement had a significant negative correlation with vertebral BMD (r[Formula: see text][Formula: see text][Formula: see text]−0.933, P[Formula: see text][Formula: see text][Formula: see text]0.01). No significant difference in bone density was found between the destruction and the control groups (P[Formula: see text][Formula: see text][Formula: see text]0.05); the difference in maximum axial pullout strength between the destruction and the control groups was significant (P[Formula: see text][Formula: see text][Formula: see text]0.01). These results indicated that vertebral BMD positively correlated with the maximum shear force and negatively with the screw displacement. It is important that the corrective strength for spinal deformity may affect bone–screw interface in anterior thoracolumbar surgery.

Planning the Surgical Correction of Spinal Deformities: Toward the Identification of the Biomechanical Principles by Means of Numerical Simulation

In decades of technical developments after the first surgical corrections of spinal deformities, the set of devices, techniques, and tools available to the surgeons has widened dramatically. Nevertheless, the rate of complications due to mechanical failure of the fixation or the instrumentation remains rather high. Indeed, basic and clinical research about the principles of deformity correction and the optimal surgical strategies (i.e., the choice of the fusion length, the most appropriate instrumentation, and the degree of tolerable correction) did not progress as much as the implantable devices and the surgical techniques. In this work, a software approach for the biomechanical simulation of the correction of patient-specific spinal deformities aimed to the identification of its biomechanical principles is presented. The method is based on three-dimensional reconstructions of the spinal anatomy obtained from biplanar radiographic images. A user-friendly graphical user interface allows for the planning of the desired deformity correction and to simulate the implantation of pedicle screws. Robust meshing of the instrumented spine is provided by using consolidated computational geometry and meshing libraries. Based on a finite element simulation, the program is able to predict the loads and stresses acting in the instrumentation as well as those in the biological tissues. A simple test case (reduction of a low-grade spondylolisthesis at L3–L4) was simulated as a proof of concept, and showed plausible results. Despite the numerous limitations of this approach which will be addressed in future implementations, the preliminary outcome is promising and encourages a wide effort toward its refinement.

Implant distribution in surgically instrumented Lenke 1 adolescent idiopathic scoliosis: does it affect curve correction?

STUDY DESIGN

Retrospective review of prospective multicenter database of patients with adolescent idiopathic scoliosis who underwent posterior spinal fusion.

OBJECTIVE

To analyze implant distribution in surgically instrumented Lenke 1 patients and evaluate how it impacts curve correction.

SUMMARY OF BACKGROUND DATA

Although pedicle screw constructs have demonstrated successful surgical results, the optimal pedicle screw density and configuration remain unclear.

METHODS

A total of 279 patients with adolescent idiopathic scoliosis treated with pedicle screws were reviewed. Implant density was computed for each side of the instrumented segment, which was divided into 5 regions: distal and proximal ends (upper/lower instrumented vertebra +1 adjacent vertebra), apical region (apex ± 1 vertebra), and the 2 regions in between (upper/lower periapical). Centralized measurement of Cobb angle and thoracic kyphosis was performed on preoperative and at 1-year postoperative radiographs as well as percent curve flexibility.

RESULTS

The mean implant density was 1.66 implants per level fused (1.08 to 2) with greater available pedicles filled on the concavity (92%, 53%-100%) compared with the convex side (73%, 23%-100%, P < 0.01). The concave distal end region had the highest density with 99% of pedicles filled (P < 0.01), followed by the other concave regions and the convex distal end region (88%-94%) (P > 0.05). Other convex regions of the construct had less instrumentation, with only 54% to 78% of pedicles instrumented (P < 0.01). Implant density in the concave apical region (69%, 23%-100%) had a positive effect on curve correction (P = 0.002, R = 0.19).

CONCLUSION

Significant variability exists in implant distribution with the greatest variation on the convex side and lowest implant density used in the periapical convex regions. Only instrumentation at the concave side, particularly at the apical region, was associated with curve correction. This suggests that for a low implant density construct, the best regions for planned screw dropout may be in the periapical convexity.

LEVEL OF EVIDENCE

3.

Biomechanical analysis of iliac screw fixation in spinal deformity instrumentation

BACKGROUND

High rates of iliac screw fixation failures have been reported in spinopelvic instrumentations. The objective was to assess the iliac screw loads as functions of instrumentation variables.

METHODS

Spinopelvic instrumentations of six neuromuscular scoliosis were simulated using patient-specific modeling techniques to evaluate the intra- and postoperative iliac screw loads as functions of instrumentation variables: the combined use of sacral screws, the uses of lateral offset connectors and cross-rod connectors, and the iliac screw insertion point and trajectory.

FINDINGS

Sacral screws, lateral connectors and the iliac screw insertion point had significant effects on iliac screw axial forces (69-297N) and toggle moments (0.8-2.9Nm) (p<0.05). The addition of sacral screws made the iliac screw forces lower for some functional loads but higher for other functional loads, and resulted in an increase of intraoperative screw forces when attaching the rods onto these additional screws. When lateral offset connectors were used, the toggle moments were 16% and 25% higher, respectively for the left and right sides. Inserting iliac through the sacrum resulted in 17% lower toggle moment compared to insertion through the iliac crest. Cross-rod connectors had no significant effect on the intraoperative iliac screw forces. Postoperative functional loading had an important effect (additional 34% screw axial force and 18% toggle moment).

INTERPRETATION

It is possible to reduce the iliac screw loads by adapting instrumentation variables and strategies. Reducing the loads could decrease the risk of failure associated with iliac screw fixations.