Biomechanical comparison of sacral and transarticular sacroiliac screw fixation.

STUDY DESIGN: A detailed finite element analysis of screw fixation in the sacrum and pelvis. OBJECTIVE: To biomechanically assess and compare the fixation performance of sacral and transarticular sacroiliac screws. Instrumentation constructs are used to achieve fixation and stabilization for the treatment of spinopelvic pathologies. The optimal screw trajectory and type of bone engagement to caudally anchor long fusion constructs are not yet known. METHODS: A detailed finite element model of the sacroiliac articulation with two different bone densities was developed. Two sacral and one transarticular sacroiliac screw trajectories were modeled with different diameters (5.5 and 6.5 mm) and lengths (uni-cortical, bi-cortical and quad-cortical purchase). Axial pullout and flexion/extension toggle forces were applied on the screws representing intra and post-operative loads. The force-displacement results and von Mises stresses were used to characterize the failure pattern. RESULTS: Overall, sacroiliac screws provided forces to failure 2.75 times higher than sacral fixation screws. On the contrary, the initial stiffness was approximately half as much for sacroiliac screws. High stresses were located at screw tips for the sacral trajectories and near the cortical bone screw entry points for the sacroiliac trajectory. Overall, the diameter and length of the screws had significant effects on the screw fixation (33% increase in force to failure; 5% increase in initial stiffness). A 20% drop in bone mineral density (lower bone quality) decreased the initial stiffness by 25% and the force to failure by 5-10%. High stresses and failure occurred at the screw tip for uni- and tri-cortical screws and were close to trabecular-cortical bone interface for bi-cortical and quad-cortical screws. CONCLUSIONS: Sacroiliac fixation provided better anchorage than sacral fixation. The transarticular purchase of the sacroiliac trajectory resulted in differences in failure pattern and fixation performance.

Effect of impact velocity and ligament mechanical properties on lumbar spine injuries in posterior-anterior impact loading conditions: a finite element study.

Traumatic events may lead to lumbar spine injuries ranging from low severity bony fracture to complex fracture dislocation. Injury pathomechanisms as well as the influence of loading rate and ligament mechanical properties were not yet fully elucidated. The objective was to quantify the influence of impact velocity and ligament properties variability on the lumbar spine response in traumatic flexion-shear conditions. An L1-L3 finite element spinal segment was submitted to a posterior-anterior impact at three velocities (2.7, 5, or 10 m/s) and for 27 sets of ligament properties. Spinal injury pathomechanism varied according to the impact velocities: initial osseous compression in the anterior column for low and medium velocities versus distraction in the posterior column for high velocity. Impact at 2.7 and 5 m/s lead to higher extent of bony injury, i.e., volume of ruptured bone, compared to the impact at 10 m/s (1140, 1094, and 718 mm3 respectively), lower L2 anterior displacement (2.09, 5.36, and 7.72 mm respectively), and lower facet fracture occurrence. Ligament properties had no effect on bony injury initiation but influenced the presence of facet fracture. These results improve the understanding of lumbar injury pathomechanisms and provide additional knowledge of lumbar injury load thresholds that could be used for injury prevention. Graphical abstract Stress distribution analysis at the injury initiation and final injury pattern identification for a lumbar segment submitted to a traumatic posterior-anterior impact.

Biomechanical Analysis of Acute Proximal Junctional Failure After Surgical Instrumentation of Adult Spinal Deformity: The Impact of Proximal Implant Type, Osteotomy Procedures, and Lumbar Lordosis Restoration.

STUDY DESIGN: Computer biomechanical simulations to analyze risk factors of proximal junctional failure (PJF) following adult scoliosis instrumentation. OBJECTIVE: To evaluate the biomechanical effects on the proximal junctional spine of the proximal implant type, tissue dissection, and lumbar lordosis (LL) restoration. SUMMARY OF BACKGROUND DATA: PJF is a severe proximal junctional complication following adult spinal instrumentation requiring revision surgery. Potential risk factors have been reported in the literature, but knowledge on their biomechanics is still lacking to address the issues. METHODS: A patient-specific multibody and finite-element hybrid modeling technique was developed for a 54-year-old patient having undergone instrumented spinal fusion for multilevel stenosis resulting in PJF. Based on the actual surgery, 30 instrumentation scenarios were derived and simulated by changing the implant type at the upper instrumented vertebra (UIV), varying the extent of proximal osteotomy and the degree of LL creation. Five functional loads were simulated, and stresses and strains were analyzed for each of the 30 tested scenarios. RESULTS: There was 80% more trabecular bone with stress greater than 0.5 MPa in the UIV with screws compared to hooks. Hooks allowed 96% more mobility of the proximal instrumented functional unit compared to screws. The bilateral complete facetectomy along with posterior ligaments dissection caused a significant increase of the range of motion of the functional unit above the UIV. LL creation increased the flexion moment applied on the proximal vertebra from 7.5 to 17.5 Nm, which generated damage at the bone-screw interface that affected the screw purchase. CONCLUSION: Using hooks at UIV and reducing posterior proximal spinal element dissection lowered stress levels in the proximal junctional spinal segment and thus reduced the biomechanical risks of PJF. LL restoration was associated with increased stress levels in postoperative functional upper body flexion.

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.

Finite Element Analysis of Sacroiliac Joint Fixation under Compression Loads.

BACKGROUND: Sacroiliac joint (SIJ) is a known chronic pain-generator. The last resort of treatment is the arthrodesis. Different implants allow fixation of the joint, but to date there is no tool to analyze their influence on the SIJ biomechanics under physiological loads. The objective was to develop a computational model to biomechanically analyze different parameters of the stable SIJ fixation instrumentation. METHODS: A comprehensive finite element model (FEM) of the pelvis was built with detailed SIJ representation. Bone and sacroiliac joint ligament material properties were calibrated against experimentally acquired load-displacement data of the SIJ. Model evaluation was performed with experimental load-displacement measurements of instrumented cadaveric SIJ. Then six fixation scenarios with one or two implants on one side with two different trajectories (proximal, distal) were simulated and assessed with the FEM under vertical compression loads. RESULTS: The simulated S1 endplate displacement reduction achieved with the fixation devices was within 3% of the experimentally measured data. Under compression loads, the uninstrumented sacrum exhibited mainly a rotation motion (nutation) of 1.38° and 2.80° respectively at 600 N and 1000 N, with a combined relative translation (0.3 mm). The instrumentation with one screw reduced the local displacement within the SIJ by up to 62.5% for the proximal trajectory vs. 15.6% for the distal trajectory. Adding a second implant had no significant additional effect. CONCLUSION: A comprehensive finite element model was developed to assess the biomechanics of SIJ fixation. SIJ devices enable to reduce the motion, mainly rotational, between the sacrum and ilium. Positioning the implant farther from the SIJ instantaneous rotation center was an important factor to reduce the intra-articular displacement. CLINICAL RELEVANCE: Knowledge provided by this biomechanical study enables improvement of SIJ fixation through optimal implant trajectory.

Strain rate dependent behavior of the porcine spinal cord under transverse dynamic compression.

The accurate description of the mechanical properties of spinal cord tissue benefits to clinical evaluation of spinal cord injuries and is a required input for analysis tools such as finite element models. Unfortunately, available data in the literature generally relate mechanical properties of the spinal cord under quasi-static loading conditions, which is not adapted to the study of traumatic behavior, as neurological tissue adopts a viscoelastic behavior. Thus, the objective of this study is to describe mechanical properties of the spinal cord up to mechanical damage, under dynamic loading conditions. A total of 192 porcine cervical to lumbar spinal cord samples were compressed in a transverse direction. Loading conditions included ramp tests at 0.5, 5 or 50 s-1 and cyclic loading at 1, 10 or 20 Hz. Results showed that spinal cord behavior was significantly influenced by strain rate. Mechanical damage occurred at 0.64, 0.68 and 0.73 strains for 0.5, 5 or 50 s-1 loadings, respectively. Variations of behavior between the tested strain rates were explained by cyclic loading results, which revealed behavior more or less viscous depending on strain rate. Also, a parameter (stress multiplication factor) was introduced to allow transcription of a stress-strain behavior curve to different strain rates. This factor was described and was significantly different for cervical, thoracic and lumbar vertebral heights, and for the strain rates evaluated in this study.

Morphometrics of the entire human spinal cord and spinal canal measured from in vivo high-resolution anatomical magnetic resonance imaging.

STUDY DESIGN: Measurements of cervical and thoracolumbar human spinal cord (SC) geometry based on in vivo magnetic resonance imaging and investigation of morphological "invariants." OBJECTIVE: The current work aims at providing morphological features of the complete in vivo human normal SC and at investigating possible "invariant" parameters that may serve as normative data for individualized study of SC injuries. SUMMARY OF BACKGROUND DATA: Few in vivo magnetic resonance image-based studies have described human SC morphology at the cervical level, and similar description of the entire SC only relies on postmortem studies, which may be prone to atrophy biases. Moreover, large interindividual variations currently limit the use of morphological metrics as reference for clinical applications or as modeling inputs. METHODS: Absolute metrics of SC (transverse and anteroposterior diameters, width of anterior and posterior horns, cross-sectional SC area, and white matter percentage) were measured using semiautomatic segmentation of high resolution in vivo T2*-weighted transverse images acquired at 3 T, at each SC level, on healthy young (N = 15) and older (N = 8) volunteers. Robustness of measurements, effects of subject, age, or sex, as well as comparison with previously published postmortem data were investigated using statistical analyses (separate analysis of variance, Tukey-HSD, Bland-Altman). Normalized-to-C3 parameters were evaluated as invariants using a leave-one-out analysis. Spinal canal parameters were measured and occupation ratio border values were determined. RESULTS: Metrics of SC morphology showed large intra- and interindividual variations, up to 30% and 13%, respectively, on average. Sex had no influence except on posterior horn width (P < 0.01). Age-related differences were observed for anteroposterior diameter and white matter percentage (P < 0.05) and all postmortem metrics were significantly lower than in vivo values (P < 0.001). In vivo normalized SC area and diameters seemed to be invariants (R > 0.74, root-mean-square deviation < 10%). Finally, minimal and maximal occupation ratio were 0.2 and 0.6, respectively. CONCLUSION: This study presented morphological characteristics of the complete in vivo human SC. Significant differences linked to age and postmortem state have been identified. Morphological "invariants" that could be used to calculate the normally expected morphology accurately, were also identified. These observations should benefit to biomechanical and SC pathology studies. LEVEL OF EVIDENCE: N/A.

Biomechanics of thoracolumbar junction vertebral fractures from various kinematic conditions.

Thoracolumbar spine fracture classifications are mainly based on a post-traumatic observation of fracture patterns, which is not sufficient to provide a full understanding of spinal fracture mechanisms. This study aimed to biomechanically analyze known fracture patterns and to study how they relate to fracture mechanisms. The instigation of each fracture type was computationally simulated to assess the fracture process. A refined finite element model of three vertebrae and intervertebral connective tissues was subjected to 51 different dynamic loading conditions divided into four categories: compression, shear, distraction and torsion. Fracture initiation and propagation were analyzed, and time and energy at fracture initiation were computed. To each fracture pattern described in the clinical literature were associated one or several of the simulated fracture patterns and corresponding loading conditions. When compared to each other, torsion resulted in low-energy fractures, compression and shear resulted in medium energy fractures, and distraction resulted in high-energy fractures. Increased velocity resulted in higher-energy fracture for similar loadings. The use of a finite element model provided quantitative characterization of fracture patterns occurrence complementary to clinical and experimental studies, allowing to fully understand spinal fracture biomechanics.