Effect of heterotopic ossification after bryan-cervical disc arthroplasty on adjacent level range of motion: A finite element study.

BACKGROUND: Quantitative bone re-modelling theories suggest that bones adapt to mechanical loading conditions. Follow-up studies have shown that total disc replacement (TDR) modifies stress patterns in the bones, leading to heterotopic ossification (HO). Although there are a few studies on HO using finite element models (FEM), its effect on the adjacent levels and change in range of motion (ROM) have not been adequately investigated. This study interfaces the HO using bone re-modelling algorithm with a finite element solution and investigates the subsequent changes in segmental ROM. METHODS: A FEM of the human cervical spine (C3-C7) was developed for this study, with material properties obtained from literature. The motion of the segments in the sagittal, frontal and transverse planes under combined loading conditions of 1 Nm moment and 73.6 N compression were validated against experimental corridors. The natural disc between the C5-C6 segment was replaced with the Bryan artificial cervical disc, and changes in sagittal ROM were compared before and after HO. The process of HO was simulated using a bone remodelling algorithm using strain energy density (SED) as the mechanical stimuli. RESULTS AND CONCLUSION: Our study demonstrates the feasibility of using SED calculations from the flexion-extension loading conditions for prediction of HO after ADR. The current findings suggest that the nature of trabecular stresses, and the subsequent rate and location of HO formation could differ based on the geometric design and nature of constraint for different artificial discs. The Bryan disc significantly reduced ROM at the implanted level in flexion. However, in extension, ROM increased at the implanted level and decreased slightly at the adjacent levels. After HO, ROM drastically reduced at the implanted level in both extension and flexion, and showed a minor increase in the adjacent levels, indicating that biomechanical behavior of high-grade HO is similar to a fused segment, thereby reducing the intended and initial motion preservation.

Sensitivity studies of pediatric material properties on juvenile lumbar spine responses using finite element analysis.

The objective of the study was to determine the sensitivity of material properties of the juvenile spine to its external and internal responses using a finite element model under compression, and flexion-extension bending moments. The methodology included exercising the 8-year-old juvenile lumbar spine using parametric procedures. The model included the vertebral centrum, growth plates, laminae, pedicles, transverse processes and spinous processes; disc annulus and nucleus; and various ligaments. The sensitivity analysis was conducted by varying the modulus of elasticity for various components. The first simulation was done using mean material properties. Additional simulations were done for each component corresponding to low and high material property variations. External displacement/rotation and internal stress-strain responses were determined under compression and flexion-extension bending. Results indicated that, under compression, disc properties were more sensitive than bone properties, implying an elevated role of the disc under this mode. Under flexion-extension moments, ligament properties were more dominant than the other components, suggesting that various ligaments of the juvenile spine play a key role in modulating bending behaviors. Changes in the growth plate stress associated with ligament properties explained the importance of the growth plate in the pediatric spine with potential implications in progressive deformities.

Validation efforts and flexibilities of an eight-year-old human juvenile lumbar spine using a three-dimensional finite element model.

The objective of this study was to develop a finite element model of the lumbar spinal column of an eight-year-old human spine and compare flexibilities under pure moments, adult, and pediatric loading with different material models. The geometry was extracted from computed tomography scans. The model included the cortical and cancellous bones, growth plates, ligaments, and discs. Adult, adolescent, and pediatric material models were used. Flexion (8 Nm), extension (6 Nm), lateral bending (6 Nm), and axial rotation (4 Nm) moments representing adult loads were applied to the three material models. Pediatric loading (0.5 Nm) was applied under these loadings to the eight-year-old spine using adult and pediatric material models. Flexibilities depended on spinal level, loading mode, and material model. Outputs incorporating the pediatric material model responded with increased flexibilities compared to the adult and adolescent material models, with one exception. This was true for the adult and pediatric loading conditions. While the sagittal and coronal bending responses were not considerably different between the adult and pediatric loadings, axial rotation responses were greater under the adult loading. This model may be used to determine intrinsic responses, such as stresses and strains, for improved characterizations of the juvenile spine behavior.