Lumbar Disc Degeneration Affects the Risk of Rod Fracture Following PSO; A Finite Element Study.

STUDY DESIGN: Finite element (FE) study. OBJECTIVE: Pedicle subtraction osteotomy (PSO) is a surgical method to correct sagittal plane deformities. In this study, we aimed to investigate the biomechanical effects of lumbar disc degeneration on the instrumentation following PSO and assess the effects of using interbody spacers adjacent to the PSO level in a long instrumented spinal construct. METHODS: A spinopelvic model (T10-pelvis) with PSO at the L3 level was used to generate 3 different simplified grades of degenerated lumbar discs (mild (Pfirrmann grade III), moderate (Pfirrmann grade IV), and severe (Pfirrmann grade V)). Instrumentation included eighteen pedicle screws and bilateral primary rods. To investigate the effect of interbody spacers, the model with normal disc height was modified to accommodate 2 interbody spacers adjacent to the PSO level through a lateral approach. For the models, the rods' stress distribution, PSO site force values, and the spine range of motion (ROM) were recorded. RESULTS: The mildly, moderately, and severely degenerated models indicated approximately 10%, 26%, and 40% decrease in flexion/extension motion, respectively. Supplementing the instrumented spinopelvic PSO model using interbody spacers reduced the ROM by 22%, 21%, 4%, and 11% in flexion, extension, lateral bending, and axial rotation, respectively. The FE results illustrated lower von Mises stress on the rods and higher forces at the PSO site at higher degeneration grades and while using the interbody spacers. CONCLUSIONS: Larger and less degenerated discs adjacent to the PSO site may warrant consideration for interbody cage instrumentation to decrease the risk of rod fracture and PSO site non-union.

A Comparative Biomechanical Analysis of Various Rod Configurations Following Anterior Column Realignment and Pedicle Subtraction Osteotomy.

OBJECTIVE: The objective of this study was to compare the biomechanical differences of different rod configurations following anterior column realignment (ACR) and pedicle subtraction osteotomy (PSO) for an optimal correction technique and rod configuration that would minimize the risk of rod failure. METHODS: A validated spinopelvic (L1-pelvis) finite element model was used to simulate ACR at the L3-4 level. The ACR procedure was followed by dual-rod fixation, and for 4-rod constructs, either medial/lateral accessory rods (connected to primary rods) or satellite rods (directly connected to ACR level screws). The range of motion (ROM), maximum von Mises stress on the rods, and factor of safety (FOS) were calculated for the ACR models and compared to the existing literature of different PSO rod configurations. RESULTS: All of the 4-rod ACR constructs showed a reduction in ROM and maximum von Mises stress compared to the dual-rod ACR construct. Additionally, all of the 4-rod ACR constructs showed greater percentage reduction in ROM and maximum von Mises stress compared to the PSO 4-rod configurations. The ACR satellite rod construct had the maximum stress reduction i.e., 47.3% compared to dual-rod construct and showed the highest FOS (4.76). These findings are consistent with existing literature that supports the use of satellite rods to reduce the occurrence of rod fracture. CONCLUSION: Our findings suggest that the ACR satellite rod construct may be the most beneficial in reducing the risk of rod failure compared to all other PSO and ACR constructs.

Sacroiliac joint stabilization using implants provide better fixation in females compared to males: a finite element analysis.

PURPOSE: This study's objective was to assess biomechanical parameters across fused and contralateral sacroiliac joints (SIJs) and implants during all spinal motions for both sexes. Various SIJ implant devices on the market are used in minimally invasive surgeries. These implants are placed across the joint using different surgical approaches. The biomechanical effects of fusion surgical techniques in males and females have not been studied. METHODS: The validated finite element models of a male, and a female spine-pelvis-femur were unilaterally instrumented across the SIJ using three screws for two SIJ implants, half threaded and fully threaded screws placed laterally and posteriorly to the joint, respectively. RESULTS: Motion and peak stress data at the SIJs showed that the female model exhibited lower stresses and higher reduction in motion at the contralateral SIJ in all motions than the male model predictions with 84% and 71% reductions in motion and stresses across the SIJ. CONCLUSION: Implants exhibited higher stresses in the female model compared to the male model. However, chances of SIJ implant failure in the female patients are still minimal, based on the calculated factor of safety which is still very high. Both lateral and posterior surgical approaches were effective in both sexes; however, the lateral approach may provide a better biomechanical response, especially for females. Moreover, implant design characteristics did not make a difference in the implants' biomechanical performance. SIJ stabilization was primarily provided by the implants which were the farthest from the sacrum rotation center.

Mechanical performance of traditional distraction-based dual growing rod constructs.

BACKGROUND: Growing rod constructs are an important contribution in the treatment of children with early onset scoliosis even though these devices experience high rates of rod fracture. The mechanical performance of traditional, distraction-based dual growing rod constructs is not well understood, and mechanical models for predicting device performance are limited. PURPOSE: Two mechanical models were developed and used to determine the mechanical performance of various growing rod configurations by increasing construct complexity. STUDY DESIGN/SETTING: Mechanical bench testing and finite element (FE) analysis. METHODS: Static and dynamic compression bending tests were based on an ASTM F1717 method modified to accommodate dual growing rod constructs. Six construct configurations were tested, mechanical properties were recorded, and statistical analyses were performed to determine significant differences between groups: (1) no connectors (rods only), (2) side-by-side connectors, (3) side-by-side connectors plus 4 crosslinks, (4) (40-mm long tandem connectors, (5) 80-mm long tandem connectors, and (6) 80-mm long tandem connectors plus 4 crosslinks. FE analysis was used to predict the stress distribution within the constructs. RESULTS: The static results indicated greater stiffness, yield load, and peak load as the axial connector length increased (side-by-side to 40 mm tandem to 80 mm tandem). The dynamic results showed similar cycles to failure for side-by-side and tandem connector (40 and 80 mm) construct configurations without crosslinks. Crosslinks shifted the location of rod fracture observed experimentally and significantly reduced the fatigue life of the construct. The flexibility of the construct decreased significantly as the axial connector length increased. FE predictions were highly consistent with the experimentally measured values and provided information on stress distribution within the rod for comparison to experimental fracture locations. CONCLUSIONS: This is the first study to evaluate mechanical performance of various configurations of pediatric growing rod constructs using preclinical models. The current study is consistent with a previous retrieval study in that rigid constructs lacking flexibility (ie, higher stiffness and lower displacement), such as those with 80-mm tandem connectors and multiple crosslinks, demonstrated decreased mechanical performance as shown through both experimental and computational models. Additionally, the experimental and computational findings suggest that surgeons should strategically consider the number of interconnecting components and subsequent stress concentrations along the posterior side of the rod. For example, changing the placement of crosslinks to low stress regions of the construct or not using crosslinks in the construct are options.

Sex Specific Sacroiliac Joint Biomechanics During Standing Upright: A Finite Element Study.

STUDY DESIGN: The comparison of sacroiliac joint (SIJ) angular motions, pelvis ligaments strain, load sharing, and stress distribution across the joint for male and female spine-pelvis-femur models using finite element analysis. OBJECTIVE: To quantify biomechanical parameters at SIJ for all motions for both male and female models. SUMMARY OF BACKGROUND DATA: SIJ has been recognized as a main source of pain in 13% to 30% of patients with low back pain. It is shown that the SIJ rotation and translation in different planes are not exceeding 2° to 3° and 2 mm, respectively. Due to limitation of in vivo and in vitro studies, it is difficult to quantify certain biomechanical parameters such as load-sharing and stress distribution across the joint. Finite element analysis is a useful tool which can be utilized to understand the biomechanics of the SIJ. METHODS: The validated finite element models of a male and a female lumbar spine-pelvis-femur were developed from computer tomography (CT) scans. The models were used to simulate spine physiological motions. The range of motion, ligament strains, load sharing, and stress distribution across the left and right SIJs were compared between male and female models. RESULTS: Motions data at SIJs demonstrated that female model experienced 86% higher mobility in flexion, 264% in extension, 143% in left bending, and 228% in right bending compared with the male model. The stresses and loads on SIJs were higher on the female model compared with the male model. Female model ligaments underwent larger strains compared with the male model ligaments. CONCLUSION: Female SIJ had higher mobility, stresses, loads, and pelvis ligament strains compared with the male SIJ which led to higher stress across the joint, especially on the sacrum under identical loading conditions. This could be a possible reason for higher incidence of SIJ pain and pelvic stress fracture in females. LEVEL OF EVIDENCE: N/A.