In Silico Meta-Analysis of Boundary Conditions for Experimental Tests on the Lumbar Spine

Combined Rigid-Flexible Multibody Analysis Reveals Reduced Pedicle Screw Loads in Short-Segment Fixation for Decompressed Lumbar Spine Stabilization

BACKGROUND

Spinal cord compression in patients with vertebral metastases often requires surgical decompression with spinal fixation. Recent studies reported increased implant failures due to mechanical complications, raising concerns about current clinical practices. Long-segment fixation (Lf) is commonly employed to enhance mechanical stability and reduce the severity of pedicle screw failure. The study investigates how the number of vertebral levels involved in fixation affects the loads on pedicle screw anchorages in a fatigue-related displacement domain.

METHOD

Using a rigid-flexible multibody approach, a non-linear T12-S1 model was employed to simulate two fixation types following L3 posterior decompression surgery: Lf spanning two levels above and below the decompression site (L1, L2, L4, and L5) and a short-segment fixation (Sf) involving only adjacent vertebrae. Internal reactions at the rod-pedicle screw anchorages were estimated in terms of pullout, shear forces, and bending moments. The range of motion analysed (flexion: 22°, extension: 8°, lateral bending: 12°, axial rotation: 5°) was confined to the "Cone of Economy", representing a small-displacement volume where loads are assumed cyclically exchanged.

RESULTS

Lf exhibited up to fivefold higher reactions than Sf, with a heterogeneous shear force distribution: middle screws appeared shielded, while extremity screws were overloaded (~400 N, comparable to experimental fatigue strength). Pullout forces remained within safe limits (< 150 N).

CONCLUSIONS

The rigid-flexible multibody approach effectively estimated internal loads in the implant-spine constructs under dynamic conditions. The findings highlight the long-term implications of Lf, demonstrating that involving more vertebral levels triggers adverse loads on pedicle screws, potentially compromising implant durability.

In Silico Evaluation of the Primary Stability of Acetabular Revision Cups: Standard Versus Locking Screws

Given the increasing complexity of revision total hip arthroplasty, ensuring optimal stability of acetabular revision cups (ARCs) is crucial, especially in cases of bone stock loss. In this study, the primary stability of ARCs was investigated by modeling various configurations of screw placements, including all standard, all locking, and mixed setups, under different load scenarios. Finite element (FE) models were implemented to evaluate the stress distribution and micromotions at the bone-prosthesis interface, simulating three primary load cases: "walking," "stairs-down," and "stand-up." The results indicated that the stairs-down load case generated the highest von Mises stresses and sliding distances, marking it as the most critical load scenario. Configurations using all standard screws showed higher peak stresses and reduced stable areas, compared to those utilizing locking screws. While the locking screws provided a stiffer connection, the mixed screw configuration offered a balanced performance by combining the compression benefits of standard screws with the rigid fixation of locking screws. Configurations with a single standard screw complemented by locking screws showed enhanced stability, suggesting this combination could be advantageous in clinical applications. This study underscores the importance of screw type and placement in ensuring the primary stability of ARCs. Locking screws are recommended for use when feasible. For ARCs allowing both screw types, a mixed configuration starting with a standard compression screw followed by locking screws appears optimal. Future research should further explore various combinations of screw geometries and lengths to refine these findings and enhance surgical outcomes in acetabular revisions.

A rig for in vitro testing of the lumbar spine and pelvis simulating posterior, anterior and oblique trunk muscles

Numerous research questions require in vitro testing on lumbar spine and pelvis specimens. The majority of test setups apply forces and torques via the uppermost vertebral body with the lowermost vertebral body fixed and have been validated for kinematics and intradiscal pressure. Models without simulation of muscle traction may produce valid data only for testing conditions for which they have been validated. In vitro test setups with simulation of muscle traction would appear to be useful for conditions beyond such conditions. The aim of the present study was to describe and validate a test rig for the lumbar spine that applies the forces directly to the vertebral bodies via artificial muscle attachments and thus includes the stabilising effects of the muscles known from the literature. The artificial muscle attachments were chosen to get a stable fixation of the pulleys on the cadaver. The location of force application was as close as possible to the physiological footprint of the muscle on the bone. Three paired muscles were combined by individual linear actuators and simulated under force control (posterior, anterior and oblique trunk muscles). An optical 3D motion capture system (GOM, Zeiss, Germany) was used to measure the reposition of the entire lumbar spine and the sacrum against the ilium. At the same time, the force applied to all simulated muscles was recorded. All muscle attachments could be loaded up to a maximum force of 1 kN without failure. The following reposition of the lumbar spine could be generated by the simulated muscle traction keeping the force below each muscle's individual strength: extension 18°, flexion 27°, lateral bending 33°, axial rotation 11°. The effects on lumbar spine reposition of the individual trunk muscles differed depending on the direction of movement. The anterior trunk muscles were the most acting for flexion/extension, at 0.16 ± 0.06°/N, while the oblique trunk muscles were the most acting for lateral bending (0.17 ± 0.16°/N) and axial rotation (0.10 ± 0.14°/N). The maximum nutation of the sacroiliac joint (SIJ) was on average 1,2° ± 0,2°. The artificial muscle attachments to the vertebral bodies proved to be withstand physiologically occurring forces. The range of motion generated in the test rig was physiological. The SIJ nutation determined and the direction of action of the muscle groups correspond to literature data. The order of the individual muscle effects on lumbar spine reposition corresponds to the distance between the muscle insertions and the physiological centre of rotation. In conclusion, taking into account the limitations, the lumbar spine test rig presented here allows the analysis of movements of the lumbar spine and pelvis resulting directly from simulated muscle tractions and thus enables a test environment close to in vivo conditions.

Understanding the role of head size and neck length in micromotion generation at the taper junction in total hip arthroplasty

Modular hip implants allow intra-operative adjustments for patient-specific customization and targeted replacement of damaged elements without full implant extraction. However, challenges arise from relative micromotions between components, potentially leading to implant failure due to cytotoxic metal debris. In this study magnitude and directions of micromotions at the taper junction were estimated, aiming to understand the effect of variations in head size and neck length. Starting from a reference configuration adhering to the 12/14 taper standard, six additional implant configurations were generated by varying the head size and/or neck length. A musculoskeletal multibody model of a prothesized lower limb was developed to estimate hip contact force and location during a normal walking task. Following the implant assembly, the multibody-derived loads were imposed as boundary conditions in a finite element analysis to compute the taper junction micromotions as the relative slip between the contacting surfaces. Results highlighted the L-size head as the most critical configuration, indicating a 2.81 μm relative slip at the mid-stance phase. The proposed approach enables the investigation of geometric variations in implants under accurate load conditions, providing valuable insights for designing less risky prostheses and informing clinical decision-making processes.

Multibody Models of the Thoracolumbar Spine: A Review on Applications, Limitations, and Challenges

Numerical models of the musculoskeletal system as investigative tools are an integral part of biomechanical and clinical research. While finite element modeling is primarily suitable for the examination of deformation states and internal stresses in flexible bodies, multibody modeling is based on the assumption of rigid bodies, that are connected via joints and flexible elements. This simplification allows the consideration of biomechanical systems from a holistic perspective and thus takes into account multiple influencing factors of mechanical loads. Being the source of major health issues worldwide, the human spine is subject to a variety of studies using these models to investigate and understand healthy and pathological biomechanics of the upper body. In this review, we summarize the current state-of-the-art literature on multibody models of the thoracolumbar spine and identify limitations and challenges related to current modeling approaches.

Cross-link augmentation enhances CFR-PEEK short fixation in lumbar metastasis stabilization

Introduction: Spinal stability plays a crucial role in the success of the surgical treatment of lumbar vertebral metastasis and, in current practice, less invasive approaches such as short constructs have been considered. Concurrently, carbon fiber-reinforced (CFR) poly-ether-ether-ketone (PEEK) fixation devices are expanding in oncologic spinal surgery thanks to their radiotransparency and valid mechanical properties. This study attempts to provide an exhaustive biomechanical comparison of different CFR-PEEK surgical stabilizations through a highly reproducible experimental setup. Methods: A Sawbones biomimetic phantom (T12-S1) was tested in flexion, extension, lateral bending, and axial rotation. An hemisome lesion on L3 vertebral body was mimicked and different pedicle screw posterior fixations were realized with implants from CarboFix Orthopedics Ltd: a long construct involving two spinal levels above and below the lesion, and a short construct involving only the levels adjacent to L3, with and without the addition of a transverse rod-rod cross-link; to provide additional insights on its long-term applicability, the event of a pedicle screw loosening was also accounted. Results: Short construct reduced the overloading onset caused by long stabilization. Particularly, the segmental motion contribution less deviated from the physiologic pattern and also the long-chain stiffness was reduced with respect to the prevalent long construct. The use of the cross-link enhanced the short stabilization by making it significantly stiffer in lateral bending and axial rotation, and by limiting mobiliza-tion in case of pedicle screw loosening. Discussion: The present study proved in vitro the biomechanical benefits of cross-link augmentation in short CFR-PEEK fixation, demonstrating it to be a potential alternative to standard long fixation in the surgical management of lumbar metastasis.