Biomechanical evaluation of a novel intervertebral disc repair technique for large box-shaped ruptures.

Objective: The purpose of this study was to analyze the feasibility of repairing a ruptured intervertebral disc using a patch secured to the inner surface of the annulus fibrosus (AF). Different material properties and geometries for the patch were evaluated. Methods: Using finite element analysis, this study created a large box-shaped rupture in the posterior-lateral region of the AF and then repaired it with a circular and square inner patch. The elastic modulus of the patches ranged from 1 to 50 MPa to determine the effect on the nucleus pulposus (NP) pressure, vertical displacement, disc bulge, AF stress, segmental range of motion (ROM), patch stress, and suture stress. The results were compared against the intact spine to determine the most suitable shape and properties for the repair patch. Results: The intervertebral height and ROM of the repaired lumbar spine was similar to the intact spine and was independent of the patch material properties and geometry. The patches with a modulus of 2-3 MPa resulted in an NP pressure and AF stresses closest to the healthy disc, and produced minimal contact pressure on the cleft surfaces and minimal stress on the suture and patch of all models. Circular patches caused lower NP pressure, AF stress and patch stress than the square patch, but also caused greater stress on the suture. Conclusion: A circular patch with an elastic modulus of 2-3 MPa secured to the inner region of the ruptured annulus fibrosus was able to immediately close the rupture and maintain an NP pressure and AF stress similar to the intact intervertebral disc. This patch had the lowest risk of complications and produced the greatest restorative effect of all patches simulated in this study.

Failure analysis of a locking compression plate with asymmetric holes and polyaxial screws.

Locking compression plates (LCP) with asymmetrical holes and polyaxial screws are effective for treating mid-femoral fractures, but are prone to failure in cases of bone nonunion. To understand the failure mechanism of the LCP, this study assessed the material composition, microhardness, metallography, fractography and biomechanical performance of a retrieved LCP used for treating a bone fracture of AO type 32-A1. For the biomechanical assessment, a finite element surgical model implanted with the intact fixation-plate system was constructed to understand the stresses and structural stiffness on the construct. Also, to avoid positioning screws around the bone fracture, different working lengths of the plate (the distance between the two innermost screws) and screw inclinations (±5°, ±10° and ±15°) were investigated. The fracture site of the retrieved LCP was divided into a narrow side and broad side due to the asymmetrical distribution of holes on the plate. The results indicated that the chemical composition and microhardness of the LCP complied with ASTM standards. The fatigue failure was found to originate on the narrow side of the hole, while the broad side showed overloading characteristics of crack growth. When the screws were inserted away from the region of the bone fracture by increasing the working length, the stress of the fixation-plate system decreased. Regardless of the screw insertion angle, the maximum stress on the LCP always appeared on the narrow side, and there was little change in the structural stiffness. However, angling the screws at -10° resulted in the most even stress distribution on the fixation-plate system. In conclusion, the LCP assessed in this study failed by fatigue fracture due to bone nonunion and stress concentration. The narrow side of the LCP was vulnerable to failure and needs to be strengthened. When treating an AO type 32-A1 fracture using an LCP with asymmetrical holes and polyaxial screws, inserting the screws at -10° may reduce the risk of implant failure and positing screws around the fractured area of the bone should be avoided.

Biomechanical evaluation of a short-rod technique for lumbar fixation surgery.

Objective: The purpose of this study was to analyze the stability and instrument-related complications associated with fixation of the lumbar spine using the Short-Rod (SR) technique. Methods: Using finite element analysis, this study assessed the stability of a bilateral lumbar fixation system when inserting the pedicle screws at angles of 10°, 15°, and 20° to the endplate in the sagittal plane. Using the most stable construct with a screw angle, the model was then assessed with different rod lengths of 25, 30, 35, and 45 mm. The optimal screw inclination angle and rod length were incorporated into the SR model and compared against traditional parallel screw insertion (pedicle screws in parallel to the endplate, PPS) in terms of the stability and risk of instrument-related complications. The following parameters were evaluated using the validated L4-L5 lumbar finite element model: axial stiffness, range of motion (ROM), stress on the endplate and facet joint, von-Mises stress on the contact surface between the screw and rod (CSSR), and screw displacement. Results: The results showed that the SR model with a 15° screw inclination angle and 35 mm rod length was superior in terms of construct stability and risk of complications. Compared to the PPS model, the SR model had lower stiffness, lower ROM, less screw displacement, and lower stress on the facet cartilage, the CSSR, and screws. However, the SR model also suffered more stress on the endplate in flexion and lateral bending. Conclusion: The SR technique with a 15° screw inclination and 35 mm rod length offers good lumbar stability with a low risk of instrument-related complications.

Biomechanical changes at the adjacent segments induced by a lordotic porous interbody fusion cage.

Biomechanical changes at the adjacent segments after interbody fusion are common instigators of adjacent segment degeneration (ASD). This study aims to investigate how the presence of a lordotic porous cage affects the biomechanical performance of the adjacent segments. A finite element model (FEM) of a lumbar spine implanted with a lordotic cage at L3-L4 was validated by in-vitro testing. The stress distribution on the cage and range of motion (ROM) of L3-L4 were used to assess the stability of the implant. Three angles of cage (0° = non-restoration, 7° = normal restoration and 11° = over-restoration) were modelled with different porosities (0%, 30% and 60%) and evaluated in the motions of flexion, extension, lateral bending and rotation. The ROM, intervertebral disc pressure (IDP) and facet joint force (FJF) were used to evaluate biomechanical changes at the adjacent segments in each model. The results indicated that porous cages produced more uniform stress distribution, but cage porosity did not influence the ROM, IDP and FJF at L2-L3 and L4-L5. Increasing the cage lordotic angle acted to decrease the ROM and IDP, and increase the FJF of L4-L5, but did not alter the ROM of L2-L3. In conclusion, changes in ROM, IDP and FJF at the adjacent segments were mainly influenced by the lordotic angle of the cage and not by the porosity. A larger angle of lordotic cage was shown to reduce the ROM and IDP, and increase the FJF of the lower segment (L4-L5), but had little effect on the ROM of the upper segment (L2-L3).

In Vitro Analysis of Wearing of Hip Joint Prostheses Composed of Different Contact Materials.

Cobalt-chromium-molybdenum alloy (CoCrMo) and ceramic are the two most common materials for the femoral head in hip joint prostheses, and the acetabular liner is typically made from ultra-high molecular weight polyethylene (UHMWPE), highly cross-linked polyethylene (XLPE), or highly cross-linked polyethylene blended with Vitamin E (VEXLPE). The selection of suitable materials should consider both wear performance and cost-effectiveness. This study compared the wear rate between different friction pairs using a hip joint simulator and then recommended a suitable prosthesis based on the corresponding processing technology and cost. All wear simulations were performed in accordance with ISO 14242, using the same hip joint simulator and same test conditions. This study found that when using the same material for the femoral head, the XLPE and VEXLPE liners had a lower wear rate than the UHMWPE liners, and the wear rate of the XLPE liners increased after blending with Vitamin E (VEXLPE). There was no significant difference in the wear rate of XLPE when using a CoCrMo or ceramic head. Considering the wear rate and cost-effectiveness, a CoCrMo femoral head with an accompanying XLPE liner is recommended as the more suitable combination for hip prostheses.

Challenges of pre-clinical testing in orthopedic implant development.

The market for orthopedic implants is growing rapidly with the increasing prevalence of orthopedic diseases in an aging society. Different designs and materials have been developed over the years and have, in general, shown excellent results in pre-clinical testing. However, there have been incidences of serious complications when novel implants or materials are put into clinical use, with some well-known cases being metallosis in patients implanted with metal-on-metal hip replacements and osteolysis from polyethylene wear debris generated in hip and knee joint replacements. Unforeseen factors related to new designs, materials and surgical techniques can lead to different outcomes for pre-clinical testing and clinical use. While often an excellent indicator of a device's performance in clinical settings, pre-clinical testing does sometime fail to predict critical flaws in implant development. This article aims to explore the gaps in the current approach to testing. The ISO international standard of pre-clinical testing should be modified to more adequately capture actual clinical use of the implant and simulate daily activities. This article will also introduce modern methods for implant development, such as FEM, 3D printing and computer-aided orthopedic surgery, which can be widely applied to improve pre-clinical testing procedures and reduce the incidence of surgical malalignment by analyzing biomechanical performance, planning surgical procedure and providing surgical guide.