Aims

In this study, we aimed to explore surgical variations in the Femoral Neck System (FNS) used for stable fixation of Pauwels type III femoral neck fractures.

Implant manufacturers develop new products to improve existing fracture fixation methods or to approach new fracture challenges. New implants are commonly tested and approved with respect to their corresponding predecessor products, because the knowledge about the internal forces and moments acting on implants in the human body is unclear. The aim of this study was to evaluate and validate implant internal forces and moments of a complex physiological loading case and translate this to a standard medical device approval test. A finite elements model for a transverse femur shaft fracture (AO/OTA type 32-B2) treated with a locked plate system (AxSOS 3 Ti Waisted Compression Plate Broad, Stryker, Kalamazoo, USA) was developed and experimentally validated. The fractured construct was physiologically loaded by resulting forces on the hip joint from previously measured in-vivo loading experiments (Bergmann et. al). The forces were reduced to a level where the material response in the construct remained linear elastic. Resulting forces, moments and stresses in the implant of the fractured model were analysed and compared to the manufacturers’ approval data. The FE-model accurately predicted the behaviour of the whole construct and the micro motion of the working length of the osteosynthesis. The resulting moment reaction in the working length was 24 Nm at a load of 400 N on the hip. The maximum principle strains on the locking plate were predicted well and did not exceed 1 %. In this study we presented a protocol by the example of locked plated femur shaft fracture to calculate and validate implant internal loading using finite element analysis of a complex loading. This might be a first step to move the basis of development of new implants from experience from previous products to calculation of mechanical behaviour of the implants and therefore, promote further optimization of the implants’ design

Background. High cup abduction angles generate increased contact stresses, higher wear rates and increased revision rates. However, there is no reported study about the influence of cup abduction on stresses under head lateralisation conditions for ceramic-on-Ceramic THA. Material and method. A finite elements model of a ceramic-on-ceramic THA was developed in order to predict the contact area and the contact pressure, first under an ideal regime and then under lateralised conditions. A 32 mm head diameter with a 30 microns radial clearance was used. The cup was positioned with a 0°anteversion angle and the abduction angle was varied from 45° to 90°. The medial-lateral lateralisation was varied from 0 to 500 microns. A load of 2500 N was applied through the head center. Results. For 45° abduction angle, edge loading appeared above a medial-lateral separation of 30 μm. Complete edge loading was obtained above 60 μm medial-lateral separation. For 45 degrees inclination angle, as the lateralisation increased, the maximal contact pressure increased from 66 MPa and converged to an asymptotic value of 205 MPa. A higher inclination angle resulted in a higher maximum contact pressure. However, this increase in contact pressure induced by higher inclination angle, became negligible as the lateral separation increased. Discussion Both inclination angle and lateral displacement induced a large increase in the stresses in Ceramic-on-Ceramic THA. Edge loading appeared for a small lateralisation. The influence of acetabular inclination angle became negligible for a lateral displacement above 240 μm, as the stresses reached an asymptotic value

Purpose of the study: Implantation of the acetabular socket with high inclination generates increased contract stress, wear and revision rate for total hip arthroplasty (THA). Study of ceramic-on-ceramic THA explants has revealed a high wear rate in bands, suggesting a microseparation effect generating edge loading. There have not been any studies examining the influence of the cup inclination on the contact pressures in ceramic-on-ceramic THA exposed to microseparation between the head and the cup. Material and methods: A finite elements model of a ceramic-on-ceramic hip prosthesis was developed with ABAQUS in order to predict the surface contact and the distribution of the contract pressures, first during ideal centred function then under conditions of microseparation. A 32mm head and a radial clearance head (30μm) were used. The cup was positioned in zero anteversion and 45, 65, 70, and 90° anteversion. Progressive microseparation (0 to 500 μm) was imposed. A 2500N loading force was applied to the centre of the head. Results: For 45° inclination, edge loading appeared for mediolateral separation greater than 30 μm and became complete for 60 μm separation. When edge loading appeared, the contact surface was elliptic. The length of the lesser axis converged towards 0.96mm; the greater axis towards 8.15mm, respectively in the anteroposterior and mediolateral directions. For inclinations of 45°, the contact pressure was 66 Mpa for the centred force. As the mediolateral separation increased, the maximal contact pressure increased, converging towards an asymptotic value of 205 MPa. Increasing the inclination angle of the cup generated an increase in the maximal contact pressure. However, this increase in contact pressure generated by the increasing inclination angle was negligible if the microseparation increased. Discussion: Cup inclination and mediolateral laxity increase stress forces of ceramic-on-ceramic THA and should be avoided. However, the influence of the cup inclination becomes negligible beyond a separation value of 240 μm, the stress forces already having reached their asymptotic value

Purpose of the study: Knowledge of the normal kinematics of the knee joint, and particularly the femoropatellar joint, is indispensable for evaluating prosthetic implants. Accurate measurements are however necessary, especially for patellar tracking. The purpose of this study was to propose a new experimental set up for analysis of the knee joint and to validate its pertinence in terms of accuracy and incertitude. Materials and Methods: Eight anatomic specimens of non-embalmed healthy knees were tested on the new setup with a fixed femur and a tibia left free to move. The flexion-extension movement was created by applying force to the quadriceps tendon and resistance to the distal end of the tibia. The femorotibial and femoropatellar kinematics were monitored with an infrared optoelectronic tracking system after acquisition of the bone geometry and the position of the markers on stereoradiographs coupled with a specific 3D reconstruction software. The landmarks used to interpret the kinematic measurements were calculated from the reconstructions of anatomic specimens. Incertitude linked to the determination of these landmarks was assessed as was its impact on the kinematic measurements. Results: Trials were run on eight knees to validate the experimental setup and study knee kinematics during flexion-extension movements. Method-related measurement incertitude was less than 0.2° in rotation (1 SD) and less than 0.9 mm in translation (1 SD) for the tibia and less than 0.2° in rotation (1 SD) and 0.6 mm in translation (1 SD) for the patella. Quantitative analysis was completed by an animation to visualise any anomalies under different angles. Discussion: This protocol which couples 3D imaging with a kinematic analysis enables real time tracking of the bone pieces during the experimental trials. This in vitro setup produces femoropatellar and tibial kinematics in agreement with data in the literature. Observations will enable better understanding of femoropatellar function and provide objective data on potential kinematic anomalies. Conclusion: This experimental evaluation combining bone geometry and kinematic monitoring specifically designed for the knee joint should enable objective evaluation of implants and a validation of personalised finite elements models of the knee

Introduction. Uncemented porous coated acetabular components have gained more research emphasis in recent years compared to their cemented counterparts, largely owing to the natural biological fixation they offer. Nevertheless, sufficient peri-prosthetic bone ingrowth is essential for long-term fixation of such uncemented acetabular components. The phenomenon of bone ingrowth can be predicted based on mechanoregulatory principles of primary bone fracture healing. Literature review reveals that the surface texture of implant plays a major role in implant-bone fixation mechanism. A few insilico models based on 2-D microscale finite elements (FE) were reported in literatures to predict the influence of surface texture designs on peri-prosthetic bone ingrowth. However, most of these studies were based on FE models of dental implants. The primary objective of this study, therefore, is to mechanobiologically predict the influence of surface texture on bone- ingrowth in acetabular components considering a novel 3-D mesh-shaped surface texture on the implant. Materials/Methods. The 3-D microscale model [Fig.1] of implant-bone interface was developed using CATIA. ®. V5R20 software (DassaultSystèmes, France) and was modelled in ANSYS V15.0 FE software (Ansys Inc., PA, USA) using coupled linear elastic ten-noded tetrahedral finite elements. The model consists of cast-inbeaded mesh textured implant having finely meshed inter-bead spacing. Linear, elastic and isotropic material properties considering Young's modulus of 210 GPa and Poisson's ratio of 0.3 for stainless steel implant were employed in the model. Boundary of bone was assumed to be rich in Mesenchymal Stem Cells(MSC) with periodic boundary conditions at contralateral surfaces. The linear elastic material properties in the model were updated iteratively through a tissue differentiation algorithm that works on the principle of mechanotransduction driven by local mechanical stimuli, e.g. hydrostatic pressure and equivalent deviatoric strain. Results. Results indicate that bone ingrowth is inhibited upon increasing the inter-bead spacing and upon decreasing the bead aspect ratio. It has been observed that there is a predominant influence of bead spacing diameter on the peri-acetabular bone ingrowth. The increase in bead spacing diameter has led to increased bead height that is found to promote higher bone ingrowth with an increase in average Young's modulus of neo-tissue layer. Conclusions. The present study focussed on the development of a new texture on the implant surface and to study the influence of surface texture on bone-ingrowth in acetabular components. Since there is a promising increase in average Young's modulus of the newly formed tissue layer, it predicts the increase in stiffness of the newly formed tissue. The increase in tissue stiffness reveals that, there is not much inhibition in bone ingrowth after the employment of the acetabular implant. The numerical study based on mechanoregulatory algorithm considering the appropriate mechanical stimuli responsible for bone ingrowth, reveals that, compared to hemispherical beaded surface texture, mesh shaped surface texture provides an improved fixation of the acetabular component. For any figures or tables, please contact the authors directly