Patient-specific design process and evaluation of a hip prosthesis femoral stem.

INTRODUCTION: There are several commercially available hip implant systems. However, for some cases, custom implant designed based on patient-specific anatomy can offer the patient the best available implant solution. Currently, there is a growing trend toward personalization of medical implants involving additive manufacturing into orthopedic medical implants' manufacturing. METHODS: This article introduces a systematic design methodology of femoral stem prosthesis based on patient's computer tomography data. Finite element analysis is used to evaluate and compare the micromotion and stress distribution of the customized femoral component and a conventional stem. RESULTS: The proposed customized femoral stem achieved close geometrical fit and fill between femoral canal and stem surfaces. The customized stem demonstrated lower micromotion (peak: 21 µm) than conventional stem (peak: 34 µm). Stress results indicate up to 89% increase in load transfer by conventional stem than custom stem because the higher stiffness of patient-specific femoral stem proximally increases the custom stem shielding in Gruen's zone 7. Moreover, patient-specific femoral stem transfers the load widely in metaphyseal region. CONCLUSION: The customized femoral stem presented satisfactory results related to primary stability, but compromising proximo-medial load transfer due to increased stem cross-sectional area increased stem stiffness.

Research: Design and Analysis of a Low-Stiffness Porous Hip Stem.

Two major problems are associated with total hip replacement: 1) stress shielding and 2) the adverse tissue reaction to certain elements of the implant material. In this regard, a porous implant provides lower stiffness and vacancies for bone ingrowth, making it more suitable for the human bone compared with a solid stem. Moreover, second-generation titanium biomedical alloys, such as TNZT (Ti35Nb7Zr5Ta) and TMZF (Ti12Mo6Zr2Fe), have been introduced to prevent the adverse tissue reactions related to aluminum and vanadium elements of the popular Ti6Al4V alloy. In the current work, an analysis was performed based on uniaxial compression testing of cubic Ti6Al4V structures of different porosities to predict the governing equations that relate the relative density of the structure to the mechanical properties of the structure according to the Gibson-Ashby model. A numerical study was conducted to evaluate the change in stress distribution obtained by incorporating the new titanium alloys in porous hip stem implants. Implants modeled with the mechanical properties of TNZT and TMZF showed a minimum safety factor of 1.69 and 3.02, respectively, with respect to the yield strength. The results demonstrated an increase in the equivalent von Mises stresses and maximum principal elastic strain up to 7% and 15%, respectively, compared with the porous Ti6Al4V implant and up to 108% and 156%, respectively, compared with the solid Ti6Al4V implant.

Pre-clinical evaluation of the mechanical properties of a low-stiffness cement-injectable hip stem.

In total hip arthroplasty (THA), the femoral stem can be fixed with or without bone cement. Cementless stem fixation is recommended for young and active patients as it eliminates the risk of loss of fixation at the bone-cement and cement-implant interfaces. Cementless fixation, however, suffers from a relatively high early revision rate. In the current research, a novel low-stiffness hip stem was designed, fabricated and tested. The stem design provided the option to inject biodegradable bone cement that could enhance initial stem stability. The stem was made of Ti6Al4V alloy. The proximal portion of the stem was porous, with cubic cells. The stem was fabricated using electron beam melting (EBM) technology and tested in compression and bending. Finite-element analysis was used to evaluate stem performance under a dynamic load representing a stair descending cycle and compare it to the performance of a solid stem with similar geometry. The von Mises stresses and maximum principal strains generated within the bone increased after porous stem insertion compared to solid stem insertion. The low-modulus stem tested in this study has acceptable mechanical properties and generates strain patterns in bone that appear compatible with clinical use.