Abstract
INTRODUCTION
Total hip arthroplasty (THA) is a very successful orthopaedic treatment with 15 years implant survival reaching 95%, but decreasing age and increasing life expectancy of THA patients ask for much longer lasting solutions. Shorter and more flexible cementless stems are of high interest as these allow to maintain maximum bone stock and reduce adverse long-term bone remodeling.1 However, decreasing stem length and reducing implant stiffness might compromise the initial stability by excessively increasing interfacial stresses. In general, a good balance between implant stability and reduced stress shielding must be provided to obtain durable THA reconstruction.2
This finite element (FE) study aimed to evaluate primary stability and bone remodeling of a new design of short hip implant with solid and U-shaped cross-section.
MATERIALS AND METHODS
The long tapered Quadra-H stem and the short SMS implants (Medacta International, Castel San Pietro, Switzerland) were compared in this study (Figure 1). A FE model of a femur was based on calibrated CT data of an 81 year-old male (osteopenic bone quality). Both titanium alloy implants were assigned an elastic modulus of 105 GPa and the Poisson's ratios were set to 0.3. Initial stability simulations included the hip joint force and all muscle loads during a full cycle of normal walking as calculated in AnyBody software (Anybody Technology AS, Denmark), whereas the remodeling simulation used the peak loads from normal walking and stair climbing activities. Initial stability results are presented as micromotions on the implant surface with a threshold of 40 µm.3 Bone remodeling outcomes are represented in a form of simulated Dual X-ray Absorptiometry (DEXA) scans and the quantitative bone mineral density (BMD) changes in 7 periprosthetic zones.
RESULTS
The U-shaped SMS implant showed slightly higher micromotions (2.7% surface area exceeding 40 µm) than the Quadra-H stem (0.2%), whereas micromotions of solid SMS were considerably higher (8.4%) (Figure 2). The largest micromotions were found on medial side of all implants.
The smallest bone loss one year post-operatively was predicted around the U-shaped SMS implant. Proximal zones (1, 6 and 7) showed the largest bone loss with average of 9.9%, 11.8% and 12.8% for the U-shaped SMS, solid SMS and Quadra-H respectively (Figure 3). The bone remodeling prediction for the Quadra-H stem was in good agreement with clinical DEXA measurements (overall bone loss of 5.5% vs. 5.7).
CONCLUSION
The U-shaped SMS implant is clearly superior to its solid version and has potential to provide comparable initial stability as the long Quadra-H stem and considerably better long-term bone stock preservation.
