Abstract
During conventional hip arthroplasties, the diseased femur is rigidified using a metallic stem. The insertion of the stem induces a change in the stress distribution in the surrounding femur, and the bone remodels; this stress distribution is a direct result of the stem stiffness characteristics. Healthy healing of the femur requires that the bone be loaded as naturally as possible. If the bone is not loaded appropriately, it can resorb which may result in stem loosening and revision. Although current rigid metallic femoral stems are very successful, a poor stress distribution may become a critical problem for younger patients as the stem/femoral bone construct will be subjected to higher loads for longer times, and since remodelling is faster, loosening can occur earlier. Reduced stiffness stems have therefore been investigated, but early failures have been reported due to increased movements, poor initial stability and the low proximal stiffness of the stem. A novel biocompatible carbon fibre reinforced plastic (CFRP) stem has been developed in light of these past experiences1. Using a series of analytical models and experimental validation tests1, the fibre type and architecture have been tailored along and across the stem to achieve healthy bone remodelling and proximal strength of the construct. In addition, a biocompatible hydroxyapatite coating was specifically designed to enhance interface strength and stability2. The present study describes the mechanical behaviour of this novel stem with particular emphasis on the stem/bone interface. 4 static and 29 fatigue tests were performed according to ISO7206; these tests were complemented by acoustic emission monitoring to identify failure mechanisms3. A stress versus number of cycles to failure (SN) curve was obtained to describe the fatigue behaviour (i) under constant amplitude cycling at various load levels and (ii) incorporating rest periods and overloads. In addition, a mechanical test was designed to characterise the motions between the bone and the stem during sinusoidal fatigue loading (5000 cycles, 0.2–2kN, 1Hz). Two linear variable differential transformers measured the vertical and horizontal displacements at the stem/ bone interface in the proximal region. 3 tests were performed on CFRP stems and 3 on a metallic stem. The CFRP stem exceeded the standard requirements. The SN curve showed good repeatability across the loading spectrum. The inclusion of overloads/static loads during fatigue had a beneficial effect on the stem endurance. This is attributed to the development of microcracks, which dissipate the load, and to creep of the resin. The amplitude of recoverable motion observed at the interface during each load cycle was similar for both types of stem (20mm and 4mm in the horizontal and vertical directions respectively) and remained below the recommended limit4. Composite materials offer high design flexibility. This has been exploited in the development of a compliant, mechanically tailored biocompatible hip stem for femoral reconstruction, and could provide an answer to hip replacement for younger, more active patients.
Correspondence should be addressed to Dr Carlos Wigderowitz, Honorary Secretary of BORS, Division of Surgery & Oncology, Section of Orthopaedic & Trauma Surgery, Ninewells Hospital & Medical School Tort Centre, Dundee, DD1 9SY.
References –
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2 Thompson et al, Mat Sc-Med1999, 10. Google Scholar
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