The disadvantage of removing a well-fixed femoral stem are multiple (operating time, risk of fracture, bone and blood loss, recovery time and post-op complications. Ceramic heads with titanium adapter sleeves (e.g. BIOLOX®OPTION, Ceramtec) are a possibility for putting a new ceramic head on slightly damaged used tapers. ‘Intolerable’ taper damages even for this solution are qualitatively specified by the manufacturers. The aim of this study was to determine the fracture strength of ceramic heads with adapter sleeves on stem tapers with such defined damage patterns. Pristine stem tapers (Ti-6Al-4V, 12/14) were damaged to represent the four major stem taper damage patterns specified by the manufacturers:
‘Truncated’: Removal of 12.5% of the circumference along the entire length of the stem taper at a uniform depth of 0.5mm parallel to the taper slope. ‘Slanted’: Removal of 33.3% of the proximal diameter perimeter with decreasing damage down to 3.7mm from the proximal taper end. ‘Cut’: Removal of the proximal 25% (4mm) of the stem taper. ‘Scratched’: Stem tapers from a previous ceramic fracture test study with a variety of scratches and crushing around the upper taper edge from multiple ceramic head fractures. The ‘Control’ group consisted of three pristine tapers left undamaged. BIOLOX®OPTION heads (Ø 32mm, length M) with Ti adapter sleeves were assembled to the damaged stem tapers and subjected to ISO7206-10 ultimate compression strength testing. The forces required to fracture the head were high and caused complete destruction of the ceramic heads in all cases. The ‘Truncated’ group showed the lowest values (136kN ± 4.37kN; Fig. 3). Forces were higher and similar for the ‘Cut’ (170kN ± 8.89kN), ‘Control’ (171.8 ± 16.5kN) and ‘Slanted’ (173kN ± 21.9kN) groups, the ‘Scratched’ group showed slightly higher values (193kN ± 11.9kN). The Ti adapter sleeves were plastically deformed but did not fail catastrophically. The present study suggests that manufacturer's recommendations for removal of a well fixed femoral stem could be narrowed down to the ‘Truncated’ condition. Even this might not be necessary since the fracture load is still substantially higher than the ASTM standard requires. Surgeons should consider to keep stems with larger taper damages as previously thought and spare the patient from stem revision. The greatest reservation regarding adapter sleeves is the introduction of the new metal-on-metal interface between stem and sleeve, which could possibly facilitate fretting-corrosion, which is presently one of the major concerns for modular junctions (3). Clinically such problems have not been reported yet. Ongoing FE-simulations are performed to investigate whether micromotions between stem and head taper are altered by the investigated damages.
Total hip replacement is one of the most successful orthopaedic surgeries, not least because of the introduction of modular systems giving surgeons the flexibility to intraoperatively adapt the geometry of the artificial joint to the patient's anatomy. However, taper junctions of modular implants are at risk of fretting-induced postoperative complications such as corrosion, which can lead to adverse tissue reactions. Interface micro-motions are suspected to be a causal factor for mechanical loading-induced corrosion, which can require implant revision. The aim of this study was to determine the micro-motions at the stem-head taper interface during daily activities and the influence of specific material combinations. The ball heads (ø 32mm, 12/14, size L, CoCr or Al2O3) were quasi-statically assembled to the stems (Ti or CoCr, Metha, Aesculap AG, Germany, v=0.5 kN/s, F=6 kN, n=3 each, 10° adduction/ 9° flexion according to ISO 7206-4) and then loaded sinusoidally using a material testing machine (Mini Bionix II, MTS, USA, Figure 1). The peak forces represented different daily activities [Bergmann, 2010]: walking (2.3 kN), stair climbing (4.3 kN) and stumbling (5.3 kN). 2,000 loading cycles (f=1 Hz) were applied for each load level. Six eddy-current sensors, placed between stem and head, were used to determine the displacement (interface micro-motion and elastic deformation) between head and stem (Figure 1). A finite element model (FEM) based on CAD data was used to determine the elastic deformation of the prostheses for the experimentally tested activities (Abaqus, Simulia, USA). Tie-junctions at all interfaces prevented relative movements of the adjacent surfaces. The resultant translations at the centre of the ball head were determined using a coordinate transformation and a subsequent subtraction of the elastic deformation.Introduction
Materials & Methods
