Wear and corrosion between head and stem tapers of modular hip implants have recently been related to clinical failures, possibly due to high friction moments in poorly lubricated joints [1–2]. In-vivo measurements have revealed reversing joint friction moments in the hip during a gait cycle [3], which may foster relative motion between the modular components. Blood, soft tissue or bone debris at the taper interface during assembly can lead to decreased stability or increased stress concentrations due to non-uniform loading [4]. The purpose of this study is to investigate the influence of taper contamination and the assembly force on the seating characteristic of the head on the stem incorporating realistic reversing joint friction moments. Cobalt chrome heads (M-SPEC, 36mm, +1.5mm; n=5) were assembled on titanium femoral stems (Corail 12/14, both components Depuy Synthes; n=5) by quasistatic axial push-on forces (F=0.5kN, 1kN, 2kN). Heads were modified by milling a flat plane, to which the joint load was applied alternately to point A and point B for 20 cycles to provide reversing moments (heel-strike FA=1971N, MA=5.4Nm; toe-off FB=807N, MB=4.6Nm; Fig. 1). All 6 degrees of freedom of relative displacement between head and stem were determined in the unloaded state and after each loading cycle. A coordinate measurement machine (accuracy ±2µm) was used to determine the components positions. Pull-off forces were measured after the last loading cycle. Each taper was tested in pristine condition and then contaminated with a bone chip (1.7±0.2mg).Introduction
Methods
Micromotions between stem and neck adapter depend on prosthesis design and material coupling. Based on the results of this study, the amount of micromotion seems to reflect the risk of fretting-induced fatigue in vivo. Bimodular hip prostheses were developed to allow surgeons an individual reconstruction of the hip joint by varying length, offset and anteversion in the operation theatre. Despite these advantages, the use of these systems led to a high rate of postoperative complications resulting in revision rates of up to 11% ten years after surgical intervention. During daily activities taper connections of modular hip implants are highly stressed regions and contain the potential of micromotions between adjacent components, fretting and corrosion. This might explain why an elevated number of fretting-induced neck fractures occurred in clinics. However, some bi-modular prostheses (e.g. Metha, Aesculap, Ti-Ti) are more often affected by those complications than others (e.g. H-Max M, Limacorporate, Ti-Ti or Metha, Ti-CoCr) implying that the design and the material coupling have an impact on this failure pattern. Therefore, the purpose of this study was to clarify whether clinical successful prostheses offer lower micromotions than those with an elevated number of in vivo fractures.Summary
Introduction
