Abstract
Introduction
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.
Methods
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).
Results
Contaminated tapers exhibited significantly larger seating than clean samples for all assembly forces (p<0.001, Fig. 2). Higher assembly forces led to decreased translation and rotation (p<0.001). Pull-off forces remained constant (F=890±99N) and independent of assembly force (p=0.303), or contamination condition (p=0.192). No further seating of the head on the stem taper was seen for the 20th load cycle.
Discussion
This study shows that contamination, even in very small quantities, can increase initial translation and rotation, which could initiate corrosion. This can be countered, to some extent by applying sufficiently high assembly forces. It is noted that no further seating was measurable by the 20th loading cycle for any assembly condition but this does not rule out increased cyclic motion, particularly in contaminated interfaces [5]. The final pull-off forces were independent of the magnitude of the assembly force but might rather be related to the maximum joint load, suggesting that the assembly force should be at least as great as the maximum joint load.
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