Abstract
Limitations of the range of motion (ROM) of total hip prostheses lead to impingement causing dislocation and material failure. Due to wear, the femoral head penetrates polyethylene (PE)-sockets by about 0,1 mm/year (ceramic on PE) and 0,2–0,6 mm/year (metal on PE). Wear rate increases with steep acetabular cup position. In contrast to polyethylene, wear of alumina-ceramic cups appears to be independent from inclination angle and is only about 0,001 mm/year. Wear and design features may restrict the artificial joint mobility. The purpose of this study is to determine the effects of head penetration on ROM in relation to different cup positions.
Computer simulation was carried out with a three-dimensional CAD-program. 3-D models of modular cup, spherical head, and stem with cylindrical neck and 12/14 taper were generated. The femoral head was shifted 0, 1, 2, and 5 mm towards the pole of the cup. According to mean direction of penetration measured in retrieved PE-sockets, femoral head was also moved 0, 1, 2, and 5 mm in vertical direction. The joint motions were measured at different cup positions.
The study demonstrates that ROM is clearly reduced by increasing head penetration. After 2 mm penetration, e.g. maximum flexion is reduced by approx. 15° at 45° cup inclination. Restriction of flexion is more pronounced in the vertical penetration path. If the socket is placed in more horizontal position, less ROM of flexion, extension and abduction is observed. With steeper cup positions ROM of flexion increases but, as well as risk of dislocation, wear and penetration rate of PE sockets increase.
Modern hip prostheses should provide sufficient joint movements, precise implant positioning and low wear bearing couples avoiding penetration of femoral head. Additionally, design aspects like liner geometry, head-neck ratio have to be considered preventing impingement, dislocation or early failure by aseptic loosening.
