Friction in total hip joint prosthesis measured in vivo during walking.

Friction-induced moments and subsequent cup loosening can be the reason for total hip joint replacement failure. The aim of this study was to measure the in vivo contact forces and friction moments during walking. Instrumented hip implants with Al2O3 ceramic head and an XPE inlay were used. In vivo measurements were taken 3 months post operatively in 8 subjects. The coefficient of friction was calculated in 3D throughout the whole gait cycle, and average values of the friction-induced power dissipation in the joint were determined. On average, peak contact forces of 248% of the bodyweight and peak friction moments of 0.26% bodyweight times meter were determined. However, contact forces and friction moments varied greatly between individuals. The friction moment increased during the extension phase of the joint. The average coefficient of friction also increased during this period, from 0.04 (0.03 to 0.06) at contralateral toe off to 0.06 (0.04 to 0.08) at contralateral heel strike. During the flexion phase, the coefficient of friction increased further to 0.14 (0.09 to 0.23) at toe off. The average friction-induced power throughout the whole gait cycle was 2.3 W (1.4 W to 3.8 W). Although more parameters than only the synovia determine the friction, the wide ranges of friction coefficients and power dissipation indicate that the lubricating properties of synovia are individually very different. However, such differences may also exist in natural joints and may influence the progression of arthrosis. Furthermore, subjects with very high power dissipation may be at risk of thermally induced implant loosening. The large increase of the friction coefficient during each step could be caused by the synovia being squeezed out under load.

In vivo hip joint loads during three methods of walking with forearm crutches.

BACKGROUND: Patients with osteoarthritis, joint implants or fractures use crutches in order to reduce lower limb loading. However, insufficient information exists on how much the loading is then in fact reduced. This situation was studied by using seven patients who had instrumented hip implants. METHODS: Part I: To investigate the effectiveness of forearm crutches, crutch and hip joint contact forces were measured in seven patients with instrumented hip prostheses. Additionally, the bending moments in the implant neck and torsion around its stem were determined. Reductions of peak loads during 3, 4, and 2-point gaits were compared with loads present when walking without crutches. Part II: This examines joint load reduction during a 4-point gait from one to 12 weeks post-operatively. FINDINGS: Part I: During a 3, 4, and 2-point gait, the joint force was 17, 12, and 13% lower than it was while walking without crutches. The corresponding reductions of the bending moment were 16, 11, and 12%, while the maximum torque decreased by 19, 21, and 10%. Part II: The reductions of contact forces in comparison with walking without crutches were highest during the first 4 weeks after surgery. One and 4 weeks post-operatively, the force maximum was 21 and 8% lower than it was after 3 months. When compared with the initial values of the 1st week, crutch forces decreased by 28% in the 4th week and by 38% in the 3rd month. INTERPRETATION: Average reductions of the joint load by more than 20% are achieved only during the first 4 post-operative weeks. Because fractures are in most cases relatively stable after 6 weeks, and bone ingrowth into implant interfaces is nearly finished after this time, a single crutch and a 2-point gait can be prescribed during the 5th and 6th post-operative week.