S326 Short Talk ST-8 Implant and Joint

THE RISK OF ACETABULAR CUP TURN-OUT DUE TO FRICTION MOMENTS

Nicholas Bishop, Michael Morlock

Biomechanics Section, TUHH Hamburg University of Technology, Germany

Introduction The rate of failure of modern press fit acetabular cup implantations due to turn-out has been found to be as much as 28% at 10 years [Jiranek, 2004]. This can only occur when the joint moment exceeds the shear capacity of the implant-bone interface. The joint moment arises from frictional shear forces acting between the ball and cup. Shear capacity of the implant-bone interface is achieved by press fit. With a large interference the cup does not seat completely and shear transfer occurs at the cup equator. A lower interference allows polar contact, and shear transfer can then occur in the region where the joint load presses the cup against the bone. The relative capacities of these two modes are unknown. However, if an equatorial press fit relaxes or no press-fit is achieved, load transfer will default to the friction capacity of the cup-bone interface due to the joint force alone. This must resist the joint moment acting between the head and the cup, which is also dependent on the joint force. In this study measured joint moments were compared with the theoretical cup capacity due to friction resistance from the joint force only, with no press-fit interference. Methods In a previous study [Bishop, 2007] friction moments were measured under pysiological joint load and displacement during gait. 9 modern hip joints were tested, with different materials (metal / ceramic / polyethylene) and diameters (28-55mm). Parameters varied were: Frequency (0.5-2Hz), swing load (100-500N) and rest period (10-30s). The measured time-varying moments M(t) were compared with the friction resistance moment estimated from the shear friction force due to the time-varying joint load F(t) acting over the radius r of the implant bone interface. The risk factor of failure was calculated as:

(t).r.F(t)M

MM

Risk(t)joint

joint

capacity

joint

μ is the friction coefficient at the implant bone interface, set to 0.4 [Grant, 2006]. A risk value greater than 1 implies failure. The maximum Risk was recorded for each tested cycle and also the Risk

at maximum moment. Their temporal relation to the load curve (gait cycle) was also recorded. Results The maximum risk of cup turn-out occurs during the swing phase of gait, when the joint load is minimum (Figure 1, dark grey). In contrast, the Risk at maximum joint moment (during stance phase) was lower (light grey).

2000N

0N

Joint Force

Mjoint

0% 100%21% 65%

0

0

Risk

Proportion of gait from healstrike

(example curve)

(example curve)

2000N

0N

Joint Force

Mjoint

0% 100%21% 65%

0

0

Risk

Proportion of gait from healstrike

(example curve)

(example curve)

Figure 1: The Risk at maximum Mjoint occurs at mean 21% (±17% SD) of gait (light grey); Maximum Riskoccurs at mean 65% (±9% SD) of gait (dark grey). Risk increased (to greater than 1) with decreasing swing phase load and gait frequency, but was not influenced by rest period.

Discussion The risk of cup turn was found to be greatest during the swing phase of gait, when the joint force is lowest. This is somewhat counterintuitive and is a consequence of the assumption that cup stability decreases with decreasing joint load. This assumption must be tested experimentally. Currently, cup stability, in terms of turn-out moment is usually tested under a high simulated joint load. Turn-out moments at low joint loads may be a better indicator of cup failure risk. References Jiranek et al, CORR, (418):172-8, 2004. Bishop et al, MedEngPhys, 2007(in review). Grant et al, J Biomech, 40(5):1158-64, 2004.

Journal of Biomechanics 41(S1) 16th ESB Congress, Short Talks, Tuesday 8 July 2008