Pushing Ceramic-on-Ceramic in the most extreme wear conditions: A hip simulator study

Hip revision surgery for fractured ceramic components may represent the worst-case wear scenario due to the high risks of recurrent dislocations, instability, and third body wear. The ideal bearing choice for the new prosthetic articulation is still subject of debate, while alumina matrix composite (AMC) articulations offer theoretical superior performances; the present work was designed to test the wear behaviour of ceramic on ceramic articulations (liner and head) in a worst-case scenario by adding ceramic third-body particles to the test lubricant with combined walking and subluxation cycles in a hip joint simulator. Therefore, we performed an in vitro study aiming to assess how does AMC articulation perform with 1) third-body particles added to the test environment and 2) under subluxation stresses.

We hypothesised that AMC articulations offer superior performances in such worst conditions.

A hip simulator test was designed to analyse how AMC articulation performs with third-body particles added to the test environment and under subluxation stresses. Two different load patterns including level walking and subluxation of the ceramic liner were applied. The test fluid lubricant was contaminated by adding coarse ceramic particles during the first 2 million cycles and fine ceramic particles from 2 to 4 million cycles. Group 1 consisted of an alumina matrix composite articulation (liner and head); group 2 consisted of an alumina liner and an alumina matrix composite head. A control group consisting of an alumina ceramic liner articulated against an alumina matrix composite head was provided and only axially loaded. The liners of groups 1 and 2 were tested at an in vivo angle of 45° in the medial lateral plane (inclinationangle), which corresponds to an angle L=30° relative to the ISO standard fixated position used for in vitro testing. All mass measurements were performed using a high precision balance (Sartorius BP211D). During each examination, images on dedicated location of the bearing surfaces were taken using a digital microscope.

Mean cumulative wear of 0.09mg per million cycles between 2 and 4 million cycles was detected in group 2, and this value was significantly lower (p=0.016) in comparison with the average value in group 1 (0.21mg per million cycle). This result can be explained in light of a possible transformation phase of zirconia in AMC liners, probably due to excessive stress during subluxation cycles. However, wear levels observed are close to the gravimetric measurement detection limit of the Sartorious Balance (about 0.1-0.2mg); therefore, wear can be considered negligible in all groups.

Our results confirm that AMC couplings perform very well even in the worst-case wear scenario. Since AMC articulations revealed 25% lower cumulative wear respect to AMC on cross linked polyethylene in same simulator setup, AMC articulations should be considered the bearing of choice in revision surgery in light of the high risk of recurrent dislocations, instability, and third body wear.

III, prospective case-control study, in vitro.