Abstract
Summary Statement
A new 28mm-diameter ceramic-on-ceramic (COC) acetabular bearing couple (Biomet Orthopedics) showed extremely low wear, even under adverse microseparation conditions∗. The wear results are similar or more favorable than those reported for clinical retrievals and wear testing of similar ceramic bearings.
Introduction
A new acetabular shell and ceramic insert design (Biomet) incorporates features to help prevent malalignment during implantation, while still providing secure fixation within the acetabular shell. The incorporation of Biolox® Delta (zirconia toughened alumina, CeramTec) material should provide improved wear resistance over pure alumina ceramics. The goal of this study is to evaluate the wear durability of this system for standard and microseparation testing.
Materials & Methods
The 28 mm diameter ceramic heads and inserts (CeramTec) were seated on taper spigots and within acetabular shells (Biomet), respectively. Six sets of parts were tested for 5M cycles of standard hip wear testing (ISO 14242) and an additional six sets of parts for 2M cycles of microseparation testing. The microseparation testing protocol included a steep cup angle (60° in-vivo), side load, and reduced axial load to induce head-liner separation. The lateral displacement was increased from 0.5mm, to 1mm, and then to 2mm in order to replicate wear features observed in extreme situations of clinical retrievals[1]. The parts were weighed (gravimetric wear rates) and photographed throughout the test. SEM, transformation, and wear debris analyses were completed.
Results
The steady-state wear rate throughout standard testing was 0.0094 +/− 0.0029 mm3/106 cycles (+/-95% CI). The initial 0.5mm microseparation distance (0–1M cycles) showed no signs of wear. Most heads showed wear stripes after increasing to 1.0mm (1–1.5M cycles), and then all test parts showed stripes after increasing to 2mm. The increased visibility in wear stripes correlated with an increased level of measured wear. For the 2mm separation-distance testing interval, the wear rate was 0.178 +/− 0.052mm3/106 cycles.
Discussion/Conclusion
The lack of wear stripes during 0.5mm of microseparation is an indication of the strength of the implants. A distance of 1–2mm is an extreme level of microseparation and the 60° in-vivo cup inclination created an even worse-case situation for wear; however, the implants showed excellent mechanical strength and low wear rates. SEM and transformation analyses showed minimal wear and evidence of stress-induced ceramic toughening. Microseparation testing at another lab [2] has shown a similarly low wear rate (0.5 mm3/106 cycles) for Biolox® Delta ceramic, with Biolox® Forte (alumina ceramic, without zirconia) showing a considerably higher wear rate (6.3mm3/106 cycles). The standard testing wear rate (0.0094+/-0.0029 mm3/106 cycles) was much lower than the average wear rate (0.69+/-0.63 mm3/106 cycles) of several COC implant retrievals by Walter [1]. The 28mm steady-state wear rate of this test is better than or equal to the wear rate (0.0101 mm3/106 cycles) observed in other 28mm COC systems.∗∗
∗Ceramic-on-Ceramic articulation is not cleared for use in the United States.
∗∗Laboratory results are not necessarily indicative of clinical performance.
