Abstract
Introduction
Third body wear caused by contaminated bearing environment with debris that may have been generated by a worn or fractured revised bearing coupling, but also originated by generation of metal/cement particles during the primary or revision surgery, may be a relevant issue for the implant life.
Objectives
To evaluate the wear behavior of a last generation alumina matrix ceramic composite (AMC) bearing in a worst case scenario consisting of highly contaminated test lubricant with alumina particles in a hip joint simulator study.
Methods
AMC (BIOLOX®delta - Group 1) and alumina (BIOLOX®forte - Group 2) liners with an internal diameter of 32mm were articulated against AMC BIOLOX®OPTION heads (all CeramTec GmbH, Plochingen). Coarse alumina particles (D(50):60µm) and fine alumina particles (D(50):0.30µm) with a concentration of 48mg/ml were used as environmental contamination of the first 2mlc and the second 2mlc, respectively. All components were tested according to ISO14242-1(2012) using the EndoLab®hip joint simulator. Each group consisted of 3 couples plus one axially loaded control consisting of a 32mm AMC head on an alumina liner. The test fluid was exchanged every 500'000 cycles. Two different test regimes describing level walking and subluxation of the insert from the femoral head were used as test input. The first regime included 500 standard walking cycles followed by a second block of 5 subluxation cycles. Head subluxation is assumed to increase the number of third-body particles that enter the otherwise closely conforming articular bearing space, when compared to level-walking cycles alone. To maintain the particle suspension in the test fluid, the test fluid of each test chamber was circulated by peristaltic pumps. The wear rate was determined by gravimetric method. The surface of ball heads was subjected to visual inspection every 500'000 cycles by optical and laser microscope. The depth of scratches was measured by the laser microscope.
Results
After 2 million load cycles with the test fluid contaminated with coarse alumina particles, no significant wear was detected by the gravimetric method. The qualitative surface inspection by laser microscope individuated an increasing, but low concentration of sparse tiny scratches about 40nm deep on ball head surface at every simulator stop. Nevertheless, after loading the chambers with fine alumina particles, bearing surface wear seemed to increase consistently, since opaque areas appeared after 3 million load cycles, but the wear-rate remained close to the gravimetric measurement detection limit (about 0.1–0.2mg) indicating the still extremely low wear-rate of the tested ceramic couplings.
Conclusions
This study confirms the high wear and scratch resistance of AMC used in THA. Even in heavily contaminated environment with hard alumina particles, which may occur after fracture of a ceramic component, gravimetry wasn't able to detect significant wear. The visual inspection by laser microscope exhibited only slight damaged surface characterized by tiny scratches and more opaque areas in the main wear zone as result of using fine alumina particles. Ceramic-on-ceramic is a safe bearing even in the case of revision for ceramic fracture, maintaining the excellent wear resistance of this bearing.
