Abstract
Total knee arthroplasty is a well established treatment for degenerative joint disease with good clinical results. However, complications may occur due to a biological response to polyethylene wear particles, leading to osteolysis and aseptic loosening, as well as local and systemic hypersensitivity reactions triggered by metal ions and particles such as chromium, cobalt and molybdenum. Moreover, there is an increasing demand on the performance of these implants, as this treatment is also performed in heavier, younger and middle-aged adults who have a significant physical activity and higher life expectancy. The purpose of the following study was to compare the wear characteristics and performance of a zirconium nitride (ZrN) coated knee implant, designed for patients with metal ion hypersensitivity, against the clinically established cobalt-chromium (CoCr) version under a high demanding activities wear simulation.
Medium size AS Columbus® DD (Aesculap AG, Tuttlingen, Germany) femoral and tibial components with a ZrN surface were tested in comparison with the cobalt-chromium version Columbus® DD. For both groups, ultra-high-molecular weight polyethylene (UHMWPE) gliding surfaces (size T3, high 10 mm) were used. Wear simulation was performed on a load controlled 4 station knee wear simulator (EndoLab GmbH, Thansau, Germany) capable of reproducing loads and movement of daily activities measured in vivo (Bergmann et al, 2014) on 8 patients and normalized to a patient weight of 100 kg (Schwiesau et al, 2014). The load profiles were applied for 5 million cycles in a combination of 40% stairs up, 40% stairs down, 10% level walking, 8% chair raising and 2% deep squatting. Test serum was changed every 0.5 million cycles and all the components were cleaned and analyzed according to ISO 14243-2:2009(E). The gliding surfaces were evaluated for gravimetric wear and wear patterns, femur components analyzed for scratches and the test medium analyzed for metal ion concentration (cobalt, chromium, molybdenum and zirconium) using ICP-MS according to ISO 17294-2.
The present study showed a wear rate reduction for the ZrN group (1.01 ± 0.29 mg/million) in comparison with the CoCr group (2.40 ± 1.18 mg/million cycles). The articulation surface of the ZrN coated femurs remained polished after the testing period, whereas the uncoated femurs showed wear scratches. Furthermore, the metal ion release from the ZrN coated implants was reduced orders of magnitude in comparison with the CoCr implants through the entire test. These results demonstrate the efficiency of ZrN coated knee implants to reduce wear as well as to prevent metal ion release in the knee joint.
