Introduction

Beneath infection, instability and malalignment, aseptic tibial component loosening remains a major cause of failure in total knee arthroplasty (TKA) [1]. This emphasizes the need for stable primary and long-term secondary fixation of tibial baseplates. To evaluate the primary stability of cemented tibial baseplates, different pre-clinical test methods have been undergone: finite element analysis [2], static push-out [3,4] or dynamic compression-shear loading [5] until interface failure. However, these test conditions do not reflect the long-term endurance under in vivo loading modes, where the tibial baseplate is predominantly subjected to compression and shear forces in a cyclic profile [5,6].

To distinguish between design parameters the aim of our study was to develop suitable pre-clinical test methods to evaluate the endurance of the implant-cement-bone interface fixation for tibial baseplates under severe anterior (method I) and internal-external torsional (method II) shear test conditions.

Modular hip prostheses were introduced to optimize the intra-surgical adaptation of the implant design to the native anatomy und biomechanics of the hip. The downside of a modular implant design with an additional modular interface is the potential susceptibility to fretting, crevice corrosion and wear. For testing hip implants with proximal femoral modularity according to ISO & ASTM, sodium chloride solutions are frequently used to determine the fatigue strength and durability of the stem-neck connection. The present study illustrate that the expansion of standard requirements of biomechanical testing is necessary to simulate metal ion release as well as fretting and crevice corrosion by using alternative test fluids. To assess the primary stability of tibial plateaus in vitro, different approaches had been undergone: cement penetration depth analysis, static tension or compression loading until interface failure. However, these test conditions do not reflect the in vivo physiologic loading modes, where the tibial plateau is predominantly subjected to combined compression and shear forces. The objectives were to evaluate the impact of the tibial keel & stem length on the primary stability of a posterior-stabilised tibial plateau under dynamic compression-shear loading conditions in human tibiae.

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.

INTRODUCTION

Highly cross linked polyethylenes (HXPE) have to be treated thermally after irradiation to eliminate residual free radicals. By adding vitamin E in the polyethylene powder a post-irradiation thermal treatment is not necessary.

In this review the correlation between the intrinsic properties and the long-term stability of Vitelene® as a high performance material for artificial hip articulation will be displayed.