Introduction

Wear plays a key role in the clinical outcome of total hip replacements (THR). In addition, increased frictional moment can stress the implant interfaces which may lead to high torsional loadings in the intermodular taper junction (fretting) and cup loosening and to the development of noise (squeaking). Against the background of larger head diameters (increased range of motion and decreased risk of dislocation), the friction induced by the joint articulation is of particular interest. As of now, the investigation of friction with the use of relevant joint kinematics and loadings are limited to numerical studies. Experimental approaches use simplified models which do not take into consideration complex activities. Thus, with the aim of this study is the identification of articular frictional moments that consider critical in vivo loading conditions and kinematics as well as the clinical cup inclination, head size and clearance of ceramic-on-ceramic hip bearings.

The numbers of anatomic total shoulder joint replacements (ATSR) is increasing during the past years with encouraging clinical results. However, the survivorship of ATSR is lower as compared to total knee and hip replacements. Although the reasons for revision surgery are multifactorial, wear-associated problems like loosening are well-known causes for long-term failure of ATSR. Furthermore there is lack of valid experimental wear tests for ATSR. Therefore the purpose of this study was to define experimental wear testing parameters for ATSR and to perform a wear study comparing ceramic and metallic humeral heads.

Kinetic and kinematic data were adopted from in-vivo loading measurements of the shoulder joint (orthoload.com) and from several clinical studies on shoulder joint kinematics. As activity an ab/adduction motion of 0 to 90° in combination with an ante/retroversion while lifting a load of 2 kg has been chosen. Also a superior-inferior translation of the humeral head has been considered. The wear assessment was performed using a force controlled AMTI joint simulator for 3×10⁶ cycles (Fig. 1) and polyethylene wear has been assed gravimetrically.

The studied ATSR (Turon^TM, DJO Surgical, USA) resulted in a polyethylene wear rate of 62.75 ± 1.60 mg/10⁶ cycles in combination with metallic heads. The ceramic heads significantly reduced the wear rate by 26.7 % to 45.99 ± 1.31 mg/10⁶ (p<0.01). The wear scars dimensions were in good agreement to clinical retrievals.

This study is the first that experimentally studied the wear behavior of ATSR based on clinical and biomechanical data under load controlled conditions. In term of wear the analyzed ATSR could clearly benefit from ceramic humeral heads. However, in comparison to experimental wear studies of total knee and hip replacements the wear rate of the studied ATSR was relatively high. Therefore further research may focus on optimized wear conditions of ATSR and the hereby described method may serve as a tool to evaluate a wear optimization process.