Purpose

Total shoulder arthroplasty (TSA) has become a successful treatment option for degenerative shoulder disease. With the increasing incidence in primary TSA procedures during the last decades, strategies to improve implant longevity become more relevant. Implant failure is mainly associated with mechanical or biological causes. Chronic inflammation as a response to wear particle exposure is regarded as a main biological mechanism leading to implant failure. Metal ions released by fretting and corrosion at modular taper connections of orthopedic implants can cause cell-mediated hypersensitivity reactions and might lead to aseptic loosening. Modularity is also commonly used in total shoulder replacement. However, little is known about metal ion exposure in patients following TSA. The objective of this study was to determine in-vivo blood metal ion levels in patients after TSA and to compare blood metal ion levels to control subjects without metal implants.

Introduction

The frequency of revision hip arthroplasty is increasing with the increasing life expectancy and number of individuals treated with joint replacement. Newer porous implants have been introduced which may provide better treatment options for revision arthroplasty. These may require cementation to other prosthesis components and occasionally to bone, however, there is currently no information on how these porous implants interface with cement.

As there are many reports describing avascular reactions to metal debris (ARMD) after Metal-on-Metal Hip Arthroplasty (MoMHA), the use of MoMHA, especially hip resurfacing, is decreasing worldwide. In cases of ARMD or a rise of metal ion blood levels, revision is commended even in pain free patients with a well integrated implant. The revision of a well integrated implant will cause bone loss. As most of the patients with a hip resurfacing are young and a good bone stock is desirable for further revision surgeries, the purpose of this study was to evaluate the stability of a cemented polyethylene cup in a metal hip resurfacing cup. Two different hip resurfacing systems were investigated in this study (ASR™, DePuy Orthopaedics, Leatherhead, UK; Cormet™, Corin Group, Cirencester, UK).

Six different groups were formed according to the treatment and preparation of the cement-cup-interface (table 1). Before instilling cement in groups 1, 3, 5 the surface, which was contaminated with blood, was cleaned just using a gauze bandage. In groups 2, 4, 6 saline, polyhexanid and a gauze were used to clean the surface prior to the cement application. In group one and two the polyethylene cup (PE) was cemented either into Cormet™ or ASR™, just the ASR™ was further investigated in group three to six. A monoaxial load was applied while the cup was fixed with 45 degrees inclination (group 1–4) and 90 degrees inclination (group 5, 6: rotatory stability) and the failure torque was measured. In contrast to group 1 and 2, the cement penetrated the peripheral groove of the ASR™ in groups 3–6. The mean failure torque of five tests for each group was compared between the groups and the implants.

The ASR™ showed mean failure torque of 0.1 Nm in group one, of 0.14 Nm in group two, of 56.9 Nm in group three, of 61.5 Nm in group four, of 2.96 Nm in group five and of 3.04 Nm in group six. The mean failure torque of the Cormet™ was 0.14 Nm both in groups one and two (table 2). In groups 1–6 there were no significant differences between the different preparations of the interface. Furthermore, in groups 1 and 2 there were no significant differences between the Cormet™ and the ASR™. The mean failure torque of group 4 was significant increased compared to group 3 (p=0.008).

We saw an early failure of the cement fixation due to the smooth surface of the Cormet™ and the ASR™ components in groups 1, 2, 5, 6. In contrast to other hip resurfacing cups the ASR™ has a peripheral groove, which was not cemented except in groups 3 and 4 and therefore the lever-out failure torque was significant increased in these groups. Nevertheless, the groove did not provide stability of the cement-PE compound in case of rotatory movements. In conclusion we do not recommend the use of these methods in clinical routine. The complete removal of hip resurfacing components seems to be the most reasonable procedure.

Background

Migration analysis after total joint arthroplasty are performed using EBRA analysis (Krismer et al., 1997) or - more accurate but also much more cost-intensive and time-consuming – via radiostereometric analysis (RSA). For the latter, additional radiographs from two inclined perspectives are needed in regular intervals in order to define the position of the implant relative to tantalum bone markers which have been implanted during surgery of the artificial joint (Fig. 1). Modern analysis software promises a migration precision along the stem axis of a hip implant of less than 100 μm (Witvoet-Brahm et al., 2007). However, as the analysis is performed semi-automatically, the results are still dependent on the subjective evaluation of the X-rays by the observer. Thus, the present phantom study aims at evaluating the inter- and intra-observer reliability, the repeatability as well as the precision and gives insight into the potential and limits of the RSA method.

In cases of poor bone quality intraoperative torque measurement might be an alternative to preoperative dual energy x-ray absorptiometry (DXA) to assess bone quality in Total Hip Arthroplasty (THA). 14 paired fresh frozen human femurs were included for trabecular peak torque measurement. We evaluated an existing intraoperative torque measurement method to assess bone quality and bone strength. We modified the approach to use this method in total hip arthroplasty (THA), which has not been published before. Since there are several approaches used in THA to exposure the hip joint, we decided to prefer the measurement in the femoral head which allows every surgeon to perform this measurement. Here a 6.5 × 23 mm blade was inserted into the proximal femur without harming the lateral cortical bone (figure 1). Further tests of the proximal femur evaluated the results of this new method: DXA, micro-computed tomography (μCT) and biomechanical load tests. Basic statistical analyses and multiple regressions were done. In the femoral head mean trabecular peak torque was 4.38 ± 1.86 Nm. These values showed a strong correlation with the values of the DXA, the μCT and the biomechanical load test. In comparison to the bone mineral density captured by DXA, the results of the intraoperative torque measurement showed a superior correlation with high sensitive bone quality evaluating methods (mechanical load tests and micro-computed tomography). Hence, the use of this intraoperative torque measurement seems to be more accurate in evaluating bone strength and bone quality than DXA during THA. The torque measurement provides sensitive information about the bone strength, which may affect the choice of implant in cases of poor bone stock and osteoporosis. In clinical use the surgeon may alter the prosthesis if the device indicates poor bone quality. Furthermore, we assume that the disadvantages associated with DXA scans like radiation exposure or errors caused by potential extraosteal sclerosis and interindividual soft-tissue artifacts could be excluded.