Introduction

Clinically relevant attributes of an orthopedic bearing material include its strength, oxidative stability, and wear resistance. Recent reductions in bearing wear and oxidation have been realized by crosslinking (HXLPE), and through the incorporation of α-tocopherol (VE). VE infusion has improved the oxidative stability of HXLPE in vitro [1] and in vivo (as evidenced by retrievals) [2]. However, concern has been raised that adding VE may increase the frictional torque of bearings, potentially increasing the risk of trunnionosis [3]. This study compares the 3D frictional torques of HXLPE vs Vitamin-E HXPLE, against 28 mm and 36 mm metal and ceramic heads.

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.

INTRODUCTION

The reported revision rate for THA is below 10% at 10 years. Major factors for revision are aseptic loosening or dislocation of the articulating components. CoC bearings in total hip arthroplasty (THA) have demonstrated very low wear rates. Due to producing the least number of wear particles of any articular bearing used for THA, osteolysis is very rarely observed. Zirconia-platelet toughened alumina (ZPTA) has improved toughness and bending strength while maintaining all other advantageous properties of alumina. Consequently, its clinical fracture rate is minimal and wear resistance is superior to alumina.

Introduction:

Failure of the polyethylene glenoid component is the most common complication of Total Shoulder Arthroplasty (TSA) and accounts for a majority of the unsatisfactory results after this procedure. Nowadays, most of the shoulder prostheses consist of metal on polyethylene bearing components. Repetitive contact between the metal ball and the polyethylene socket produces progressive abrasion of the implant if the moving part is made of polyethylene. Its debris may then lead to an active osteolysis and implant loosening. Failure of the glenoid component is often manifested clinically by pain, loss of function, and the presence of a clunking noise and leads to revision surgery.

The use of ceramic balls aims at the reduction of this phenomenon. In many studies regarding knee and hip replacement it has been shown that the use of ceramic on polyethylene is more beneficial in terms of polyethylene wear and failure, when compared to metal on polyethylene. This is to our knowledge the first study to address in direct comparison wear in both TSA and RTSA.

As allergic reactions to implant wear are gaining more attention [4], the incorporation of ceramic materials to device design appears to be a promising development. In particular, ceramic femoral components of total knee replacements have been designed to produce less wear under standard [1] and adverse [5] implant conditions.

Whereas the wear reduction effect of ceramics is generally accepted for hip implants, the corresponding effect for knee implants is not proven. Ezzet et al. reported a wear reduction of 42% for standard wear conditions [2] and of 55% for adverse wear conditions [3] when compared to a geometrically identical CoCr femoral component. In contrast to these findings, an analysis of the EndoLab® database has indicated wear rates of ceramic knee implants that are comparable to traditional low wear material couplings (Figure 1), and are within the range of clinically established devices.

The purpose of this study was to directly compare two TKR designs, one fixed bearing and one mobile bearing, each made of traditional CoCr to one made of alumina matrix composite (BIOLOX®delta, CeramTec, Germany) ceramic material. The BPK-S Rotating Platform System (Peter-Brehm, Germany; Figure 2) and the MULTIGEN PLUS fixed bearing (Lima, Italy) were knee simulator wear tested according to ISO 14243-1 (2002). A total of three specimens plus one loaded soak control for each group (four groups in total) was subjected to 5 million standard gait cycles. The anterior-posterior (AP) and internal-external rotational (IE) motion of the implants resulting from the external load application of this force controlled test was recorded continuously. Wear was determined gravimetrically. The surface appearance of contact areas was analyzed by light microscopy and particle analysis was performed according to ISO 18129. For the mobile bearing groups, a mean wear rate of 2.47 mg per million cycles (StdDev. 0.38) was determined for the CoCr implant and of 1.10 mg per million cycles (StdDev. 0.46) for the BIOLOX®deltaimplant (Figure 3). The total AP and IE motion of the two groups did not differ. However, motion during stance phase was considerably higher for the ceramic group, indicating reduced frictional resistance (data not shown).

For the fixed bearing groups, a mean wear rate of 12.01 mg per million cycles (StdDev. 3.28) was determined for the CoCr implant and of 1.78 mg per million cycles (StdDev. 0.40) for the BIOLOX®delta implant.

Based upon the EndoLab® experience the ceramic total knee replacements tested perform as good as the best performing metallic total knee replacements. However it can be concluded that for the two implant systems tested the wear rate is reduced by more than 50% by using ceramic on polyethylene articulation when compared to an identical cobald crome design.

Introduction

From a tribological point of view and clinical experience, a ceramic-on-ceramic bearing represents the best treatment option after rare cases of ceramic component fracture in total hip arthroplasty (THA). Fractured ceramic components potentially leave small ceramic fragments in the joint capsule which might become embedded in PE acetabular liners.

Some mobile bearing knee replacement designs have shown truly excellent long-term clinical results. The higher laxity of a mobile bearing helps reduce the shear forces and torques transmitted to the prosthesis-bone interface, and this could only help reduce the risk of loosening. Some argue that self-alignment of a mobile bearing rotationally can produce more central patellar tracking. However, the most commonly assumed benefit of mobile bearings is the reduction in contact stress, which is typically expected to reduce fatigue and wear. In a rotating platform TKR for example, wear is also expected to be less because the rolling/sliding motion is separated from the transverse rotational motion onto two separate articulating surfaces, thus less cross-paths and less wear. Such expectations may have dominated the thinking and perhaps even clouded the expectations of TKR wear test engineers. Such wear reduction however has not really been categorically proven clinically.

This paper combines in-vitro wear results from two separate laboratories, one in Nebraska USA and one in Germany. These two (industrially unattached labs) possess between them a very large set of in-vitro wear testing results across the widest variety of fixed and mobile bearing TKR designs. Fortunately, the wear testing methodology using the force-control regime used in the two labs was largely similar, and was highly consistent within each lab. The fixed and the mobile bearings were subjected to the exact same force fields, allowing their Anterior-Posterior translation and internal-external rotation kinematics to vary based on the individual TKR design.

Tens of implant designs have been tested, both fixed and mobile, in total (bycondylar) form and unicompartmental, of various sizes. Some mobile bearings had rotating platforms and some were rotating-translating. Some of the tests specifically compared mobile to fixed bearing tibial components using identical femoral components. Between both labs, and across all tests, no statistically significant difference resulted in wear between fixed and mobile bearings. Yet, such differences did clearly feature with known superior bearing materials (for wear) and other favored design features. Also, generally, the force-control test methodology has proven highly discriminatory in its simulation and measurement of wear as a potential clinical failure mode.

The take home message to test engineers is to expect the wear of both mobile and fixed bearings to depend more on the detailed design and materials of the TKR than on the mobility of the bearing. The results of this study re-confirm the need for wear testing to be performed prior to any clinical use on all implant designs, despite seemingly similar predicates or success of some mobile bearings.