UHMWPE wear against roughened oxidized zirconium and CoCr femoral knee components during force-controlled simulation
Introduction
The primary factor in reducing the longevity of total joint replacements is recognized to be the wear of ultra-high molecular weight polyethylene (UHMWPE) and its sequelae, including osteolysis, implant loosening and implant instability [1], [2], [3], [4]. Most modern multi-component total knee replacement (TKR) designs rely on the articulation of a hard metallic femoral component against a softer UHMWPE tibial insert to facilitate joint function. This articulation produces UHMWPE wear which is dependant upon a host of factors, including but not limited to tibial/femoral material properties, implant geometric contact stresses, joint lubricant properties, joint motions and repetition of load bearing activity [5], [6], [7]. One wear related factor that has come under recurrent scrutiny has been the surface roughness of the femoral component as it articulates against the UHMWPE surface [8], [9], [10], [11]. Surface roughness is known to affect the production of wear, the surface frictional coefficients, and the modes of bearing lubrication that are present in total knee joint systems, with smoother bearing counterfaces promoting fluid-film lubrication, less restricted motion and lower wear rates [12], [13], [14], [15], [16], [17], [18].
Clinical retrieval studies and long-term wear testing simulations of TKR components almost universally observe the increased roughening of femoral component surfaces, with evidence of mild-to-severe scratching and pitting noted [5], [19], [20], [21], [22], [23]. Even with mild scratching, basic studies have shown that the inclusion of even a few clinically relevant scratches on a metallic surface can significantly increase the wear rate of the UHMWPE counterface by as much as 70-fold [9], [11]. Despite the efforts by manufacturers to create smooth metallic surface finishes, retrieval studies have shown that these implants can become quickly scratched or damaged [24], [25], [26], thus increasing the potential for UHMWPE wear. Even before implantation, scratches from surgical instrumentation have been cited as the first possibility of insult to the femoral surface [27]. Once implanted, third body wear debris as a result of inadequate lavage of surgical bone chips or inadequate containment of bone cement can seriously damage the surfaces of a femoral component design [21], [28], [29], [30], [31]. Femoral component materials that are better suited to resist surface roughening due to surgical tooling, third body abrasion and surface scratching therefore offer a fundamental method by which to reduce the potential for UHMWPE wear.
Recently, oxidized zirconium (OxZr) femoral components were introduced into the total hip and total knee replacement market. OxZr components offer the potential benefit of a smoother, harder zirconia ceramic bearing surface combined with the toughness of an underlying fracture resistant metallic zirconium alloy body [32]. The zirconia surface minimizes corrosion, and readily attracts synovial fluid to aid in joint lubrication [33], [34], while the metallic body alleviates many historical concerns over the potential fracture of bulk ceramic components. Clinically, OxZr components have been shown to produce less adhesive friction against UHMWPE than CoCr alloys [35], and offer a more rapid return of flexion and attainment of functional milestones in patients compared to CoCr components [36]. Experimentally, these OxZr components have been shown to produce as much as 85% less UHMWPE wear in TKR's than conventional CoCr femoral components [37] during un-roughened displacement-control simulator wear testing conditions. Regardless of femoral component bearing material selection however, implant surfaces are likely to be exposed to critical wear related factors such as surgical insult, third body wear debris and bone cement. It is therefore necessary to investigate how these OxZr femoral surfaces will perform under more clinically abrasive conditions.
Initial studies have reported that roughened OxZr TKR femoral components produce less UHMWPE wear than roughened CoCr TKR femoral components under identical ranges of imposed simulated motion [38]. Because surface roughness is known to affect surface friction however, it is likely that significant differences in TKR femoral component surface roughness will affect resulting in vivo kinematic TKR performance. Two of the most fundamental measures of TKR system performance are the motion characteristics of the bearing system during gait, and the resulting wear rate of the UHMWPE component over time. Therefore, for TKR wear testing studies that wish to study the effect of TKR design on resulting TKR motion, or the effect of TKR wear on changes to TKR motion, it is critical to let the TKR bearing system determine how the implant responds to imposed loading and activity, and to allow the resulting wear or roughness of the bearing system to continuously effect the resulting kinematic performance of the bearing coupling.
In this study, a standardized roughening methodology was used to evaluate the roughening resistance of CoCr and OxZr femoral components to initial third body roughening. These femoral implants were then used in a full scale total knee replacement wear testing simulation to quantify the effect of this femoral surface roughening on the wear rate of UHMWPE in vitro. Force-controlled wear testing simulation was utilized to quantify the effect of femoral component material selection on the resulting TKR kinematic and kinetic performance during gait simulation, and the UHMWPE bearing counterface wear rates over time. We hypothesized that OxZr femoral components were more resistant to extrinsic roughening conditions than CoCr femoral components, and that this roughening resistance would translate into a decrease in the production of UHMWPE wear during functional simulation, with notable changes in TKR kinematic and kinetic performance over time.
Section snippets
Materials and methods
For this study the Genesis™ II Knee System (Smith & Nephew Inc., Memphis, Tennessee) was chosen, as it is available with both an oxidized zirconium (OxZr) femoral component (Oxinium™) and a conventional cobalt chromium (CoCr) alloy (Co–28% Cr–6% Mo) femoral component. Each knee femoral component (Size 5, Left) was coupled with a cruciate-retaining UHMWPE (EtO-sterilized, ram-extruded, GUR 1050) bearing surface insert and a titanium alloy (Ti–6% Al–4% V) baseplate. These OxZr femoral components
Weight loss results
After 5 million cycles of wear testing, the roughened OxZr femoral components were found to reduce the rate at which UHMWPE wear debris was generated by 82.0% when compared to the roughened CoCr femoral components (Fig. 2). The average per-million weight loss rates for the roughened OxZr and the roughened CoCr groups were 16.6 ± 7.6 and 92.0 ± 24.4 mg per million cycles, respectively (p < 0.00001). This translates to a 5.60-fold increase in wear rate for the roughened CoCr group over the roughened
Discussion
Polyethylene wear debris formation continues to be a primary factor in the reduced longevity of total knee replacements [43], [44], [45], [46]. A common finding in most laboratory total knee joint testing is the formation of “scratches” on both the polyethylene and the femoral surface. These femoral surface scratches are often cited as possibly being the result of third body wear from loose bone cement particles or inadvertent contact with surgical tooling. Studies that specifically address
Acknowledgements
Funding for this work was provided by the National Science Foundation (grant numbers CMS-96061859 and EPS-9871943), The Fullerton Foundation Inc., and Smith & Nephew Inc.
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