Each year, approximately 500,000 patients, including those with rheumatoid arthritis and osteoarthritis, undergo total hip, knee and other joint replacement surgery in the United States.  The clinical performance of implanted devices can be followed and studied.  In addition, a fraction of these implants are removed each year for reasons such as infection, osteolysis, or mechanical failure of the components.  The removed or revised components can be preserved for analysis.  They provide a rich resource with which to examine the influence of surgical, patient, design, and material factors on the long-term performance of these devices.

The overall objective of the Center for the Evaluation of Implant Performance is to correlate evidence of the mechanical behavior of implanted and retrieved devices with clinical, material, design, and manufacturing variables.  This objective is achieved, in part, by collaborating as the key secondary site in a multi-center Hip and Knee Retrieval Repository Program based at Drexel University under the direction of Steven Kurtz, Ph.D. 

Current implant performance topics of study include:

Development of tools for the clinical evaluation of wear of UHMWPE total hip replacement components

Over the past 30 years, improvements in biomaterials, implant design and surgical technique have resulted in dramatic improvements in the reliability and success of total hip arthroplasty.  However, wear of the ultra high molecular weight polyethylene (UHMWPE) articular surface of these joint replacement components, and the resultant osteolysis has been recognized as a important factor affecting the long term success of total hip arthoplasty.  This in vivo wear can be measured from radiographs using computer-assisted techniques.

One tool available (Hip Analysis Suite) uses edge detection to measure femoral head penetration from standard clinical anterior-posterior pelvic radiographs.  We have collaborated  with the developers of Hip Analysis Suite to evaluate and refine this sophisticated research tool. This has resulted in a more user friendly and accurate technique for clinical evaluation of the wear performance of UHMWPE hip replacement components.

Clinical wear performance of crosslinked UHMWPE

Studies of patients having primary total hip arthroplasty (THA) using highly crosslinked UHMWPE formulations show excellent clinical outcomes and very low radiographic wear.  Utilizing Hip Analysis Suite, we have been following the clinical wear performance of highly crosslinked UHMWPE as a function of material and design variables in THA (i.e., different UHMWPE formulations and femoral head diameter). 

Clinical wear performance of UHMWPE paired with a Zirconia ceramic vs. a Co-Cr-Mo alloy femoral head in THA

In vitro laboratory studies suggest that articular surface UHMWPE wear in THA may be significantly reduced when the femoral head bearing surface is ceramic rather than metallic (e.g., Co-Cr-Mo alloy).  The highly polished surface finish that can be achieved with ceramics, their low wear rate, and the formation of a fluid film on the surface of ceramics all suggest that UHMWPE should experience less wear, and hence debris generation that could lead to osteolysis, when coupled with a ceramic rather than metallic bearing. Utilizing Hip Analysis Suite, we have been following the clinical wear performance of ceramic-on-polyethylene versus metal-on-polyethylene in THA.  The purpose of this study is to establish the rates of radiographic wear and incidence of osteolysis in THA using zirconia ceramic and conventional Co-Cr-Mo alloy femoral head components.  This is a randomized clinical trial.

Wear, mechanical performance, and degradation of retrieved  UHMWPE components

UHMWPE has been used successfully as a bearing material in total joint replacement components for over 40 years. However, damage to these components has been recognized as a significant clinical problem limiting the lifetime of joint arthroplasties.  In this regard, long-term complications, such as osteolysis and aseptic loosening of metal-backed acetabular components, have been related to an immunologic response provoked by polyethylene wear debris.New formulations of UHMWPE, including highly crosslinked and thermal or antioxidant stabilized variations, have been and continue to be introduced into clinical practice.  Implant retrieval analysis is an integral tool in tracking the natural history of the wear, mechanical performance (including fracture), and degradation of current and historical generations of these UHMWPE components.  

Corrosion and fretting damage of retrieved metallic components

Contemporary joint replacement devices are modular in nature, in part because it allows surgeons greater intraoperative flexibility and ability to restore patient biomechanics. However, corrosion and metal release at the modular junction of the femoral head/stem taperin THA has occurred in the past, though it was thought to have been addressed. Recent design trends of metal-on-metalbearings, large femoral heads, adapter sleeves, and modular necks have recently re-introduced implant corrosion as a major clinical concern, due to the severe consequences of adverse local tissue reactions (ALTRs), including hypersensitivity, tissue necrosis, and pseudotumors.  Similar observations of ALTR secondary to taper corrosion have also recently been reported in total knee arthroplasty.Implant retrieval analysis is an integral tool in tracking the natural history of corrosion and fretting damage in contemporary modular metallic alloy components.

New Methodologies to Determine Fatigue Crack Propagation and Initiation in 2nd Generation Crosslinked UHMWPE Formulations

Recent work has demonstrated that crack propagation in a number of formulations (uncrosslinked and highly crosslinked) of orthopaedic grade ultra-high molecular weight polyethylene (UHMWPE) occurs in an essentially non-cyclic manner, even under applied cyclic loading.  This has been demonstrated through an observed cyclic frequency independence and a predictable waveform dependence of fatigue crack propagation velocities in uncrosslinked and first generation crosslinked UHMWPEs.

     We have found that for UHMWPE the material rankings are preserved whether crack propagation occurs under a cyclically applied load or a statically applied load.  From a practical point of view, static loading may be preferable to cyclic loading as it can be conducted with less intensive oversight and closed-loop test frames would not be strictly necessary (i.e., a frame that applies a dead load would be sufficient).  Scientifically, the intrinsic processes that control crack behavior are quasi-static; therefore, quasi-static experiments more accurately extract the phenomena of interest.  Quasi-static tests yield crack propagation velocity and fracture instability, as do conventional cyclic tests, but can also reveal the time to crack initiation.  Understanding the time to crack initiation from design-type stress concentrations would be of use in predicting the fracture resistance of UHMWPE joint replacement components; to-date, this has been an elusive quantity to capture.  In the presence of a stress concentration, UHMWPE behaves in a brittle manner, therefore, understanding the crack initiation resistance of this material is of keen interest.

     In this regard, we have also found that crack initiation time and crack velocity in two generic crosslinked UHMWPE formulations (remelted 65 kGy and remelted 100 kGy) depend on the applied static load in a manner well predicted from analytical models.  In addition, the mechanism of crack initiation has been found to be one that can occur as a single-layer initiation process or a multi-layer initiation process and this affects the time to crack initiation.  This also depends on the notch root radius.

     Overall, quasi-static crack propagation experimental methodologies appear to be more informative in a wide variety of ways to traditional cyclic fatigue crack propagation experiments in the evaluation of UHMWPE materials.  We are presently exploring this avenue of research with a 2nd generation highly crosslinked/antioxidant UHMWPE.  The ultimate goal of this research is to document the correspondence of cyclic and static testing techniques such that future crack propagation testing can be conducted solely under constant loading, and also to establish the additional value of crack initiation mechanistic behavior as understood through our unique constant load testing methodology.