One example of the potential use in orthopedics is in the hip joint, where a need for new materials exists. Statistics indicate that today’s commonly used materials simply aren’t performing well enough. More than 190,000 total hip arthroplasties were performed in 2002 in the United States alone.2 But it is estimated that by 2012, as many as 21% of all hip arthroplasty procedures will be surgical revisions of the implant.3,4 Such a high revision rate means better implant materials are needed. Traditionally an artificial hip consists of a metallic femoral head and an acetabular cup usually made of ultrahigh molecular weight polyethylene (UHMWPE), a material that has been used clinically for over 40 years. Several studies have concluded that a primary cause of implant loosening is osteolysis (or bone resorption) caused by wear debris generated at the articulating surface of the polyethylene acetabular cup. The goal, therefore, is to develop an alternate material for use in orthopedics that does not generate wear debris of a type that induces osteolysis.5,6 This article makes the case for using the polymer polycarbonate-urethane (PCU) as a replacement for UHMWPE in orthopedic applications.
Improving Upon UHMWPE
Figure 1. (click to enlarge) Stress-Strain Curve showing UHMWPE compared to polycarbonate-
urethane (Bionate 80A, Polymer Technology Group). UHMWPE data is courtesy
of S.M. Kurtz, and PCU data is courtesy of Polymer Technology Group.
Polycarbonate-urethane acetabular component from the TriboFit Hip System.
Courtesy of Active Implants Corp.
For potential orthopedic applications, a series of tests indicate that the PCU material developed for biostable cardiovascular applications also has properties that are equal to or better than the UHMWPE currently used as an articulating material in artificial joints. The first test, using a friction measurement, assessed four potential load-bearing plastic materials for use in artificial joints. From this data, PCU was found to have the lowest friction properties of the four materials in the study.9 Compared to UHMWPE, PCU was found in two studies to have lower friction properties.10,11 PCU also will be an easier material to lubricate than UHMWPE, because PCU is a hydrophilic material, while UHMWPE is hydrophobic.12 Most importantly, as a potential biomaterial for orthopedic wear applications, PCU has been found to have equal to or better wear properties than UHMWPE.2,10,13 And finally, the modulus of elasticity of PCU is similar to cartilage, unlike UHMWPE, which is about 70 times stiffer.14,15 Based on all this information, PCU has already been selected for several orthopedic applications.
Similarities to Natural Cartilage
Active Implants uses polycarbonate-urethane for its TriboFit acetabular buffer.
Courtesy of Active Implants Corp.
This favorable situation leads to a great reduction or elimination in wear at the articulating surface because the natural compliant layers allow a fluid film to generate between the two opposing articular cartilage surfaces. PCU appears to mimic this natural process and does not appear to develop the same type of damage as UHMWPE, as shown in in vitro and in vivo studies.5,6 Preclinical data show that friction levels are low and sufficiently thick levels of lubrication exist to prevent contact between the surfaces. These results suggest that the low modulus of the PCU can simulate the function of articular cartilage in the natural joint and allow a layer of synovial fluid to form between the surfaces of the articulating prostheses. Also, as with the natural joint, should third-body wear particles be introduced between the interface of the articulating surfaces, either the particle will roll out off the surface because the surface is so compliant, or the third-body will embed in the polymer surface. Lack of third-body wear was observed in a four-year sheep study of PCU acetabular cups.16 All of this information and discussion is compelling evidence that PCU can recreate an effective fluid-film lubricating layer within the joint and is a potential material for use in joint replacement.5,6
Table 1. (click to enlarge) Comparison of Typical UHMWPE and Polycarbonate-urethanes (Bionate, Polymer Technology Group).
The case for the use of polycarbonate-urethane in orthopedic applications is compelling. PCU has good wear properties, good compatibility with natural tissues, and is easy to lubricate. For those reasons and others, it appears to be a viable alternative to the traditional UHMWPE weight-bearing material that has been used for more than 40 years in orthopedic joint replacement prostheses.
Richard W. Treharne, PhD, is vice president of orthopedic research with Active Implants Corp. (Memphis). Alex H. Greene, is an assistant research engineer with Active Implants.
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