Medical Device Link.

IMPLANT MATERIALS

The use of polyurethane in medical applications has been well-documented.¹ But there is one member of the polyurethane family — the polycarbonate-urethane subset ­— that has great potential for use in orthopedics. In this article, we will explain why.

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.

Alternative materials to UHMWPE were studied and ranked according to their biocompatibility and mechanical properties, for use as an acetabular bearing surface in an artificial hip prosthesis, by a team led by F.P. Quigley in 2002. Quigley and his colleagues determined that, based on these factors, the polyurethane elastomers appear the most promising.⁷ The evolution of segmented polyurethane elastomers has been studied extensively in biomedical applications because of the combination of excellent biocompatibility and mechanical properties of polyurethanes.⁸ Poly(ester urethanes) (PEUs) were the first generation of polyurethanes used in medical devices, but were found unsuitable for long-term implants because of significant hydrolytic degradation of the polyester soft segment in vivo.8 After replacing the ester linkage of PEUs with an ether bond, PEUs have been used successfully for the past 20 years.⁸ However, in several long-term studies it has been shown that the polyether soft segment of PEUs is susceptible to oxidation in vivo.⁸ Any time the human body recognizes a material that is not itself, the normal self-preservation response is to have cells try to encapsulate it. Because they are relatively large, these cells are often called giant cells, and the reaction is called a “foreign body giant cell” response. Foreign body giant cells have been determined to initiate oxidative biodegradation of PEU via oxygen radicals abstracting a proton from the alpha-methylene position of the polyether, resulting in chain scission and/or crosslinking of the polyurethane.⁸ In vitro studies performed by Khan et al. conclude that PCU displays the best overall resistance to hydrolytic degradation, in vivo stress cracking, metal ion oxidation, and calcification as compared to other medical grade polyurethanes.

Polycarbonate-urethane acetabular component from the TriboFit Hip System. Courtesy of Active Implants Corp.

PCUs have more oxidatively stable polycarbonate soft segment. Several in vitro and in vivo studies have demonstrated that PCUs are significantly more biostable than PEU.⁸ PCU is synthesized from a methylene di (-phenyl isocyanate) hard segment chain extended with butane diol and a poly(1,6 hexyl 1,2 ethyl carbonate) soft segment. PCUs are currently being used in vascular grafts, artificial heart valves, and pacemaker leads.^5,6

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.

It is the case for the use of PCU material as an articulating surface that is the most intriguing. For a material to be selected for this use, it must match the properties of the natural tissue at the surfaces of articulation joints, which are both elastomeric and hydrophilic. The tissue provides both an articulating surface that protects the bone and a source of lubrication to improve motion within the joint.^5,6 Future designs of artificial joints should strive not only to replace but to imitate the natural joint. The natural joint is lined with a layer of synovial fluid that completely lubricates and separates the articulating surfaces of the femoral head and the acetabular cup.⁸ Two engineering concepts referred to as “elastohydrodynamic lubrication” (EHL) and “squeeze-film action” are the main mechanisms of action in the natural joint.⁸ During the stance phase, EHL predominates when pressure is generated in the lubricant by an entraining motion between the two joint surfaces. Squeeze-film action predominates at heel strike in the walking phase; the two surfaces move towards each other, squeezing the fluid out of the joint space. Both mechanisms are enhanced by elastic deformation in the joint surfaces.⁸ The natural joint operates under a full fluid-film lubrication regime, where the load is carried by the synovial fluid, resulting in minimal contact between the articulating surfaces.

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

Wear

Table 1. (click to enlarge) Comparison of Typical UHMWPE and Polycarbonate-urethanes (Bionate, Polymer Technology Group).

In three studies PCU was reported to have less wear than UHMWPE.^2,10,13 One theory is that PCU has the ability to allow the formation of a fluid film to separate the two surfaces, which reduces the contact pressure and decreases the friction.² An acetabular cup with PCU as the bearing surface showed great promise in in vivo experiments.⁶ The PCU cup showed no evidence of significant wear at four years post implantation. This data provides compelling evidence for the use of PCU in orthopedic load bearing/wear applications. The articulating surfaces were in excellent condition, suggesting that there was good joint function over the implant period. Environmental scanning electron microscopy and surface profilometry showed minimal normal wear. Supporting animal study findings revealed no significant soft tissue damage, wear particle generation, or inflammatory response. The absence of significant physical or chemical degradation suggests that the prototype PCU acetabular cup functioned well in a sheep total hip arthroplasty model during a four-year period of study. The PCU material showed excellent biostability in regard to maintenance of molecular weight. The absence of wear debris confirms the histological findings, which show no soft tissue damage or inflammatory response. The lack of third-body wear damage on most of the retrieved cups and the absence of third-body particles in the articular surface confirm laboratory studies suggesting that PCU acetabular cups can perform a similar function as the natural joint by allowing synovial fluid-film lubrication.^5,6

Conclusion

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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