Polymer Biomaterials Offer Possibilities for Orthopedic Implants

Pioneering joint manufacturers are utilizing an engineering polymer that overcomes shortcomings of alternative bearing materials.

Since the 1950s and John Charnley’s introduction of the low friction hip prosthesis, metal-on-polyethylene bearings have remained the gold standard in terms of the long-term performance of orthopedic implants, and have represented the largest share of the orthopedic implant market. In spite of decades of use and large amounts of research and development on these materials, some problems still exist with this material combination. Prime among these are wear-related osteolysis and thick implants leading to healthy bone removal during surgery. The use of a high-performance polymer material may present an opportunity to create a step change in this sector.

Since it was introduced in 1999, PEEKOPTIMA polymer (an implantable grade of polyether ether ketone polymer from Invibio® Biomaterial Solutions) has been used in over 3 million implanted devices globally. This material offers high stiffness and strength, excellent biocompatibility, and biostability. Early adopters of PEEK-OPTIMA saw success in the spinal fusion cage market by taking advantage of the material’s performance properties, such as: reduced stiffness, increasing load transfer, and the material radiolucency, which improves fusion assessment using radiographs. Sports medicine and cranio-maxillofacial surgery are areas in which medical device manufacturers have been able to exploit properties of the material, including its radiolucency and relatively high strength-to-weight ratio.

Arthroplasty and implantable bearings have been significantly slower in adopting this new material, perhaps in fear of adopting new technology too quickly. However, this has not stopped some pioneering manufacturers from exploiting aspects of the material that make it very attractive for use in bearing applications. As a result, the number of devices in clinical use has steadily increased over the last 10 years.

Overview of Bearing Materials

Moving from metal-on-polyethylene, the last two decades saw a resurgence in the popularity of metal-on-metal total joints. The interest in this sprang from improved wear rates over polyethylene, the ability to make larger joint sizes in the hip to increase joint stability, and the possibility to remove only minimal amounts of healthy tissue from the acetabulum (effectively resurfacing the damaged joint). Problems have arisen, however, due to wear debris sensitivity and metallic ion release into the surrounding tissue. This has caused large numbers of patients to experience dramatic soft and hard tissue death, leading to so-called pseudotumors and the need for joint revision.

The issues that these couplings were intended to address remain unsolved. An ability to create a resurfacing implant is an attractive prospect, particularly in younger patient cohorts, where underlying bone remains healthy, leaving the option to revise to a total hip at a later date. This type of implant requires a thinwall section that can only be achieved using a stiff and strong material.

Another high-performing section of bearing materials that has seen a return to orthopedics is ceramics. These materials have been paired against polyethylenes, metal, or as a self-mating material. The reports for these have been similar to those of metallic components but with reduced wear and reduced patient sensitivity to the material and any wear debris. However, ceramics are not without their problems. Production of these materials is not a simple route and incurs a high cost, often resulting in significantly higher device pricing. Additionally, due to the brittle nature of the material, there have been reports of implant fracture both during insertion and during service. This brittle nature also restricts design options and does not allow the thin-sectioned resurfacing implants mentioned above.

The PEEK-OPTIMA biomaterial and its derivative compounds offer the potential to address shortcomings of alternative materials. This type of engineering polymer offers a combination of strength and stiffness that opens up many options for the manufacture of innovative implant designs. This has been seen not only in the hip joint, but also in other joints where research data and commercialized devices have been generated. Joint manufacturers making everything from knees and ankles to elbows and fingers have examined the potential of PEEK-OPTIMA.