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.
Examples of Applications
Hip implants have shown evidence of high polyethylene wear rates in a variety of devices and scenarios. The wear debris produced in this manner is typically of sub-micron size and therefore produces an inflammatory response, often leading to bone resorption. This bone resorption then leads to a secondary process often resulting in a loosened implant and the need for revision surgery.
Other polyethylene material properties also limit the design options for devices made with the material. For example, its low stiffness does not provide structural rigidity in thinner parts. Subsequently, polyethylene acetabular cups must be either reinforced using a metallic shell, or designed to have thick section as in the case of cemented monoblock cups. By having this thicker cross-section in the device, the surgeon is faced with one of two options: Either to use a femoral im plant with a smaller head diameter which introduces higher potential for dislocation, or to remove large quantities of healthy bone from the acetabulum.
In 2007, the MITCH PCR™ implant from Stryker (Fig. 1) used the properties of MOTIS® (a milled pitch carbon-fiber reinforced compound of PEEK-OPTIMA) in a unique design. In testing of the device to 25x106 cycles using a 54mm head diameter hip simulator by the University of Durham, a steady state wear rate of 1.13 mm3/million cycles was seen.1 This wear rate is comparable to those reported for metal-on-metal bearings and more than an order of magnitude lower than those seen in metal-on-polyethylene bearings of a smaller size. Debris from these tests has been shown in several studies to elicit a similar biological response as polyethylene on a like-for-like dosing level. Additionally, the structural integrity of the MOTIS material allows a cup that is just 3.5 mm thick and incorporates a horseshoe design that mimics the natural geometry of the meniscal cartilage in the acetabulum. After more than three years of implantation in 25 patients, this joint replacement has not been revised for any issues related to wear of the primary bearing surfaces.
Finger joints have been developed using both image contrast grade PEEK-OPTIMA (a compound with Barium Sulphate to give visibility on radiographs) in a self-mating, hinged metacarpophalangeal (MCP) implant from Mathys and PEEK-OPTIMA CFR in a self-mating, proximal interphalangeal (PIP) implant from Zrinski™. Both devices have been used clinically in Europe and continue to perform very well. Also in common, these implants utilize another feature of the PEEKOPTIMA material — the ability to be coated for improved osseointegration. On simulator testing, the Zrinski implant produced very little wear.
Rotating Hinge Knee
Another area of use for self-mating PEEK-OPTIMA carbon-fiber-reinforced (CFR) is in the internal bearing components of a rotating hinge knee from Aesculap (Fig. 2). The EnduRo knee uses the material in the internal bushings that are designed with an under cut in the bearing sleeve so that dislocation is ruled out. Previous attempts were made to manufacture these types of implants from polyethylene, but the abrasive wear in the bush bearing could lead to premature loosening and dislocation. The use of PEEK-OPTIMA CFR combined with the hinge mechanism and joined cone design are designed to prevent dislocation compared to traditional hinge mechanism types that use ultra-high-molecular-weight polyethylene (UHMWPE). The EnduRo knee system has been commercially available in Europe since January 2010, achieving more than 2,000 prostheses implanted to date. It received FDA 510(k) clearance in December 2010 with the first U.S. implantation in November 2011. Use of PEEK-OPTIMA-based biomaterials in a rotating hinge knee represents a significant step in the acceptance of the material for implantable bearing applications.
Incumbent bearing materials have proven over the years to be flawed in a number of areas such as the wear rates and the debris products causing bioreactivity problems. PEEK-OPTIMA and CFR PEEK-OPTIMA compounds have been able to address these concerns in testing and in clinical studies. PEEK-OPTIMA-based biomaterials are expected to continue expanding arthroplasty applications in more innovative products. The opportunity presented by a high-performance, injection-moldable biopolymer means that low-cost, high-performance devices will continue to make themselves available to an ever-expanding market.
- Scholes, S. C., I. A. Inman, et al. (2008). Proc Inst Mech Eng H 222(3): 273-283.