High-performance polymer VESTAKEEP® PEEK, a proven implant material for medical technology, is being explored as a material for complex joint prosthesis systems. Evonik, a specialty chemicals company based in Germany, is drawing on the expertise of medical specialists at the world-recognized Center for Knee and Hip Replacement at Massachusetts General Hospital in Boston (USA). If the innovation succeeds, the service life of VESTAKEEP-based joint prostheses could be significantly extended, consequently reducing revision surgery or years of pain therapy. The success of PEEK-based hip prostheses for use in veterinary medicine from the Swiss company KYON supported Evonik’s innovation approach.

Joints are complex movement systems that fulfill important anatomical functions and are constantly exposed to heavy stress. Evonik is pursuing the approach of analyzing the weak points of the joint prosthesis systems already established on the market in human medicine and developing a solution with its polyetheretherketone high-performance polymer (PEEK). Evonik is looking into the use of VESTAKEEP in human joint prostheses to improve the quality of life for patients. For example, the company has learned to understand PEEK as a material component in complex joint prosthesis systems that can be integrated into existing technologies according to the modular principle.

Hip joint replacement operations have long been among the most standard operations. In the group of OECD countries, an average of 182 procedures were performed per 100,000 population in 2017, according to the Health at a Glance 2017 Report. This figure was 30 percent higher than 10 years earlier. Today’s joint prosthesis systems are convincing in terms of availability and reliability.

However, friction partners — so-called inlays such as those between the head and the cup of a hip prosthesis anchored in the bone — are a primary weak point of current technologies. The tribological PEEK biomaterial could make the decisive difference in the future and extend the service life of a hip prosthesis fourfold.

Tribiological Test of PEEK for Joint at MGH in Boston

The specialty chemicals company’s determination is expressed in its close cooperation with the Massachusetts General Hospital (MGH) in Boston. Evonik draws on the expertise of medical specialists from MGH’s globally recognized center for knee and hip replacement. Evonik has tested its PEEK biomaterial for tribological properties in its professional laboratories. The main purpose of the collaboration is to identify parts of artificial joint systems at which PEEK can provide an added value to patients by increasing the lifetime of artificial joint systems. This covered, for example, reduced stress shielding due to elasticity of the polymer versus metals or tribological properties for reduced wear.

Table 1. For tribological properties, MGH used its specifically designed Pin-on-Disc simulator to mimic the forces and tribological behavior in knees and hips. In these studies, PEEK was used as a tribological partner against established existing materials.

Evonik first analyzed the performance of VESTAKEEP PEEK biomaterial in comparison to the existing market benchmarks as follows: For tribological properties, MGH used its specifically designed Pin-on-Disc simulator to mimic the forces and tribological behavior in knees and hips. In these studies, PEEK was used as a tribological partner against established existing materials (see Table 1 and Figure 1).

Pin-on-Disc Wear Testing at MGH

Fig. 2 - The use of PEEK can reduce the wear and increase the lifetime of artificial joints.

As a result, the use of PEEK reduces the wear and increases the lifetime of the artificial joints (see Figure 2):

  • PEEK on XLPE material pair shows lower wear compared to market standard pair CoCr on XLPE — i.e., replacing CoCr with PEEK.

  • CoCr on PEEK material pair shows lower wear compared with market standard pair CoCr on UHMWPE — i.e., replacing UHMWPE with PEEK.

Orhun K. Muratoglu, PhD, director of the Harris Orthopaedics Laboratory and director of the Technology Implementation Research Center (TIRC) at MGH and professor of orthopedic surgery at Harvard Medical School notes, “At the Harris Orthopaedics Laboratory, we used our state-of-the art in vitro wear testing machines to evaluate the performance of medical-grade PEEK formulations for orthopedic joint replacement applications such as hip and knee implants and found that UHMWPE-PEEK pair has similar or better performance than that of the gold standard UHMWPE-CoCr pair.”

The investigation results show the suitability of PEEK in load-bearing joint parts in addition to the main research and development focus on tribological material pairs for joint prothesis.

Besides excellent tribological properties of VESTAKEEP PEEK, the proven biomaterial for implants provides bonelike mechanical properties that leads to reduced stress and therefore reduced stress shielding. Using a finite element analysis (FEA), Evonik’s polymer experts have simulated stress shielding by comparing a femoral knee implant of PEEK with CoCr. Stress shielding refers to the reduction in bone density (osteopenia) as a result of removal of typical stress from the bone by an implant (for instance, the femoral component of a hip or knee prosthesis). The investigation results demonstrate the suitability of PEEK in load-bearing joint parts in addition to the main research and development focus on tribological material pairs for joint prothesis (see Figure 3).

Following the promising results from the PEEK-for-Joints study at MGH, Evonik’s materials experts are taking further steps to get the high-performance polymer VESTAKEEP approved for medical use in complex joint prothesis systems. The regulatory approval procedure in human medicine is strictly oriented toward added value for patients.

Evonik plans to provide sufficient evidence of this added value, i.e., the extension of the service life of a hip prosthesis and thus a significant reduction in revision procedures, in order to identify partners for the further development steps. The specialty chemicals company aims to continue the wear testing at MGH with other material combinations, e.g., ceramics, followed by clinical evaluation and eventually integration and combination with existing joint prothesis solutions.

Innovation Scenario from Veterinary Medicine

Evonik’s PEEK biomaterial is already being used successfully in the latest generation of hip prosthesis systems developed by Swiss company KYON for pets such as dogs and cats. At the heart of the innovative technology is a friction partner made of Evonik’s VESTAKEEP PEEK biomaterial with an additional carbon-fiber-reinforced PEEK ring between the ceramic head and the cup. KYON’s unpublished data indicate that linear wear is reduced by a factor of seven with ceramic on PEEK compared to conventional pairings.

Approximately 6,000 successfully performed hip replacement procedures without a single revision due to inlay wear and just as many satisfied dogs — including true champions (agility dogs) — as well as five years of careful documentation confirm the success of KYON’s PEEK-based hip replacement system. Among other things, Evonik’s polymer experts would like to transfer the material expertise and application understanding from veterinary applications with potential medical device manufacturers to human medicine.

The use of VESTAKEEP PEEK as a material component in human joint prostheses is an innovative scenario and once again demonstrates the possibilities of high-performance materials in modern medicine.

If the breakthrough is successful, a new quality in the treatment of hip arthrosis would be available to human medicine. Considering the more than 300 million cases of hip and knee osteoarthritis worldwide in 2017, as published by Annals of the Rheumatic Diseases, this would be a significant medical achievement.

This article was written by Marc Knebel, Head of the Medical Systems Market Segment at Evonik, Darmstadt/Essen, Germany. For more information, visit here .