A Hip or Knee replacement surgery is conducted when the current hip has been damaged by wear and tear (aging) or due to unanticipated events. Constant wear and tear lead to Degenerative Osteoarthritis. Unexpected trauma to the joint leads to Hip or Knee Fracture. More than 500,000 Hip and Knee replacement surgeries are conducted in the US annually.
Two kinds of materials are used in Hip and Knee replacement surgeries – metals and polymers. While metals and ceramics have been the long-standing materials used for Knee and Hip Joints, polymers are becoming more mainstream in these applications as well. Polymers are long-chain organic molecules that are made of repeating chain-units (‘mono’-mers) and find use in a variety of plastics, adhesives and coatings applications.
The traditional Hip or Knee replacement surgery includes the use of a ball and socket joint, wherein the ball is made of metal or polymer (plastic) material, and the socket is typically made of a metal. However, the use of ceramics in this joint is declining. In addition, in some cases, a plastic bearing can be used for additional mechanical stability. The metal provides the integration to the bone and the rigidity while the plastic provides the mobility and flexibility to the joint. Both the metal and the plastic should be bio-compatible for these applications.
The metallic hip joints typically employed for this application are Titanium based, and the plastic usually utilized is Ultra-High-Molecular-Weight Polyethylene or UHMWPE based. These joints have a typical expected life of approximately ten years. New advances in technology have resulted in the development of new materials for use in Hip and Knee joint surgeries– both for longer life and for lighter weight. Some of the recent innovations in this space include the use of composites (metal, polymer, or ceramic based) or the utilization of a highly cross-linked UHMWPE.
UHMWPE is among the most abrasion resistant plastic materials available. In addition, UHMWPE is also among the few plastic materials approved for implant applications in the body. This is a unique plastic that melts but does not flow and leaves no residue in the application (which is critical for “in the body” applications). It is molded by methods that are closer to ceramic processing techniques rather than conventional polymer processing methods and then machined into the appropriate shape for the socket.
Ball and socket joints have gotten lighter with the use of plastics to replace metals (with no deleterious effect) or by making the joints smaller and safer. Further innovations are expected in this space to enhance functionality without sacrificing performance, which would give way to a future of an even more stable, bionic man.
Written by:Karthik Vaideeswaran PhD, Polymer Science
Steven M Kurtz et al., “Prevalence of Primary and Revision Total Hip and Knee Arthroplasty in the United States (1990–2002),” Journal of Bone and Joint Surgery, American, 2005.
Steven M Kurtz et al., “Meeting the Joint Replacement Challenge with UHMWPE”, MDDI March 2005.
MC Forster, “Survival Analysis of Primary Cemented Total Knee Arthroplasty; Which Designs Last?” Journal of Arthroplasty, Vol. 18, No. 3, S1, 265–270, 2003.