By Dr. Michael M. Lynch, MD Coastal Orthopaedics LLP
During knee replacement surgery, an orthopaedic surgeon will replace damaged knee surfaces with artificial implants. Most of the time, all of the surfaces are replaced, but in some instances, only part of the total bearing surface of the knee warrants replacing. This is known as a partial knee replacement. This can be either the inner or outer half of the knee (unicondylar replacement), or isolated to the knee cap and its bearing surface on the femur (patellofemoral replacement).
Most commonly implants are made of metal alloys covering the femur and tibia bone with a plastic insert placed between them. A small patella “button”, usually made of plastic, is used to resurface the contacting part of the kneecap. Alternatively some devices are made of ceramic in lieu of metal.
There are many manufacturers of total knee implants and up to 150 designs on the market today. While there is general agreement amongst surgeons, engineers, and researchers on the basics of implant design, there are many variabilities within these parameters. These include: materials for metal, ceramic, and plastic components, whether the plastic insert is fixed to the tibia, or rotates freely between the components or which ligaments are sacrificed, and which remain; the use of acrylic cement to aid in implant fixation versus the use of ingrowth technology where the bone is encouraged to grow into the metal implant and size variation, and gender specific implants are also a consideration.
The materials used for implants must meet certain criteria. They must be biocompatible (exist in the body without an adverse reaction. They must be strong enough to withstand large loads without breaking (this from both onetime large loads such as in a fall or twist of the leg) but also have durability over the long term from the repetitive cyclical loading to which they are subject. For instance, while titanium is a tremendous material for making golf clubs, in knee implant design it proves too “soft” over the long term, leading to deformity and wear. Polished cobalt chrome metal against high molecular weight