Archive Total Joint 2016

Understanding Your Implant Options

What you need to know before sifting through the countless versions of hip and knee hardware.

Paul Manner, MD, FRCSC

BIO

Getting an implant DEEP DIVE Look to factors other than surgical outcomes to differentiate available implants.

Hip and knee implants are essentially created equal when it comes to reducing joint pain and improving post-op function. So how do you and your surgeons make sense of the hundreds of choices in prostheses that vary in material and design? Matching the right implant to the right patient generally boils down to these key considerations.

In the knee
Knees are often thought of as a "hinge" joint, since they bend and straighten like a hinged door. In reality, though, the knee is much more complex. While original implants used a simple door-hinge design, newer models more closely mimic the rolling and gliding motion of the joint. Most standard knee implants are made up of 3 components that cover different bone surfaces:

  • The metal femoral component curves around the lower end of the femur.
  • The tibial component is made up of a metal platform with a polyethylene cushion insert. It is attached to the top surface of the tibia.
  • The patellar piece is a dome-shaped implant made of polyethylene that covers the back surface of the patella and mimics the shape of the kneecap.
  • Typically, manufacturers design these components to work as metal-on-plastic (usually cobalt-chromium or titanium with polyethylene). This provides smooth movement and minimal wear. There are currently more than 150 knee implant designs on the market, so when choosing which device to use surgeons consider several key differences, including:
  • Cruciate-retaining vs. posterior-stabilized. Cruciate-retaining implants let the surgeon preserve the patient's posterior cruciate ligament, if it's still intact. These implants have a small groove that allows for flexion of the ligament. Posterior-stabilized implants let the surgeon remove the ligament and replace it with a cam-and-post system that prevents the thighbone from sliding too far forward on the shinbone when the knee is bent.
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