Types of Knee Arthroplasties
Generally speaking there are two main categories of knee athroplasty:
- Total knee replacement
Unicompartmental prostheses replace only one compartment and are indicated for patients with predominantly unicompartmental arthritis. This replacement is especially useful for young patients who want to engage in high-level activities. It is used most frequently in the medial compartment for osteoarthritis, but can also be used for patients with focal osteonecrosis and secondary osteoarthritis. In the 1993, unicompartment prosthesis fell out of favor with many surgeons for a variety of reasons, but mostly due to the success of total joint prostheses. Interest has been revived due to newer minimally invasive surgical techniques and good long-term outcomes.
Total knee replacements (TKR) can be further categorized by mechanical stability into:
- Constrained or hinged
In most patients undergoing total knee replacement, the anterior cruciate ligament is no longer competent. During surgery it is removed. The posterior cruciate ligament (PCL) is frequently competent, and may be retained.
Non-constrained or cruciate retaining prostheses are designed to use the PCL, and referred to as PCL-sparing prostheses. Non-constrained prostheses are highly successful and the most common type. The components are not linked and rely on the patient's own ligaments and muscles for stability.
Non-constrained total knee prosthesis. The femoral and tibial components are independent of each other. A groove is present in the posterior aspect of the tibial prosthesis (arrow) for the native PCL.
Semi-constrained or cruciate substituting prostheses are used when PCL retention is not possible or if the surgeon prefers to remove it and use a more stable prosthesis. These protheses have more inherent stability by virtue of a variety of design techniques, including placement of large central anterior tibial spine that articulates with a rectangular box-like opening between the condyles of the femoral component. The posterior portion of the box is formed by a transverse metal cam that prevents posterior tibial subluxation when the knee is in extension.
Semi-constrained TKR viewed from the front, back and side. A central tibial polyethylene spine articulates with a transverse femoral cam.
Both PCL retaining and PCL substituting knee replacements are very successful, with success rates over 95% in 10-year follow-up studies.4, 5
Constrained or hinged prostheses have tibial and femoral components that are linked together with a hinged mechanism. They are used when the knee is highly unstable and the ligaments will be unable to support other types of prostheses. This is rarely a first choice option. Constrained prostheses are reserved for severely damaged knees. Typically they are used in elderly patients undergoing revision arthroplasty. Due to the higher amount of mechanical stress put on hinged prostheses, they are not expected to last as long as other less-constrained designs.
AP and lateral radiographs of a hinged prosthesis.