Partial knee replacement is not a new technique and was popular back in the 1980s given that many patients have arthritis that is limited to a single compartment in the knee. Early designs had a poor track record and high revision rate causing many surgeons to abandon this concept in the late 1980s and 1990s. Improvements in prosthesis design and a better understanding of failure mechanisms have led to a return partial knee replacement over the past 15 years and we now have a better understanding of who might benefit from this option.
What?: The knee is divided into 3 compartments: the medial tibiofemoral compartment, lateral tibiofemoral compartment and patellofemoral compartment (behind the knee cap). While some patients may develop arthritis diffusely throughout the knee joints, up to 1/3 of patients may only develop it in one area of the knee. This is most commonly the medial tibiofemoral compartment on the inside of the knee. When this is the case and conservative treatment options have failed to provide lasting relief, a partial, or unicompartmental, knee replacement can resurface only the diseased area of the joint leaving the remainder of the joint in its native state.
Why?: The advantages of the partial knee replacement include the following. 1) it is less invasive and can be performed through a smaller incision without any disruption of the quadriceps tendon; 2) it preserves both the anterior and posterior cruciate ligaments which may give the knee greater stability and a more natural feel; 3) it requires less postoperative physical therapy to regain full range of motion.
Disadvantages of the partial knee replacement mainly related to a slightly higher failure rate compared to full knee replacement. This usually occurs either because the remainder of the knee develops arthritis over time or because the implant loosens from the bone. Failures of partial knee replacements generally occur earlier or late. Early failures occur in the first two years and often relate to improper patient selection or surgical errors relating to poor implant placement or overstuffing the joint. The latter causes arthritis to develop in the opposite normal compartment. These methods of failure can be avoided by proper patient selection and good surgical technique. Late failures tend to occur between 10-20 years after surgery and relate more to wear of the implant over time.
Who? Success of partial knee replacement requires careful patient selection to ensure a good long-term outcome. Key criteria include the following: 1) arthritis isolated only one compartment of the knee; 2) intact anterior cruciate ligament; 3) no history of inflammatory arthritis such as rheumatoid or psoriatic arthritic; 4) no flexion contracture; 5) a passively correctible deformity (this means the leg can be straightened to neutral alignment with a manual force applied.
How? Partial knee replacement is less invasive than total knee replacement because the knee cap does not need to be dislocated to approach only one portion of the knee. There is less bone and less soft tissue work which tends to speed the recovery in terms of achieving postoperative range of motion. We still use our opioid-sparing AVATAR comprehensive care program which involves both regional and local anesthetics at the time of surgery.When? If the above criteria are applied rigorously, patients can achieve an excellent outcome with partial knee replacement. Long-term survivorship studies recently published from the United Kingdom Joint Registry shows 89% survivorship at 10 years and 80% at 15 years. While this is not as successful as total replacement which has a 94% 15 year survival rate, it may be sufficient for patients in their mid-70s to 80s, or for younger patients where it is buys time prior to needing revision to a full knee replacement.