Not every arthritic knee needs the whole joint resurfaced. In a well selected group of patients, replacing only the worn compartment preserves the rest of the knee, including the cruciate ligaments, and generally produces a knee that feels more natural.
The difficulty is that partial knee replacement has a narrower set of correct patients, and its results depend heavily on getting that selection right.
What a partial replacement does
The knee has three compartments: medial, lateral and patellofemoral. A unicompartmental or partial replacement resurfaces only the affected one, usually the medial compartment, and leaves the healthy cartilage and both cruciate ligaments intact.
Because less bone is removed and the ligaments are preserved, patients often report earlier recovery, better proprioception and a knee that feels closer to their own.
Who is suitable for a partial replacement
The arthritis should be genuinely confined to one compartment, confirmed on examination and weight-bearing radiographs. The anterior cruciate ligament should be functionally intact. The deformity should be correctable rather than fixed, and range of movement should be reasonably preserved with a good degree of flexion and minimal fixed flexion.
Inflammatory arthritis such as rheumatoid disease is not suitable, because the disease affects the whole joint lining rather than one compartment.
Who needs a total replacement
Patients with arthritis in two or three compartments, a deficient or torn anterior cruciate ligament, a significant fixed deformity, substantial stiffness, or inflammatory arthritis are better served by total knee replacement.
Total replacement is also the more predictable operation when the pattern of wear is uncertain, and it remains the procedure with the longest and most robust track record.
The trade-off patients should understand
Partial replacement offers a more natural feeling knee and a quicker early recovery, but carries a higher rate of later revision, principally because arthritis can progress in the compartments that were left alone.
Total replacement is a larger operation with a slightly longer rehabilitation, but has very reliable long-term survivorship and removes the possibility of progression elsewhere in the joint.
Neither is simply better. The right answer depends on the knee in front of the surgeon.
How the decision is confirmed
Clinical examination establishes the deformity, its correctability, ligament status and range of movement. Weight-bearing radiographs, including a skyline view of the kneecap, show the distribution of wear. The final confirmation of compartment status is made at the time of surgery.
A patient being considered for partial replacement should be consented for the possibility of a total replacement, in case the joint is found to be more widely affected than expected.
A detailed comparison is available here: https://www.mayurajcc.com/knee-replacement-guides/total-vs-partial-knee-replacement
Questions worth asking
Ask which compartments are affected on your X-ray, whether your deformity is correctable, whether your cruciate ligament is intact, and what the plan is if the joint looks different at surgery.