Partial knee replacement.

Not every arthritic knee needs a full knee replacement.

The knee has three compartments — the inner (medial), the outer (lateral), and the joint between the kneecap and thigh bone. In some people arthritis affects only one, usually the inner compartment, while the rest of the knee and the ligaments remain healthy.

Mako partial knee replacement, also called unicompartmental knee replacement, replaces only the part of the knee affected by arthritis. The healthy compartments, the cruciate ligaments and as much normal bone as possible are preserved.

Patient selection is the critical factor, and the suitable group is genuinely narrower than for total knee replacement.

Partial knee replacement may be considered when arthritis is confined to one compartment, most commonly the medial compartment, and the remaining joint surfaces and key ligaments are sufficiently healthy.

It is generally not suitable where arthritis affects more than one compartment, where the anterior cruciate ligament is deficient, where there is significant fixed deformity, or in some inflammatory arthritis.

Symptoms, examination findings and imaging all matter. Some patients who appear suitable on an X-ray have more widespread disease or ligament damage that makes total knee replacement a better option.

A partial knee replacement is a smaller operation because only one part of the joint is resurfaced. Because the cruciate ligaments are preserved, the knee retains more of its natural movement pattern than after a total knee replacement, where one or both cruciates are removed. Early recovery can be quicker in appropriately selected patients.

The trade-off is important to understand. Australian and international registry data consistently show a higher revision rate for partial knee replacement than for total knee replacement. One contributing factor is that conversion of a partial replacement to a total replacement may be considered at a lower threshold than a major revision of a total knee replacement. Even so, the difference in revision rates is genuine and should form part of the decision.

Arthritis may also develop in another compartment over time, which is the other reason a partial replacement may later need converting.

Partial knee replacement is technically demanding. The implants are smaller, the margins of error are finer and only part of the joint is being resurfaced. Accurate component positioning and careful patient selection are therefore important to the result.

Mako can be particularly useful in this setting because the CT-based plan defines the limited area of bone to be prepared and helps the surgeon reproduce the intended implant position while preserving the unaffected parts of the knee.

A Mako-protocol CT scan is used to create a three-dimensional model of the knee. Dr Limbers plans the implant size and position and assesses how much bone needs to be removed from the affected compartment.

During surgery, knee movement and ligament tension are assessed and the plan can be refined so the new components work with your remaining natural joint surfaces and ligaments.

Through a smaller incision, Dr Limbers exposes the affected compartment while preserving the unaffected parts of the knee. Your anatomy is registered to the Mako system and the plan confirmed. The robotic arm assists with precise preparation of the damaged bone within the planned boundaries. Trial components are assessed for movement, stability and balance before the final implants are secured.

• Preservation of unaffected bone and joint surfaces
• Retention of the cruciate ligaments
• A smaller operation than total knee replacement
• Patient-specific CT-based planning
• Precise preparation of a limited area of bone
• Potential for a more natural-feeling knee in suitable patients

Patients are usually encouraged to stand and walk shortly after surgery. Recovery can be quicker than after total knee replacement, though this varies, and swelling, discomfort and weakness are still expected during the early weeks.

Rehabilitation focuses on knee movement, quadriceps strength, balance and progressive walking.

View the Partial Knee Replacement Rehabilitation Protocol →

Infection, blood clots, stiffness, persistent pain, fracture, implant loosening or wear, progression of arthritis elsewhere in the knee, nerve or blood vessel injury, and the need for revision or conversion to total knee replacement.