Unicompartmental Knee Replacement
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Direct answer
A unicompartmental knee replacement (UKA) resurfaces the worn medial compartment and leaves the cruciate ligaments, lateral side and most of the patellofemoral joint untouched, so kinematics and proprioception feel closer to a normal knee than after total knee arthroplasty. It sits between high tibial osteotomy and TKA in the algorithm: the right operation for the older-but-not-elderly patient with isolated medial compartment disease, an intact anterior cruciate ligament and a correctable varus. Cemented mobile-bearing designs — the Oxford phase 3 — dominate the teaching; in high-volume hands survival exceeds 90 percent at ten years, with lateral compartment progression and bearing dislocation the characteristic failures.
What you must remember
- Oxford selection criteria (quoteable): age over 60, weight under 82 kg (13 stone), low-demand lifestyle, intact ACL, and no rest pain or inflammatory arthritis; modern practice loosens the age and weight dogma but never the ACL and correctable-deformity requirements.
- Correctability test: varus under about 15 degrees that corrects fully on valgus stress, fixed flexion deformity under 10 degrees, and flexion beyond 100 degrees.
- Non-negotiable contraindications: ACL deficiency, inflammatory arthritis, fixed deformity, lateral compartment disease; patellofemoral arthritis is acceptable in medial UKA — a standard contrast with TKA thinking, because the medial bearing tolerates medial facet wear.
- Mobile versus fixed bearing: the Oxford mobile bearing gives congruent contact and low wear but introduces bearing dislocation as a UKA-specific complication; fixed bearings accept less congruent geometry with no dislocation risk.
- Advantages over TKA: less bone resection, faster rehabilitation (day-care surgery in metro centres), better stair descent and proprioception, lower perioperative morbidity, and an easier revision to TKA.
- Characteristic complications: bearing dislocation (closed reduction or bearing exchange), lateral compartment progression — the commonest late revision cause — component loosening, and tibial plateau fracture during preparation.
- Survival: more than 90 percent at ten years in high-volume series; volume matters — surgeon learning-curve failures dominate early published registries.
- Indian practice reality: UKA implants often cost as much as or more than TKA sets in Indian markets, so volumes stay low outside large centres; the Oxford system still dominates exam questions because it is the most studied arthroplasty design worldwide.
The three-way comparison
Examiners love the middle patient, so reason through the algorithm rather than reciting lists. A 45-year-old labourer with medial pain, 8 degrees of varus and good flexion wants an HTO — he needs decades of load-bearing, his arthroplasty would wear out, and his activity would destroy a UKA bearing. A 62-year-old brisk walker with the same pattern, an intact ACL and a correctable deformity is the UKA candidate: enough years lived that a bearing will outlast the demand, enough function wanted that a total knee feels excessive. A 72-year-old with a 12-degree fixed flexion contracture, tricompartmental changes and no ACL to speak of gets a TKA, because UKA cannot balance a fixed contracture and HTO would leave her with lateral disease unaddressed. The age numbers are conventions, not walls — the ligament, the correctability and the compartment count carry the real decision weight.
Where the criteria bite
The trap is quoting Oxford criteria without knowing why they exist. The mobile bearing subluxes posteriorly without an ACL, which is why ACL deficiency is absolute. Correctable deformity matters because a UKA has no soft-tissue release to speak of — it cannot convert a fixed varus into a balanced knee the way a TKA can. Students also forget that obesity was written into the original criteria as a bearing-load worry, not an absolute bar, and that "patellofemoral arthritis is a contraindication" loses marks when the question specifies medial facet involvement. If a viva pushes further, the expected sequence is: why not HTO at 62? — because correction fades and the osteotomy compromises a future arthroplasty; why not TKA at 62? — because less resection and better proprioception are worth having when the criteria allow them.
Frequently asked questions
Which ligament must be intact for a UKA and why?
The anterior cruciate ligament; a mobile bearing needs the AP stability the ACL provides, and without it the bearing subluxes or dislocates posteriorly.
What is the commonest cause of UKA revision in the long term?
Progression of arthritis in the lateral (untreated) compartment; loosening and bearing dislocation come next.
Why is patellofemoral arthritis not always a contraindication?
In medial UKA only medial patellar facet involvement is acceptable disease; significant lateral facet wear with trochlear disease is the problem the exam is probing for.
How does UKA recovery compare with TKA?
Faster — less bone and soft-tissue trauma allows day-case or short-stay rehabilitation, earlier knee motion and better reported function in the first year.
Can a failed UKA be revised to TKA?
Yes, easily; residual bone stock is usually adequate, though lateral compartment bone loss or bearing-related defects may need augments or small grafts.