Unicompartmental Knee Replacement (UKR/Oxford)

Indication. Symptomatic, single-compartment (almost always anteromedial) osteoarthritis with full-thickness, bone-on-bone cartilage loss in that compartment, a functionally intact ACL, a passively correctable intra-articular deformity, and preserved full-thickness cartilage in the opposite compartment — that has failed non-operative care. Partial-thickness cartilage loss is NOT an indication. Specific indications include anteromedial OA (the great majority of cases), isolated lateral compartment OA (use a domed/biconcave or fixed-bearing lateral design), spontaneous osteonecrosis of the knee (SONK) confined to one compartment, and selected post-traumatic or post-meniscectomy unicompartmental OA with an intact ACL. Two schools of selection — know both for the viva. The conventional Kozinn-Scott criteria (1989) have been challenged by modern Oxford evidence, which restricts the real requirements to an intact ACL and correctable bone-on-bone disease:

Absolute contraindications (medial mobile-bearing Oxford UKR): - ACL deficiency — the unconstrained bearing relies on the ACL for anteroposterior control; deficiency causes anterior translation, edge-loading and dislocation. (Some fixed-bearing medial designs are used in selected ACL-deficient knees, but this is controversial — state it as a caveat, not the default.)

• Inflammatory arthritis (rheumatoid, psoriatic, gout) — polyarticular disease will progress in every compartment.
• Tricompartmental / generalised osteoarthritis.
• A fixed (non-correctable) deformity — varus or valgus that does not correct passively implies soft-tissue contracture.
• Full-thickness eburnated lateral patellofemoral facet with grooving, and partial-thickness medial cartilage loss only (not yet bone-on-bone) — a high failure and pain rate. Relative contraindications / cautions. Lateral-compartment OA treated with a flat mobile bearing (higher dislocation — use a domed/biconcave or fixed-bearing lateral design); severe obesity or very high-impact occupational/sporting demand; chondrocalcinosis (relative only — challenged by Pandit unless florid inflammatory CPPD); a prior high tibial osteotomy (altered anatomy and bone stock); and age less than 60 (registries show higher revision in the young, largely from greater activity and longer time at risk — relative, not absolute). The bearing decision (pre-operative planning). The other genuine choice is mobile versus fixed bearing:

Advantages over TKR. A minimally invasive approach (6 to 8 cm versus 15 to 20 cm), bone-stock preservation, faster recovery (often same-day discharge), better proprioception and kinematics, lower perioperative morbidity, and a more natural-feeling knee. Alternatives. Total knee replacement (more predictable, addresses all compartments, longer recovery, more bone resection); high tibial osteotomy (younger, higher-demand patients under 60, preserves the native joint); cartilage restoration for focal defects in young patients (limited evidence in OA); and non-operative care (weight loss, activity modification, physiotherapy and strengthening, intra-articular injections, bracing for varus thrust, NSAIDs and analgesia). Consent. Counsel specifically for the revision trade-off — UKR carries roughly double the reoperation/revision rate of TKR but lower mortality, complications and length of stay — for the possibility of converting to a TKR on the table if intra-operative findings are unsuitable, and (for a mobile bearing) for a small risk of bearing dislocation. If the diagnosis is uncertain pre-operatively, always consent for a possible TKR.

The goal: through a minimally invasive medial arthrotomy, resurface only the worn compartment with a congruent mobile bearing, restoring a balanced and stable joint while preserving the ACL, the opposite compartment and the bone stock for any future TKR. The exposure and the go/no-go intra-operative assessment are laid out in full below.

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The structures at risk through these steps — the five danger zones:

Rehabilitation | Phase | Timing | Mobilisation & ROM | Key milestones | |-------|--------|---------------------|----------------| | Immediate | Day 0 to 1 | WBAT immediately; active-assisted flexion/extension same day | Ice and elevation, multimodal analgesia, DVT prophylaxis; often same-day or overnight discharge | | Early | Week 1 to 6 | Progress to full weight-bearing; target 0 to 120 degrees | Quadriceps and proprioception work; driving at 2 to 3 weeks; wound check at 2 weeks | | Intermediate | Week 6 to 12 | Achieve 0 to 135 degrees (usually exceeds TKR) | Progressive resistance; standing AP and lateral films at 6 weeks | | Late | 3 to 12 months | Low-impact sport (golf, swimming, cycling) at 3 months | Avoid running and jumping; review at 3, 6 and 12 months, then annually | Most patients report a "normal" or "near-normal" knee that recovers faster and with better proprioception than after TKR. Pooled survival is about 93 to 94 percent at 10 years and 89 percent at 15 years for medial Oxford UKR (Mohammad 2018; Hamilton 2017); registries show roughly double the revision rate of TKR (Liddle 2014), most often for lateral OA progression, aseptic loosening or bearing dislocation — but conversion to TKR is generally straightforward when bone stock is preserved. Complications

Epidemiology & survivorship. Anteromedial osteoarthritis is the commonest pattern of knee OA and the substrate for the great majority of UKRs. For the medial Oxford Phase 3 mobile-bearing UKR, pooled survival is about 93 to 94 percent at 10 years and 89 percent at 15 years (Mohammad 2018), reproducible outside the designer centres; the National Joint Registry reports Oxford mobile-bearing survival of about 95 percent at 10 years, while the AOANJRR records a cumulative revision rate of about 14 percent at 10 years (Oxford the commonest design). Crucially, outcomes depend on disciplined indications more than on volume — surgeons whose UKR usage is at least 20 percent of their knee arthroplasties (ideally over 30 percent) have low revision rates regardless of raw caseload (Hamilton 2017). The bearing biomechanics. The Oxford mobile bearing is fully congruent — a spherical femoral surface on flat-on-flat polyethylene gives a large contact area and low wear, and the bearing self-centres through range. It depends on the ACL for anteroposterior control, which is why ACL deficiency is an absolute contraindication and its characteristic failure mode is bearing dislocation. A fixed bearing locks the polyethylene to the tibial baseplate: more forgiving and no dislocation, but less congruent and so higher contact stress.

Why it works — the selection evidence. TOPKAT (Beard, Lancet 2019) showed no clinically important difference in Oxford Knee Score between partial and total replacement at 5 years, with PKR both more effective and less expensive. The NJR propensity-matched comparison (Liddle, Lancet 2014) found UKR had roughly double the revision rate of TKR but LOWER mortality, complications, readmissions and length of stay at every timepoint — the trade-off to frame in consent. Modern selection rests on Pandit et al (2011), who showed that weight, age, activity, PFJ status and chondrocalcinosis should not be considered contraindications for the Oxford UKR, leaving an intact ACL and correctable bone-on-bone disease as the true requirements.