Unicompartmental Knee Arthroplasty (UKA) — Indications & Outcomes

Unicompartmental knee arthroplasty (UKA) resurfaces a single compartment of the knee — medial (most common, accounting for approximately 80% of UKA), lateral, or patellofemoral — while preserving the unreplaced compartments, the cruciate ligaments, and the normal proprioceptive and kinematic properties of the knee. When properly indicated, UKA provides outcomes equivalent or superior to TKA in terms of patient satisfaction, function, and recovery, with a lower complication profile and better preservation of bone stock. The most critical factor determining success is patient and disease selection.

• Incidence: UKA accounts for approximately 8–10% of all knee arthroplasties performed in the UK (NJR data); the Oxford Partial Knee (Biomet) is the most commonly used medial UKA implant in the UK and has the largest evidence base; the proportion of UKA varies internationally — higher in the UK than in the USA
• Advantages over TKA: preserves bone stock (facilitates easier conversion to TKA if required); preserves cruciate ligaments (more physiological kinematics); shorter recovery; lower complication rates (lower blood loss, DVT, infection, peri-prosthetic fracture); faster return to activity; better proprioception; more natural feeling knee
• The fundamental trade-off: UKA provides better early outcomes but has a higher revision rate than TKA (NJR 10-year revision rate approximately 10–15% for UKA vs 4–5% for TKA); however, conversion of a failed UKA to TKA is generally straightforward and achieves outcomes similar to primary TKA

• The Oxford criteria for medial UKA: the criteria developed by the Oxford group and validated in large cohort studies: (1) anteromedial OA pattern (full-thickness medial compartment cartilage loss with preserved lateral compartment and ACL); (2) correctable varus deformity; (3) ACL intact; (4) flexion >100° and flexion contracture <15°; (5) no severe patellofemoral OA causing symptoms; these criteria were intentionally broadened by the Oxford group — advanced age, obesity, and activity level are NOT contraindications in the current Oxford evidence base
• The "anteromedial OA" pattern is key — in anteromedial OA, the medial tibial cartilage wears anteriorly first; on the frog-leg lateral view, the posterior medial tibial cartilage is often preserved; the ACL is intact and the deformity is passively correctable (the remaining cartilage acts as a hinge); this is the ideal UKA pattern

• NJR survivorship: 10-year revision rate for UKA approximately 10–15%; the Oxford medial UKA has among the best survivorship data of any UKA implant; lateral UKA has a higher revision rate than medial UKA (approximately 2× higher); the higher UKA revision rate vs TKA is partly explained by the less aggressive selection criteria used for UKA, leaving the patient susceptible to disease progression in unreplaced compartments
• Most common reasons for UKA revision: disease progression (OA in the lateral or patellofemoral compartment — the most common reason); aseptic loosening (tibial component more common); bearing dislocation (mobile bearing designs); polyethylene wear; unexplained pain; fracture; infection (lower rate than TKA)
• Conversion UKA to TKA: generally straightforward; outcomes of revision UKA to TKA are similar to primary TKA outcomes; this is a key advantage of UKA — bone stock preservation facilitates revision; however, if the tibial baseplate is undersized, an augmented tibial component may be needed at conversion
• Registry data interpretation: the NJR captures all revisions; UKA has a higher crude revision rate than TKA; however, when adjusted for age and sex (UKA patients are typically younger and more active), the UKA vs TKA revision rate difference narrows considerably

• Lateral UKA: technically more demanding than medial UKA due to the different kinematics of the lateral compartment (lateral femoral condyle rolls back further in flexion); the lateral compartment is convex-on-convex (femur and tibia both convex) — different from medial (convex femur on concave tibia); mobile bearing lateral UKA requires a different bearing design; higher revision rate than medial; indicated for isolated lateral compartment OA (less common than medial OA)
• Patellofemoral arthroplasty (PFA): replaces only the patellofemoral joint — an onlay (trochlear) component and patellar button; indicated for isolated symptomatic patellofemoral OA in a young patient with a preserved medial and lateral tibiofemoral compartment; historically poor results with first-generation implants; modern implants (e.g., Avon PFA) have improved outcomes and acceptably low revision rates at 10 years; the trochlear component must be well positioned to avoid patellar maltracking; disease progression into the tibiofemoral compartments remains the most common reason for revision

• UKA in the younger and more active patient: the Oxford group and registry data support UKA in younger (<60 years) and more active patients — the outcomes are excellent and the higher activity level does not significantly increase revision risk in most series; the Liddle et al. analysis of the NJR showed that patients undergoing UKA had higher activity levels and better functional outcomes at 5 years compared to matched TKA patients; UKA should not be withheld from younger patients on the basis of age or activity level alone
• The tibial component overhang problem: in UKA, oversizing the tibial tray to maximise bone coverage risks overhang into the soft tissues of the medial compartment; medial overhang causes pes anserinus bursitis and soft tissue irritation; the tray should sit within the cortical rim of the tibia; slight undersizing (with central fixation) is preferable to overhang
• Medial tibial plateau fracture after UKA: a rare but recognised complication; the tibial keel slot created for the tibial component weakens the medial tibial plateau; fracture is more common when the tibial tray is too vertically oriented, creating a stress riser; conservative management for non-displaced fractures; ORIF or revision for displaced fractures
• Unexplained pain after UKA: a challenging problem; workup — exclude PJI (aspirate), assess component positioning (CT for rotation, weight-bearing X-rays for alignment), assess for bearing problems, assess adjacent compartments for progression; overstuffing the medial compartment (tibial component too thick) produces pain and restricted flexion; a systematic approach mirrors the approach to the dissatisfied TKA

• Oxford UKA criteria: anteromedial OA pattern + correctable varus + intact ACL + flexion >100° + flexion contracture <15° + no symptomatic PFJ OA; age, obesity, activity NOT contraindications in the Oxford evidence base
• Mobile bearing (Oxford): ACL MUST be intact — absent ACL = bearing dislocation anteriorly; conforming at both surfaces; lower wear stress; fixed bearing tolerates ACL deficiency
• NJR: UKA 10-year revision ~10–15%; TKA ~4–5%; UKA has higher revision rate but conversion UKA → TKA achieves similar results to primary TKA; bone stock preserved
• Most common UKA revision reason: disease progression in unreplaced compartments (lateral OA, PFJ OA); then aseptic loosening
• Lateral UKA: technically harder; convex-on-convex geometry; higher revision rate than medial; lateral femoral condyle rolls back further in flexion — different bearing design required for mobile bearing
• PFA: isolated PFJ OA in young patients; Avon PFA; disease progression to tibiofemoral compartments is the main failure mode
• Anteromedial OA pattern: medial tibial cartilage wears anteriorly; posterior tibial cartilage preserved on frog-leg lateral; ACL intact; correctable varus — ideal UKA patient
• Bearing dislocation (mobile bearing): rare (0.5–1%); caused by ACL deficiency or inadequate posterior tibial slope; requires revision
• Medial tibial plateau fracture: rare complication of UKA; caused by stress riser at tibial keel; ORIF or revision for displaced fractures
• UKA in younger patients: supported by Oxford evidence and NJR data; age and activity NOT contraindications; better outcomes at 5 years vs matched TKA patients in Liddle et al.