Unicompartmental Knee Arthroplasty (UKA)

Unicompartmental knee arthroplasty (UKA) resurfaces a single compartment of the knee — most commonly the medial compartment — while preserving the cruciate ligaments, the native lateral (or medial) compartment, and the patellofemoral joint. When correctly indicated and performed, UKA provides excellent functional outcomes with faster rehabilitation, lower perioperative mortality, and better patient-reported outcomes than total knee arthroplasty (TKA) in the short and medium term, while preserving the option of future TKA if required.

• Medial UKA accounts for approximately 90% of UKA procedures; lateral UKA approximately 8%; patellofemoral arthroplasty (PFA) approximately 2%
• UKA as a proportion of all knee arthroplasty: approximately 8–12% in the UK (NJR); much lower in other countries; significant variation between surgeons and centres
• Lower perioperative mortality, faster recovery, better proprioception, and superior range of motion compared to TKA — these advantages are well-established in registry data and RCTs; trade-off is higher revision rate (approximately double that of TKA at 10 years in registry data)
• The revision penalty: NJR data shows higher revision rate for UKA vs TKA at 10 years; however, most revisions are to TKA and are technically straightforward; when accounting for reoperation rate (including manipulations), UKA and TKA have more comparable overall reoperation rates
• High-volume surgeon and centre effect: revision rate for UKA is strongly inversely correlated with surgeon volume — lower volume surgeons have significantly higher revision rates; UKA should be concentrated in high-volume units

Careful patient selection is the single most important determinant of UKA outcome. The Oxford criteria for medial UKA, developed with the Oxford Phase 3 mobile-bearing UKA, represent the most widely used selection framework.

• Age, weight, and activity level are NOT absolute contraindications in the Oxford criteria — evidence shows equivalent outcomes in young, heavy, and active patients; the common assumption that UKA is only for elderly low-demand patients is not supported by modern data from high-volume centres
• Inflammatory arthritis: absolute contraindication — polyarticular disease; bilateral compartment involvement; potential for progression
• Chondrocalcinosis: relative contraindication — increased risk of progression to lateral compartment OA; some centres avoid UKA in confirmed CPPD

• Mobile bearing dislocation: most feared UKA-specific complication; bearing displaces anteriorly when ACL is absent or in extension instability; presents as sudden severe pain and inability to flex knee; requires open or arthroscopic relocation; revision to TKA if recurrent
• Oxford UKA results from high-volume centres (Oxford, Nuffield): 95% survival at 10 years, 91% at 20 years — among the best reported UKA survivorship worldwide; considerably lower in NJR registry data (≈82–85% at 10 years) due to lower-volume surgeon inclusion

• Minimally invasive approach (MIS): standard for Oxford UKA — small medial parapatellar incision (8–10 cm); patellar eversion not required; preserves patellofemoral mechanics; faster recovery
• Tibial resection: flat cut perpendicular to the tibial axis in the coronal plane; approximately 3–4 mm of bone resected; preserve tibial bone stock; do not resect excessively — reduces fixation surface
• Femoral component positioning: mill-guided femoral preparation; component placed to restore native joint line; flexion-extension gap balance essential
• Alignment target for medial UKA: aim for slight undercorrection of varus — target 2–3° of residual varus (not full neutral correction); overcorrection to neutral or valgus shifts load to the lateral compartment, accelerating lateral OA progression and increasing revision risk
• Cement fixation: standard for Oxford UKA and most fixed-bearing systems; cementless Oxford UKA now available — early data shows comparable fixation with potential bone stock preservation benefit
• Robotic-assisted UKA (MAKO): increasingly popular; pre-operative CT-based plan; intraoperative haptic boundary guidance; evidence of improved component positioning accuracy; equivalent or superior early functional outcomes; higher cost

• Revision UKA to TKA: generally straightforward — tibial defect from UKA component is usually small; standard primary TKA components used in most cases; augments or stems occasionally needed; outcomes of revision to TKA are inferior to primary TKA but acceptable

• Lateral UKA: less commonly performed; lateral compartment OA is less common and often develops secondary to valgus deformity; indications similar to medial UKA; ACL integrity required for mobile bearing; femoral component positioning more complex due to lateral condyle anatomy; results comparable to medial UKA in specialist hands
• Patellofemoral arthroplasty (PFA): resurfaces trochlea and patella only; for isolated patellofemoral OA with intact tibiofemoral compartments; modern inlay designs (Journey PFJ, Femoro-patella Vialla) show better tracking than older onlay designs; 10-year survivorship approximately 75–80% NJR — lower than medial UKA; risk of progression to tibiofemoral OA is the main failure mode
• PFA indication: isolated patellofemoral OA confirmed on Merchant view and MRI; no medial or lateral compartment OA; failed conservative management including physiotherapy, patellofemoral taping, and unloader bracing

• The NJR revision penalty vs Oxford results discrepancy: NJR shows 10-year revision rate for UKA approximately twice that of TKA; however, Oxford series and other high-volume centres report 95% survivorship — this gap is explained by the strong volume-outcome relationship for UKA; low-volume surgeons drag registry data down; the procedure itself, properly performed, has excellent survivorship
• Medial tibial plateau fracture: most common intraoperative complication; occurs during tibial saw cut if too deep a resection is made medially, or if excessive force applied to the chisel; identify and fix immediately with screws ± bone graft; revise to TKA if comminuted; prevent by careful pre-operative templating and gentle surgical technique
• Component overhang: medial overhang of the tibial component causes medial soft tissue impingement and pain — one of the most common causes of unexplained medial pain after UKA; visible on AP X-ray; revision or arthroscopic trimming if symptomatic
• Cementless Oxford UKA: phase IV design with cementless tibial and femoral components; early data shows equivalent fixation to cemented at 5 years; potential advantage of bone stock preservation and easier revision; longer-term data awaited
• Bilateral simultaneous UKA: more controversial than bilateral TKA — blood loss and anaesthetic exposure lower than bilateral TKA; most units perform staged bilateral UKA; simultaneous bilateral UKA acceptable in selected low-risk patients at high-volume centres

• Oxford criteria: AMOA + intact ACL + correctable varus + medial disease only + flexion >90° + FFD <15°
• Age, weight, activity level NOT absolute contraindications in Oxford criteria — modern evidence supports UKA in young, active, heavier patients
• Intact ACL mandatory for mobile-bearing UKA — absent ACL = bearing dislocation risk
• Alignment target: 2–3° residual varus — undercorrect; overcorrection to neutral shifts load to lateral compartment
• Mobile bearing dislocation: anterior displacement, ACL deficiency; open/arthroscopic relocation; revision if recurrent
• Oxford Phase 3 survivorship: 95% at 10 years high-volume; 82–85% NJR (volume effect explains discrepancy)
• Most common revision cause: disease progression to lateral compartment OA (30–40%)
• Revision UKA to TKA: straightforward in most cases; minimal bone loss; outcomes inferior to primary TKA but acceptable
• Inflammatory arthritis: absolute contraindication to UKA
• Robotic UKA: improved component positioning accuracy; equivalent or superior early outcomes; higher cost