Unicompartmental vs Total Knee Arthroplasty — Indications & Outcomes

The choice between unicompartmental knee arthroplasty (UKA) and total knee arthroplasty (TKA) for isolated single-compartment knee osteoarthritis is one of the most debated questions in arthroplasty surgery. UKA resects and replaces only the affected compartment (medial, lateral, or patellofemoral), preserving cruciate ligaments, the opposite compartment, and native bone stock. TKA replaces all three compartments of the tibio-femoral and typically the patellofemoral joint. UKA offers theoretical advantages — more physiological kinematics, faster recovery, lower perioperative morbidity, and preserved bone stock for future revision. However, UKA has a consistently higher revision rate than TKA in national joint registries, largely due to disease progression in the unreplaced compartment and a narrower patient selection window.

• Classic Kozinn and Scott criteria for UKA (1989): the original selection criteria for medial UKA — (1) isolated medial compartment OA; (2) intact ACL (essential — loss of ACL leads to abnormal kinematics and medial compartment overloading after UKA, accelerating tibial loosening); (3) correctable varus deformity ≤10° (the deformity must be passively correctable — a fixed varus deformity indicates contracture of the lateral structures and is a relative contraindication); (4) no inflammatory arthritis; (5) age >60 years and low activity level; (6) BMI <32 (obesity was listed as a relative contraindication — though modern evidence has challenged this); (7) no patellofemoral OA symptoms (though modern evidence suggests the patellofemoral criteria have been overstated)
• Evolution of criteria — the Oxford criteria (Goodfellow and O`Connor): the Oxford Phase 3 UKA was designed for anteromedial OA (OA of the medial compartment with a bone-on-bone medial compartment, preserved lateral compartment, and intact ACL); the Oxford group advocate a more liberal selection — intact ACL, full-thickness medial OA (bone on bone), correctable varus, preserved lateral compartment; they argue that age, obesity, and patellofemoral OA are not contraindications if these core criteria are met; the `Oxford criteria` have significantly broadened UKA indications in contemporary practice
• Lateral UKA: less common than medial UKA (~10% of UKA); lateral compartment OA has a different pathoanatomy — the lateral compartment has greater mobility (lateral femoral condyle translates more than medial during flexion) and the deformity is typically valgus; lateral UKA requires careful understanding of lateral compartment kinematics; Oxford mobile bearing lateral UKA and fixed bearing designs are used; outcomes are broadly comparable to medial UKA in selected patients

• Aseptic loosening: the most common mode of failure in fixed bearing UKA; tibial component loosening is more common than femoral loosening; caused by micromotion at the bone-implant interface from malalignment, undersizing, or poor bone quality; risk is reduced by accurate component positioning (robotic UKA evidence — see robotic arthroplasty article) and appropriate patient selection
• Disease progression in the unreplaced compartment: OA develops in the lateral compartment (for medial UKA) or patellofemoral joint over time; the rate of progression is approximately 1% per year; patients with pre-operative lateral or patellofemoral changes are at higher risk; this is the second most common cause of revision and the main argument for the higher long-term revision rate of UKA vs TKA
• Mobile bearing dislocation (Oxford UKA): the Oxford UKA uses a mobile polyethylene bearing that glides on the tibial platform and on the femoral component; bearing dislocation occurs when the bearing spins out of position — typically due to ACL laxity or deficiency (allowing abnormal tibio-femoral kinematics that displace the bearing); this is why an intact functional ACL is an absolute requirement for mobile bearing Oxford UKA; bearing dislocation requires urgent revision to replace the bearing or convert to TKA
• Unexplained pain: a distinct failure mode of UKA (and TKA); the patient has persistent pain without identifiable mechanical or infective cause; more common after UKA than TKA; may reflect ongoing patellofemoral or contralateral compartment symptoms that were not identified pre-operatively
• Tibial plateau fracture: a rare but recognised complication of UKA — the tibial component peg or keel can create a stress riser in the proximal tibia; a periprosthetic tibial fracture may occur especially with malaligned tibial components or excessive tibial resection

• The UKA paradox: UKA consistently produces superior patient-reported functional outcomes and higher satisfaction compared to TKA in well-selected patients, yet has a higher revision rate; this apparent contradiction is partly explained by the lower threshold for UKA revision — a UKA with unexplained pain or mild disease progression is more readily revised to TKA than a TKA would be revised (the revision of UKA to TKA is less complex and carries lower patient risk, so surgeons and patients have a lower revision threshold for UKA); the higher revision rate of UKA therefore reflects the lower revision threshold, not necessarily inferior implant performance
• The Oxford group argument: Liddle et al. and Murray et al. from the Oxford group argue that the apparent survival disadvantage of UKA in registry data is partly a statistical artefact — UKA cases that fail and are revised are captured in the denominator; the denominator of `at-risk` UKA cases in registries includes many cases performed by low-volume surgeons on suboptimal patient selections; high-volume specialist UKA centres report 10-year survival rates of 95–98% — comparable to TKA; UKA outcomes are highly volume-dependent

• UKA indications (Kozinn & Scott): isolated compartment OA; intact ACL; correctable varus ≤10°; no inflammatory arthritis; age >60; BMI <32; no PF symptoms; Oxford criteria more liberal — bone-on-bone medial OA + intact ACL + correctable deformity; age/obesity/PF OA less strict contraindications
• ACL integrity: ABSOLUTE requirement for mobile bearing Oxford UKA; ACL deficiency → abnormal kinematics → bearing dislocation → failure; also important for fixed bearing (though less absolute)
• NJR revision rates: UKA ~8–12% at 10 years vs TKA ~4–6% at 10 years; UKA consistently higher; main causes — disease progression, aseptic loosening, bearing dislocation, unexplained pain
• UKA advantages: superior PROMs and satisfaction; faster recovery; day-case feasible; lower 90-day mortality; preserves bone stock (revision to TKA = primary TKA complexity); more natural knee feel
• The UKA paradox: better function + higher revision rate; lower revision threshold (UKA→TKA is easier and safer than TKA revision); high-volume specialist centres report 95–98% 10-year survival comparable to TKA
• Mobile bearing dislocation: Oxford UKA specific; ACL deficiency most common cause; urgent revision required; reinforces ACL as absolute requirement
• Disease progression: ~1% per year in unreplaced compartment; main driver of late UKA revision; pre-existing lateral or PF OA increases risk; select patients without lateral compartment involvement
• Revision UKA to TKA: straightforward — comparable to primary TKA in most cases; bone stock preserved; stems and augments rarely required; contrast with TKA revision which is significantly more complex
• Robotic UKA: strongest evidence for robotic arthroplasty; reduces component positioning outliers; NJR showing lower revision rates for robotic UKA approaching TKA rates
• Lateral UKA: ~10% of UKA; valgus deformity; lateral compartment more mobile; Oxford lateral and fixed bearing designs; comparable outcomes to medial UKA