Introduction / Definition

Unicompartmental Knee Replacement (UKR) is a targeted arthroplasty procedure designed to resurface isolated single-compartment arthropathy of the knee. The procedure can be performed for medial compartment osteoarthritis (anteromedial osteoarthritis, AMOA), lateral compartment disease, or isolated patellofemoral osteoarthritis.

Compared to total knee replacement (TKR), UKR preserves both cruciate ligaments, the opposing tibiofemoral compartment, and the patellofemoral joint. This confers key advantages, including more physiological knee kinematics, enhanced proprioception, faster rehabilitation, lower perioperative morbidity and mortality, and higher patient-reported functional satisfaction scores.

Epidemiology

• Approximately 8% to 10% of all primary knee arthroplasties performed in national registries (such as the UK National Joint Registry) are UKRs, although up to 40% to 50% of knee arthroplasty candidates anatomically meet the criteria for partial replacement.
• Medial compartment UKR accounts for approximately 90% of all unicompartmental replacements; lateral UKR accounts for roughly 5% to 10%.
• High-volume centres and specialist surgeons achieve lower revision rates and 10-year implant survivorship rates exceeding 90% to 95%, whereas registry-wide UKR revision rates remain approximately double that of primary TKR, predominantly influenced by low surgeon caseload, lower threshold for revision, and aseptic loosening.

Relevant Anatomy & Biomechanics

• Cruciate Ligament Competence: Successful medial UKR requires a functionally intact anterior cruciate ligament (ACL). In full-thickness AMOA, erosion is restricted to the anterior and middle thirds of the medial tibial plateau, as an intact ACL prevents posterior tibial subluxation. Posteromedial tibial bone loss indicates ACL deficiency, which allows abnormal anterior-posterior shear stresses and leads to early mechanical failure.
• Collateral Ligaments: The medial collateral ligament (MCL) must not be formally released during medial UKR. Soft tissue balance is maintained through precise bone resections and implant thickness selection rather than ligament release.
• Kinematics: Medial UKR maintains the physiological "screw-home" mechanism and physiological femorotibial roll-back governed by intact cruciate guides.

Classification & Indications

Classic Kozinn & Scott Criteria (1989)

Historically restrictive criteria that defined early indications:

• Age > 60 years
• Body weight < 82 kg
• Low activity demands / sedentary lifestyle
• Angular deformity < 10° varus or < 5° valgus (passively correctable)
• Flexion contracture < 5° with minimum arc of flexion ≥ 90°
• Absence of patellofemoral or lateral compartment symptoms
• Intact ACL

Modern Oxford Indications (Expanded Criteria)

Subsequent extensive long-term cohort studies have substantially challenged and revised these historical limits:

• Age & Weight: Age (< 60 years) and elevated BMI (> 30 or even > 40 kg/m²) are no longer considered absolute contraindications; implant survivorship has been shown to be comparable across weight categories.
• Patellofemoral Joint (PFJ) Arthritis: Asymptomatic PFJ osteoarthritis, medial facet erosion, or trochlear wear are acceptable. Only severe lateral patellar facet bone-on-bone disease with lateral subluxation or grooving is a contraindication.
• Chondrocalcinosis: Asymptomatic chondrocalcinosis without widespread inflammatory disease is not a contraindication.

Absolute Contraindications

• Active intra-articular or systemic sepsis
• Inflammatory arthropathy (e.g., rheumatoid arthritis)
• Functionally incompetent ACL (for standard mobile-bearing medial UKR)
• Full-thickness chondral loss in the opposing (lateral) tibiofemoral compartment
• Fixed (non-correctable) varus deformity > 10° or valgus > 5°
• Fixed flexion contracture > 15°

Clinical Presentation & Examination

• History: Pain well-localised to the involved compartment during weight-bearing activities (e.g., medial joint line pain on level walking, stairs). True mechanical catching or locked knee suggests associated meniscal tearing or loose bodies.
• Alignment: Assess limb axis while standing. Deformity must be passively correctable on manual stress testing (demonstrating preservation of contralateral joint space and absence of structural contractures).
• Ligamentous Examination: Confirm ACL integrity using the Lachman and anterior drawer tests. Assess collateral ligament competence (MCL and LCL stability at 0° and 30° flexion).
• Patellofemoral Examination: Assess for localized anterior knee pain, patellar tracking, and specific lateral facet tenderness.

Investigations

• Plain Radiographs (Standard Views):
  • Weight-bearing anteroposterior (AP) and true lateral views
  • 45° flexion posteroanterior (Rosenberg view) to detect posteroinferior femoral condylar chondral loss
  • Skyline (Merchant) view to assess lateral patellar tilt, subluxation, and lateral facet preservation
• Stress Radiographs:
  • Valgus stress views in 20° flexion: Confirms full thickness cartilage loss medially, full thickness preservation of lateral compartment cartilage, and correctability of varus deformity.
• Magnetic Resonance Imaging (MRI):
  • Generally reserved for equivocal presentations; useful to verify ACL integrity, osteonecrosis (SONK), and the condition of cartilage in the unaffected compartments.

Management

Non-Operative Management

• First-line for mild-to-moderate disease:
  • Patient education, weight reduction, and low-impact aerobic exercise
  • Unloader knee bracing (valgus unloader for medial compartment disease)
  • Non-steroidal anti-inflammatory drugs (NSAIDs) and intra-articular corticosteroids or hyaluronic acid

Operative Management

Surgical Approaches

• Minimally Invasive Medial Parapatellar Approach: Extends from the superior pole of the patella to approximately 1–2 cm distal to the tibial joint line. Subvastus or limited parapatellar capsulotomy avoids eversion of the extensor mechanism, preserving quadriceps function.
• Lateral Subvastus or Parapatellar Approach: Used for isolated lateral UKR, which features differing kinematics (mobile anterior-posterior lateral translation).

Bearing Designs

• Mobile-Bearing (e.g., Oxford UKR):
  • Fully conforming, unconstrained spherical femoral component articulating with a free-floating ultra-high-molecular-weight polyethylene (UHMWPE) bearing on a flat tibial tray.
  • Maximises contact area, dramatically minimising contact stress and linear polyethylene wear.
  • Requires strict ligamentous balance and an intact ACL to prevent bearing dislocation.
• Fixed-Bearing UKR:
  • Polyethylene insert is locked into the tibial baseplate; femoral geometry is typically flat or multi-radius.
  • Tolerates minor ACL laxity better than mobile bearings and eliminates bearing dislocation risks.
  • Associated with higher local contact stresses and theoretically increased long-term wear rates.

Fixation Options

• Cemented Fixation: Historical gold standard with predictable long-term survivorship.
• Cementless Fixation: Porous titanium and hydroxyapatite-coated components. Minimises operative time, eliminates cement extrusion loose bodies, and demonstrates reduced early radiolucent lines compared to cemented interfaces.

Intra-Operative Alignment Principles

• Under-correction is deliberate: Reconstruct to the patient's pre-arthritic constitutional alignment (typically 1° to 3° of residual varus).
• Overcorrection into mechanical valgus shifts load into the lateral compartment, predisposing to rapid contralateral osteoarthritis progression.
• Avoid penetrating the posterior tibial cortex with pins or aggressive sagittal recuts to prevent catastrophic tibial plateau fracture.

Complications

• Aseptic Loosening: Most common mode of long-term failure (historically accounts for 25% to 45% of revisions). Radiolucent lines around cemented tibial components require careful differentiation between physiological stress-shielding (surrounded by sclerotic lines < 2 mm) and pathological loosening.
• Osteoarthritis Progression: In the unresurfaced compartments (predominantly lateral compartment), often accelerated by surgical overcorrection.
• Bearing Dislocation: Specific to mobile-bearing designs (incidence ~0.5%–1.5% medially, higher laterally). Commonly precipitated by retained cement fragments, ligamentous laxity, improper component seating, or gap imbalance.
• Periprosthetic Tibial Fracture: Associated with excessive tibial resection depth, multiple re-drilled guide pin holes, cortical perforation, or forceful impaction.
• Infection: Overall rates are substantially lower than TKR (< 1%).

Exam Pearls

• Registry vs RCT Paradox: Large randomised controlled trials (e.g., TOPKAT) demonstrate equivalent clinical efficacy, superior cost-effectiveness, and fewer perioperative medical complications for UKR compared with TKR, yet national registries demonstrate consistently higher long-term revision rates for UKR (attributed to a lower threshold for revision and surgeon volume effects).
• The ACL Rule: An intact, functionally competent ACL is mandatory for mobile-bearing medial UKR; posteromedial bone loss signifies ACL deficiency and is a contraindication.
• Safe Correction Target: Aim for slight undercorrection (leaving 1°–4° of constitutional varus). Never correct into valgus.
• Physiological Radiolucency: A thin (< 2 mm), stable radiolucent line bounded by a sclerotic margin under the tibial tray on postoperative radiographs is a common fibrocartilaginous adaptation in cemented UKR and does not denote aseptic loosening in an asymptomatic patient.