Unicompartmental Knee Replacement (UKR/Oxford)

Step 1: Patient Positioning and Preparation

Position: Supine with operative leg free to flex and extend. Leg hanging off side of table or in leg holder.

Lateral post: Position at mid-thigh level to allow valgus stress during tibial preparation.

Tourniquet: Optional - many surgeons prefer bloodless field for visualization (250-300 mmHg).

Draping: Standard knee draping allowing full access from mid-thigh to ankle.

Step 2: Skin Incision and Arthrotomy

Incision: MINIMALLY INVASIVE 6-8cm medial parapatellar incision (contrast with 15-20cm for TKR).

Start at superior pole of patella, extend distally over medial border to 1cm below joint line.

Approach options:

• Medial parapatellar: Standard, versatile
• Subvastus: Preserves extensor mechanism, better proprioception, limited in obese patients

Arthrotomy: Capsular incision at joint line. Minimal or NO patellar eversion needed.

Fat pad: Preserve to protect patellar blood supply.

Step 3: Intraoperative Assessment (CRITICAL DECISION POINT)

ACL INTEGRITY: Test with Lachman - ABSOLUTE requirement. If deficient, ABORT UKR and convert to TKR.

Opposite compartment: Assess cartilage - Outerbridge grade 2 acceptable, grade 3-4 is relative contraindication.

Patellofemoral joint: Assess cartilage and tracking - significant disease suggests TKR better option.

Deformity: Confirm correctable to neutral with manual stress.

Decision: If ACL deficient OR tricompartmental disease - STOP procedure and convert to TKR.

Step 4: Osteophyte Removal

Remove ALL peripheral osteophytes systematically:

• Anterior femoral osteophytes - prevent impingement in extension
• Posterior femoral osteophytes - prevent impingement in flexion
• Tibial rim osteophytes - medial and lateral
• Tibial spine osteophytes - clear around ACL attachments

Purpose: Reveals true joint space, allows assessment of correctable deformity, prevents bearing impingement.

Caution: Preserve ACL tibial and femoral attachments - stay peripheral to ligament insertions.

Step 5: Tibial Resection (CRITICAL STEP)

Principles for Mobile Bearing (Oxford):

• MINIMAL resection - aim to remove only enough to seat the thinnest bearing, referencing the tibial saw guide off the intact lateral plateau, NOT off the eroded medial surface
• The cut is made in two planes: a vertical sagittal cut just medial to the ACL insertion / tibial spine, and a transverse (horizontal) cut set with a small posterior slope (about 7°) by the tibial cutting guide, in the coronal plane perpendicular to the tibial axis
• Common misconception to AVOID stating: the Oxford bearing is a flat-on-flat unconstrained design - it has NO built-in slope, so the surgeon must deliberately reproduce the natural posterior slope with the guide
• Preserve the tibial eminence and the ACL footprint

Technique:

1. Set the extramedullary tibial guide parallel to the tibial shaft with the appropriate posterior slope dialled in (about 7°, neutral coronal alignment)
2. Make the vertical sagittal cut with a reciprocating saw directed toward the head of the fibula / hip, hugging the medial side of the tibial spine to protect the ACL
3. Make the transverse cut with the oscillating saw, removing minimal bone
4. Retrieve the resected fragment as a single piece to confirm depth

Why the small posterior slope: it reproduces native tibial slope, optimises flexion-gap kinematics and avoids impingement; excessive slope endangers the ACL and posterior cortex.

Step 6: Tibial Component Positioning and Trialing

Position principles:

• Cover entire tibial resection surface
• Sit on strong cortical rim (avoid overhang)
• Vertical alignment matches tibial shaft axis
• Rotation neutral (parallel to anteroposterior axis)

Technique:

1. Place tibial trial on resected surface
2. Position medially (for medial UKR) to cover cut bone
3. Check no overhang (soft tissue irritation)
4. Verify vertical alignment with rod down tibial shaft
5. Mark position with drill holes or marker

Oxford technique: Use sizing spoon to determine optimal tibial component size.

Step 7: Femoral Preparation and Sizing

Sizing principle: Match native femoral condyle size (AP dimension). Do NOT oversize.

Oxford technique (unique feature):

1. Femoral drilling guide sits on tibial resection surface
2. Links femoral component position to tibial cut
3. Maintains balanced flexion-extension gaps automatically
4. Pin placement determines femoral component rotation and AP position

Steps:

1. Place femoral sizing guide on exposed condyle
2. Select size matching native condyle AP dimension
3. Position drilling guide on tibial cut
4. Insert guide pins for femoral component
5. Prepare femoral condyle with milling/sawing per technique
6. Check no anterior femoral notching (fracture risk)

Step 8: Bearing Selection (CRITICAL FOR MOBILE BEARING)

Measurement technique:

1. Insert trial femoral and tibial components
2. Flex knee to 90° (CRITICAL - measure in flexion not extension)
3. Use spacer blocks/feeler gauges to measure gap
4. Select bearing thickness that fills gap with slight tension

Target balance:

• Gap in 90° flexion: Snug fit with no gross laxity
• Extension: 1-2mm opening acceptable (mobile bearing self-centers)
• Should resist distraction but not be over-tight

Mobile bearing specifics:

• Polyethylene bearing moves freely between components
• Self-centering design accommodates slight laxity
• Too thick: Stiffness, pain, cartilage damage opposite compartment
• Too thin: Instability, dislocation, abnormal wear

Step 9: Trial Reduction and Assessment

Systematic trial assessment:

1. Insert trial components with appropriate bearing thickness
2. Take knee through full ROM (0° extension to 135° flexion)
3. Check bearing stability - should not dislocate or move excessively
4. Assess for impingement - anterior in extension, posterior in flexion
5. Patella tracking - should track smoothly without lateral tilt
6. Gap opening in extension - 1-2mm laxity acceptable for mobile bearing

Red flags requiring adjustment:

• Bearing dislocation on ROM - undersized or malpositioned
• Unable to achieve full extension - overstuffed or anterior impingement
• Tight in flexion - oversized bearing or posterior impingement
• Gross instability - undersized bearing

Step 10: Bone Preparation and Cementation

Preparation:

1. Remove trial components
2. Pulse lavage bone surfaces thoroughly (removes debris, blood, fat)
3. Dry with suction and gauze - cement bonds best to dry bone
4. Pack wound to keep field dry during cement mixing

Cementation sequence:

1. Mix cement to appropriate consistency
2. Cement tibial component FIRST
   • Apply cement to bone surface
   • PRESSURIZE with finger/pressurizer into cancellous bone
   • Seat tibial component in marked position
   • Hold firmly until cement sets
3. Cement femoral component SECOND
   • Apply cement to bone cuts
   • Pressurize into cancellous bone
   • Seat component, ensure no cement interposition
   • Hold position until cement cures
4. Remove ALL excess cement meticulously
   • Anterior recess
   • Posterior capsule
   • Intercondylar notch
   • Tibial cut margins

Step 11: Final Bearing Insertion and Assessment

Bearing insertion:

1. Ensure all cement fully cured
2. Remove any remaining debris
3. Copious irrigation
4. Insert final polyethylene bearing (size determined during trialing)
5. Bearing should click into position (mobile bearing designs)

Final assessment:

• Full ROM: 0° extension to 135° flexion
• Stable bearing through full ROM - no dislocation
• No impingement in extension or flexion
• Patella tracks smoothly
• Bearing self-centers during movement
• No clicking or catching

Step 12: Closure and Post-procedure Protocol

Closure:

1. Copious irrigation (3L minimum)
2. Achieve hemostasis (minimize hematoma)
3. Repair capsule carefully with absorbable sutures
4. Repair retinaculum and subcutaneous layer
5. Skin closure (subcuticular or staples)
6. Infiltrate local anesthetic (improves pain control)
7. Sterile dressing and wool-and-crepe

Drain: Usually not required (minimally invasive approach with less dead space).

Post-procedure:

• Weight bearing as tolerated IMMEDIATELY
• ROM exercises same day
• Often outpatient or 1-night stay
• Early mobilization to prevent stiffness
• DVT prophylaxis per protocol

Immediate Post-operative (Day 0-1)

• Mobilization: WBAT immediately with crutches/walker
• ROM exercises: Begin same day - active assisted flexion/extension
• Ice and elevation: Reduce swelling
• Pain control: Multimodal analgesia (paracetamol, NSAIDs, opioids prn)
• DVT prophylaxis: LMWH or DOAC per local protocol
• Discharge: Often same day or overnight stay (contrast with 3-5 days for TKR)

Early Phase (Week 1-6)

• Weight bearing: Progress to full weight bearing, wean aids
• ROM: Target 0-120° by 6 weeks
• Physiotherapy: Quadriceps strengthening, proprioception exercises
• Activities: Stairs, return to driving at 2-3 weeks (right knee sooner)
• Follow-up: Wound check at 2 weeks, remove sutures/staples

Intermediate Phase (Week 6-12)

• ROM: Achieve 0-135° (usually exceeds TKR outcomes)
• Strength: Progressive resistance exercises
• Function: Return to light activities, walking unlimited
• X-rays: Standing AP/lateral at 6 weeks to assess components

Late Phase (3-12 months)

• Activities: Return to low-impact sports (golf, swimming, cycling) at 3 months
• High-impact: Avoid running, jumping sports (increases failure risk)
• Function: Most patients report "normal" or "near-normal" knee (better than TKR)
• Follow-up: Review at 3, 6, 12 months, then annually

Long-term Surveillance

• Annual X-rays: Assess for progression of OA, component loosening
• Survival: pooled ~93-94% at 10 years and ~89% at 15 years for medial Oxford UKR (Mohammad et al; Hamilton et al), with results dependent on correct selection and surgeon usage
• Revision: registries show roughly double the revision rate of TKR (Liddle et al), most often for lateral OA progression, aseptic loosening or bearing dislocation; conversion to TKR is generally straightforward when bone stock is preserved

References

1. National Joint Registry for England, Wales, Northern Ireland and the Isle of Man (NJR) 2024 Annual Report. UK National Joint Registry. Available at: https://www.njrcentre.org.uk - Comprehensive registry data showing Oxford mobile bearing UKR with 95.3% implant survival at 10 years.

2. Beard DJ, Davies LJ, Cook JA, et al. The clinical and cost-effectiveness of total versus partial knee replacement in patients with medial compartment osteoarthritis (TOPKAT): 5-year outcomes of a randomised controlled trial. Lancet. 2019;394(10200):746-756. Level 1 evidence comparing UKR to TKR in appropriate patients.

3. Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) 2024 Annual Report. Adelaide: AOA. Available at: https://aoanjrr.sahmri.com - Australian-specific UKR data showing 14.2% cumulative revision rate at 10 years, Oxford mobile bearing most common design.

4. Mohammad HR, Strickland L, Hamilton TW, Murray DW. Long-term outcomes of over 8,000 medial Oxford Phase 3 Unicompartmental Knees - a systematic review. Acta Orthop. 2018;89(1):101-107. Systematic review demonstrating excellent long-term survivorship with appropriate patient selection.

5. Kozinn SC, Scott R. Unicondylar knee arthroplasty. J Bone Joint Surg Am. 1989;71(1):145-150. Classic paper establishing patient selection criteria for UKR - age over 60, weight under 82kg, low demand.

6. Goodfellow JW, O'Connor JJ, Murray DW. The Oxford Meniscal Unicompartmental Knee. J Knee Surg. 2010;23(1):3-9. Foundational paper on Oxford mobile bearing design principles and biomechanics.

7. Pandit H, Jenkins C, Gill HS, et al. Unnecessary contraindications for mobile-bearing unicompartmental knee replacement. J Bone Joint Surg Br. 2011;93-B(5):622-628. Challenges traditional contraindications and expands indications based on modern evidence.

8. Lygre SH, Espehaug B, Havelin LI, Furnes O, Vollset SE. Does patellofemoral joint status influence survival of unicompartmental knee arthroplasty? Acta Orthop. 2010;81(3):337-343. Registry study examining impact of PFJ arthritis on UKR outcomes.

9. Edmondson M, Atrey A, East D, et al. Survival analysis and functional outcome of the Oxford unicompartmental knee replacement up to 11 years follow up at a District General Hospital. J Orthop. 2015;12(Suppl 1):S105-S110. Real-world outcomes from district hospital showing reproducibility of registry results.

10. Hamilton TW, Rizkalla JM, Kontochristos L, et al. The Interaction of Caseload and Usage in Determining Outcomes of Unicompartmental Knee Arthroplasty: A Meta-Analysis. J Arthroplasty. 2017;32(10):3228-3237. Meta-analysis demonstrating importance of surgeon experience and patient selection for UKR outcomes - higher volume surgeons have better results.