Unicompartmental Knee Arthroplasty (UKA)

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Definition: Unicompartmental Knee Arthroplasty (UKA) involves replacing only the damaged compartment of the knee (medial or lateral), preserving the cruciate ligaments and the healthy compartments.

Epidemiology:

• Prevalence: Accounts for 10-15% of all knee replacements in most developed registries.
• Utilization: Highly variable. Usage ranges from 5% to 50% depending on surgeon philosophy and country. In Australia, utilization has stabilized around 8%.
• Gender: Equal distribution.
• Age: Bimodal distribution.
  • Young Active: Under 55. Goal is bone preservation and high function.
  • Elderly Frail: Over 80. Goal is less morbidity, quicker recovery, and lower risk of medical complications (MI/Stroke) compared to TKA.

Rationale: By preserving the ACL and PCL, UKA maintains near-normal knee kinematics and proprioception. This translates to a "forgotten knee" feeling more often than TKA.

• Kinematics: Retains the screw-home mechanism and femoral rollback.
• Function: Patients walk faster and have better gait analysis scores than TKA patients.
• Recovery: Shorter length of stay and faster return to work.

The Medial Compartment:

• Wear Pattern: Anteromedial wear is typical in varus OA with an intact ACL. The femoral wear is on the distal condyle, and tibial wear is central/posterior (often with a 'cup' defect).
• Obligatory Rotation: The tibia internally rotates during flexion. UKA designs must accommodate this screw-home mechanism.
• Meniscus: The medial meniscus is excised. The implant (polyethylene) effectively replaces meniscal function.

Kinematics (The ACL):

• Anterior Translation: The ACL prevents anterior tibial translation.
• In UKA: Without an ACL, the tibia subluxes anteriorly, causing eccentric loading on the posterior aspect of the UKA insert ("Rocking Horse").
• Result: Early loosening and failure. Hence, ACL integrity is non-negotiable.

Alignment Goals:

• Resurfacing: The goal is to resurface the joint line to its pre-disease state (constitutional varus).
• Coronal Plane: Tibial cut usually perpendicular to tibial mechanical axis. Femoral component aligned to restore joint line height.
• Sagittal Plane: Tibial slope must match native slope (usually 7 degrees posterior) to balance the flexion gap.

Keys to Classification for UKA:

• "Kissing Lesion": Bone-on-bone contact is ideal (confirms full thickness loss). Pain relief is more predictable when bone-on-bone is present than when some cartilage remains.
• "Anteromedial Wear": On lateral X-ray, wear should be anterior. Posterior wear suggests ACL deficiency.

History:

• Pain Location: "One finger test". Patient should point directly to the medial joint line. Generalized pain or retropatellar pain is a "red flag".
• Start Up Pain: Typical of OA.
• Mechanical Symptoms: Locking/Catching (meniscal) fits with UKA pathology.
• Instability: Giving way? (Suggests ACL deficiency - Contraindication).
• Patellofemoral Symptoms: Significant pain on stairs or rising from a chair? (Relative contraindication if severe).

Physical Examination:

• Gait: Antalgic? Varus thrust? (Thrust suggests dynamic instability).
• Alignment: Standing varus. Is it mild/moderate or severe?
• Range of Motion:
  • Check extension (FFD under 15 deg?). Fixed flexion is hard to correct.
  • Check flexion (Need greater than 110 deg for surgery exposure).
• Ligaments:
  • Lachman/Anterior Drawer: MUST be stable. A firm endpoint is required.
  • MCL: Valgus stress test. Must be competent (especially for mobile bearing). Open at 30 deg flexion but solid endpoint.
• Correctability: With knee in 20 deg flexion, apply valgus stress. Does the varus correct to neutral? If rigid, TKA is better.
• Other Compartments: Palpate lateral joint line and PF joint. Should be pain-free. Crepitus in PF joint with pain is concerning.

Standard Series:

1. Weight-Bearing AP: Assess medial joint space narrowing. Look for "bone-on-bone".
2. Lateral: Assess posterior wear (ACL status) and patellofemoral osteophytes.
3. Skyline: Assess PF joint. Lateral facet wear? (Lateral facet wear is contraindication. Medial facet is debatable).
4. 45-Degree PA (Rosenberg): Most sensitive view for posterior wear.

Stress Views:

• Valgus Stress X-ray: Confirms lateral compartment cartilage thickness is maintained. Confirms correctability of varus. This is the "Gold Standard" investigation for candidacy.
• Varus Stress X-ray: To check if medial gap opens (MCL integrity) - less common.

MRI:

• Not routinely needed if X-rays are classic.
• Role: Verify ACL integrity if Lachman is equivocal. Check lateral cartilage if stress view not available. Check PF cartilage.
• Pearl: MRI often "over-calls" damage. Normal aging cartilage changes in lateral compartment on MRI do not necessarily preclude UKA if X-ray is normal.

Bone Scan (Spect-CT):

• Can help identifying the "pain generator" if X-rays are mild (e.g., Grade 2 OA with unexplained severe pain).

Bearing Dislocation (Mobile Bearing):

• Cause: Flexion gap too loose, or MCL injury, or impingement on osteophytes.
• Presentation: Sudden pain, locking, lump.
• Treatment: Operative. Exchange bearing for thicker one (if loose).

Periprosthetic Fracture:

• Tibial: Vertical shear fracture through the keel slot. Usually intra-operative or stress fracture post-op.

Progression of OA:

• The most common "late" cause of failure. Lateral compartment wears out over 10-15 years.
• Requires conversion to TKA.

Survival:

• Excellent survivorship in designer series (Oxford greater than 95% at 10 years).
• Registry data shows lower survivorship (85-90% at 10 years) compared to TKA.
• Why?: Lower threshold to revise (painful UKA is easily revised to TKA). TKA revision is a bigger deal, so surgeons/patients tolerate more pain before revising.

Function:

• UKA consistently scores better on Forgotten Knee Score (FKS).
• Better ROM (usually greater than 120 compared to 110 for TKA).
• More normal gait pattern.

Registry Data (AOANJRR):

• UKA usage in Australia is stable (approx 8% of primary knees).
• Revision Burden: 12.8% at 15 years (vs 6% for TKA). Significantly higher for UKA.
• Revision Diagnosis: Aseptic loosening is #1 (30%), Progression of disease #2 (28%).
• Robotic UKA: Increasing adoption. AOANJRR data shows slightly lower revision rate for robotic UKA at short term (2 years), but long term data pending.
• Surgeon Volume: Strong correlation. Surgeons doing greater than 20/year have 50% lower revision rate than those doing less than 5/year.

Guidelines:

• ACSQHC Clinical Care Standard for Knee OA: Recommend discussion of higher revision rate vs better functional scores.