Unicompartmental Knee Arthroplasty (UKA)

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 under 5% to over 40% depending on surgeon philosophy and country, with most large registries clustering around 8-12% of primary knee arthroplasties.
• 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.

1. The registry-versus-designer-series paradox:

• Designer centres report ~96-99% 10-year survival (Pandit 2011), while national registries report ~85-90%. The gap is partly real (case selection, learning curve, low-volume surgeons) and partly an artefact: a painful UKA is far easier to revise to a primary TKA than to revise a TKA, so the threshold to "revise" is lower. Revision should not be read as a pure measure of clinical failure.

2. Cemented vs cementless fixation:

• Cementless mobile-bearing UKA was introduced to reduce radiolucent lines and aseptic loosening. Some series and registry signals suggest lower revision for cementless designs, but the evidence is not yet definitive and cementing errors remain a recognised cause of early failure.

3. Robotic / navigated UKA:

• Robotic assistance improves component-positioning accuracy and reduces outliers; early registry data hint at lower short-term revision. However, long-term survival benefit and cost-effectiveness are unproven, and the dominant determinant of outcome remains correct patient selection.

4. Patellofemoral OA as a contraindication:

• Historically any PFJ disease excluded UKA. The Oxford group's work suggests that medial PFJ changes and even some lateral facet damage WITHOUT severe lateral grooving do not compromise medial UKA outcome. Severe lateral PFJ facet wear with grooving remains a contraindication. This area is genuinely debated.

5. UKA vs HTO in the young, active patient:

• For the young heavy labourer, HTO preserves bone and permits unrestricted activity but has a longer recovery and complicates future arthroplasty. UKA offers faster, more predictable pain relief. A prior HTO markedly worsens later UKA survival (66% vs 96% at 10 years; Rees 2001), so sequencing matters.

6. Expanding indications:

• Modern anatomical (rather than demographic) selection has extended UKA to younger, heavier and more active patients. Whether this broadening will hold up in long-term registry survival — or simply shift more revisions into the data — is still being resolved.

Global epidemiology & utilisation:

• UKA accounts for roughly 8-12% of primary knee arthroplasty in most major registries, but usage varies enormously (under 5% to over 40%) between surgeons and health systems — a function of surgeon selection philosophy rather than disease prevalence.
• Isolated anteromedial OA with an intact ACL — the ideal UKA substrate — is present in a substantial minority of patients otherwise listed for TKA, so most registries report UKA as "under-used" relative to the ideal indication pool.

Registry evidence (side by side):

Guideline positions (side by side):

Historical selection criteria (Kozinn & Scott):

• The classic restrictive criteria (age over 60, weight under 82 kg, low activity, deformity under 10-15 degrees) were popularised by Kozinn & Scott. Modern evidence (TOPKAT, designer series) shows these age/weight limits are NOT independent predictors of failure when anatomical criteria are met — the field has moved toward anatomical, not demographic, selection.

High- vs limited-resource practice variation:

• High-resource settings: robotic/navigated UKA increasingly used to improve component positioning; early registry signals suggest comparable-to-lower short-term revision, but long-term and cost-effectiveness data remain immature. Day-case/outpatient UKA is well established.
• Limited-resource settings: UKA's smaller implant, shorter stay, lower transfusion need and faster rehabilitation are attractive, but the technique-sensitivity and need for adequate UKA volume mean outcomes depend heavily on surgeon experience; in low-volume centres TKA may be the more reproducible choice.