Residency · Residency · Orthopedic Surgery
Unicompartmental Knee Arthroplasty: Patient Selection and Outcomes
Introduction
Unicompartmental knee arthroplasty (UKA) is a bone-conserving alternative to total knee arthroplasty for patients with isolated single-compartment disease. When performed in appropriately selected patients, UKA offers faster recovery, more natural knee kinematics, and improved patient satisfaction compared to TKA. However, strict adherence to patient selection criteria is essential to avoid premature failure and revision.
Indications
UKA is indicated for isolated medial or lateral compartment osteoarthritis or osteonecrosis that has failed conservative management including activity modification, anti-inflammatory medications, injections, and bracing. Medial UKA is far more commonly performed than lateral UKA, reflecting the higher prevalence of medial compartment disease. Spontaneous osteonecrosis of the knee (SONK) affecting a single compartment is an excellent indication.
Patient Selection Criteria
Classic Criteria (Modified Kozinn and Scott)
The classic criteria require a correctable deformity (varus or valgus that corrects passively on stress radiographs), intact cruciate ligaments (a competent ACL is traditionally considered essential for medial UKA), minimal patellofemoral disease (no significant bone-on-bone changes), a preserved opposite compartment (intact articular cartilage in the uninvolved compartment), range of motion with a flexion contracture less than 10 to 15 degrees and flexion greater than 90 degrees, and a BMI historically recommended below 30 to 35 kg/m squared (though modern data suggest outcomes may be acceptable in moderately obese patients).
Expanded Indications (Oxford Group)
The Oxford group has broadened indications to include patients with anterior knee pain, patellofemoral cartilage changes (if not bone-on-bone), and ACL deficiency (for lateral UKA). Age and activity level are not absolute contraindications, and outcomes depend more on disease pattern than on demographic factors.
Contraindications
Contraindications include inflammatory arthritis (rheumatoid, psoriatic), tricompartmental disease, fixed varus or valgus deformity exceeding 15 degrees, ligamentous instability (particularly ACL deficiency for medial UKA), and prior high tibial osteotomy (relative contraindication).
Implant Design
Fixed-Bearing UKA
In fixed-bearing designs, the polyethylene insert is locked into the tibial tray. This design is more forgiving of minor component malalignment and carries a lower risk of bearing dislocation.
Mobile-Bearing UKA
In mobile-bearing designs, the polyethylene meniscal bearing is free to glide on a polished tibial tray. This achieves full congruency in all positions, reducing polyethylene contact stress and wear rates, but carries a risk of bearing dislocation (1 to 4%). The Oxford UKA is the most widely studied mobile-bearing design with excellent long-term survivorship. Precise surgical technique and ligament balance are required to prevent dislocation.
Surgical Technique Highlights
A minimally invasive approach through a medial or lateral parapatellar arthrotomy (7 to 10 cm incision) avoids disruption of the extensor mechanism or eversion of the patella. Tibial resection depth should not exceed 6 to 8 mm below the deepest point of the defect. Femoral component positioning must avoid impingement on the ACL or the opposite compartment. Bearing stability is confirmed through a full range of motion intraoperatively. Robotic-assisted UKA (such as MAKO) improves accuracy of bone preparation and component positioning, reducing outliers.
Alignment Considerations
The goal is to undercorrect the deformity and restore alignment to the constitutional (pre-disease) state. Overcorrection transfers load to the opposite compartment and accelerates its degeneration. For medial UKA, residual varus of 2 to 3 degrees is the target.
Outcomes
Survivorship
Ten-year survivorship ranges from 90 to 98% in high-volume centers and national registries. Fifteen-year survivorship exceeds 85 to 90% with modern designs and proper patient selection. Finnish and Swedish registries report lower survivorship than single-center studies, highlighting the importance of surgeon volume.
Functional Outcomes
Patients consistently report a more natural knee feeling compared to TKA. Range of motion averages 125 to 130 degrees compared to 115 to 120 degrees with TKA. Rehabilitation and return to activity are faster, and proprioception is better preserved due to retained cruciate ligaments.
Comparison with TKA
UKA has lower perioperative complication rates with reduced blood loss, lower DVT risk, and lower infection rate. However, it has a higher revision rate in most registry data, primarily due to progression of arthritis in the remaining compartments. Revision of UKA to TKA generally yields outcomes comparable to primary TKA.
Modes of Failure
The most common long-term cause of failure is progression of arthritis in the opposite or patellofemoral compartment. Other modes include aseptic loosening of the tibial component, bearing dislocation (in mobile-bearing designs), polyethylene wear, unexplained pain (emphasizing the importance of proper patient selection), and overcorrection leading to accelerated contralateral compartment degeneration.
Clinical Pearls
The single most important factor in UKA success is patient selection; an intact ACL, correctable deformity, and isolated compartment disease are the cornerstones. Undercorrecting the alignment to preserve the opposite compartment is essential, as overcorrection is the most common technical error leading to early failure. Surgeon and hospital volume significantly impact UKA outcomes, with high-volume centers consistently reporting superior survivorship. Conversion of a failed UKA to TKA is generally straightforward and yields results comparable to primary TKA when performed before significant bone loss occurs.
References
- Liddle AD, Pandit H, Judge A, Murray DW. Patient-reported outcomes after total and unicompartmental knee arthroplasty: a study of 14,076 matched patients. Bone Joint J. 2015;97-B(6):793-801.
- Mohammad HR, Strickland L, Hamilton TW, Murray DW. Long-term outcomes of over 8,000 medial Oxford Phase 3 Unicompartmental Knees. Bone Joint J. 2018;100-B(8):1031-1037.
- Kozinn SC, Scott R. Unicondylar knee arthroplasty. J Bone Joint Surg Am. 1989;71(1):145-150.
- Blyth MJG, Smith JR, Anthony IC, et al. Robotic arm-assisted versus conventional unicompartmental knee arthroplasty. Bone Joint J. 2022;104-B(5):541-548.
For the intern
If you're called at 3am about a patient who underwent unicompartmental knee arthroplasty (UKA) earlier that day, focus on the basics: assess neurovascular status (ensure intact pulses, capillary refill, and sensation), check the surgical drain output if present, and manage pain appropriately. Bearing dislocation in mobile-bearing UKA presents with acute mechanical symptoms and inability to flex—this requires urgent imaging and orthopedic consultation. Routine post-operative concerns like breakthrough pain, nausea, or Foley issues can be managed immediately. Questions about weight-bearing status, physical therapy progression, anticoagulation dosing, and discharge planning can wait until morning rounds when the attending can provide patient-specific guidance.
Common pitfalls
- Overlooking patient selection errors: The most common reason for UKA failure is operating on the wrong patient. If the disease extends beyond one compartment, if the ACL is torn (for medial UKA), or if there's inflammatory arthritis, the patient wasn't a candidate—this leads to early failure and unhappy outcomes.
- Assuming all knee replacements are the same: UKA patients mobilize faster and have fewer restrictions than total knee arthroplasty (TKA) patients. Don't delay discharge or rehabilitation based on TKA protocols.
- Missing bearing dislocation: In mobile-bearing designs, sudden mechanical symptoms with a palpable clunk require immediate imaging—don't dismiss this as routine post-operative pain.
- Ignoring the importance of undercorrection: Overcorrecting alignment destroys the opposite compartment rapidly. Residual mild varus after medial UKA is intentional, not a surgical error.
<image> Create a simple decision algorithm flowchart with three main panels arranged vertically.
Panel 1 (top): Shows a knee diagram with three compartments (medial, lateral, patellofemoral) labeled, with an arrow pointing to a decision box asking "Isolated single compartment disease?"
Panel 2 (middle): Two pathways branching from "Yes" and "No". The "Yes" pathway leads to four sequential checkboxes listing: "Correctable deformity (<15°)", "Intact ACL (for medial UKA)", "ROM >90°, flexion contracture <10-15°", and "No inflammatory arthritis". The "No" pathway leads to a box stating "Consider TKA instead".
Panel 3 (bottom): From the checklist, an arrow labeled "All criteria met" points to a green box reading "UKA candidate", while an arrow labeled "Any criterion failed" points to an orange box reading "Reconsider—high revision risk".
Use simple icons, minimal text, and clear arrows to show the decision flow for UKA patient selection. </image>