Scott, J Arthroplasty 2024 - Unicompartmental Knee Arthroplasty: Lessons Learned after 50 Years of Experience - Article Summary

Unicompartmental Knee Arthroplasty: Lessons Learned after 50 Years of Experience

Scott·J Arthroplasty·2024·1 citations·Review article

Expert perspective from Richard Scott, who performed his first UKA in 1973, synthesizing 50 years of lessons on patient selection, implant design, alignment, and technique. The paper traces the evolution from the original Kozinn-Scott criteria through the 2015 liberalized consensus and distills the technical principles that determine UKA success or failure.

Key Findings

UKA fails at approximately 1% per year versus 0.3–0.5% per year for TKA — roughly double the rate. This trade-off is acceptable because UKA preserves both cruciate ligaments, offers faster recovery, and converts to TKA more successfully than either osteotomy or a prior TKA.
The original Kozinn-Scott criteria limited ideal candidates to about 6% of arthroplasty patients. The 2015 consensus (6 surgeons, combined >8,000 UKAs) liberalized the criteria, expanding the pool to 10–15% by removing obesity and young age as absolute contraindications and adding MRI-confirmed medial femoral condyle osteonecrosis as an indication.
Fixed-bearing UKAs must be non-conforming (round-on-flat). Component retrievals show the prosthetic wear pattern reproduces the preoperative arthritic wear pattern. Conforming fixed bearings resist this natural path, generating constraint forces that loosen fixation. Only mobile-bearing designs can be conforming, since the bearing itself absorbs the kinematic mismatch.
Ideal post-operative alignment is 2–5° anatomic valgus (a few degrees of mechanical varus). Overcorrection shifts load to the opposite compartment and drives early degeneration there. Deformity is corrected by peripheral osteophyte removal only. Formal medial release is not tolerated by this procedure.
ACL status determines bearing choice. An absent ACL is an absolute contraindication to mobile-bearing UKA unless reconstructed. For fixed-bearing UKA, acceptability depends on the tibial wear pattern:
- Anterior or central wear: may proceed
- Posterior tibial wear: contraindicated
- Posterior tibial slope should not exceed 5° in ACL-deficient knees.
Lateral UKA is performed at a 1:10 ratio versus medial, always requires a fixed-bearing design, and carries higher patellar impingement risk. A mini-lateral incision is avoided because it compromises intraoperative TKA conversion. A medial parapatellar approach with patellar subluxation preserves that option.
Using a single tibial fixation pin is preferred over multiple pins. Multiple fixation holes predispose the proximal tibia to postoperative stress fracture. A known complication documented with 4-pin jig systems.

Board PearlFixed-bearing UKAs must be non-conforming (round-on-flat); conforming fixed bearings generate loosening forces the predetermined wear pattern cannot accommodate.

Supplemental

Unicompartmental Knee Arthroplasty: Lessons Learned after 50 Years of Experience.