Unicompartmental knee arthroplasty to total knee arthroplasty conversion: assuring a primary outcome

Converting unicompartmental knee arthroplasty (UKA) to total knee arthroplasty can be difficult, and specialized techniques are needed. Issues include bone loss, joint-line, sizing, and rotation. Determining the complexity of conversion preoperatively helps predict the need for augmentation, grafting, stems, or constraint. We examined insert thickness, augmentation, stem use, and effect of failure mode on complexity of UKA conversion. Fifty cases (1997-2007) were reviewed: 9 implants (18%) were modular fixed-bearing, 4 (8%) were metal-backed nonmodular fixed-bearing, 8 (16%) were resurfacing onlay, 10 (20%) were all-polyethylene step-cut, and 19 (38%) were mobile bearing designs; 5 knees (10%) failed due to infection, 5 (10%) due to wear and/or instability, 10 (20%) for pain or progression of arthritis, 8 (16%) for tibial fracture or severe subsidence, and 22 (44%) due to loosening of either one or both components. Complexity was evaluated using analysis of variance and chi-squared 2-by-k test (80% power; 95% confidence interval). Insert thickness was no different between implants (P=.23) or failure modes (P=.27). Stemmed component use was most frequent with nonmodular components (50%), all-polyethylene step-cut implants (44%), and modular fixed-bearing implants (33%; P=.40). Stem use was highest in tibial fracture (86%; P=.002). Augment use was highest among all-polyethylene step-cut implants (all-polyethylene, 56%; metal-backed, 50%; modular fixed-bearing, 33%; P=.01). Augmentation use was highest in fracture (86%) and infection (67%), with a significant difference noted between failure modes (P=.003). Failure of nonmodular all-polyethylene step-cut devices was more complex than resurfacing or mobile bearing. Failure mode was predictive of complexity. Reestablishing the joint-line, ligamentous balance, and durable fixation are critical to assuring a primary outcome.