Bearing thickness of unicompartmental knee arthroplasty is a reliable predictor of tibial bone loss during revision to total knee arthroplasty.

BACKGROUND

Bone defects during revision procedures for failed UKA represent a challenge even for the most experienced surgeons; therefore, an accurate preoperative planning remains essential to prevent dramatic scenarios in the surgical theatre.

HYPOTHESIS

Our hypothesis is that bearing thickness used in original UKA represents a reliable predictor of severe tibial bone loss, requiring a metallic augment or constrained implant, during revision to TKA.

PATIENTS AND METHODS

Forty-two patients who underwent a total knee arthroplasty from failed UKA were identified from our institutional database and evaluated clinically using the Knee Society Score (KSS). A multivariate logistic regression analysis was performed using the presence of tibial augments or the need of varus-valgus constrained (VVC) prosthesis as depend variables, and patients' gender, age at revision procedure, side (medial or lateral), UKA tibial tray (all-polyethylene or metal back), bearing thickness (composite thicknesses of the metal-backed tray and insert or all-polyethylene tibial component ≤8mm or more than 8mm) and cause of failed UKA as independent variables.

RESULTS

A posterior-stabilized prosthesis was used in 27 cases (64.3%). An augment was necessary in 12 patients (28.6%). Initial bearing thickness greater than 8mm was associated with greater likelihood of a VVC implant (OR=11.78, 95% CI, 1.6583 to 83.6484, p=0.0137) and a tibial augment (OR=9.59, 95% CI, 1.327 to 69.395, p=0.0251). Tibial tray design, patients' gender or age during revision surgery, side or cause of failure were not associated to increased risk of augmentation or constrained implants.

DISCUSSION

Surgeons should be aware of the particular challenges that the conversion of a UKA to a TKA presents and be prepared to address them intraoperatively, with particular care to proper bone loss manage. Satisfying results can be achieved at mid-to-long term follow-up, if these procedures are planned accurately, and a precise analysis of failed UKA components, in particular bearing thickness, represents a helpful support in this context.

LEVEL OF EVIDENCE

IV, retrospective case series.