Magnetic resonance imaging provides additional utility in the preoperative cartilage assessment of patients undergoing medial unicompartmental knee arthroplasty.

Purpose: For unicompartmental knee arthroplasty (UKA), patient selection using correct indications can optimise postsurgical outcomes. The current gold standard for assessing eligibility is with radiographs; however, magnetic resonance imaging (MRI) may allow for more accurate assessments of cartilage damage. This study aimed to evaluate the utility of MRI for preoperative assessment of medial UKA patients by (1) comparing osteoarthritis severity of the medial, lateral and patellofemoral (PF) compartments when assessed using MRI compared with standard radiographs, and (2) investigating associations of these two assessments with postoperative clinical outcomes.

Methods: This study had ethical approval. A retrospective review was performed for 88 primary medial UKA between 1 January 2017 and 31 December 2021. The main outcome measures were preoperative cartilage loss and patient-reported clinical outcomes. Preoperative cartilage loss was recorded using the International Cartilage Repair Society (ICRS) classification using MRI, and Kellgren-Lawrence (K-L) scores from radiographs. Patient-reported clinical outcomes were measured using preop, early (6-week) and late (1- or 2-year) Oxford Knee Score (OKS) change scores.

Results: The use of MRI has improved accuracy over radiographs. In the medial compartment, 37 (44%) patients had less severe radiographic K-L scores (1-3); however, all patients had the most severe MRI ICRS scores (4). For patients with mild K-L scores (0 and 1), 20 (43%) and 7 (78%) patients had more severe ICRS scores (3 and 4) within their lateral and PF compartments, respectively. No associations were found between ICRS or K-L scores and OKS for any compartments.

Conclusions: Assessment of medial cartilage thickness loss using MRI provides additional utility over standard radiographs in preoperative assessments of medial UKA patients. However, evidence of disease in the PF compartment assessed using MRI should not be considered a contraindication for UKA.

Level of evidence: Level III, retrospective cohort study.