Summary

This retrospective cohort study demonstrates that using MRI to confirm full thickness cartilage loss leads to good early outcomes following UKA in patients who show a preserved joint space on plain film radiograph.

Abstract

Background

Poor outcomes for medial UKA have been reported in patients who have partial thickness cartilage loss(1). Consequently, medial UKA is only recommended for those patients with bone on bone osteoarthritis.
Currently, the role of surgery in patients with symptomatic osteoarthritis and early to moderate radiographic changes on plain radiographs is not clear. Waiting for sufficient time to pass in order to demonstrate progression to “bone on bone” arthritis on a plain radiograph exposes the patient to a prolonged period of painful disability.
In our practice we use MRI to determine the extent and severity of disease in the medial compartment including the presence of any full thickness cartilage loss on either the femur or tibia and the presence of subchondral bone changes (oedema, cysts). It is our belief that these findings on MRI are sufficient to warrant proceeding to UKA in cases where bone on bone disease cannot be established by plain film radiographs.

Methods

Local permission was given for this retrospective cohort study. Our institutional database contains detailed information on UKAs performed in our hospital and is linked with the United Kingdom’s government led Hospital Episode Statistics datasets, including Patient Reported Outcome Measurements (HES-PROMS). It currently holds complete data on 325 subjects who have undergone UKA since 2012.
Pre-operative radiographs were reviewed for all subjects and a total of 62 subjects had radiographs, which showed a preserved joint space on the medial side (Kellgren Lawrence grade 2 or 3). All cases had an MRI scan pre-operatively which confirmed full thickness cartilage loss. A control group, matched for age, gender, co-morbidity and baseline PROM was selected from the remaining subjects (n=155). All these subjects demonstrated full thickness cartilage loss on plain radiographs (Kellgren Lawrence 4). Oxford knee scores (OKS) were retrieved pre-operatively and at 6 months post-operatively.
Multiple logistic regression was performed to identify if there was a significant difference in outcome between the two groups and to determine if any pre-operative variables had a significant impact on outcome.

Results

The mean OKS improved in both groups from baseline (Controls = 21.7, Cases =20.9) to 6 months post-operatively (Controls =41.3, Cases =38.7). Regression analysis revealed no significant difference in the change in OKS between the two groups (p=0.15). There was no difference in the percentage of patients who reported a “good” , “very good” or “excellent” outcome (p=0.38).

Discussion

This study demonstrates that using MRI to confirm full thickness cartilage loss leads to good early PROMs following UKA in patients who show a preserved joint space on plain film radiograph. These outcomes are equivalent to controls that demonstrate full-thickness disease on plain film radiographs. To our knowledge this is the first study to demonstrate the value of MRI in this context.

1) Pandit et al. Unicompartmental knee replacement for patients with partial thickness cartilage loss in the affected compartment. The Knee. 2011. 18 (3):168-171.