The incidence and natural history of pain after medial unicompartmental knee replacement

Introduction

Medial Oxford unicompartmental knee replacement (UKR) is an established and successful treatment for patients with end-stage antero-medial knee osteoarthritis. However, following the operation some patients have pain, which compromises their functional outcome. The true burden of this pain is not known. The reported incidence in individual studies is as low as 1% and as high as 50%. The joint registers demonstrate pain as a cause of revision in 38% (New Zealand) and 23% (England and Wales) UKR patients.

The aim of this study was analyse the incidence and natural history of pain after Oxford UKR and its impact on long-term outcome.

Methods

Prospectively collected clinical data from 1101 patients with cemented Oxford UKR, performed over the last 12 years at the Nuffield Orthopaedic Centre, Oxford, was used to analyse the incidence and progression of pain as demonstrated by the response to question number 1 of the Oxford Knee Score (OKS). In addition to the detailed OKS, the change in the overall OKS (∆ OKS), pain score (PS) and the change in PS (∆PS); specific pain scores [painDETECT and Intermittent and Constant Osteoarthritis Pain (ICOAP)] were also collected. The standardised response means (SRM’s) were calculated to measure responsiveness of a scoring system to detect change.

Over the past 13 years, surgical technique for UKR has been modified. The changes include use of an alternative surgical saw and ensuring adequate coverage of the medial tibial plateau with a component of largest possible size. To analyse whether these changes have influenced the incidence of pain, data from two matched groups (120 patients in each arm) from different decades were compared. The ‘old group’ comprised of patients who had surgery from 1998-2001, while the ‘new group’ had surgery from 2008-2011.

Results

Overall, the incidence of post-operative severe pain was 3% at six weeks and 2% at one year. The incidence remained unchanged at subsequent follow-ups.

In the ‘old group’, the incidence of severe pain was 5% which was greater than the ‘new group’ which had a 2.3% incidence of severe pain. Patients with likely or probable pre-operative neuropathic pain achieved a worse outcome. Furthermore, the patients with moderate or severe pain at one year had a significantly worse pre-operative neuropathic pain score. The question 1 of the OKS and the PS had the highest SRMs (1.87 and 1.83 respectively) and the painDETECT score had the lowest (0.7) SRM.

Discussion

This study demonstrates that, following Oxford UKR, pain is a rare but important complication. The functional outcome of Oxford UKR, however, has improved over the last 13 years. As the indications have not changed, the improvement is probably due to modifications in the surgical technique and due to improvements in instrumentation. Patients with probable pre-operative neuropathic pain tend to do badly after Oxford UKR and the use of the painDETECT score helps to some extent in identifying such patients. Question number 1 of OKS and PS are more valuable in detecting the change in outcome between the pre-operative and follow-up states than the ICOAP and painDETECT scores.