Persistent pain after Oxford Unicompartmental Knee Replacement: Incidence, natural progression and outcome assessment

Introduction

Besides being an otherwise successful operation, some patients continue to experience pain post-operatively after Oxford unicompartmental knee replacement (UKR). The exact incidence and true burden of this pain is not known but the joint registers demonstrate that 38% of UKR revisions in New Zealand and 23% in England and Wales were due to unexplained pain. Some individual studies have reported an incidence as high as 50%. There has not been a large prospective study to examine the incidence of pain after UKR. The natural history of this pain, its aetiology and its impact on long-term outcome is also not known.

Objective

The aim of this study was to analyse the incidence of pain after Oxford UKR and its progression with time. The study also evaluated whether the incidence has changed over the years and whether the use of specific pain questionnaires help in identifying patients that are at risk of developing persistent pain post-operatively.

Methods

The data was collected prospectively over the last 13 years from 1101 patients undergoing cemented Oxford UKR. The incidence and progression of pain was analysed by assessing the patients’ 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. To analyse whether the incidence of pain has changed over the past 13 years, data from two matched groups (120 patients in each arm) were analysed. The ‘old group’ comprised of patients who had surgery from 1998-2001, while the ‘new group’ had surgery from 2008-2011.

Results

The incidence of post-operative severe pain was 3% at six weeks, 2% at one year and there was little change at subsequent follow-ups. A similar trend was seen with OKS, ∆ OKS, PS and ∆ PS. The question 1 of OKS and the PS had the highest SRMs (1.87 and 1.83 respectively) and the painDETECT score had the lowest (0.7) SRM. The incidence of severe pain in the ‘old group’ was 5% which reduced to 2.3% in the ‘new group’. 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 significantly worse pre-operative neuropathic pain score.

Discussion

This study demonstrates that the functional outcome of Oxford UKR has improved over the last 13 years. There are number of factors that may contribute to this effect but since the indications have not changed, the improvement is probably due to modifications in the surgical technique and due to improvements in instrumentation. The incidence of severe pain post-operatively is 3% at six weeks and 2% at one year and at a medium-term follow up. Patients with probable pre-operative neuropathic pain tend to do badly 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.