Natural history of pain following unicompartmental knee replacement

Medial unicompartmental knee replacement (UKR) is an alternative to total knee replacement for osteoarthritis and has been shown to have better functional outcomes in clinical studies. However the revision rate is higher and revisions are often attributed to unexplained pain. Proponents of UKR suggest that this pain is likely to improve over the first postoperative year, but this has not been demonstrated empirically. The aim of this study was to define the natural history of pain following UKR, and to determine the factors affecting incidence of, and recovery from, postoperative pain. 191 knees in 183 patients underwent UKR (Oxford UKR, Biomet, Bridgend UK) in a single institution. Mean age was 65.2 years (36.6-86.5) and 52% of knees were in females. Oxford Knee Scores were recorded on each patient preoperatively, at six weeks and one year post-op. At each time, pain was classified as mild/none, moderate, or severe. Pain was classified as ‘explained’ if an obvious cause was present (eg infection or trauma). Where no other cause was found, pain was classified as ‘unexplained’. Patient factors (body mass index (BMI), age, gender) and surgeon grade were recorded. At six weeks, pain was severe in 7/191 knees (3.7%) and moderate in 51/191 (27.2). At one year pain was severe in 6/191 (3.1%) and moderate in 27/191 (14.1%). 73/191 (38%) reported pain at either time point; 56/73 (77%) were unexplained. Pain improved between 6 and 52 weeks (one way ANOVA, p<0.05 for all outcome measures, across all time intervals), regardless of whether it was explained or not. The incidence of unexplained pain was unaffected by age, BMI or surgeon grade. Women were more likely to experience unexplained pain than men (Chi Squared test, p=0.02). Neither age, gender, BMI nor surgeon grade affected the progression of pain beyond 6 weeks. This study demonstrates that unexplained pain after UKR is likely to improve in the first postoperative year. Whilst women are more slightly more likely to experience unexplained pain at 6 weeks, neither age nor BMI affected the incidence of pain. Neither age, gender nor BMI affected the progression of this pain beyond six weeks.