Papers of the Week

Persistent pain following knee arthroplasty occurs in up to 20% of patients and may require ongoing analgesia, including extended opioid administration. A comprehensive secondary analysis was performed from results of a study that considered persistent postoperative pain in 242 patients who underwent unilateral knee arthroplasty using a standardised enhanced recovery programme. Opioid prescribing for 12 months before and 12 months after surgery was evaluated and converted to oral morphine equivalents. Demographic, functional, psychological and pain questionnaires were completed along with quantitative sensory testing and genetic analysis. Forty-nine percent of patients had at least one opioid prescription in the 12 months before surgery. Opioid prescriptions were filled in 93% of patients from discharge to 3 months and in 27% of patients ≥6 months after surgery. Persistent opioid use ≥6 months after surgery was strongly associated with pre-operative opioid use (RR 3.2, p < 0.001 (95%CI 1.9-5.4)). The median (IQR [range]) oral morphine equivalent daily dose was 3.6 (0.9-10.5 [0-100.0]) mg pre-operatively, 35.0 (22.5-52.5 [4.6-180.0]) mg in hospital, 12.8 (5.1-24.8 [0-57.9]) mg from discharge to 3 months and 5.9 (4.5-12.0 [0-44.5]) mg at ≥6 months following surgery. Predictors of increased daily oral morphine equivalent ≥6 months after surgery included increased average daily oral morphine equivalent dose compared with previous values (lag), increased body mass index and three or more comorbid pain sites. Persistent opioid use was not associated with the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain (RR 1.003, p = 0.655, 95%CI 0.65-1.002) or WOMAC function (RR 1.001, p = 0.99, 95%CI 0.99-1.03) outcomes 6 months after surgery. There was no association between persistent opioid use and pre-operative quantitative sensory testing results or psychological distress. Pre-operatively, patients with a higher body mass index, more comorbid pain sites and those who had filled an opioid prescription in the last 12 months, were at increased risk of persistent opioid use and a higher oral morphine equivalent daily dose ≥ 6 months after surgery. Strategies need to be developed to limit dose and duration of persistent opioid use in patients following knee arthroplasty surgery.