Predictors of the use of analgesic drugs 1 year after joint replacement: a single-center analysis of 13,000 hip and knee replacements

BACKGROUND: Pain persists in a moderate number of patients following hip or knee replacement surgery. Persistent pain may subsequently lead to the prolonged consumption of analgesics after surgery and expose patients to the adverse drug events of opioids and NSAIDs, especially in older patients and patients with comorbidities. This study aimed to identify risk factors for the increased use of opioids and other analgesics 1 year after surgery and focused on comorbidities and surgery-related factors. METHODS: All patients who underwent a primary hip or knee replacement for osteoarthritis from 2002 to 2013 were identified. Redeemed prescriptions for acetaminophen, non-steroidal anti-inflammatory drugs (NSAIDs), and opioids (mild and strong) were collected from a nationwide Drug Prescription Register. The user rates of analgesics and the adjusted risks ratios for analgesic use 1 year after joint replacement were calculated. RESULTS: Of the 6238 hip replacement and 7501 knee replacement recipients, 3591 (26.1%) were still using analgesics 1 year after surgery. Significant predictors of overall analgesic use (acetaminophen, NSAID, or opioid) were (risk ratio (95% CI)) age 65–74.9 years (reference < 65), 1.1 (1.03–1.2); age > 75 years, 1.2 (1.1–1.3); female gender, 1.2 (1.1–1.3); BMI 30–34.9 kg/m(2) (reference < 25 kg/m(2)), 1.1 (1.04–1.2); BMI > 35 kg/m(2), 1.4 (1.3–1.6); and a higher number of comorbidities (according to the modified Charlson Comorbidity Index score), 1.2 (1.1–1.4). Diabetes and other comorbidities were not significant independent predictors. Of the other clinical factors, the preoperative use of analgesics, 2.6 (2.5–2.8), and knee surgery, 1.2 (1.1–1.3), predicted the use of analgesics, whereas simultaneous bilateral knee replacement (compared to unilateral procedure) was a protective factor, 0.86 (0.77–0.96). Opioid use was associated with obesity, higher CCI score, epilepsy, knee vs hip surgery, unilateral vs bilateral knee operation, total vs unicompartmental knee replacement, and the preoperative use of analgesics/opioids. CONCLUSIONS: Obesity (especially BMI > 35 kg/m(2)) and the preoperative use of analgesics were the strongest predictors of an increased postoperative use of analgesics. It is remarkable that also older age and higher number of comorbidities predicted analgesic use despite these patients being the most vulnerable to adverse drug events.