A118G polymorphism of OPRM1 gene caused different morphine consumption in female patients after total knee replacement

We intended to clarify the effect of gender and A118G polymorphism of Opioid Receptor μ1 (OPRM1) on the required morphine for patients to maintain Visual Analogue Scale ≦ 3 for post-operative pain control after total knee replacement (TKR). With approval from Institutional Review Board (IRB) and obtainment of informed consents, 111 patients undergoing primary TKR were enrolled without violating the exclusion criteria, including previous history of allergy to morphine, alcohol or substance abuse, use of psychotropic medications, morbid obesity of body mass index (BMI) > 40 kg/m2, chronic pain with evidence of central sensitization, and uncooperative use of patient controlled analgesia (PCA) device. The amount of PCA-delivered morphine for pain control and the ‘demand’ (the count of button pushing for requiring pain medication from PCA) were recorded at 3, 6, 12, 24, 36, 48 and 72 h after the operation. One-way ANOVA with post-hoc test by Scheffe's method was adopted for statistical analysis of each variant (AA, AG, GG). The side-effects (nausea, vomiting and sedation) from morphine were recorded. P