Characterization of Patients with Difficult-to-Treat Acute Pain Following Total Knee Arthroplasty Using Multi-Modal Analgesia

Objective: To determine the efficacy of post-operative pain control over the first 24 hours in patients undergoing unilateral total knee arthroplasty (TKA) using a multi-modal analgesia protocol including femoral nerve catheters (FNC). Materials and Methods: 119 patients who underwent unilateral TKA between 2009 and 2010 under regional anesthesia with FNC were studied. Electronic charts were reviewed for numerical rating scale (NRS) pain scores at baseline, from the time the patients entered the post-operative care unit and every 4 hours thereafter until FNC was discontinued at 24 hours post-surgery. Opioid usage was also recorded during the same time period. Results: Analysis of average NRS pain scores from all patients demonstrated that 69% had NRS pain scores ≤ 5 (‘low pain’ (LP) group) and 31% had NRS pain scores ≥ 6 (‘high pain’ (HP) group). Time analysis showed that HP patients’ high pain scores persisted for 24 hours post-surgery and they were characterized by being younger when compared to the LP patients.The majority of HP patients were female. Further analysis demonstrated that the average body mass index (BMI) of the female HP patients was significantly greater than females with LP. Conclusions: In spite of a diverse multi-modal analgesia protocol designed for TKA surgery, 31% of our patients had ‘difficult-to-treat’ pain (NRS pain scores ≥ 6) for 24hours postsurgery. Our analysis implicates age, pre-operative pain scores, female gender and obesity as potential risk factors for experiencing insufficient pain control with the currently evaluated multimodal pain protocol post-TKA surgery.