Pressure pain thresholds in children before and after surgery: a prospective study

Background and aims This prospective study aimed to assess pressure pain thresholds (PPTs) by pressure algometry and the correlation to postoperative pain in children undergoing orthopaedic surgery. We hypothesized, that the PPTs would decline immediately after elective orthopaedic surgery and return to baseline values at follow-up. Methods Thirty children aged 6-16 years were included. PPTs and intensity of pain (Numerical Rating Scale, NRS) were assessed 3-6 weeks before surgery (baseline), 1-2 h before surgery (Day 0), the first postoperative day (Day 1) and 6-12 weeks after surgery (Follow-up). Results A significant difference of PPTs between the four assessments was seen using the Friedman test for detecting differences across multiple tests and Wilcoxon signed-rank test with a Bonferroni adjustment. The changes in PPTs between baseline (PPTcrus = 248 kPa, PPTthenar = 195 kPa) and day 1 (PPTcrus = 146 kPa, PPTthenar = 161 kPa) showed a decline of PPTs as hypothesized (Zcrus = 2.373, p = 0.018; Zthenar = 0.55, p = 0.581). More surprisingly, a significant decrease in PPTs between baseline and day 0, just before surgery (PPTcrus = 171 kPa, PPTthenar = 179 kPa), was also measured (Zcrus = 2.475, p = 0.013; Zthenar = 2.414, p = 0.016). PPTs were positively correlated to higher age, weight and height; but not to NRS or opioid equivalent use. Conclusions Children undergoing orthopaedic surgery demonstrate significant changes in PPTs over time. The PPTs decrease significantly between baseline and day 0, further decreases the first day postoperatively and returns to baseline values at follow-up. This suggests that other factors than surgery modulate the threshold for pain. Implications Awareness of pressure pain thresholds may help identify children with affected pain perception and hence improve future pain management in children undergoing orthopaedic surgery. Factors as for example anticipatory anxiety, psychological habitus, expected pain, catastrophizing, distraction, physical activity, patient education and preoperative pain medication might play a role in the perception of pain and need further investigation.