Modeling Individual Recovery after Peripheral Nerve Injury in Rats and the Effects of Parturition

Background: Recovery from pain after surgery exhibits large interindividual variability, with very slow recovery equated to chronic pain. Surgical injury in the postpartum period modestly increases initial recovery after major nerve injury. In this study, the authors use a nerve injury that recovers over 2 to 3 months and apply growth curve modeling to further understand the effect of the postpartum period on speed of recovery. Methods: Withdrawal threshold to mechanical stimulus on the hind paw was determined in 41 Sprague-Dawley rats before and for 10 weeks after partial spinal nerve ligation. Age-matched male and female rats and postpartum females with pups or those separated from pups at delivery were studied. Growth curve analyses were applied to model recovery after surgery despite varying timing of measurements across groups and missing data, and these results were compared with those of two-way repeated-measures ANOVA. Results: The recovery time course was similar between males and females. In contrast, recovery was hastened in the postpartum groups, with nonoverlapping 95% CIs of modeled trajectories between days 6 and 66 after surgery. CIs were more precise at most time periods with growth curve analysis compared with ANOVA. Conclusions: The authors describe a method of analysis to quantify recovery from hypersensitivity after surgery in rats with several distinct advantages over traditionally used methods. Study results do not support a sex difference in trajectory of recovery but confirm and extend previous observations that injury at the time of obstetric delivery is associated with an abnormally rapid recovery.