Abstract
Objectives:
Chronic rotator cuff tear (RCT) presents a significant pain burden leading to substantial functional impairment. Despite advances in treatment, pain management remains a critical aspect of quality of life. Up to 20% of patients are on opioids for RCT-related pain and continue use even after surgery; however, they pose substantial risks due to their potential for addiction and adverse effects. Thus, understanding the underlying neurologic pain pathways associated with chronic RCTs is essential for developing effective therapeutic strategies. The objective of this study was to demonstrate the presence of sensory neuron injury and neuropathic pain after RCT through in a mouse model and determine the effectiveness of various classes of pain medications, such as nonsteroid anti-inflammatory drugs (NSAIDs), gabapentin, and opioids. Further, due to the overall degenerative effects of muscle fatty infiltration (FI), we also sought to investigate the relationship of rotator cuff muscle FI and shoulder pain in this model. We hypothesized that (1) RCTs cause sensory neuron injury and neuropathic pain that can be relieved by gabapentin and (2) isolated muscle FI cause pain and shoulder function deficits similar to RCTs.
Methods:
Adult mice were divided into four surgical groups (n=6/group): control, sham surgery, complete tendon transection of right rotator cuff (TT), and TT with transection of the suprascapular nerve (TT+DN). After 4 weeks, these mice were sacrificed and dorsal root ganglion (DRG) were harvested and stained for ATF3, an injury marker for sensory neurons. To test the effects of various pain medications on RCT neuropathic pain, separate adult mice underwent the TT+DN procedure (n=7), and after 12 weeks, they were treated with intraperitoneal injection of gabapentin (30mg/kg), buprenorphine (3.25mg/kg), or ketorolac (10mg/kg). In a separate set of experiments to assess the impacts of FI on shoulder function and pain, adult mice underwent unilateral right supraspinatus glycerol injection to induce muscle FI without rotator cuff tendon or nerve injury (n=5). Complete supraspinatus and infraspinatus TT and TT+DN were also conducted separate groups of mice as controls (n=4 in each). To define the possible neuropathic pain in these mice, gabapentin was administered intraperitoneally (30mg/kg) at 2 weeks and 4 weeks. Gait and pain metrics, namely stride length and forepaw weightbearing ratio, were recorded via BlackBox, a novel AI-based gait analysis system, and analyzed with DeepLabCut, an open-source software that employs next-generation machine-learning technology. Unpaired t-tests were used for statistical analysis between experimental groups. Two-way analysis of variance with Tukey post hoc significant difference test was used in the case of multiple comparisons. Significance was considered when p<0.05. All data is presented in the form of mean ± SD.
Results:
Harvesting DRG from mice revealed increased ATF3 expression in the TT and TT+DN groups (Fig 1). In the follow-up experiment with 7 mice that underwent TT+DN, we observed a decline in forepaw weightbearing (Baseline: 1.043 ± 0.189 vs. Week 12: 0.563 ± 0.148, p<.05) and right forepaw stride length (Baseline: 1.895cm ± 0.150 vs. Week 12: 1.342cm ± 0.594, p<.05) (Fig 2 A, E). While treatment with ketorolac or buprenorphine did not alter forepaw weightbearing, we found that gabapentin treatment led to significant improvements in weightbearing (Week 12: 0.563 ± 0.148 vs. post-gabapentin: 0.883 ± 0.168, p<.05) (Fig 2 B, F). Gabapentin and ketorolac had no effect on stride length, but buprenorphine resulted in notable improvements in weightbearing (Week 12: 0.563 ± 0.148 vs. 1hr post-buprenorphine: 0.741 ± 0.123, p<.05) and stride length (Week 12: 1.342cm ± 0.594 vs. 1hr post-buprenorphine: 2.222cm ± 0.301, p<.05) (Fig 2B-D, F-H). When assessing the impacts of FI, at 4 weeks post-glycerol injection, mice exhibited a significant reduction in right forepaw stride length (1.633cm ± 0.150 vs. 2.644cm ± 0.392, p = 0.003) and weightbearing (0.678 ± 0.121 vs. 1.151 ± 0.062, p = 0.027), compared to baseline (Figure 3A, B). Following gabapentin administration, both right forepaw stride length (2.473cm ± 0.224 vs. 2.644cm ± 0.392, p = 0.468) and weight bearing (1.100± 0.061 vs. 1.151 ± 0.062, p = 0.531) improved to levels comparable to baseline. Interestingly, there similar functional outcomes between glycerol-injected mice and TT mice (p>0.05), while the TT+DN group performed significantly worse (p<0.05). Overall, gabapentin relieved pain and improved shoulder function in all groups.
Conclusions:
Our study demonstrates the correlation between rotator cuff injury and neuropathic pain, as strongly indicated by the upregulation of ATF3 in the DRG. Similar functional responses to gabapentin between glycerol-injected and rotator cuff injury mice also shows a clear relationship between FI and pain after rotator cuff injury. While we did see improvement in stride length with the administration of buprenorphine, this is likely confounded by the opioid-induced euphoric effects. However, the improved weightbearing and stride length outcomes with gabapentin clearly indicates its effectiveness for pain relief in this model. Altogether, these findings suggest that (1) rotator cuff injuries can induce a neuropathic-like response leading to neural sensitization and chronic pain and (2) gabapentin, given its positive effects on functional outcomes, can serve as a potential therapeutic option for managing pain associated with rotator cuff injuries. Further investigation is needed to elucidate the mechanisms by which rotator cuff injuries induce neural sensitization and chronic pain. Studies focusing on the role of ATF3 upregulation in the DRG and its downstream effects could provide crucial insights into the development of neuropathic pain following rotator cuff injuries.
