Acromioclavicular Disk as a Potential Source of Pain in AC Joint Injuries

Abstract

Background: Injuries of the acromioclavicular joint (ACJ) are common shoulder injuries that often lead to pain and dysfunction of the affected shoulder. Regardless of operative or nonoperative treatment, a relatively large number of patients remain symptomatic and experience pain. However, the specific source of persistent pain in the ACJ remains ambiguous. Purpose: To investigate the presence of sensory nerve fibers or pain-generating neurotransmitters within the intra-articular disk of the ACJ to determine its potential role as an independent pain generator in ACJ disorders. Study Design: Descriptive laboratory study. Methods: Twelve paired ACJs from 6 fresh human cadavers (mean age, 56 years; range, 41-82 years) were harvested and freed from surrounding soft tissues, leaving only the ACJ capsule intact. The specimens were placed in 4.5% formaldehyde fixative for a minimum of 48 hours. Coronal plane sections were obtained and demineralized in EDTA for a week, embedded in paraffin for 12 hours, and dehydrated overnight. With a rotation microtome, 2-μm sections were cut and stained with hematoxylin and eosin to investigate tissue architecture and confirm the presence of a fibrocartilaginous intra-articular disk. The sections were immunohistochemically stained with antisera against S100, neuropeptide Y (NPY), and substance P (SP) to detect for neural tissue. Additionally, a nerve fiber count per 10 high-power fields representing an area of 0.2 mm2 was conducted for S100 stains. All sections were examined for the presence of positive immunoreactivity to S100, NPY, and SP. Results: The presence of a fibrocartilaginous intra-articular disk could be observed in all 12 examined ACJs. In all specimens, an immunoreactivity to S100, NPY, and SP could be observed within the superior peripheral region of the intra-articular disk. High-power field nerve counts of the S100 stains revealed a mean ± SD of 7.9 ± 2.28 nerves per 10 high-power fields (range, 4-12). Conclusion: The documented immunoreactivity to S100, NPY, and SP indicates the presence of somatic and autonomic nerve fibers within the intra-articular disk of the ACJ. Clinical Relevance: Confirming the presence of nerve fibers within the intra-articular disk of the ACJ suggests that the disk itself could be an independent source of pain after injury and thus a possible explanation for recalcitrant pain after treatment.