Histological structure of entheses in the sacroiliac joint-related ligaments: implications for pain generation

The sacroiliac joint (SIJ) consists of an articular surface and a large posterior ligamentous region, both essential to its function. However, the detailed histological structure of the posterior ligaments, including enthesis characteristics, is not well understood, which obscures our understanding of SIJ-associated pain. This study aimed to determine their histological features and potential contribution to pain generation. The SIJ-related ligaments, including the ligament-bone attachment sites (the sacrotuberous ligament [STL]: the sacrum site and the ischial tuberosity site, the sacrospinous ligament [SSP]: the ischial spine site, the long posterior sacroiliac ligament [LPSL]: the posterior superior iliac spine [PSIS] site and the third sacral tuberosity site, and the short posterior sacroiliac ligament: medial site of the PSIS) were harvested from 10 human cadaveric donors. Histological staining (hematoxylin-eosin and Alcian blue) and immunohistochemistry for aggrecan, collagen type Ⅱ, and PGP9.5 were performed to assess fibrocartilage presence and innervation. Alcian blue positivity was most frequent at the ischial tuberosity of the STL (100%) and the third sacral tuberosity of the LPSL (70%). Immunohistochemical staining for aggrecan and collagen type II confirmed the presence of extracellular matrix components of fibrocartilage in these regions, with concordance rates of 85.7% and 90.0%, respectively, compared to Alcian blue staining. Adipose tissue and PGP9.5-positive nerve fibers were observed adjacent to the ligament-bone attachment site. The ligament-bone attachments of SIJ-related ligaments, especially the STL and LPSL, exhibit fibrocartilaginous structures consistent with classical entheses. Additionally, the existence of nerve fibers in the adipose tissue around the SIJ-related ligaments indicated a potential source of pain.