Nociceptive nerve fibers in the sacroiliac joint in humans

Background and Objectives: A positive response to sacroiliac joint intra-articular infiltration with local anesthetics is used to confirm sacroiliac joint pain. However, current anatomical and histological knowledge concerning the anatomy of pain perception within the sacroiliac joint intra- and peri-articular structures is insufficient to explain the efficacy of this infiltration, because of the use of unspecific histochemical visualization techniques. Methods: In this study, immunohistochemistry for calcitonin gene-related peptide (CGRP) and substance P was used to trace nociceptive fibers and receptors in the anterior and interosseous sacroiliac ligaments obtained from 5 human cadavers without history of sacroiliac joint pain. Results: Microscopic analysis of stained slides showed presence of CGRP and substance P immunoreactive fibers. Thick, wavy, formed bundles were observed in dense and loose connective tissue, whereas single, beaded nerve fibers, occasionally ramified, were observed more frequently in the dense connective tissue and next to blood vessels. Based on their morphologic features, these immunoreactive structures were classified as receptors type IV. Additionally, receptors type II were found in anterior and interosseous ligaments, which contained CGRP or substance P immunoreactive free nerve endings. Conclusions: We conclude that the presence of CGRP and substance P immunoreactive fibers in the normal anterior capsular ligament and interosseous ligament provides a morphological and physiological base for pain signals originating from these ligaments. Therefore, diagnostic infiltration techniques for sacroiliac joint pain should consider extra- as well as intra-articular approaches.