The influence of experimentally induced osteoarthrosis on articular nerve fibers of the sheep temporomandibular joint

Aims: To study the effect of experimentally induced osteoarthrosis, or non-inflammatory degenerative changes, on the innervation of the sheep temporomandibular joint (TMJ) through the use of indirect immunohistochemistry and image analysis quantification. Methods: Bilateral condylar scarification was performed in 8 sheep, which were killed at 16 weeks post-operation; 3 unoperated sheep served as controls. Tissues from 8 osteoarthrotic joints and 4 control joints were processed for the immunostaining with antisera for protein gene product 9.5 (PGP 9.5), substance P (SP), calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP), and tyrosine hydroxylase (TH). An additional 10 joints were decalcified to study the morphologic changes induced by the condylar abrasion. Results: Osteoarthrotic changes were commonly, seen in the anterior and lateral regions of the joint and included fibrosis, peripheral osteophyte formation, cysts, and erosion of articular surfaces. In the osteoarthrotic joints, the distribution of PGP 9.5-, CGRP-, and SP-immunoreactive (IR) nerve fibers was similar to that observed for control joints in the capsule, synovium, and capsule/disc junction. There were statistically detectable decreases in the percent surface area of IR nerve fibers in the capsule for both PGP 9.5 and CGRP in arthrotic joints compared with control joints. The lateral and anterior regions of the capsule had greater density of PGP 9.5- and CGRP-IR nerve fibers than other parts of the capsule in both control and arthrotic joints, and the medial capsule was poorly innervated in all joints. Immunostaining for substance P was always weaker. Conclusion: This study suggests that while inflammatory arthritis has a marked influence on the density of sensory and autonomic nerve fibers in synovium in a variety of joints in different species, experimentally induced non-inflammatory osteoarthrosis in the sheep TMJ also leads to a depletion of the density of nerve fibers in the capsule, especially in the lateral part of the joint. Further work is required to determine whether other parts of the joint, such as synovium and marrow, respond differently to experimentally induced osteoarthrosis.