Sustained Unilateral Occlusal Alteration Is Associated with Bilateral and Asymmetric Changes in MMP-2, MMP-3, and MMP-9 Immunoreactivity in the Rat Temporomandibular Joint

Background: Abnormal occlusal loading may alter temporomandibular joint (TMJ) homeostasis and promote extracellular matrix remodelling. Matrix metalloproteinases (MMPs) are key regulators of extracellular matrix turnover, but the bilateral response of different TMJ fibrocartilaginous components to a sustained unilateral occlusal disturbance remains insufficiently characterised. Objective: To characterise MMP-2, MMP-3, and MMP-9 immunoreactivity in the glenoid cartilage, condylar cartilage, and articular disc of both temporomandibular joints following sustained unilateral occlusal alteration in rats. Methods: Twenty-five female Wistar rats were included in the experimental design. The study comprised a 4-week control group (n = 4), a 24-week control group (n = 9), and an experimental group (n = 12). In experimental animals, sustained unilateral occlusal alteration was created at 8 weeks of age by application of light-cured composite to the three right maxillary molars and maintained for 16 weeks. MMP-2, MMP-3, and MMP-9 were detected immunohistochemically in the glenoid cartilage, condylar cartilage, and articular disc of both TMJs. DAB immunostaining intensity was quantified by grayscale densitometry; lower grayscale values corresponded to stronger immunoreactivity. Results: Sustained unilateral occlusal alteration was associated with stronger MMP-2, MMP-3, and MMP-9 immunoreactivity in the glenoid cartilage, condylar cartilage, and articular disc relative to 24-week controls (P < 0.01 for the comparisons reported in the original analysis). The increase was bilateral but consistently greater in the TMJ ipsilateral to the occlusal alteration (P < 0.05 or P < 0.01, depending on marker and tissue). The original analysis also documented maturation-related differences for MMP-2 in glenoid cartilage and for MMP-9 in glenoid and condylar cartilage, whereas MMP-3 showed no significant age-related differences. Conclusions: Sustained unilateral occlusal alteration was associated with a bilateral but asymmetric increase in the MMP immunoreactivity profile of TMJ fibrocartilaginous tissues. The concordant involvement of MMP-2, MMP-3, and MMP-9 across the glenoid cartilage, condylar cartilage, and articular disc supports a broad alteration of matrix-remodelling proteins following altered occlusal loading. These findings refer to immunoreactivity and should not be interpreted as direct evidence of increased enzymatic activity or matrix proteolysis.