Bilateral Temporomandibular Joint Changes Following Unilateral Posterior Occlusal Alteration in Rats: A Longitudinal MRI and Histomorphometric Study

Background/Objectives: Whether a unilateral posterior occlusal alteration produces bilateral temporomandibular joint (TMJ) changes, and how imaging findings relate to tissue-level alterations, remains incompletely characterised. This study aimed to characterise the bilateral TMJ response to a sustained unilateral posterior occlusal alteration in rats using longitudinal magnetic resonance imaging (MRI) and endpoint histological and histomorphometric assessment. Methods: Twenty-eight female Wistar rats were allocated to an experimental group (n = 12), a baseline control group (n = 4) and an age-matched control group (n = 12). Light-cured composite resin was applied to the three right maxillary molars of experimental animals and maintained for 16 weeks. Baseline MRI was obtained from the separate 8-week control group, whereas experimental and age-matched control animals underwent bilateral MRI at 16 and 24 weeks of age. Cartilage and disc structure, thickness and cell density were assessed histologically at the endpoint. Results: MRI demonstrated a progressive bilateral response. At 24 weeks, 11 of 12 experimental animals showed bilateral joint-space narrowing; condylar flattening affected 9 of 12 TMJs on each side, and reduced disc size was present in all experimental TMJs. Histology and histomorphometry showed a more intense ipsilateral response, with cartilage and disc thickening, loss of normal cartilage organisation and reduced cell density. Contralateral tissue architecture remained preserved, although disc thickness was greater than in age-matched controls. Conclusions: Unilateral posterior occlusal alteration produced a bilateral but asymmetric TMJ response: imaging abnormalities progressed in both joints, while tissue remodelling was most pronounced ipsilaterally. The combined findings support early load-related degenerative remodelling and show that unilateral occlusal disturbance can affect both TMJs to different degrees.