Reducing dietary loading decreases mouse temporomandibular joint degradation induced by anterior crossbite prosthesis

Abstract

Purpose: Loading on temporomandibular joint (TMJ) originates from the contraction of jaw-closing muscles. Variations in food properties, such as volume or hardness, will initiate different levels of masticatory muscle contraction via periodontal-muscular reflex, leading to differences in levels of occlusal force and TMJ loading. Soft diet will cause growing rats a thinning cartilage, enhanced cartilage catabolic activity and decreased subchondral bone volume fraction in condyles. Aberrant biomechanical stimulation from abnormal dental occlusion plays an important role in TMJ osteoarthritis (OA) development. Incisors of rodent animals function more than other teeth because when a rodent catches or cuts foods they used to move the mandible in the sagittal direction following the incisal guidance. Unilateral anterior crossbite prosthesis will heavily interfere with this function because the totally opposite guidances will be provided by the normal incisor pair and the crossbite incisor pair while the TMJ can support only one pattern of them at the same time. A small-size diet may diminish the requirement of the cutting function of incisors, and thus may reduce the harmful biomechanical stimulation from the anterior crossbite prosthesis. Hence, in the present study, we exposed the mice that received the unilateral anterior crossbite prosthesis to a small-size versus a large-size diet. The aim was to test whether mice receiving soft diet have a decreased TMJ degradation induced by the unilateral anterior crossbite prosthesis using our reported methods. Methods: Forty 6-week-old female C57BL/6J mice were randomly divided into two experimental and two control groups (n = 10), one experimental and one control group received small-size diet, less than 3mm thick and the other two groups received large-size diet, about 12.5mm in diameter and 15-20mm in length. For experimental groups, metal tubes made of pinheads were bonded onto the mouse left maxillary and mandibular incisors to create unilateral anterior crossbite relationship. For the control groups, the mice underwent the similar procedures but no metal tubes were bonded. No differences in degrading changes were found between the left side and right side TMJs in the experimental mice as we reported. Left side TMJ tissue blocks from six mice of each group were fixed, decalcified and embedded in paraffin. Sagittal sections were prepared for Hematoxylin & Eosin (H-E) staining, toluidine blue staining, Safranin O staining, tartrate-resistant acid phosphatase (TRAP) staining and immunohistochemical staining of Collagen II, OPG and RANKL. For each group, the condylar cartilages and subchondral bones of the other 14 TMJs were respectively separated and preserved at -80°C. Four to five condyles from 3 or 4 different mice were pooled to create a single sample of the cartilage or subchondral bone respectively for RNA extraction to compare the mRNA expression levels of Col2a1, Aggrecan, ADAMTS-5, RANK, OPG, RANKL. Differences were set at 0.05 level (α=0.05). Results: Thinner and degraded cartilage, reduced cartilage cellular density, decrease in toluidine blue staining (reflecting reduced proteoglycan levels) in cartilage, decreased expression levels of Collagen II and Aggrecan, loss of subchondral bone assessed by histomorphometry, and enhanced osteoclast activity were observed in both experimental groups. However, the cartilage degradation phenotype was less severe and cartilage ADAMTS-5 mRNA was lower while OPG/RANKL ratio in both cartilage and subchondral bone was higher in the small-size diet than the large-size experimental group. In contrast, no differences of histomorphology and OPG/RNAKL expression were found in the two control groups. Conclusions: The current findings suggest that a lower level of functional loading by providing small-size diet could reduce TMJ degradation induced by the biomechanical stimulation from abnormal occlusion.