Loss of Masticatory Function Affects Growth and Development of the Mandibular Condyle in Rats

Abstract

The effects of childhood masticatory function loss and soft foods on the mandibular condyle have been the subject of much research. However, the corresponding bone turnover is not fully understood. The purpose of the present study was to clarify the effects of a lack of teeth and a soft food diet during the growth period on bone turnover in the mandibular condyle. We divided 3-week-old Wistar rats into the following three groups: 1) Extraction group: The maxillary molars were extracted at the age of 4 weeks, and animals were fed powdered standard feed. 2) Powder group: Animals were fed powdered standard feed without tooth extraction. 3) Control group: Animals were fed solid standard feed without tooth extraction. Non-decalcified thin-slice specimens of sagittal sections of the mandibular condyle were obtained at the age of 20 weeks for histological analysis. We used micro-CT analysis and bone histomorphometry to measure bone volume (BV), bone mineral content (BMC), bone mineral density (BMD), bone microstructure, bone resorption, and osteogenesis in the mandibular condyle, and we compared the results among groups. In the extraction and the powder groups, we found deformation and disruption of the arrangement of chondrocytes, coagulation of chondrocytes, and duplication of the tidemark in the cartilage. We also found an increase in multinuclear osteoclasts in the cancellous bone. We found a reduction in BV, BMC, and BMD in the extraction and powder groups compared to the control group, as well as a reduction of bone volume, a lowering of osteogenesis parameters, and an increase in bone resorption parameters in the secondary cancellous bone. These results suggest that a lack of teeth and a soft food diet during the growth period cause a decline in bone microstructure, a decrease in osteogenesis, and an increase in bone resorption.

Functional Analysis of Rhythmic Jaw Movements Evoked by Electrical Stimulation of the Cortical Masticatory Area During Low Occlusal Loading in Growing Rats

“…Rats exhibit a smaller mandibular condyle and thinner cartilage when fed a soft diet during growth (Kato et al, 2015). Previous studies have reported that there was a regressive change in the trabecular structure when being fed a soft diet, which can reduce the resistance of the mandibular condyle to mechanical loading, and increase the occurrence temporomandibular deformation (Ogawa et al, 2016;Kono et al, 2017). In addition, alterations in occlusal stimuli followed by consumption of a soft diet may result in a decrease in the response of the mechanoreceptors of the temporomandibular joint (Ishida et al, 2009).…”

Mandibular condyle changes in rats with unilateral masticatory function

“…Soft diet and molar extraction are conditions leading to a lower masticatory function. 12,19,20 Although it has not been verified in this study, based on other studies, [20][21][22][27][28][29]32 we assumed that extracting the maxillary right molars of young and adult rats could create a unilateral masticatory function, which in turn could cause changes to the condylar process.…”

“…Young rats were 4 weeks old, which corresponds to the initial period of usage of the teeth, which finish erupting at 21 days, 36 while adult rats were 26 weeks old. Many studies report similar ages when evaluating young and adult rats 10–18,21,22,27–29,31,32 …”

“…Micro‐CT has also been used to study condylar changes on rats with a diminished masticatory function 22,27–30 . To the best of our knowledge, however, there are only two references in the literature 31,32 in which an in vivo micro‐CT examination was used to examine the effects of a decreased masticatory function on rats, induced by molar extraction and soft diet.…”

Influence of Masticatory Functional Loss on the Remodeling of Alveolar Bone in Rats

“…Similarly, our previous research showed masticatory hypofunction in rats, due to extraction of maxillary molars and a diet of powdered feed, was associated with reduced mandibular volume, bone density, mandibular height, and mandibular condyle thickness [7] [8]. Additionally, we indicated that reduced masticatory function led to deformation and cellular disorders in the condylar cartilage, and diminished ossification and heightened bone resorption in the secondary cancellous bone of the condylar head, resulting in slower ossification, less bone volume, and reduced trabecular thickness [9]. There are also reports on the effects of occlusal stimuli on the alveolar bone: In rats, a soft diet, tooth extraction or the insertion of a bite-opening appliance were associated with a thinner [10] [11] [12] [13] and higher alveolar process [6] [10] [13] [14] [15], and sparser mandibular alveolar architecture [15] [ 16].…”