Condylar Degradation from Decreased Occlusal Loading following Masticatory Muscle Atrophy

Abstract

Objective The masticatory muscles are the most important contributor to bite force, and the temporomandibular joint (TMJ) receives direct occlusal loading. The present study aimed to investigate condylar remodeling after masseter muscle atrophy in rats. Methods Sixty 5-week-old female Sprague-Dawley rats were divided into the following 3 groups: the control group, soft diet (SD) group, and botulinum toxin (BTX) group. The cross-sectional area (CSA) of the masseter muscles was investigated as well as atrogin-1/MuRF-1 expression. Changes in the condylar head were evaluated by H-E, toluidine blue staining, and contour measurements. The biomechanical sensitive factors PTHrP Ihh, Col2a1, and ColX of condylar cartilage were detected by immunohistochemical staining and western blotting. Furthermore, micro-CT and tartrate-resistant acid phosphatase (TRAP) staining were performed to determine the osteopenia in subchondral bone. Results The histological and protein analysis demonstrated muscle hypofunction in the SD and BTX groups. Condylar cartilage contour was diminished due to different treatments; the immunohistochemistry and protein examination showed that the expressions of PTHrP, Ihh, Col2a1, and ColX were suppressed in condylar cartilage. A steady osteoporosis in subchondral bone was found only in the BTX group. Conclusion The current results suggested that a steady relationship between muscular dysfunction and condylar remodeling exists.

Masseter muscle atrophy impairs bone quality of the mandibular condyle but not the alveolar process early after induction

“…In this study, significant bone loss of BoNTA‐right condyles was observed after 2 weeks. This is consistent with the potential mandibular bone deterioration after BoNTA intervention suggested by studies in humans and demonstrates that the degradation of the mandibular condyle is established earlier than reported by other studies using female mice, male and female rats and female rabbits . These animal studies only assessed bone tissue after 4 weeks and later.…”

“…In addition, during this initial phase, nerve sprouting is evident in the lower limb muscles of rats, the levator auris longus muscle from the neck of mice and the masseter muscle of non‐human primates, which may explain the early masticatory function recovery seen in adult rodents after 10 days . After one single injection, BoNTA rapidly paralyses the masseter, followed by a severe loss of muscle mass . If this BoNTA‐induced masseter muscle atrophy is performed during rodent development, the subchondral bone of the mandibular condyle is deteriorated after 4 weeks .…”

“…After one single injection, BoNTA rapidly paralyses the masseter, followed by a severe loss of muscle mass . If this BoNTA‐induced masseter muscle atrophy is performed during rodent development, the subchondral bone of the mandibular condyle is deteriorated after 4 weeks . Also, at the same time, significant bone loss in both mandibular condyle and alveolar process has been found in adult animals such as male rats, female rabbits and female mice using unilateral BoNTA injection in the masseter muscle.…”

Mandibular Bone Loss after Masticatory Muscles Intervention with Botulinum Toxin: An Approach from Basic Research to Clinical Findings

“…A significant reduction in bone remodeling processes of the samples from the experimental side was detected in young adult female mice as a decrease in both osteoclast activity (measured with TRAP staining) [63,66] and bone mineralization (assessed through alkaline phosphatase staining and fluorescent dyes for mineralized bone) after four weeks [63]. In contrast to these findings, Shi et al reported, in adult female rats, a significant increase of the TRAP staining in the subchondral bone of mandibular condyles four weeks after bilateral injection of BoNT/A in the masseter muscles, when compared with samples from animals bilaterally injected with saline solution [64]. However, different techniques were implemented to assess TRAP staining: In the samples from mice [63,66], a fluorescent approach was employed using TRAP-positive pixels for quantification, whereas the study with rats used an immunohistochemical procedure with multinucleated positive TRAP cells quantitation [64].…”

Influence of age and gender on sex steroid receptors in rat masticatory muscles

“…The representative of this group is the Digastric (Anterior and Posterior) muscle. The main difference between the human and rat temporomandibular articulation is the biomechanics of movement and the type of incident load resulting from the different nutritional diets among species, albeit a static load is essential for the growth and maintenance of the temporomandibular joint 19–22 . Therefore, physiological experiments have some limitations.…”