Jaw-muscle activity changes after the induction of osteoarthrosis in the temporomandibular joint by mechanical loading.
OBJECTIVE
To examine the effect of mechanical loading on the induction of temporomandibular joint osteoarthrosis (TMJ OA).


METHODS
Mechanical stress was applied to the rat TMJ by forced jaw opening of 3 hours a day for 5 days. The electromyographic (EMG) activity of the masseter and digastric muscles was continuously monitored by radio-telemetry. It was characterized by the total time each muscle was active (duty time), the number of bursts, and the average burst length. For histologic analysis, rats were sacrificed before, immediately after, and 3 weeks after the period of forced jaw opening.


RESULTS
The condylar cartilage revealed OA-like lesions with a decrease in the number of chondrocytes immediately after forced jaw opening. Three weeks later, the OA-like lesions were repaired to some extent. After the forced jaw opening, the duty time of the masseter increased, whereas the duty time of the digastric decreased significantly (P < .01) at the 5% activity level. Three weeks later, the masseter duty time had decreased and the digastric duty time had slightly increased, returning to the levels observed before forced jaw opening.


CONCLUSIONS
These results suggest that mechanical overloading of the TMJ induced OA-like lesions with a simultaneous influence on jaw muscle activity, especially at the low activity level. This might imply that muscle activity adapted to reduce the effects of (forced) joint overloading.
Kawai
Nobuhiko
N
Department of Orthodontics and Craniofacial Developmental Biology, Hiroshima University Graduate School of Biomedical Sciences, Hiroshima, Japan. kamo@hiroshima-u.ac.jp
Tanaka
Eiji
E
Langenbach
Geerling E J
GE
van Wessel
Tim
T
Sano
Ryota
R
van Eijden
Theo M G J
TM
Tanne
Kazuo
K
D000818:Animals; D002358:Cartilage, Articular / Q000473:pathology; D003799:Dental Stress Analysis; D004576:Electromyography; D008297:Male; D008406:Masseter Muscle / Q000503:physiopathology*; D009334:Neck Muscles / Q000503:physiopathology; D010003:Osteoarthritis / Q000503:physiopathology*; D051381:Rats; D017208:Rats, Wistar; D013314:Stress, Mechanical; D013686:Telemetry; D013705:Temporomandibular Joint Disorders / Q000503:physiopathology*