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Cementum crack formation by repeated loading in vitro.

BACKGROUND
In this study, the hypothesis was adopted that fatigue destruction may develop on the cementum of the root surface because of the repeated occlusal stresses loaded on the tooth as in the case of prolonged occlusal trauma. The purpose of this study is to clarify whether cemental fatigue destruction occurs when repeated stresses are loaded on the occlusal surface of a tooth.


METHODS
The teeth used were five human mandible premolars from individuals 14 to 22 years of age. The teeth were freshly extracted in orthodontic treatment. They were free of decay and fillings, and their roots were straight. One half of the root was embedded in cold curing acrylic resin, and a compressive load of 5.0 kgf was repeatedly applied along the tooth axis in water 1 million times with a speed of one time per second. Changes in the tooth surface were observed by taking photographs with a stereoscopic microscope and a digital camera after every 100,000 loadings. Cracks were dyed with 2% methylene blue solution, and crack areas were measured using gray-scaled and binary-processed photographs. Data were analyzed using image analysis software.


RESULTS
It was confirmed that cracks significantly developed on the buccal, mesial, and distal surfaces but not on the lingual surface; crack areas showed a tendency to significantly increase after 500,000 loadings; extension of the crack from the cemento-enamel junction to the root apex increased with time, and the average extension was 0.67 +/- 0.12 mm after 1 million loadings.


CONCLUSION
This study revealed that cementum cracks developed in the cervix by repeated loadings and extended toward the root apex with time, suggesting that repeated stresses by occlusion or strong stresses such as by bruxism may trigger the development of site-specific attachment loss, which is one of the potential factors inducing periodontal diseases.