Dynamics of tissue changes found after mechanical loading of the rat incisor. II. A three-dimensional longitudinal study of the histopathologic aspects

The study comprised 60 lower left rat incisors subjected to 2 weeks of loading (19.7 +/- 1.6 gm) and 5 control teeth. After the loading period, 10 rats were killed with the springs in situ, and the remainder were killed in groups of 5 at intervals of 1 through 10 weeks after the springs were removed. The teeth were cross-sectioned serially (2 microns). The distance of each section from the apex and the time of its tissue formation in relation to load removal were calculated. Bone remodeling and dental and periodontal trauma were plotted according to their location on the tooth axis and circumference. Vigorous bone remodeling continued throughout the observation period, apposition and resorption sites intermingling around the tooth. Directionality, as expressed by the resorption/apposition ratio, for the formerly tensed and stressed areas was 0.61 +/- 0.07 and 1.79 +/- 0.52, respectively. However, in some of the groups directionality was negligible or reversed. New lesions of dentin (folding and resorption) were formed for 6 weeks after loading, whereas new damage to the enamel organ and enamel matrix appeared until week 9. The integrity of the PDL deteriorated for the first 8 weeks, as evidenced by edema, cell loss, scarring, and collagen fiber disorientation. Necrosis occurred throughout the recovery period. Fiber to bone attachment was disrupted by formation of big sinusoids adjacent to the bone. The frequency of most lesions decreased toward the end of the observation period. It is suggested that tissue recovery proceeds in subsiding cycles of tissue injury/tissue repair, a process that is much more prolonged than is generally believed.