Recovery of the dental and periodontal tissues of the rat incisor following application of continuous intrusive loads: A long-term study

The present investigation deals with the functional recovery of the rat incisor periodontium following application of optimal intrusive loads for a period of 2 weeks and addresses the relationship between function and integrity of dental and periodontal tissues. The daily rate of eruption served as an indicator of the functional capacity of the rat periodontal ligament (PDL), while the condition of the PDL was evaluated by means of serial histologic sections. The left mandibular incisors of thirteen experimental and seven control rats were shortened to prevent occlusion, and calibrated loads of 19 ± 1.6 grams were applied to the teeth of the experimental animals. Following removal of the springs, the daily rate of eruption of all teeth was measured during a recovery period of 80 days. In all experimental animals, prolonged impairment of the eruptive potential had occurred: In 50% the rate of eruption was fully restored 8 weeks after removal of the loads, and in these animals the histologic evaluation showed an almost normal PDL. In the remaining 50% gross impediment in the eruption rate persisted throughout the entire experimental period. The intratissue forces operating during the recovery period had sufficient strength to cause a degree of periodontal injury equivalent to that ensuant on direct loading. Extensive newly developed necrotic lesions were evident in the initially stretched mesial PDL. Areas of dentin resorption, denuded cementum, and injured enamel appeared a considerable time after load removal. Periodontal injury included disorientation and loss of attachment of collagen fibers, presence of scarlike tissue, and infiltration of inflammatory cells. A positive relationship existed between individual response of tooth movement under loading, extent of injuries, and functional impediment.