The response of supraalveolar gingival collagen to orthodontic rotation movement in dogs.
An orthodontically rotated tooth relapses toward its pretreatment position. Explanations for this phenomenon have been given after light microscopic studies, according to which it had been assumed that stretched supraalveolar gingival fibers pulled back the tooth and brought about relaxation of the stretched fibers. The rotational relapse, however, can be prevented by supraalveolar fiberotomy of the gingiva around the tooth. This investigation was initiated to reevaluate the validity of the hitherto assumed causes for the relapse, by obtaining ultrastructural data on the response of collagen fibers after orthodontic intervention. Lateral maxillary incisors in the dog were rotated with bonded fixed appliances. The teeth were divided into groups according to different orthodontic procedures. Scanning and transmission electron microscopic analyses were performed on gingival samples after proper processing. Analyses of the untreated control samples showed well-organized, parallel, and densely packed thick bundles of collagen fibers, interconnected with thin fibers. After rotation-followed-by-retention, the gingival fibers were torn, ripped, disorganized, and laterally spaced and of increased diameter. Thus it was concluded that all these patterns are incompatible with stretching. Also, an increased number of elastic fibers were seen in proximity to the torn collagen fibers. After gingival fiberotomy, most fibers resumed the appearance of the organized pattern of large fiber bundles similar to those seen in the controls.