pical root resorption observed mid-or posttreatment is occasionally of passing interest to the clinician, but usually of little import. Fortunately, truly severe resorption that threatens the longevity of the tooth or forces a halt to treatment is rare. The fact is, however, that orthodontic tooth movement does directly cause irreversible resorption of the root. As the movers of the teeth, it is incumbent for us to know as much as possible about the causes, effects, and prevention of this phenomenon.
The orthodontic literature contains many reports of clinical and laboratory investigations of root resorption. The principal difficulties in studying root resorption are the infrequency of severe shortening and the many possible factors that can be associated with the condition. Studies vary considerably in their findings on the same variables because of sample size and selection, exclusion of important factors, measurement methods, and other problems. [1][2][3][4][5] The purpose of this study was to examine previously postulated diagnostic and treatment factors and new variables (such as ethnicity) in a large clinical setting to determine which factors are associated with severe root resorption. In contrast to most previous investigations that used panoramic films to examine the maxillary anterior teeth only, we measured and assessed the 24 teeth from first molar to first molar with periapical radiographs. The ultimate goal of this comprehensive study was to develop a diagnostic index that is based on the significant findings from this large data set.
Part I of this series will discuss the findings of this investigation with respect to incidence and diagnostic factors. Part II will conclude with a discussion of treatment factors and clinical recommendations.
MATERIAL AND METHODS
Six university-educated, private orthodontic practitioners with at least 10 years of full-time experience provided pre-and posttreatment full-mouth radiographs and lateral cephalometric radiographs for 868 patients from their practices. These patients were selected from blocks of consecutively treated patients. All of the patients were treated between 1980 and 1996 with complete maxillary and mandibular fixed appliances. A consistency in treatment philosophy and mechanics was established for each practitioner during the period in which records were collected. Not all teeth were visible or measurable on all records.
Demographic information that was collected included age at start of treatment, gender, office, and ethnicity. Treatment variables were extractions, slot size, wire type, elastic use, months in treatment, orthognathic surgery, and transverse treatments. Dental characteristics included root length, root shape, crown length, overjet, overbite, presence of previous endodontic treatment, his-From the