Etiology of elderly crowding analyzed by crown angulation and inclination

Objective: Crowding of varying severity is often seen in lower arches of elderly with many remaining teeth. Lower arch crowding adversely affects long-term stability of orthodontic treatment. This study was designed to quantify crown angulation and inclination in elderly aged over 80 with 20 or more teeth remaining (8020 achievers) and test the hypothesis that lower arch crowding results from changes in tooth axis associated with occlusal changes. Methods: Dental casts of twenty 8020 achievers (10 males, 10 females, average age 82 years, average number of 28.2 remaining teeth) were digitized with 3D laser scanner, reconstructed into 3D images and measured with analytical software. Crown angulation and inclination were measured relative to occlusal plane formed by central incisors edge and buccal cusp tips of first molars. These measurements were compared with those of young adults with normal occlusion enrolled in Sebata-Motegi study. Results: Crowns of lower centrals, laterals, canines and first premolars were significantly more mesially angulated in 8020 achievers than young adults (P<0.01-0.001). 8020 achievers had significantly greater labial crown inclinations of upper canines(P<0.001), lower central and lateral incisors than young adults (P<0.05). Upper arch crowding was significantly less than lower arch crowding in 8020 achievers (P<0.001). These results suggest that labial inclination of upper canines may increase with age, creating room for incisors and thereby reducing opportunity of upper anterior crowding, whereas lower arch length may increase with mesial tipping and labial inclination of anterior and mesial tipping of first premolar teeth, contributing to lower anterior crowding, indicating that lower anterior crowding is unlikely to be caused by lingual tipping of incisors. Conclusion: Measurement of crown angulations and inclinations in 8020 achievers revealed differences in tooth axis from young adults and suggested changes in tooth axis with age as possible cause of anterior crowding.