Radiographic assessment of congenitally missing teeth in orthodontic patients.

OBJECTIVE: To estimate the prevalence of congenitally missing teeth (CMT) in patients evaluated for orthodontic care. A sample of 668 panoramic radiographs from Mexican population was reviewed from orthodontic healthy patients, 9-20 years old. METHOD: Statistical tests were used to compare maxillary and mandibular hypodontia in right and left quadrants between males and females. RESULTS: Prevalence was 27% when all teeth were included, and was 2.7% when third molars were excluded. A significant difference was found between maxillary and mandibular symmetrical CMT in third maxillary molars, lateral incisors and second premolars (P < 0.001), and in mandibular third molars and central incisors (P < 0.001). CONCLUSIONS: We confirmed that occurrence of CMT in the permanent dentition of a Mexican population most often affects third molars, followed by maxillary lateral incisors and then mandibular second premolars.

[Relaxation of an elastomeric chain under constant elongation]. / Relajación de una cadena elastomérica bajo elongación constante.

A modality of elastomeric chain, Power Chain II (Ormco Co) was studied in vitro in order to observe the behavior of the force it exerts under constant elongation. Eight groups of ten samples each of the elastomeric material were placed at two different distances, 25 and 35 mm, and then proceeded to measure their magnitude at different time intervals (initial force, after 24 hours, a week and a month) under four varying conditions: cut in three and four links, pre-elongated and not pre-elongated. All specimens were immersed in saline water at a constant 37 degrees C (98.6 degrees F). Relaxation of the force observed after 24 hours was, for all groups, 52.85%, similar to that found in other studies. Although decrease of residual force was lower in the pre-elongated groups, larger residual magnitude was observed in the non pre-elongated groups.