Mandibular third molar eruption in relation to available space as assessed on lateral cephalograms.

It has previously been suggested that the distance between the Xi point (Ricketts 1972) and the distal aspect of the mandibular second molar (M2), measured on lateral cephalometric head films, should be at least 25 mm by adulthood to allow for the normal eruption of the third molars (M3s) in the lower jaw. It was the aim of this investigation to test the validity of this statement in two groups of young adults (mean age 25 years). The individuals in the first group had complete dentitions including M3s in good occlusions. In the second group all M3s had been extracted due to failure to erupt. Mandibular length and morphology and the dimension of the ramus was also studied in the same material. All measurements were made on standard lateral head films. The distance Xi-M2 was found to be significantly greater in individuals with complete dentitions as compared with individuals who had impacted M3s. Furthermore, this variable exhibited significantly greater values in males than in females in both investigation groups. However, in nearly fifty per cent of the subjects with complete dentitions, the M3s had erupted despite a Xi-M2 distance of less than 25 mm. Mandibular length in males was significantly greater in the complete group than in the extraction group. In females no such difference was recorded. The angular measurements used in this study did not differ significantly between the complete and the extraction groups.