Association between Björk's structural signs of mandibular growth rotation and skeletofacial morphology.

The aim of this study was to apply Björk's structural signs of mandibular growth rotation to answer the questions: (1) Is a hyperdivergent or hypodivergent skeletofacial growth pattern characterized by a specific mandibular morphology? (2) Are severe skeletofacial hyperor hypodivergencies recognized more easily than mild ones? (3) Are skeletofacial hyper- or hypodivergencies recognized more easily in older than in younger subjects? Mandibular cuttings from lateral head films of 135 Class I or Class II subjects were surveyed twice by nine observers. Of the 135 subjects, 95 subjects exhibited a large (ML/NSL > 38 degrees) and 40 a small (ML/NSL < 26 degrees) mandibular plane angle. Using the structural signs of mandibular growth rotation, the observers had to categorize the subjects as having either a high-angle or low-angle skeletofacial morphology. In 14% (13 of 95) of the subjects with a large ML/NSL angle, the skeletofacial hyperdivergency was recognized in all registrations, but in 19% (18 of 95), the hyperdivergency was identified in less than half of the registrations. In 63% (25 of 40) of the subjects with a small ML/NSL angle, the skeletofacial hypodivergency was recognized in all registrations, whereas in only 2.5% (one of 40), the hypodivergency was identified in less than half of the registrations. There was no association between the degree of hypo- or hyperdivergency or the age of the subjects and the number of correct registrations. Using the structural method of Björk, it was difficult to categorize the subjects as having either a hyperor hypodivergent skeletofacial morphology. However, hypodivergency was recognized more easily than hyperdivergency.