The geometry of the arbitrary hinge axis as it relates to the occlusion

Abstract

I n order to program a dental articulator to duplicate the hinge opening and closure of the mandible, it is necessary to make a hinge axis record and transfer this information to the articulator. The transverse hinge axis point may be located most precisely by a kinematic process, or its location may be estimated by some arbitrary method. Several studies have been made to determine the relationship between the kinematic or transverse hinge axis (THA) and the arbitrary hinge axis (AHA) points. Schallhorn’ found that in over 95% of his subjects the true center of rotation was within 5 mm of the arbitrary center of rotation. Lauritzen’s and Bodner’s’ study indicated that 33% of the arbitrary axis points were within 5 mm of the true hinge axis. By redefining the location of their arbitrary center, 70% of the true axis points occurred within 5 mm of the new arbitrary center. Teteruck and Lundeen3 compared the accuracy of the earpiece face-bow with that of a conventional AHA location. Their studies showed that 33% of the arbitrary axis locations were within 6 mm of the true hinge axis point, while 56% of the earpiece face-bow axis locations occurred within 6 mm of the true hinge axis point. Walker4 studied the discrepancies between the arbitrary and the true hinge axis location. He found that only 20% of all arbitrary points were within 5 mm of the true axis points. Weinberg5 was the only investigator to actually consider the magnitude of the occlusal error produced by an error in the location of the true hinge axis point. His calculations, however, were based on only one theoretical axis location, which was in error by 5 mm anterior to the true hinge axis point. While it is recognized that the location of an arbitrary axis point is usually in error, its significance at the occlusal level is the only important issue. This study was designed to determine the influence of the AHA on the relationship of the mandible to the maxillae.