Determining the relationship of an arbitrary and true hinge axis

Objectives: The objective of this study was to compare the difference of the relationship of an arbitrary hinge axis location and a true hinge axis.

Methods: Eight subjects (six men and two women with an age range of 25 to 50) at the University of Texas Health Science Center were selected to evaluate this relationship.  This location was established on a line twelve mm anterior to the top of the tragus and the most anterior part of the external auditory meatus  on a line with the anterior point being 43mm superior to the distal incisal line angle of the maxillary right central incisor.  A (D) kinematic facebow was attached to a mandibular clutch and the patient’s mandible was guided into a centric relation position.  The (D) kinematic facebow was adjusted until the exact location of the true hinge axis was established.  The distance between the arbitrary and true hinge axis was measured using a digital caliper.

Results: A statistical significant difference p less than 0.05 using a paired t-test was observed between the arbitrary hinge axis and the true hinge axis.  The means for the right and left arbitrary hinge axis was 12.00 mm and 12.00mm.  The means for the right and left true hinge axis was 4.08mm and 4.38mm and the range of the right hinge axis was 3.22 to 9.27 and the left hinge axis 2.78 to 9.11.

Conclusions: Although there are similarities between the arbitrary hinge axis and the true hinge axis, there was a difference between these locations.  This study showed the mean distance from the arbitrary hinge axis to the true hinge axis was less than the 5 mm currently accepted difference for use of the arbitrary facebow; however the range showed some of the true hinge axis findings were more than this 5mm accepted difference.