Terminal hinge axis--Hobo point--Frankfurt horizontal relations

Aim of this investigation was to determine the spatial relations of the terminal hinge axis of the temporomandibular joints toward Frankfurt horizontal. The sample consisted of 50 examinees. Terminal hinge axis was located, using Lauritzen's technique, on the left side of face for each examinee. Arbitrary point, as proposed by Sumiya Hobo, was measured and point Orbital was defined by palpation. On designated points steel pellets 0.25 mm in diameter were stocked. Lathero--latheral cephalogram was obtained for each examinee. Cephalograms were traced down on a transparent foil and points Orbital, THA--terminal hinge axis, Hobo and Porion--as the uppermost point of the metal ear rod of the cephalostat, were designated. Connecting points Orbital and Porion, Frankfurt horizontal was defined, and shortest distance toward points THA and Hobo, as well as between them were measured. Lines parallel and perpendicular to Frankfurt horizontal were drawn through point THA. In a co-ordinate system defined in such a way, upper left quadrant was first, upper right second, lower right third, and lower left was fourth. Observation was made in which quadrant Hobo point was located. Measured values were statistically evaluated. Results revealed that Hobo point is located in first quadrant in 5 cases, in second in 7, third quadrant in 18, and in fourth in 20 cases. That means that Hobo point is located lower to the THA point in majority of our population. Points THA and Hobo were not identical in any case. Mean values of variables showed that investigated points were inferior to the FH. Measures of variability revealed great variability of both points toward FH, as well as for their mutual distance. It could be pointed out that THA point should be defined kinematically during prosthodontic procedures, and that orientation in articulator's space should not be according to the FH.