Standard and limit values for the symmetry of articulation parameters in the temporomandibular joint area - evaluations of the associated project of the SHIP study

The articulation parameters, especially the horizontal condylar inclination angle (HCI), the Bennett angle (BA), and the immediate side shift (ISS) were determined in 259 subjects (100 males; 159 females) of the associated project with reference to the population-representative baseline study (Study of Health in Pomerania, SHIP 0). The evaluations were based on a clinical functional status and electronic motion recording with the ultrasonic measuring system Jaw Motion Analyser (JMA, Zebris, Isny, Germany). The reference plane, to which all measured values were represented and the HCI calculated, was the hinge axis infraorbital plane (HA-IOP). The HCI was determined after an excursive movement with a length of 4 mm to the HA-IOP in the sagittal view and the BA after a mediotrusive excursion movement of 6 mm in the horizontal view to the midsagittal plane. For the standard and limit values, the average value in addition to the standard deviation and the 10th and 90th percentile value (10th percentile value, 90th percentile value) were determined: HCI right 52.1 ± 10.14 degrees (39.4 degrees, 64.0 degrees), HCI left 53.1 ± 9.67 degrees (42.3 degrees, 67.0 degrees), BA right 15.2 ± 7.53 degrees (6.7 degrees, 25.0 degrees), BA left 14.2 ± 7.84 degrees (5.4 degrees, 24.1 degrees). The HCI was approximately 2 to 3 degrees larger in females (males: right 50.5 ± 9.47 degrees, left 51.9 ± 8.99 degrees; females: right 53.1 ± 10.42 degrees, left 53.8 ± 10.03 degrees). Likewise, the BA in the age group ≥ 40 years (males: right 14.4 ± 6.62 degrees, left 13.1 ± 7.14 degrees; females: right 17.0 ± 9.02 degrees, left 16.9 ± 8.72 degrees). The latter proved to be statistically significant in the t test for independent samples, assuming variance equivalence on the right, with P = 0.009, and with rejection of the variant equivalence on the left, with P = 0.002. The right and left HCI and BA joint values showed highly significant linear dependence at P 0.001, but rather low, however, for the HCI with r2 = 0.175 for the HCI and r2 = 0.228 for the BA. In 46% of cases, the right and left HCI values differed up to 5 degrees only; a further 20.9% were in an interval difference of between 5 and 10 degrees. The following results were shown for the differences in the BA: 56.4% of the cases were between 0 and 5 degrees, and 26.2% were in the interval ranges of 5 to 10 degrees. ISS occurred in 18.1% of cases on the right side of the joint, and in 27.8% of cases on the left side. On both sides of the joint it was significantly more frequent in the age group ≥ 40 years with assumed variance equality than in the age group 40 years (P = 0.002 right, P = 0.003 left). The groups relating to the Helkimo index (HI) did not differ significantly in all function-specific parameters. If it is assumed that there is no significant influence on the occlusion if the HCI values differ by 7 to 8 degrees from the average value, only approximately one third of all cases (35.1%) were characterized by a purely average value setting in the articulator. In 41.7% of cases, one joint value was situated outside the average value range; in 23.2% of the cases both values were outside the average value range. Without a measurement of the condylar path inclination, however, it is impossible to decide to what extent the HCI deviates from the average value, and which joint side is larger or smaller than the other and to what extent. These results suggest that in extensive and complex cases, the articulator should be adjusted according to individual, function-specific joint values. Keywords: population-based values of condylar adjustment, population-based standard and limit values for the adjustment of joint values, articulator, articulation, sagittal condylar inclination, horizontal condylar inclination angle (HCI), Bennett angle (BA)