The significance of errors in hinge axis location

Accuracy of a predetermined transverse horizontal mandibular axis point

STATEMENT OF PROBLEM

The transverse horizontal mandibular axis point may be located most precisely by a kinematic process. However, an anatomical method of locating the axis is also an acceptable technique, and an easily determined point that is consistently close to the kinematic axis would simplify transfer of the arc of rotation from the patient to the articulator.

PURPOSE

This in vivo study compared the location of an anatomically predetermined hinge axis point with the determined kinematic axis.

MATERIAL AND METHODS

Forty subjects (27 males, 13 females; 23 to 47 years of age) with functionally acceptable occlusion and no detectable clinical signs of temporomandibular disorders participated in the study. The earpiece alignment flags on a mechanical SAM Axiograph III combination flag/face-bow were used to locate the right and left predetermined hinge axis points, 10 mm anterior to the earpiece. The right and left kinematic center of rotation was located as described by Lauritzen and confirmed with the PC Axiotron electronic Axiograph to within 0.25 mm. All points were transferred to 1 mm(2) grid paper on the subject's skin. The distance between each predetermined and kinematic point was measured +/-0.25 mm. Wilcoxon and Mann-Whitney tests were used to examine differences between the left and right axis points and potential significant differences between genders at a significance level of P<.05. The number of occurrences and the distance of the predetermined axis points from the kinematic axis also were described.

RESULTS

The mean distance between points was 1.1 mm on the right (range 0.0 to 3.0 mm), 1.2 mm on the left (range 0.0 to 3.0 mm), and 1.1 mm for all 80 points (+/-0.63). More than 96% of the predetermined points were within 2 mm of the kinematic axis, and 67% were within 1 mm. There was no significant difference between the right and left points and no significant differences based on gender.

CONCLUSION

Within the limitations of this study, the results suggest that the predetermined axis point is well within the clinical norm for estimated location of the transverse horizontal mandibular axis.

Occlusal plane discrepancies generated by transverse horizontal axis deviations

STATEMENT OF PROBLEM

Any deviation from the patient's rotational axis of the mandible during closure and the arc of closure on an articulator has been reported to produce occlusal discrepancies. These discrepancies can affect diagnostic planning procedures and the occlusal relationships of restorations.

PURPOSE

This study investigated the magnitude and types of occlusal errors produced by deviations between the patient's transverse horizontal axis and the axle on the articulator.

MATERIAL AND METHODS

A simplified monoplane model was diagrammatically analyzed for deviations from the model's true transverse horizontal axis. Axis deviations of 5 mm were used in 8 different directions from the true transverse horizontal axis of the model. Changes in the orientation of the mandibular plane to the maxillary plane were illustrated diagrammatically. These changes were quantified by geometric and trigonometric assessments.

RESULTS

All axis deviations produced mandibular plane orientation errors with angular plane malalignment to the maxillary plane on complete closure. These axis deviations also produced anterior-posterior shifting of the mandibular plane relative to the maxillary plane.

CONCLUSION

Several different types of planar shifting and angular discrepancies were demonstrated from deviations away from the true transverse horizontal axis.

A color-enhanced aid for location of the transverse horizontal opening and closing axis of the mandible

This article describes the use of a device for assisting in identification of the direction of the arc of rotation while locating the transverse horizontal opening and closing axis of the mandible. Translucent disks are mounted on the point of the styli of a kinematic face-bow and centering magnets are aligned with them, with no overlapping of the colors. The direction of the arcing movement is enlarged by the overlapping of the translucent disk over the centering magnet. The different color value generated by the overlapping identifies the arcing direction. After correction on the microadjustable arm, the centering magnet is realigned on the translucent disk. This sequence is repeated until the disk no longer overlaps with the centering magnet during the opening and closing movements of the mandible. This device uses the different color generated by mixing the color of the disk against a template of darker color. The disk enlarges the point of the stylus, thus magnifies the direction of the arc of rotation, and potentially reduces the clinical time necessary to locate the points of pure rotation.

Computer simulation of occlusal discrepancies resulting from different mounting techniques

The effect of arbitrary mounting of maxillary casts on occlusal relationships was investigated in this study. Maxillary casts of 31 volunteers were mounted on an articulator by use of two split cast bases. This mounting was done first with the arbitrary face bow and second with a hinge bow. Three reference points were defined and measured on each maxillary cast with a three-dimensional digitizer. The measurements were taken from the arbitrarily mounted cast and from the cast mounted according to the hinge axis. Opening and closing movements that were transferred according to the hinge axis. Opening and closing movements that were transferred from the articulator to the mouth of the patient were simulated by a computer based on measurements of the reference points. The results revealed that the use of an arbitrary face bow causes a deviation of the hinge-axis points from the precise axis of more than 5 mm in 77% of the cases. Resulting occlusal errors depended on the angles between the arbitrary and precise axes and the direction of the axis shifts. The occlusal error is roughly proportional to the shift or tilting of the hinge axis in millimeters or degrees. For a given deviation of the arbitrary and precise axes, the occlusal error is proportional to the record height. For a record height of 2 mm or more, an occlusal error of more than 0.1 mm will occur. An average occlusal error of more than 0.1 mm would most likely lead to the necessity of extensive selective grinding of occlusal discrepancies in the patient's mouth.

The use of the maxillary interocclusal splint as a mouthpiece for the mouthstick prosthesis

In the past, the maxillary and mandibular teeth have been firmly indexed on the mouthpiece prosthesis, which can produce significant occlusal discrepancies. The construction of a mouthpiece consisting of a maxillary interocclusal splint is described. The two advantages for this type of mouthpiece are (1) it can be adjusted clinically to accommodate errors in the mounting of the casts and the laboratory processing of the prosthesis, and (2) the use of the maxillary interocclusal splint will reduce the likelihood of the development of masticatory muscle spasms.

Application of a geometric principle for locating the mandibular hinge axis through the use of a double recording stylus

The hinge axis of the mandible has been determined by using double recording styli at a distance of 2 to 4 inches from the estimated axis area. Visible arcs in pairs on a plain recording surface become the basis for a geometric construction by which the true axis of rotation can be identified. The theoretical basis for this method of axis location is sound and practical as applied and the clinical results indicate that the procedure is accurate.

Location of the terminal hinge axis and its effect on the second molar cusp position

Incorrect location of the terminal hinge axis of 5 and 8 mm to the anterior, posterior, superior, and inferior was examined. With jaw relation records 3 and 6 mm thick at the incisors, the errors in cusp height at the second molar ranged from 0.15 mm open space to 0.4 mm excess height. The mesiodistal error ranged from 0.51 mm toward the distal to 0.52 mm toward the mesial. While the mesiodistal component to the error has been calculated in the past with some accuracy, the values obtained have varied because different anatomic dimensions were used. In addition, the vertical component of the error in cusp height as illustrated in Fig. 2 was not considered and/or not subjected to in-depth calculation.

An investigation of the reproducibility of face-bow transfers

An investigation was conducted to determine the reproducibility of the position of a maxillary cast in two widely-used semi-adjustable articulators when a face-bow transfer was repeated ten times for the same individual. The results obtained with the Dentatus AEB, Whipmix Quickmount and Almore transfer bows were compared. The variations in cast position rarely exceeded +/- 1.0 mm in any of three planes of space and although the results suggested a ranking in accuracy of the three instruments, all were considered clinically acceptable. On the basis of the results observed, face-bow transfer appears to be a reliable clinical procedure and accuracy in the determination of reference locations is unlikely to be negated by subsequent transfer errors, provided that appropriate care is employed.