Abstract
A systematic study was carried out using plastic model skulls to quantify the accuracy of the transfer of face bow registration to the articulator. A standard Dentatus semi-adjustable articulator system was compared to a purpose built orthognathic articulator system by measuring the maxillary occlusal plane angles of plastic model skulls and of dental casts mounted on the two different types of articulators. There was a statistically significant difference between the two systems; the orthognathic system showed small random errors, but the standard system showed systematic errors of up to 28°.
Orthognathic model surgery is used to produce occlusal wafers as templates to provide intra-operative guidance in positioning the jaws during orthognathic surgery. The model surgery is commonly carried out on dental models mounted on a semi-adjustable articulator using the appropriate face-bow. During model surgery the casts are detached from the mounting plaster on the articulator, moved by the distances prescribed by the surgeon and re-attached to the mounting plaster. The occlusal wafers are then constructed on the remounted casts.
Casts mounted using conventional articulators and face-bows do not reproduce the patients’ occlusal plane angles . The discrepancy in mounting the cast will produce errors in moving the casts during model surgery and hence to the articulated position. W alker et al. described an improved orthognathic articulator designed to reproduce the patient’s occlusal plane angle accurately in order to produce a more accurate surgical position. The improved system was compared with a Dentatus semi-adjustable system in a small group of patients and the improved system was shown to reproduce the patients’ occlusal plane angle more accurately. The use of a small patient group precluded a systematic study to assess the accuracy of the mounted casts and of the occlusal wafers prepared on the improved orthognathic articulator. These were the objectives of the current paper which evaluated the accuracy of mounting maxillary casts on articulators (Part 1) and of the occlusal wafers prepared on the articulators (Part 2).
Materials and methods
A systematic study was undertaken using five plastic model skulls mounted at different angles between the occlusal and horizontal planes. The plastic models (K-Med, UK) used were similar but not identical.
Mounting the plastic skull onto the measuring device
A mounting plate that could be locked onto a camera support was attached to the underside of the skull using cold cure acrylic resin. An aluminium rod, 4 cm in diameter and 14.5 cm long, was fixed with a screw into the centre of a base plate which could be levelled using spirit levels. The upper end of the rod had a camera support plate attached to it, which incorporated a ball and socket joint that could be rotated to obtain a variable skull position and then locked into position ( Fig. 1 ).
A bone screw was inserted through each condylar head and then into the medial side of the articular fossa to ensure that a hinge movement occurred into centric occlusion. A line was drawn on the maxilla to represent the position of a Le Fort I osteotomy cut. Four brass plates representing bone plates were adapted to the pyriform aperture and zygomatic areas of the maxilla, with a bridge across the Le Fort I cut line. Two holes were drilled on the left and right sides above and below the osteotomy cut on each side at the zygomatic buttress and the pyriform aperture. These were for the insertion of 2 mm diameter cortical titanium screws to fix the bone plates to the maxilla. The upper and lower holes were sufficiently far apart to allow for a 5 mm maxillary impaction. A centric registration of the bite of the upper and lower dental arches was taken in rubber base impression material and removed. The lower portion of the maxilla was detached from the plastic skull by carrying out the Le Fort I osteotomy cut and it was re-attached to the skull using the pre-fixed bone plates in the centric bite position. This was the standard preparation for every skull to allow maxillary movement at the Le Fort I level, the accuracy of predicting this movement is considered in Part 2 of this study.
The prepared skull was mounted on the camera support and the antero-posterior angle of the skull, and hence of the horizontal-maxillary plane angle, was adjusted to simulate different natural head positions as follows. The upper portion of the skull (i.e. the skull vault) was removed, and the remaining part of skull was levelled medio-laterally and antero-posteriorly. A flat plane was placed across the levelled skull and a protractor angle finder was used to record the initial antero-posterior angle of the cut surface ( Fig. 2 A ). Using the attached handle to the skull the natural head position was set to one of five values: −20°, −10°, +10°, +15° and +20° ( Fig. 2 B). The skull vault was re-attached once the skull was fixed in the required angular position. A levelled circular spirit level was fixed on to the top of the skull using cold cure acrylic to allow detection and correction of any alteration in angulation during measurement.
Duplicate impressions were taken of the maxillary and mandibular dentition using silicone-duplicating material (Metrosil, Metrodent Ltd., Huddersfield, UK), which were then cast in a hard stone according to the manufacturer’s specification. Using a standard Dentatus face-bow with an orbital pointer one maxillary cast was mounted on a Dentatus semi-adjustable articulator (Dentatus AB, Sweden), which is widely used for orthognathic surgery planning, Another cast was mounted on the improved orthognathic articulator using an Improved face-bow, to which were attached spirit levels instead of an orbital pointer .
The maxillary occlusal plane angles of the mounted casts were measured relative to the true horizontal using a flat plane and a protractor adapted with a sliding fit on the right-angled stand ( Fig. 3 ).
The full process was repeated and 10 sets of measurements were obtained for each articulator system.
Results
The differences between the maxillary occlusal plane angle of the skull and the mounted models for both the standard and improved orthognathic articulators are shown in Fig. 4 . The histograms show that the errors for the standard articulator ranged from −19° to 28° (mean −5.62° with a standard deviation of 16.37°); the errors for the improved orthognathic articulator ranged from −0.75° to 0° (mean −0.15° with a standard deviation of −0.51°).
The errors produced by the improved orthognathic articulator were consistently smaller than those of the standard articulator. The difference was clearly demonstrated in a Bland Altman plot of the individual results ( Fig. 5 ); the plot shows the mean values (bold line) and the 95% confidence limits for the data from both articulators. The errors for the 10 models mounted on the improved orthognathic articulator were all smaller than the errors of the models mounted on the standard articulator. The consistency of the results made the difference between the articulators statistically significant (binomial test , P < 0.01.).
