The errors produced by occlusal wafers constructed on casts of the teeth mounted on a standard articulator and an improved orthognathic articulator were investigated by carrying out simulated orthognathic surgery on plastic skulls. The wafers were used to relocate the position of the maxillae of the skulls. The vertical and horizontal displacements of the maxillae were determined from measurements of the positions of markers on the skull and teeth. Comparison of the magnitudes of the actual and intended movements showed that wafers constructed on the standard articulator had systematic prediction errors of up to 5 mm, but the improved orthognathic articulator showed much smaller random errors. There was a statistically significant improvement in overall accuracy in predicting maxillary Le Fort I position with the use of the improved orthognathic articulator which the authors recommend for clinical use.
Occlusal wafers are used during orthognathic surgery as templates to provide intraoperative guidance in repositioning the jaws. The wafers are generally produced by model surgery carried out on dental models mounted on an articulator. The accuracy of the surgical outcome depends on the accuracy with which the articulator reproduces the patient’s occlusal plane angle .
The first part of this report showed that maxillary casts were more accurately mounted on an improved articulator than on a conventional Dentatus ARL semi-adjustable articulator (Dentatus AB, Sweden) (referred to as the standard articulator in this report). This report compared the accuracy of simulated orthognathic surgery using perioperative wafers made using the improved orthognathic articulator system and the standard Dentatus articulator system.
Materials and methods
A systematic study was undertaken using plastic model skulls mounted at different angles between the occlusal and horizontal planes. The plastic models used (K-Med, UK) were similar but not identical. The method of supporting the skulls, the preparation of the skulls for simulated orthognathic surgery and for mounting duplicate dental casts on an improved orthognathic articulator and a conventional semi-adjustable Dentatus articulator are described in the first part of this report .
Construction of occlusal wafers and simulated surgery
Perioperative wafers used to guide maxillary osteotomy at Le Fort I level were constructed on each articulator and incorporated each of three maxillary displacements: forward 10 mm; forward 10 mm and up (impaction) 5 mm; forward 10 mm and down (downgraft) 5 mm.
Circular aluminium discs were used to ensure that the vertical displacements of the maxilla were accurate and reproducible. When mounting the maxillary casts, a 5 mm thick aluminium disc was positioned between the underside of the articulator upper cross member and the mounting disc ( Fig. 1 ). Removal of the disc impacted the cast by 5 mm. Replacing the 5 mm disc with another, 10 mm thick, produced a 5 mm downgraft.
The maxillary cast was advanced horizontally using a hole in the upper cross member of each articulator, which was 10 mm in front of the hole used to mount the cast initially. The mandibular cast was articulated using a centric rubber based registration bite taken from the plastic skull. The mandible was fixed in centric occlusion using a silicone putty matrix and an acrylic wafer was fabricated in the preplanned position.
Orthognathic surgery was simulated by detaching the maxilla from the model mounted plastic skull by removing the custom-made bone plates from the pyriform aperture and zygomatic buttress . The maxilla was repositioned using the perioperative wafer constructed on the standard articulator. The mandible was maintained in a fixed position, the wafer was fitted to the mandibular teeth and then the detached maxilla was repositioned with the dentition occluding with the wafer and mandibular teeth. Dental sticky wax was used to seal the maxilla in place and re-attach it to the skull ( Fig. 2 ). This procedure was then carried out using a wafer constructed from the casts mounted on the improved articulator.
The mandible was de-rotated with a thicker wafer when there was a downgrafted displacement. The procedure was carried out twice for each of the five angles, producing 10 sets of results for each articulator .
Measurement of maxillary displacements
The accuracy of the simulated surgery was assessed by measuring the position of six maxillary reference points on the plastic skulls ( Fig. 2 ). The vertical movement was measured anteriorly between a reference point at the nasion and the left central incisal tip, and on the left and right sides between paired anterior and posterior reference points placed above and below the Le Fort I cut line on the plastic skull. The antero-posterior movement was measured using a reference point on the upper left central incisor.
A horizontal 8 mm rod with a pointer at one end was attached to the vertical rod, using a clamp that allowed the horizontal rod to be moved antero-posteriorly and vertically. The antero-posterior measurement was taken by bringing the pointed tip of the horizontal rod into contact with the reference point on the upper left central incisor and by measuring the length of rod protruding from the clamp using an electronic digital calliper with a resolution of 0.001 mm. Vertical measurements were made using a vertical height calliper (Chesterman, Sheffield, UK) with an analogue Vernier scale with a resolution of 0.5 mm ( Fig. 3 ).
Duplicate measurements of the positions of the reference points before and after simulated surgery allowed the calculation of the displacement of the maxilla produced by surgery. Twenty paired measurements of the antero-posterior position of the central incisors were analysed to assess the accuracy of the measurements.
The mean difference between repeated measurements was 0.0925 mm (range 0.03–0.24 mm); values that were considered acceptable.
The prediction errors of maxillary movements at Le Fort I level (the difference between the measurements of the actual and intended movement of the maxilla) were calculated. A positive or negative sign was used to indicate whether the movement was greater or smaller than the planned movement. Using a sign to denote the direction of movement was useful for displaying the nature of the differences, but produced problems in ranking the results for the Wilcoxon non-parametric statistics used to analyse the data. All the negative errors were ranked as smaller than the positive values, whatever their magnitude. The statistical comparisons were, therefore, carried out using the absolute values of the errors, without a positive or negative sign.
Antero-posterior differences at the central incisor
The magnitude of the errors produced using wafers produced on the standard and improved orthognathic articulators are shown in Fig. 4 . The differences between the predicted and actual movements guided by the constructed wafer for the standard articulator ranged from −4.96 mm to +2.56 mm (mean −1.03 mm with a standard deviation of 1.621 mm). The errors for the improved articulator ranged from −0.75 mm to +1.50 mm (mean 0.11 mm with a standard deviation of 0.569 mm). Both histograms were asymmetrical; the discrepancies between the maxillary predicted and actual movements using the standard articulator were mostly negative values (under-advancement of the maxilla) whereas with the improved orthognathic articulator these discrepancies were mostly positive (over advancement of the maxilla). This exacerbated the problem of ranking the results for the statistical analysis.
The values of the absolute errors are shown in Fig. 5 . The absolute differences between the predicted and actual maxillary movements for the standard articulator ranged from 0 mm to 4.96 mm (mean 1.53 mm with a standard deviation of 1.136 mm); for the improved orthognathic articulator these differences ranged from 0 mm to 1.50 mm (mean 0.47 mm with a standard deviation of 0.324 mm).
Using either the signed or absolute values for the differences between the actual and the predicted movements showed that the improved orthognathic articulator had smaller mean discrepancies than the standard articulator.
The Wilcoxon comparison of the absolute measures indicated that the standard articulator showed larger errors in 24 of the 30 paired comparisons. The value of the Wilcoxon parameter, z , was −3.877; the difference in the errors of the two articulators was highly significant ( p < 0.001).
The vertical errors measured at the central incisors are shown in Fig. 6 . The differences between the predicted and the actual maxillary movements for the standard articulator ranged from −4 mm to 7 mm (mean 0.45 mm with a standard deviation of 2.647 mm); for the improved orthognathic articulator the discrepancies ranged from −2 mm to 4 mm (mean 0.33 mm with a standard deviation of 1.003 mm).