Bamber, M. Anwar; (1995) An evaluation of orthognathic surgery planning techniques. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A review of the literature revealed little controlled evaluation of the various stages of preoperative Orthognathic surgery planning and associated problems, and none in the UK. Hence investigations were carried out into: a) the reproducibility of two facebow transfer systems; the Denar Slidematic and the Dentatus type AEB. A novel method was developed to determine the variation in three dimensional spatial position and orientation of the maxillary cast, using a custom written computer graphics programme in 'C' language. This novel parallelepiped envelope of movement volume analysis (PEMVA) enabled realistic comparison to be made for each facebow system, operator and skeletal type, and will have important applications in future research. Both facebow/articulator procedures showed poor reproducibility. However, the Denar was relatively better than the Dentatus (p=0.001). b) to assess the occlusal plane to Frankfort plane angle transfer reproducibility. The results showed that the Denar facebow articulated the models close to the normal range, 8° ±4°. Whereas, the Dentatus facebow mounted the models with a steeper occlusal to model Frankfort plane angle, which was close to the patient's cephalometric occlusal - Frankfort plane angle, with relatively poor reproducibility. In the transverse occlusal plane, both facebow systems; the Denar with ear plug and the Dentatus with arbitrary hinge axis location method were reproducible. c) the effect of posture and anaesthesia on the occlusal registration for Orthognathic surgery planning. This was done by measuring the distances between the centric occlusion and centric relation. Displacement vectors and 3D graphics computed the position of the centric occlusion representing each condyle and from this point centric relation records were projected in various axes. The differences between postures were significant in almost all skeletal groups in the anteroposterior plane; conscious upright v conscious supine, and conscious supine v anaesthetized supine p<0.05, and conscious upright v anaesthetized supine (positions normally used in Orthognathic planning and surgery, respectively) highly significant p=0.001. In the superoinferior plane the differences were highly significant in Class II div.2, p=0.009. This important finding would explain discrepancies created by rigid fixations of the mandible. d) a comparative study of the Lockwood Keyspacer system and the Eastman anatomically-orientated model surgery technique. This was done in two stagesyl) model surgery and error measurements, 2) simulated osteotomy and measurement of an outcome. A significant deviation from the treatment plan was found in both techniques. Model surgery mean (mm) ± SD the vertical plane, Lockwood -0.8 ± 1.6, Eastman 0.0 ± 1.0 (p=0.00), the anteroposterior plane, Lockwood, 1.2 ± 1.8, Eastman -0.1 ± 1.4 (p=0.05). Thus the Eastman was significantly better than Lockwood. The mean errors (mm) in the reproducibility, after the simulated osteotomy excluding 'mandibular' autorotation, were; vertical plane; Lockwood; -0.5 ± 1.5 and Eastman: 0.3 ± 1.(p=0.00), anteroposterior plane; Lockwood; 0.03 ± 1.5 and Eastman; -0.3 ± 1.0 (p > 0.05). The Eastman technique was significantly better than Lockwood in the vertical plane, the clinical advantage proved to be in the better orientation provided by the Eastman technique for bimaxillary procedures. e) the maxillary dental midline and transverse asymmetry correction was also tested. The preoperative dental midline deviation mean (mm) was 1.9 ± 1.3 and four weeks postoperative, 0.3 ± 0.7 (p=0.00). The mean (mm) occlusal cant preoperative was 1.9 ± 1.1 and postoperative 0.2 ± 0.6 (p=0.00). This study showed that dental midline and transverse asymmetry can be easily corrected with rotation of the maxilla even in Le Fort II osteotomies. f) a comparison of two intermediate wafers; a thick wafer, made before and a thin wafer made after articulator "mandibular autorotation". In 74% of the cases both wafers located the maxilla in the same position and in 26% there was up to 2mm difference in maxillary position. These were cases where impaction was greater than 6mm. This difference between the two groups was significant (P=005). This study showed that the mean (mm) anteroposterior error in the surgical reproducibility of the preoperative planning, within the normal range of hinge axis error (6mm), and normal range of maxillary vertical movements (2-10mm), was 1.6 ± 0.5 in 26% of the cases, and remaining cases had error < 1.0mm, and the maximum error was 2mm. The principal problem is reconciling articulator planning to the anaesthetized supine patient when using mandibular rigid fixation. These investigations have important implications for improving orthognathic surgical planning and outcome.

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