A comparison of two 3-dimensional mandibular superimposition techniques and a 2-dimensional superimposition technique against Bjork’s structural method

Abstract

Objectives: (1) compare two rapid voxel-based 3-dimensional registration methods against Bjork’s structural method, (2) compare a 2-dimensional method – utilising the mental reversal line described by Enlow and overlay of the mandibular canal – against the structural method, (3) account for vertical facial types and (4) account for the time interval between T1 and T2 CBCT acquisitions. The primary outcome measure for each method is mandibular growth rotation relative to the cranial base. Materials and methods: Paired CBCTs taken at a mean interval of 54.8 ± 16.8 months (range: 27 to 103) were assessed from a sample of 70 patients. These included: 44 females and 26 males; the mean age at T1 was 11.0 ± 2.0 years (range: 7 to 15) and T2 was 15.6 ± 1.9 years (range: 12 – 20). By vertical facial pattern, the sample was comprised of 43% brachyfacial, 30% mesofacial and 27% dolichofacial types. Three mandibular superimposition methods were compared against Bjork’s structural method: (1) 2D-M1 method – 2-dimensional landmark method, (2) 3D-M1 method – a voxel-based 3-dimensional method based on a previous reported method, and (3) 3D-M2 – a voxelbased 3-dimensional method incorporating a region of reference comprising of the mental reversal line above Pogonion, and the internal symphyseal structures within the anterior half of the symphysis. Following superimposition, the relative change in cranial base lines in sagittal view between T1 and T2 were measured for true mandibular rotation. Agreement between methods was assessed with Lin’s concordance correlation coefficient (95% confidence interval), Bland-Altman’s limits of agreement and the Bradley-Blackwood test. Results: Lin's concordance correlation coefficients (95% confidence interval) ranged between 0.924 (0.889, 0.958) for the 2D-M1 method, 0.695 (0.579, 0.812) for the 3D-M1 method, and 0.965 (0.948, 0.981) for the 3D-M2 method. Bland-Altman limits of agreement (95% LoA) were wide for all but the 3D-M2 method. Finally, the Bradley-Blackwood test of equality of means and variances was significant in all except the 3D-M2 method. Conclusions: For time intervals between CBCT volume acquisitions greater than three years, the use of the 2D-M1 and 3D-M1 methods are not recommended. There was high concordance between the 3D-M2 method and Bjork’s structural method when assessing mandibular growth rotation using relative changes in cranial base lines. The high concordance was displayed across all vertical facial types and for all time differences between first and second CBCT data acquisitions.