An Alternative to Angle ANB in the Assessment of Sagittal Discrepancy: A Case for W angle, M-axis and G-axis based on a Secondary Analysis of Clinical Trial Data - Special Issue

Introduction: The objective was to test if W angle was a superior method of measuring skeletal change over angle ANB. The design was to re-analyse the results of a previously performed prospective, randomized controlled clinical trial with two parallel groups. The setting was the Orthodontic department, Royal Preston District General Hospital, Preston, United Kingdom. Method: The participants were 26 patients randomly selected from a previous trial, aged 9-17 years undergoing treatment for Class II division 1 malocclusion (ANB > 5˚, OJ > 6mm). The intervention was two alternative designs of Twin-block appliance, allocated randomly. One with a Southend clasp on the upper and lower central incisors and one without. The appliances were identical in all other aspects. The outcome measured was the skeletal and dental changes from lateral cephalometric radiographs before and after treatment. Randomization was accomplished using a computer generated sequence. Allocation concealment was achieved with sequentially numbered, opaque, sealed envelopes. Blinding was applicable to outcome assessment only. Results: Based on a power calculation for W angle, 26 patients were randomly selected from 41 in the original trial. 13 patients were analysed in the Southend group and 13 in the Non-Southend group. Baseline characteristics were similar between the groups and the trial status is completed. No harms were observed as a result of treatment. There was a statistically significant change in W angle in the Southend group (5.4˚) and the Non-Southend group (2.8˚) (p=0.000). This was greater than that for angle ANB in the Southend group (-3.5˚) and Non-Southend group (-2.6˚) (p=0.004). There was a 2.6˚ difference in treatment change between the Southend and Non-Southend groups for W angle but only 0.9˚ for angle ANB. There was a total reduction to class I normative values for W angle (54.4˚ Southend, 52.2˚ Non-Southend group, norm 51-56˚). This was not so for angle ANB (3.9˚ Southend, 4.5˚ Non-Southend, norm 2.0-2.5˚). Conclusions: The elimination of confounding factors that influence angle ANB especially change in incisor angulation meant that W angle demonstrated a greater differentiation of skeletal response both within and between the Southend and Non-Southend groups. M-axis and G-axis are stable with growth and unchanged by treatment.