Longitudinal assessment of the Z-angle

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Date

2016

Authors

Kaka, Muhammed

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Abstract

Background: Several analytical soft-tissue reference lines have been derived to aid in orthodontic treatment planning. One of these is the Z-angle (Merrifield), which is a measure of the most protrusive lip to Frankfort horizontal. While there is multitude of information on the changes in the soft tissue of the lips, nose and chin, little is available on the longitudinal changes associated with the Z-angle. Objective: To assess the longitudinal changes in the Z-angle from six to 18 years of age. Methods: Records of 50 subjects were selected from the Denver and Michigan growth data based on Class I occlusion and good radiographs. Due to poor quality radiographs, the final sample was narrowed down to 34 subjects (17 male and 17 female). Five time points were evaluated: six, nine, 12, 15 and 18 years of age. Primary variables: Upper lip Z-angle (ULZ) and Lower lip Z-angle (LLZ). A 3rd variable was created from the first two, diffz as the difference between ULZ and LLZ. Other measurements included Total Chin thickness (TotChin) and upper (ULE) and lower (LLE) lip positions with respect to the Eline. General linear models (GLM) in SAS were used to assess interaction between time and gender and determine differences over time (α=0.05). Results: GLM revealed no interaction between gender and time (p>0.05) for all variables except for TotChin. Diffz changed non-significantly from six to 18 years of age. However, both Z-angles increased over time and so did TotChin. ULE and LLE became more negative. Conclusion: The Z-angle became more positive from childhood to adolescence. This was partly due to an increase in TotChin and a decrease in lip protrusion (ULE, LLE). diffz increased but non-significantly, which meant the upper lip became more retrusive compared to the lower lip.

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A research project submitted to the postgraduate committee in partial fulfillment of the requirements for the degree of Master of Dentistry in the branch of orthodontics Johannesburg, South Africa

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Z-angle

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