Longitudinal assessment of the Z-angle
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.
Description
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
Keywords
Z-angle