EFFICACY OF TEMPORARY FIXED RETENTION FOLLOWING ...

Sexual Dimorphism
The patient’s sex is included in the statistical analyses because this aspect
of the sample can have confounding effects on the interpretations if not
controlled. Males are characteristically larger than females as a statistical
average, males grow more during adolescence and for a longer time following
adolescence than girls (e.g., Tanner et al. 1962; van der Linden 1986; Ursi et al.
1993), and so sexual dimorphism can influence the results if left uncontrolled.
For several of the variables, such as dental arch dimensions, patient’s sex is
largely a nuisance variable, where its interpretation has no important clinical
consequence. For other variables, such as whether one sex is more prone to
relapse, knowledge of the patient’s sex can provide clinical insights.
For each of the three examination periods, a two-way factorial analysis of
variance was used as a screening device for each of the 32 variables to assess
whether the type of retention (fixed versus removable) affects the results while
controlling for patient’s sex. The same was done for the in-treatment changes
and the posttreatment changes. All of these results are listed in Appendix B.
Again, “sex” was included to control for this known source of variation, and for
completeness, we summarize the differences in sexual dimorphism in this
section. Appendix C lists the results of testing whether the amount of change
depends on the type of retention, with patient’s sex and time-out-of-treatment as
covariates. We list descriptive statistics for these variables in Appendix D.
Prior literature (e.g., Moorrees 1959; Knott 1961; 1972; De Koch 1972;
Bishara et al. 1989) led us to expect that some dental arch dimensions would be
statistically larger in boys at the start of treatment (Table D-1), but results show
that most of the significant variables actually reflect the worse malocclusions in
boys. Of the 17 significant variables, only maxillary intermolar width,
mandibular intermolar width, and anterior segment perimeter are due to boys
being larger than girls; the 14 other variables reflect access-to-treatment
differences, where boys seeking orthodontic treatment have a more severe
malocclusion than girls (Blair and Harris 2008; Glassell and Harris 2008). That is,
epidemiologically, there are very few sex differences between boys and girls in
the frequency or severity of malocclusion (e.g., Kelly and Harvey 1977). In
contrast, most orthodontic practices are composed of a majority of girls (often
with a sex ratio of 2-to-1) because girls (and their parents) are more aware of and
more concerned with minor occlusal issues (Anderson 2010). The severity of the
malocclusion has to be somewhat greater for boys to seek treatment.
This pattern is evident in these results (Table D-1), where, as examples,
the mean status of overbite, overjet, ANB, AOBO, and the Curve of Spee are all
larger (clinically more severe) in boys than girls. This suggests to us that minor
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