EFFICACY OF TEMPORARY FIXED RETENTION FOLLOWING ...

Total chin, which is mostly a soft tissue measurement but does include a
portion of the bony chin, also was significant (P = 0.01). Total chin was greater at
pretreatment (Figure 4-6) in the fixed group, although there was barely a
millimeter difference between the two groups (14.74 and 13.67 mm).
It is not likely that these differences at the pretreatment examination
governed the orthodontist’s decision concerning whether to use fixed retention at
the end of treatment, but they do suggest that some occlusal conditions may
influence the decision. It also is likely (Table A-1) that these differences
evaluated at the start of treatment carried through to affect the posttreatment
status—and that this latter condition controlled the decision about kind of
retention.
More of the variables (7 denoted with an asterisk) were significantly
different at the end of treatment (Table A-2). This in and of itself might indicate
that orthodontists were attuned to which cases warranted fixed retention. Of
note, incisor depth and overjet, which were significant at the pretreatment
examination, persisted as significant predictor variables at the end of treatment.
Incisor depth and overjet had been reduced in both groups during treatment, yet
remained significantly greater in the fixed group at the end of treatment.
Total chin also continued to be statistically significant between groups at
the end of treatment (P = 0.02). The fixed retention group maintained a greater
average value of 16.4 mm versus 15.4 mm in the removable group.
Incisor irregularity, which intuitively can be regarded as a proximate
reason for using a fixed appliance, was greater in those receiving a fixed
appliance (Figure 4-7), though the difference between the means seems small (0.5
mm and 0.2 mm). Even though the mean difference was small, this difference is
statistically significant. Those receiving a fixed retainer were more likely to
exhibit greater Incisor Irregularity at the end of treatment. Overbite also was
significantly larger in the fixed group at the posttreatment examination.
Table A-2 lists the Curve of Spee and reveals that the average curve is
small in both groups (Figure 4-8), but the value is actually almost twice as great
in the fixed group (0.8 mm) as in the removable group (0.4 mm). While the mean
difference between the two groups is trivial (~ 0.31 mm), the distributions show
that cases with deeper curves are more likely to be fitted with a fixed retainer.
Molar correction also occurs on this list (Figure 4-9). The key point is that
the median for the removable group is appreciably closer to zero (closer to a
Class I relationship) than the median of the fixed group, meaning that cases in
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