EFFICACY OF TEMPORARY FIXED RETENTION FOLLOWING ...

Table A-4 addresses the question of which variables changed differently
during the course of treatment depending on the type of retention used. The
Curve of Spee changed more during treatment in the removable group. There
was no difference at the start of treatment; both groups averaged 2.2 mm of
curve, so the difference is due to the orthodontists’ effectiveness in flattening the
curve during treatment. In other words, less correction of the Curve of Spee
enhanced the likelihood that the orthodontist would use fixed retention.
Incisor segment ratio appears as a significant predictor based on the intreatment
changes. This variable (ISR) increased only slightly during treatment
(~ 0.1) in the fixed group, but increased substantially (~ 2.3) in the removable
group.
Six variables achieved statistical significance when the posttreatment
changes are evaluated (Table A-5). The question addressed here is: Which
variables changed (“relapsed”) more in one group than the other following the
active phase of treatment? Maxillary intermolar width is one such variable.
Intermolar width decreased less than a millimeter on average in both groups, but
significantly more in the removable group (ca. -0.4 versus -0.8 mm).
The anterior discrepancy increased slightly in both groups, but the
increase was almost double in the removable group (ca. 0.5 versus 0.9 mm).
Interestingly, incisor segment ratio, which substantially increased in the
removable group during treatment, decreased (-0.8) during the posttreatment
examination period versus a continued increase (~ 0.7) in the fixed group.
Perhaps the most notable changes observed during the posttreatment
period were seen in Incisor Irregularity (Figures 4-15, 4-16 and 4-17). During the
average of ~ 16 years between the posttreatment and recall examinations,
mandibular Incisor Irregularity increased an average of ~ 1.5 mm in the fixed
group and 2.3 mm in the removable group. This is suggestive evidence that
fixed retention does inhibit long-term relapse even though it is only in place for
the first two years following treatment. That is, one could argue that the
lessened amount of relapse following treatment is due to the interval of fixed
retention. While marginally significant statistically (P = 0.0247), though, the
difference between the groups averages only 0.8 mm, so even if true, the effect of
fixed retention is very small and certainly not of any real clinical consequence.
Incisor segment depth (ISD) is the one other variable that achieved
statistical significance (Table A-5). This marginally significant difference
between groups (P = 0.0315) occurred because there was virtually no change
after treatment in the group with fixed retention ( x = -0.1 mm) but the Hawley-
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