Second, based on the results of the multiple regression analysis, the data were divided into two groups regarding pretreatment crown size: one consists of fully crowned and open-grown, and the other consists of 70% live crown, 80% live crown, and 90% live crown. The results of the segmented regression are depicted in Figure 2.
In the group of fully crowned and open-grown, a breakpoint was found at 67% crown removed, but not significantly supported by score test (p = 0.78). The first segmented line (β = -0.81, 0 < severity < 67) was significantly negative (p < 0.001) (the left panel of Figure 2).
On the other hand, in the group of 70-90% live crown, a breakpoint was found at 53% crown removed and significantly supported by score test (p < 0.001). The first segmented line (β = -0.16, 0 < severity < 53) was not significant (p = 0.14) (the right panel of Figure 2).