Fig. 4.

Correlation between the four indices and the spring constant k

The spring constant k showed negative correlation with the area index [ρ = –0.52 (p < 0.050)], but no correlation with the other indices [Feret’s D; ρ = –0.31, circularity; ρ = 0.25, centroid – CoM; ρ = –0.23]. The k-value of Target 360 ultra was 2.5 times that of i-ED complex ∞ SilkySoft. The spring constant k was proportional to the stock wire diameter to the fourth power and inversely proportional to the cube of the primary coil diameter, representing the coils’ stiffness.

Feret’s D: Feret’s diameter

CoM: centroid center of mass