The phenomenon of tooth surface twist refers to the torsion of tooth surface along the direction of tooth width. As shown in Fig. (a), there is uneven resection on both sides symmetrical about the center of tooth width at the same height. As shown in figure (b), the modified twisted tooth surface is compared with the standard tooth surface. In order to characterize the degree of distortion of the tooth surface along the tooth width direction, the tooth surface twist coefficient r is defined as

The closer the twist coefficient is to 1, the smaller the twist degree is.

The tooth surface distortion coefficient r is consistent with the definition of the tooth surface distortion degree by the tooth surface uniformity coefficient rce