67

During cast surveying (Fig. 37, p. 27) corresponding undercuts were mentioned. These undercuts are of such crucial importance for clasp retention that we will go into special detail here.

67b

Corresponding means converging or diverging. On lower molars one finds lingually, as a rule, converging undercuts (Fig. 67 a). On upper molars one finds buccally, as a rule, diverging undercuts (Fig. 67 b). If the undercuts run parallel, no retention occurs.

68

This last statement is best explained using the following example: in the vertical axis, lower canines have no undercuts. If the path of insertion is the same as the anatomical, vertical axis, then no retention can be achieved with clasps (Fig. 68).

69

If the tooth is tipped ventrally, an undercut will be present. A retentive arm placed in this newly developed undercut will still have no retention. When the clasp is lifted the reciprocal arm on the lingual surface immediately loses contact with the tooth. The buccal retentive arm glides along the slanting surface; the clasp can be removed from the tooth without undergoing any elastic deformation (Fig. 69).

If two teeth are present, but their undercuts have parallel guiding planes, the situation will not be improved.

70

Consequently, no clasp retention can be achieved with parallel undercuts. Corresponding undercuts are mandatory for clasp-type anchorage. In this context, corresponding refers to undercuts which either converge or diverge (Fig. 70). It is also possible that one surface stands vertically and that the second surface converges or diverges with respect to the first.