55

By saddle-open clasp we mean any clasp whose opening (the space between retentive arm and reciprocal arm or dental rest) faces the denture saddle. A saddle-open clasp also serves to increase indirect retention. Its mode of action is best explained by using the case of lone molars remaining in the lower arch. If an E-clasp with mesial rest and arms pointing distally were employed, this would be regarded as a saddle-closed clasp. When lifting forces are encountered in the anterior region of the arch the partial denture will rotate around the mesial rest and the clasp arms will be displaced in a gingival direction away from the height of contour (Fig. 55a + b).

56

If, on the other hand, a saddle-open clasp such as the ring clasp with a distal rest is employed, anterior lifting forces will result in a rotation around the distal rest. The retentive arm here will be displaced occlusally toward the height of contour and thus resistance will be encountered (Fig. 56 a + b).

57

In the case of a Kennedy Class II, Mod. I, a molar ring clasp with a lingually placed retentive arm serves as an indirect retainer for the free- end saddle of the opposite side (Fig. 57).