The use of the semiprecision rest in removable partial dentures

Abstract

The requirements of retainers for removable dentures are more effectively fulfilled with a deep rest retainer than with a shallow rest design. A deep rest retainer may be of the precision or the semiprecision type. In designing a semiprecision rest, four aspects of it must be taken into consideration: (1) the occlusal form, (2) the proximal form, (3) the gingival floor form, and (4) the proximal surface placement. When a semiprecision rest is used, primary retention can be attained by either a lingual arm or a spring device. If a lingual arm is used, the degree of retention can be augmented by modifying the contour of the lingual surface of the abutment casting. Possible modifications are an exaggerated undercut contour, a horizontal groove, and a dimple in a casting. Various spring devices can be used depending on whether the deep rest is placed on the proximal surface adjacent to the edentulous area or on the proximal surface away from the edentulous region. All types of preparations for abutment castings should have a full shoulder on the proximal surface that is to receive the deep rest. The shoulder should turn into the buccal and lingual contours of the preparation. The wax patterns of the abutment castings should have proximal surfaces that are parallel to each other and have enough bulk to permit milling of the selected form of semiprecision rest. The lingual surface should have its greatest contour close to the gingival margin of the tooth on the half that is to be contacted by the rigid section of the lingual arm. The greatest contour of the other half of the lingual surface should be closer to the occlusal surface. Impressions for the construction of a semiprecision removable partial denture can be made in rubber or silicone elastic materials in conjunction with plaster. The technique used varies depending on the arrangement of the remaining teeth. The two main arrangements encountered are (1) a free-end ridge on one side and a tooth-bounded ridge on the opposite side, and (2) bilateral free-end edentulous ridges. In both situations, the ridge areas are registered by the elastic material in a spaced custom-made tray, and the seats for the abutment castings are registered in plaster or rapid-setting artificial stone.