Retentive areas

The retentive areas located below the survey line on the buccal and lingual surfaces of suitable abutment teeth also play a role in determining the path of insertion.

During prosthesis planning, the rigid components of the prosthesis should be positioned above the survey line, while only flexible components are located below it.

The retentive area has three dimensions: mesiodistal, occlusogingival, and buccolingual. Among these, the buccolingual dimension is the most critical, as the retentive clasp arm must flex to the extent of this dimension when engaging and disengaging from the undercut area—highlighting the importance of undercut measurement. This region on the abutment tooth surface is triangular in shape, with the apex at the height of contour and the base contacting the gingival tissues.

During the determination of the path of insertion, an effort is made to obtain similar amounts of undercut on the terminal abutment teeth. When it is necessary to use teeth lacking sufficient undercut as abutments, the tooth surface may need to be reshaped through enameloplasty or restored with a crown to create an appropriate contour.

Retention must be effective in relation to all possible paths of insertion. The forces acting against the prosthesis and causing dislodgement of the clasp assemblies act perpendicular to the occlusal plane. In the maxilla, the occlusal plane is parallel to the ground and remains stable as long as the patient is seated upright. Therefore, dislodging forces on maxillary prostheses act in a vertical direction. In a functioning mandible, as in the maxilla, the occlusal plane is not static and follows a vertical arc curvature. A dislodging force on a mandibular prosthesis caused by sticky food occurs when the mandible opens. For this reason, regardless of the path of insertion, the retentive clasp arm must be effective against dislodging forces acting perpendicular to the occlusal plane.