RPD Manual 11 - Removable Prosthodontics - Dalhousie University

Surveying, Path of Insertion, Guiding Planes - 18
B. Interferences - A path of insertion must be selected so that the prosthesis may be inserted
and removed without encountering tooth or soft tissue interferences. Tooth interferences are
usually encountered where rigid elements of the partial denture would require placement in
areas of undercut. Since rigid elements do not flex, such a partial denture will not seat.
Tooth interferences are often encountered in the bucco- or linguo-occluso-proximal point
angles, where the height of contour is high and where rigid portions of the clasp assemblies
must be placed.
Since only the terminal end of the retentive arm
flexes (circled area), the clasp on the left will
lock on the cast. The rigid portion of the clasp
will not release from the undercut. The clasp on
the right will release since only the flexible tip is
in the undercut.
When soft tissue undercuts exist (i.e. mandibular tori, residual ridge undercuts), the rigid
denture base can cause abrasion, irritation and /or ulceration as it is forced over the tissue.
As such, a path of insertion that involves interferences should only be selected if the
interferences can be eliminated by tooth preparation or reasonable blockout of the master
cast. If the interferences cannot be eliminated or minimized, then a different path of
insertion must be considered, even if less desirable guiding plane and retentive areas must
be selected.
Soft tissue undercuts require that the tissue be impinged
upon as the major connector or denture base passes over it.
If a large undercut is blocked out, the connector or base will
be remote from the tissue and collect food and debris.
C. Esthetics. A path of insertion should be selected to provide the most esthetic placement of
artificial teeth and the least amount of visible metal on the abutment teeth. Ensure that the
retentive undercut and the height of contour are not placed too far occlusally, so that the
retentive clasp and that anterior proximal plates are close to the gingival contours making
these components as inconspicuous as possible.
D. Guiding planes. Guiding planes are flat surfaces prepared on abutment teeth. Flat, rigid
elements of the partial denture (bracing elements and proximal plate minor connectors) fit
against these surfaces to ensure that a partial denture seats along one path of insertion.
Guiding planes are used to control and limit the directions of movement of a removable
partial denture as it is being inserted, removed or while it is in function. To do this, bracing
elements or proximal plates should, whenever possible, be the initial portions of the
partial denture to contact the abutments. In this way, the teeth are stabilized from the
potential moving forces as retentive elements of direct retainers flex over the heights of
contour into the retentive undercuts.