A path of insertion (or removal) is the path along which a prosthesis is placed (or removed) intraorally. A removable partial denture is usually fabricated to have a single path of insertion or removal from the mouth.
A single path of insertion is advantageous because it:
1. Equalizes retention on all abutments
2. Provides bracing and cross-arch stabilization of teeth
3. Minimizes torquing forces of the partial denture
4. Allows the partial denture to be removed without encountering interferences
5. Directs forces along the long axes of the teeth
6. Provides frictional retention from contact of parallel surfaces on the teeth
In order to provide a single path of insertion for a partial denture, some axial surfaces of abutments must be prepared so that they parallel the path of insertion. These parallel surfaces are called guiding planes.
Guiding planes are prepared wherever rigid components of a partial denture contact abutment teeth.
Specifically, guiding planes should usually be prepared for:
1. Proximal plates
2. Bracing arms
3. Rigid portions of retentive clasps
The dental surveyor is a diagnostic instrument used to select the most favorable path of insertion and aid in the preparation of guiding planes. It is an essential instrument in designing removable partial dentures. The act of using a surveyor is referred to as surveying.
Selecting the Path of Insertion of a Removable Partial Denture
A path of insertion is selected to provide the best combination of retentive undercuts and parallel surfaces for ALL ABUTMENTS. Use the following steps to do so:
STEP 1 Place the cast on the surveyor table and orient the plane of occlusion relatively horizontal. The final tilt of the cast for the ideal path of insertion is seldom more than 10° from this position.
STEP 2 Place the analyzing rod against the axial surface of a proposed abutment teeth (any tooth adjacent an edentulous space). The tip of the rod should be at the level of the free gingival margin. The point where the tooth touches the analyzing rod is greatest convexity (bulge) of a tooth and is called the height of contour.
STEP 3 Tilt the cast to gain maximum parallelism of axial surfaces of all of the proposed abutments. Maximum parallelism is present when the heights of contour of all teeth and all surfaces are as close as possible to the same position occluso-gingival. An additional check for maximum parallelism is that equal amounts of undercut are present on all abutments and all abutment surfaces. Check the mesial and distal tooth surfaces while tilting the cast anterior-posteriorly (A-P). While maintaining the same A-P tilt check facial and lingual parallelism. Lock the tilt of the cast when maximum parallelism is achieved.
STEP 4 Use an undercut gauge to check for adequate and relatively equal retentive, undercuts for retentive arms on all abutments. Alter the tilt of cast if required.
STEP 5 Change the tilt of the cast if there are any major soft tissue interferences (i.e. mandibular tori, residual ridge undercuts), or if the selected path of insertion will cause an esthetic problem (i.e. clasp would have to be placed to far incisally on the facial surface of an anterior tooth, as when the height of contour or required depth of undercut is too close to the incisal or occlusal surface).
STEP 6 Lock the diagnostic cast in position on the surveying table and mark the heights of contour on the denture abutments and soft tissues with the carbon marker. When marking the heights of contour, ensure that the carbon tip follows close to the free gingival margin so that you do not register a false height of contour.
STEP 7 Tripod the diagnostic cast so that the selected path of insertion may be easily found for future reference.
Selection and Preparation of Guiding Planes:
a. A path of insertion is selected.
b. The number and position of guiding planes is selected.
c. With the diagnostic cast as a guide, parallel surfaces are prepared intraorally with straight cylindrical burs (#1156 or #557L or equivalent cylindrical bur). The surveyed cast should be nearby for comparison, so that the bur can be placed in the same relationship to the tooth as the analyzing rod makes with the diagnostic cast.
d. Guiding planes should be at least 1/2 to 1/3 of the axial height of the tooth (generally a minimum of 2 mm in height). Use a light sweeping stroke continuing past the bucco- and the linguo proximal line angles. Reduction should follow the bucco-lingual curvature of the tooth, rather than slicing straight across the tooth. Guide planes for distal-extension cases should be slightly shorter to avoid torquing of the abutment teeth. Lingual guiding planes for bracing or reciprocal arms should be 2-4 mm and ideally be located in the middle third of the crown, occluso-gingivally. Use a good finger rest to establish parallel planes.
e. If tooth surfaces selected for guiding planes are already parallel to the path of insertion, little if any tooth modification may be necessary.
f. The prepared surfaces are polished rubber wheels or points.
g. Guiding planes are the first features prepared intraorally. If occlusal rest seats are prepared initially, placement of a proximal guiding plane will remove some of the rest seat preparation, and result in a narrowed rest with a sharp occluso-proximal angle.
