13.2 Removable Partial Denture Design, Surveying, Clasping & RPI System

13.2 Removable Partial Denture Design, Surveying, Clasping & RPI System

Removable Partial Dentures (RPDs) restore masticatory function, aesthetics, and arch stability while safeguarding remaining teeth and oral tissues. For the ADC Written Examination, candidates must demonstrate precise knowledge of Kennedy classification, Applegate's governing rules, surveyor mechanics, major connector selection, direct retainer physics, and the biomechanics of distal extension design (specifically the RPI system).

---

1. Kennedy Classification & Applegate's Governing Rules

The Kennedy system categorizes partially edentulous arches based on the location of edentulous spaces relative to remaining teeth.

The Four Kennedy Classes

• Class I: Bilateral edentulous areas located posterior to the remaining natural teeth (distal extension).
• Class II: Unilateral edentulous area located posterior to the remaining natural teeth (distal extension).
• Class III: Unilateral edentulous area with natural teeth remaining both anterior and posterior to it (tooth-borne bounded span).
• Class IV: A single, bilateral edentulous area located anterior to the remaining natural teeth, crossing the dental midline.

Applegate's 8 Rules for Classification

1. Classification should follow, rather than precede, extractions that might alter the final design.
2. If a third molar is missing and not to be replaced, it is not considered in the classification.
3. If a third molar is present and is to be used as an abutment, it is considered in the classification.
4. If a second molar is missing and is not to be replaced, it is not considered in the classification.
5. The most posterior edentulous area(s) always determines the classification.
6. Edentulous areas other than those determining the classification are referred to as modification spaces and are designated by their number.
7. The extent of the modification space is not considered—only the number of additional edentulous areas.
8. There can be no modification spaces in Class IV arches. Any edentulous area lying posterior to the single bilateral anterior space changes the classification to Class I, II, or III.

---

2. Component Parts & Biomechanical Functions

An RPD consists of major connectors, minor connectors, direct retainers (clasps), indirect retainers, rests, and denture bases.

Major Connectors: Mandibular & Maxillary Selection

Major connectors unite components from one side of the dental arch to the other, providing cross-arch stability and rigidity.

• Mandibular Major Connectors:
  • Lingual Bar: Standard choice. Requires a minimum of 7 to 8 mm vertical clearance between the floor of the mouth and the free gingival margin (bar height 4–5 mm, plus 3–4 mm gingival margin clearance).
  • Lingual Plate: Indicated when vertical clearance is $<7\text{ mm}$, high floor of mouth, presence of mandibular tori, or when remaining anterior teeth are periodontally compromised and require future tooth additions.
• Maxillary Major Connectors:
  • Palatal Strap: Single wide band; ideal for short tooth-borne bounded spans (Class III).
  • Broad Palatal Plate: Covers maximum palatal vault; indicated for Class I arches with severe residual ridge resorption.
  • Anteroposterior (AP) Palatal Strap: Rigid structural box design; indicated when a palatal torus is present or in long-span Class I/II arches.
  • Horseshoe (U-shaped): Least rigid maxillary connector; prone to flexing under load. Indicated only when a large, inoperable palatal torus extends to the posterior border.

---

3. Direct Retainers, Rest Seats & Reciprocation

Direct retainers provide resistance to dislodgement along the path of insertion.

Direct Retainer Types

• Suprabulge Clasps (Occlusally Approaching):
  • Cast Circumferential (Akers) Clasp: Standard clasp for tooth-borne RPDs; engages 0.25 mm (0.01 inch) undercut in the gingival third opposite the origin.
  • Ring Clasp: Wraps almost $360^\circ$ around an isolated, tilted molar abutment.
  • Embrasure (Double Akers) Clasp: Used on adjacent teeth in bounded spans without edentulous spaces on that side.
• Infrabulge Clasps (Gingivally Approaching / Bar Clasps):
  • I-Bar, T-Bar, Y-Bar: Approaches the undercut from the mucosal direction. Requires 3 mm clearance from gingival margin. Contraindicated in deep mucosal undercuts or tissue impingement.

Rest Seat Preparation & Support

Rests transfer vertical occlusal forces down the long axis of abutment teeth.

• Occlusal Rest: Spoon-shaped, rounded triangular outline; marginal ridge lowered by 1.5 mm; floor slopes toward center of tooth ($<90^\circ$ angle between rest and minor connector).
• Cingulum Rest: Inverted 'V' or shelf preparation on maxillary canines; avoids occlusal interference.

Principle of Reciprocation

During insertion and removal, as the retentive arm flexes over the height of contour, it exerts lateral forces on the abutment tooth. To prevent tooth movement, a rigid reciprocal arm located above the height of contour must contact the tooth before the retentive arm flexes over the survey line.

---

4. Model Surveying & The RPI System

Surveying analyzes the master cast to establish the path of insertion, heights of contour, and undercuts.

Objectives of Surveying

1. Identify parallel Guiding Planes along proximal tooth surfaces to direct insertion and removal.
2. Locate heights of contour and measure retentive undercuts (0.25 mm for cast cobalt-chromium; 0.50 mm for wrought wire).
3. Identify soft tissue or bony interferences (tori, tipped teeth).
4. Record spatial orientation of the cast via Tripoding.

The RPI System for Distal Extension (Kennedy Class I & II)

Distal extension bases rely on dual support: rigid periodontal ligament of abutment teeth (moves $\sim 0.1\text{ mm}$) and viscoelastic mucosal tissue (displaces $1.0\text{--}3.0\text{ mm}$). Under occlusal load, the denture base depresses into the mucosa, creating a lever arm that can torque abutment teeth.

The RPI System overcomes destructive lever forces through three specific components:

• R = Rest: Placed on the mesio-occlusal surface of the primary abutment tooth (rather than distal occlusal), moving the fulcrum point anteriorly.
• P = Proximal Plate: Positioned on the distal surface of the abutment, contacting the lower 1/3 of the guiding plane. Relief is placed below the guiding plane contact.
• I = I-Bar Clasp: Located at or anterior to the greatest prominence on the buccal surface, engaging a 0.25 mm undercut in the gingival third.

Biomechanical Action Under Load

When occlusal force depresses the distal extension base:

1. The denture rotates around the mesial rest fulcrum.
2. The proximal plate moves inferiorly and anteriorly into the distal gingival trough, disengaging from the tooth surface.
3. The I-bar clasp moves anteriorly and inferiorly into the mesial undercut area, disengaging from the tooth.
4. Result: Abutment tooth receives non-destructive vertical forces rather than harmful distal tipping torque (prevents Class I lever action).