9.2 Fixed & Removable Prosthodontics & Occlusion

Removable Partial Denture (RPD) Design and Biomechanics

Designing removable partial dentures requires a systematic approach to balance support, retention, stability, and tissue preservation.

Kennedy Classification and Applegate's Governing Rules

The Kennedy Classification System divides partially edentulous arches into four principal classes:

• Class I: Bilateral edentulous areas located posterior to the remaining natural teeth (bilateral distal extension).
• Class II: Unilateral edentulous area located posterior to the remaining natural teeth (unilateral distal extension).
• Class III: Unilateral edentulous area with natural teeth remaining both anterior and posterior to it (tooth-bound edentulous space).
• Class IV: A single, bilateral edentulous area located anterior to the remaining natural teeth, crossing the mid-sagittal line.

To standardize classification application, Applegate formulated 8 mandatory rules:

1. Classification should follow, rather than precede, any planned tooth extractions.
2. If an third molar is missing and not to be replaced, it is omitted from classification considerations.
3. If a third molar is present and is to be utilized as an abutment, it is included in the classification.
4. If a second molar is missing and is not to be replaced, it is omitted from the classification.
5. The most posterior edentulous area (or areas) always determines the primary classification category.
6. Edentulous areas in addition to the primary classifying space are designated as modification spaces and are referred to by their number (e.g., Class II modification 1).
7. The extent of the modification space is irrelevant; only the number of additional edentulous spaces is counted.
8. Kennedy Class IV can have no modification spaces. Any additional posterior edentulous space automatically changes the primary classification to Class I, II, or III.

Component Architecture and Structural Functions

An RPD framework comprises several functional components:

                              [RPD Framework]
                                     │
      ┌──────────────────────────────┼──────────────────────────────┐
      ▼                              ▼                              ▼
[Major Connector]            [Rest Seats & Rests]           [Direct Retainers]
(Unifies framework;          (Provides Support;             (Provides Retention;
 broad rigid support)         transmits axial force)         engages undercuts)

Major Connectors

• Maxillary Major Connectors: Must terminate at least 6 mm away from the free gingival margin (or fully cover the palate as a plate). Options include the Single Palatal Strap (ideal for Class III), Anterior-Posterior (A-P) Palatal Strap (versatile for Class I and II), and Full Palatal Plate (maximum support for severely resorbed ridges).
• Mandibular Major Connectors: Must clear the lingual gingival margins by at least 3 mm to 4 mm. The Lingual Bar requires a minimum of 7 mm to 8 mm of vertical space between the floor of the mouth and the gingival margin (4 mm bar height + 3 mm gingival clearance). If vertical space is <7 mm or lingual tori exist, a Lingual Plate (supported by rest seats at each end) is indicated.

Rest Seats and Support

Rest seats transfer vertical occlusal forces down the long axis of abutment teeth:

• Occlusal Rests: Prepared in enamel, rounded spoon-shaped, 1.0 mm to 1.5 mm deep, occupying 1/3 of the intercuspal width. The marginal ridge is reduced to provide clearance, and the floor of the rest seat must slope centrally toward the tooth center (<90° angle with minor connector) to prevent slipping.
• Cingulum Rests: Inverted 'V' or crescent shape prepared on maxillary canines.

Direct Retainers (Clasps) and Reciprocation

Direct retainers flex over the survey line (height of contour) to engage undercut areas:

• Cast Cobalt-Chromium (Co-Cr) Clasps: Rigid; require a shallow undercut of 0.25 mm (0.010 inches).
• Wrought Wire Clasps: Flexible; require a deeper undercut of 0.50 mm (0.020 inches).
• Reciprocation Principle: A rigid reciprocating arm (or plate) must be located on the opposite side of the tooth above the survey line. It must make contact with the tooth before the retentive arm flexes over the height of contour during insertion to neutralize lateral destructive forces.
• Indirect Retention: Essential in Class I and Class II RPDs to prevent the distal extension base from lifting off the ridge due to sticky foods. Indirect retainers (rests) are positioned on the opposite side of the fulcrum line (an axis passing through the most posterior principal rest seats), as far anteriorly as possible.

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Occlusion and Articulator Kinematics

Understanding mandibular positions and movements is essential for complex fixed and removable restorations.

Mandibular Reference Positions

• Centric Relation (CR): A physiological, muscularly independent reference position where the mandibular condyles articulate in the most anterior-superior position against the posterior slopes of the articular eminences, resting against the avascular central zone of their articular discs. CR is completely independent of tooth contact and is clinically reproducible.
• Centric Occlusion (CO) / Maximum Intercuspation (ICP): The position of maximum intercuspation of opposing teeth. In approximately 90% of the population, ICP does not coincide with CR, displaying an average anterior-superior slide of 0.5 mm to 1.5 mm from the retruded contact position (RCP) to ICP.

Posselt's Envelope of Motion

Mandibular motion in the sagittal plane is described by Posselt's Envelope of Motion, illustrating border movements from terminal hinge axis rotation to maximum opening, protrusion, and maximum intercuspation.

(RCP: Retruded Contact Position / CR)
  │   
  │    (Terminal Hinge Axis arc: 20-25 mm pure rotation)
  ▼     
(ICP)   ▼
  │    (Pure Rotation Limit)
  │      \n  ▼       ▼ (Translation + Rotation to Maximum Opening: 40-50 mm)
(Protrusion) ──► (Maximum Opening)

Functional Occlusal Schemes

• Mutually Protected Occlusion (Canine Guidance): In ICP, posterior teeth provide vertical support, protecting anterior teeth from heavy axial loads. During lateral excursive movements, the canine teeth disocclude all posterior teeth on both working and non-working sides.
• Group Function: On the working side, the canine together with premolars (and sometimes the mesiobuccal cusp of the first molar) contact simultaneously during lateral excursion, distributing lateral forces. Non-working (balancing) side contacts are completely absent.
• Bilateral Balanced Articulation: Simultaneous contact of all teeth on working and non-working sides in all excursive movements. Mandatory in complete removable dentures to maintain denture base stability.

Articulators and Facebow Transfers

Articulators simulate jaw mechanics:

• Arcon Articulators (ARticulator CONdyle): Condylar spheres are attached to the lower member (mandible) and condylar guidance slots are on the upper member (maxilla), mirroring human anatomy. The angle between the condylar guidance track and the maxillary occlusal plane remains constant when the vertical dimension is adjusted.
• Non-Arcon Articulators: Condylar guidance slots are located on the lower member while condylar spheres are attached to the upper member.
• Facebow Transfer: Records the spatial position of the maxillary arch relative to the transverse horizontal (terminal hinge) axis and a reference plane (such as the Frankfurt horizontal plane), transferring this relationship to a semi-adjustable articulator.

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Fixed Prosthodontics and Restoration Preparation

Successful fixed prosthodontic preparations balance structural preservation, mechanical resistance, and marginal integrity.

Biomechanical Principles of Tooth Preparation

• Total Occlusal Convergence (TOC): The angle between opposing axial walls. The ideal taper is 6° to 10°. Tapers exceeding 15° exponentially compromise retention and resistance forms.
• Retention Form: Prevents displacement of the restoration along its path of insertion. Height-to-width ratio should be at least 0.4.
• Resistance Form: Prevents dislodgement by oblique or rotational forces. Enhancements include axial grooves and boxes.
• Occlusal Reduction: Requires 1.5 mm to 2.0 mm of clearance on functional cusps (palatal of maxillary, buccal of mandibular) incorporating a functional cusp bevel, and 1.0 mm to 1.5 mm on non-functional cusps.

Preparation Margins and Material Specifications

Impression Techniques and Provisionalization

Pre-impression tissue management requires dual-cord gingival retraction techniques (e.g., a smaller #00 cord placed deep in the sulcus for fluid control, capped by a larger #1 cord impregnated with aluminium chloride for physical expansion). Elastomeric impression materials of choice include Addition Silicone (Vinyl Polysiloxane / VPS) and Polyether. Provisional crowns (fabricated from autopolymerizing PMMA or bis-acryl composite) preserve pulpal vitality, maintain interproximal/occlusal contacts, and protect periodontal health.