35.2 Intraoral, Bitewing and Occlusal Projections

2. Intraoral Radiography: Paralleling vs Bisecting Angle Techniques

Periapical radiography aims to produce an anatomically accurate image of the crown, root apex, and supporting periodontium of a tooth with minimal geometric distortion.

Intraoral Periapical Geometry Comparison

    A. Paralleling Technique                    B. Bisecting Angle Technique

      X-Ray Beam ────────>                      X-Ray Beam ────────>
                                                               ╲
      ┌───────┐  Perpendicular                   Tooth Axis     ╲  Bisecting Plane
      │ Tooth │  (90°) to both                        │          ╲    (Beam 90° to this)
      │       │                                       │   θ  / ╱ ╲
      │       │                                       │     / /   ╲
      │       │      ┌──────────┐                     │ θ  / /     ╲ ┌─────────┐
      │       │      │ Receptor │                     │   / /       ╲│ Receptor│
      └───────┘      │          │                     └───┴──────────┴─────────┘
                     └──────────┘                                    Receptor close to crown
       Parallel Alignment:                           Angle θ = Angle θ (Isometry)
       Receptor parallel to tooth                    Prone to elongation / foreshortening

The Paralleling Technique (Long-Cone Technique)

• Geometric Rule: The image receptor (sensor or phosphor plate) is positioned inside the mouth strictly parallel to the long axis of the tooth. The central ray of the primary X-ray beam is directed perpendicular ($90^\circ$) to both the tooth long axis and the receptor plane in both horizontal and vertical dimensions.
• Need for Increased Object-to-Receptor Distance: Due to the anatomical contour of the hard palate and the shallow muscular floor of the mouth, a flat receptor cannot remain parallel to the tooth if placed in direct contact with it. To achieve parallelism, the receptor must be positioned deeper within the oral cavity (e.g. towards the middle of the palate or tongue space).
• Compensation via the 'Long Cone' (Focal Spot-to-Skin Distance): Placing the receptor at a distance from the tooth causes divergent X-ray photons to produce geometric magnification and penumbra (loss of edge definition). To counteract this magnification and maintain sharpness, the X-ray tubehead must feature an extended spacer cone delivering a focal spot-to-skin distance (FSD) of not less than 200 mm (ideally $300\text{ mm}$). This ensures that only nearly parallel central photons strike the tooth and receptor.
• Instrumentation: Mandates the use of rigid film/sensor holders with external beam-aiming guide rings (e.g., Rinn XCP system). The holder fixes the receptor relative to the tooth, and the aiming ring guides the collimator cone into precise orthogonal alignment, preventing cone-cutting.

The Bisecting Angle Technique

• Geometric Rule (Cieszynski's Rule of Isometry): Two triangles are identical if they share two equal angles and a common side. The receptor is placed directly against the lingual or palatal aspect of the tooth crown. An imaginary plane is constructed that bisects the angle formed between the long axis of the tooth and the long axis of the receptor. The central X-ray beam is aimed strictly perpendicular to this imaginary bisecting line.
• Limitations and Clinical Errors:
  • Operator Subjectivity: The long axis of the tooth root and the bisecting line must be visually estimated, making precision and repeatability exceptionally poor.
  • Patient Finger Retention (Banned): Patients should never be asked to hold receptors with their fingers. This introduces finger irradiation, receptor bending (curved image distortion), and receptor displacement.
  • Vertical Angulation Errors:
    • Over-angulation (Beam too steep): Directing the central ray at too steep an angle causes foreshortening (teeth appear unnaturally short and compressed).
    • Under-angulation (Beam too flat): Directing the central ray at too flat an angle causes elongation (teeth appear unnaturally stretched, roots projected off the image).
  • Anatomical Superimposition: In the maxillary molar region, the steep vertical angulation required by the bisecting technique frequently projects the dense radiopaque zygomatic buttress directly over the maxillary molar roots, masking periapical pathology.

Head-to-Head Comparison: Paralleling vs Bisecting Angle

3. Bitewing & Occlusal Projections

Bitewing & Occlusal Radiographic Overview
  │
  ├── Horizontal Bitewing ───> Interproximal caries detection (premolar & molar crowns)
  │                            Alveolar crest monitoring in mild-to-moderate bone loss (≤5 mm)
  │
  ├── Vertical Bitewing ─────> Periodontal assessment in moderate-to-severe bone loss (>5 mm)
  │                            Prevents truncation of alveolar crest in severe periodontitis
  │
  └── Occlusal Views ────────> Maxillary Standard/Anterior (+65° to +70°): Canines, mesiodens
                               Mandibular True Cross-Sectional (90°): Submandibular duct stones
                               Mandibular Oblique (-45°): Mandibular pathology / fracture

Bitewing Radiography: Horizontal vs Vertical

Bitewings capture the crowns, cervical regions, and alveolar crests of both the maxillary and mandibular arches on a single image receptor.

• Beam Alignment: The central ray is directed through the interproximal contact points parallel to the occlusal plane, with a slight vertical angulation of +5° to +8° downwards. This positive angulation compensates for the curve of Monson and the slight lingual tilt of mandibular posterior teeth, preventing overlap of opposing cusps.
• Horizontal Bitewings:
  • Standard projection. Receptor placed with its long axis horizontal.
  • Primary Indications: Early detection of interproximal enamel and dentinal caries; evaluation of existing restorations (marginal overhangs, recurrent caries); monitoring alveolar bone crest height in health or mild-to-moderate periodontitis ($< 5\text{ mm}$ bone loss).
• Vertical Bitewings:
  • The receptor is oriented with its long axis vertical.
  • Primary Indication: Indicated in patients with moderate to severe generalized periodontitis exhibiting horizontal or vertical bone loss greater than 5 mm.
  • Diagnostic Rationale: In severe periodontitis, the alveolar bone crest resorbs far apically. On a conventional horizontal bitewing, the crestal margin falls below the inferior border of the receptor, resulting in an unreadable image. Vertical bitewings provide additional vertical coverage ($41\text{ mm}$ height), capturing deep infrabony defects and furcations without requiring full periapicals.

Occlusal Projections

Occlusal radiographs utilize a large Size 4 receptor ($57 \times 76\text{ mm}$) placed on the occlusal plane.

• Maxillary Standard (Anterior) Occlusal:
  • The central ray is directed downwards through the bridge of the nose at an angle of +65° to +70° to the receptor.
  • Indications: Detecting unerupted maxillary canines or incisors; locating supernumerary teeth (e.g. mesiodens); assessing palatal clefts; visualizing anterior maxillary cysts and fractures.
• Mandibular True (Cross-Sectional) Occlusal:
  • The patient reclines with the neck extended; the central ray is directed at 90° perpendicular to the receptor and floor of the mouth, entering through the midline between the submental triangles.
  • Indications: Gold-standard 2D projection for visualizing radiopaque sialoliths (salivary calculi) within the submandibular duct (Wharton's duct); detecting foreign radiopaque bodies in the floor of the mouth; assessing true buccolingual cortical expansion or lingual plate perforation caused by cysts or tumours.
• Mandibular Anterior Oblique Occlusal:
  • The central ray is angled at -45° upwards through the chin symphysis. Visualizes periapical status of lower anterior teeth when periapical holders cannot be tolerated, and assesses anterior mandibular trauma.