14.2 Facial Bones, Orbits, Zygomatic Arches & Paranasal Sinuses

14.2 Facial Bones, Orbits, Zygomatic Arches & Paranasal Sinuses

Facial Bones & Orbital Positioning

Radiographic imaging of the facial skeleton evaluates fourteen bones—two nasal bones, two maxillae, two zygomatic bones, mandible, two lacrimal bones, two palatine bones, two inferior nasal conchae, and the vomer—along with the bony orbits. Precise positioning is vital for detecting facial fractures, orbital wall disruptions, and inflammatory disease.

1. Parietacanthial Projection (Waters Method)

• Clinical Purpose: Primary screening projection for facial bones, maxillary sinuses, nasal septum, and zygomatic arches.
• Patient & Part Position: Seated or standing facing the upright IR. Extend the neck, placing the chin against the IR so that the Mentomeatal Line (MML) is perpendicular to the IR.
  • This position forms an angle of 37° between the Orbitomeatal Line (OML) and the IR (the OML forms a 53° angle with the central ray).
  • Align the mid-sagittal plane (MSP) perpendicular to the grid.
• Central Ray (CR): Direct the CR perpendicular to the IR, exiting at the acanthion.
• Diagnostic Criteria & Radiographic Evaluation:
  • The petrous ridges/pyramids are projected immediately inferior to the floor of the maxillary sinuses.
  • Open maxillary sinuses, inferior orbital rims, zygomatic bone, and nasal septum visualized without petrous superimposition.

2. Modified Parietacanthial Projection (Modified Waters Method)

• Clinical Indication: Specifically designed for evaluation of orbital floor blowout fractures and foreign body localization.
• Positioning Modification: Adjust head extension so that the Orbitomeatal Line (OML) forms a 55° angle with the IR (MML is angled slightly lower than in standard Waters).
• Central Ray (CR): Direct the CR perpendicular to the IR, exiting at the acanthion.
• Diagnostic Criteria: Projects the orbital roofs and floors free of distortion, demonstrating the inferior orbital rim and orbital floor in profile to detect subtle trapdoor fractures or orbital herniation into the maxillary sinus.

3. PA Axial Facial Bones (Caldwell Method)

• Positioning: Forehead and nose against IR, OML perpendicular to IR, MSP perpendicular.
• Central Ray: Angled 15° caudal, exiting at the nasion.
• Diagnostic Criteria: Petrous ridges located in the lower one-third (1/3) of the orbits, demonstrating the orbital rims, maxillae, nasal septum, and zygomatic bones.

4. Lateral Facial Bones

• Positioning: Affected side of face against IR. Align MSP parallel to IR, Interpupillary Line (IPL) perpendicular to IR, and IOML parallel to the transverse plane.
• Central Ray: Perpendicular to IR, entering at the zygoma (midway between outer canthus and EAM).
• Diagnostic Criteria: Superimposed orbital roofs, mandibular rami, and facial structures in profile.

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Zygomatic Arches & Mandibular Positioning

Zygomatic Arch Projections

1. Submentovertex (SMV) Projection
   • Positioning: Hyperextend neck until IOML is parallel to IR. MSP perpendicular.
   • Central Ray: Perpendicular to IOML, centered midway between zygomatic arches (1.5 inches inferior to symphysis).
   • Diagnostic Criteria: Bilateral temporal processes and zygomatic arches demonstrated free of superimposition.
2. Tangential Projection (May Method)
   • Clinical Indication: Evaluates a single zygomatic arch when depressed or fractured.
   • Positioning: Hyperextend neck with IOML parallel to IR. Rotate head 15° toward the affected side, then tilt head 15° away from the affected side.
   • Central Ray: Perpendicular to IOML, centered to the zygomatic arch of interest.
   • Diagnostic Criteria: The single zygomatic arch is projected free of parietal bone and mandibular superimposition.
3. AP Axial Projection (Towne Method for Arches)
   • Positioning: OML perpendicular to IR. CR angled 30° caudal (or 37° to IOML), entering 1 inch superior to glabella. Demonstrates bilateral zygomatic arches projecting laterally.

Mandible Projections

1. PA Mandible (0° Central Ray)
   • Positioning: Forehead and nose against IR. OML perpendicular to IR.
   • Central Ray: Perpendicular to IR, exiting at the junction of the lips. Demonstrates mandibular rami and body symmetrically.
2. PA Axial Mandible
   • Positioning: Forehead and nose against IR. OML perpendicular.
   • Central Ray: Angled 20° to 25° cephalad, exiting at the acanthion. Demonstrates the condyloid processes and mandibular rami.
3. Axiolateral Oblique Mandible
   • Positioning & Head Rotation: Patient seated or semi-prone. Place mandible parallel to IR and tilt head 15° toward IR (or angle CR 25° cephalad). Adjust head rotation to isolate specific regions:
     • 0° Rotation (True Lateral): Demonstrates the mandibular ramus.
     • 10° to 15° Rotation toward IR: Demonstrates the mandibular body.
     • 30° Rotation toward IR: Demonstrates the mandibular body in full profile.
     • 45° Rotation toward IR: Demonstrates the mandibular mentum (symphysis).

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Paranasal Sinuses: Mandatory Positioning Rules

The Golden Rule of Sinus Radiography

ALL PARANASAL SINUS RADIOGRAPHS MUST BE PERFORMED WITH THE PATIENT ERECT AND USING A HORIZONTAL CENTRAL RAY.

• Physiological Rationale: The paranasal sinuses (frontal, ethmoid, sphenoid, and maxillary) are air-filled cavities within the cranial and facial bones. Pathological states often produce exudate or hemorrhage.
• Why Horizontal Beam is Mandatory: An erect position combined with a horizontal X-ray beam allows fluid to pool at the bottom of the sinus cavity, demonstrating a distinct air-fluid level. If the patient is radiographed in a recumbent position or with an angled vertical beam, the fluid coats the anterior/posterior sinus walls, appearing as diffuse opacification and completely masking air-fluid levels.

Standard Sinus Projections

1. Parietacanthial Projection (Waters Method - Sinuses)
   • Positioning: Patient erect, chin extended against upright IR, MML perpendicular to IR (OML 37° to IR).
   • Central Ray: Horizontal beam perpendicular to IR, exiting at the acanthion.
   • Open-Mouth Waters Variant: Have the patient open their mouth wide while maintaining MML perpendicularity. Demonstrates the sphenoid sinus projected through the open oral cavity, alongside the maxillary sinuses.
2. PA Axial Projection (Caldwell Method - Sinuses)
   • Positioning: Patient erect, tilt grid table 15° OR tilt patient head forward so OML is 15° to a vertical IR.
   • Central Ray: Horizontal beam exiting at the nasion. Demonstrates frontal sinuses superior to frontonasal suture and anterior ethmoid cells; petrous ridges lie in the lower third of the orbits.
3. Lateral Sinus Projection
   • Positioning: Patient erect in true lateral position (MSP parallel, IPL perpendicular to IR).
   • Central Ray: Horizontal beam centered 0.5 to 1 inch posterior to the outer canthus.
   • Diagnostic Criteria: All four sinus groups (frontal, ethmoid, sphenoid, and maxillary) are demonstrated, with the sphenoid sinus clearly visualized in profile beneath the sella turcica.
4. Submentovertex (SMV) Sinus Projection
   • Positioning: Patient erect, hyperextend neck so IOML is parallel to vertical IR.
   • Central Ray: Horizontal beam perpendicular to IOML entering 1.5 inches inferior to symphysis. Demonstrates sphenoid and ethmoid sinuses.

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Facial & Paranasal Sinuses Positioning Summary