🩺 TMJ Radiography: Open & Closed Mouth Projections for Dysfunction

TMJ Radiography: Open & Closed Mouth Projections for Dysfunction - Survival Guide Cheat Sheet

The Bottom Line

• TMJ radiography via open and closed mouth transcranial projections is fundamental for assessing the condyle-fossa relationship in static (closed) and dynamic (open) states.
• It aids in diagnosing structural dysfunction, internal derangement (indirectly), degenerative joint disease, and provides a baseline for treatment planning and monitoring.
• Critical for understanding condylar position, range of motion, and translation.

Key Mechanisms

• Purpose of Open & Closed Mouth Views

  • Closed Mouth Projection: Visualizes the condylar head within the glenoid fossa in centric occlusion. Assesses static condylar position, joint space, and early degenerative changes.
  • Open Mouth Projection: Visualizes condylar translation relative to the articular eminence during maximal comfortable opening. Evaluates condylar mobility, range of motion, and identifies hypomobility, hypermobility, or subluxation.

• Standard Projection: Lateral Oblique Transcranial (TMJ View)

  • Principle: Utilizes specific angulation to project the contralateral side out of the region of interest, focusing on the TMJ closest to the receptor.
  • Advantages: Readily available, relatively low cost, useful for initial assessment of bony components.
  • Limitations: Two-dimensional representation of a three-dimensional joint, prone to distortion and superimposition; provides limited direct information on soft tissue (disc).

• Anatomical Structures of Interest

  • Condyle: Articulating head of the mandibular ramus.
  • Glenoid Fossa: Concave depression in the temporal bone where the condyle articulates.
  • Articular Eminence: Convex bony prominence anterior to the glenoid fossa, over which the condyle translates during opening.
  • External Auditory Meatus (EAM): Posterior landmark for positioning.

Pre-Procedure Checklist

• Patient Preparation

  • Remove all metallic objects: Earrings, dentures, hairpins, glasses, necklaces, hearing aids.
  • Explain procedure: Clearly communicate instructions and the importance of holding still.
  • Informed Consent: Obtain prior to exposure.
  • Assess Opening: Evaluate patient's ability to achieve maximal comfortable opening without excessive pain.

• Equipment Setup

  • X-ray Unit: Confirm appropriate kVp (e.g., 70-80), mA (e.g., 7-10), and exposure time (e.g., 0.8-1.2 seconds); adjust based on manufacturer guidelines and patient size.
  • Positioning Device: Use a head holder or cephalostat for stabilization if available.
  • Receptor: Digital sensor or film cassette positioned correctly for the size of the TMJ area.

Procedure: Lateral Oblique Transcranial Projection

• General Patient Positioning

  • Patient Position: Seated or standing comfortably, back straight.
  • Head Position:
    • Mid-sagittal plane perpendicular to the floor.
    • Frankfort plane (line from EAM superior border to infraorbital rim) parallel to the floor.
    • Rotation: Head rotated approximately 15-20 degrees towards the side being examined (away from the X-ray tube).

• Receptor Placement

  • Place flat against the side of the head, centered over the TMJ being examined.
  • Superior border just above the orbit, anterior border just anterior to the condyle.

• Central Ray (CR) Angulation and Entry/Exit Points

  • Horizontal Angulation: Approximately 20-25 degrees anteriorly (from posterior to anterior) relative to the receptor plane.
  • Vertical Angulation: Approximately 10-15 degrees caudally (from superior to inferior) relative to the receptor plane.
  • Entry Point: Approximately 2 inches (5 cm) superior and 0.5 inch (1.25 cm) posterior to the contralateral EAM.
  • Exit Point: Ipsilateral TMJ area (closest to the receptor).

• Closed Mouth Projection

  • Instruction: Patient gently bites down with teeth in centric occlusion.
  • Acquisition: Maintain stillness and expose.

• Open Mouth Projection

  • Instruction: Patient opens mouth as wide as comfortably possible and holds still. A bite block may be used to maintain maximum opening if needed.
  • Acquisition: Maintain stillness and expose, typically using the same CR angulation as the closed mouth view.

Post-Procedure Evaluation & Interpretation

• Image Quality Assessment

  • Clarity: Evaluate for sharpness and absence of motion blur.
  • Density & Contrast: Ensure optimal visibility of bony structures.
  • Artifacts: Check for superimposition, patient jewelry, or processing errors.
  • Anatomical Coverage: Ensure the entire condyle, glenoid fossa, and articular eminence are clearly visible.

• Interpretation (Closed Mouth)

  • Condylar Position: Assess the condyle's relationship within the glenoid fossa (e.g., centric, anterior, posterior, superior).
  • Joint Space: Observe the superior joint space for uniformity or narrowing.

• Interpretation (Open Mouth)

  • Condylar Translation: Evaluate the degree of anterior movement of the condyle relative to the articular eminence.
  • Normal Translation: Condyle should typically translate to or just beyond the crest of the articular eminence.
  • Hypomobility: Condyle does not translate fully to the eminence.
  • Hypermobility/Subluxation: Condyle translates significantly beyond the eminence, potentially indicating instability.

Red Flags / Warnings

• Common Errors Leading to Non-Diagnostic Images

  • Improper Patient Positioning: Head tilt, rotation, or failure to maintain the Frankfort plane will lead to distortion and superimposition.
  • Incorrect CR Angulation: Causes superimposition of mastoid process, zygomatic arch, or petrous portion of the temporal bone over the TMJ.
  • Insufficient Mouth Opening: Prevents accurate assessment of condylar translation; may require patient re-instruction or use of a bite block.
  • Motion Artifact: Patient movement during exposure will result in a blurred image, necessitating a repeat and increased radiation dose.

• Clinical Considerations

  • Patient Discomfort: If the patient experiences acute pain during positioning or opening, modify the procedure or consider alternative imaging modalities (e.g., MRI for soft tissue, CBCT for detailed bone).
  • Limited Opening (Trismus): May preclude adequate open mouth views. Document the limitation.
  • Radiation Dose: Always adhere to the ALARA (As Low As Reasonably Achievable) principle. Minimize repeat exposures.
  • Diagnostic Limitations: Transcranial views are screening tools. For definitive diagnosis of disc displacement, soft tissue pathologies, or complex bony anomalies, Magnetic Resonance Imaging (MRI) or Cone Beam Computed Tomography (CBCT) may be indicated.
  • Bilateral Views: Always obtain bilateral open and closed views for comparison, even if symptoms are unilateral.