Head
54 Temporomandibular Joints (TMJs)
Anatomy
Students should be able to identify the following structures on diagrams and radiographs of the TMJ region:
- Mandible
- Body
- Ramus
- Condylar process (condyle + neck)
- Coronoid process
- Mandibular notch
- Mentum
- Temporal Bone Structures
- Mandibular (glenoid) fossa
- Articular tubercle (eminence)
- Petrous portion (pars petrosa)
- Joint Spaces
- Temporomandibular joint space (articulation of condyle with mandibular fossa)
- Soft Tissue / Functional Landmarks (radiographic relevance)
- Occlusal plane (teeth alignment reference)
- External acoustic meatus (EAM) – for localization
- Mastoid process (positional landmark)
- Positional/Reference Lines
- Orbitomeatal line (OML)
- Acanthiomeatal line (AML)
- Infraorbitomeatal line (IOML)
- Interpupillary line (IPL)
- Midsagittal plane (MSP)
Routine Projections
- AP Axial (Towne Method) – closed and open mouth (when not contraindicated)
- Axiolateral (Schüller Method) – both sides, open and closed
- Axiolateral Oblique (Modified Law Method) – both sides, open and closed
AP Axial Projection (Towne Method)
CR Location & Positioning
- SID: 40 inches
- Patient position: Supine or seated upright, posterior skull against Bucky.
- Adjustments: MSP perpendicular to IR; OML perpendicular to IR. Expose once with mouth closed (posterior teeth in contact), and once with mouth open unless contraindicated.
- CR: 35° caudad, centered midway between TMJs, entering ~3 inches (7.6 cm) above the nasion.
- Pt. Instructions: Keep teeth gently together for closed-mouth; open as wide as possible without protruding mandible for open-mouth (unless trauma).
- Exposure: Suspend respiration.
Evaluation Criteria
- Coverage: Both TMJs and condylar processes included.
- Rotation checks: Symmetry of condyles relative to mastoid and petrous portions; MSP aligned.
- Motion checks: Sharp margins of mandibular condyles and fossae.
- Technique checks: Adequate contrast to show condyle position relative to mandibular fossa and articular eminence.
- Clinical aim: Compare condyle position in closed- vs open-mouth; detect condylar displacement, subluxation, or fracture.
Axiolateral Projection (Schüller Method)
CR Location & Positioning
- SID: 40 inches
- Patient position: Semi-prone or upright lateral, affected side closest to IR.
- Adjustments: MSP parallel to IR; IPL perpendicular to IR; center ½ inch (1.3 cm) anterior to EAM. Expose with mouth closed and open (unless contraindicated).
- CR: 25°–30° caudad, entering ½ inch (1.3 cm) anterior and 2 inches (5 cm) superior to upside EAM.
- Pt. Instructions: Keep still; open mouth only on second exposure unless trauma.
- Exposure: Suspend respiration.
Evaluation Criteria
- Coverage: TMJ anterior to EAM visualized; both open- and closed-mouth images obtained.
- Rotation checks: Condyle centered anterior to EAM with minimal superimposition.
- Motion checks: Clear margins of condyle, fossa, and articular tubercle.
- Technique checks: Adequate contrast to show condylar excursion between open- and closed-mouth images.
- Clinical aim: Demonstrates condylar excursion; detects ankylosis, displacement, or fracture.
Axiolateral Oblique Projection (Modified Law Method)
CR Location & Positioning
- SID: 40 inches
- Patient position: Semi-prone or upright oblique, affected side closest to IR.
- Adjustments: Rotate MSP 15° toward IR; IPL perpendicular to IR; AML parallel to transverse axis of IR. Closed- and open-mouth exposures obtained.
- CR: 15° caudad, centered to exit through TMJ closest to IR, entering ~1.5 inches (3.8 cm) superior to upside EAM.
- Pt. Instructions: First exposure with posterior teeth in contact, second with mouth open (unless contraindicated).
- Exposure: Suspend respiration.
Evaluation Criteria
- Coverage: TMJ, mandibular condyle, and neck included.
- Rotation checks: Proper 15° obliquity shows TMJ free of superimposition from opposite side.
- Motion checks: Sharp outlines of mandibular condyle and fossa.
- Technique checks: Enough contrast and brightness to distinguish condyle position relative to articular tubercle.
- Clinical aim: Evaluates condylar excursion with less superimposition than Schüller; useful for detecting subtle dislocations.
Quick Reference Table – TMJs
| Projection | CR Location & Angle | Patient Position | Key Evaluation Points |
|---|---|---|---|
| AP Axial (Towne) | 35°–42° caudad, midway between TMJs (3″ above nasion) | Supine/upright; MSP ⟂ IR; OML ⟂ IR (or IOML + 7°) | Closed-mouth: condyle in fossa; Open-mouth: condyle below pars petrosa |
| Axiolateral (Schüller) | 25°–30° caudad, ½” anterior & 2″ superior to upside EAM | Semi-prone/upright; MSP ∥ IR; IPL ⟂ IR | Closed: condyle in fossa; Open: condyle below articular tubercle |
| Axiolateral Oblique (Modified Law) | 15° caudad, exiting TMJ closest to IR | Semi-prone/upright; MSP rotated 15° toward IR; IPL ⟂ IR; AML ∥ transverse IR | Closed: condyle in fossa; Open: condyle moves out under articular tubercle |
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Hows & Whys of TMJ Radiography
Positioning
- Why must the posterior teeth (not incisors) be in contact for the closed-mouth AP Axial projection?
Occluding on the posterior teeth prevents the mandible from being protruded, keeping the condyles seated in the fossae. - Why is the mouth opened as wide as possible for the open-mouth AP Axial?
It demonstrates condylar excursion out of the mandibular fossa and beneath the articular eminence. - Why is the open-mouth position contraindicated in trauma?
Attempting mandibular movement could displace fracture fragments or worsen injury. - Why is the OML positioned perpendicular to the IR for the AP Axial?
To standardize skull base positioning so the CR angle accurately projects the condyles below the petrous ridges. - Why is the CR angled 35° caudad for the AP Axial?
This angle directs the condyles beneath the pars petrosa, allowing visualization of their position in the fossae. - Why is the IPL perpendicular and MSP parallel for the Schüller (Axiolateral) projection?
To avoid tilt and rotation, ensuring accurate lateral visualization of the TMJ. - Why is a 25°–30° caudad CR angle used for the Schüller?
It projects the TMJ anterior to the EAM, reducing superimposition from the temporal bone. - Why is the head rotated 15° toward the IR in the Modified Law projection?
This oblique rotation opens the TMJ joint space and prevents overlap of the opposite side. - Why are both open- and closed-mouth images required in TMJ studies?
Comparing both positions shows the condylar excursion, which is important for diagnosing ankylosis, subluxation, or displacement.
Technique & Image Evaluation
- How do you confirm correct CR angulation and chin positioning on an AP Axial TMJ?
The condyles and TMJs should appear below the pars petrosa in the open-mouth view, and minimally superimposed in the closed-mouth view. - How do you know the MSP was aligned correctly on the AP Axial?
The condyles appear symmetrical on both sides of the skull. - How do you check for motion on any TMJ projection?
The condylar margins and mandibular fossae should appear sharp, with no blurring. - What error is indicated if the condyles appear too high and obscured by the petrous ridges on an AP Axial?
The chin was not flexed enough, or the CR angle was insufficient. - What error is indicated if the condyles appear foreshortened or superimposed by the posterior skull base on an AP Axial?
The chin was flexed too much, or the CR angle was excessive. - How do you confirm proper obliquity on the Modified Law projection?
The TMJ of interest is projected free from overlap of the opposite side, with the condyle centered within the fossa. - How do you know the condyle moved normally between open- and closed-mouth views?
Closed-mouth: condyle lies within the mandibular fossa.
Open-mouth: condyle lies inferior to the articular tubercle. - What finding indicates abnormal condylar excursion on open-mouth views?
The condyle remains in the fossa instead of moving anteriorly below the articular eminence, suggesting ankylosis or restricted motion.