Head
Temporomandibular Joints (TMJs)
Anatomy
Students should be able to identify the following structures on themselves, the patient or radiographic images:
Temporomandibular Joint (TMJ), mandibular fossa of the temporal bone, external acoustic meatus (EAM), Mandible (Body, Ramus, Condylar process, Coronoid process, Mandibular notch, Mentum, Temporal Bone (Mandibular fossa, Articular tubercle (eminence), Petrous portion), Mastoid process, Occlusal plane.
Topographical Landmarks & Positioning Lines
- Orbitomeatal line (OML): From outer canthus to EAM; primary baseline for Towne and Schüller projections.
- Infraorbitomeatal line (IOML): From infraorbital margin to EAM; alternate baseline when OML cannot be aligned.
- Acanthiomeatal line (AML): From acanthion to EAM; useful for modified projections.
- Interpupillary line (IPL): Perpendicular to IR for lateral or oblique TMJ views to prevent tilt.
- Midsagittal plane (MSP): Divides skull into equal halves; perpendicular for PA and AP axial projections.
Routine Projections (ARRT Required)
AP Axial Projection (Towne Method)– closed and open mouth
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 (what anatomy must be included and how you verify it’s complete): Both TMJs and condylar processes included.
- Rotation checks (how symmetry or alignment tells you if positioning is correct): Symmetry of condyles relative to mastoid and petrous portions; MSP aligned.
- Motion checks (how sharpness confirms patient cooperation/exposure timing): Sharp margins of mandibular condyles and fossae.
- Technique checks (what contrast, density, soft tissue visibility, and artifacts to look for): Adequate contrast to show condyle position relative to mandibular fossa and articular eminence.
- Clinical aim (when applicable, e.g., reflux, obstruction, displacement, etc.): Compare condyle position in closed- vs open-mouth; detect condylar displacement, subluxation, or fracture.
Axiolateral Projection (Schüller Method) – both sides, open and closed
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) – both sides, open and closed
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 |
.
Hows & Whys of TMJ Radiography
Anatomy
- How many joints form the TMJ, and what type of joint is it?
Two joints—one on each side—form a synovial diarthrodial (hinge–gliding) joint between the mandibular condyle and the mandibular fossa of the temporal bone. - How does the TMJ move during mouth opening?
The condyle rotates within the mandibular fossa, then glides forward and downward along the articular eminence. - How does the articular disc function within the TMJ?
It divides the joint into upper and lower compartments, allowing separate gliding and rotational movements for smooth jaw motion. - How can pathology alter condylar movement?
Ankylosis, arthritis, or displacement of the articular disc can restrict excursion and cause deviation or asymmetry in motion. - How do the temporal bone and condyle relate to the EAM?
The TMJ lies just anterior to the external acoustic meatus, an important surface landmark for centering and CR angulation.
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.
Clinical Applications
- Why are open- and closed-mouth views essential in TMJ evaluation?
They provide a dynamic assessment of joint motion, condylar translation, and potential disc displacement or ankylosis. - Why is bilateral imaging always performed?
Pathologies are frequently asymmetric; both sides must be compared to establish normal motion range and symmetry. - Why is careful patient instruction important before exposures?
TMJ imaging requires specific, reproducible positions and controlled motion during open- and closed-mouth views to ensure diagnostic comparison. - Why might a physician request tomography, CT, or MRI after radiographic TMJ studies?
If bony alignment appears normal but symptoms persist, advanced imaging evaluates the disc, ligaments, or soft-tissue pathology. - Why is radiography still valuable despite advanced imaging options?
It provides a low-dose, functional assessment of condylar position and mobility and remains a standard baseline in TMJ evaluation.
