The temporomandibular joint (TMJ) is one of the most complex and frequently used joints in the human body. As the articulation between the mandibular condyle and the temporal bone's mandibular fossa, the TMJ opens and closes thousands of times a day — and when something goes wrong, the pain can be debilitating.
TMJ imaging is a specialized skill on the ARRT radiography exam and an essential tool in clinical practice for diagnosing condylar fractures, TMJ dislocation, arthritis, and internal derangement of the joint. Unlike routine mandible series, TMJ imaging requires paired open and closed mouth views — a unique technical requirement that tests your understanding of both anatomy and positioning.
In this guide, you'll learn the three standard TMJ projections — axiolateral (Schüller method), PA transoral, and AP axial (modified Towne) — with exact centering points, CR angles, and evaluation criteria. You'll also master the critical skill of comparing open vs closed mouth images to assess condylar translation.
For foundational head and neck positioning, see our Skull X-Ray Positioning Guide and Mandible X-Ray Positioning Guide.
The TMJ is a diarthrodial, hinge-gliding joint — able to both hinge (rotate) and glide (translate). Understanding its anatomy is essential for correct positioning and image evaluation.
| Structure | Description | Positioning Relevance |
|---|---|---|
| Mandibular condyle | Rounded head of the mandibular ramus | Primary structure visualized on all TMJ projections |
| Mandibular fossa | Concave depression in the temporal bone | Houses the condyle at rest (closed mouth) |
| Articular eminence | Bony ridge anterior to the fossa | Condyle translates forward onto this in open mouth |
| Articular disc (meniscus) | Fibrocartilage disc between condyle and fossa | Not directly visible on X-ray; inferred from spacing |
| Capsular ligament | Fibrous joint capsule | Distended in effusion — may widen joint space |
| External auditory meatus (EAM) | Ear canal opening in the temporal bone | Key palpable landmark for CR centering |
The TMJ is described as a diarthrodial hinge-gliding joint — a classic registry term. Know that the mandibular condyle translates forward onto the articular eminence during opening, and the articular disc (meniscus) sits between the condyle and fossa. Anterior disc displacement is the most common form of internal derangement and the most frequently tested pathology related to TMJ imaging.
The TMJ lies approximately 2.5–3 cm superior and 2 cm anterior to the external auditory meatus (EAM) — this is your key palpable landmark for centering on all TMJ projections. For comparison, the EAM landmark and surrounding skull anatomy are also covered in our Facial Bones X-Ray Positioning Guide.
A TMJ X-ray series is typically ordered when a patient presents with:
For guidance on managing trauma patients in the X-ray room, see our Trauma Radiography Principles guide.
A complete TMJ series consists of four images: two projections (one lateral/oblique and one frontal), each taken with the mouth open and closed. The specific combination varies by institution, but the most common approach uses:
The key to TMJ imaging is comparison. Each open-mouth image is compared against its closed-mouth counterpart to assess the range of condylar translation. A normal condyle moves from the mandibular fossa (closed) forward onto the articular eminence (open). Limited or absent translation suggests internal derangement, while excessive translation suggests hypermobility. Always label each image as "Open" or "Closed" — this seems obvious, but unlabeled TMJ images are a common cause of repeat exposures.
The Schüller method (also called the Law method in some texts) is the most frequently performed TMJ projection. It provides a lateral-oblique view of the TMJ closest to the image receptor, with the opposite TMJ projected superiorly and posteriorly out of the field of interest.
| Parameter | Detail |
|---|---|
| Position | Erect or recumbent; true lateral with side of interest closest to IR |
| Head position | Head in true lateral (MSP parallel to IR); interpupillary line perpendicular to IR |
| SID | 40" (102 cm) |
| CR | 15–20° caudad, entering 2.5–3 cm superior and 2 cm anterior to the EAM on the side of interest |
| CR landmark | If the EAM can be palpated, the centering point is 2 finger-breadths above and 1 finger-breadth anterior to the canal opening |
| kVp | 70–80 (digital) |
| Grid | Yes (8:1 or 10:1) |
| Breathing | Suspend respiration |
| AEC | Not recommended (tight collimation) |
| Mouth Position | Patient Instruction | What It Shows |
|---|---|---|
| Closed | "Close your mouth gently and keep your teeth together." | Condyle seated in the mandibular fossa at rest; evaluates baseline position and joint space |
| Open | "Open your mouth as wide as you comfortably can and hold it still." | Condyle translated forward onto the articular eminence; evaluates range of motion and reduction of dislocations |
Key positioning points:
Evaluation criteria:
The Schüller method axiolateral TMJ uses a 15–20° caudad CR angle — a frequently tested distinction from the mandibular axiolateral oblique, which uses 25° cephalad. Remember: TMJ = angled down (caudad), mandible = angled up (cephalad). The centering point is also different: TMJ is centered at the joint (2.5 cm above and 2 cm anterior to the EAM), while the mandibular oblique is centered at the body/ramus. These distinctions are classic exam traps.
The PA transoral projection provides a frontal view of both TMJs simultaneously. The patient opens their mouth wide, and the CR passes through the open mouth to project the condyles through the beam path without superimposition of the skull base.
| Parameter | Detail |
|---|---|
| Position | Erect or prone; forehead and nose against IR |
| Part | OML perpendicular to IR; MSP perpendicular to IR; mouth wide open |
| CR | Perpendicular, entering posteriorly and exiting through the open mouth at the TMJ level |
| Centering | Level of the lips (the CR passes through the open oral cavity to reach the TMJs) |
| SID | 40" (102 cm) |
| kVp | 75–85 (digital) |
| Grid | Yes (8:1 or 10:1) |
| Breathing | Suspend respiration |
Key positioning points:
Evaluation criteria:
The modified Towne projection for TMJ is an alternative frontal view that can be used when the patient cannot hold their mouth open for the PA transoral approach. It visualizes the condyles through the foramen magnum, similar to the standard mandibular Towne but with specific TMJ centering.
| Parameter | Detail |
|---|---|
| Position | Supine or erect |
| Part | OML perpendicular to IR; MSP perpendicular to IR; mouth closed (or open for comparison view) |
| CR | 30–35° caudad to OML, entering at the glabella |
| Centering | CR directed to exit at the level of the TMJs (approximately 2.5 cm above the EAM) |
| SID | 40" (102 cm) |
| kVp | 75–85 (digital) |
| Grid | Yes (8:1 or 10:1) |
Key positioning points:
Evaluation criteria:
| Projection | Patient Position | CR | Centering Point | Key Angle |
|---|---|---|---|---|
| Axiolateral TMJ (Schüller) | True lateral, side of interest on IR | 15–20° caudad | 2.5 cm above + 2 cm anterior to EAM | 15–20° caudad |
| PA Transoral TMJ | Prone/erect, forehead + nose on IR | Perpendicular | Level of lips (through open mouth) | 0° |
| AP Axial TMJ (Mod. Towne) | Supine/erect | 30–35° caudad | Glabella, exits at TMJ level | 30–35° caudad to OML |
For a complete reference on adjacent skull and facial bone anatomy, see our Skull X-Ray Positioning Guide and Cervical Spine Positioning Guide.
| Error | Cause | Fix |
|---|---|---|
| Both TMJs superimposed (axiolateral) | CR angle too shallow or head rotated away from IR | Increase caudad angle to 15–20°; verify true lateral head position |
| Condyle not visible (axiolateral open) | Insufficient mouth opening or CR not centered to TMJ | Verify patient opened wide enough; re-check centering 2.5 cm above + 2 cm anterior to EAM |
| Asymmetric condyles (PA transoral) | Head tilt (MSP not perpendicular) | Ensure MSP is truly vertical; correct tilt before re-exposure |
| Skull base superimposing condyles (PA transoral) | Mouth not open wide enough | Re-instruct patient; use bite block if available |
| Condyles not projected through foramen magnum (Towne) | CR angle too shallow | Increase to 30–35° caudad; verify OML ⟂ IR |
| Motion blur (open mouth views) | Patient unable to hold open mouth steady | Use shortest exposure time; provide chin support if possible; consider closed-mouth-only protocol |
| Unlabeled images | Technologist forgot to mark Open vs Closed | Use lead markers or digital annotation before releasing images |
The diagnostic value of the TMJ series comes from the side-by-side comparison of open and closed mouth images. Here's what the referring clinician and radiologist are looking for:
| Finding | Closed Mouth | Open Mouth | Clinical Significance |
|---|---|---|---|
| Normal | Condyle centered in fossa; uniform joint space ~2–3 mm | Condyle translates to or near the articular eminence | Normal range of motion; intact disc function |
| Anterior disc displacement | Condyle may appear posteriorly displaced in fossa | Limited or absent translation; condyle stays posterior | Disc has slipped forward; most common internal derangement |
| Fracture | Condylar head may be displaced, angulated, or foreshortened | No translation or asymmetric translation | Condylar neck fracture is common in falls and MVAs |
| Dislocation | Condyle anterior to the eminence; cannot close mouth fully | Condyle remains anterior — no reduction | Condyle is "stuck" forward; requires manual reduction |
| Degenerative joint disease | Narrowed joint space; osteophytes on condyle or fossa | Limited translation; irregular articular surface | Osteoarthritis, rheumatoid arthritis, or psoriatic arthritis |
A non-reducing anterior disc displacement (the disc stays forward of the condyle during opening) presents with limited opening and a characteristic "closed lock." On X-ray, the closed mouth view may look normal, but the open mouth view shows the condyle unable to translate forward. This is the most common TMJ internal derangement and a frequent clinical question on ARRT exams. Always compare both images before deciding if translation is normal.
A standard TMJ series includes four images: an axiolateral (Schüller method) projection of the affected or both TMJs with mouth open and closed, plus a frontal projection (PA transoral or modified Towne) also with mouth open and closed. Each image must be clearly labeled as "Open" or "Closed." Bilateral axiolateral views are obtained if both sides are symptomatic.
The CR is angled 15–20° caudad, centered 2.5–3 cm superior and 2 cm anterior to the EAM on the side of interest. This angulation separates the TMJ of interest from the opposite side. Remember: TMJ axiolateral = caudad (downward); mandibular axiolateral = cephalad (upward) — this is a frequently tested difference.
While the mandible series (Towne, PA axial) does show the condyles, it is not a substitute for a dedicated TMJ series. The mandible series is optimized for showing the body, ramus, and teeth — it uses different CR angles and does not routinely include open mouth views. The TMJ series specifically targets the joint space and condylar translation, which are the key diagnostic indicators for TMJ pathology.
The direction of the CR angle is opposite: TMJ uses 15–20° caudad (angulated downward), while the mandibular oblique uses 25° cephalad (angulated upward). The centering point also differs: TMJ targets the condyle/fossa at the skull base, while the mandibular oblique targets the mandibular body/ramus further inferiorly. Additionally, the TMJ view requires paired open/closed mouth exposures, whereas the mandibular oblique does not.
CT provides superior bony detail of the TMJ, including 3D reconstruction of condylar fractures and degenerative changes. However, CT does not assess functional condylar translation as effectively as serial open/closed mouth radiographs. MRI is the gold standard for evaluating the articular disc (meniscus) and soft tissues. In practice, CT is used for complex trauma, MRI for internal derangement, and X-ray for initial screening and functional assessment.
Question 1: What CR angle is used for the axiolateral (Schüller method) TMJ projection?
Question 2: A patient presents with suspected TMJ dislocation. What finding would you expect on the open mouth axiolateral view?
Question 3: Where is the centering point for the axiolateral TMJ (Schüller method) projection?
Question 4: A technologist obtains axiolateral TMJ images but notices that both TMJs are superimposed on the image. What is the most likely cause?
Question 5: Which of the following is the primary advantage of the PA transoral projection over the AP axial (modified Towne) for TMJ imaging?