Modified Law View TMJ X-Ray: Axiolateral Oblique Positioning Notes
The Modified Law View TMJ X-Ray Method (15°/15° Axiolateral Oblique Projection) is a specialized diagnostic view designed to demonstrate the Temporomandibular Joint (TMJ) in profile.
By combining a 15⁰ head rotation toward the image receptor with a 15⁰ caudal central ray angulation, the dense petrous bone and TMJ of the upside are projected away from the joint of interest. This “double 15⁰” angle geometry minimizes the geometric distortion and severe elongation seen in higher single-angle projections (such as the 25°-30° Schüller method).
1. Clinical Indications & Contraindications
Detailed Indications:
- Temporomandibular Joint Dysfunction (TMD):
- Closed-Mouth Phase: Demonstrates the mandibular condyle seated within the mandibular (glenoid) fossa to detect joint space narrowing, subchondral sclerosis, osteophytes, and erosive changes of osteoarthritis.
- Open-Mouth Phase: Assesses functional condylar translation (excursion). Detects condylar hypomobility (fibrous/bony ankylosis, anterior disc displacement without reduction) or hypermobility/subluxation (where the condyle travels excessively anterior to the articular eminence).
- Trauma & Fractures:
- Assessment of condylar neck, subcondylar, and intracapsular head fractures.
- Detection of joint dislocation (anterior, posterior, or lateral displacement).
- Neoplastic & Fibro-Osseous Lesions:
- Evaluation of osteochondromas, condylar hyperplasia/hypoplasia, and synovial chondromatosis.
Contraindications & Limitations:
- Unstable Cervical Spine Injury: Standard head rotation and prone/seated lateral postures are contraindicated until the C-spine is cleared.
- Articular Disc Soft-Tissue Staging (Limitation): 2D plain film views show only bony contours and joint space; MRI is the gold standard for soft-tissue TMJ articular disc displacement and internal derangement.
2. Technical Factors & Exposure Physics
| Parameter | Recommended Specification | Radiographic Rationale |
| Patient Posture | Erect (Seated) or Semi-Prone | Erect seated provides superior stabilization during jaw movements |
| Image Receptor (IR) | 18 × 24 cm (Portrait) | Small format centered over the downside joint of interest |
| SID | 100 to 115 cm ( 40 to 44 inches) | Standard distance balances spatial sharpness and beam divergence |
| Grid Ratio | 8: 1 to 12: 1 focused grid | Essential to absorb scatter generated across dense cranial structures |
| Tube Potential (kVp) | 70 – 78 kVp | High bone-to-soft tissue contrast for fine joint spaces |
| mAs Range | 15 – 25 mAs | Calibrated based on patient head thickness (Center AEC active) |
| Focal Spot | Small (0.6 mm) | High spatial sharpness required to visualize delicate articular cortex |
| Marker | “R” or “L” + “Closed” / “Open” | Essential to document the side of interest and functional phase |
| Breathing | Suspended respiration | Eliminates respiratory and swallowing motion artifacts |
3. Step-by-Step Patient & Part Positioning
Modified Law Method: Patient Positioning
Axiolateral Oblique (15°/15°)
Central Ray: 15° Caudad Tube Tilt
Enters 1.5″ superior to upside EAM
Enters 1.5″ superior to upside EAM
▼
MSP Rotated 15° Toward Image Receptor
(From true lateral toward face contact)
(From true lateral toward face contact)
AML Parallel (//) to Transverse Axis of IR
(Acanthiomeatal line level alignment)
(Acanthiomeatal line level alignment)
▼
[ Image Receptor Surface ]
CR exits through downside TMJ
CR exits through downside TMJ
1
Dual 15° Principle (15°/15°)
Combines a 15° face rotation toward the IR with a 15° caudal tube angle to project the TMJ cleanly.
2
Superimposition Clearance
Projects the upside temporal bone downward and anteriorly away from the downside condyle and fossa.
3
AML Baseline Alignment
Acanthiomeatal line is kept parallel to the transverse axis to standardize condylar head positioning.
4
Bilateral Open/Closed Protocol
Standardized comparisons evaluate condylar seating in the fossa (closed) vs. anterior excursion (open).
- Patient Preparation: Remove all metallic and radiopaque objects (earrings, hairpins, clips, spectacles, dentures, hearing aids, and necklaces).
- Base Posture: Patient sits erect facing the vertical Bucky (or lies semi-prone on the table). Place the TMJ of interest closest to the receptor surface.
- The “15°/15°” Alignment Protocol:
- Starting Point: Position the patient’s head in a true lateral position with the affected side flat against the Bucky.
- Head Rotation (MSP): Rotate the patient’s face 15⁰ toward the image receptor (so the Mid-Sagittal Plane forms a 75⁰ angle with the IR plane).
- Acanthiomeatal Line (AML): Adjust chin flexion/extension until the AML is parallel to the transverse axis of the IR (or IOML parallel to the long axis).
- Interpupillary Line (IPL): Must remain perpendicular to the transverse plane to avoid unwanted lateral tilt.
4. Functional Open vs. Closed Mouth Technique
- Closed-Mouth Exposure:
- Instruct the patient to close their mouth gently with the posterior molars in light, natural contact. Avoid aggressive jaw clenching.
- Open-Mouth Exposure:
- Instruct the patient to open their mouth as wide as comfortably possible (dropping the lower jaw vertically) without rotating or twisting the head.
- Bilateral Protocol: A complete TMJ examination routinely consists of 4 exposures:
- Right TMJ — Closed Mouth
- Right TMJ — Open Mouth
- Left TMJ — Closed Mouth
- Left TMJ — Open Mouth
5. Central Ray (CR), Tube Angle & Collimation
- Central Ray (CR) Tube Angle:15⁰ Caudad (angled downwards toward the feet).
- Rationale: Combining a 15⁰ caudal tube angle with a 15⁰ head turn creates a compound angle that clears the upside mandibular condyle and petrous bone without severe foreshortening.
- CR Entry Point: Enters the cranium approximately 4 cm (1.5 inches) superior to the upside (elevated) External Auditory Meatus (EAM).
- CR Exit Point: Passes obliquely through the skull to exit directly at the downside (dependent) TMJ touching the image receptor.
- Collimation: Four-sided tight collimation restricted to a 7.5 × 7.5 cm (3 × 3 inches) field centered over the downside joint.
6. Positioning Errors & Corrective Actions
| Error Type | Radiographic Sign on Film | Clinical Impact | Correction |
| Insufficient Head Rotation (< 15⁰) | Upside petrous temporal bone superimposes over the anterior aspect of the downside TMJ. | Obscures the articular eminence and anterior joint space. | Rotate head a full 15⁰ toward the IR. |
| Excessive Head Rotation (> 15⁰) | Mandibular ramus and body are projected too far anteriorly, foreshortening the condylar head. | Condylar morphology and joint space width appear distorted. | Rotate the head back to maintain an exact 15⁰ turn. |
| Insufficient Caudal Angle (< 15⁰) | Upside condyle and petrosa superimpose directly over the downside joint space. | Total failure of joint isolation. | Increase the caudal tube angle to a full 15⁰. |
| Head Tilt (IPL not perpendicular) | Mandibular condyle appears tilted/angled inside the glenoid fossa. | Creates false asymmetry in superior joint space width. | Realign the IPL perpendicular to the plane of the IR. |
7. Comparison: Modified Law Method vs. Schüller Method
| Technical Parameter | Modified Law Method | Schüller Method |
| Head Rotation | 15⁰ toward the IR | 0⁰ (True Lateral) |
| Central Ray Angle | 15⁰ Caudad | 25⁰ to 30⁰ Caudad |
| Geometric Distortion | Minimal (Lower tube angle preserves anatomy) | Moderate to High (Steep angle elongates joint) |
| Petrous Ridge Separation | Achieved via combination of rotation + tilt | Achieved entirely by steep caudal tube angle |
8. Radiographic Anatomy & Quality Evaluation Criteria
TMJ: Closed vs. Open Mouth Evaluation
Functional Dynamics
Closed-Mouth Phase
Open-Mouth Phase
Downside TMJ Profile (Free of Superimposition)
1
Seated in Glenoid Fossa (Closed)
Mandibular condyle rests symmetrically within the fossa with intact superior/posterior joint spaces.
2
Anterior Translation (Open)
Condyle moves inferiorly and anteriorly to align below the apex of the articular eminence.
3
Clear Downside TMJ Profile
Contralateral temporal bone is projected away, leaving the joint space completely unobstructed.
4
External Acoustic Meatus (EAM)
Acoustic canal serves as the posterior reference landmark for measuring condylar displacement.
- Clear Downside TMJ Isolation (Primary Hallmark):
- The downside TMJ is clearly demonstrated in profile, free of superimposition from the upside temporal bone and cervical spine.
- Closed-Mouth Assessment:
- Mandibular condyle is seated centrally within the mandibular (glenoid) fossa.
- Clear, uniform radiolucent joint space is visible surrounding the anterior, superior, and posterior borders of the condylar head.
- Open-Mouth Assessment:
- Mandibular condyle translates anteriorly and inferiorly, positioning itself directly beneath or slightly anterior to the crest of the articular eminence.
- Cortical Sharpness:
- Sharp cortical margins and distinct trabecular architecture of the condylar head, mandibular notch, and glenoid fossa without motion blur.
