Procedures · Head, Spine and Pelvis

Mandible and TMJ Positioning Series

Study the mandible and temporomandibular joints through all four routine projections: axiolateral oblique, PA, and two AP axial views.

Mandible and TMJ

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The routine mandible and tmj series

The projections this region is examined with. Set them up yourself in Practice positioning.

Axiolateral oblique mandible

Axiolateral oblique projection · head rotated 15° toward the image receptor · central ray 25° cephalad

Demonstrates: The ramus, the condyloid and coronoid processes, the body and the mentum of the mandible NEAREST the image receptor — this projection images one half of the mandible at a time, which is why both sides are examined for comparison. Three separate faults each have their own readout, and they are worth separating because they are corrected in three different places: the opposite mandible superimposed on the ramus of interest means the CENTRAL RAY ANGLE was too shallow; the cervical spine superimposed on the ramus means the NECK was not extended enough; and a foreshortened ramus and body mean the HEAD ROTATION was wrong for the part being examined.
Part position: Erect or recumbent. Place the head in a true lateral position with the side of interest against the image receptor, and if the patient can manage it close the mouth and bring the teeth together. Extend the neck slightly, which lifts the angle of the mandible clear of the cervical spine. Then rotate the head toward the receptor by the amount that lays the part of interest flat against it: a true lateral with no rotation for the ramus, 15° for the general survey, 30° for the body and 45° for the mentum.
Central ray: Angled 25° cephalad, centered to pass through the mandibular region of interest — directed from beneath the mandible on the side away from the image receptor.
Why it is positioned this way: The mandible is a single U-shaped bone, so a plain lateral lands its two halves exactly on top of each other and neither can be read. The 25° cephalad angle is what separates them: it projects the far half upward and clear of the near half, leaving the side against the receptor free to be read. That angle is a relationship between the beam and the PART, not a number on the tube — which is why the same projection appears with two different tube readings. Angling the head about 15° open inferiorly, done on a muscular patient so the shoulder is not projected over the mandible, already supplies 15° of it, and the tube then contributes only 10°. Both routes arrive at the same 25°.

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Rotation is chosen by the part, not by habitThe degree of obliquity is the one thing the technologist changes between one axiolateral and the next, and it follows the anatomy: the ramus lies close to the coronal plane, so a true lateral with no rotation already has it against the receptor; the body runs forward and inward from the angle, so it needs about 30°; and the mentum sits across the front of the arch, so it needs about 45°. The 10° to 15° general survey is the compromise that shows the whole bone reasonably rather than any one part well.

Trauma takes the beam, not the patientA patient who cannot be turned is imaged with a horizontal beam instead, with the receptor placed parallel to the mandible and the central ray angled 25° cephalad. The part relationship is unchanged — what moves is the equipment rather than the head — which is the general pattern for trauma work throughout the skull.

What the panoramic image replacedOrthopantomography — a panoramic image made by rotating the tube and receptor around the patient — shows the whole mandible and both joints on one image, and where it is available it has largely displaced this series for survey work. It is listed as a SPECIAL projection rather than a routine one. Its own position criteria are worth knowing: the mandible should appear oval, the symphysis projected slightly below the angles, and the upper and lower teeth slightly apart and not superimposed.

PA mandible

Posteroanterior projection · central ray 90° to the image receptor

Demonstrates: The rami and the lateral portion of the body, with the two sides compared against each other. No rotation is proved by the rami appearing symmetric and lying LATERAL to the cervical spine. The midbody and the mentum are only faintly visible here because they are superimposed on the cervical spine — that is a property of the projection rather than a fault, and it is the reason the axiolateral exists alongside it.
Part position: Erect or prone, with the forehead and nose resting against the imaging device. Tuck the chin until the orbitomeatal line lies at 90° to the image receptor, and set the midsagittal plane at 90° to the midline of the grid so the head neither rotates nor tilts.
Central ray: 90° to the image receptor, centered to the midsagittal plane to exit at the junction of the lips.
Why it is positioned this way: This is the frontal comparison view, and its value is bilateral symmetry: a displaced ramus or a widened angle shows up against its opposite number in a way no lateral can demonstrate, because a lateral superimposes the two sides. What it cannot do is show the midbody and the mentum, which the cervical spine sits behind. So the PA and the axiolateral are not alternatives — between them they cover the whole bone, and the routine asks for both.

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The optional PA axial, and what the angle buysThe routine allows a PA axial in place of the straight PA, with the central ray angled 20° to 25° cephalad to exit at the acanthion. The angle lifts the condyloid processes clear of the mastoid processes, so the joint region and the heads of the condyles become visible through them and the condyloid processes are well shown, though slightly elongated. Choose it when the proximal rami or the condyles are the question; the straight PA is the better view of the body.

Why the chin is tuckedBringing the orbitomeatal line to 90° to the receptor is what fixes the relationship between the beam and the mandible on a projection where the tube is not angled at all. Without the chin tucked, the mandible sits obliquely to the beam and both the rami and the condyles are foreshortened by an amount nothing on the image reveals — the head simply looks slightly wrong rather than obviously mispositioned.

AP axial mandible (Towne method)

Anteroposterior axial projection · central ray 35° caudad to the orbitomeatal line

Demonstrates: The condyloid processes of the mandible and the temporomandibular fossae, both sides on one image. No rotation is proved by the condyloid processes appearing symmetric. This is the projection for a fracture of the condyloid process — the part of the mandible the axiolateral shows obliquely and the PA hides behind the mastoid.
Part position: Erect or supine, with the posterior skull resting against the imaging device. Tuck the chin until the orbitomeatal line lies at 90° to the image receptor, and set the midsagittal plane at 90° to the midline of the grid so the head neither rotates nor tilts.
Central ray: Angled 35° caudad to the orbitomeatal line, centered to the midsagittal plane 1 inch (2.5 cm) superior to the glabella.
Why it is positioned this way: The condyles sit deep, directly in front of the mastoid portions of the temporal bones, and every frontal projection that leaves the tube square buries them there. A steep caudad angle is what pulls them out: it projects the condyles downward away from the dense mastoid and separates them from the skull base, so their relationship to the fossae can be read. That steepness is also why the reference line matters so much here — the angle is measured to the ORBITOMEATAL LINE, not to the tabletop, so a chin that is not properly tucked changes the projection even though the tube reads 35°.

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When the chin will not tuckIf the patient cannot bring the orbitomeatal line square to the receptor, the infraorbitomeatal line is brought square instead and the central ray is increased by 7° to 42° caudad. The two lines differ by about 7° to 8°, so the substitution keeps the same 35° relationship between the beam and the skull and demonstrates the same anatomy. This 7° swap recurs throughout the skull chapter and is worth learning once.

A steeper 40° for the fossaeWhen the temporomandibular FOSSAE rather than the condyles are the area of interest, the angle steepens to 40° to the orbitomeatal line, which further reduces superimposition of the fossae by the mastoid portions of the temporal bone. Note that 35° is the routine figure and 40° is this specific variant — the two are sometimes quoted together as a range, which loses the distinction.

AP axial temporomandibular joints (modified Towne method)

Anteroposterior axial projection · central ray 35° caudad to the orbitomeatal line · closed mouth

Demonstrates: The condyloid processes of the mandible and the temporomandibular fossae — the two halves of the joint and the relationship between them. A correctly positioned image shows the condyloid processes symmetric and lateral to the cervical spine, with the condyle-to-fossa relationship clear on both sides.
Part position: Erect or supine, with the posterior skull resting against the imaging device. Tuck the chin until the orbitomeatal line lies at 90° to the image receptor, and set the midsagittal plane at 90° to the midline of the grid so the head neither rotates nor tilts. The routine image is made with the mouth closed — and closed means the POSTERIOR teeth in contact, not the incisors.
Central ray: Angled 35° caudad from the orbitomeatal line, centered midway between the joints 3 inches (7.5 cm) superior to the nasion.
Why it is positioned this way: This is the same part position and the same 35° central ray as the AP axial mandible on the previous tab, and comparing the two is the point: what makes it a different examination is the CENTRING POINT — 3 inches above the nasion rather than 1 inch above the glabella — and the collimation, which is tight across the two joints rather than down over the jaw. The receptor orientation in the book records the same difference, landscape for the joints and portrait for the mandible. It is a clean example of two exams that share a projection and are separated by the field.

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Closed mouth means the back teeth touchFor the closed-mouth image the posterior teeth must be in contact rather than the incisors. Occluding on the incisors puts the mandible into protrusion, which carries the condyles forward out of the fossae — so an image made that way shows a displaced condyle that is an artefact of how the patient was asked to bite, not a finding. This is a small instruction with a large consequence and it is easy to leave out.

The open-mouth comparison, and when not to do itSome departmental protocols take this projection in both closed-mouth and open-mouth positions for comparison, when the patient's condition allows. The open-mouth image is what demonstrates range of motion: the condyle should move forward to the articular tubercle at the anterior margin of the fossa. The mouth must NOT be opened when a fracture is suspected, because the movement can displace fragments, and a trauma patient is examined without any stress movement of the mandible at all.

The lateral joint views are SPECIAL, not routineThe projections most associated with the joints — the axiolateral oblique by the modified Law method, with the face rotated 15° toward the receptor and the central ray 15° caudad, and the axiolateral by the Schuller method, a true lateral with the central ray 25° to 30° caudad — are listed as SPECIAL rather than routine, and both are normally taken as an open- and closed-mouth pair. They show one joint at a time, the one nearest the receptor, which is what the frontal projection here cannot do.

A further 5° if the joints are still buriedIncreasing the central ray by a further 5° may better demonstrate the fossae and the joints. The reason is the same one that drives the 40° variant on the mandible tab: the denser petrous and mastoid portions of the temporal bone lie behind the joints, and more caudad angle moves them apart.

Procedures lessonSkull / facial / sinuses