Skull, Facial Bones, and Sinuses — Limited X-Ray / Radiologic Technology

Objective. Students will identify the cranial and facial bones, major skull landmarks, and paranasal sinuses on anatomical models and on standard skull and facial bone radiographic images.

Overview: Why Skull and Facial Bone Anatomy Matters

The skull is one of the most complex anatomical regions you will image. A thorough knowledge of cranial and facial bone anatomy is essential before you ever position a patient. Without it, you cannot recognize what you are seeing on an image, cannot explain positioning to a patient, and cannot catch errors before they leave the room. Every projection you perform must be grounded in a clear mental picture of the underlying structures.

The Cranial Vault

Eight bones form the cranium, the bony case that encloses and protects the brain.

Frontal bone — forms the forehead and the superior margin of the orbits. The glabella is the smooth midline prominence between the superciliary arches; it is a key surface landmark for positioning.
Parietal bones (2) — form the superior and lateral walls of the cranium. They meet at the sagittal suture and join the frontal bone at the coronal suture.
Temporal bones (2) — contain the external auditory meatus (EAM), mastoid process, styloid process, and the petrous portion, which houses the inner ear structures. The EAM and the mastoid tip are frequent positioning reference points.
Occipital bone — forms the posterior cranium and the base. The foramen magnum is the large opening through which the brainstem passes. The external occipital protuberance (inion) is a posterior surface landmark.
Sphenoid bone — a butterfly-shaped bone at the base of the skull visible only on base and special projections. The sella turcica, a saddle-shaped depression in the body of the sphenoid, houses the pituitary gland and is a key landmark on lateral skull images.
Ethmoid bone — lies between the orbits, forming part of the nasal cavity roof and the medial orbital walls. The cribriform plate transmits fibers of the olfactory nerve (CN I).

Major cranial sutures you must know: sagittal (between parietals), coronal (frontal–parietal), lambdoidal (parietal–occipital), and squamosal (temporal–parietal).

Facial Bones

Fourteen bones make up the face.

Maxillae (2) — form the upper jaw and the hard palate; each contributes to the floor and medial wall of the orbit and houses the maxillary sinuses within its body.
Zygomatic bones (2) — form the cheekbones and portions of the lateral orbital walls and floors. The zygomatic arch extends posteriorly to articulate with the temporal bone.
Nasal bones (2) — small, paired bones forming the bridge of the nose. They are among the most frequently fractured facial bones.
Mandible — the only movable bone of the skull; forms the lower jaw. Landmarks include the body, two rami, condylar processes (articulate at the temporomandibular joints), and the mental protuberance (chin).
Orbits — formed by seven bones: frontal, zygomatic, maxillary, palatine, lacrimal, ethmoid, and sphenoid. The orbital floor is thin and is the site of the classic blowout fracture when pressure is transmitted from a blow to the eyeball.
Additional facial bones: lacrimal (2), inferior nasal conchae (2), palatine (2), vomer (1).

Paranasal Sinuses

The paranasal sinuses are air-filled cavities within cranial and facial bones that communicate with the nasal cavity. They are important both anatomically and clinically because sinusitis and tumors occur here.

Maxillary sinuses — the largest; located within the bodies of the maxillae, lateral to the nasal cavity. They are best demonstrated in the Waters (PA axial) projection.
Frontal sinuses — located in the frontal bone above the orbits; variable in size and often asymmetric. Best seen on the PA Caldwell projection and lateral.
Ethmoid sinuses (air cells) — multiple small cells between the orbits. They appear as a group and are best shown on the PA Caldwell and lateral projections.
Sphenoid sinuses — located in the body of the sphenoid bone; best demonstrated on the lateral projection and the submentovertex (SMV) view.

For sinus imaging, patients are positioned erect whenever possible. An upright position allows air-fluid levels — a key sign of infection or trauma — to settle and become visible. Positioning a sinus patient recumbent will hide fluid levels and reduce diagnostic value.

Key Projections and What They Show

| Projection | Primary Structures Demonstrated |

|---|---|

| PA Caldwell (15° caudal angle, OML perpendicular to IR) | Frontal and anterior ethmoid sinuses, orbital margins |

| Waters (PA axial, MML perpendicular to IR) | Maxillary sinuses, orbital floors, zygomatic arches |

| Lateral skull/sinuses | All four sinus groups in profile; sella turcica; sphenoid sinus |

| Submentovertex (SMV) | Sphenoid sinuses, ethmoid cells, zygomatic arches, base of skull |

| PA skull | Frontal bone, petrous ridges, facial bones |

Positioning Lines and Patient Safety

Three baseline lines are used throughout skull positioning:

Orbitomeatal line (OML) — from outer canthus of eye to center of EAM.
Infraorbitomeatal line (IOML) — from infraorbital margin to EAM; approximately 7–8° below OML.
Mentomeatal line (MML) — from mental point (chin) to EAM; used in the Waters projection.

Always ask about pregnancy before skull imaging. Collimate to the area of clinical interest. Use gonadal and breast shielding when practical and when it does not obscure anatomy. Immobilize carefully — skull studies require precise angulation, and motion blur will degrade diagnostic quality and may require a repeat exposure, increasing dose. Never force a patient's neck into position; report any signs of cervical pain or limited range of motion to the supervising radiologist or ordering provider before proceeding.

Key terms10

GlabellaThe smooth midline bony prominence on the frontal bone between the superciliary arches, just above the root of the nose; used as a surface landmark in skull positioning.

Orbitomeatal line (OML)An imaginary line drawn from the outer canthus of the eye to the center of the external auditory meatus; the standard baseline for most skull and head projections.

Sella turcicaA saddle-shaped depression in the body of the sphenoid bone at the base of the skull; it houses the pituitary gland and is a key landmark on lateral skull radiographs.

Waters projectionA PA axial skull projection in which the mentomeatal line is perpendicular to the image receptor; it is the primary view for demonstrating the maxillary sinuses and orbital floors.

PA Caldwell projectionA PA skull projection with a caudal central ray angle that projects the petrous ridges below the orbital floors; it best demonstrates the frontal sinuses and anterior ethmoid air cells.

Blowout fractureA fracture of the thin floor of the orbit caused by sudden increased intraorbital pressure, classically from a blunt blow to the eye; the orbital floor depresses into the maxillary sinus.

Paranasal sinusesAir-filled cavities within the frontal, maxillary, ethmoid, and sphenoid bones that communicate with the nasal cavity; they appear radiolucent on radiographs and opacify with fluid or infection.

Submentovertex (SMV) projectionA skull projection taken with the central ray directed from below the chin toward the vertex of the skull; it demonstrates the base of the skull, sphenoid sinuses, ethmoid air cells, and zygomatic arches.

Petrous ridgesThe dense, pyramid-shaped portions of the temporal bones that house the inner ear structures; their position on a PA skull image is controlled by central ray angulation and serves as a quality indicator.

Air-fluid levelA horizontal line visible on an upright radiograph where air and fluid meet inside a sinus cavity; its presence indicates sinusitis, hemorrhage, or other pathology and is only detectable when the patient is erect.

In class: Use a disarticulated or labeled skull model to locate and name all cranial bones (frontal, parietal, temporal, occipital, sphenoid, ethmoid), major foramina, and facial bones (maxillae, zygomae, nasal bones, mandible, orbits); sketch each surface view and label from memory. · Compare Waters, PA Caldwell, lateral skull, submentovertex (SMV), and paranasal sinus radiographic images to atlas diagrams; annotate each to identify sinuses (maxillary, frontal, ethmoid, sphenoid), orbital floors, and cranial bones demonstrated. · Complete a case-based matching activity: given three simulated clinical orders (suspected maxillary sinus infection, suspected orbital floor blowout, suspected nasal bone fracture), identify the appropriate projections and the anatomy each projection best demonstrates.
Materials: Articulated and disarticulated skull models, Merrill's Atlas, projected or printed skull and facial bone radiographs, case-based activity worksheets
Check: Score ≥ 80% on a 40-item anatomy identification quiz using unlabeled skull and sinus radiographs, plus correct selection of projections for all three case-based scenarios.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.