A frontal facial blow can crack the ethmoid roof. The crista galli points you to the cribriform plate. The tiny fila olfactoria passing through it are what shear first.
Board move: ethmoid roof or crista galli fracture plus anosmia or clear rhinorrhea → CN I and cribriform plate until proven otherwise.
Challenge before reveal
A 29-year-old mechanic is evaluated in the emergency department after striking his forehead on a windshield. He is alert but says his coffee "has no smell." Vital signs are stable. Examination shows forehead swelling, broken incisors, and clear fluid dripping from the right naris when he leans forward. CT face shows a fracture through the ethmoid roof near the crista galli. Which structure is the most likely source of the sensory deficit?
Interactive · Sieve gate
Odor reaches olfactory epithelium. Tiny CN I bundles climb through cribriform holes into the olfactory bulbs. The plate is the sieve.
Map the trap
Crista galli is the signpost. Cribriform plate is the danger zone.
Tap the tabs. The point is not memorizing another skull list. It is seeing why this exact fracture picks CN I instead of every other cranial nerve.
The roof
The cribriform plate is the thin ethmoid roof over the nasal cavity. It is full of tiny holes for the olfactory fila. Thin bone plus many holes means frontal facial trauma can crack it.
The ridge
The crista galli is the midline ridge projecting upward between the paired cribriform plates. A crista galli fracture means the injury lives at the same ethmoid roof neighborhood as CN I.
The wire
Olfactory receptor neurons send small unmyelinated bundles upward through the cribriform foramina. Those bundles synapse in the olfactory bulb sitting above the plate.
The symptom
Shear the bundles or bruise the bulb and the patient loses smell. Tear the dura in the same roof and clear rhinorrhea can appear as CSF drains into the nose.
Why the others fail
CN III, IV, V1, and VI cluster at the superior orbital fissure. V2 exits rotundum. V3 exits ovale. The middle meningeal artery uses spinosum. Vagus exits jugular. None of those landmarks are ethmoid roof.
The board shortcut
Do not start at the symptom list. Start at the bone. Ethmoid and cribriform point to smell. Sphenoid foramina point to eye movement or trigeminal branches. Temporal and occipital point to jugular or internal acoustic meatus.
Challenge before reveal
Run the foramen passport.
Pick the doorway from the clue. The feedback tells you why that landmark belongs or gets thrown out.
Q1Facial trauma, ethmoid roof fracture, anosmia, and clear nasal drainage. Which doorway?
Q2Now the patient has ptosis, a dilated pupil, and the eye rests down and out after orbital apex trauma. Which doorway?
Doorway rule loaded.
Passport drill
Sieve = smellCribriform means many holes. If smell fibers cross many holes, trauma there knocks out smell.
SOF = eye movers plus V1Superior orbital fissure carries III, IV, V1, VI. If the eye will not move or corneal afferent drops, think sphenoid fissure.
Jugular = swallow and voiceIX, X, XI exit jugular. Dysphagia, hoarseness, absent gag efferent, or weak shoulder shrug lives posteriorly, not ethmoid.
clinical walkthrough
Twenty-five original skull-base vignettes.
One at a time. Answers shuffle. Right-click or long-press to cross out. Select stem text to highlight it. The deck will not repeat until it exhausts the bank.
Vignette 1 of 25
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Medically reviewed by Kaitlyn Cocuzzo, MD and Fatima Ali, DO · Last reviewed June 2026
Bone Wizardry is an independent educational resource for visual learning in the medical sciences. It is not affiliated with, endorsed by, or sponsored by any licensing or examination board, contains no real or recalled examination questions, and does not guarantee any educational or examination outcome.
