Five dysfunctions of the sphenobasilar synchondrosis: two physiologic, three traumatic. Know the axes, the naming rules, and which ones can happen normally.
A student palpates a patient's cranium and finds the left greater wing of the sphenoid is superior compared to the right. The sphenoid and occiput appear to rotate around an anteroposterior axis in opposite directions. What is this dysfunction?
Toggle between flexion and extension to see what every structure does in each phase.
The sphenobasilar synchondrosis moves superiorly. Picture pushing up on a rubber ball from below: it squishes wider and shorter. That is cranial flexion.
Temporal and parietal bones rotate outward, spreading the vault. When the ball squishes wider, the sides flare out. Same thing.
As it widens transversely, the front-to-back distance shrinks. The ball gets fatter but shorter. Wider + shorter = flexion.
The sacral base tips posteriorly (counternutation = sacral flexion). The sacrum rocks back as the cranium widens. They are coupled through the dural tube.
The SBS descends inferiorly. Now let go of the ball: it springs back to tall and narrow. The skull narrows transversely and lengthens AP. Opposite of flexion.
Temporal and parietal bones rotate inward. The sides pull back in as the vault narrows. Everything tightens and lengthens.
As the skull narrows, the AP distance increases. Narrower + longer = extension. The skull stretches out front-to-back.
Sacral base tips anteriorly (nutation = sacral extension). S in Spine: the spine straightens as the sacrum nutates. Extension phase tightens and lengthens everything.
CRI is the palpable rhythm of the PRM. Slower than respiration, faster than heart rate variability.
Toggle between extension and flexion. Watch the SBS, spinal cord, and sacrum move together.
Tap each tab. Know the naming rule, the axis, and whether it is physiologic or traumatic.
The sphenoid and occiput rotate around the anteroposterior (AP) axis in OPPOSITE directions. One greater wing goes up, the other goes down.
Naming: Named for the HIGH greater wing of the sphenoid. If the left wing is superior, it is a left torsion.
Example: Left torsion = left greater wing superior, right greater wing inferior. The sphenoid and occiput are twisting around the AP axis like wringing a towel.
Classification: Physiologic. Can occur during normal cranial motion. No trauma required.
The sphenoid and occiput sidebend in the SAME direction but rotate around vertical axes in OPPOSITE directions. This creates a convexity on one side of the SBS.
Naming: Named for the convexity (the sidebend direction). If the SBS is convex to the left, it is a left SB-R.
Example: Left SB-R = SBS convex to the left. Both bones sidebend left (same direction), but they rotate around their vertical axes in opposite directions.
Classification: Physiologic. Can occur during normal cranial motion.
The sphenoid base and occiput base shift vertically in opposite directions around a transverse axis. One goes up, the other goes down.
Naming: Named for the direction the sphenoid base moves relative to the occiput.
Superior vertical strain: sphenoid base moves UP, occiput base moves DOWN. The SBS is "kinked" upward at the sphenoid end.
Inferior vertical strain: sphenoid base moves DOWN, occiput base moves UP.
Classification: Non-physiologic. Results from trauma (e.g., a blow to the face or forehead).
The sphenoid and occiput shift laterally in opposite directions. This is a shear force: one bone goes left while the other goes right.
Naming: Named for the direction the sphenoid shifts. If the sphenoid goes left, it is a left lateral strain.
Mechanism: Think of a lateral blow to the head. The sphenoid and occiput slide past each other like tectonic plates.
Classification: Non-physiologic. Requires trauma.
The SBS is jammed together. No flexion or extension can occur at the synchondrosis. The cranial mechanism is essentially locked.
Naming: Just "compression." No left or right, no superior or inferior. The joint is stuck.
Mechanism: Most commonly from birth trauma (difficult delivery, forceps). Can also result from severe head trauma at any age.
Clinical: The CRI will be diminished or absent on palpation. The cranium feels rigid, with no inherent motion at the SBS.
Classification: Non-physiologic. Always trauma.
Tap each dysfunction to sort it into the correct bucket. Get all five right.
Tap each card to reveal what happens to the bone during cranial flexion.
Unpaired / midline
In FlexionUndergoes flexion. The greater wings move laterally and inferiorly. The SBS rises.
Board noteMidline bones flex and extend. They do NOT externally/internally rotate. The sphenoid is the key bone for naming torsion, vertical strain, and lateral strain.
Unpaired / midline
In FlexionUndergoes flexion. The occiput moves with the sphenoid at the SBS. The squamous portion moves posteriorly.
Board noteOcciput and sphenoid together define the SBS. Their relative motion determines the dysfunction type.
Paired
In FlexionUndergoes external rotation. The mastoid tips move medially and the petrous portions widen the skull laterally.
Board notePaired bones externally rotate in flexion, internally rotate in extension. Always. This is the opposite of what midline bones do (which flex/extend).
Paired
In FlexionUndergoes external rotation. The parietals flare laterally, widening the cranial vault.
Board noteAlong with the temporals, the parietals demonstrate the "Flexion Flares" pattern: paired bones rotate outward during flexion.
Unpaired / midline
In FlexionUndergoes flexion with the other midline bones. Descends slightly with sphenoid flexion.
Board noteRarely tested directly, but know it is midline and follows the flexion/extension pattern, not external/internal rotation.
Unpaired / midline
In FlexionUndergoes flexion. Moves with the sphenoid as the SBS rises.
Board noteLike the ethmoid, rarely a standalone question. Its value is knowing it belongs in the midline group.
The gold standard hand position for cranial diagnosis. Know exactly where every finger goes.
25 board-style vignettes with clue highlights, per-choice breakdowns, and tap-through teaching chains.