What is SBS compression and how does it present?

SBS compression is a nonphysiologic, traumatic strain in which an anterior-to-posterior force jams the sphenoid into the occiput at the sphenobasilar synchondrosis. Classic mechanisms are a forehead or windshield blow and a fall onto the vertex. It presents as a low-amplitude cranial rhythmic impulse, chronic headaches, and often depressed mood.

What is the difference between a torsion and a sidebending rotation strain?

Both involve an anteroposterior axis. In a torsion the sphenoid and occiput rotate in opposite directions around one anteroposterior axis, and it is named for the superior (high) greater wing. In a sidebending rotation the bones rotate the same way around one anteroposterior axis and also sidebend around two vertical axes, producing a convexity on one side, and it is named for that convex or full side.

Which mechanism of injury causes a lateral strain?

A lateral strain is a traumatic strain classically produced by a blow to the side of the head or by birth molding. The sphenoid and occiput shift the same way around two vertical axes, sheared at the synchondrosis, so the head takes on a parallelogram shape. It is named for the side the base of the sphenoid shifts toward.

Cranial Strain Patterns: Reading the Mechanism of Injury (Medical Education)

Foundation 01

Two bones, one cartilage joint, three ways to move

Before any strain makes sense, you need the part that strains: the sphenobasilar synchondrosis (SBS), where the body of the sphenoid meets the basilar occiput.

Superior view of the base of the skull showing the sphenoid bone anteriorly and the occipital bone posteriorly — Cranial base from above: the sphenoid (front, butterfly) meets the occiput (back, around the foramen magnum) at the SBS. Henry Gray, Anatomy of the Human Body (1918), public domain, via Wikimedia Commons.

What the SBS actually is

The sphenoid in front and the occiput behind are joined by a wedge of cartilage at the floor of the skull. In a child it is a true growth plate; in an adult it is fused but, in the osteopathic cranial model, still has a tiny rhythmic give. Think of it as two rocking chairs sharing one hinge: when the hinge flexes and extends gently with the cranial rhythm, both bones rock together in a coordinated way.

A strain pattern is simply the hinge getting held in a position it should pass through, usually after a force pushes it there. Name the position, and you have named the strain.

The three axes

Every strain is described by the axis the bones move around. There are only three kinds. Learn these and the whole list falls out.

Transverse axis

A left-to-right axis through each bone. Movement here rocks the bones forward and back. This is the axis of normal flexion and extension, and of the traumatic vertical strain.

Anteroposterior (A-P) axis

A front-to-back axis down the midline. Rotation here tips one side up and the other down. This is the axis of torsion and of the rotation part of sidebending rotation.

Vertical axes

Two up-and-down axes, one per bone. Movement here swings the bones side to side. This is the axis of lateral strain and of the sidebending part of sidebending rotation.

Interactive 02

Watch the sphenoid and occiput strain

Tap a strain. The two bones move, the working axis lights up, and the readout tells you the axis, the naming rule, whether it is physiologic, and the force that causes it. Lavender buttons are physiologic. Red buttons are traumatic.

Rest: the cranial rhythm

Physiologic

Axis: The bones rock gently around their transverse axes with the cranial rhythmic impulse.
Named for: Nothing yet. Pick a strain to lock the naming rule.
Mechanism: No force. This is the baseline both bones return to.

Superior view of the isolated sphenoid bone showing the body and greater wings — The sphenoid alone. The greater wings (where your index fingers sit in the vault hold) report what the front of the SBS is doing. Henry Gray, Anatomy of the Human Body (1918), public domain, via Wikimedia Commons.

Framework 03

Physiologic: motions the SBS is allowed to make

These three are normal directions of motion. They become a diagnosis only when the SBS gets stuck in one. They can follow trauma, but the body performs them on purpose every cycle.

Flexion and extension

The sphenoid and occiput rotate in opposite directions around their two transverse axes. In flexion the skull shortens front to back and widens side to side; in extension it lengthens and narrows. This is the baseline rocking of the cranial rhythm.

Memory hook: flexion is named for the SBS riding superior, extension for the SBS riding inferior. Opposite directions, transverse axes.

Torsion

The sphenoid and occiput rotate in opposite directions around one anteroposterior axis. One greater wing rides high, the other rides low, like wringing a towel along its length.

Naming rule: a torsion is named for the side of the superior (high) greater wing. Right wing up equals a right torsion.

Sidebending rotation

Two motions at once: the bones sidebend around their two vertical axes and rotate the same way around one anteroposterior axis. The result is a convexity, a fullness, on one side of the head, and that side rotates down toward the table.

Naming rule: named for the side of the convexity, the full side where the vault-hold fingers spread apart. Right fullness equals a right sidebending rotation.

Framework 04

Nonphysiologic: motions that only trauma creates

The body never does these on its own. Each one needs a force, and the direction of that force is written into the strain. This is where the windshield case lives.

Vertical strain (superior or inferior)

The sphenoid and occiput rotate around their transverse axes in the same direction, so the base of the sphenoid is held superior or inferior relative to the occiput.

The board fork: same axes as flexion and extension (transverse), but the SAME direction instead of opposite. Same plane, wrong partner.

Lateral strain (the parallelogram head)

The sphenoid and occiput shift the same way around their two vertical axes, sheared at the synchondrosis. From above the head looks like a parallelogram.

Naming rule: named for the side the base of the sphenoid shifts toward. Classic forces are a blow to the side of the head and birth molding.

SBS compression (the pin of this page)

An anterior-to-posterior force jams the sphenoid straight into the occiput at the synchondrosis. No rotation. No clean axis. Pure approximation. The classic mechanisms are a forehead or windshield blow and a fall onto the vertex.

The presentation is the giveaway: a low-amplitude cranial rhythmic impulse (the bones are jammed, so they barely move), chronic headaches, and often a depressed, flat mood. Our 47-year-old passenger is textbook compression.

External surface of the occipital bone with the foramen magnum — The occiput: the back half of the synchondrosis that the sphenoid jams into during a compression. Henry Gray, Anatomy of the Human Body (1918), public domain, via Wikimedia Commons.

Decision tool 05

Read the blow, choose the strain

This is the skill the case demanded: given a mechanism of injury, name the most likely pattern. Tap the answer for each. Two of these point to the same strain. That is the lesson.

A fall straight onto the top of the head (the vertex)

A vertex fall drives force straight down the midline into the SBS. The sphenoid jams into the occiput. Axial force into the joint equals compression.

A forehead striking a windshield (the case)

An anterior-to-posterior blow drives the sphenoid straight back into the occiput. Same destination as the vertex fall, different doorway. Any force that approximates the SBS is compression.

A blow to the side of the head from a falling object

A lateral force shears the sphenoid and occiput sideways at the joint, the same direction around two vertical axes. The head becomes a parallelogram. Side force equals lateral strain.

Molding of an infant skull during a difficult delivery

Birth molding and forceps deform the soft cranial base in odd directions and classically produce a vertical strain (a lateral strain is also common). Molding forces make nonphysiologic strains, vertical strain first among them.

0 of 4 correct

Decision tool 06

Why the SBS, and not the suture or the neck?

A forehead-to-windshield blow could plausibly hurt three different places. The exam writer wants you to localize to the right one. Predict first, then tap each option to check yourself.

A 47-year-old unrestrained passenger struck his forehead on the windshield four weeks ago. He now has a progressive, constant headache, a normal neurologic exam, no cervical fracture, and a low-amplitude cranial rhythm. Which structure best explains the picture?

Tap your prediction, then read why

The discriminator: focal suture pain stays local, an OA dysfunction lives in the neck, and only a force carried all the way to the base gives the low-amplitude, whole-head compression picture.

Self-test 07

The whole list, one row at a time

Predict each hidden cell, then tap it to check. Work one row at a time, the way you will recall it on test day.

Strain	Type	Axes	Named for / clue
Flexion / extension	Physiologic	Two transverse, opposite directions	SBS superior (flexion) or inferior (extension)
Torsion	Physiologic	One A-P, opposite directions	The superior (high) greater wing
Sidebending rotation	Physiologic	Two vertical plus one A-P, same direction	The convex (full) side
Vertical strain	Traumatic	Two transverse, same direction	Sphenoid base superior or inferior
Lateral strain	Traumatic	Two vertical, same direction (shear)	Side the sphenoid base shifts to; parallelogram head
SBS compression	Traumatic	No axis; A-P approximation	Low-amplitude rhythm, chronic headache, depression

Reveal a whole row at once by tapping any cell in it.

Board walkthrough 08

Seven original board-style vignettes

One at a time, shuffled, never repeating until the bank is exhausted. Right-click or long-press a choice to cross it out; double-click or double-tap to highlight it. Answer, then tap through the teaching chain.

VIGNETTE 1 OF 7