Arm septa !
The arm septa are two sheets inside the upper arm that separate the elbow benders in front from the triceps behind; the radial and ulnar nerves cross them.
Official name: Medial and lateral intermuscular septa of the arm
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What it is
Two fascial walls that split the arm into front and back compartments.
What it does
Separate the elbow benders from the triceps; the radial and ulnar nerves pierce them.
Key facts
- Attaches
- Medial and lateral supracondylar ridges of the humerus to the brachial fascia
- Function
- Separate the flexor and extensor compartments; the radial nerve pierces the lateral septum, the ulnar nerve the medial (some texts: it passes behind its upper edge)
Relations
- In front
- Anterior compartment: biceps, brachialis, coracobrachialis and the musculocutaneous nerve
- Behind
- Triceps; the radial nerve and its collateral artery behind the lateral septum above, the ulnar nerve behind the medial septum below
- Medial
- Brachial artery, basilic vein and median nerve run along the front of the medial septum
- Lateral
- Brachioradialis and ECRL arise in front of the lateral septum; the radial nerve, once through, runs between brachialis and brachioradialis
How to find it
They cannot be felt. The lateral septum lies roughly along a line from the deltoid insertion to the lateral epicondyle, the medial along the medial bicipital groove to the medial epicondyle; in the lateral approach to the distal humerus the radial nerve is found where it crosses the lateral septum.
On imaging
On axial MRI or CT the medial and lateral septa run from the humeral borders to the brachial fascia, splitting the arm into anterior and posterior compartments. The radial nerve pierces the lateral septum in the lower third of the arm, where shaft fractures can trap it, and ultrasound can follow it there. The ulnar nerve passes behind the medial septum toward the elbow.
Clinical notes
- Holstein–Lewis fracture: a spiral fracture of the distal third of the humeral shaft can trap the radial nerve at the septum, causing wrist drop
- The lateral arm flap is based on the posterior radial collateral artery running along the lateral septum
- Anterior transposition of the ulnar nerve: the distal medial septum is removed so the moved nerve does not kink against its sharp edge
Test yourself
Think of the answer, then tap the question to see it.
Which nerve pierces the lateral intermuscular septum of the arm, and why does it matter?
The radial nerve, from back to front in the lower arm; tethered there, it is at risk in distal humeral shaft fractures (Holstein–Lewis).
Which nerve passes through the medial intermuscular septum of the arm, and with which artery?
The ulnar nerve, from front to back about mid-arm, with the superior ulnar collateral artery.
Why is the lower medial intermuscular septum excised in anterior transposition of the ulnar nerve?
So the moved nerve does not kink over it.
Educational information, not medical advice. For any symptom, see a health professional.
Its names
- Official Latin (TA2)
- septum intermusculare mediale brachii TA2 2542
- Official Spanish (DeCS)
- Tabiques intermusculares medial y lateral del brazo
- English (TA2)
- Medial and lateral intermuscular septa of the arm
Related
Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Arm Muscles.
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Representative model, for educational purposes only.
