Why does a supracondylar fracture lead to a median nerve injury and not the ulnar? According to one of Gray's Anatomy Review's practice questions, it was the median. Doesn't the median nerve pass in the arm between the Biceps and the Brachialis, while the ulnar nerve passes directly on the medial epicondyle? Doesn't this association of the ulnar nerve with the bone directly, put it at risk to injury when the bone is fractured?
1 Answer
Nerve injury in supracondylar fractures of the humerus
Supracondylar fractures of the humerus are the most common type of elbow fracture in children.
Both median and ulnar nerve injury can occur with supracondylar fractures.
Anatomy
A supracondylar fracture is a fracture through the distal humerus, proximal to the condyle.
The image above shows an x-ray demonstrating a supracondylar fracture of the humerus.
The median nerve runs anterior to the humerus, and the ulnar nerve runs posteromedially, as shown below.
Both nerves have a high chance of being injured due to their proximity to the distal humerus.
Note that the median nerve has a significant branch called the anterior interosseous nerve.
I found a 2010 meta-analysis that compared the risk of nerve injury in different types of supracondylar fracture.
They define two main types of fracture, based on the angulation of the distal part (mimicking the terminology of elbow movements).
Flexion: Where the distal part of the fracture is angulated anteriorly.
Extension: Where the distal part of the fracture is angulated posteriorly.
Of nerve injury associated with extension-type fractures, anterior interosseous neurapraxia ranks highest, whereas of flexion-type neuropathy, ulnar nerve injury predominates.
Nerve injury can also be iatrogenic, due to fixation surgery:
We confirm that medial pinning carries the greater overall risk of nerve injury as compared with lateral-only pinning and that the ulnar nerve is at risk of injury in medially pinned patients. We additionally suggest that lateral pinning carries neurapraxic risk with respect to the median nerve.
Summary
Median nerve injury: More common with extension type fractures (especially the anterior interosseous branch) and with lateral pinning surgery.
Ulnar nerve injury: More common with flexion type fractures and with medial pinning surgery.
