What is the pathophysiology of ulnar neuropathy?

Updated: Jun 08, 2018
  • Author: Charles F Guardia, III, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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Answer

As the elbow moves from extension to flexion, the distance between the medial epicondyle and the olecranon increases by 5 mm for every 45° of elbow flexion. Elbow flexion places stress on the medial collateral ligament and the overlying retinaculum. The shape of the cubital tunnel in cross-section changes from round to oval, with a 2.5-mm loss of height, because the cubital tunnel rises during elbow flexion and the epicondylar groove is not as deep on the inferior aspect of the medial epicondyle as it is posteriorly.

The cubital tunnel’s loss in height with flexion leads to a 55% volume decrease in the canal, which causes the mean ulnar intraneural pressure to increase from 7 mm Hg to 14 mm Hg. [44, 45] A combination of shoulder abduction, elbow flexion, and wrist extension results in the greatest increase in cubital tunnel pressure, with ulnar intraneural pressure increasing to about 6 times normal. [46, 47, 48, 49, 37]

Traction and excursion of the ulnar nerve also occur during elbow flexion, as the ulnar nerve passes behind the axis of rotation of the elbow. With full range of motion of the elbow, the ulnar nerve undergoes 9-10 mm of longitudinal excursion proximal to the medial epicondyle and 3-6 mm of excursion distal to the epicondyle. [50] In addition, the ulnar nerve elongates by 5-8 mm with elbow flexion.

In addition to prior cadaver and surgical studies of ulnar nerve motion, recently developed sonographic methods facilitate monitoring the motion in the intact arm. [51] Interestingly, in patients with ulnar neuropathies, the nerve is somewhat more motile than in individuals with normal ulnar nerves.


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