Olecranon bursitis and posterior interosseous nerve entrapment

Ultrasound revealed:

• student's elbow (olecranon bursitis)
• acute medial epicondylitis
• chronic lateral epicondylitis
• posterior interosseous nerve entrapment at the arcade of Frohse
• a small posterior joint effusion
• bilateral dislocation of the ulnar nerve over the medial epicondyle on elbow extension implying congenital rather than traumatic absence of the Osborn retinacula.

The PIN entrapment explains the finger drop and radial deviation of the wrist on extension. The dislocation of the ulnar nerve (a form of cubital tunnel syndrome) and being right-handed may explain the right-sided ulnar neuropathy.

The tricepsandtriceps and biceps tendon insertions were unremarkable as were the anterior joints, median nerve and the anterior, posterior and oblique ulnar collateral ligaments.

In light of the posterior joint effusion (and trauma) an X-ray was performed. It was unremarkable aside from the swelling at the olecranon bursa.

The olecranon bursa was drained the following day and sent for pathology. The results showed that the fluid was blood. No fluid, leucocytes or organisms were detected.

Fluid within the olecranon bursa and surrounding hyperaemia. Fluid forced out of the posterior joint on extension of the elbow from a flexed position.

Swollen, hypoechoic common extensor tendon origin, no neovascularisation (not shown). The pathological change extended across the width of the tendon complex. Note the slight concavity to the contour of the tendon, which may imply chronic change, and the pathological radial collateral ligament deep it.

Note the hypoechoic, fusiform thickening and deviation from the normal course of the right PIN compared to the left. There were numerous findings as listedmay be a small band of scar tissue at the right arcade of Frohse causing the entrapment, although this is not conclusive.