Olecranon bursitis and posterior interosseous nerve entrapment

Case contributed by Brendan Cullinane
Diagnosis almost certain


Referral for aspiration of the swollen right olecranon bursa following trauma. The patient was a right-handed male plumber. Clinical assessment and questioning of the patient revealed a swollen but non-tender olecranon bursa, finger drop and radial deviation of the wrist on extension, pain on compression over the right arcade of Frohse but not the left and ulnar neuropathy. No previous imaging, so proceeded with a complete workup of his right elbow.

Patient Data

Age: 30
Gender: Male

US right elbow


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

Swollen, hypoechoic common extensor tendon origin, no neovascularization (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 may be a small band of scar tissue at the right arcade of Frohse causing the entrapment, although this is not conclusive.

Hypoechoic change within the common flexor tendon origin. Minor amount of neovascularity (not shown) and point tenderness probably implying acute change.

The hypoechoic change spans the posterior half of the CFTO, thus involving all of the FCU and part of the FDS fibers.

The ulnar nerve is shown dislocating out of the cubital tunnel over the medial epicondyle on elbow extension bilaterally. There is minor oligofascicular change to the normal polyfascicular pattern of the nerve. Note the medial subluxation/dislocation of the triceps muscle over the olecranon and into the cubital tunnel helping force the ulnar nerve out.

Case Discussion

Ultrasound revealed:

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 triceps 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, leukocytes or organisms were detected.

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