In idiopathic cubital tunnel syndrome, ulnar nerve excursion and instability can be reduced by repairing Osborne's ligament after simple decompression.

Osborne's modified decompression involves repairing Osborne's ligament beneath the ulnar nerve after simple decompression for idiopathic cubital tunnel syndrome. In this retrospective interrupted time series, 31 patients underwent modified simple decompression and 20 patients underwent conventional simple decompression. In the modified simple decompression group, the ulnar nerve length was measured at operation in full elbow flexion and extension before and after repair of Osborne's ligament. Ulnar nerve instability during elbow motion was measured using ultrasonography before operation and at 12 months after operation. In patients treated by modified simple decompression, the ulnar nerve length in full elbow flexion reduced significantly after repair of Osborne's ligament. At 12 months after surgery, the grade of ulnar nerve instability was lower in the modified simple decompression group than in the conventional simple decompression group. The clinical outcomes did not differ significantly between the groups at 24 months after operation. Level of evidence: III.