Subluxation-related ulnar neuropathy (SUN) syndrome related to distal radioulnar joint instability.

Ulnar neuropathy coexistent with distal radioulnar joint (DRUJ) instability has previously been observed in our practice. The aim of this study was to define this phenomenon and investigate the hypothesis that the cause of this intermittent, positional ulnar neuropathy is related to kinking of the ulnar nerve about the DRUJ. Ulna neuropathy was present in 10/51 (19.6%) of a historical cohort of patients who presented with DRUJ instability. Nine subsequent patients with DRUJ instability and coexistent ulnar neuropathy underwent 3-T magnetic resonance imaging to better understand the mechanism of the observed syndrome. Both 3D qualitative and quantitative analyses were used to assess the presence of nerve 'kinking', displacing the nerve from its normal course and causing nerve compression/distraction in the distal forearm and Guyon's canal. Results of the quantitative analysis were statistically significant (p < 0.05). The clinical features of the condition have been delineated and termed subluxation-related ulnar neuropathy or SUN syndrome. The imaging study was a level II diagnostic study.