Background: Diagnostic stress testing of ulnar collateral ligament (UCL) injuries of the thumb metacarpophalangeal (MCP) joint is pivotal to determining treatment. Comparison to the uninjured extremity and fluoroscopy-assisted examination are readily available modalities in the assessment of these patients, with 5-10° differences impacting treatment. Comparative examination, however, assumes that both extremities are normally equal, which has never been verified experimentally.