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Objectives The objective of this paper was to biomechanically investigate rotational stability of the thumb after ulnar collateral ligament (UCL) and accessory collateral ligament (ACL) disruption and repair at the metacarpal joint of the thumb. Methods Twelve fresh frozen adult cadaveric thumbs were used. The torsion test was performed under constant rotation of 1/s through 30 arc of metacarpal phalangeal (MCP) joint. The torsional resistance was determined for four categories: first no intervention of the UCL structures (control), next with the proper UCL cut at the distal insertion, then with the additional ACL ligament cut, and lastly with the repair of only the proper UCL. The decrease on the amount of torsional rigidity for each of the last three categories was determined and compared. Each thumb was used as its own control. Significance of the differences in each test categories was statistically determined.