Assessment of rotational instability with disruption of the accessory collateral ligament of the thumb MCP joint: a biomechanical study.

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.

RESULTS

After the proper UCL was cut, the torsional rigidity of the MCP joint was reduced 35.18 +/- 17.56% (p < 0.001). When, additionally, the ACL was cut, the torsional rigidity of the MCP joint was further reduced to 49.34 +/- 16.82% (P < 0.001). After repair of only the proper UCL, the torsional rigidity of the MCP joint improved, but still showed a considerable reduction from controls. The amount of reduction was not consistent among specimens and was 13.52 +/- 16.40%.

CONCLUSIONS

The ACL ligament is a contributor of rotary stability as well as a provider of lateral stability. Leaving the ruptured ACL unrepaired causes some residual rotating instability and that may lead to future rotational instability of the MCP joint.