The proper ulnar collateral ligament(PCL) runs from the metacarpal head to the volar aspect of proximal phalanx and resists ulnar stress with the thumb MCPJ in flexion. The accessory ulnar collateral ligament(ACL) lies palmar to the proper ligament, and insets inserts onto the volar plate. The volar plate and ACL function as the principle restraints to ulnar stress with the thumb MCPJ in extension.
The function of the ulnar collateral ligaments is shown in Illustration A.(Please note the distal phalanx of the thumb has been removed in Illustration A.) A Stener lesion is described by displacement of the distal end of the completely ruptured UCL such that it comes to lie superficial and proximal to the adductor aponeurosis. This is shown in Illustration B.
Thrikannad and Wolff report a case of distal pull-off of the ulnar collateral ligament (UCL) of the thumb MCPJ with two fracture fragments. They identify the need to look for a second fragment of bone in these injuries, where an apparently undisplaced fracture is noted at the base of the proximal phalanx. They suggest that this second fragment probably indicates the location of the distal end of the UCL and may identify a Stener lesion. A radiographic example from their paper is shown in Illustration C.
Newland, in his review article on Gamekeeper's Thumb, states that criteria for judging what constitutes a complete tear vary from 15 deg to 45 deg difference with respect to the opposite side. He goes on to state, however, that many authors choose an absolute value of >35 degrees of joint laxity compared to the contralateral side when judging a tear to be complete or incomplete. When an acute tear is identified, surgical repair is recommended.
Thirkannad S, Wolff TW. The ''two fleck sign'' for an occult Stener lesion. J Hand Surg Eur Vol. 2008 Apr;33(2):208-11.
PMID:18443066 (Link to Abstract)
Newland CC. Gamekeeper's thumb. Orthop Clin North Am. 1992 Jan;23(1):41-8.
PMID:1729668 (Link to Abstract)