Role of the Juncturae Tendinum in Preventing Radial Subluxation of the Extensor Communis Tendons after Ulnar Sagittal Band Rupture: A Cadaveric Study

Background. Radial subluxation of the extensor communis tendons at the metacarpophalangeal (MCP) joints is a rarely reportedvinjury. These injuries have proved difficult to reproduce in cadaveric studies and have a low biomechanical likelihood of occurrence due to the ulnar direction of pull of the extensor communis tendons. It has been suggested that the juncturae tendinum may have a stabilising role, preventing radial subluxation after ulnar sagittal band rupture; however this has not been established. Methods. 40 cadaveric digits were dissected to reveal the extensor mechanism around the MCP joints. The ulnar sagittal bands were released and then the juncturae tendinum divided, in stages, before observing for radial subluxation or dislocation during finger flexion. Results. Radial subluxation of the extensor tendon was observed in only one digit after complete ulnar sagisttal band release. When all the fingers were flexed, after the juncturae tendinorum were divided, four additional tendons subluxed radially and a fifth tendon dislocated in this direction. When the digits were then flexed individually, there were eight unstable tendons in total. Conclusions. The juncturae tendinum appear to have a role in stabilising the extensor communis tendons at the MCP joints and preventing radial subluxation after ulnar sagittal band rupture. 1. Background Ulnar subluxation of the extensor communis tendons at the metacarpophalangeal (MCP) joints occurs chronically in the rheumatoid hand, for which surgical repair is a well-established technique [1]. Acute traumatic disruption also occurs and has become commonly termed as “boxer’s knuckle”. Descriptions of the mechanism of tendon subluxation in boxer’s knuckle have concentrated on damage to the radial sagittal bands, with ulnar subluxation at the MCP joints being the resultant deformity, and pain on forming a closed fist often the presenting complaint of patients. Traumatic radial subluxation occurs infrequently, and only a few cases have been reported. It has occurred in boxers after repetitive punching trauma in five cases [2]. It has also been described as having occurred after a single well-executed karate punch, once after “flicking a spider,” and one congenital case has been reported [3–5]. Subluxation in the ulnar direction is found most commonly in Boxer’s knuckle, and two cadaveric studies have confirmed the biomechanical likelihood of this occurrence [6, 7]. The ulnar direction of pull of the extensor communis tendon, from its origin at the lateral epicondyle of the humerus, is at its greatest at the