0614: Pulleys, Plates and Other Paraphernalia - Anatomy and Unique Injuries to Fingers

Abstract

The digits of a hand have 17 articulations and unique tendon anatomy. Assessment of injuries to these structures requires an understanding of the anatomy. Finger Flexor Mechanism At the level of the wrist, the FDS and FDP tendons are contained within a common synovial sheath. This ends at the level of the mid metacarpals except for the little finger where it is usually continuous with its digital sheath. An effusion around the little finger flexor tendons within the digit may reflect more proximal pathology. The digital flexor tendons are held in osseo-aponeurotic canals by fibrous sheaths from the metacarpal heads to the base of the terminal phalanx. Thickened zones of the sheath are termed pulleys, which may be annular or cruciate. The most clinically important of these are the A1 and A2 pulley. Thickening of the A1 pulley is one of the many aetiologies of trigger finger. The A2 pulley has recently been described in cadaver study as hyperechoic, but in-vivo is hypoechoic. Disruption of the A2 pulley results in “bowstringing” of the flexor tendons. A4 pulley ruptures are also relatively common. Finger Extensor Mechanism There is a fibrous expansion on the dorsum of each digit, which extends distally from the metacarpal head and blends with the extensor digitorum tendon. The tendon is stabilized on the metacarpal head by components of the extensor hood, including sagittal bands which extend anteriorly to the volar plate. Dorsal hood injuries usually involve the radial sagittal band, resulting in ulnar subluxation of the common extensor and ulnar deviation of the proximal phalanx. As the extensor digitorum tendon approaches the PIP joint, it divides into an axial part, which inserts into the base of the middle phalanx and two collateral slips. Disruption of the axial portion results in a Boutonniere deformity. The collateral slips of the extensor tendon then unite to be attached to the dorsal aspect of the base of the distal phalanx. Disruption of this attachment results in mallet finger. Articulations The volar plate is a fibrocartiliginous plate that forms the floor of the MCP, PIP and DIP joints. At the PIP joint it provides significant stability and disruption, almost universally at its distal attachment, predisposes to recurrent dorsal dislocation. The collateral ligaments of the articulations are thick cords which attach to a posterior tubercle at the head of the proximal structure and pass obliquely forwards to the base of the distal structure - an important concept in identifying these structures. Sonographic findings in collateral ligament injuries will be discussed with emphasis on the thumb UCL and Stener lesion.