Dislocation of the interphalangeal (IP) joints is one of the most common orthopedic injuries seen in the Emergency Department (ED).1-3 Most of these injuries occur during athletic activities. The proximal interphalangeal (PIP) joint is especially susceptible to injury during ball-handling sports.2-5 Among dislocations, IP joint injuries are second only to shoulder dislocations in incidence.4 While IP joint dislocations are generally easy to reduce, improperly treated injuries can result in chronic pain, swelling, restricted range of motion, deformity, and early degenerative arthritis.1,6,7 Emergency Physicians must be proficient in diagnosing and treating IP joint dislocations.
ANATOMY AND PATHOPHYSIOLOGY
The bicondylar conformation of the PIP joint creates an inherently stable hinge joint limited to flexion and extension within a range from 0° to 120°.1,8,9 Additional stability comes from the complex of ligaments and tendons which form a box around the joint (Figure 107-1). The elements of this complex include the volar plate, lateral and collateral accessory ligaments, and the dorsal extensor tendons.10 The volar plate’s dense, fibrous distal aspect attaches firmly to the middle phalanx while its more membranous proximal portion is continuous with the synovial reflection. This conformation resists dorsal dislocation at the joint.4 The three bands of the extensor tendon mechanism (i.e., the central slip with a lateral band on each side) provide dorsal support that resists joint dislocation (Figures 107-2 and 107-3). Lateral collateral ligaments bridge the PIP on the radial and ulnar sides, stabilizing it against lateral dislocation.3 Most dislocations are the result of hyperextension injuries. This results in the distal bone displacing dorsally. This damages the volar plate. The volar plate can be interposed in the joint space making the dislocation irreducible. This can sometimes be unlocked during the reduction with hyperextension.
A schematic drawing of the box complex surrounding the PIP joint.
Dorsal view of the extensor mechanism.
Lateral view demonstrating the anatomy of supporting structures. The volar plate and collateral ligaments form a box around three sides of the joint, while the extensor mechanism (consisting of central and lateral slips) lies along the dorsal aspect of the joint.
The less commonly dislocated IP joints (i.e., finger distal interphalangeal [DIP] and thumb IP joints) are similar in anatomy to one another. They are more broad-based than the PIP and range from 0° in extension to 90° in flexion with no significant lateral or rotary movement.3 The distal phalanx in both joints is firmly attached to the skin, accounting for the high percentage of open dislocations involving these joints.