Biomechanics of the Volar Plate of the Proximal Interphalangeal Joint: A Dynamic Ultrasonographic Study

Abstract

To study the biomechanics of the volar plate of the proximal interphalangeal (PIP) joint using ultrasonography. The subjects were 20 normal and 5 pathological digits. We included as normal subjects 20 digits (10 index and 10 middle fingers) of 10 hands of 10 volunteers who had no history of trauma or any other hand-related pathological conditions. To clarify the biomechanical effect of the A3 pulley for the volar plate motion, we included a total of 5 digits (1 index, 2 middle, and 2 ring) of 4 hands of 4 consecutive patients who had flexor tendon repair surgeries in zone II with the A3 pulley left open as pathological subjects. Transducers were applied volarly to the PIP joint and the ultrasonographic plane was kept in the central axis of the digits to display the volar plate and the flexor tendons in the same image. We recorded cine videos during active joint motion. We also obtained sequential static images for morphological analysis. The motion of the volar plate of the normal PIP joint was as follows: when joint flexion started, the volar plate slid proximally to lie parallel to the condylar slope of the proximal phalanx. In 30° of flexion, the flexor tendons tightened and the distal portion of the volar plate moved volarward. In 45° or more of flexion, the distal portion protruded over the lip of the middle phalanx. The lip rolled dorsalward in the recess in return. In the flexor tendon repair cases, the distal portion of the volar plate did not move volarward at all, even when the joint flexed 45° or more. The main body of the volar plate showed a marked wavelike or sigmoid deformity. To characterize the biomechanics of the volar plate of the PIP joint, we propose the following 3 sequential phases: sliding, elevating, and rolling in the recess. The A3 pulley may serve as an elevator of the volar plate, triggering the next phase of rolling in the recess. The recess can provide some articulation which the lip of the middle phalanx can roll in.