The Infrapatellar Plica ‐ a New Intra‐Articular Ligament of the Knee

Abstract

The infrapatellar plica (IPP) is considered to be an embryological remnant, a synovial fold of no mechanical importance. Its tissue characterization has not been described in the English literature. We had noted during knee arthroscopy that the IPP appeared ligament-like, and showed mechanical behavior, stretching and relaxing with motion, and holding the FP against the distal femur. Seeking to determine its gross and microscopic structure and link it to the observed behavior, we dissected 12 cadaver knees, noting for each the gross anatomy, histology of the IPP and attached fat pad (FP). According to the classification by Kim, the IPP types were: 8 separate, 1 split, 2 fenestrated, and 1 vertical septum. The IPP is a non-isometric ligament, with typical ligamentous structure at its femoral attachment (FA), a typical enthesis, and its rope-like central zone (CZ), primarily dense fibrous bundles of collagenous tissue. The structure of the attachment of the IPP to the FP at the central body (CB) is unique and consists of a splaying out of finger-like collagenous septa that merge with those of the FP. There is abundant elastase staining consistent with the distortion associated with knee motion. Lodged in the axillae of these septa are fat lobules similar to those of the FP, but containing a rich plexus of neurovascular bundles, which show wavy redundancy also correlating with the observed mechanical behavior. The IPP/CB/FP complex can be linked to chronic anterior knee pain in that arthroscopic release of the FA relieves the pain in most.