Kinematics of the Fat Pad, its Central Body, and the Infrapatellar Plica, Newly Described as a Non‐Isometric Ligament of the Knee

Abstract

We have described the infrapatellar plica (IPP) of the knee as a non-isometric ligament, attaching the fat pad (FP) to the femur. We had noted during knee arthroscopy that the IPP showed mechanical behavior, stretching and relaxing with motion, and holding the FP against the distal femur. We sought to visualize the kinematics of this behavior using fluoroscopy in cadavers and, in an Institutional Review Board approved study of human volunteers undergoing arthroscopy. With radiographic contrast injected in the IPP, we demonstrated in cadavers and in vivo, that the FP, highly innervated central body (CB), and IPP rotate around the femoral attachment (FA) of the IPP. The fluoroscopic films show a sequence starting in flexion, of stretch and distortion of the IPP and CB, then relaxation throughout mid arc and, remarkable stretch and distortion approaching full extension. This confirmed the non-isometric nature of the IPP. The overlying FP as seen in the second figure was tethered centrally by the IPP, and distorted throughout. Release of the IPP at the FA, which has been shown to reduce or eliminate anterior knee pain (AKP), eliminated the distortion of the FP. This kinematic, demonstrating mechanical perturbation of innervated central structures in the knee, introduces a new central vector, altering traditional patellofemoral biomechanics and suggests an altered concept in the causation of knee pain.