Vasti Medialis Nerve Blocks Alter in Vivo Knee Joint Kinematics in Females: Implications for Patellofemoral Pain Syndrome

INTRODUCTION Patellofemoral (PF) pain syndrome (PFPS) is one of the most common problems of the knee, characterized by idiopathic anterior knee pain aggravated by deep knee flexion, prolonged sitting, and repetitive flexion/extension. The most widely accepted theory in regards to the source of PF pain is that a force imbalance around the knee leads to PF static malalignment and dynamic maltracking. In turn, this causes elevated joint contact stresses, which ultimately results in PF pain. The source of this force imbalance is still open to debate with some postulating the cause to be delayed timing or loss of strength in the vasti medialis (VM) or an imbalance in the passive structures [1]. Yet, numerous studies refute these claims as well [2]. Unfortunately, muscle force cannot be measured directly, without highly invasive techniques. Thus, the force imbalance between the VM and vasti lateralis (VL), reported previously, is based on electromyographic recordings (EMG), from which muscle force can be approximated based on a series of assumptions. An earlier study clearly demonstrated that, in patients with PFPS, a muscle may not contribute to useful work even if it is active [3]. In such an instance the assumptions required to estimate muscle force from EMG are not valid. Therefore, the purpose of this study was to determine how a loss of force in the VM alters the dynamic control of 3D in vivo PF kinematics during a volitional extension task.