Muscle Strength Loss and Its Effect on Knee Cap Motion in Volunteers With Anterior Knee Pain

Background: \- Researchers are interested in how the muscles affect the movement of the knee cap. These muscles may be related to different kinds of knee pain that are not caused by an injury or a disease. Imaging studies such as magnetic resonance imaging (MRI) can be used to look at these muscles. To study these muscles and how they move, researchers will use MRI to look at healthy individuals and individuals with knee cap pain. Objectives: \- To study how changes to the muscles around the knee can influence knee pain. Eligibility: * Individuals between 18 to 55 years of age who have knee cap pain that cannot be explained by a specific injury or disease. * Healthy volunteers between 18 and 55 years of age. Design: * Participants will be screened with a physical exam and medical history. * This study requires two visits. Each visit will use standard MRI sequences to take images of the knee in motion and at rest. * On the first visit, the MRI scan will look at the knee in its natural state. Participants will move the knee up and down for 1 to 3 minutes at a time during the scan. * On the second visit, a local anesthetic agent will be injected into the muscle of the thigh. The anesthetic will block this muscle from generating force for 2 or 3 hours. Participants will move the knee up and down for 1 to 3 minutes at a time during the MRI scan.

Chronic idiopathic patellofemoral pain, a potential precursor to osteoarthritis, is one of the most common problems of the knee. It is characterized by anterior knee pain that is aggravated by deep knee flexion, prolonged sitting, and repetitive flexion/extension. The most widely accepted theory in regards to the source of this pain is that a force imbalance around the knee leads to static patellofemoral malalignment and dynamic patellofemoral maltracking. In turn, this malalignment and maltracking leads to elevated joint contact stresses, which ultimately leads to patellofemoral pain. Current understanding of patellofemoral maltracking is typically focused on static 2D alignment (lateral tilt and displacement). Yet, patellofemoral pain is most often exacerbated during dynamic events and the patella has complete six degrees of freedom in its movement. More recent studies have been able to quantify patellofemoral kinematics during volitional dynamic tasks and demonstrate that the maltracking problem is more complex than originally postulated. Specifically, in patients with patellofemoral pain, altered kinematics are not limited to excessive patellar lateral translation and tilt, but include excessive patellar superior translation along with excessive flexion and valgus rotations. Flexion and valgus are sagittal and coronal plane rotations that cause the superior pole of the patella to shift anteriorly and medially, respectively. Recent work has shown that altered force balance around the knee can indeed lead to maltracking. However, the question remains whether correcting an existing force imbalance around the knee can normalize patellar kinematics and/or reduce pain. The purpose of this study is to determine how temporary iatrogenic loss of force in the vastus lateralis muscle alters the aberrant patellar kinematics in subjects with chronic idiopathic patellofemoral pain.