STRESS RADIOGRAPHS OF THE PATELLOFEMORAL JOINT

Patellar instability is usually diagnosed on the basis of the clinical presentation without radiographic confirmation. In the present report , we describe a new radiographic method to demonstrate patellar instab ility. Axial radiographs were made of the patellofemoral joint of nine ty individuals (180 knees) and were then repeated while a medial or la teral force was applied to the patella, The applied force was kept con stant with use of a specially designed instrument. The ninety individu als were divided into four groups on the basis of the clinical finding s: normal, lateral instability, medial instability, and multidirection al instability. Stress radiographs differentiated the four groups and confirmed the clinical diagnosis in all patients who had unilateral sy mptoms. A four-millimeter increase in medial or lateral excursion of t he patella of the symptomatic knee compared with the patellar excursio n of the asymptomatic knee was significant (p < 0.0001). Stress radiog raphs offer a simple method for the measurement of force-displacement relationships in the patellofemoral joint and for the demonstration of patellofemoral instability.