Load Bearing Characteristics of the Patello-Femoral Joint

Abstract

This study of normal patello-femoral biomechanics defines some functional specifications which may be useful in future total knee prosthesis design. Serial lateral X-rays of 15 fresh knees and their patellar mechanisms at several flexion angles provided definition of the direction of the resolved patello-femoral forces. Assuming that the patella acts as a frictionless pulley, the magnitude of the patello-femoral forces during several activities was calculated using data from Morrison (1970) and Smidt (1973). It ranged between 421 and 3420 newtons for the various activities and for isometric exercise. A methylene blue contact print technique was used to measure the bearing areas. These data indicate that between 13 and 38 per cent of the patellar surface bears joint loadings. Patello-femoral contact stresses were calculated to range from 1.28 to 12.6 N/mm2. A 696 new-ton man climbing stairs would, for example, generate a patello-femoral force of 1754 newtons and would experience patello-femoral contact stresses between 3.73 and 6.87 N/mm2. Stress values were equal to or in excess of anticipated tibial-femoral stresses. The high patello-femoral load values, the small bearing surfaces, and the consequent significant stress magnitudes indicate the need for caution in development of a patello-femoral joint prosthetic replacement.