Kinematic and dynamic axial computerized tomography of the normal patellofemoral joint

Abstract

Fourteen normal volunteers with no history suggesting previous or current knee pathology underwent axial computed tomographic examination of the patellofemoral joint. There were 11 men and 3 women, whose ages ranged from 10 to 46 years (average 25 years). Axial images were obtained at 0°, 10°, 20°, 30°, 40°, and 60° flexion both with and without contraction of the thigh muscles. Thus, 12 images were obtained for each individual. The CT scanner was focused at the midpatellar level prior to each image. Three measurements were made on 24 knees for each individual: congruence angle (CA), patellar tilt angle (PTA), and sulcus angle (SA). PTA increased slightly from 0° to 20°, and decreased slightly with more flexion (not significant, NS). The lower limit of PTA was usually 9°–10°: it was not lower than 7° in any knee position. Muscle contraction increased PTA slightly at each degree of flexion (NS). Mean CA was +18.3° (SD 20.8°) at 0°, which means that normal individuals may have CAs as high as +39° at full extension. There was a gradual decrease in CAs with knee flexion. The mean values became negative between 20° and 60° flexion. Contraction of the thigh muscles caused lateralisation of the patella except at 30° and 40° flexion. This lateral pull was statistically significant at full extension (P<0.01) and at 10° flexion (P<0.05). The SA decreased gradually as the flexion of the knee increased. Angles at 0°, 10°, and 20° flexion were significantly higher than those at 40° and 60° flexion (P<0.05). This study shows that CA, PTA and SA change depending on the degree of flexion of the knee, and that these angles show wide variations in the normal population. One should not rely on axial images taken at full extension, as this may erroneously lead to a diagnosis of subluxation in a normally tracking patella. The values obtained in this study may provide a basis for determining the type of patellar instability at different knee positions, and thus give a better profile or patellar tracking. This is a new concept. Besides, comparison of dynamic values obtained in this study with the ones in abnormal patellofemoral joints may also reveal useful information.