The effect of knee flexion and quadriceps contraction on the axial view of the patella.

Axial views of the patella are standard in the investigation of anterior knee symptoms, and many investigators use congruence angles and patellar tilt in determining the diagnosis and appropriate treatment. A variety of different methods are used to obtain axial radiographs of the patella, but they are static representations of a dynamic phenomenon. The effect of quadriceps contraction on these indices has not been well defined. The purpose of this study was to compare the 30 degree axial view of the patella with the 45 degree axial view of the patella and to study the effect of quadriceps contraction on the 30 degree axial radiograph of the patella. Forty knees with normal patellofemoral joints and 20 knees with patellar subluxation were radiographed using 30 degree axial views of the patella with the quadriceps relaxed and contracted and with 45 degree axial views with the quadriceps relaxed. There was no significant difference in the sulcus or congruence angles between the 30 and 45 degree axial view in either the control or the patellar subluxation group. Contraction of the quadriceps did not cause a significant or consistent alteration in the congruence angles in either group. With quadriceps contraction, 55% of patients will centralize the patella, whereas 40% will actually increase lateral tracking or subluxation. In conclusion, the 30 degree axial view of the patella is equivalent to the 45 degree axial view in the assessment of patellar subluxation. Quadriceps contraction does not alter the congruence angles obtained in these axial views in a significant or consistent manner.(ABSTRACT TRUNCATED AT 250 WORDS)