Background: Radiographic assessment of the patella after total knee
arthroplasty is typically performed with use of static, unloaded views that
may not reproduce the in vivo patellofemoral kinematics. The purpose of the
present study was to evaluate and validate the reliability and reproducibility
of a weight-bearing radiographic assessment of the patellofemoral joint in
patients who have undergone total knee arthroplasty.
Methods: Radiographs were made for 100 knees in sixty-nine patients
who had undergone total knee arthroplasty. Radiographic assessment of the
patellofemoral joint was performed with use of both the standard Merchant
axial view and a modification of that view. The Merchant axial view was
modified by positioning the standing patient in the semi-squatted position
with the knees in 45° of flexion. The relationship between the x-ray
source, the angle of incidence on the joint, and the cassette position was
kept unchanged from the original view. The standing position and consequent
muscle involvement were the only differences.
Results: Compared with the standard Merchant axial view, the
weight-bearing axial view showed a number of patellofemoral tracking changes.
Specifically, lateral tilt and subluxation of the patella were significantly
reduced; the rate of exposed, uncovered patellar bone contact with the femoral
trochlea was significantly increased; and radiographic evidence of maltracking
was more closely correlated with clinical symptoms.
Conclusions: An axial weight-bearing radiographic view with the
patient in the semi-squatting position was developed to reproduce
patellofemoral joint loading. This view demonstrates that the position of the
patella, as seen on the standard unloaded Merchant view, changes during
squatting. Utilization of this axial weight-bearing view to evaluate total
knee arthroplasty may provide additional information over standard
Level of Evidence: Prognostic Level I. See Instructions
to Authors for a complete description of levels of evidence.