chapter 4:imaging the patellofemoral joint
DETERMINING SUBLUXATION AND TILT USING TOMOGRAPHIC IMAGES
Measurements of the patellar tilt angles or congruence angles are routinely taken from midpatellar transverse tomographic images. The purpose is to assess patellar alignment just after the patella has engaged the femoral sulcus (15 degrees), in the midrange of knee flexion (30 and 45 degrees), and at flexion consistent with maximal or near‑maximal patellar contact stress (60 degrees).
First, suitable images are obtained that are consistent with excellent technique, emphasizing reproduction of normal standing alignment, midpatellar transverse cuts, and straight through the femur so that the femoral sulcus and posterior condyles are well visualized (Fig. 4.20).
Next, lines are drawn with a fine grease pencil along the lateral patellar facet and posterior femoral condyles (Fig. 4.21).
This angle should always be greater than 7 degrees even in full extension, and has been 12 to 14 degrees or more on the 15‑ to 20‑degree knee flexion tomographic slice in the knees of asymptomatic controls.
After measuring the tilt angles, the congruence angles are determined. In our modification of Merchant's technique (18), the femoral sulcus angles are drawn (lines C and D) (Fig. 4.23) on the same images for which patellar tilt was just determined (if necessary, the previous lines are removed). The sulcus angle is measured on each image and bisected (line E) (Fig. 4.23).
The congruence angle (Fig. 4.25, angle a) is created by lines E and F and is our preferred index of subluxation (as recommended earlier by Merchant et al ).
Using CT, we (31) noted that the congruence angle consistently became 0 or negative by 10 degrees of knee flexion in the knees of 10 age‑matched asymptomatic volunteers. We consider a patella to be congruent when it centers in the trochlea (as determined by CT) by 15 to 20 degrees of knee flexion.
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