chapter 4:imaging the patellofemoral joint
COMPUTERIZED TOMOGRAPHY (CT)
To define patellar tracking accurately, CT offers sequential images at any degree of knee flexion using the midtransverse patella as a stable plane of reference. Unlike tangential radiographs, CT images through specific segments of the femoral trochlea, omitting image overlap and distortion. Consequently, one can define accurately the relationship between a specific reference plane (midtransverse patella) and its exact counterpart on the femur, the femoral trochlea (Fig. 4.14). Particularly helpful also is the possibility of tomographic sectioning into the posterior femoral condyles where the anatomy is quite symmetrical (30), providing a reliable reference plane (Fig. 4.15) for determining patellar tilt as compared with a line drawn across the variable anterior femoral trochlear (Fig. 4.16) margins. With sequential tomographic images from 0 to 60 degrees of knee flexion, one can appreciate early engagement of the patella into the trochlea by 10 to 20 degrees of knee flexion in normal patellofemoral function (31‑33). Under normal circumstances, the patella should not exhibit tilt or subluxation through the subsequent range of knee motion. Using CT criteria, it is possible to note a variety of different tracking patterns that are not as well defined using standard radiographs. Also, CT with contrast has proved helpful in the appraisal of patellar articular cartilage (34). With its more sensitive reference planes for determining tilt, CT has revealed three basic tracking abnormalities (see Fig. 4.17). Stanciu et al (35) confirmed the improved sensitivity of CT of the patellofemoral joint compared with standard radiographs.
Within each of these categories, there is additional variability as to degree and correctability. Studies have shown that normal patellae engage the trochlea and are centered, without tilt, by 10 to 20 degrees of knee flexion (31). The patella normally sits lateral to the trochlea with the knee extended. One study has suggested evaluation of subluxation using CT with the knee extended (36), but the patella is mostly above the trochlea, at this point making accurate evaluation of patellar alignment less reliable until 10 to 20 degrees of knee flexion. Using CT, a patella that is still significantly tilted or subluxated by 20 degrees of knee flexion is malaligned. Using the sensitive CT criteria, some subtle abnormalities may be detected even at 10 degrees of knee flexion, but in our experience these changes should be sustained to 15 to 20 degrees of knee flexion to control for variability in technique. Upon further flexion of the knee, subluxation or tilt may improve or worsen. By examining a technically well‑done CT study of patellofemoral tracking, one can gain clear insight into the tracking pattern of each individual patient, leaving little to the imagination. It is extremely important, however, that these studies be performed in a standardized, accurate, and reproducible manner, centered on the midtransverse patella. Classification of tilt and/or subluxation becomes particularly important in developing a prognosis and appropriate treatment plan. Also, CT is extremely helpful in the evaluation of patellofemoral realignment or lateral release (30). Figures 4.18 and 4.19 show the patterns of subluxation and tilt exhibited by patients. Evaluation of patellofemoral congruence with the quadriceps both contracted and relaxed may improve sensitivity selectively (37). At 30 to 40 degrees of knee flexion, quadriceps contraction causes slight medialization of the patella and a relative decrease of the congruence angle. Dynamic CT of the patellofemoral joint is possible also (38).
Performing Patellofemoral CT
The patient is examined in a comfortable standing position to evaluate distances between the medial malleoli and femoral condyles. Rotation of the feet is also noted. The patient is then placed in the scanner gantry, reproducing this normal standing alignment as closely as possible using bolsters. Beginning at neutral knee flexion/extension (not full extension, because some patients may hyperextend), sequential CT slices are taken, centering the beam on the midtransverse patella (which is labeled with a marking pen) of both knees simultaneously. This is best accomplished with the patient in a lateral decubitus position. It is important to use the same reference plane for sequential tomographic slices, and it is extremely important to define the midtransverse patella because tomographic cuts at different levels will give less accurate indications of tilt. Tomographic cuts may be taken as desired, but it is currently our preference to obtain tomographic slices at 0, 15, 30, 45, and 60 degrees of knee flexion. By so doing, one can accurately assess the patella as it enters the trochlea and progresses to a point of maximal contact stress. Progression and/or correction of tilt or subluxation can then be determined. Once this technique is mastered, the entire study can be obtained in approximately 20 minutes at a cost that is about the same as a full set of four knee radiographs.
Our approach to analyzing a CT study will be described in some detail, emphasizing the differentiation between subluxation and tilt.
Inside Chapter 4: