chapter 4:imaging the patellofemoral joint
MAGNETIC RESONANCE IMAGING (MRI)
MRI has been used extensively to evaluate the patellofemoral joint (41‑44). Because it gives a direct picture of hydrated structures, it is the most effective noninvasive modality currently available to view patellar articular cartilage (Fig. 4.30). Ghelman and Hodge (45) reviewed the role of MRI for the patellofemoral joint. Nakanishi et al (46) noted that MRI is very useful in the evaluation of cartilage injury. These researchers noted also, however, that early changes observed by arthroscopy were "underestimated" from magnetic resonance images. Therefore, MRI was more useful for evaluating moderate to advanced patellar cartilage damage. Conway et al (47) made similar observations. McCauley et al (48) noted that chondromalacia patellae patients have focal defects in patellar cartilage on T2‑weighted magnetic resonance images. Many previous studies on detection of chondromalacia have used T1‑weighted images. Brown and Quinn (49) further emphasized the value of T2‑weighted images for detecting chondromalacia, and these authors reiterated the lack of MRI sensitivity in detecting soft, but unbroken, articular cartilage. Nonetheless, as imaging techniques improve and as computer software packages improve, sensitivity for detecting articular cartilage lesions may also continue to improve. At the current time, however, arthroscopic (or open) evaluation of patellar articular cartilage remains superior to MRI.
MRI of the patella for intra‑articular fractures and neoplasms may be very helpful, however. Figure 4.31 shows a hemangioma of the quadriceps muscle immediately above the patella in a patient with anterior knee pain. Whenever there is a need specifically for soft tissue imaging, MRI is the method of choice.
MRI with progressive knee flexion is possible, and kinematic MRI (50), even in this range, is interesting. From a practical point of view, however, static tomographic images are more useful for accurately measuring tilt and subluxation. Some clinicians and radiologists believe that a more subjective interpretation of images is equally valid, but one cannot deny the benefits of objective criteria of abnormal alignment using carefully established normal limits.
CT is less expensive, although this may change. CT currently offers greater ability to obtain tomographic slices at varying knee flexion angles. I continue to recommend CT for evaluation of patellar alignment when history, physical examination, and well-done plain radiographs yield insufficient information. I have found that more sophisticated imaging techniques are less necessary with improved clinical examination skills and precise radiographs. Nonetheless, in selected patients, MRI can be extremely helpful. Shellock et al (51) introduced a "positioning device" that permits loading of the knee and normalized stress to the patellofemoral joint during imaging. This loadbearing platform, together with kinematic MRI, may provide some interesting insight into functional behavior of the patellofemoral joint. Using this positioning system, he later demonstrated the effect of bracing on patellar position (Shellock,F, Mullin, M, Stone, K, Coleman,M, and Crues, J, Kinematic MRI of the effect of bracing on patellar position, J. Athletic Training 35(1): 44-49, 2000)
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