Imaging of the knee has changed dramatically during the past 10 years as a result of enhanced imaging capabilities but also owing to a better appreciation of the pathology or injury, and thus planning of surgical intervention or other treatment. The primary challenges at the knee include multiple joints, weight-bearing functions, and a variety of anatomic structures. Because clinicians always attempt to gain the greatest assurance of detail, they have often accepted the use of magnetic resonance imaging (MRI) as requisite to models of "best practice." Importantly, the use of plain radiography coupled with appropriate physical examination provides very acceptable levels of sensitivity and specificity for most routine clinical examinations (O'Shea, 1996). The use of MRI is best applied in complex patients (multiple injuries) or where structural tissue differentiation is desired, particularly if surgical planning can be enhanced. The most common approach for MRI use is to use T1-weighted images to outline basic anatomic detail and T2-weighted images to better define specific structures (particularly soft and fibrous tissues) and to provide greater contrast. A very exciting evolution is to use additional modifications such as high-resolution proton density–fast spin echo (FSE) to elucidate and map articular cartilage changes which occur early in the "disease process" and thus permit clinicians hopefully to treat patients better based on predicted outcomes (Figure 9–1).
In this T2-weighted FSE technique, note the heterogeneous appearance of the signal from the articular cartilage weight-bearing area in the femoral condyle. Subtle changes of signal intensity can be indicative of early alterations in the functional status of the articular cartilage.
The knee joint proper (tibiofemoral joint) is divided into the medial and lateral compartments for evaluative processes. The medial compartment is larger and transmits more than half of the weight-bearing loads to the tibia, thus rendering it more susceptible to arthritic changes. This is coupled with the lateral compartment being less stable and allowing greater amounts of rotation. These actions are functionally defined by bony architecture as the medial side presents as a convex femur articulating with a concave tibia, whereas the convex lateral femoral condyle sits atop a flat or convex lateral tibia. The menisci sit between these opposing structures, enhancing the congruence or articulation and allowing better weight-bearing loads to be dispersed (greater area of contact, lesser per unit area of loading). The flexion/extension movements are controlled via both the bony articulation and the complex ligamentous structures while the musculature provides the ability to move but also to absorb and dissipate functional loading impacts. The musculature performs through the patellofemoral joint (patella and underlying femoral sulcus) in a pattern of motion controlled by both soft tissues (specific ligaments, capsule, and musculature) as well as the level of bony congruence and orientation of the patella with the sulcus. Specialized views are used to attempt to give data referring to these patterns with moderate success.