313 10 Knee A. Radiologic Perspective: Magnetic Resonance Imaging of the Knee Theodore T. Miller In most radiology practices, the knee is the most commonly imaged joint in the appendicular skeleton. There is wide varia- tion in the field strengths of clinical magnets (ranging from 0.2 to 3 tesla [T]), the configuration of the magnets (open or closed), the sequences (e.g., T1, T2, proton density, con- ventional spin echo, fast spin echo, gradient echo), and slice thicknesses used to image the knee, as well as a wide variety in the skill of the radiologist interpreting the images. All of these factors have bearing on the accuracy of the magnetic res- onance (MR) examination 1 as well as the anecdotal usefulness of this modality to the referring physician. Nonetheless, Glynn et al., 2 using Medicare data on reimbursements between 1993 and 1999, found a 145% increase in performance of magnetic resonance imaging (MRI) of the lower extremity and a 54.5% decrease in performance of diagnostic arthroscopy of the knee, suggesting an increasingly greater reliance by clinicians on MRI to provide diagnostic information. Similarly, Bryan et al. 3 found that the use of MRI in the diagnostic evaluation of patients with chronic knee complaints significantly reduced the need for surgery. While careful physical examination is as accurate as MRI for tears of the menisci and rupture of the anterior cruciate liga- ment (ACL), 4–6 a survey of members of the ACL study group showed that 44% of respondents routinely order preopera- tive MR examinations for patients with suspected ACL injury and 51% order MR examinations for patients with suspected posterior cruciate ligament (PCL) injury. 7 In cases in which the physical examination is equivocal or multiple injuries are present, MRI does have a significant effect on surgical decision making, 8–11 and 63% of the respondents of the ACL study group order MR examinations for patients with multiple ligament injury. 7 The discerning use of MRI by orthopedic surgeons was described by both Sherman et al. 12 and Bernstein et al., 13 who found that patients referred by nonorthopedic surgeons had a statistically significant higher rate of normal MR examinations than patients referred by orthopedic surgeons. This chapter discusses the normal anatomy, mechanisms of injury, MRI appearances of injury, and, where appropriate, the postsurgical appearance of the structures in and around the knee joint. Menisci Anatomy The menisci are crescent-shaped wedges of fibrocartilage whose purpose is to provide increased contact area between the rounded femoral condyles and flat tibial plateau, thereby providing increased stability to the femorotibial articulation, distributing axial load, absorbing shock, and distributing syno- vial fluid to nourish the adjacent hyaline articular cartilage of the condyles and plateau. Looking down on the menisci, the lateral meniscus is C-shaped and is uniform in thickness and size in cross section throughout. The medial meniscus is slightly larger, less tightly curved, and the posterior horn is thicker and wider than the body and anterior horn. Thus, on sagittal images the anterior and posterior horns of the lateral meniscus are equal in size while the posterior horn of the medial meniscus is larger than that of the anterior horn. The blood supply of the meniscus comes from the joint capsule, but only the peripheral approxi- mately 20% of the adult meniscus is vascularized, which has implications for the management of tears. The entire periphery of the medial meniscus is attached to the joint capsule. In con- trast, the anterior horn and body of the lateral meniscus are entirely attached to the joint capsule, but the posterior horn is focally separated from the joint capsule and connected to it by superior and inferior popliteomeniscal fascicles, which form an oblique tunnel called the popliteal hiatus, through which runs the popliteus tendon (Fig. 10.1). In addition to their cap- sular attachment, the menisci are attached to the tibial plateau by fibrous bands at the roots of their anterior and posterior horns. The anterior horns of the menisci are also connected to each other by the transverse meniscal ligament, and the attachment site of this ligament on the meniscus can some- times mimic a meniscal tear (Fig. 10.2). The meniscofemoral ligaments of Humphry and Wrisberg are anatomically inconstant structures that run from the lateral aspect of the medial femoral condyle to the posterior horn of the lateral meniscus. The Humphry ligament lies anterior to the PCL, while the Wrisberg ligament is posterior to the PCL (Fig. 10.3).