Chapter 13

### INTRODUCTION

Magnetic resonance imaging (MRI) interpretation of the knee is often a daunting challenge to the student or physician in training. After all, an entire year of fellowship training is dedicated to musculoskeletal imaging. To overcome this challenge, it is imperative for both the medical student and resident in training to achieve a fundamental understanding of the structural and functional anatomy of the knee. Thus, the initial goal of this chapter is to review the complex anatomic relevant components of the knee. After establishing this anatomic knowledge base, a second goal of this chapter is to provide a systematic approach for interpretation of MRI as it applies to the clinical presentation of internal derangement of the knee. The term “Internal Derangement” is used to cover a group of disorders that affect the normal functioning ligaments, tendons, or cartilage (including the meniscus) of the knee articulation. From a technical standpoint, this is ideally performed with a high field strength 1.5 Tesla or 3 T MRI scanner. Three conventional MRI planes that are utilized to evaluate the knee include sagittal (oblique), coronal, and transaxial planes. While a detailed explanation of MRI protocols and MR physics is beyond the scope of this text, fast spin echo (FSE) MRI is most commonly utilized for MRI of the knee. Specifically, in the coronal and sagittal planes, T1, T2, and intermediate-weighted proton density FSE sequences are often utilized. Fat saturation is employed on some of the fluid-weighted sequences to better detect the presence of edema in the soft tissues or bone marrow. A spin echo or gradient echo sequence is often acquired in the transaxial plane. MRI of the knee may be performed without contrast. Alternatively, utilization of gadolinium contrast agent may be utilized and with regard to internal derangement; this is particularly useful in evaluating the postoperative knee. Administration of gadolinium for an MR arthrogram may be employed. This may be performed with either a direct or indirect technique. With the direct MR arthrogram, a 1:200 dilution of gadolinium in approximately 30 mL of sterile saline is percutaneously injected into the knee joint using a 22-gauge needle. The indirect MR arthrogram is performed after an intravenous injection of gadolinium contrast followed by exercise allowing leakage of contrast agent from the synovium into the joint space and then subsequent MR image acquisition.1

### ANATOMY

#### Patellofemoral Articulation

The patella is the largest sesamoid bone in the body, and the osseous contour of its articular surface is composed of a lateral facet, a pointed apex, a medial facet, and a smaller odd facet located at its far medial periphery. The patella articulates with the anterior femur that is characterized by a midline trochlear groove or sulcus, a medial facet, and a lateral facet. The articular surfaces of these patellar and femoral facets are covered with hyaline articular cartilage. A group of tendon fibers composed of the vastus medialis, vastus lateralis, ...

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