■ Describe the anatomy and function of each of the principal stabilizers of the medial arch and how their dysfunction leads to AAFD. ■ Assess foot malalignment with standard radiographic metrics and recognize imaging findings that indicate damage to the supporting structures of the foot. ■ Explain the principles of clinical staging of AAFD and the most commonly used treatment options for each stage.

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Atraumatic disorders of skeletal muscles include congenital variants; inherited myopathies; acquired inflammatory, infectious, or ischemic disorders; neoplastic diseases; and conditions leading to muscle atrophy. These have overlapping appearances at magnetic resonance (MR) imaging and are challenging for the radiologist to differentiate. The authors organize muscle disorders into four MR imaging patterns: (a) abnormal anatomy with normal signal intensity, (b) edema/inflammation, (c) mass, and (d) atrophy, highlighting each of their key clinical and imaging findings. Anatomic muscle variants, while common, do not produce signal intensity alterations and therefore are easily overlooked. Muscle edema is the most common pattern but is nonspecific, with a broad differential diagnosis. Autoimmune, paraneoplastic, and drug-induced myositis tend to be symmetric, whereas infection, radiation-induced injury, and myonecrosis are focal asymmetric processes. Architectural distortion in the setting of muscle edema suggests one of these latter processes. Intramuscular masses include primary neoplasms, metastases, and several benign masslike lesions that simulate malignancy. Some lesions, such as lipomas, low-flow vascular malformations, fibromatoses, and subacute hematomas, are distinctive, but many intramuscular masses ultimately require a biopsy for definitive diagnosis. Atrophy is the irreversible end result of any muscle disease of sufficient severity and is the dominant finding in disorders such as the muscular dystrophies, denervation myopathy, and sarcopenia. This imaging-based classification, in correlation with clinical and laboratory data, will aid the radiologist in interpreting MR imaging findings in patients with atraumatic muscle disorders. RSNA, 2018.

Describe the anatomy and function of the static and dynamic stabilizers of the AC joint.Recognize the imaging findings of various pathologic processes that affect the AC region, including acute injury of the joint and distal clavicle, distal clavicle osteolysis, and degenerative disorders.Discuss the treatment options and surgical techniques used to manage AC disease and the most common associated complications.

Anterior knee pain is a common complaint that can be caused by a wide spectrum of disorders affecting the many varied tissues at the anterior knee. The anatomy and pathologic conditions of the anterior knee can be approached systematically by organizing the region into four layers: (a) superficial layer of fat, fascia, and bursae; (b) functional layer composed of the extensor mechanism and patellar stabilizers; (c) intracapsular extrasynovial layer containing the fat pads; and (d) intra-articular layer. The superficial layer is composed of delicate tissues that are predisposed to blunt and penetrating trauma, irritation, and infection. The extensor mechanism forms the functional layer, is responsible for knee extension and patellar stabilization, and is engaged in repetitive movements; overuse disorders dominate in this layer. The fat pads of the anterior knee are discussed collectively as an extracapsular extrasynovial layer, functioning to improve congruence and protect the articular surfaces during motion. Diseases involving the fat pads can be primary or secondary to pathologic conditions in the rest of the joint. The synovial lining and cartilage surface are in the fourth and final intra-articular layer; pathologic conditions are centered around arthritides and internal derangement. Symptoms in the anterior knee may be due to conditions affecting one or more of these interrelated layers. © RSNA, 2018 • radiographics.rsna.org Dyan V. Flores, MD Catalina Mejía Gómez, MD Mini N. Pathria, MD Abbreviations: ACL = anterior cruciate ligament, FNS = focal nodular synovitis, MPFL = medial patellofemoral ligament, PD = proton density RadioGraphics 2018; 38:2069-2101 ). © RSNA, 2018After completing this journal-based SA-CME activity, participants will be able to:This journal-based SA-CME activity has been approved for AMA PRA Category 1 Credit TM . See rsna.org/learning-center-rg.