Giant cell tumor (GCT) of bone is generally a benign tumor composed of mononuclear stromal cells and characteristic multinucleated giant cells that exhibit osteoclastic activity. It usually develops in long bones but can occur in unusual locations. The typical appearance is a lytic lesion with a well-defined but nonsclerotic margin that is eccentric in location, extends near the articular surface, and occurs in patients with closed physes. However, GCT may have aggressive features, including cortical expansion or destruction with a soft-tissue component. Fluid-fluid levels, consistent with secondary formation of aneurysmal bone cysts, are seen in 14% of cases. GCT can mimic or be mimicked by other benign or malignant lesions at both radiologic evaluation and histologic analysis. Rarely, GCT is associated with histologically benign lung metastases or undergoes malignant degeneration. In the past, the mainstay of treatment was surgical, primarily consisting of curettage with cement placement, with recurrence rates of 15%-25%. Recurrence is suggested by development of progressive lucency at the cement-bone interface. Other complications include pathologic fracture and postoperative infection. Denosumab, a monoclonal antibody that targets the osteoclastic activity of GCT, has produced 90% tumor necrosis in early studies, results indicative of promise as a potential adjuvant therapy.

Intraarticular masses can be classified as noninfectious synovial proliferative processes (lipoma arborescens, synovial osteochondromatosis, pigmented villonodular synovitis, rheumatoid arthritis), infectious granulomatous diseases (tuberculous arthritis, coccidioidomycosis arthritis), deposition diseases (gout, amyloid arthropathy), vascular malformations (synovial hemangioma, arteriovenous malformations), malignancies (synovial chondrosarcoma, synovial sarcoma, synovial metastases), and miscellaneous (cyclops lesion). Knowledge of articular anatomy aids the radiologist in localizing masses to the joint space. Some joints have complex anatomy with contiguous or adjacent bursae, recesses, and tendinous connections from which masses may originate or into which masses may extend. Many of the diseases causing intraarticular masses have specific imaging characteristics, especially on magnetic resonance images, and knowledge of these characteristics will allow for a more confident diagnosis.

Sclerosing bone dysplasias are skeletal abnormalities of varying severity with a wide range of radiologic, clinical, and genetic features. Hereditary sclerosing bone dysplasias result from some disturbance in the pathways involved in osteoblast or osteoclast regulation, leading to abnormal accumulation of bone. Several genes have been discovered that, when disrupted, result in specific types of hereditary sclerosing bone dysplasia (osteopetrosis, pyknodysostosis, osteopoikilosis, osteopathia striata, progressive diaphyseal dysplasia, hereditary multiple diaphyseal sclerosis, hyperostosis corticalis generalisata), many of which exhibit similar pathologic mechanisms involving endochondral or intramembranous ossification and some of which share similar underlying genetic defects. Nonhereditary dysplasias include intramedullary osteosclerosis, melorheostosis, and overlap syndromes, whereas acquired syndromes with increased bone density, which may simulate sclerosing bone dysplasias, include osteoblastic metastases, Paget disease of bone, Erdheim-Chester disease, myelofibrosis, and sickle cell disease. Knowledge of the radiologic appearances, distribution, and associated clinical findings of hereditary and nonhereditary sclerosing bone dysplasias and acquired syndromes with increased bone density is crucial for accurate diagnosis.

The knee is an intricate joint with numerous tendinous, ligamentous, and meniscal attachments, which make it particularly vulnerable to complex injuries after trauma. A variety of avulsion fractures of the knee can occur, including Segond and reverse Segond fractures; avulsions of the anterior and posterior cruciate ligaments; arcuate complex avulsion; iliotibial band avulsion; avulsions of the biceps femoris, semimembranosus, and quadriceps tendons; Sinding-Larsen-Johansson syndrome; and Osgood-Schlatter disease. These fractures often have a subtle appearance at conventional radiography, which is typically the first imaging modality performed in these cases. Advanced imaging modalities, particularly magnetic resonance imaging, are helpful and can provide valuable additional information for adequately defining the extent of damage. The onus is on the radiologist to identify the pattern of injury and to understand the substantial underlying damage that it frequently represents. Conveying this information to the referring clinician is crucial and represents the first step toward additional evaluation and probable orthopedic referral. By recognizing the significance of these injuries at initial presentation, radiologists can facilitate appropriate patient work-up and prevent the chronic morbidity associated with delayed treatment.

The posteromedial corner of the knee (PMC) is an important anatomic structure that is easily seen but often overlooked on magnetic resonance (MR) images. Whereas the posterolateral corner has been referred to as the "dark side of the knee" by some authors owing to widespread lack of knowledge of its complex anatomy, even less is written about the PMC; yet it is as important as the posterolateral corner in multiligament injuries of the knee. The PMC lies between the posterior margin of the longitudinal fibers of the superficial medial collateral ligament (MCL) and the medial border of the posterior cruciate ligament (PCL). The anatomy of the PMC can be complex and the literature describing it can be confusing, at times oversimplifying it and at other times adding unnecessary complexity. Its most important structures, however, can be described more simply as five major components, and can be better shown with illustrations that emphasize the anatomic distinctions. Injuries to the PMC are important to recognize, as disruption of the supporting structures can cause anteromedial rotational instability (AMRI). Isolated PMC injuries are rare; most occur in conjunction with injuries to other important stabilizing knee structures such as the anterior cruciate ligament (ACL) and PCL. Unrecognized and unaddressed injury of the PMC is one of the causes of ACL and PCL graft failures. Recognition of PMC injuries is critical, as the diagnosis will often change or require surgical management.