 Introduction/Background Acute bone and soft-tissue injuries to the knee may result from low- or high-energy trauma and are commonly seen in emergency departments as well as in outpatient practices [1]. The most common acute knee injuries result from a direct blow, a fall, or a twisting injury [2,3]. The fracture risk increases with age, likely secondary to decreased bone mineral density, increased frequency of blunt injury, and inability to protect the knee during a fall [3]. An estimated 6.6 million knee injuries presented to emergency departments in the United States from 1999 through 2008, for a rate of 2.29 knee injuries per 1,000 population. Prompt and accurate diagnosis facilitates adequate management and may prevent potential complications [1,4]. After thorough history and clinical examination, radiographs are usually the initial imaging modality in the evaluation of the acutely injured knee [1]. Adequate clinical examination and appropriate application of the established decision-making rules can reduce the number of radiographic studies in the setting of acute knee injuries with a potential benefit of reducing health care costs and decreasing radiation exposure to the patient [1,5,6]. Treatment options for acute traumatic knee injuries depend on the severity of injury and patient factors and include conservative or surgical management [1]. Special Imaging Considerations For the purpose of this document, point-of-care ultrasound (US) is not discussed or listed in the variant tables. For the purposes of distinguishing between CT and CT angiography (CTA), ACR Appropriateness Criteria topics use the definition in the ACR-NASCI-SIR-SPR Practice Parameter for the Performance and Interpretation of Body Computed Tomography Angiography (CTA) [7]. All elements are essential: 1) timing, 2) reconstructions/reformats, and 3) 3-D renderings. Standard CTs with contrast also include timing issues and recons/reformats. Only in CTA; however, is 3-D rendering a required element.
Acute Trauma to the Knee.  Introduction/Background Acute bone and soft-tissue injuries to the knee may result from low- or high-energy trauma and are commonly seen in emergency departments as well as in outpatient practices [1]. The most common acute knee injuries result from a direct blow, a fall, or a twisting injury [2,3]. The fracture risk increases with age, likely secondary to decreased bone mineral density, increased frequency of blunt injury, and inability to protect the knee during a fall [3]. An estimated 6.6 million knee injuries presented to emergency departments in the United States from 1999 through 2008, for a rate of 2.29 knee injuries per 1,000 population. Prompt and accurate diagnosis facilitates adequate management and may prevent potential complications [1,4]. After thorough history and clinical examination, radiographs are usually the initial imaging modality in the evaluation of the acutely injured knee [1]. Adequate clinical examination and appropriate application of the established decision-making rules can reduce the number of radiographic studies in the setting of acute knee injuries with a potential benefit of reducing health care costs and decreasing radiation exposure to the patient [1,5,6]. Treatment options for acute traumatic knee injuries depend on the severity of injury and patient factors and include conservative or surgical management [1]. Special Imaging Considerations For the purpose of this document, point-of-care ultrasound (US) is not discussed or listed in the variant tables. For the purposes of distinguishing between CT and CT angiography (CTA), ACR Appropriateness Criteria topics use the definition in the ACR-NASCI-SIR-SPR Practice Parameter for the Performance and Interpretation of Body Computed Tomography Angiography (CTA) [7]. All elements are essential: 1) timing, 2) reconstructions/reformats, and 3) 3-D renderings. Standard CTs with contrast also include timing issues and recons/reformats. Only in CTA; however, is 3-D rendering a required element.