CHAPTER 6: Imaging the Knee

1. INTRODUCTION: FAST KNEE FACTS

❑ The Pathologies

• Why Is the Knee So Frequently Injured? The arthrokinematics of the knee allow for great stability combined with great mobility. This mechanical compromise, combined with its exposed location between the two longest bones of the body, predisposes it to various sports injuries, trauma, and degenerative pathologies.

• Sports Injuries/Trauma In the United States, knee pain results in more than 1.3 million annual visits to emergency departments and more than 12 million annual visits to physician offices.

• Osteoarthritis (OA) is the most common form of joint disease in humans and is a leading cause of disability among the elderly. More than one-half million total knee replacements are performed annually in the United States.

• Neoplasms—in children, neoplasms have a predilection for the distal femur and proximal tibia metaphyseal regions. Increased incidence of malignant bone cancer coincides with adolescent growth spurts and represent up to 11% of all childhood cancers. Most common are osteosarcoma (56%) and Ewing's sarcoma (34%).

❑ The Imaging Choices

• Radiographs adequately demonstrate most fractures and dislocations at the knee as well as nontraumatic disorders such as the various arthritides.

• Computed tomography (CT) provides optimal visualization of complex fracture characteristics such as tibial plateau fracture depression, articular defects, and fragmentation.

• Magnetic resonance imaging (MRI) is best for evaluating injuries to the ligaments, menisci, articular cartilage, and osteochondral fractures and osteochondritis dissecans.

• Ultrasonography's role in the evaluation of acute knee pain is generally limited to assessment of the extensor mechanism, joint effusion, and popliteal cyst.

• In the evaluation of neoplasms, radiographs help determine what advanced imaging to choose next, if any. Bone scans show the distribution and activity of the lesion in the skeleton. MRI evaluates the intraosseous and extraosseous extent of the lesion.

❑ The Available Guidelines

• Clinical Decision Rules (CDRs): The Ottawa Knee Rules and Pittsburgh Rules for Knee Trauma are evidence-based guidelines that serve to minimize unnecessary emergency room radiography of the knee.

• ACR Appropriateness Criteria: These evidence-based criteria identify the most appropriate imaging study to be ordered for specific patient presentations. Currently, 6 presentations of traumatic knee pain and 11 presentations of nontraumatic knee pain have been researched.

• Diagnostic Imaging Pathways: Clinical decision trees have been established for the assessment of (1) nontraumatic knee pain, (2) posttraumatic knee pain, and (3) suspected osteomyelitis.

2. REVIEW OF ANATOMY

3. AVAILABLE IMAGING GUIDELINES: ACR APPROPRIATENESS CRITERIA, CLINICAL DECISION RULES, DIAGNOSTIC IMAGING PATHWAYS

• Appropriateness Criteria are evidence-based guidelines developed by expert consensus of professional groups to assist clinicians in choosing which imaging modality to order for a specific clinical condition. The American College of Radiology (ACR) publishes criteria for both acute traumatic ...