CHAPTER 7: Imaging the Ankle

1. INTRODUCTION: FAST ANKLE FACTS

❑ The Pathologies

• Trauma—in adults, the ankle is considered the most frequently injured joint in the body.

• Sprains—inversion force accounts for most traumatic conditions at the ankle. The degree of injury depends on the direction and magnitude of the applied force. Injuries can range in severity from minor overstretching of ligaments to ligamentous rupture, fracture, and dislocation.

• Fractures at the ankle—basic classifications include unimalleolar, bimalleolar, and trimalleolar (the posterior rim of the tibia is the “third” malleolus). Treatment goals are to reestablish the ankle mortise for optimal function and stability.

• Fractures at the foot—these are described by location as fractures of the hindfoot (talus and calcaneus), midfoot, and forefoot.

  • ◗ Hindfoot—the calcaneus is most often injured owing to falls from heights.

  • ◗ Midfoot—isolated injuries are rare owing to the relative immobility of the midfoot. More often seen are fracture–dislocation injuries at the transverse tarsal or tarsometatarsal joints (Lisfranc joint) where the midfoot articulates with the more mobile hindfoot and forefoot.

  • ◗ Forefoot—metatarsals are frequently injured by objects dropped on the foot. Phalanges are often injured by “stubbing” mechanisms.

  • ◗ Stress fractures—these commonly occur at the 2nd, 3rd, 4th, and 5th metatarsals; navicular; calcaneus; and distal fibula.

• Osteomyelitis—infection of bone can occur anywhere in the body but is often seen at the foot. Younger patients can develop osteomyelitis in a metatarsal via a hematogenous route from infection elsewhere in the body. Older patients may develop osteomyelitis because of vascular insufficiency or complications of a diabetic ulcer.

❑ The Imaging Choices

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

• Computed tomography (CT) provides optimal visualization of complex fractures, especially in characterizing fractures of the hindfoot or in localizing the position of fracture fragments.

• Magnetic resonance imaging (MRI) is best for evaluating tendon abnormalities, osteochondral injury, joint instability, impingement syndromes, and occult Lisfranc injury, and for diagnosing osteomyelitis.

• Musculoskeletal ultrasound (MSUS) is used for assessment of tendon abnormalities, penetrating trauma with a foreign body, and Morton's neuroma.

❑ The Available Guidelines

• Clinical Decision Rules: The Ottawa Ankle Rules are evidence-based guidelines to help first-contact clinicians determine whether radiographs are needed to rule out a fracture.

• ACR Appropriateness Criteria: Currently, eight presentations of acute trauma to foot, seven presentations of chronic ankle pain, and four presentations of suspected osteomyelitis in the patient with diabetes mellitus have been researched.

• Diagnostic Imaging Pathways: Clinical decision trees have been established for the assessment of (1) acute ankle sprain, (2) diabetic foot ulcers, and (3) suspected peripheral vascular disease.

2. REVIEW OF ANATOMY

3. AVAILABLE IMAGING GUIDELINES: CLINICAL ...