An ankle X-ray is a common diagnostic tool that uses small doses of radiation to create a two-dimensional image of the dense tissues within the ankle joint. This imaging method is primarily used to assess the integrity of the three bones that form the ankle: the distal ends of the tibia and fibula, and the talus. Understanding the appearance of a normal X-ray is the foundation for recognizing potential fractures, dislocations, or degenerative changes. A normal image serves as a baseline, helping healthcare professionals confirm skeletal stability and proper alignment following an injury or in the presence of chronic pain.
Standard X-ray Views
Because the ankle is a complex three-dimensional joint, a single image cannot fully capture its anatomy, necessitating a series of specific projections. The standard examination typically involves three distinct views: the Anteroposterior (AP), the Lateral, and the Mortise view. Each orientation is designed to highlight different aspects of the joint and minimize anatomical overlap.
The AP view, taken from the front, captures the ankle in its natural anatomical position, which helps evaluate the general alignment of the leg bones with the talus. However, this view causes overlap between the distal fibula and tibia, obscuring the joint space. The Lateral view is taken from the side, providing a profile of the entire joint and allowing assessment of the front-to-back relationship of the bones.
The Mortise view requires the foot to be internally rotated by about 15 to 20 degrees. This rotation projects the fibula away from the joint line, effectively “opening up” the ankle joint space, known as the mortise. This projection provides the clearest, unobstructed view of the joint’s articulation and confirms proper spacing and alignment.
The Bony Structure
The appearance of healthy bone on an X-ray is characterized by its density and smooth contours. A normal bony cortex, the hard outer layer of the bone, appears as a continuous, uniformly bright white line due to its high calcium content. This bright line should be traced without any interruptions, breaks, or irregularities, as these could signal a fracture.
The internal spongy bone, or trabecular bone, found within the core of the bones, displays a structured pattern. This structure is less dense and appears slightly grayer than the cortex, but it should show a distinct, organized mesh-like arrangement. The distal tibia features the medial malleolus, a bony prominence that appears smooth, while the lateral malleolus (lower end of the fibula) extends further down. The trochlea, or dome, of the talus, which articulates with the tibia, must also have a smooth and unbroken surface.
Joint Spaces and Alignment
A normal ankle X-ray demonstrates a precise spatial relationship between the tibia, fibula, and talus, which is defined by the joint spaces. Cartilage, the smooth tissue covering the bone ends, is not visible on an X-ray, so the physical space it occupies appears as a dark, uniform gap. This gap, the tibiotalar joint space, should be consistently symmetrical and congruent across the entire width of the joint, indicating healthy cartilage thickness and proper weight distribution.
In an adult, the joint space between the talus and the distal tibia and fibula should measure no more than 4 millimeters in width. The ankle mortise, the socket formed by the malleoli and the tibial plafond, must cradle the talus tightly and symmetrically. The space between the distal tibia and fibula, measured one centimeter above the joint, should be less than 6 millimeters wide.
The specialized Mortise view confirms the stability of the tibiofibular syndesmosis, the strong ligamentous connection between the tibia and fibula. Proper alignment is confirmed when the lateral shoulder of the talus lines up perfectly with the lateral margin of the tibial articular surface. A normal X-ray shows the talus sitting securely and centrally within its socket, with no tilting or displacement.
Normal Soft Tissue Appearance
While X-rays excel at imaging bone, the surrounding soft tissues, such as muscles, fat, and skin, are also captured and provide important context. Soft tissues appear in varying shades of gray or dark gray, a stark contrast to the bright white of the bone. The expected contour of the skin and muscle should be intact, without excessive bulging or loss of definition, which would suggest swelling.
Fat pads, localized collections of adipose tissue, appear as distinct, dark lines or triangles because fat is less dense than muscle or fluid. The most notable is Kager’s fat pad, a triangular area of lucency located in the posterior ankle. On a normal lateral X-ray, Kager’s fat pad is sharply marginated and lucent. The absence of excessive soft tissue density around the medial and lateral malleoli suggests that no significant injury or effusion is present.