An X-ray, or radiograph, uses electromagnetic radiation to create an image of the dense structures inside the foot. This quick, non-invasive process is a primary diagnostic tool in podiatry and orthopedics for evaluating pain, trauma, and chronic conditions. The resulting image is a two-dimensional, grayscale picture where tissues are distinguished by how much radiation they absorb. Bone, being the densest tissue due to its high calcium content, appears bright white, while soft tissues appear in varying shades of gray. This contrast allows medical professionals to assess the skeletal integrity and overall alignment of the foot and ankle.
Understanding the Imaging Views
A single X-ray image is rarely sufficient for a complete diagnosis because the foot’s many bones overlap when viewed from only one angle. Therefore, a series of images is taken from multiple perspectives, typically comprising three primary views.
One standard projection is the Dorsoplantar (DP) or Anteroposterior (AP) view, where the X-ray beam passes from the top of the foot to the sole. This top-down view is effective for visualizing the alignment of the toes and the shafts of the metatarsal bones in the forefoot.
The Lateral view is taken from the side, providing a profile perspective. This view is essential for assessing the height of the longitudinal arch and the alignment of the ankle and heel joints.
The third primary image is the Oblique view, taken with the foot positioned at an angle, usually between 30 and 45 degrees. This angulation separates the bones of the midfoot, such as the cuboid and navicular, which often overlap in the DP view. The oblique projection is useful for detecting subtle fractures.
Mapping the Foot’s Skeletal Structure
The foot contains 26 bones, and their appearance on an X-ray is determined by their density and shape. The bones are the brightest white structures on the film, a characteristic known as radiopacity. The outside layer, the cortical bone, appears as a thin, uniform white line, while the spongy inner tissue shows a slightly less dense, trabecular pattern.
The forefoot is made up of the phalanges (toe bones) and the five long metatarsals. On the DP view, the metatarsal shafts are clearly visible, extending toward the midfoot.
The midfoot consists of five irregularly shaped tarsal bones that articulate closely, forming the complex arches of the foot:
- Navicular
- Cuboid
- Medial cuneiform
- Intermediate cuneiform
- Lateral cuneiform
The hindfoot is formed by the two largest tarsal bones: the talus, which connects the foot to the leg, and the calcaneus, which forms the heel. These large bones appear as expansive white masses, especially visible on the lateral view where their relationship determines the arch angle.
The joints, where the bones meet, are not visible as solid structures. Since the cartilage covering them is soft tissue, it allows X-rays to pass through easily. Instead, the joints appear as distinct, dark spaces between the bright white bone ends.
Interpreting Common Abnormalities
Pathological conditions and injuries alter the foot’s normal X-ray appearance, providing visual evidence for diagnosis. An acute bone fracture shows up as a dark, irregular line that breaks the continuity of the dense white bone cortex. This dark line, called a radiolucency, represents the gap where the X-ray beam passed through the bone without being absorbed. Stress fractures, which are tiny hairline cracks from repetitive strain, are subtle and may require specialized views or follow-up imaging.
Degenerative changes, most commonly associated with arthritis, alter the joint spaces. As protective cartilage wears away, the dark gap between the bones narrows, known as joint space narrowing. Friction between the bone ends can also stimulate the growth of small, abnormal projections called bone spurs or osteophytes. These appear as extra white bone growth around the joint margins.
Misalignment and deformities are diagnosed by assessing the abnormal relationships between the bones. For instance, a bunion (hallux valgus) is identified by an increase in the angle between the first metatarsal and the phalanx of the big toe.
In cases of flat feet, the lateral X-ray often reveals a collapse of the normal arch, with the midfoot bones lying closer to the ground plane. Foreign objects, such as metal or glass shards, appear extremely bright white because they are often denser than bone, making them easily identifiable within the gray soft tissues.