The bony bumps on either side of your ankle are called the malleoli, which are naturally prominent parts of your anatomy. The lateral malleolus is the outer prominence of the fibula bone, and the medial malleolus is the inner prominence of the tibia bone. This article explores why these bones, or the area around them, might appear larger or more visible than they have in the past.
Identifying Your Ankle Structure
The ankle joint, or talocrural joint, is formed by the articulation of three bones: the tibia, the fibula, and the talus. The medial and lateral malleoli serve as the anchors for this joint, extending downward to cup the talus bone and provide necessary stability. The size and specific shape of these bony projections are determined by genetics and can vary significantly from person to person.
For some individuals, the malleoli are simply large by nature, or the soft tissues like fat and muscle surrounding the joint are minimal. This combination means the bone will always appear prominent even in a state of perfect health. The lateral malleolus, which descends further than the medial malleolus, often appears more noticeable on the outside of the ankle. Understanding this baseline anatomy helps distinguish between a natural contour and a true, acquired change in structure.
Prominence Caused by Soft Tissue Swelling
Many instances of a “protruding ankle bone” are actually a visual illusion created by swelling in the surrounding soft tissues. When ligaments, tendons, or the joint capsule become inflamed, the tissue expands, flattening the normal contours of the foot and ankle. This expansion heightens the visual contrast, making the bony malleolus seem to jut out more dramatically than usual.
This soft tissue swelling, or edema, is a common response to minor trauma, such as a low-grade ankle sprain where ligaments are stretched but not torn. The body sends an influx of fluid and inflammatory cells to the injured site, leading to a temporary increase in size. Overuse injuries, like tendonitis from repetitive activity, can also cause localized inflammation around the malleoli, resulting in a similar effect.
Systemic conditions can also contribute to this appearance through fluid retention. Conditions like heart failure, kidney disease, or venous insufficiency cause fluid to pool in the lower extremities due to gravity and impaired circulation. This non-traumatic, generalized edema causes the skin and subcutaneous tissue around the ankle to become puffy, which highlights the bony prominence of the malleoli.
Structural Changes and Chronic Deformities
When the prominence is not due to temporary swelling, it often results from a permanent change to the bone structure itself. One common structural alteration is the formation of osteophytes, or bone spurs, which are bony growths that develop along the edges of joints, often in response to chronic stress or arthritis. These spurs can form on the tibia or talus, causing a true increase in the size of the bone and leading to a noticeable bump.
Another cause of acquired prominence is post-traumatic malunion, which occurs when an ankle fracture heals improperly. If the fractured tibia or fibula is not perfectly realigned, the resulting bone may heal in a slightly rotated or shortened position. This misalignment can shift the entire ankle mortise, sometimes causing the medial malleolus to become conspicuously prominent and contributing to chronic discomfort.
Accessory ossicles, such as the Os Trigonum, are small extra bones that can also be responsible for a posterior prominence. The Os Trigonum forms when a secondary growth center on the back of the talus fails to fuse with the main bone during adolescence. While often asymptomatic, this small bone can become compressed or irritated, causing painful swelling and a distinct bump at the back of the ankle, a condition known as posterior ankle impingement.
A frequently misidentified cause of posterior prominence is Haglund’s Deformity, often called a “pump bump.” This condition is a bony enlargement on the back of the heel bone, the calcaneus, not the true ankle malleoli. This growth develops where the Achilles tendon attaches, causing irritation of the soft tissue and bursa, leading to a visibly enlarged and inflamed bump at the top of the heel.
When to Seek Medical Consultation
It is important to seek a medical evaluation if the bony prominence is accompanied by signs suggesting a more serious underlying issue. If the protrusion appeared suddenly with severe pain or the inability to bear weight, professional attention is warranted, as these symptoms can indicate an acute injury like a fracture or severe ligament tear.
Seek medical consultation if the prominence is accompanied by any of the following signs:
- Sudden appearance combined with severe pain.
- Inability to bear weight on the affected foot.
- Signs of infection, including warmth, spreading redness, or a fever.
- Swelling or pain that persists for several weeks despite home care.
- Any change in skin sensation, such as numbness or tingling.