The medial malleolus is the prominent bony projection commonly known as the inner ankle bone. It marks the inner boundary of the ankle joint, which is a complex hinge connecting the foot and leg. Its location makes it particularly susceptible to specific types of stress and fracture, often associated with twisting injuries.
The Anatomy of the Medial Malleolus
The medial malleolus is not a separate bone but is an integral part of the tibia, the main weight-bearing bone of the lower leg. It forms the most distal projection on the medial (inner) side of the tibia, extending downward in a pyramidal shape to create the characteristic bony bump.
The opposing bony prominence, the lateral malleolus, belongs to the fibula, the thinner bone of the lower leg. Together, the medial malleolus and the lateral malleolus bracket the ankle bone, known as the talus. These two projections form the ankle mortise, a deep socket that stabilizes the joint.
Location and Relationship to the Ankle Joint
The medial malleolus forms the inner wall of the talocrural joint, the formal name for the ankle joint. Its lateral side features a smooth, slightly concave articular facet. This cartilage-covered area articulates directly with the medial surface of the talus, allowing for smooth movement.
The medial malleolus is shorter and positioned higher than the lateral malleolus, which helps define the overall geometry of the ankle socket. A broad groove, known as the malleolar sulcus, is situated on its posterior border. This groove serves as a pathway for important tendons, such as the tibialis posterior and the flexor digitorum longus, as they pass into the foot.
Structural Role in Ankle Stability
The medial malleolus maintains the structural integrity and stability of the ankle joint. Its primary function is providing the attachment site for the strong, fan-shaped Deltoid Ligament, also known as the medial ligament. This ligament is composed of multiple bands, including the anterior tibiotalar, tibiocalcaneal, and posterior tibiotalar ligaments, which anchor the malleolus to the talus, calcaneus, and navicular bones.
The Deltoid Ligament is robust and acts as a major stabilizer, preventing the foot from rolling excessively outward, a movement known as eversion. Because the ligament is so strong, a severe force attempting eversion often results in an avulsion fracture, where the ligament pulls a piece of the medial malleolus bone off instead of tearing the ligament itself.
Beyond ligament attachment, the medial malleolus acts as a physical block that restricts the lateral shift and external rotation of the talus within the ankle mortise. This bony restriction, coupled with the tension provided by the Deltoid Ligament, ensures the ankle remains aligned.