The posterior malleolus is the back edge of the bottom of the tibia, or shinbone. The tibia ends in a slightly flared, concave joint with two slight knobs. The knob situated over the heel bone, at the back of the ankle, is the posterior malleolus.
This part of the bone provides connection points for tendons to stretch from the bottom of the tibia to the bones in the ankle. It’s difficult for a person to feel this bone through the skin because it's covered by fibrous connective tissue. The best way for it to be observed is through an X-ray.
Although it is small, the posterior malleolus is a strong, load-bearing bone. When standing, humans often rest most of their weight on their heels, compressing the tendons around the it. This dense little knob takes the weight and distributes it throughout the ankle with the help of the medial and lateral malleoli, as well as the connective tissue surrounding all three bones.
The medial malleolus is a prominent knob that is located on the inside of the ankle, and the lateral malleolus sticks out on the outside of the ankle. Together, the three malleoli create a join that is stable yet flexible and is perfect for rotation and fast movement. All of the malleoli should be treated with care, however.
Although the ankle is meant to twist, bend and rotate freely, rough treatment can quickly bruise and fracture the bones inside. Fast, hard turns in which the ankle twists beyond its limit will often crack the posterior malleolus, along with one or more bones in the ankle. Blunt trauma aimed at the back of the ankle, such as a kick or falling objects, also can cause serious damage. Athletes and people who work in hazardous areas, such as construction workers, are prone to posterior malleolus injuries.
Broken ankles can occur in any of the malleolus bones, but fractures in the posterior one are particularly devastating. The bone in the back of the ankle takes and distributes most body weight, so breaks in this area can be very painful. A person who has a broken posterior malleolus often cannot walk until the break is fully healed.
Breaks in the posterior malleolus might also threaten the tendons around it. Sharp, fractured bone could slice into the tendons, further injuring the ankle. If this little bone is broken and separated, not just fractured, the bone could push against the inside of the tendon connections and tear them. Someone who has a possible injury to the posterior malleolus should not try to stand or walk, because this could cause more damage. Instead, someone with an injured ankle should be lifted or carried to a place where he or she can be examined and treated.