The os peroneum is a small, accessory bone found within the foot, often discovered incidentally during imaging studies. It is located in the mid-foot area, near the outside edge where the arch begins to curve. While present in a segment of the population, it typically remains unnoticed unless an injury occurs. The primary concern for those experiencing discomfort is understanding what causes the bone to become symptomatic and painful.
Anatomy and Function of the Os Peroneum
The os peroneum is a sesamoid bone, meaning it is embedded within a tendon rather than being connected directly to other bones in a traditional joint. It is situated within the distal portion of the peroneus longus tendon, which runs along the outer side of the ankle and passes underneath the cuboid bone in the mid-foot. At this point, the tendon abruptly changes direction before crossing the sole of the foot.
The peroneus longus muscle is responsible for eversion (outward movement of the foot) and assists in plantar flexion (pointing the foot downwards). The os peroneum acts as a mechanical pulley where the tendon curves under the cuboid bone. By elevating the tendon’s line of action, the ossicle increases the muscle’s leverage, improving its mechanical advantage for foot movement.
Acting as a gliding surface, the os peroneum helps the tendon move smoothly and protects the peroneus longus tendon from excessive friction or compression against the cuboid bone. This protective role is significant because the tendon is subjected to high forces and a sharp angle change as it passes through the groove of the cuboid. The accessory bone is an integrated part of this biomechanical system, ensuring efficient muscle function during walking and running.
Developmental Origins of the Os Peroneum
The existence of the os peroneum, like other sesamoid bones, is attributed to the body’s response to localized mechanical stresses. These bones develop within tendons that experience significant tension and compressive forces against an underlying bone. The formation process involves the development of fibrocartilage within the tendon tissue at the point of high stress, which then ossifies into bone.
The os peroneum is not a universal finding, present in its fully ossified form in approximately 5% to 30% of the population. Some research suggests the precursor tissue may be present in all individuals, but only fully ossifies into a visible bone in a minority. The accessory bone can be found in one foot or both, and it varies in size and shape among individuals.
A developmental variant is the bipartite or multipartite os peroneum, where the bone is naturally split into two or more distinct pieces. This is a common finding, occurring in up to 30% of cases where the ossicle is present. This division is due to incomplete fusion of the bone’s ossification centers during development. While not initially problematic, a bipartite os peroneum can be mistaken for an acute fracture and is more susceptible to separation if the surrounding tendon is injured.
Specific Conditions Leading to Os Peroneum Pain
When the os peroneum becomes painful, the condition is often referred to as Painful Os Peroneum Syndrome. Discomfort is localized to the lateral mid-foot and arises from several pathological mechanisms involving the bone and its surrounding tendon. The most direct cause of acute pain is an os peroneum fracture, which typically occurs from a sudden, forceful contraction of the peroneus longus muscle. This contraction often happens during an acute ankle inversion injury or a twisting motion, compressing the bone against the cuboid.
A fracture can be a simple break or a separation, known as diastasis, of a pre-existing bipartite os peroneum. Diastasis involves the traumatic widening of the gap between bone fragments, often indicating significant stress or injury to the surrounding tendon. If the fracture or diastasis is accompanied by a tear or rupture of the peroneus longus tendon, the os peroneum can be pulled away from its normal position, a sign known as proximal migration.
Chronic strain or overuse can lead to peroneal tendinopathy, which is degeneration or chronic inflammation of the tendon. This constant stress on the peroneus longus tendon, where the ossicle is embedded, results in localized pain and tenderness. Repetitive friction and pressure where the tendon wraps around the cuboid, especially in individuals with high-arched feet, may lead to wear and tear of the gliding surface, contributing to chronic pain.
Another mechanism for pain is os peroneum impingement, which occurs when the bone or its fragments are unstable or entrapped. This instability is more likely with a bipartite configuration, as movement between the fragments irritates the surrounding tendon and soft tissues. Chronic inflammatory changes associated with tendinopathy can also involve tenosynovitis, which is inflammation of the sheath surrounding the peroneus longus tendon. This spectrum of issues, from acute trauma to chronic degeneration, explains why the accessory bone may suddenly begin to hurt.