The term 'sesamoid', coined by Galen, is derived from the flat, oval seeds of the Sesanum indicum plant.
The largest sesamoid bone in the body is the patella, a sesamoid within the extensor mechanism of the knee.
The peroneus longus muscle plantar flexes and everts the foot. A sesamoid bone called the os peroneum may be present within the peroneus longus tendon. Many support the idea that the os peroneum is always present, but the os peroneum is completely ossified in only about 20% of the population. It may also be cartilaginous or fibrocartilaginous in nature. The os peroneum is multipartite in some individuals.
In this case, a 23 year old female presented with lateral ankle pain.
(A) Oblique axial intermediate and (B) Oblique axial T2 fatsat images depict globular areas of abnormal signal red arrows) within the peroneus longus tendon. There is adjacent soft tissue edema (yellow arrows)
Examination of a lateral radiograph of this area reveals a markedly enlarged, multipartite os peroneum (red arrows):
(A) Oblique axial intermediate and (B) Oblique axial T2 fatsat images immediately proximal to the enlarged, fragmented os peroneum identify focal longitudinal splits (red arrows) within the peroneus longus tendon.
This is an example of painful os peroneum syndrome, with superimposed partial tearing of the peroneus longus. Painful os peroneum syndrome is associated with a spectrum of conditions that includes one or more of the following: (1) acute os peroneum fracture or a diastasis of a multipartite os peroneum, either of which may result in a discontinuity of the peroneus longus tendon; (2) chronic os peroneum fracture or diastasis of a multipartite os peroneum with callus formation; (3) attrition or partial rupture of the peroneus longus tendon, proximal or distal to the os peroneum; (4) frank rupture of the peroneus longus tendon with discontinuity proximal or distal to the os peroneum; (5) enlarged peroneal tubercle on the lateral aspect of the calcaneus which entraps the peroneus longus tendon and/or the os peroneum during tendon excursion (Foot Ankle Int. 15:112-24, 1994).
One must be careful not equate abnormal signal within the peroneus longus tendon at the level of the calcaneocuboid joint with a tendon tear. I have had more than one phone call from a foot and ankle surgeon asking why a radiologist commented on a peroneus longus tear, and did not mention the os peroneum, which was clearly present on x-rays.
While the os peroneum is obvious on x-ray, it is often much less apparent on MRI, which is insensitive to the presence of calcification and mineralized bone (unless it contains fatty marrow).
When abnormal signal is present in the peroneus longus tendon at the level of the calcaneocuboid joint on MRI, one should consider the possibility of os peroneum syndrome, and correlate with conventional radiographs.
Vic David MD