Comparative anatomy and significance of the sesamoid bone of the peroneus longus muscle (os peroneum).

The os peroneum is found in only a few primate families and seems to be completely absent in the non-primate pentadactyl mammals, in the Prosimii and in the Platyrrhinii (New World monkeys). In the Cercopithecidae (Catarrhinii, Old World monkeys) and the Hylobatidae, the os peroneum is a coffee bean-shaped constant, large and regular bone. The lateral surface of the bone is convex in all directions and non-articular. The medial surface is covered with hyaline cartilage and articulates by means of a synovial joint with the corresponding facet of the cuboid bone. The histological structure and the mode of ossification of the os peroneum are identical to that of other short bones of the skeleton. The os peroneum of the Cercopithecidae and Hylobatidae is an example of a new skeletal element that has appeared in a tendon subject to unusual mechanical stress. In the case of the peroneus longus tendon the stress is due to repetitive friction because of the functional importance of this muscle in the adduction and pseudo-opposability of the hallux. This osseous element is genetically fixed and hereditarily transmitted. Its mode of appearance is analogous to that accepted for the origin of the patella. In the Pongidae, the os peroneum is absent or rare. In man, this bone is relatively infrequent (approx 20% of mature individuals) and its shape is most irregular. In this case, the os peroneum appears as a regressive form of the typical bone observed in the above families, which is in the process of disappearing. Besides fundamental genetical factors, this regression is probably in relation to the disappearance of the functional importance of the peroneus longus muscle to the loss of the hallux opposability. Thus the mechanical factors cannot be dissociated from the genetic and phylogenetic factors in explaining the appearance and the regression of the os peroneum.