Os Intermetatarseum: Unusual Variant

Abstract

Os intermetatarseum is described as an accessory bone of rather rare occurrence (1, 2, 5). Almost all of the cases thus far recorded have been rather diminutive osseous bodies lying usually between the bases of the first and second metatarsals. Sometimes the bone appears as spindle-shaped, projecting a half or a third of the distance to the heads of the other metatarsals. One well known authority writes in a recent text (5): “The os intermetatarseum is a small accessory bone lying between the bases of the first and second metatarsal bones; it is sometimes attached to the second metatarsal bone and projects in such a way as to suggest the appearance of a rudimentary metatarsal bone. It is seen only rarely and does not give rise to symptoms.” A search of the literature tends to confirm this observation as to symptoms, though in one case occurring at the New York Orthopaedic Dispensary and Hospital, such a bone occurring between the first and second metatarsals was considered to play an etiologic röle in the production of a hallux valgus (6). The case here presented is of interest because of the unusual situation of an os intermetatarseum; it appears in the space between the fourth and fifth metatarsals, articulates distally with the phalanx, the “true” fourth metatarsal head appearing to be distinctly extra-articular, and tapers to a small proximal end, which appears free. The “true” fourth metatarsal articulates with the tarsus at its base. It would appear that the “true” fourth metatarsal lies below, the accessory metatarsal above. A further anomaly is present in that the fourth toe appears to have only two phalanges. On physical examination the foot appeared to show some increase in thickness in the region of the fourth metatarsal. The fourth toe was shorter than the corresponding toe on the opposite foot. A callus was present over the fourth metatarsal head region on the plantar surface of the foot. The patient, who was a woman fifty years of age and who had been sent for examination to rule out fracture in a case of sprained ankle, gave only an ambiguous history of any previous symptoms referable to the foot. She was questioned further because of an incidental finding of Freiberg's disease (deforming metatarsophalangeal osteochondritis) in the second metatarsal head. If there had ever been an acute phase in the latter condition, the history gave little clue to it. Relative to Freiberg's disease, Hauser (5) makes the comment: “The condition is believed to be the result of mechanical strain and abnormal repeated traumas that occur at the heads of the second and third metatarsal bones. It is usually seen in cases of pes valgoplanus associated with metatarsus latus. “Since the second metatarsal bone is generally the longest, it is exposed to increased weight bearing and trauma.”