Fused form of os intermetatarseum

Clinical History

A 37-year-old female patient was admitted to the orthopaedic clinic with persistent polyarthralgia (ankle and elbow), leg and ankle oedema and underwent foot X-ray, where a bone formation between proximal portion of first and second metatarsus was observed (Figures 1-3). Followed up at rheumatology consultation, the patient also presented temporomandibular joint arthralgia.

Imaging Findings

Besides degenerative alterations, namely in talus and navicular bones, a bone formation between proximal portions of the first and second metatarsus was observed on plain radiographs (AP, lateral and oblique projections) (Figure 1-3). This bone formation seems to be in relation with the second metatarsal base on dorsomedial surface. Foot CT (Figures 4-6) confirms the presence of this bone formation as an apparent exostosis process in the dependence of the dorsomedial surface of second metatarsal base with 30mm of long axis. This bone formation presented characteristics of an incomplete metatarsus, including shape, orientation and presence of narrow bone inside of the cortical bone, compatible with os intermetatarseum in fused form with the second metatarsal base. A ligament tissue was observed between the distal intermetatarseum part and first phalanx.
Discrete hallux valgus (a complication of this condition [1]) signals were present, namely a sclerotic metatarsophalangeal articular surface and discrete phalanx deviation, and only the intermetatarsal angle measurement was superior to normal.

Discussion

Os intermetatarseum is a rare accessory bone of the foot, usually found between first and second metatarsal bones [1, 2, 3]. According to several authors the incidence range is 1.2 to 14% [3]. This disparate range of incidence probably is related with heritability, what make this variant more frequent in some populations [1].

Commonly, os intermetatarseum are asymptomatic, but rare cases had been reported as painful, once they caused anterior tarsal tunnel syndrome in relation with overuse or trauma of the dorsal part of the foot [1, 4, 5]. Therefore, mostly they are diagnosed accidentally, as it happened in this case.

The os intermetatarseum can present various forms: free-standing type, a completely independent ossicle with no articular connections with neighbour bones; articulating type, presents a synovial joint with first metatarsal, second metatarsal, first cuneiform or even with two or three of these bones; or fused form, the rarest type of os intermetatarseum, where the accessory metatarsal is fused with first metatarsal base, second metatarsal base or first cuneiform [2].

In first instance, plain X-ray projections are used to find and describe. On the other hand, CT scan gives a better description of os intermetatarseum. Therefore, those are useful tools in the diagnosis of this normal variant. Os intermetatarseum shows up in the image as a bone formation, located commonly between the first and the second metatarsal. The relation with neighbour structures depends on the type.

The fused form of os intermetatarseum, although rare, is the most easily recognizable form because it tend to present long osseous projections [2]. On plain radiographs it could be observed as an apparent exostosis between first and second metatarsal in dependence of the first or second metatarsal base, or even the first cuneiform. In this patient it was observed in dependence of the second metatarsal bone (Figures 1-3). Due to a high overlap of structures in the metatarsal bases, plain radiographs could not distinguish the fused form of other forms of os intermetatarseum. In CT scan, the imaging findings are the same, but we could better describe the relation between neighbouring structures, and also describe with precision the type of os intermetatarseum present, as it is possible to observe in Figures 4 to 7. CT scan also contributes to rule out false positives. MRI, even not performed in this patient, could have an important role in the study of the ligament tissue (tendon-like form) between distal part of the os intermetatarseum and first proximal phalanx.

Differential Diagnosis List

Final Diagnosis

Os intermetatarseum (fused form)

URL:	https://www.eurorad.org/case/13138
DOI:	10.1594/EURORAD/CASE.13138
ISSN:	1563-4086