Teaching Case

Α 45-year-old man, trained in martial arts, was referred to our hospital with 1 month ankle pain. Physical examination revealed swelling of the ankle without any inflammation sings. Imaging studies were requested with the suspicion of an osteochondral lesion.

No fracture was evident at plain radiography. A poorly defined os naviculare was detected (Fig. 1). The CT examination did not show any fracture or osteochondral talar lesion. An anatomical variant consisting of a multiossicle Type 1 os naviculare was shown (Fig. 2, 3).
Due to non-specific findings to explain the persistent ankle pain, an MRI examination was performed with a 1.5 Tesla scanner. MRI showed bone marrow oedema located in the anterior talus which enhanced following intravenous administration of gadolinium contrast (Fig. 4). No fracture line or other subarticular focal lesion was disclosed within the oedematous area. In both CT and MRI, the os naviculare was located eccentrically within the distal course of the posterior tibialis tendon.
In conclusion, pain was assigned to transient osteoporosis of talus (transient bone marrow oedema syndrome) and the respective anatomic variant was an incidental finding.

A. It is accepted that the accessory navicular bone (AN) is an anatomic variant. AN represents failure of the navicular secondary ossification centre to unite during childhood. There are few reports supporting that AN may be inherited. AN is considered as the most common accessory bone of the foot [1, 2]. It is classified in 3 types. Type 1 is a small, accessory bone usually located in the medial posterior aspect of the navicular within the posterior tibialis tendon (PTT). Type 2 is a larger bone with an irregular shape which is attached to the navicular with a synchondrosis. Type 3 is a fused accessory bone, which is producing a cornuate navicular [3]. A multiossicle appearance AN has been recently reported, as it was in our case [1].

B. Type 1 and 3 are usually asymptomatic. In contrast, Type 2 is often symptomatic. Young athletes may present with a symptomatic AN during sports activities. In this clinical scenario, the physical examination typically reveals tenderness over the prominence on the medial aspect of the midfoot. Symptomatic AN is typically seen in childhood or in early adulthood as a result of pressure of the accessory bone against shoes. Types 1 and 2 AN are associated with a progressive flattening of the longitudinal arch [4].
AN could be seen incidentally in routine radiologic examinations. The 45ο external oblique view is required in order to better differentiate the AN from a small fracture. In the presence of pain and tenderness in the medial aspect of the foot, the proper interpretation of the imaging studies will obviate further imaging or inaccurate treatment [3]. Radiologist should suggest the clinical impact of an anatomic variant. In the case presented herein, the multiossicle os naviculare type I, was irrelevant to the clinical symptoms.

C. Type 1 AN is a small ossicle within the distal PTT. On CT and MRI, the presence of the os within the tendon is diagnostic.

D. AN type I requires no treatment if asymptomatic. If symptomatic, preoperative MRI will guide the tendon reconstruction. In our case, pain was attributed to bone marrow oedema and thus treatment was focused to alleviate weight bearing.

E. The multiossicle appearance of AN type I, has been recently reported. Its presence is unlike to cause symptoms. Proper recognition of this entity will obviate unnecessary therapeutic procedures.