A Case of Chronic Foot Pain with a Rare Diagnosis of Synostosis of Second and Third Metatarsals

We describe a case of a 63-year old lady with a history of chronic forefoot pain since the age of 13 years. The patient had her first presentation as a child after sustaining an injury. The pain and swelling persisted for many years, requiring several surgical operations throughout the years.

The patient had several radiological investigations in her lifetime with her pathology being brought to attention by the consultant radiologist. The plain radiograph (figure 1) demonstrated severe osteoarthritis (OA) in the first metatarsal joint (MTPJ). The second and third metatarsal bones appeared fused with abnormal interosseous communication at the level of the shaft and chronic periosteal thickening. Appearances were longstanding. A CT scan (figure 2) was subsequently organised to evaluate this further. The CT scan confirmed moderate OA changes in the first MTPJ with loss of joint space, subchondral sclerosis, osteophytes and subchondral cysts. The CT also confirmed an ‘incidental’ fusion of the second and third metatarsals bones with areas of cortical and marrow continuity consistent with congenital metatarsal synostosis. Mild degenerative OA changes with osteophytosis was noted in the second and third MTPJs.

Metatarsal synostosis is a rare condition with an incidence of less than 1% [1]. The commonest area includes the tarsals, involving the calcaneonavicular and talocalcaneal joints ^[2]. Metatarsal synostosis is exceptionally rare with most cases in the literature reporting the coalition of base of the fourth and fifth or the first and second metatarsal bones [3-5]. The cases reported in the literature are predominantly of children or young adults. Our case is the first to describe synostosis of the 2^nd and 3^rd metatarsal, with other cases describing a fusion of the 2^nd-4^th metatarsal along with tarsometatarsal coalition [4,7]. Interestingly, it is the only reported case in a middle-aged individual with a longstanding history of a painful foot, consequently undergoing multiple surgical procedures.

Histologically there are three types of coalitions; fibrous (syndesmosis), cartilaginous (synchondrosis) and osseous (synostosis), with synostosis further classified as congenital or acquired [7]. Congenital coalitions are formed by the fusion of accessory ossicles and/or from the failure of differentiation and segmentation of primitive mesenchyme. Acquired coalition can occur as a result of arthritis, infection, neoplasm and trauma [8].

The commonest presentation is mid-foot pain. With ongoing stress to the surrounding joints, patients later present with stiffness, osteoarthritis, and joint deformities [9].

Radiologically both Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) are invaluable methods of assessment.  CT scans are used to differentiate the more straightforward osseous coalitions with the non-osseous. The former is demonstrated as continuity between the two bones. The non-osseous coalitions are demonstrated through abnormal narrowing, joint space irregularity, and subchondral cyst formation, easily misinterpreted as degenerative changes risking a delayed or misdiagnosis. MRI is an important tool in differentiating between the three different types of coalitions. In osseous coalitions, the bone marrow signal is continuous across the fused bones, demonstrated as a high signal intensity on T1-weighted images and low signal intensity on T2-weighted fat-suppressed images. The non-osseous coalitions demonstrate non-specific changes [10]. In addition to radiology, the patient’s clinical presentation and examination must be taken into consideration when reaching this diagnosis.

The management of metatarsal synostosis includes conservative management with analgesia, orthotics, with the only curative measure being operative management [10].

Our case illustrates the importance of detecting this abnormality as an important differential of mid-foot pain to avoid any undue investigations and subsequent extensive surgical interventions as noted in this patient.

Written informed patient consent for publication has been obtained.