An incidental finding of an os sustentaculi with a concomitant talocalcaneal synchondrosis after a fracture of the internal malleolus in an 11-year-old child: A case report

The sustentaculum tali is a bony eminence at the medial surface of the calcaneus that articulates with the middle calcaneal articular surface of the talus and provides an important support for the medial column of the foot. Its upper articular surface is concave, and its lower surface has a groove for the tendon of the flexor hallucis longus. The sustentaculum tali gives attachment to the plantar calcaneonavicular (spring) ligament on its anterior margin and the tibiocalcaneal ligament and the medial talocalcaneal ligament (part of the deltoid ligament) on its medial margin [1-4].


Introduction
The sustentaculum tali is a bony eminence at the medial surface of the calcaneus that articulates with the middle calcaneal articular surface of the talus and provides an important support for the medial column of the foot. Its upper articular surface is concave, and its lower surface has a groove for the tendon of the flexor hallucis longus. The sustentaculum tali gives attachment to the plantar calcaneonavicular (spring) ligament on its anterior margin and the tibiocalcaneal ligament and the medial talocalcaneal ligament (part of the deltoid ligament) on its medial margin [1][2][3][4].
The os sustentaculi (OS) is a rare accessory bone located at the posterior end of the sustentaculum tali which is usually attached to the calcaneus by fibrous or fibrocartilaginous tissue. The first report of this accessory bone was published by Pfitzner in 1896 [5]. Since then a few case reports have appeared discussing the clinical manifestations and radiographic features of this bony variant [6][7][8]. We report on the case of an unusual medial malleolar fracture in a patient presenting an OS with a concomitant talocalcaneal synchondrosis. Due to subtalar motion restriction an abnormal force may have been applied to the ankle mortise causing the atypical fracture.

Case report
An 11-year-old boy was admitted to our hospital after a right ankle sprain. The mechanism of injury was a combination of eversion movement and external rotation. He immediately complained of severe pain over the medial malleolus. Clinical examination determined swelling and bruising on the medial side of his ankle and full weight bearing was impossible. Anteroposterior and lateral radiographs revealed a fracture of the anterior aspect of the medial malleolus. A CTscan was conducted to accurately evaluate the size and the morphology of the fracture. The exam confirmed the radiographic findings but revealed also an os sustentaculi with a concomitant incomplete talocalcaneal coalition (Figures 1 and 2).
Open reduction and internal fixation of the malleolar fracture was performed under general anaesthesia using an anteromedial approach; the fragment was stabilized with a 2.7 mm screw. The ankle was then immobilized in an under-the-knee cast for 6 weeks, followed by physiotherapy instructed rehabilitation.

Discussion
Talocalcaneal coalition (TCC) is defined as a union between the talus and the calcaneus, usually associated with an abnormal hypertrophy of the medial aspect of the talus and the sustentaculum tali. TCC is diagnosed in less than 1% of the general population [8]. However, as the bony variation is often asymptomatic the real prevalence is probably much higher [9]. In the paediatric population, talocalcaneal coalition is of congenital origin and results from a failure of mesenchymal segmentation of calcaneus and talus [10]. Usually, this condition becomes symptomatic in the second decade of life [11][12]. A TCC can present itself as a synostosis, as a synchondrosis, or as a syndesmosis [13][14][15][16]. Furthermore, it can be classified according to its location, as intraarticular (affecting either anterior, medial, or posterior facets) or extraarticular (usually posteromedial) [17][18]. Recently, a new classification of TCC has been established using multiplanar CT or MRI scans [16]. Surprisingly, the scans showed that an os sustentaculum occurred more often in conjonction with a TCC than previously thought. Furthermore, the coalition was frequently determined between the sustentaculum tali and the os sustentaculum, and between the os sustentaculum and the talus [16] as was the case in our patient.
The mechanical characteristics of the ankle are complex [19][20][21], due to the two contributing joints. The principal motion of the talocrural joint is in the sagittal plane, whereas the subtalar joint moves in the frontal plane [10]. The subtalar complex is defined as a polyaxial articulation, whose range of motion accounts for 70% to 90% of the overall range of motion of the ankle complex [19]. Therefore, patients suffering from subtalar joint coalitions display a severe restriction of subtalar joint motion. In the absence of subtalar joint motion an excessive rotational stress is exerted on the talocrural joint, especially during inversion movements. This mechanical stress may cause peroneal spastic flatfoot [22][23][24] or may result in the development of a ball and socket joint at the level of the talocrural joint [10]. It is also known that a motion restriction at the subtalar joint increases the mechanical stress on collateral ligaments of the ankle which can result in frequent ankle sprains [9].
The injury mechanism in our patient was classified as pronationeversion-external rotation stage I in the Lauge-Hansen classification. The morphologic features of the fracture corresponded to an avulsion fracture of the anterior tibiotalar and tibionavicular ligaments from the anterior aspect of the medial malleolus. It is unanimously recognized that the main cause of an isolated injury of the anterior tibiotalar and tibionavicular ligament is pronation and external rotation of the hind & midfoot. The TCC in our patient restricted the range of motion in the subtalar joint and we therefore believe that the rotational force was not absorbed properly. Thus, the excessive rotational force may have been transmitted to the talocrural joint, leading to an overload of the superficial deltoid ligament, which resulted in an avulsion fracture of the anterior aspect of the medial malleolus.
In our review of the published data, we found only a few studies that described unusual fracture patterns of the ankle in patients presenting with a TCC. Imade et al reported the case of a 16-year-old male who experienced a bimalleolar fracture of the ankle, classified as a supination-adduction type in the Lauge-Hansen classification [25]. Compared to the typical supination-adduction type fracture, the authors considered that the TCC may have caused more severe injury as the medial malleolar fracture was positioned more laterally and had changed to a spiral fracture [25]. Godoy and al reported the case of 53-year-old female who sustained an isolated intraarticular tibial pilon fracture, without lesion of the medial and lateral malleoli, and intact syndesmosis. In this case, computed tomography revealed both talocalcaneal and talonavicular coalitions [10].
In conclusion, we consider that patients with TCC should undergo a full radiographic evaluation as altered biomechanics can cause atypical fractures and increase the risk of more significant ankle injuries.

Conflicts of Interest
We have no conflict of interest to declare.

Funding Statement
The authors received no specific funding for this work.