Introduction
Isolated Lateral Talar Process (LTP) fractures are rare injuries, first reported by Marottoli in 1942, as quoted by Nicholas et al. [1]. Hawkins in 1965 [2], classified LTP fractures into three types. Type I consisted of a single large fragment. Type II was a comminuted fracture whereas; type III was a small or "chip" fracture of the tip of the LTP (Fig. 1). Another classification widely used is the one proposed by McCrory and Bladin in 1996 [3], who subdivided LTP fractures in chip fractures (type I); large-fragment fractures (type II) and comminuted fractures (type III).
These injuries were generally associated with motor vehicle accidents, falls from a height or inversion injuries [2]. Starting from the 70s, as snowboarding became more popular, an increased incidence of LTP fracture was reported in athletes participating in this sport. Kirkpatrick et al. [4] prospectively documented that in 3213 snowboarding injuries 2.3% were LTP fractures. The unexpectedly high incidence of LTP fracture in snowboarders led to the term “snowboarder’s fracture” [1].
On physical examination, patients with an LTP fracture usually present with point tenderness and marked swelling on the lateral aspect of the ankle, just distal to the lateral malleolus. On standard radiographic examination, the fracture is difficult to be appreciated and needs a certain amount of awareness in order to identify it [5]. As a consequence, this injury is frequently misdiagnosed as a lateral ankle sprain and overlooked at the initial presentation [3,6,7].
As far as the treatment is concerned, if the fracture is not displaced, it may be treated with immobilisation in a boot or cast for 6-8 weeks [2]. For comminuted or displaced fragments more than 2 mm, surgical reduction and fixation of the fracture should be attempted [8-10]. If this is not possible, early excision of the fragment(s) should be performed [7,10]. Late or missed treatment, nonunion, malunion, and overgrowth are associated with poor outcome resulting in pain, functional impairment and subtalar osteoarthritis [7].
On the other hand, isolated Sustentaculum Tali (ST) fractures are also uncommon and often missed upon the first presentation [11-13]. Due to the strong trabecular structure and thick cortical bone, solitary fractures of the ST without additional calcaneal injuries occur in less than 1% of all calcaneal fractures [14]. More frequently, they are associated with fractures of the medial facet of the subtalar joint, subtalar dislocations, or they are a part of more complex os calcis fractures [15,16].
Patients with ST fractures present with pain on the medial aspect of the hindfoot just distal and anterior to the medial malleolus. Pain might be elicited by passively moving the great toe. In standard radiographs, it is difficult to diagnose an ST fracture [17]. Therefore, a high index of suspicion is needed, especially, in patients with a history of subtalar dislocation, talar fracture, mid foot injuries or a fall from a height. The diagnosis is finally made by performing a CT scan, which not only helps identify the Sustentaculum fracture but also identifies additional injuries. Surgery is indicated if the fragment is displaced more than 2 mm, if the medial facet is depressed, if there is a tendon entrapment or if the fracture involves also the posterior facet of the calcaneus [14].
Materials and methods
Between August 2010 and May 2017, at our institution, we assessed four patients (all male) who sustained an LTP fracture associated or not with an ST injury.
They all complained about pain and inability to bear weight on their injured leg. After documentation of the patients' demographic data and side of the injured foot, the mechanism of injury was inquired. A patient reported a twisting injury to his foot while playing football and three patients reported an axial impact of their foot after a fall from a height. Physical examination revealed marked swelling as well as point tenderness around the region of the lateral malleolus. The posterior tibial and pedal pulses were present, and no neurologic deficit was recorded. Standard anteroposterior and lateral radiographs of the ankle were taken. Due to irregularities at the contour of the LTP in both views, a CT scan was requested (Fig. 2). On the CT scan, a large displaced McCrory-Bladin type II LTP fracture was noticed in two patients. In one of them, due to the presence of a talar beak sign, further MRI was performed to rule out a concomitant tarsal coalition. In the other two patients, CT scan helped diagnose a combined injury; a comminuted relatively undisplaced McCrory-Bladin type III LTP fracture in association with a large ST fracture in one patient and a severely comminuted McCrory-Bladin type III fracture with a concomitant small avulsion ST fracture in the other. Their foot was temporarily immobilised in a back slab and placed on a Brown’s splint.
All patients underwent surgery as soon as the swelling has subsided, no more than ten days. A tourniquet was placed at the thigh and was inflated at 300 mmHg. Prophylactic antibiotic was administered before the induction of anaesthesia.
Of the four patients, in three (75%), the LTP was accessed openly through a lateral hockey stick incision, with the patients in a lateral decubitus position. In the patient with the associated tarsal coalition, the fragment was reduced and fixed with a staple, whereas, in the other patient with the McCrory-Bladin type II LTP fracture, the fragment was anatomically reduced and fixed with two cortical 1.5 mm mini-fragment screws.
In the patient with the comminuted LTP and the combined ST fracture, the LTP fragments were minimally displaced and adequately big to consider internal fixation with three cortical 1.5 mm mini-fragment screws. The concomitant ST fracture was then stabilised with three cortical 1.5 mm mini-fragment screws.
Finally, in the patient with the severely comminuted McCrory-Bladin type III fracture, the LTP fragments were excised endoscopically. Standard posterolateral and posteromedial portals were created according to van Dijk to access and remove an oversized os trigonum [18]. An accessory lateral middle portal just distal and anterior to the tip of the fibula was created on the lateral foot as described by Frey et al. [19] to remove the fragments of the fractured LTP (Fig. 3). The concomitant ST avulsion fracture was considered too small to be removed or fixed.
Post-operatively, a back slab was applied, and prophylactic anticoagulation (Innohep 0.45 [Tinzaparin]; LEO Pharmaceutical Inc) was administrated for six weeks. At discharge, the back slab was exchanged with a full cast, and the patients were ordered not to bear weight. After three weeks, a walking boot was applied and partial weight bearing of 15 to 20 kg was commenced. Patients began range-of-motion exercises avoiding inversion and eversion of the hindfoot. Progression to full weight bearing and muscle-strengthening exercises begun six weeks after surgery.
Results
All patients were assessed at 3, 6 and 12 months after operation, and annually thereafter. The mean follow-up time was 27 months (range, 2-60 months). Patients were assessed clinically (pain, ankle and subtalar ROM) and radiologically. Evaluation of functional result was done using the American Orthopaedic Foot and Ankle Society (AOFAS) hindfoot score and the Foot and Ankle Disability Index [20,21].
At the most recent follow-up, the mean AOFAS score was 93.6 (range, 87 to 100) and the mean FADI score was 89.3 (range, 82.4 to 99; Table 1). One patient (33.3%) was extremely satisfied with the functional result, as he returned to the same level of activities before injury, without reporting pain, swelling, or subjective limitation of hindfoot motion. The other two patients (66.6%) were very satisfied with the outcome, as they returned to their usual activities with having mild, occasional pain only during their recreational activities.
At radiological evaluation, in all patients, fractures appeared united within six months after the surgery.
At their latest follow-up (2 and 5.5 years after the injury) mild osteoarthritic changes at the talofibular joint and the medial talocalcaneal facets were observed (Fig. 4).
Discussion
An LTP fracture associated with ST fracture is a very rare injury, and there are only isolated references in a small number of case series.
F. von Knoch et al. [22] in 2007 reported one combined injury in 23 snowboarders with an LTP fracture. Mark Gatha et al. [23] in a small case series of 4 patients that sustained an ST fracture reported one combined LTP fracture. Dürr et al. [14] in 2013 reported that over the course of 15 years, they treated operatively 31 patients for ST fractures. Accompanying injury to the LTP was seen in 23% of these patients. In our series, half of the patients had a combined fracture, although, no safe conclusions can be drawn, as the number of our cases is limited. Larger scale studies or retrospective analysis of existing series might reveal an increased incidence of this combined entity.
As far as the mechanism of the combined injury is concerned, it seems to be multifactorial and not fully defined. In literature, there are few reports describing the mechanism of each fracture in isolation and only scarce references of their association.
Sustentaculum Tali is the most stable part of the calcaneus, and high energy is needed in order to be fractured. It is a general belief that isolated ST fractures occur from axial loading and inversion of the hindfoot. Wuelker and Zwipp [13] by studying the fracture anatomy of the axially loaded calcaneus observed that with the hindfoot in inversion (varus) an isolated fracture of the sustentaculum could be produced. Gatha et al. [23] also report that the mechanism of injury seems to involve high-energy axial and varus loading with some component of rotation.
More controversy exists regarding LTP fracture. Dimon [24] postulated that a possible mechanism involves compression caused by dorsiflexion and slight external rotation of the foot. He suggested that these movements lock the posterior facet of the calcaneus behind the corresponding facet of the talus, and, as dorsiflexion of the foot continues, the anterolateral portion of the articular surface of the talus is sheared off and displaces. In addition, Huson [25] pointed out that heel inversion causes a lateral shift of the head of the talus and an upward shift of the lateral process of the talus on the posterior articular process of the calcaneus. The joint surfaces of the posterior articulation are no more congruous, the subtalar joint opens and, if the foot is forced into dorsiflexion while it is in this position, a force is exerted on the lateral process.
Based on this study, Hawkins [2] formulated the suggestion that lateral process fracture is caused by forced compression of the talus when the inverted foot is severely dorsiflexed. This explanation was also shared by other future clinical and anatomical studies.9,11,20 However, Boon et al. [26] in their anatomical study, demonstrated that also external rotation was a key factor in producing this type of fracture.
On the other hand, Funk et al. [27] refuted the consolidated mechanism of the involved injury. By subjecting dynamic inversion or eversion to ten axially loaded and dorsiflexed cadaveric leg specimens, they suggested that eversion and not inversion was necessary to produce an LTP fracture. They also stated that Boon’s results were non-contradictory to theirs. Eversion and external rotation of an axially loaded dorsiflexed ankle may be independent injury mechanisms for an LTP fracture. Indeed, they explain that during a fall in real-world traumas, forces and torques will be applied to the ankle along continually changing non-Cartesian vectors, and torque about a combined eversion/dorsiflexion/external rotation axis is possible.
It can be concluded, therefore, that the mechanism of injury of the combined fractures resembles a subtalar joint subluxation. As Huson noted in 1961 [25], heel inversion causes a lateral shift of the head of the talus and incongruity of the posterior subtalar joint articulation. He suggested that, if the foot is forced into dorsiflexion while it is in this position, an LTP fracture may occur. However, as Boon et al. [26] stated, dorsiflexion and inversion in an axially loaded foot is not enough to produce an LTP fracture, but when the talocalcaneal congruency is disrupted, an external rotation force is also needed.
On the other hand, Funk et al. [27] in their cadaveric study noted that by subjecting their specimens in axial loading, eversion and ankle dorsiflexion, all resulted LTP fractures were intra-articular (McCrory-Bladin type II, III). Since the aforementioned fractures involved the posterior talocalcaneal joint surface, they postulated that these fractures have been caused by localised compression of the subtalar joint surface beneath the lateral process. Interestingly, in their experiments, no extraarticular LTP avulsion fractures were produced (McCrory-Bladin type I), probably because another mechanism of injury is needed to cause this type of LTP injury.
Based on these observations, we suggest that the combined fracture of the LTP and ST may result from two possible mechanisms. In both mechanisms, the common key is the forced axial loading, as from a fall from a height, motor vehicle accident or sports injury. If then, the axially loaded foot is subjected to continuous inversion, an ST fracture happens first, resulting in spontaneous subtalar joint subluxation. By applying more inversion, dorsiflexion and external rotation, the LTP could also fail (McCrory-Bladin type I). Another possible mechanism may involve continuous eversion in an axially loaded dorsiflexed foot. This time, by exercising compression on the subtalar articular surface, the LTP could fail first (McCrory-Bladin type II, III), leading again to subtalar joint subluxation. If the oblique axial force continues, then an ST fracture may occur.
Conclusion
Isolated ST and LTP fractures are not common entities in clinical practice and literature. Moreover, they are frequently missed and misdiagnosed as ankle sprains. Recently, the awareness of these two fractures has inclined due to the introduction of sports such as snowboarding and the increasing number of road traffic accidents. Nevertheless, their combination is still met only in isolated cases.
The mechanism of this combined injury is not clear yet. It seems that the common denominator of these injuries is forced axial loading and subtalar subluxation. Indeed, in all cases, a loss of talocalcaneal congruity, leading to subtalar instability and subluxation, is needed to produce this entity. Once the articular surfaces start to move, if the force responsible for the instability continues to exert, the combined injury might occur. We propose that ST fractures in association with a McCrory-Bladin type I LTP fracture may be caused by continuous inversion in an axially loaded, inverted, dorsiflexed and externally rotated foot. On the other hand, ST fractures in combination with a McCrory-Bladin type II or III LTP fracture may result from continuous eversion in an axially loaded and dorsiflexed foot. In reality, the subluxation of the subtalar joint creating the combined injury might be more common than generally thought. In our series, half of the patients had a combined fracture, whereas, as mentioned before, Dürr et al. [14] in 2013, reported that almost a quarter of their patients had an association of an ST and an LTP fracture. Thus, when an LTP fracture is encountered, a meticulous study of the CT scan images is indispensable, in order not to miss a possible ST fracture and vice versa.