Discussion
Cranial mid-body axis fractures associated with a disruption of the vertebral canal can be fatal injuries. Due to the relatively confined nature of the vertebral canal through the axis, mild displacement is sufficient to result in spinal cord damage and neurological deficits (Nixon 2020). Although plain radiography remains the most appropriate screening examination for acute injuries of the cervical spine, CT enables more accurate evaluation of fracture displacement and spinal cord impingement (El-Khoury et al. 1995). Standing CT examination should therefore be considered if the procedure can be carried out safely despite the patient’s likely neurological symptoms.
In human medicine, fractures of the axis involving the odontoid process have been classified by Anderson and D’Alonzo in three categories (Anderson and D’Alonzo 1974). Type I fractures involve the proximal tip of the odontoid process. Type II fractures occur at the junction of the dens and the C2 vertebral body. Type III fractures extend into the vertebral body (Niemeier et al. 2018).
Treatment of odontoid fractures aims to re-establish anatomical alignment and provide adequate stabilization of the fracture to enable bone healing. Both surgical and conservative management are common in human medicine (Shears and Armitstead 2008). Surgical management of odontoid fractures includes posterior C1-C2 fusion techniques and anterior screw fixation of the odontoid process itself (Shears and Armitstead 2008). Conservative approaches include the application of a cervical collar (with or without prior traction) and the use of an external fixation device (Halo device) (Shears and Armitstead 2008). Conservative management is only advised for non- or minimally displaced odontoid fractures (displacement inferior to 4mm). Axis fractures with a displacement superior to 6mm have a very high non-union rate when treated conservatively (86%). Consequently, internal fixation is always preferred over conservative management for cases with obvious displacement (Greene et al. 1997). Overall, results from meta-analysis studies have shown that internal fixation results in a higher fusion rate compared to external immobilization (Nourbakhsh et al. 2009).
In equine, splinting techniques have been described to manage fractures of the cranial articulation of the axis, not involving the odontoid process, in foals (Nixon 2020). For all other axis fracture types, especially in adult horses, external copation does not achieve fracture stabilization (Nixon 2020). Although successful conservative treatment of displaced cranial mid-body axis fractures has been reported, conservative management carries a substantial risk of non-union and remains therefore controversial (Florman et al. 2022). Fracture displacement, insufficient stabilization and comminution are amongst the main predisposing factors for non-union (Buckley and Richard 2018). One differentiates between stable and unstable non-unions. Stable non-unions, also referred to as fibrous non-unions, are characterized by the formation of fibrotic tissue that might offer protection from threatening motion (Florman et al. 2022). Stable non united fractures are well aligned and immobile on dynamic imaging studies (Florman et al. 2022). One must keep in mind, that callus formation is not always a sign of successful bone healing. Radiography can show excess bone at the fracture site but if there is no bridging callus at the bone ends, this is referred to as a hypertrophic nonunion (Buckley and Richard 2018). Non-union fractures with marked malalignment and dynamic instability clearly pose a substantial risk of neurologic catastrophe. Even when non-unions appear to be stable, they still carry the risk of delayed myelopathy caused by an excessive callus formation or a loss of stability (Pommier et al. 2020).
An ideal treatment for odontoid fracture in horses should achieve reduction and stable fixation of the fracture to enable bone healing (Vos et al. 2008). Surgical repair of odontoid process fractures has been described in foals using compression plating (McCoy et al. 1984) and Steinmann pin fixation (Owen and Smith-Maxie 1978). To the best of the authors’ knowledge, no successful surgical treatment for cranial mid-body axis fractures in mature horses has previously been reported. Internal fixation has been performed in this case using a 4.5mm equine locking compression T-plate. This implant, recently developed by DePuy Synthes in cooperation with the Large Animal Veterinary expert group of the AOVET foundation, was initially designed for physeal fractures of the proximal tibia in foals (Lischer et al. 2018) and has been successfully used for the internal fixation of tarsometatarsal subluxations, tarsometatarsal and distal intertarsal joint arthrodesis, partial carpal arthrodesis and luxation of the atlantoaxial joint (Curtiss et al. 2018; Keller et al.2015; Lambert et al. 2023; Schulze et al. 2019). The equine T-Plate, which incorporates locking technology, is intended to enhance fixation in short fracture segments (Lischer et al.2018). The three stacked combi holes that are arranged in the head of the plate allow insertion of locking head screws with a length of up to 50mm without interference of the screw tips (Schulze et al.2019). This feature of the plate enabled, in this case, to increase bone purchase in the cranial fragment of the axis.
Creating adequate stability is the main concern when performing internal fixation as it is the limiting factor to achieve the ultimate goal of fracture repair: Early and safe mobilization of the injured area and patient as a whole (Mukhopadhaya and Jain 2019). The 4.5mm T-LCP was used in this case as a bridging “internal fixator”. The authors decided to use almost exclusively locking head screws to privilege construct stability over compression. Locking screws enable angular as well as axial stability, eliminating the possibility for the screw to toggle, slide or dislodge and thereby strongly reducing the risk of postoperative loss of reduction (Wagner 2003). Locking head screws also have better resistance against bending and torsion forces in cancellous and osteoporotic bone compared to cortical screws (Wagner 2003). The second cervical vertebra contains a high amount of cancellous bone (Barone 2009), which supports the use of locking head screws. Since the stability of a locking construct does not rely on compression between the plate and the bone, precise anatomical contouring of the plate is not necessary (Nixon 2020). However, the further the plate is from the surface of the bone, the greater the bending moment on the locked screws, increasing the chance of screw failure and instability (Ahmadet al. 2007). Contouring the plate adds an additional advantage in terms of construct stability: it results in divergent and convergent screw directions enhancing the pullout strength of the screws compared to a construct in which all screws have an identical direction (Gautier and Sommer 2003). Furthermore, using an LCP as an “internal fixator” eliminates the risk of a loss of primary reduction: When locking head screws are tightened, they “lock” to the threaded screw hole, stabilizing the fragments without pulling the bone to the plate. Unlike other screws, locking screws make it impossible for screw insertion to alter achieved reduction (Nixon 2020).