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).