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Case Report: Traumatic Fracture of the Axis with concurrent Atlantoaxial Subluxation in a Labrador Retriever

fig. 1— lateral radiograph of cranial cervical spine. There is stepping of the neural canal with dorsal displacement of the main portion of Ce2.

A six-month-old, male, entire, Labrador Retriever presented with peracute onset tetraplegia following a collision with another dog five hours previously.  The referring veterinarian had noted increased tone in the thoracic limbs, transient anisocoria and poor pain perception in all four limbs, but mentation had remained normal.

Examination

General physical examination was unremarkable apart from guarding of the cervical spine.  There was evidence of voluntary movement of the tail, but complete plegia of all four limbs.  Muscle tone was increased and segmental spinal reflexes were brisk in all limbs.  In addition, mentation and cranial nerves were normal, suggestive of a spinal cord lesion in the Ce1-5 segments.  Superficial pain perception was present by the time of the author’s assessment.

Further Investigations

fig. 2 — ventrodorsal radiograph cranial cervical spine. Note; the dens and atlantoaxial articulation appear intact. The fracture is difficult to visualise on this view. The ET tube is still in situ; optimal imaging was sacrificed so as to minimise the risk of further cord trauma.

Survey spinal radiology was performed (figs. 1 & 2) , taking great care to keep the cervical spine and head supported throughout the entire anaesthetic period.  This revealed fracture of the vertebral body of the axis caudal to the cranial articular facets.  The dens and atlantoaxial articulation appeared intact, whereas the remainder of the axis was displaced dorsally consistent with rupture of the interarcuate ligament.  There was significant stepping of the neural canal.

Treatment

fig. 3 — postoperative lateral radiograph showing reduction and ventral stabilisation with screws and cement

Emergency stabilisation was achieved via a standard ventral, midline approach1,  taking care to preserve the vascular pedicle to the right thyroid gland.  The fracture was reduced using a combination of fine Hohmann retractors and a feline stifle distractor; rotation/ distraction was achieved by placing one tip of the retractor into the cranial metaphyseal fragment of the axis and the other tip into a hole drilled into the ventral surface of the caudal fragment.  There was significant haemorrhage from the venous sinuses at this point, which ceased on successful reduction of the fracture.

The reduced fracture was stabilised with four diverging, 2.4mm titanium screws placed within each fragment.  These were then linked with a plug of bone-cement.  Exogenous bone graft (Osteoallograft Orthomix – Fine) was also placed around the fracture and cement plug to promote callus formation.  The surgical wound was closed routinely.  Postoperative radiology (fig. 3, left) revealed successful reduction.  However, the two most caudal screws appeared to breach the neural canal; in order to assess any potential compromise of neural structures, postoperative MRI scans were obtained (fig. 4, below).  These revealed fat-signal dorsal to the cord, indicating no on-going compression.

Outcome

fig. 4 — transverse T1W MRI slice at level of caudal fixation screws:A linear void representing a screw can be seen traversing the vertebral body ventral to the neural canal, but the fat signal dorsal to the cord is still present, indicating no significant compression of the cord at this level.

The patient regained voluntary motor limb function within 12 hours of surgery and was ambulatory after three days.  Exercise was restricted for three weeks after surgery, before a further three week program of increasing chest-harness lead walks.  At the time of writing, mild proprioceptive deficits remain in the right pelvic limb.

Discussion

Atlantoaxial subluxation usually presents in juvenile toy-breeds secondary to hypoplasia/ fracture of the dens or failure of the dorsal interarcuate ligaments.  Nevertheless, owners will often report an inciting, minor, traumatic event, such as dental work or flexing of the neck during a recent physical examination (e.g. to investigate neck pain—see last edition of Downs Diary).  In contrast, supraphysiological derangement of the anatomically normal cranial cervical spinal column is rarely seen in practice, let alone managed surgically; most cases that present with significant neurological impairment as a result of unstable fracture/ luxations of this region of the spine suffer respiratory arrest at the time of injury or shortly thereafter when moved from the scene of the accident.

Preoperative MRI was not performed in this case to minimise the risk of displacing potentially unstable fragments; advanced neuroimaging was not thought likely to yielded any additional information over and above what was gained from the clinical assessment and orthogonal radiographic views. However, non-ferrous implants should ideally be used in all, non-routine spinal surgery so as to permit diagnostic postoperative scanning where indicated.

At the time of fracture planning, consideration was given to dorsal stabilisation of the Ce1-2 subluxation.  This can be acomplished by threading orthopaedic wire from the spinous process of Ce2, to encircle the dorsal arch of Ce1, thus mimicking the ruptured interarcuate ligament2.  However, the passage of wires along the neural canal risks iatrogenic cord trauma and in this age of patient, rapid fracture healing was anticipated by ventral compartment stabilisation alone.

Summary

Surgery is rarely indicated in cases of traumatic, cranial cervical spinal cord injury; the majority of such patients present with stable, concussive/ contusion-type lesions which are usually amenable to conservative management.  However, a successful outcome is possible with unstable fracture/ luxations with timely, careful assessment and stabilisation (including appropriate management at the onset of signs).

References

1.  Piermattei D.L. (1993) An Atlas of Surgical Approaches to the   Bones and Joints of the Dog and Cat  London, Saunders 46-9.

2.  Sharp NJ & Wheeler SJ (2005) Atlantoaxial subluxation in   Small Animal Spinal Disorders.  Diagnosis and Surgery p.161-  180 Elsevier Mosby, London.