Atlantoaxial instability (AAI)
Atlantoaxial instability (AAI) occurs secondary to disruption of the stabilizing ligamentous structures and associated dorsal displacement of the axis, and may result in acute or chronic compressive cervical myelopathy. Congenital AAI is well described in immature toy dog breeds, and is commonly observed in Yorkshire terriers, Chihuahuas, miniature and toy poodles, Pomeranians, and Pekingese. Traumatic AAI can occur in any breed of dog and at any age, but the majority of cases are associated with an underlying congenital or developmental instability, such as hypoplasia, aplasia or dorsal angulation of the dens, incomplete ossification of the atlas or dens, and loss or absence of atlantoaxial (AA) ligaments.
Management of AAI remains controversial. Conservative treatment with neck brace immobilization for 6 weeks, exercise restriction and avoidance of excessive neck movement for the remainder of the dog’s life has been described. Nonetheless, surgical therapy is often considered necessary and a variety of techniques have been described to achieve decompression of the spinal cord and stabilization of the AA joint. Both ventral and dorsal surgical fixation have been used with comparable success. Ventral procedures described include the use transarticular lag screws, screws or pins with or without polymethylmethacrylate (PMMA) and plates.
Plate and multiple implant fixation have been described with higher success rates and fewer complications. In addition, the use of a mini-plate for the stabilization of AAI was effective in resolving clinical signs in three toy breed dogs.
The ventral AAI hook plate A-LOX, was constructed such that 2 small hooks and 2 screw holes aligned perpendicular to the long axis of the plate were present in the cranial part of the plate. The caudal part of the plate was composed of 2 screw holes aligned in a sagittal plan. The position, the course of the hook and the angle between the cranial and caudal parts of the plate were designed based on anatomic and CT pilot studies to allow reduction with distraction of the atlantoaxial joint.
For fixation with the ventral AAI hook plate, the ventral AA joint capsule is excised. The cartilage of the atlantoaxial joint is then removed. The atlantoaxial joint is distracted and the hooks of the plate are then inserted within the ventral arch of the atlas on both sides of the dens under AA distraction. After ventral AA alignment is restored, the plate is secured with 4 locking screws (2 in the medial margin of the wings of C1 and 2 in the median part of the body of C2).