Small Animal Spinal Disorders: Diagnosis and Surgery

Atlantoaxial subluxation was first reported in dogs nearly fifty years ago. Since that time a better understanding of the aetiologies predisposing to joint laxity and instability has been achieved. Surgeons however are still trying to address the problems associated with stabilizing this joint which by nature is often required in small juvenile dogs. This review describes the various techniques used, discussing the associated benefits and complications thereby allowing the clinician to make an informed decision on the best treatment for the individual patient.

A review of canine atlantoaxial joint subluxation.

Disregarding atlantoaxial instability in toy breed dogs associated with dens malformation and cervical spondylomyelopathy; cervical vertebral malformations are rare and poorly characterised in veterinary medicine and consequently treatment strategies and clinical outcome are sparsely documented. Electronic clinical records at our veterinary referral hospital between April 2009 and November 2018 were searched for patients presented with cervical myelopathy secondary to an underlying suspected vertebral malformation/instability. Nine dogs met the inclusion criteria. Two dogs were diagnosed with atlantoaxial pseudoarthrosis, two dogs with a syndrome similar to Klippel-Feil in humans, two dogs with congenital cervical fusion, two dogs with congenital C2-C3 canal stenosis and deficiencies of the dorsal arch of the atlas and laminae of the axis and one with axial rotatory displacement. Tetraparesis, proprioceptive deficits, cervical hyperesthesia and cervical scoliosis were the most common clinical signs. The axis was the most commonly affected vertebrae (8/9 patients). Patients diagnosed with Klippel-Feil-like Syndrome were the younger (average of 262.5 days) and patients diagnosed with fused vertebrae the oldest (average of 2896 days) in our studied population (average of 1580.8 days). Cervical vertebral malformations are rare, or alternatively, being underdiagnosed in veterinary medicine. Patients diagnosed with Klippel-Feil-like Syndrome had a successful medium and long-term outcome with conservative management. Surgical treatment was often indicated for the other conditions presented in this study due to spinal instability and/or myelopathy. Stabilisations via ventral approaches were revealed to be safe. Multicentre and prospective studies are necessary in veterinary medicine to better characterise clinical outcomes in cervical vertebral malformations.

/pdf/cervical-vertebral-malformations-in-9-dogs-radiological-46n9r96fa1.pdf

Cervical vertebral malformations in 9 dogs: radiological findings, treatment options and outcomes.

The biomechanical properties of the atlanto-axial joint in a young Yorkshire Terrier dog with spontaneous atlantoaxial instability were compared to those of another young toy breed dog with a healthy atlantoaxial joint. The range-of-motion was increased in flexion and lateral bending in the unstable joint. In addition, lateral bending led to torsion and dorsal dislocation of the axis within the atlas. On gross examination, the dens ligaments were absent and a longitudinal tear of the tectorial membrane was observed. These findings suggest that both ventral and lateral flexion may lead to severe spinal cord compression, and that the tectorial membrane may play a protective role in some cases of atlantoaxial instability.

Biomechanical properties of the atlantoaxial joint with naturally-occurring instability in a toy breed dog. A comparative descriptive cadaveric study.

Morphometric and functional study of the canine atlantoaxial joint.

OBJECTIVE: 

To compare the biomechanical properties of a ventral transarticular lag screw fixation technique, a new dorsal atlantoaxial instability (AAI) clamp, and a new ventral AAI hook plate under sagittal shear loading after transection of the ligaments of the atlantoaxial joint.

STUDY DESIGN: 

Cadaveric biomechanical study.

ANIMALS: 

Canine cadavers (n = 10).

MATERIALS AND METHODS: 

The occipitoatlantoaxial region of Beagles euthanatized for reasons unrelated to the study was prepared leaving only ligamentous structures and the joint capsules between the first 2 cervical vertebrae (C1 and C2). The atlanto-occipital joints were stabilized with 2 transarticular diverging positive threaded K-wires. The occipital bone and the caudal end of C2 were embedded in polymethylmethacrylate and loaded in shear to a force of 50 Newtons. The range of motion (ROM) and neutral zone (NZ) of the atlantoaxial joint were determined after 3 loading cycles with atlantoaxial ligaments intact, after ligament transection, and after fixation with each implant. The testing order of implants was randomly assigned. The implants tested last were subjected to failure testing.

RESULTS: 

All stabilization procedures decreased the ROM and NZ of the atlantoaxial joint compared to transected ligament specimens. Only stabilization with transarticular lag screws and ventral plates produced a significant reduction of ROM compare to intact specimens.

CONCLUSION: 

Fixation with transarticular lag screws and a ventral hook plate was biomechanically similar and provided more rigidity compared to dorsal clamp fixation. Further load cycling to failure tests and clinical studies are required before making clinical recommendations.

Biomechanical Evaluation of the Stabilizing Function of Three Atlantoaxial Implants Under Shear Loading: A Canine Cadaveric Study.

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Biomechanical Evaluation of the Stabilizing Function of Three Atlantoaxial Implants Under Shear Loading: A Canine Cadaveric Study.