33rd Annual Scientific Meeting proceedings

Introduction
Several congenital, developmental, degenerative, traumatic, and septic disease processes of the equine atlantooccipital (AO) joint have been described. These include occipitoatlantoaxial malformation, subluxation, osteochondrosis, osteoarthritis, fracture, haematogenous and traumatic septic arthritis and osteomyelitis, or sepsis secondary to extension from guttural pouch mycosis^1-4. Symptoms of AO joint disease include abnormal overextended head carriage and stiffness, localized pain, swelling, and apprehension to handling, riding problems and/or ataxia^1,3.

Limitations of conventional imaging modalities in the AO region have precluded precise visualisation of lesions. However, advanced tomographic imaging techniques like MRI and CT allow accurate diagnosis of AO joint pathology^1,2,4,5. CT arthrograms have visualised the joint outline in all dimensions and identified that joint distension medially protrudes into the spinal canal, displacing the dura to the contralateral side⁶. Communication between the ventral compartments has been reported in older horses.

Dorsal and lateral sites for puncture of the AO synovial cavity have been identified^1,7, and surgical access via an arthroscopic approach has been reported in cadaver necks and one clinical case⁶. Case reports indicate that surgical treatment, including debridement and joint lavage, is more likely than medical therapy to be successful in septic disease processes^2,3,6.

Arthroscopic approach
The AO joint is composed of dorsal and ventral pouches, approximately corresponding to the transition of the angle of the articulating surfaces from craniodorso-caudoventral to caudodorso-cranioventral. The dorsal pouch is arthroscopically accessible.

Using cadaver necks, two arthroscopic approaches to the dorsal pouch have been described; Dorsal: dorsal to the longissimus capitis muscle tendon, directed laterocraniodorsal to mediocaudoventral; Ventral: ventral to the tendon, at the level of the most lateral edge of the atlas wing, directed laterocranio-mediocaudal.

The dorsal approach is associated with a higher risk of penetration of the dura compared to the ventral. Intra-articular positioning can be reliably achieved with ultrasound-guidance.

Intraarticular anatomy
Systematic examination of the dorsal pouch is performed while maneuvering the arthroscope along the dorsocranial occipital condyle and atlas, from medial to lateral, in an approximately horizontal plane, continuing in a ventral direction at the craniolateral articular margin of the atlas. The narrowing of the joint capsule ventrally precludes access to the ventral pouch. Structures consistently visualised include:

1. The convex articular cartilage surface of the dorsocranial occipital condyle
2. The cranial concave articulating surface of the atlas from medial to lateral and ventral
3. The joint capsule insertions to the cranial atlas
4. The synovium and ligaments of the medial and lateral joint outpouchings
5. The ventral narrowing of the joint space

With appropriate instrument portals and arthroscopic triangulation, approximately of 50% of the articular surface of the dorsocranial occipital condyle and 15% of the dorsocranial atlas is accessible⁶.

Clinical experience and application
The expansive joint capsule of the dorsal pouch allows for adequate triangulation and accessibility using conventional equine arthroscopic equipment, enabling debridement of dorsally located pathology, e.g. septic lesions^3,6. One foal with unilateral septic osteomyelitis of the occipital condyle and septic arthritis of the AO joint was treated successfully by arthroscopic lavage and debridement with no post-operative complications and at follow-up 1.5 years post-surgery, the colt had made a full recovery⁶. Inaccessibility of the ventral joint pouch precludes arthroscopic management of pathology in this location, thus limiting the number of potential clinical cases.

The approach to the AO joint is technically challenging, and, if not planned meticulously, introduction of sleeve and trocar may cause iatrogenic cartilage damage or even damage to the spinal cord. It is therefore recommended that the procedure is only performed by experienced arthroscopy surgeons and that ultrasound-guided distension of the joint is performed prior to introduction of the arthroscopic sleeve and trocar.

References

1. Tietje S. Diagnostik und Befunde am Atlanto-occipitalgelenk des Pferdes. Proceedings of the Deutschen Veterinärmedizinischen Gesellschaft. 1996;279-288.
2. Janicek JC, Kramer J, Coates JR, et al. Intracranial abscess caused by Rhodococcus equi infection in a foal. J Am Vet Med Assoc. 2006;228:251-253.
3. Van Galen G, Busoni V, Sandersen C, et al. A case of periarticular abscess and suppurative arthritis of the atlanto-occipital joint. Equine Vet Educ. 2009;21:340-343.
4. Barba M, Lepage OM. Diagnostic utility of computed tomography imaging in foals: 10 cases (2008-2010). Equine Vet Educ. 2013;25:29-38.
5. Gutiérrez-Crespo B, Kircher PR, Carrera I. 3 Tesla magnetic resonance imaging of the occipitoatlantoaxial region in the normal horse. Vet Radiol Ultrasound. 2014;55:278-285.
6. Wright L, Puchalski SM, Kristoffersen M, et al. Arthroscopic approach and intra-articular anatomy of the equine atlantooccipital joint. Veterinary Surgery. 2018;47:756-767.
7. Gollob E, Edinger H, Stanek C, et al. Ultrasonographic investigation of the atlanto-occipital articulation in the horse. Equine Vet J. 2002;34:44-50.