33rd Annual Scientific Meeting proceedings

The articular process joints (APJs) of the cervical spine are paired diarthrodial articulations between the cranial and caudal articular processes of the second to seventh cervical vertebrae. The joint surfaces are oval in shape and lie in an oblique plane to the spinal canal. APJs have a large joint capsule to accommodate neck movement and contain synovial folds which form a normal soft tissue component of the intra-articular space (Thomsen et al 2012). Important soft tissue structures lie in close proximity, with the medial joint capsule protruding into the spinal canal, particularly when the joint is effused. The ventral joint capsule is in close association with the ventral ramus of the associated cervical nerve, which becomes larger in the caudal neck. Mean joint volume is reported as 2.3cm³ at C2/C3, increasing caudally to 3.9cm³ at C6/C7 (Claridge et al 2010), however the joint allows distension beyond this volume. Movement occurs in dorsoventral flexion/extension, axial rotation and lateral bending (Clayton 1989). Additionally, there is craniocaudal translational movement of the joint surfaces in a cranio-caudal direction (Shulze et al 2021).

The arthroscopic anatomy of these joints was first described in cadavers and 3 clinical cases in 2014 (Pepe et al). Needle arthroscopy has been demonstrated in the standing horse to examine the cranial portion of the C5/6 and C6/7 APJs (Peres-Nogues et al 2020). In addition, 8 clinical cases of articular process joint (APJ) arthroscopy have been described (Shulze et al 2021, Tucker et al 2017, Tucker et al 2021), the purpose of surgery being to remove osteochondral fragments or loose bodies in order to relieve neck pain. Today, the technique is being performed in small numbers, but is not in widespread use, most likely due to the limited indication, requirement of CT and unfamiliarity with the technique.

The articular processes of C3/4 to C6/7 can be safely surgically approached from a dorsolateral trajectory via either a cranial or caudal approach, depending on the location of the lesion. Routine arthroscopic equipment and intraoperative ultrasound +/- radiography is required. An overview of the surgical approaches will be described, along with images to demonstrate normal and pathologic intra-articular anatomy. The surgical approach must be image guided and may be made via stab incision or a ‘cut down’, there being advantages and disadvantages to each.   The 3D shape of the synovial cavity of the joint is described by Claridge et al. (2010) providing an indication of the areas accessible using a 4mm diameter endoscope. There is just sufficient space cranially to allow for creation of an arthroscope and instrument portal to enable standard arthroscopic technique, however the presence of relatively large fragments can limit access until after their removal. In the caudal outpouching, access is limited and in our experience, removal of the generally large fragments is more accurately described as ‘arthroscopically guided’ and performed via mini-arthrotomy.

Case examples will be presented, to highlight features of case selection, presenting clinical signs, imaging findings, technique considerations and outcomes. The aetiology of the formation of osteochondral loose bodies will briefly be discussed.

In summary, articular process joint arthroscopy is challenging but can be safely achieved in the horse.  The indications for surgery are few, but the technique offers a definitive treatment for those cases with neck pain caused by the presence of what are often large intra-articular fragments.  It is important that significant additional pathology is ruled out prior to surgery, in order to maximise the chances of treatment success, with CT offering the most accurate method of achieving this, whilst also facilitating surgical planning.

References

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