34th Annual Scientific Meeting proceedings

Cervical fusion via ventral stabilization has been used to cervical vertebral compressive myelopathy (CVCM) for nearly four decades.^1-7 CVCM causes extradural spinal cord compression resulting various neurologic grades and eventual loss of athletic function and quality of life for many horses.⁸ Vertebral stabilization aims to fuse together two or more vertebrae to allow for healing as a single, solid bone. When successful, this procedure often results in a reduction or resolution of neurological or pain related clinical symptoms by eliminating painful motion, decreasing inflammation, and a restoration of stability within the spine. 

Interestingly, the first interbody device for spinal fusion was developed for the horse and then translated to humans in the late 1970’s.^1,2  Since that time, a similar metal cylindrical interbody device (Kerf-Cut Cylinder) has continued to be the implant of choice in equine patients, while significant advancements in interbody fusion technology have been made in the human space.⁹ Even though equine cervical stabilization is an accepted surgical treatment for CVCM, very few Surgical Centers around the world offer the procedure. The reason for this is multifactorial and includes 1) controversies related to genetics as well as safety to horse and rider, 2) cost, 3) limited caseload, 4) limited neurosurgical training, and 4) limited access to custom surgical instrumentation and implants.  With the recent growth in the equine sports medicine field, improvements in anatomical and physiological understanding of spinal cord disease processes, and the advent of advanced imaging technologies for improved diagnosis of spinal cord and/or nerve root compression, there is a recent surge in interest and focus on the equine spinal column. As a result of this, we are experiencing a rebirth in the field of equine neurosurgery.

The clinical outcomes across various publications reference improvements in neurological clinical signs in 65-85% of cases.^3-7,10 This percentage is independent of the surgical procedure performed (i.e. Kerf-cut cylinder, locking plates, custom 3D plates, and pedicle screws) which leads one to believe that stability is likely equivalent or atleast sufficient across all techniques described.  Additionally, fusion has been shown to successfully occur in all techniques.  So, if stability is sufficient to result in fusion and with a high percentage of cases improving, where should our focus turn to in order to make the greatest impact on these horses that are no longer considered lost causes or salvage cases? Cervical stabilization techniques that utilize commercially available implants without specialized instrumentation or custom-designed implants (i.e. polyaxial pedicle screws) may result in increased access to veterinary surgeons that are interested in treating horses with CVCM.

Polyaxial pedicle screws are used in human patients to anchor rods to the vertebrae, which stabilizes the spine and promotes fusion. They are commonly utilized for correction of spinal deformities (i.e. scoliosis or kyphosis), spinal fusion and spinal fractures. They are uniquely designed to be inserted into the vertebral body, which is shaped without many flat edges and not very condusive for traditional plating hardware. The “polyaxial” portion refers to the tulip shaped head of the screw that rotates in all directions. This allows the shaft of the screw to have approximately 25^o of motion in all directions. Screws can be placed into bone at varying angles to the tulip head and then secured together with a connecting rod and set screw. By tightening the set screws, the construct is “locked” in place and acts very similarly to an external fixator while still being internal to the body. 

In 2018 a proof-of-concept study in horses showed that a novel cervical stabilization technique utilizing polyaxial pedicle screws and rods was safe and efficacious.¹¹ Furthermore, a clinical trial evaluating 10 horses resulted in greater than or equal to 1 grade neurologic improvements in 6/10 cases and 6/8 cases with greater than 1 year follow up.¹² No catastrophic complications were noted. To date, polyaxial pedicle screws have been utilized safely in over 30 horses for single or double level cervical fusions.  A total of 4 pedicle screws (4.5mm diameter x 30mm or 5mm diameter x 3mm length) are required per fusion level (~ $1,750.00 of hardware per fusion level). Commercially available veterinary screws utilize standard locking plate instrumentation such as a T20 or T30 Star driver and no other specialized instrumentation is required.  From our experience, we have found polyaxial pedicle screws to be simple and user-friendly and highly compatible with the equine ventral spine. The clinical outcomes are similar to traditional cervical stabilization techniques without the need for specialized or customized implants or instruments that may result in limited access to veterinary surgeons.  In this talk, we aim to go in-depth on pedicle screws, explain their function and provide detailed procedural steps for using pedicle screws in equine cervical fusion. 

1. Wagner PC, Bagby GW, Grant BD, et al. Surgical stabilization of the equine cervical spine. Vet Surg 1979;8:7-12
2. Wagner PC, Grant BD, Bagby GW, et al. Evaluation of cervical spine fusion as a treatment in the equine “wobbler” syndrome. Vet Surg 1979;8:84-8
3. Grant BD, Barbee DD, Wagner PC, et al. Long-term results of surgery for equine cervical vertebral malformation. Proc Am Assoc Equine Pract 1985;31:91-6
4. Moore B, Reed B, Robertson J. Surgical treatment of cervical stenotic myelopathy in horses: 73 cases (1983-1992). J Am Vet Med Assoc 1993; 203: 108-12
5. Grant BD, Bagby G, Rantanen N, et al. Clinical results of kerf cylinder (Seattle Slew Implant) to reduce implant migration and fracture in hroses undergoing surgical interbody fusion. Vet Surg 2003;32:499
6. Szklarz M, Skalec A, Kirstein K et al. Management of equine ataxia caused by cervical vertebral stenotic myelopathy: a European perspective 2010-2015/ Equine Vet Educ 2018;30:370-6
7. Kuhnle C, Furst AE, Ranninger E, et al. Outcome of ventral fusion of two or three vertebrae with a locking compression plate for the treatment of cervical stenotic myelopathy in eight horses. Vet Comp Orthop Traumatol 2018;31(5):356-63
8. Reed S, Newberry J, Norton K, et al. Pathogenesis of cervical vertebral malformation in the horse. Proc Am Assoc Equine Pract 1985;31:37-42
9. Cotler JM and Cotler HB, editors. Spine Fusion Science and Technique. Springer-Verlag Publishing, New York 2011
10. Walmsley J. Surgical treatment of cervical spinal cord compression in horses: a European experience. Equine Vet Educ 2005;17:39-43
11. Aldrich E. Nout-Lomas Y, Seim HB, et al. Cervical stabilization with polyaxial pedicle screw and rod construct in horses: a proof of concept study. Vet Surg 2018;47:932-41
12. Pezzanite LM, Easley JT, Bayless R, et al. Outcomes after cervical vertebral interbody fusion using an interbody fusion device and polyaxial pedicle screw and rod construct in 10 horses (2015-2019). Equine Vet J. 2020;54:347-358