Biomechanical comparison of various implants for canine cervical distraction/fusion

Cervical vertebral column stabilization is an emerging treatment option for dogs with cervical spondylomyelopathy (CSM). The original landmark technique, first described by Bruecker in 1989 and evaluated biomechanically by Koehler in 2005, involves the placement of bicortical pins and polymethylmethacrylate (PMMA) across the affected disk space. Placement of these pins occurs from the ventral midline vertebral body and is angled laterally toward the pedicle and part of the lamina, with insertion angles ranging from 30 to 40 degrees reported. Since then, several CT imaging, as well as cadaveric studies, have raised concerns about the safety of bicortical pin placement in the cervical spine in dogs. Violation of the vertebral canal, intervertebral foramen or transverse foramen are likely, increasing the risk for damage to important neurovascular structures.

Few recent biomechanical studies have evaluated alternative fixation constructs for the canine cervical spine. Hicks et al biomechanically compared bicortical pin/PMMA fixation to transverse process screw/PMMA constructs and found both techniques to be equivalent. This technique does not rely on implants placed into the vertebral body. This study also found that bicortical pins had a 92% violation rate, again raising concern about safety with bicortical implant. Agnello et al biomechanically tested the locking compression plate (LCP) for cervical fixation and compared stabilized vertebral motion units (VMUs) to intact and post-ventral slot VMUs. In-vitro results showed that LCP fixation significantly decreased motion at the instrumented VMU. This study did not compare LCP fixation to historical or other techniques. Application of the LCP to the canine cervical spine typically limits the construct to 2 screws per vertebral body and proper pre-operative planning must be employed to ensure that the LCP will fit on the individual patient. Patient size and vertebral anatomy may preclude the use of this particular plate if not enough screws can be placed. Since the locking mechanism leads to screw insertion at a fixed angle, care must also be taken that screws will not violate adjacent intervertebral disks due to the natural slant of the disk spaces. A benefit of this type of cervical fixation is the use of an implant with much lower profile compared to PMMA constructs and the ease of implant removal if necessary.  [...]