In-vitro biomechanical comparison of SCREW-PMMA and pedicle SCREW-rod systems in dogs

In veterinary spinal surgery, vertebral instability may occur due to trauma, vertebral pathologies, or surgical interventions. Vertebral instability can result in severe spinal damage and consequent neurological impairment, significantly reducing the patient's quality of life. For this reason, implants for vertebral stabilization are continuously researched and developed in human and veterinary medicine. This PhD thesis aimed to biomechanically compare the pedicle screw-rod system, which has become the gold standard in human treatment, and the screw-Polymethyl Methacrylate (PMMA) system, which is frequently used in veterinary medicine in the thoracolumbar region of dogs. The material of this thesis study consisted of 18 dogs weighing 20 kg and over without vertebral pathology who died naturally or were euthanized. The T10-L3 segment of the dogs was separated from the soft tissues, leaving the important ligament structures intact and stored at -20°C throughout the study. The groups were divided into three randomly selected groups of six specimens: control, screw-PMMA, and screw-rod. Computed tomography was performed before and after the surgical procedures, and anatomical and pathologic findings were evaluated. The control group underwent only laminectomy, and the stabilization groups underwent laminectomy followed by implantation. The specimens were thawed on the test day and moulded with rigid polyurethane. The samples were then subjected to a dynamic flexion-extension moment in a sinusoidal waveform for 50,000 cycles at 3 Hz. After the initial dynamic loading, a static bending moment of 5 mm/min was applied to all specimens until the maximum load value was obtained and failure occurred. The stiffness and range of motion (ROM) values were calculated from the data. The most rigid groups were the screw-rod, control, and PMMA, respectively. The groups with the least range of motion were PMMA, screw-rod, and control, respectively. There was no statistically significant difference between the groups. In conclusion, it was concluded that fixation with four screws was inadequate, especially in large breed dogs, and a rigid fixation system could be obtained by increasing the number of screws to six, ideally eight. Keywords: Biomechanical comparison, Pedicle screw, PMMA, Range of motion