The Orthopaedic Innovation Centre in Canada performs biomechanical testing to determine optimal lateral mass screw fixation method in cervical spinal fusion
Cervical spinal fusion is a surgical procedure that joins two or more adjacent vertebrae in the neck to treat severe pain, disorder, trauma injury, and other spinal illnesses. A posterior fusion of the subaxial cervical vertebrae is routinely performed with screws implanted into the lateral mass regions of these vertebrae and joined with spine rods. The success of interbody fusion is greatly dependent on the level of screw fixation. Screw fixation can be affected intraoperatively by screw design, path of trajectory, purchase depth, and bone quality. However, achieving optimal screw fixation in cervical spine presents several challenges due to the small size of the lateral mass and its anatomic proximity to the vertebral artery and nerve roots. The importance of screw fixation is critical, as screw-bone failure may result in catastrophic neurologic injury and major spinal revision surgery.
Lateral mass screw fixation has been investigated by the biomedical engineers at the Orthopaedic Innovation Centre (Winnipeg, Canada) and the orthopaedic surgeons and residents from the Department of Surgery at the Health Sciences Centre and University of Manitoba (Winnipeg, Canada). They have combined orthopaedic practice with biomechanical testing to determine optimal screw fixation in subaxial cervical spinal fusion. Screw design, trajectory angle, and depth of fixation were investigated.
Research testing involved the use of artificial bone modelling (figure 1) to simulate clinical conditions (figure 2). Lateral mass screws were tested in bi-cortical cancellous filled phantom bone blocks with material characteristics similar to cervical spine. Screw fixation was determined through mechanical testing (figure 3a) by measurement of maximum axial load (N) to pull-out failure, defined as screw pull-out from bone or bone fracture (figure 3b).
Figure 1: Mechanical axial pull-out test set-up to measure maximum load to screw pull-out failure.
Figure 2. Clinical representation of lateral mass screw trajectory as demonstrated in figure 1.
Figure 3a. Mechanical axial pull-out test set-up to measure maximum load to screw pull-out failure.
Figure 3b. Maximum load-to-failure of lateral mass screw, resulting in bone block fracture.
Cortical and cancellous lateral mass screws were tested for fixation in Roy-Camille and Magerl trajectories for three purchase depths