Biomechanical study of lumbar pedicle screws: does convergence affect axial pullout strength?

We tested the hypothesis that two pedicle screws placed in convergence offer more resistance to axial pullout than do two pedicle screws placed in parallel. Eight fresh cadaveric lumbar spines, L2-L5, were harvested. Individual vertebra were then paired for testing. Into each L2 and L3 vertebra, a pair of pedicle screws were inserted. The screws were placed parallel (i.e., at 0 degrees convergence) in L2 and at 30 degrees convergence in L3, in the first pair. In the second pair of L2 and L3, this order was reversed. Alternating the convergence angle was carried out on each successive pair of L2 and L3. A section of standard longitudinal rod was attached to each pedicle screw by using top-loading three-point shear clamp fixation with tangential lock screws. Transverse connectors were attached superiorly and inferiorly to the longitudinal rods. As in L2 and L3, each L4 and L5 received a pair of screws placed at either 0 or 30 degrees convergence, with the angle of convergence alternated on each successive pair. Each pair of screws in each vertebra was tested in axial pullout at 1 mm/s. Fourteen pairs of vertebra were tested and two findings emerged: paired pedicle screws at 30 degrees of convergence offered more resistance to axial pullout (28.6% on average) and sustained higher loads at the clinical threshold of loosening (101% on average) than paired pedicle screws placed in parallel. Thus there seems to be an advantage to screws placed in 30 degrees of convergence as compared with screws in parallel.