Effects of angle and laminectomy on triangulated pedicle screws

We aimed to demonstrate the effect of angle and laminectomy on paired pedicle screws to determine whether a 90° screw angle is optimal as has been previously suggested. According to the angle between right and left screws, 28 calf vertebrae were divided into three groups and instrumented as follows: Group I: 60° screw angle; Group II: 90° angle; Group III: 60° angle with laminectomy. The screws were connected using rods and cross-fixators and tested to peak pullout force. Triangulated pedicle screws provided 76.5% more pullout strength than single screws. Most of the specimens failed through loss of convergence angle (toggling of screws on the rods) and subsequent uni- or bilateral screw pullout. Mean ± SD peak loads were: Group I: 2071 ± 622 N; Group II: 1753 ± 497 N; Group III: 2186 ± 587 N. The differences were not significant (p > 0.05). 90° triangulation was not associated with a superior pullout performance versus conventional 60° triangulation, suggesting that achieving additional triangulation angle is not necessary to obtain increased pullout strength. Laminectomy did not alter the effect of triangulation on fixation strength.