Effect of cyclical loading on the holding power of surgical screws

Although clinical failure of bone screws commonly involve pullout after repetitive lateral loading this mode of failure has rarely been reported previously. The authors examined the effect of cyclical shear force on the fixation stability and holding power of bone screws. The mechanical influence of design variables on the fatigue stability and pullout strength was studied. Different screw designs had significantly (p<0.002) different initial stiffness which was in particular sensitive to the pitch size and depth of thread. For all screws studied degradation of stiffness mostly occurred during first 2000 cycles (43% after 500, 65% after 1000, and 78% after 2000 cycles). Pullout strength was highly reduced due to cyclical loading and the loss was dependent on the screw thread design. Both degradation of fixation stiffness and fatigue damage of bone substance attributed to this loss. Rate of fatigue damage was dependent on the pitch size and decreased as the pitch size was increased. Pitch size of 1.5 mm was the best screw considering high initial holding power and lower rate of degradation with fatigue