Predrilling does not improve the pullout strength of external fixator pins: a biomechanical study

External fixator pins are thought to have improved pullout strength if inserted into both cortices and if pilot holes are drilled. We tested these hypotheses and investigated effects of osteoporosis and pin location on pin pullout strength.

Self-drilling external fixator pins were inserted using 4 techniques: 1) predrilled bicortically inserted, 2) not predrilled bicortically inserted, 3) not predrilled unicortically inserted, and 4) technique 2 with far cortex disengaged to test holding power of near cortex in isolation. Ninety-six samples were tested from four locations in 12 matched pairs of cadaveric femora [6 pairs osteoporotic (T ≤ -2.5), six pairs nonosteoporotic (T > -2.5)]. Pullout force was tested on materials testing system until failure of pin-bone interface.

Bicortical pin placement and location within femur had substantial effects on the holding power of the pins. Osteoporosis had statistically significant effect but with smaller effect size (P = 0.05). No significant difference in holding power was observed between predrilling and no predrilling techniques (P = 0.27). We observed only a trend toward "stripping effect" in near cortex with technique 4. Bicortical placement rendered 69% greater holding power than unicortical. Proximal placement rendered 56% greater holding power than distal. Osteoporotic samples had 15% less holding power than nonosteoporotic samples.

Single-cortex pin placement, osteoporotic bone, and more distal pin location decreased pin pullout strength. We did not observe a significant mechanical advantage of predrilling for modern external fixator pins. The near cortex of a pin inserted into 2 cortices without predrilling did not exhibit substantial stripping effect.