The effect of angled insertion on halo pin fixation.

This study evaluated the effect of angled insertion of halo pins on the biomechanical characteristics of the halo pin-calvarium complex. Halo pins were inserted into isolated calvarium sections at 90 degrees, 75 degrees, and 60 degrees to the surface of the bone at an insertional torque of 0.68 N-m (6 inch-pounds). Initial rigidity, load at failure, and deformation at failure of the pin-bone complex were assessed during transverse shear loading. The structural properties of the pin-bone complex were maximized at loads approaching failure when pins were inserted perpendicular (90 degrees) to the bony surface and significantly decreased at more acute angles of insertion. Perpendicular insertion of halo pins maximizes the structural properties of the complex formed by the halo pin and the calvarium. This improved structural behavior may minimize the incidence of pin loosening clinically, and may reduce the frequency of other complications currently associated with the use of the halo orthosis.