A comparison of various angles of halo pin insertion in an immature skull model

Study Design. A basic science biomechanical study involving an animal model . Objectives. To evaluate the effect of varying angles of halo pin insertion on the force generated at the pin-bone interface, and thereby the stability of the halo pin-bone interaction during insertion. Background Data. Because of variations in the shape and size of the pediatr ic skull, halo pins often are inserted at various angles rather than perpen dicular to the skull. Concern exists that the high complication rate associ ated with pediatric halo use may result in part from less than ideal struct ural properties at the halo pin-bone interface. Methods. The authors used a fetal calf skull model to simulate the thicknes s and structural properties of the pediatric skull. Halo pins were inserted at angles of 0 degrees (perpendicular), 10 degrees, 15 degrees, and 30 deg rees into skull segments via a halo ring. Load generated at the pin-bone in terface was measured using a modified mechanical testing device. Twenty tri als were conducted per angle, with the endpoint being specimen failure, pin penetration, or maximum load. Results. Mean maximum loads per unit thickness were 82.15 +/- 7.54 N/mm at 0 degrees, 68.80 +/- 4.79 N/mm at 10 degrees, 51.49 +/- 5.08 N/mm at 15 deg rees, and 42.38 +/- 3.51 N/mm at 30 degrees. There was a significant differ ence between perpendicular insertion (0 degrees) and 15 degrees and 30 degr ees angles of insertion. There was also a significant difference between th e 10 degrees and 30 degrees angles of insertion. Conclusions. Perpendicular halo pin insertion in an immature skull model wa s shown to result in increased load at the pin-bone interface. This improve d structural behavior may help to reduce the incidence of complications of halo application in children.