Halo pin loosening: a biomechanical comparison of experimental and conventional designs

Loosening of the pins is the most common complication associated with use of the halo orthosis. The purpose of this study was to test the hypothesis that a new cylindrical cutting pin tip design which minimizes damage to adjacent bone and does not rely on high axial forces to maintain fixation would perform better mechanically than conventional conical tip pins. Conventional and experimental halo pins were tested for mechanical stability in human cadaveric skull bone using a servohydraulic load frame (Model 858 Bionix, MTS Corp., Minneapolis, MN). A cyclic transverse load of +/-300 N was applied through the pins for 10,000 cycles in a sinusoidal wave form in both fully tightened and reduced axial load situations. Load-to-failure testing was also performed to determine the strength and stiffness of each configuration. Photomicrographs of thin decalcified sections through a hole formed by each pin tip were compared for gross evidence of bony damage. With the pins fully tightened, there was no statistically significant difference in the motion between the experimental design (mean +/- 95% confidence interval: 0.41+/-0.027 mm) and the conventional halo pin (0.38+/-0.075 mm). After the axial pin force was intentionally decreased, there was no significant increase in the motion of the experimental pins (0.43+/-0.032 mm), however, there was a significant increase in the motion of the conventional pins (3.15+/-2.403 mm)(p < 0.05). The failure strength of the experimental pins (2010+/-366.4 N) was significantly greater than the conventional pins (1128+/-94.5 N)(p < 0.005). The pin bone interface stiffness of the experimental pins (1728+/-144.4N/mm) was also significantly greater than that of the conventional pins (1393+/-202.6 N/mm)(p < 0.03) (Fig. 5). Qualitatively, the photomicrographs demonstrated considerably more particulate debris on the boundary of the hole formed by the conventional pin compared to the experimental pin. The data obtained herein support our hypothesis and indicate that the experimental pin design possesses biomechanical characteristics superior to current designs. These characteristics may translate into fewer complications in the clinical setting.