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 hy pothesis that a new cylindrical cutting pin tip design which minimizes damage to adjacent bone and does not rely on high axial forces to mai ntain fixation would perform better mechanically than conventional con ical tip pins. Conventional and experimental halo pins were tested for mechanical stability in human cadaveric skull bone using a servohydra ulic load frame (Model 858 Bionix, MTS Corp., Minneapolis, MN). A cycl ic transverse load of +/- 300 N was applied through the pins for 10,00 0 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. Photomicro graphs of thin decalcified sections through a hole formed by each pin tip were compared for gross evidence of bony damage. With the pins ful ly tightened, there was no statistically significant difference in the motion between the experimental design (mean +/- 95% confidence inter val: 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 st rength of the experimental pins (2010 +/- 366.4 N) was significantly g reater than the conventional pins (1128 +/- 94.5 N)(p < 0.005). The pi n-bone interface stiffness of the experimental pins (1728 +/- 144.4 N/ mm) was also significantly greater than that of the conventional pins (1393 +/- 202.6 N/mm)(p < 0.03) (Fig. 5). Qualitatively, the photomicr ographs demonstrated considerably more particulate debris on the bound ary of the hole formed by the conventional pin compared to the experim ental pin. The data obtained herein support our hypothesis and indicat e that the experimental pin design possesses biomechanical characteris tics superior to current designs. These characteristics may translate into fewer complications in the clinical setting. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.