Determination of minimum required halo pin force.

The motivation for this study is to provide design guidance for a new halo system that minimizes pin loosening. If halo pin loosening can be substantially decreased with a new halo system, then the current standard of care of overtightening halo pins will not be necessary. Accordingly, there is a need to determine the halo pin force that should be applied to ensure adequate fixation. A biomechanical test was performed using cadaver head constructs, a custom halo fixture, and a tensile testing machine in an attempt to determine the relationship between the force required to dislodge a halo ring and the initial halo pin force. Three cadaver head constructs were tested at initial pin forces of 120, 240, and 360 N. For each test, the halo was pulled from a cadaver head with a displacement rate of 2.5 mm/min until the halo ring disengaged from the head. The vertical force that caused disengagement of the halo from the head was determined from the resulting load-displacement curves. A linear regression of the data predicts disengagement forces of 80, 320, and 570 N, respectively, for initial pin forces of 120, 240, and 360 N. The 95% prediction interval of disengagement forces for initial pin forces of 120, 240, and 360 N were +/-130, 120, and 130 N, respectively. A previously published study reported the maximum vertical load on a halo orthosis during patient usage to be 186 N. The lower 95% prediction interval from this study indicates that an average initial pin force of 230 N is necessary to prevent halo pin disengagement from a 186-N vertical load.