Halo pin positioning in the temporal bone; parameters for safe halo gravity traction.

Introduction: Halo gravity traction (HGT) is increasingly used pre-operatively in the treatment of children with complex spinal deformities. However, the design of the current halo crowns is not optimal for that purpose. To prevent pin loosening and to avoid visual scars, fixation to the temporal area would be preferable. This study aims to determine whether this area could be safe for positioning HGT pins.

Methods: A custom made traction setup plus three human cadaver skulls were used to determine the most optimal pin location, the resistance to migration and the load to failure on the temporal bone. A custom-made spring-loaded pin with an adjustable axial force was used. For the migration experiment, this pin was positioned at 10 predefined anatomical areas in the temporal region of adult cadaver skulls, with different predefined axial forces. Subsequently traction force was applied and increased until migration occurred. For the load-to-failure experiment, the pin was positioned on the most applicable temporal location on both sides of the skull.

Results: The most optimal position was identified as just antero-cranial to the auricle. The resistance to migration was clearly related to the axial tightening force. With an axial force of only 100 N, which corresponds to a torque of 0.06 Nm (0.5 in-lb), a vertical traction force of at least 200 N was needed for pin migration. A tightening force of 200 N (torque 0.2 Nm or 2 in-lb) was sufficient to resist migration at the maximal applied force of 360 N for all but one of the pins. The load-to-failure experiment showed a failure range of 780-1270 N axial force, which was not obviously related to skull thickness.

Conclusion: The temporal bone area of adult skulls allows axial tightening forces that are well above those needed for HGT in children. The generally applied torque of 0.5 Nm (4 in-lb) which corresponds to about 350 N axial force, appeared well below the failure load of these skulls and much higher than needed for firm fixation.