Analysis of pin force distributions of halo orthoses.

The halo orthosis is a medical device used to immobilize the cervical spine when an injury to the region is sustained. This device consists of a halo vest, support rods, a ring, and four pins that contact the skull. A major problem experienced with the halo is pin loosening. In our previous research, we were able to model the halo orthosis as a mechanism and use kinematic principles to analyze the constraint characteristics. Using the Grubler/Kutzbach equation (a well accepted method of characterizing constraint systems), we were able to show that the halo orthosis is over-constrained which could be a possible source of pin loosening. We now present preliminary data that will show decreasing axial pin force and observed geometry changes that occur at each pin site. Biomechanical tests were performed on two cadaver skulls using a tensile loading machine that simulated the forces exerted by a patient. We were able to observe the changes in axial forces at each of the pins using load cells as cyclic transverse forces were applied. Our results show a force reduction over time at each pin site. We also discovered that the effects of creep are significant and potentially contribute to pin loosening.