Kinematic constraint analysis of halo orthoses

A kinematic constraint analysis of the four pin halo orthosis is presented. Halo orthoses are used to immobilize the cervical spine after traumatic injury. Common halo orthoses consist of four pins, a halo ring, a vest, and support rods. The head is secured relative to the halo ring by four pins. These pins are screw-like fasteners that thread into the halo ring and then engage the skull with pointed ends. The halo ring is secured relative to the torso with several rods that attach to a vest. To fully immobilize the head, the pins must constrain six degrees of freedom--three translational degrees of freedom and three rotational degrees of freedom. The constraint system for the skull relative to the halo ring is analyzed by first modeling the system as a mechanism. Then the Grubler/Kutzbach criteria are applied to characterize the nature of the constraint system. From this analysis we find that the four pin halo configuration over constrains the head relative to the halo ring. We hope to use the findings to gain insights into the problem of pin loosening, a significant problem during treatment with a halo orthosis. Once the constraint system is understood, it may be possible to explain the pin loosening problem and to use the concepts of exact kinematic constraint to address the problem.