Design and Tesing of a Halo Orthosis to Reduce Pin Loosening Using Constraint Analysis

A halo orthosis is a medical device used for restricting motion of the skull and cervical spine during accident and surgery recovery situations where minimal movement of the skull and spine is required for proper healing. Pin loosening is one of the most common complications with a commonly cited incident rate of 36%. The root cause of the loosening is related to changes in the skull geometry at the pin site and the inability of current halo systems to adapt to those changes. We hypothesize that the inability of current halo systems to adapt to these small pin site geometry changes is driven by the over constrained nature of the skull relative to the halo ring when viewed as a mechanism in the context of the Grubler / Kutzbach criteria. Accordingly, an exactly constrained 3-pin halo ring design is proposed. A prototype of the exactly constrained halo ring that is instrumented to measure pin forces was fabricated along with a skull model that simulates radial recession of the bone at the pin sites. The testing suggests that the pin forces in the exactly constrained 3-pin halo are less sensitive to the radial recession of the pin sites than the current over constrained 4-pin halo configurations. The average pin force in the over constrained 4-pin design decreased by 63% after 0.52 mm of simulated radial recession at two of the pin sites. The average pin force in the exactly constrained 3-pin design decreased by 1% after 0.52 mm of simulated radial recession at two of the pin sites.