Pin force measurement in a halo-vest orthosis, in vivo

The halo-vest is an orthosis commonly used to immobilize and protect the cervical spine. The primary complications associated with the halo-vest have been attributed to cranial pin loosening. However, the pin force history during day-to-day halo-vest wear has not previously been reported. This paper presents a new technique developed to monitor cranial pin forces in a halo-vest orthosis, in vivo. A strain gaged, open-ring halo was used to measure the compressive and shear forces produced at the posterior pin tips. The strain gages measured the bending moments produced by these forces without compromising the structural integrity of the halo-vest system. The prototype halo measured the compressive and shear force components with a resolution of ±15 and ±10 N, respectively. To test the feasibility and durability of the device, it was applied to one patient requiring treatment with a halo-vest orthosis. At the time of halo-vest application, the mean compressive force in the two posterior pins was 368 N. Over the 3 month treatment period, the compressive forces decreased by a mean of 88%. The shear forces were relatively insignificant. Using this technology future work will be aimed at determining the causes of pin loosening, optimizing vest and pin designs, and investigating the safety of more rapid rehabilitation.