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A new method of tightening halo orthosis pins that reduces initial pin force variation
Jennifer Klingenberg1, Adam Keown, David Moser
1Department of Mechanical Engineering, Rose-Hulman Institute of Technology, Terre Haute, IN 47803, USA.
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. The halo is applied to the patient by securing the halo ring to the skull with four pointed pins that are threaded into a halo ring. In an effort to produce a desired initial force in each pin, a specified torque (typically 0.9 N-m) is applied to the pin. One problem associated with the existing method is that it produces a wide range of initial pin forces despite a constant applied torque. The existing method is particularly sensitive to the presence of medical agents (e.g. antibacterial ointments or hydrogen peroxide) that may change frictional characteristics at the pin site or at the threaded hole in the halo ring. Variability in initial forces from an applied torque of 0.9 N-m (8 in-lbf) have been reported in the literature. These forces vary from 620 N (140 lbf) when hydrogen peroxide is applied to the threads and pin site up to 980 N (220 lbf) when antibacterial ointment is applied to the threads and pin site. We present a new method of tightening halo pins that reduces the variability of the initial pin force. Using the new method, a known force is applied to the pin to provide a preload. Once the appropriate preload is applied, the halo pin is secured in place relative to the halo ring and the applied force is removed leaving the pin in the final configuration. We compare the existing method and the new proposed method by examining a single pin site. The pin force data display less sensitivity to lubricants using the new direct loading technique.
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. The halo is applied to the patient by securing the halo ring to the skull with four pointed pins that are threaded into a halo ring. In an effort to produce a desired initial force in each pin, a specified torque (typically 0.9 N-m) is applied to the pin. One problem associated with the existing method is that it produces a wide range of initial pin forces despite a constant applied torque. The existing method is particularly sensitive to the presence of medical agents (e.g. antibacterial ointments or hydrogen peroxide) that may change frictional characteristics at the pin site or at the threaded hole in the halo ring. Variability in initial forces from an applied torque of 0.9 N-m (8 in-lbf) have been reported in the literature. These forces vary from 620 N (140 lbf) when hydrogen peroxide is applied to the threads and pin site up to 980 N (220 lbf) when antibacterial ointment is applied to the threads and pin site. We present a new method of tightening halo pins that reduces the variability of the initial pin force. Using the new method, a known force is applied to the pin to provide a preload. Once the appropriate preload is applied, the halo pin is secured in place relative to the halo ring and the applied force is removed leaving the pin in the final configuration. We compare the existing method and the new proposed method by examining a single pin site. The pin force data display less sensitivity to lubricants using the new direct loading technique.