The mechanics of skull traction calipers

The relationship between the torque applied to the loading screw and the load required to pull off Cone's type of skull traction caliper was established on cadavers. It was found that at a torque of 0.7 Newton metres (Nm), which corresponds to a compressive force of 13 lb (58 N) on the skull, the caliper became detached at the maximum traction load of 60 lb, but still held during this traction load at compressive forces above 18 lb (80 N). Clinically, an instrumented load cell was incorporated into one of the fixing pins on a skull traction caliper and it was found that the compressive load fell on average by 15 per cent over a period of 21 days of traction. Taking into account this fall in load with time and the limiting torque for pull-out with a traction force of 60 lb (265 N) it is suggested, for complete clinical safety, that a compressive load of 20 lb (88 N) obtained at a torque of 0.85 Nm should be applied to the loading screw of a Cone's type of caliper. Using these values in a loading analysis of the securing pins, suggestions are made as to the ideal design of skull traction calipers and the shortcomings of various designs of existing calipers are discussed, in particular the pins' lengths, compressive load and the pins' placement.