The comparative stiffness of external fixation frames.

The stiffness of 6 external fixation frames was compared by measuring the deflection which occurred at an unstable fracture, created in a tibia in vitro. All tests were repeated with the frames applied to pins brazed into a steel tube, in order to eliminate any errors caused by variation in the quality of the bone under test. The results of these tests are presented and observations are made on some design features of the frames. It was found that the 2 most critical factors which determined the stiffness of a frame were the diameter and the number of pins used to transfix the bone. Other factors, such as the stiffness of the connecting bar, were of secondary importance.