THE HYBRID RING TUBULAR EXTERNAL FIXATOR - A BIOMECHANICAL STUDY

Objective. To measure and compare the mechanical properties in bending of the four-ring, and three-ring/one-tube hybrid external fixation fr ames. Design. In vitro measurements of the mechanical behaviour of rin g and ring-tubular external fixation frames. In the latter, one ring o f the full circular frame was replaced by one tube and Schanz screws. Background. The mechanical properties of the classical Ilizarov four-r ing external fixation frames has been compared to those of other exter nal fixation frames by various authors. However, in clinical practice the hybrid fixation frame is being used with increasing frequency. The refore the mechanical properties of the fatter are of immediate intere st and clinical value. Methods. On explanted sheep tibiae with single and double osteotomies, frame stiffness in the four-point bending mode was measured at different K-wire tensions, comparing the values obtai ned from four-ring frames, to those of three-ring-tubular hybrid frame s. These measurements were made under conditions of (a) bone distracti on (BD), and (b) segment transport (ST), both at the initial and final stages of this procedure. Results. In circular frames, frame stiffnes s in bending for increasing K-wire tension showed a Gaussian distribut ion both in distraction and post-ST with an optimum at 1000 N. In ring tubular hybrid frames, however, frame stiffness showed a more linear relationship to K-wire tension. Conclusions. In the four-ring Ilizarov external fixation frame, the exchange of one ring with one tube and o ne Schanz screw both reduced frame stiffness in bending and converted to linear its relationship to K-wire tension. Relevance Under clinical conditions, the use of a similar ring tubular hybrid external fixator allows the adjustment of frame stiffness in a simple and practical wa y. This is not the case with the original ring fixation frame. (C) 199 7 Elsevier Science Ltd.