An evaluation of the bending stiffness of various tibial fixation methods

The relative stiffness of human tibial fixation by plaster casts, intramedullary rods, external fixators, and dynamic compression plates was compared to the stiffness of the intact tibia. Four-point bending was performed on each specimen, first intact, and then after sequential instrumentation, thus allowing each specimen to serve as its own control. The mean stiffnesses obtained for the bone-device complexes, as a percentage of the intact tibial stiffness, were 113 +/- 9% for the delta frame fixator, 57 +/- 14% for the dynamic compression plate, 28 +/- 2% for the unilateral external fixator (UEF) with stainless steel sidebar, 18 +/- 1% for the UEF with aluminum sidebar, 7 +/- 2% for the flexible intramedullary (IM) rods, and 6 +/- 2% for the cast. Even relatively flexible unilateral external fixators are much stiffer than cast or IM rod fixation, methods that usually lead to satisfactory union by secondary healing. If secondary union with callus formation is desirable, modifications of the external fixator may be necessary to provide more flexible fixation. This study may be one of the first attempts to quantitate the stiffness of tibial fixation in a standard cast and compare it with other devices.