Rapid pre-tension loss in the Ilizarov external fixator: an in vitro study.

BACKGROUND AND PURPOSE Wire pre-tension in the Ilizarov frame is considered to be important in order to reduce movements that can impair fracture healing. Wires will eventually lose part of their pre-tension, however. In order to gain more insight into the need for wire pre-tension, we investigated: (1) the amount of pre-ten-sion loss, (2) the relationship between pre-tension loss and bolt-tightening torque, (3) the cause of a possible loss of pre-tension, and (4) the effect of pre-tension on cyclic micromotions and total displacement of the bone segments. METHODS 3 Ilizarov configurations, with various wire pre-tensions and bolt-tightening torques, were tested 3 times with an MTS machine. For each wire, slippage and tensions were measured for the duration of the whole experiment. RESULTS A loss of wire pre-tension (up to 75%) due to slippage was found. Higher bolt-tightening torques significantly reduced the amount of pre-tension loss. Furthermore, a higher wire pre-tension reduced the maximal axial displacement of the bone fragment. There was, however, least cyclic axial micromotion when no wire pre-tension was present -- probably due to the lack of wire recoil. INTERPRETATION Wires in an experimental Ilizarov external fixator lose pre-tension within a limited period of time, and probably also in the clinical setting. This does not seem to lead to increased cyclic axial micromotions of the bone fragements. However, the question remains as to how excessive axial displacement of the bone fragment and other loading configurations will affect the process of fracture healing in a patient.