WHY FINE-WIRE FIXATORS WORK: AN ANALYSIS OF PRESSURE DISTRIBUTION AT THE WIRE-BONE INTERFACE

Abstract

Fine-wire fixator systems have been used successfully for the treatment of fractures, malunions and for limb lengthening for many years. There has been much research investigating the biomechanical properties of these systems but this has been almost entirely centred on the mechanical properties of the fixator as a whole. Our knowledge of the interactions occurring at the interface between wire and bone remains sparse. To this end we devised an experimental model to analyse the distribution of pressure in cancellous bone surrounding a tensioned wire under loading conditions. The Sawbones cancellous bone material (type 1522-11) was cut into 65x30x40 mm blocks. A 2 mm olive wire was inserted into each block, parallel to the surface and along the 65 mm dimension. The distance from the wire to the surface was variable, from 0.5mm to 5mm in a 0.5mm increment. The wire was mounted on a 150 mm ring and tensioned to 1200 N against a load cell. The ring was rigidly mounted on a material testing machine and a second bone block was incorporated into the testing machine crosshead with a universal joint. Three grades of pressure-sensitive films (Low, Superlow and Ultralow) were sandwiched in turn between the testing block and cross head. The force applied was 175 N for 5 s. The developed film was scanned into a computer and a Matlab program was developed to analyse the pressure image. The results show three phases of pressure distribution. Very close to the wire there is a polar distribution of pressure that is, the pressure is concentrated towards the entry and exit points of the wire. At a depth of 1.5mm away from the wire the pressure becomes evenly distributed along the path of the wire in a beam-loading manner. At a distance of greater than 4mm from the wire there is even distribution of pressure throughout the bone. The peak pressures (6–8 MPa) were found closest to the wire. Most of the pressure measured was less than 1 MPa, which is less than the yield strength of cancellous bone (2–7 MPa, Li and Aspden, 1997). In contrast a similar analysis using threaded half pins under the same conditions showed far higher peak pressures (20 MPa), which were present deeper in the bone specimen. The pressure was concentrated toward the pin entry site and was not well distributed throughout the pin-bone interface. These results allow us to explain why ring fixators are superior to half pin fixators when used in metaphyseal bone.