Abstract
Severe lower limb fractures may be stabilized by an External Fixation Device such as the DENHAM BAR, which is a single-sided Fixator. The feasibility of detecting healing in tibial shaft fractures using this device as a transducer was investigated The Bar was instrumented with strain gauges and calibrated in two modes to determine its reliability as a transducer. Further experimentation was carried out on a Bone / Fixator configuration to determine the limits of detectable strain in the Bar which results from firstly, a complete break, and secondly, when the bone is fully healed. The experimental work was fraught with practical difficulties and the results indicated that the bar did not behave as a reliable transducer. This suggested that direct instrumentation of the bar is not a suitable technique for clinical application. A lack of response from the strain gauges was found when the bar was in situ with fully healed bone indicating that the device behaved as a redundant structure at this stage. The Denham Bar is, however, a clinically popular device but subject to wide variation in its application. Using the device as a basis for a theoretical analysis, strain energy techniques were employed to determine whether intermediary stages of healing could be detected by related changes in the level of strain. The extent of this analysis indicated that as healing bone reaches 10 - 15% of normal strength changes in the level of healing are not detectable by the bar. The motion at the fracture site, that the device permits, was also determined from a further analysis using Beam Deflection techniques. These results were compared with an analysis of the same configuration using the PRIME STRESS PROGRAM. A number of Fixator and Physiological variables were identified, which were then selected for further analysis by the Prime Stress program to determine their relative importance and contribution to overall stiffness. From this final investigation the maximum and minimum criteria for stability were established. The analysis included a consideration of healing bone and the changes in fracture site motion which occur as the strength of CALLUS (Healing bone) approaches that of normal bone.