Resonant Frequency Analysis of the Tibia as a Measure of Fracture Healing

The Resonant Frequency (RF) of the tibia is proportional to its stiffness. As a fractured tibia heals, its RF should increase. The RF was serially determined in 74 fractured tibias (205 examinations). These were subdivided by fracture location and fixation. Fast Fourier transform software generates the RF from data obtained with an instrumented impactor and accelerometer. The RF was normalized by expressing it as a ratio of the intact tibia. This ratio is called the tibial stiffness index (TSI). A 20 point tibial fracture score (TFS) quantitated the clinical and radiographic signs of healing. For each group the paired TSI and TFS were compared by regression analysis. Except for those fractures limited to the proximal fourth of the tibia, the TSI was found to correlate significantly (p = 0.0001) with the TFS. Fractures without fixation and those with unlocked, unreamed tibial nails showed very significant correlation of TSI with TFS (p = 0.0001). RF analysis was not useful in fractures with locked or reamed tibial nails. Examination of tibia with external fixation showed significant correlation (p = 0.02) of the TSI with the TFS.