Abstract
Introduction
During broach preparation and implant insertion of the proximal femur the surgeon may be able to use audible pitch changes to judge broaching adequacy and implant position. The aim of this study was to analyse the sound produced and explain the sound spectra using acoustic physics.
Methods
A highly sensitive microphone was used to digitally record the sound made during femoral preparation and definitive implant insertion in 9 patients undergoing total hip arthroplasty. The sound data was analysed using a fast Fournier transformation spectrum analyser. The highest 4 peak spectral amplitudes of the first broach, the last strike of the final broach and the definitive implant were recorded. The sound spectra produced by striking the implant introducer in isolation were analysed in a similar manner.
Results
Analysis of the sound spectra identified the appearance of a low frequency peak (mean 892Hz) during final broaching that was not present when using the first broach. This occurred in 6 of the 9 cases and correlated with a decrease in audible pitch heard by the surgeon. The spectra from the implant insertion also showed a low frequency peak (mean 709Hz). The implant introducer when struck in isolation produced a series of peaks that correlated closely to the predicted 3rd resonant mode frequencies for a metal object of the same dimensions.
Discussion
Creating an envelope of compacted cancellous bone within the proximal femur causes the sound produced by the final broach to decrease in pitch. We postulate that this is due to vibration energy causing the femoral bone to resonate, and create longitudinal standing waves in the medullary canal. The additional spectral peaks are caused by transitional resonance vibrations from the implant and broach introducer construct.
