Abstract
When inserting a femoral stem, surgeons make use of many visual and tactile cues to be sure that the implant is correctly sized and well-seated. One such cue is the change of pitch that can be heard when the final femoral broach is inserted. This is known to be important, but has not been widely studied.
We set out to analyse the sounds produced during femoral broaching and implant fixation, and to discover whether the absence of these sounds could predict a poor fixation.
We recorded the sound of femoral broaching and definitive implant insertion, for twenty un-cemented Corail total hip replacements. Procedures were performed by the same surgeon, in the same theatre. The recordings were visualised using audio editing software, and a Fast Fourier Transform was used to identify the dominant audio frequencies.
In 19 of the 20 cases, the final strikes of the final femoral broach displayed a distinctive pattern, with the most prominent frequencies being harmonics (multiples of a fundamental frequency) which had a wavelength directly related to the length of the femoral canal. This contrasts with initial strikes, where multiple unrelated frequencies were present.
Postoperative radiographs were examined by two surgeons independently, to assess implant sizing and positioning. The one case, in which the harmonic pattern was not observed, was found on radiographs to be an undersized, varus malpositioned implant.
We demonstrate that a characteristic frequency pattern is present when impacting cancellous bone with a well-sized and well-placed femoral broach. When the pattern was absent, the broach and implant were undersized and malpositioned. We hypothesise that this pattern arises when broach and femur are vibrating as one, indicating adequate contact with, and compression of, cancellous bone.
