Abstract
Since cementless stem fixation in hip arthroplasty is becoming more and more common, the overall incidence of intraoperative femoral fractures has risen considerably. Depending on primary or revision arthroplasty, literature reports fracture rates between a few percent up to one third of the cases.
In this study, methods commonly applied in the field of structural testing were customized for this specified interference fit situation. A cementless hip system (ABG II, Stryker) was used on animal bones and biomechanical bones.
Transient excitation in the form of regular hammer strokes and sinusoidal excitation using a shaker served as an input. The output of the system under test was measured on the greater trochanter using a piezoelectric accelerometer.
The signals were digitized with a high-speed data acquisition system and analyzed in real-time with spectrum analysis software.
Analysis included threshold detection in the time domain to determine the time delay between the input and output transducer. Spectrum analysis in the frequency domain included FFT analysis and frequency response function analysis to identify shifts of fundamental frequencies and harmonics to describe the vibrational changes with increasing stability.
A digital imaging system was set up to take pictures of the metal-bone site to measure inducible displacement with each hammer impact and correlate it with the vibrometry results. Furthermore a strain gauge circularly mounted around the proximal femur monitored accurately any hairline fracture.
This study shows that changes of the vibrational spectrum are directly related to implant fit. The range of interest is well in the sonic range, which apparently is the reason for many surgeons to listen and ‘feel’ carefully during advancing the broach or the final implant into the femur.
The study is trying to extract critical vibrational parameters correlated with stability and femoral integrity. Due to the different dimensions of the tested animal bones and lack of soft tissue damping, further experiments on cadavers need to be carried out.
Vibrational spectrum analysis could prove to be a useful tool to readily assess implant stability and femoral integrity. It seems to be most beneficial in revision surgery or minimally invasive hip replacement, where the risk of femoral fractures is increased or fissures could easily be missed.
Correspondence should be addressed to Dr Carlos Wigderowitz, Honorary Secretary of BORS, Division of Surgery & Oncology, Section of Orthopaedic & Trauma Surgery, Ninewells Hospital & Medical School Tort Centre, Dundee, DD1 9SY.
