Abstract
Owing to an ageing population, the impact of unhealthy lifestyle, or simply congenital or gender specific issues (dysplasia), degenerative bone and joint disease (osteoarthritis) at the hip pose an increasing problem in many countries. Osteoarthritis is painful and causes mobility restrictions; amelioration is often only achieved by replacing the complete hip joint in a total hip arthroplasty (THA). Despite significant orthopaedic progress related to THA, the success of the surgical process relies heavily on the judgement, experience, skills and techniques used of the surgeon. One common way of implanting the stem into the femur is press fitting uncemented stem designs into a prepared cavity. By using a range of compaction broaches, which are impacted into the femur, the cavity for the implant is formed. However, the surgeon decides whether to change the size of the broach, how hard and fast it is impacted or when to stop the excavation process, merely based on acoustic, haptic or visual cues which are subjective. It is known that non-ideal cavity preparations increase the risk of peri-prosthetic fractures especially in elderly people. This study reports on a simulated hip replacement surgery on a cadaver and the analysis of impaction forces and the microphone signals during compaction. The recorded transient signals of impaction forces and acoustic pressures (≈ 80 μs - 2 ms) are statistically analysed for their trend, which shows increasing heteroscedasticity in the force-pressure relationship between broach sizes. Tikhonov regularisation, as inverse deconvolution technique, is applied to calculate the acoustic transfer functions from the acoustic responses and their mechanical impacts. The extracted spectra highlight that system characteristics altered during the cavity preparation process: in the high-frequency range the number of resonances increased with impacts and broach size. By applying nonlinear time series analysis the system dynamics increase in complexity and demand for a larger minimum embedding dimension. The growing number of resonances with similar level of the transfer function indicates a higher propensity to dissipate energy over sound; the change in embedding dimension indicates a decrease in linearity. The spectral changes as well as the altered dimension requirements indicate either an improved coupling between the bone and the broach or the onset of micro-fractures caused by growing stress levels within the bone.
Highlights
2015 about 498,660 hip, and 592,577 knee joint replacement procedures (1,123,643 in total) were registered, an increase of 2.6% and 5.9%, respectively over those in 2014 [2]
Owing to non-ideal cavity preparation, lysis and post-operative prosthesis dislocation due to insufficient stability at the time of implantation may occur which is a major cause for revision surgeries [2, 4]
Dashed vertical lines indicate fundamental resonance frequencies overlapping for two hits within ±1 kHz; bold dashed vertical lines indicate fundamental frequencies overlapping for more than 2 hits within ±1 kHz
Summary
2015 about 498,660 hip, and 592,577 knee joint replacement procedures (1,123,643 in total) were registered, an increase of 2.6% and 5.9%, respectively over those in 2014 [2]. The cavity for the stem is prepared using the impaction of broaches and a surgical hammer; here in particular the primary stability of the artificial joint is important. Primary stability is achieved through an interference-fit which strongly depends on the bone-implant contact [5]. During THA the surgeon decides when and whether to change the size of the broach, how hard and fast to impact it and when to stop the cavity preparation [7]. This decision is subjective, generally based on changes in pitch, haptic or visual cues [7]. By increasing the primary stability too far the risk of peri-prosthetic fractures grows [4, 8]
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