Aims

To investigate metallosis in patients with magnetically controlled growing rods (MCGRs) and characterize the metal particle profile of the tissues surrounding the rod.

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 3. rd. 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

Metal-on-metal hip resurfacing is commonly performed for osteoarthritis in young active patients. We have observed cystic or solid masses, which we have called inflammatory pseudotumours, arising around these devices. They may cause soft-tissue destruction with severe symptoms and a poor outcome after revision surgery. The aim of this study was to determine the incidence of and risk factors for pseudotumours that are serious enough to require revision surgery.

Since 1999, 1419 metal-on-metal hip resurfacings have been implanted by our group in 1224 patients; 1.8% of the patients had a revision for pseudotumour. In this series the Kaplan-Meier cumulative revision rate for pseudotumour increased progressively with time. At eight years, in all patients, it was 4% (95% confidence interval (CI) 2.2 to 5.8). Factors significantly associated with an increase in revision rate were female gender (p < 0.001), age under 40 (p = 0.003), small components (p = 0.003), and dysplasia (p = 0.019), whereas implant type was not (p = 0.156). These factors were inter-related, however, and on fitting a Cox proportional hazard model only gender (p = 0.002) and age (p = 0.024) had a significant independent influence on revision rate; size nearly reached significance (p = 0.08). Subdividing the cohort according to significant factors, we found that the revision rate for pseudotumours in men was 0.5% (95% CI 0 to 1.1) at eight years wheras in women over 40 years old it was 6% (95% CI 2.3 to 10.1) at eight years and in women under 40 years it was 13.1% at six years (95% CI 0 to 27) (p < 0.001).

We recommend that resurfacings are undertaken with caution in women, particularly those under 40 years of age but they remain a good option in young men. Further work is required to understand the aetiology of pseudotumours so that this complication can be avoided.