Total hip replacement (THR) is the second most performed surgical procedure with an estimated number of more than one million operations each year worldwide. This implies that, despite survival rates of 97% at 3 years  and even up to 10 years follow-up  for some prosthesis types, a large number of revision operations are needed every year, most of them because of aseptic loosening. Revision operations are more difficult to perform, carry more risk for complications and have a poorer prognosis than primary THR .
Survival rate is directly related to the long term fixation stability of the prosthesis stem . Beside the design, material composition and surface characteristics of the implant, the initial per-operative fixation of the stem in the femoral bone has a critical influence on its long term fixation stability. This is especially the case for non cemented, press-fit fixated stems. The insertion procedure results in well-defined contact areas and interface pre-stresses between the stem and the femoral bone. Under actual loading, the hip stem displacement and the femoral stress distribution will strongly depend upon these initial contact conditions. Primary hip stem stability is not only important regarding prosthesis migration, but also regarding micro movements that must be limited in order to allow interfacial bone formation and in-growth . Femoral stress distribution has a crucial influence on bone remodelling and therefore on the final strength of the bone-implant structure. Therefore the per-operative characterization of the primary stem-femur contact and the assessment of primary stem stability in the first place may help to improve the survival rate of THR.
Nowadays objective intra-operative assessment of primary stem stability is a challenge, as surgeons have to rely mainly on their clinical experience, which consists mainly of a sense of mechanical stability when exerting axial force and/or torque on the prosthesis. Moreover, excessive press-fitting of a THR femoral component can cause intra-operative fractures with an incidence of up to 30% in revision cases .
Vibration analysis has been successfully used to determine bone mechanical properties [7–9]. Clinical applications of this method were monitoring of fracture healing and in vivo assessment of bone mechanical properties [10–14]. Vibration analysis was also successfully used to quantify the fixation of oral implants . A limited number of studies prove the feasibility of detecting several forms of femoral implant loosening, in vitro and in vivo using techniques based on harmonic distortion [16–19].
In vitro, the analysis of frequency response function (FRF) was used to discriminate between well fixed and quasi-well fixed femoral stems .
This paper presents a series of cases where a per-operative vibration analysis technique was used for the mechanical characterization of the primary bone-prosthesis stability.
In a previous study we demonstrated the feasibility and validity of a vibration analysis technique for the assessment of the femur-stem fixation in vitro [21–24]. The stem insertion process was performed on a dry cadaver femur and synthetic composite femurs and the FRF change was analysed. In a recent study a finite element model was created to gain insight into the dependence of the FRF on system parameter variations .
The imperfections in the connection between a THR prosthetic stem and a femur can most sensitively be detected by observing shifts in the resonance frequency of the higher vibration modes of the femur-prosthesis system. This observation is in accordance with the work of Qi et al. who stated that the most sensitive frequency band for observing defects in the femur-prosthesis connection is above 2500 Hz .
In the present study the vibration analysis technique was applied for the per-operative assessment of fixation stability in 83 THR patients who obtained an intra-operatively manufactured prosthesis (IMP) provided by Advanced Custom Made Implants, Leuven, Belgium (see appendix 1). The IMP approach aims at optimal stem stability through a maximum fit and fill of the femoral cavity .
The objective of the present study was to apply and evaluate an endpoint criterion for the insertion of the stem by successive surgeon-controlled hammer blows. The endpoint-of-insertion criterion was based upon the Pearson's correlation coefficient R between the FRFs of two successive insertion stages.