Early periprosthetic bone remodelling around cemented and uncemented custom-made femoral components and their uncemented acetabular cups.

INTRODUCTION: Periprosthetic bone remodelling after total hip replacement may contribute to aseptic loosening of the prosthesis. The selection between cemented and uncemented fixation of the stem is mainly determined by patient's age, general constitution and CT scan-estimated bone quality; intra-operative observation may ultimately influence the choice of the fixation method. The influence of cemented versus uncemented stem fixation on periprosthetic bone remodelling around the uncemented cup has, to our knowledge, never been studied until now. METHODS: A total of 75 patients received intra-operatively manufactured stem prostheses and a standard hydroxy apatite-coated pinnacle cup. The pre-operative CT scans provides guidance for the bone quality and hence the type of stem fixation: cemented or uncemented. The influence of either type of stem fixation on periprosthetic bone remodelling around the cup and the stem was measured by bone mineral density at 6 weeks, and 3, 6 and 12 months after surgery. RESULTS: Early changes in bone mineral density were noted. The type of stem fixation had an influence on the bone remodelling of the femur and also of the pelvis. The caudal part of the acetabulum was subject to a greater loss in BMD at 12 months in the group with cemented stem fixation. Changes at 12 months correlated with the changes measured at any time point. CONCLUSIONS: The selection of the stem implant and its type of fixation in the femoral cavity (cemented or uncemented fixation) seems to have an impact on the bone mineral density of the acetabulum. Long-term clinical follow-up is required to draw conclusions regarding the influence on prosthesis survival.

A finite element analysis of the vibrational behaviour of the intra-operatively manufactured prosthesis-femur system.

In total hip replacement (THR) a good initial stability of the prosthetic stem in the femur, which corresponds to a good overall initial contact, will help assure a good long-term result. During the insertion the implant stability increases and, as a consequence, the resonance frequencies increase, allowing the assessment of the implant fixation by vibration analysis. The influence of changing contact conditions on the resonance frequencies was however not yet quantitatively understood and therefore a finite element analysis (FEA) was set up. Modal analyses on the hip stem-femur system were performed in various contact situations. By modelling the contact changes by means of the contact tolerance options in the finite element software, contact could be varied over the entire hip stem surface or only in specific zones (proximal, central, distal) while keeping other system parameters constant. The results are in agreement with previous observations: contact increase causes positive resonance frequency shifts and the dynamic behaviour is most influenced by contact changes in the proximal zone. Although the finite element analysis did not establish a monotonous relationship between the vibrational mode number and the magnitude of the resonance frequency shift, in general the higher modes are more sensitive to the contact change.

Detection of the insertion end point of cementless hip prostheses using the comparison between successive frequency response functions.

Vibration analysis is a non-destructive testing technique, which has a potential to assess the mechanical properties of the stem/femur system in total hip replacement (THR). Different methods based on vibration analysis have already been successfully used to determine bone mechanical properties, to monitor fracture healing, and to quantify the fixation of dental implants. This paper describes an in vitro study of the change in the frequency response function (FRF) of the hip stem/femur structure during implant insertion. At successive insertion stages, the FRF of the system was measured by impulse excitation on the prosthesis neck, in the range 0-5000 Hz. To quantify the difference between two successive FRF spectra, the Pearson's correlation coefficient and the cross correlation function were used. The stiffness of the implant/bone system varies during insertion, which results in a change in FRF, especially in the range of higher frequencies. If the FRF spectrum shifts to the right, then the stiffness of the implant/bone connection increases and, consequently, the stability of the implant increases as well. If the FRF does not change between two successive insertion stages, then the mechanical properties of the prosthesis-femur structure does not change; therefore, the stem-bone connection is stable and the insertion should stop to avoid intra-operative fractures. Based on the obtained results, a per-operative protocol based on FRF analysis can be designed to assess the stability of a cementless hip prosthesis, and to detect the insertion end point.

Effect of constant strain rate, composed of varying amplitude and frequency, of early loading on peri-implant bone (re)modelling.

AIM: Examine the effect of varying components of strain rate -- amplitude versus frequency -- while maintaining a constant strain rate of early controlled mechanical loading on implant stability, peri-implant bone mass and bone-to-implant contact. MATERIAL AND METHODS: Three groups of guinea-pigs received TiO2 -blasted implants in both tibiae. One week after installation test implants were loaded 5 days/week during 4 weeks. The contra-lateral implants were the unloaded controls. Strain rate was kept constant (1600 micro epsilon/s), while amplitude and frequency were varied per group. Implant stability was followed by resonance frequency analysis. Animals were sacrificed, and ground sections were prepared to rate bone-to-implant contact and bone mass. RESULTS: All implants (n=78) integrated uneventfully. A significant positive effect (p=0.03) of early loading on bone mass was observed in the distal medullar cavity. A significant difference in bone mass between test and control implants was evidenced between the groups (p=0.03 and 0.04). A significant increase in implant stability and bone-to-implant contact could not be shown. CONCLUSIONS: Early controlled stimulation of peri-implant bone is related to amplitude/frequency and not to strain rate as such, considering a constant stimulation time. An increase of bone mass around early-loaded implants was shown. This cortical bone model is most sensitive to low-frequency/high-amplitude stimulation.

Resonance frequency analysis of implants in the guinea pig model: influence of boundary conditions and orientation of the transducer.

The goal of this study was to identify the parameters that must be controlled during in vivo resonance frequency measurements with a custom Osstell transducer for custom implants in the guinea pig animal model. A numerical study and in vitro measurements were performed to determine the influence of the boundary conditions as well as the transducer orientation on the resonance frequency measured by the custom Osstell transducer. In the reported guinea pig model, the type of boundary condition, the orientation of the transducer (parallel or perpendicular to the long axis of the bone) and the length of the modelled bone have a large influence on the resonance frequency values. This implies that a follow-up in time of the stability of an implant requires the boundary conditions applied to the bone in which the implant is installed as well as the orientation of the transducer to be highly repeatable. Applying controlled boundary conditions during in vivo measurements had a highly positive influence on the repeatability of the Osstell measurements. This improves the possibility of the technique to measure changes in the implant-bone interface during healing of the implant.

Individualised, micro CT-based finite element modelling as a tool for biomechanical analysis related to tissue engineering of bone.

Load-bearing tissues, like bone, can be replaced by engineered tissues or tissue constructs. For the success of this treatment, a profound understanding is needed of the mechanical properties of both the native bone tissue and the construct. Also, the interaction between mechanical loading and bone regeneration and adaptation should be well understood. This paper demonstrates that microfocus computer tomography (microCT) based finite element modelling (FEM) can have an important contribution to the field of functional bone engineering as a biomechanical analysis tool to quantify the stress and strain state in native bone tissue and in tissue constructs. Its value is illustrated by two cases: (1) in vivo microCT-based FEM for the analysis of peri-implant bone adaptation and (2) design of biomechanically optimised bone scaffolds. The first case involves a combined animal experimental and numerical study, in which the peri-implant bone adaptive response is monitored by means of in vivo microCT scanning. In the second case microCT-based finite element models were created of native trabecular bone and bone scaffolds and a mechanical analysis of both structures was performed. Procedures to optimise the mechanical properties of bone scaffolds, in relation to those of native trabecular bone are discussed.