Leonard Pastrav

In vivo evaluation of a vibration analysis technique for the per-operative monitoring of the fixation of hip prostheses

BackgroundThe per-operative assessment of primary stem stability may help to improve the performance of total hip replacement. Vibration analysis methods have been successfully used to assess dental implant stability, to monitor fracture healing and to measure bone mechanical properties. The objective of the present study was to evaluate in vivo a vibration analysis-based endpoint criterion for the insertion of the stem by successive surgeon-controlled hammer blows.MethodsA protocol using a vibration analysis technique for the characterisation of the primary bone-prosthesis stability was tested in 83 patients receiving a custom-made, intra-operatively manufactured stem prosthesis. Two groups were studied: one (n = 30) with non cemented and one (n = 53) with partially cemented stem fixation. Frequency response functions of the stem-femur system corresponding to successive insertion stages were compared.ResultsThe correlation coefficient between the last two frequency response function curves was above 0.99 in 86.7% of the non cemented cases. Lower values of the final correlation coefficient and deviations in the frequency response pattern were associated with instability or impending bone fracture. In the cases with a partially cemented stem an important difference in frequency response function between the final stage of non cemented trial insertion and the final cemented stage was found in 84.9% of the cases. Furthermore, the frequency response function varied with the degree of cement curing.ConclusionThe frequency response function change provides reliable information regarding the stability evolution of the stem-femur system during the insertion. The protocol described in this paper can be used to accurately detect the insertion end point and to reduce the risk for intra-operative fracture.

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 Pearsons 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.

Numerical simulation of the insertion process of an uncemented hip prosthesis in order to evaluate the influence of residual stress and contact distribution on the stem initial stability

The long-term success of a cementless total hip arthroplasty depends on the implant geometry and interface bonding characteristics (fit, coating and ingrowth) and on stem stiffness. This study evaluates the influence of stem geometry and fitting conditions on the evolution and distribution of the bone–stem contact, stress and strain during and after the hip stem insertion, by means of dynamic finite element techniques. Next, the influence of the mechanical state (bone–stem contact, stress and strain) resulted from the insertion process on the stem initial resistance to subsidence is investigated. In addition, a study on the influence of bone–stem interface conditions (friction) on the insertion process and on the initial stem stability under physiological loading is performed. The results indicate that for a stem with tapered shape the contact in the proximal part of the stem was improved, but contact in the calcar region was achieved only when extra press-fit conditions were considered. Changes in stem geometry towards a more tapered shape and extra press fit and variation in the bone–stem interface conditions (contact amount and high friction) led to a raise in the total insertion force. A direct positive relationship was found between the stem resistance to subsidence and stem geometry (tapering and press fit), bone–stem interface conditions (bone–stem contact and friction interface) and the mechanical status at the end of the insertion (residual stress and strain). Therefore, further studies on evaluating the initial performance of different stem types should consider the parameters describing the bone–stem interface conditions and the mechanical state resulted from the insertion process.

Development of an acoustic measurement protocol to monitor acetabular implant fixation in cementless total hip Arthroplasty: A preliminary study

In cementless total hip arthroplasty (THA), the initial stability is obtained by press-fitting the implant in the bone to allow osseointegration for a long term secondary stability. However, finding the insertion endpoint that corresponds to a proper initial stability is currently based on the tactile and auditory experiences of the orthopedic surgeon, which can be challenging. This study presents a novel real-time method based on acoustic signals to monitor the acetabular implant fixation in cementless total hip arthroplasty. Twelve acoustic in vitro experiments were performed on three types of bone models; a simple bone block model, an artificial pelvic model and a cadaveric model. A custom made beam was screwed onto the implant which functioned as a sound enhancer and insertor. At each insertion step an acoustic measurement was performed. A significant acoustic resonance frequency shift was observed during the insertion process for the different bone models; 250 Hz (35%, second bending mode) to 180 Hz (13%, fourth bending mode) for the artificial bone block models and 120 Hz (11%, eighth bending mode) for the artificial pelvis model. No significant frequency shift was observed during the cadaveric experiment due to a lack of implant fixation in this model. This novel diagnostic method shows the potential of using acoustic signals to monitor the implant seating during insertion.

Vibration-based fixation assessment of tibial knee implants: A combined in vitro and in silico feasibility study

The preoperative diagnosis of loosening of cemented tibial knee implants is challenging. This feasibility study explored the basic potential of a vibration-based method as an alternative diagnostic technique to assess the fixation state of a cemented tibia implant and establish the methods sensitivity limits. A combined in vitro and in silico approach was pursued. Several loosening cases were simulated. The largest changes in the vibrational behavior were obtained in the frequency range above 1500 Hz. The vibrational behavior was described with two features; the frequency response function and the power spectral density band power. Using both features, all experimentally simulated loosening cases could clearly be distinguished from the fully cemented cases. By complementing the experimental work with an in silico study, it was shown that loosening of approximately 14% of the implant surface on the lateral and medial side was detectable with a vibration-based method. Proximal lateral and medial locations on the tibia or locations toward the edge of the implant surface measured in the longitudinal direction were the most sensitive measurement and excitation locations to assess implant fixation. These results contribute to the development of vibration-based methods as an alternative follow-up method to detect loosened tibia implants.

Determination of replicate composite bone material properties using modal analysis

Replicate composite bones are used extensively for in vitro testing of new orthopedic devices. Contrary to tests with cadaveric bone material, which inherently exhibits large variability, they offer a standardized alternative with limited variability. Accurate knowledge of the composites material properties is important when interpreting in vitro test results and when using them in FE models of biomechanical constructs. The cortical bone analogue material properties of three different fourth-generation composite bone models were determined by updating FE bone models using experimental and numerical modal analyses results. The influence of the cortical bone analogue material model (isotropic or transversely isotropic) and the inter- and intra-specimen variability were assessed. Isotropic cortical bone analogue material models failed to represent the experimental behavior in a satisfactory way even after updating the elastic material constants. When transversely isotropic material models were used, the updating procedure resulted in a reduction of the longitudinal Youngs modulus from 16.00GPa before updating to an average of 13.96 GPa after updating. The shear modulus was increased from 3.30GPa to an average value of 3.92GPa. The transverse Youngs modulus was lowered from an initial value of 10.00GPa to 9.89GPa. Low inter- and intra-specimen variability was found.

Vibration Analysis of the Biomechanical Stability of Total Hip Replacements

In cases of severe rheumatoid arthritis or osteoarthritis, the hip joint is substituted by an artificial joint composed of a femoral stem fitted with a spherical head that can rotate inside a cup inserted in the acetabulum. This procedure is called total hip replacement (THR) and is one of the most frequently performed orthopaedic surgeries. For a cementless femoral implant, the fixation is achieved by preparing a slightly undersized bone bed, and the implant is forcefully hammered into the bone. The initial stability at the time of surgery is one of the most important factors to establish long-term survival of the implant. With each surgical hammer blow, the fixation of the implant in the bone increases. However, introducing an implant with a diameter wider than the bones inner canal contour (i.e., press fit) introduces stresses in the cortical bone, which can cause femoral fracture. In response to mechanical excitations (i.e., hammer blows or external vibrations), a femur–implant structure will display vibration modes and frequencies, just like any other mechanical structure. Changes in material properties or boundary conditions of a femur–implant structure will change its vibration modes and frequencies, which can be obtained numerically and experimentally. Therefore, this chapter explains how to perform vibration analysis on a THR component (i.e., the femoral implant) in order to assess femur–implant stability, as well as how to analyze, present, and interpret results.

A biomechanical testing system to determine micromotion between hip implant and femur accounting for deformation of the hip implant: Assessment of the influence of rigid body assumptions on micromotions measurements

Background: Accurate pre‐clinical evaluation of the initial stability of new cementless hip stems using in vitro micromotion measurements is an important step in the design process to assess the new stems potential. Several measuring systems, linear variable displacement transducer‐based and other, require assuming bone or implant to be rigid to obtain micromotion values or to calculate derived quantities such as relative implant tilting. Methods: An alternative linear variable displacement transducer‐based measuring system not requiring a rigid body assumption was developed in this study. The system combined advantages of local unidirectional and frame–and–bracket micromotion measuring concepts. The influence and possible errors that would be made by adopting a rigid body assumption were quantified. Furthermore, as the system allowed emulating local unidirectional and frame–and–bracket systems, the influence of adopting rigid body assumptions were also analyzed for both concepts. Synthetic and embalmed bone models were tested in combination with primary and revision implants. Single‐legged stance phase loading was applied to the implant – bone constructs. Findings: Adopting a rigid body assumption resulted in an overestimation of mediolateral micromotion of up to 49.7 &mgr;m at more distal measuring locations. Maximal average relative rotational motion was overestimated by 0.12° around the anteroposterior axis. Frontal and sagittal tilting calculations based on a unidirectional measuring concept underestimated the true tilting by an order of magnitude. Interpretation: Non‐rigid behavior is a factor that should not be dismissed in micromotion stability evaluations of primary and revision femoral implants. Highlights:Implant and bone motion can be measured simultaneously at several locations.Non‐rigid behavior has a major influence on implant stability measurement results.Under a rigid body assumption, distal micromotion was overestimated by up to 49.7 &mgr;m.Under a rigid body assumption, relative rotational motion was overestimated by up to 0.12°.

A nondestructive method to verify the glenosphere-baseplate assembly in reverse shoulder arthroplasty

BACKGROUND Glenoid dissociation is a rare postoperative complication in reverse shoulder arthroplasty that has severe consequences for the patient and requires revision in most cases. A mechanically compromised Morse taper is hypothesized to be the main cause of this complication, with bony impingements and soft tissue interpositioning being cited as the most important problems. Intraoperative assessment of the taper assembly is challenging. Current methods require applying considerable torque to the glenosphere or relying on radiographs. MATERIALS AND METHODS This in vitro study demonstrates how the assembly quality can be accurately determined in a nondestructive way by exploiting the implant-specific relation between screw and Morse taper characteristics by measuring the angular rotation-torque curve. RESULTS The feasibility of the method is demonstrated on 2 reverse implant models. Several data features that can statistically discriminate between optimal and suboptimal assemblies are proposed. CONCLUSION Suboptimal assemblies can be detected using the method presented, which could easily be integrated in the current surgical workflow. Clinical recommendations based on the methods rationale are also presented, allowing detection of the most severe defect cases with surgical instruments currently in use.