Petr Henys

Evaluation of acetabular cup initial fixation by using resonance frequency principle

The clinical practice shows that the loosening of acetabular cups is more frequent than stem loosening. With standard cups, the incidence of dislocation failure is highest in the first year after arthroplasty implantation. The aim of the study was to quantitatively evaluate the implant-bone stability of a cementless acetabular cup prosthesis by using a device based on resonance frequency analysis. The evaluation of this device was done by finite element analysis and in vitro experiments. It was shown that not all the resonance frequencies can be measured by our device. The resonance frequencies vary within the range of 500–3000 Hz. The proposed power spectrum measurement gives the information about the absolute stiffness of the press-fit implant.

Impact Force, Polar Gap and Modal Parameters Predict Acetabular Cup Fixation: A Study on a Composite Bone

The balanced initial fixation of an implant makes up a crucial condition for its long-term survival. However, the quantification of initial fixation is no easy task and, to date, only qualitative assessments can be made. Although the concept of measuring fixation by means of vibration analysis is already widely used in dental implantology, the rigorous application of this method for the assessment of the fixation of femoral and acetabular components remains a challenge. Moreover, most studies on this subject have tended to focus solely on the femoral stem even though acetabular cup fixation is also important and even more difficult with respect to qualitative measurement. This study describes a comprehensive experiment aimed at assessing acetabular cup fixation. Fixation is expressed in terms of the impact force and polar gap variables, which are correlated with the modal properties of the acetabular implant during the various insertion stages. The predictive capabilities of modal frequencies and frequency functions were investigated by means of surrogate models based on the Gaussian process and functional principal component analysis. The prediction accuracy of the proposed models was in the range 82–94%. The results indicate that natural frequencies, reduced frequency, impact force and polar gap features provide great potential in terms of the prediction of implant fixation.

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.

Failure of sternal wires depends on the number of turns and plastic deformation: combined experimental and computational approach

OBJECTIVES The number of turns at the end of a wire closure is not described or discussed in any cardiosurgical guidelines. The hands-on experience of the surgeon plays a significant role. The aim of this work was to clarify the relationship between the number of turns of the suture and the resulting strength of the sternal fixation. METHODS The study was performed in 2 independent steps. The first step was a finite element simulation, where the stress and strain distribution of the sternal fixation was observed. The second step included the experimental set-up and the statistical evaluation of the results. RESULTS Our study showed that the failure force rose linearly as the number of turns increased. The lowest average measured force was 370 N (3 turns); the highest was 430 N (7 turns). The failure modes were either untwisting of the wires or rupture of the closure, which is controlled by the number of turns. As the number of turns increases, superficial cracks can occur. CONCLUSIONS Based on our results, the 5-turn option is the best solution for the closure. The failure force is still double the value reported in the literature, so there is a high safety margin for failure. The failure mode is untwisting; hence, no unexpected fracture can occur, and there is still an elastic core in the cross-section of the wire.

How does the surface treatment change the cytocompatibility of implants made by selective laser melting

ABSTRACT Introduction: The study investigates the potential for producing medical components via Selective Laser Melting technology (SLM). The material tested consisted of the biocompatible titanium alloy Ti6Al4V. The research involved the testing of laboratory specimens produced using SLM technology both in vitro and for surface roughness. The aim of the research was to clarify whether SLM technology affects the cytocompatibility of implants and, thus, whether SLM implants provide suitable candidates for medical use following zero or minimum post-fabrication treatment. Areas covered: The specimens were tested with an osteoblast cell line and, subsequently, two post-treatment processes were compared: non-treated (as-fabricated) and glass-blasted. Interactions with MG-63 cells were evaluated by means of metabolic MTT assay and microscope techniques (scanning electron microscopy, fluorescence microscopy). Surface roughness was observed on both the non-treated and glass-blasted SLM specimens. Expert Commentary: The research concluded that the glass-blasting of SLM Ti6Al4V significantly reduces surface roughness. The arithmetic mean roughness Ra was calculated at 3.4 µm for the glass-blasted and 13.3 µm for the non-treated surfaces. However, the results of in vitro testing revealed that the non-treated surface was better suited to cell growth.

Performance of radiofrequency ablation used for metastatic spinal tumor: Numerical approach:

Surgical treatment of spine metastases follows only local anatomical and biomechanical objectives. Few cases of actual solitary metastases are rather exceptional, while removal of these metastases and the primary tumor may help to eradicate the process. The aim of our subsequent numerical simulations was to find out the temperature distribution and the volume lesion in a spinal tumor. For this purpose, the parametric three-dimensional numerical model was developed. It was shown that by finite element modeling approach not only the temperature distribution but even the resulted cavity may be estimated. The numerical approach was shown as a strong tool in surgery planning.

The role of implant's surface treatment to its preload

Osseointegrated dental implants have achieved a high technology innovation level during the last 50 years. The role of surface treatment of any dental implant was mostly studied from the biocompati...

Modal frequency and shape curvature as a measure of implant fixation: A computer study on the acetabular cup

Modal parameters are often investigated in order to assess the initial fixation of an implant. Most of studies are focused on the natural frequencies and frequency response function. Usually the femoral stem is tested although the acetabular cup fixation is important as well. The results of implant stability assessment are inconsistent and seem to suggest that frequency as a stability indicator is not sufficiently sensitive. In this study the sensitivity of the modal properties to changes in the bone-implant interface was investigated with the help of the finite element method (FEM). A novel fixation index based on modal shape curvature was investigated as a potential measure of the implant fixation. Modal frequencies are sensitive to interface changes in some manner, but suffer from insensitivity to local changes at bone-implant interface. The sensitivity up to 44% of natural frequencies to stiffness change due insertion steps was observed. The tested damage indicators are able to detect localized small changes in peripheral stiffness (5% stiffness reduction) with 95% confidence under the noise up to 1%. The modal shapes and their curvatures have a great potential to be a robust fixation indicator.

Compressional stability of optical fibres: a combined experimental and computer study

Compressional stability is an important mechanical factor that influences the overall performance of a fibre. Assessment of the compressive stability and internal-stress distribution of a fibre must be performed to determine the effect of stress-induced birefringence due to the high deformation gradients in an unstable fibre. The purpose of this study is to combine experimental and finite element investigations of stress evolution and large post-buckling fibre deformations. The results of this study show that stability is greatly influenced by the initial curvature of the fibre. Increasing the deformation of the fibre leads to post-buckling behaviour that results in a fibre taking a complex shape with altered stress-field components. The results are compared with the analytical relation given by Euler’s theory of a thin beam and show that the simple analytical formula cannot predict the complicated post-buckling states and stresses.

Graft orientation influences meshing ratio

OBJECTIVES The technique of meshed skin grafting is known since 1960s. It was shown that there is a difference between the declared and real expansion ratio of the skin meshed graft. We hypothesize that the orientation of the Langers lines in a split thickness skin graft is a key parameter in the resulting expansion ratio. METHODS The skin graft meshing process was analyzed in two steps. In the first step, ex vivo uniaxial tests of human skin were performed. This served as an input for the constitutive model used for numerical simulations. In the second step, finite element analyses were performed so that stress distributions and expansion ratios could be determined. RESULTS It was shown that peaks of true stress tended to be concentrated around the vertex of the mesh pattern region for all cases. The declared expansion was impossible to obtain for all expansion ratios having the meshing incision perpendicular to the Langers lines. The highest difference between declared and real expansion ratio reaches 37%. CONCLUSIONS With regard to literature dealing with expansion of skin grafts by meshing, a high scatter amongst data results is observed. This finding was also explained by our research, demonstrating the significance of Langers lines and their relative orientation to the direction of meshing.