Massimiliano Merola

Finite Element Simulations of Hard-On-Soft Hip Joint Prosthesis Accounting for Dynamic Loads Calculated from a Musculoskeletal Model during Walking

The hip joint replacement is one of the most successful orthopedic surgical procedures although it involves challenges to overcome. The patient group undergoing total hip arthroplasty now includes younger and more active patients who require a broad range of motion and a longer service lifetime for the replacement joint. It is well known that wear tests have a long duration and they are very expensive, thus studying the effects of geometry, loading, or alignment perturbations may be performed by Finite Element Analysis. The aim of the study was to evaluate total deformation and stress intensity on ultra-high molecular weight polyethylene liner coupled with hard material head during one step. Moving toward in-silico wear assessment of implants, in the presented simulations we used a musculoskeletal multibody model of a human body giving the loading and relative kinematic of the investigated tribo-system during the gait. The analysis compared two frictional conditions -dry and wet and two geometrical cases- with and without radial clearance. The loads and rotations followed the variability of the gait cycle as well as stress/strain acting in the UHWMPE cup. The obtained results allowed collection of the complete stress/strain description of the polyethylene cup during the gait and calculation of the maximum contact pressure on the lateral edge of the insert. The tensional state resulted in being more influenced by the geometrical conditions in terms of radial clearance than by the variation of the friction coefficients due to lubrication phenomena.

Mechanical and tribological characterization of composite laminates manufactured by liquid composite molding processes

The aim of the present work is to investigate the influence of the Vacuum Assisted Resin Transfer Molding process steps on the impregnation quality of the laminates as well as on mechanical and tribological properties of the processed material. Composite laminates were realized using epoxy resin reinforced with carbon (CF) or glass continuous (GF) fibers. Two different textile architectures, namely non-crimp fabrics (UD) and woven-mat (0/90), were used and various processing conditions were employed. Optical observations revealed an unexpected trend relatively to the intra and inter bundle voids concentration with respect to the impregnation velocity, especially using UD-CF and UD-GF reinforcements and low impregnation rate. Tensile and three points bending tests highlighted the strong impact of fiber material and architecture on mechanical properties, whereas the presence of voids played a slightly influence on the fiber dominated characteristics analyzed. Tribological outcomes evidenced a reduction of the friction coefficient when the resin is reinforced by carbon or glass fibers as well as when the sliding direction of the counterbody is oriented parallel to the fiber direction.

Wear Behaviours and Oxidation Effects on Different UHMWPE Acetabular Cups Using a Hip Joint Simulator

Given the long-term problem of polyethylene wear, medical interest in the new improved cross-linked polyethylene (XLPE), with or without the adding of vitamin E, has risen. The main aim of this study is to gain further insights into the mutual effects of radiation cross-linking and addition of vitamin E on the wear performance of ultra-high-molecular-weight polyethylene (UHMWPE). We tested four different batches of polyethylene (namely, a standard one, a vitamin E-stabilized, and two cross-linked) in a hip joint simulator for five million cycles where bovine calf serum was used as lubricant. The acetabular cups were then analyzed using a confocal profilometer to characterize the surface topography. Moreover; the cups were analyzed by using Fourier Transformed Infrared Spectroscopy and Differential Scanning Calorimetry in order to assess the chemical characteristics of the pristine materials. Comparing the different cups’ configuration, mass loss was found to be higher for standard polyethylene than for the other combinations. Mass loss negatively correlated to the cross-link density of the polyethylenes. None of the tested formulations showed evidence of oxidative degradation. We found no correlation between roughness parameters and wear. Furthermore, we found significantly differences in the wear behavior of all the acetabular cups. XLPEs exhibited lower weight loss, which has potential for reduced wear and decreased osteolysis. However, surface topography revealed smoother surfaces of the standard and vitamin E stabilized polyethylene than on the cross-linked samples. This observation suggests incipient crack generations on the rough and scratched surfaces of the cross-linked polyethylene liners.

Dimensional Characterization of Prosthesis Bearings for Tribological Modelling

Prosthesis bearings are precision mechanical systems from which performance improvement represents a direct contribution to patient’s wellbeing. Spherical shape of hip prosthesis bearing approaches natural ones, but their performance become degraded in service, and the lubrication and wear mechanisms are outstanding fields of research. Because tribological phenomena are complex, attempts of modelling the bearing requires precise consideration of the boundary conditions that the real prosthesis have. Detailed experimental characterization of two ceramic hip prostheses is accomplished. Shape and roughness are measured by accurate point coordinate metrology with proper methodology through contact probing by a coordinate measuring machine, and optical measuring through confocal microscopy. The results quantify the deviation from the ideal shape and significant roughness parameters of bearing surfaces. Their influence is discussed in the spotlight of their relationship with the tribological behavior of the prosthesis. Future works direction are envisioned so geometrical boundary conditions can play an important role in prosthesis performance understanding and improvement.

Does the Hirsch Index Improve Research Quality in the Field of Biomaterials? A New Perspective in the Biomedical Research Field

Orthopaedic implants offer valuable solutions to many pathologies of bones and joints. The research in this field is driven by the aim of realizing durable and biocompatible devices; therefore, great effort is spent on material analysis and characterization. As a demonstration of the importance assumed by tribology in material devices, wear and friction are two of the main topics of investigation for joint prostheses. Research is led and supported by public institutions, whether universities or research centers, based on the laboratories’ outputs. Performance criteria assessing an author’s impact on research contribute somewhat to author inflation per publication. The need to measure the research activity of an institution is an essential goal and this leads to the development of indicators capable of giving a rating to the publication that disseminates them. The main purpose of this work was to observe the variation of the Hirsch Index (h-index) when the position of the authors is considered. To this end, we conducted an analysis evaluating the h-index by excluding the intermediate positions. We found that the higher the h value, the larger the divergence between this value and the corrected one. The correction relies on excluding publications for which the author does not have a relevant position. We propose considering the authorship order in a publication in order to obtain more information on the impact that authors have on their research field. We suggest giving the users of researcher registers (e.g., Scopus, Google Scholar) the possibility to exclude from the h-index evaluation the objects of research where the scientist has a marginal position.

Development of a Novel in Silico Model to Investigate the Influence of Radial Clearance on the Acetabular Cup Contact Pressure in Hip Implants

A hip joint replacement is considered one of the most successful orthopedic surgical procedures although it involves challenges that must be overcome. The patient group undergoing total hip arthroplasty now includes younger and more active patients who require a broad range of motion and a longer service lifetime of the implant. The current replacement joint results are not fully satisfactory for these patients’ demands. As particle release is one of the main issues, pre-clinical experimental wear testing of total hip replacement components is an invaluable tool for evaluating new implant designs and materials. The aim of the study was to investigate the cup tensional state by varying the clearance between head and cup. For doing this we use a novel hard-on-soft finite element model with kinematic and dynamic conditions calculated from a musculoskeletal multibody model during the gait. Four different usual radial clearances were considered, ranging from 0 to 0.5 mm. The results showed that radial clearance plays a key role in acetabular cup stress-strain during the gait, showing from the 0 value to the highest, 0.5, a difference of 44% and 35% in terms of maximum pressure and deformation, respectively. Moreover, the presented model could be usefully exploited for complete elastohydrodynamic synovial lubrication modelling of the joint, with the aim of moving towards an increasingly realistic total hip arthroplasty in silico wear assessment accounting for differences in radial clearances.

Influence of the Impregnation Velocity on Impregnation Quality and Mechanical Properties of Vacuum Assisted Resin Transfer Moulding (VARTM) Materials

Vacuum Assisted Resin Transfer Molding (VARTM) fabricated FRP laminates generally show significantly small void contents. The apparent difference in void content is generally attributed to the difference in the resin infusion driving force, i.e., vacuum versus injection pressure. In the present study, we contrast the influence of processing parameters on the impregnation quality of FRP laminates. Application of different vacuum pressures (500, 800 and ~103 mbar) during VARTM results in different impregnation velocity due to different compaction levels produced. Composite laminates were realized using epoxy resin reinforced with carbon (CF) or glass continuous (GF) fibers. Two different textile architectures, namely unidirectional non-crimp fabrics (UD) and woven-mat (0/90), were used and various processing conditions were employed. Optical microscope observations revealed an unexpected trend relatively to the intra and inter bundle voids concentration with respect to the impregnation velocity, especially using UD-CF and UD-GF reinforcements and low impregnation rate. Tensile and three point bending tests highlighted the strong impact of fiber material and architecture on mechanical properties, whereas the presence of voids played a slight influence on the fiber dominated characteristics analyzed.