Analysis of retrieved shoulder prostheses

In the last years the number of shoulder arthroplasties has been increasing. Simultaneously the study of their shape, size and strength and the reasons that bring to a possible early explantation have not yet been examined in detail. The research carried out directly on explants is practically nonexistent, this means a poor understanding of the mechanisms leading the patient and so the surgeon, to their removal. The analysis of the mechanisms which are the cause of instability, dislocation, broken, fracture, etc, may lead to a change in the structure or design of the shoulder prostheses and lengthen the life of the implant in situ. The idea was to analyze 22 explants through three methods in order to find roughness, corrosion and surface wear. In the first method, the humeral heads and/or the glenospheres were examined with the interferometer, a machine that through electromagnetic waves gives information about the roughness of the surfaces under examination. The output of the device was a total profile containing both roughness and information on the waves (representing the spatial waves most characteristic on the surface). The most important value is called "roughness average" and brings the average value of the peaks found in the local defects of the surfaces. It was found that 42% of the prostheses had considerable peak values in the area where the damage was caused by the implant and not only by external events, such as possibly the surgeon's hand. One of the problems of interest in the use of metallic biomaterials is their resistance to corrosion. The clinical significance of the degradation of metal implants has been the purpose of the second method; the interaction between human body and metal components is critical to understand how and why they arrive to corrosion. The percentage of damage in the joints of the prosthetic components has been calculated via high resolution photos and the software ImageJ. The 40% and 50% of the area appeared to have scratches or multiple lines due to mechanical artifacts. The third method of analysis has been made through the use of electron microscopy to quantify the wear surface in polyethylene components. Different joint movements correspond to different mechanisms of damage, which were imprinted in the parts of polyethylene examined. The most affected area was located mainly in the side edges. The results could help the manufacturers to modify the design of the prostheses and thus reduce the number of explants. It could also help surgeons in choosing the model of the prosthesis to be implanted in the patient.

Nuovo modello cinetostatico della caviglia umana.

The relevance of human joint models has been shown in the literature. They can help in diagnosis, in prostheses and ortheses design and in predicting the joints’ behavior. Recently a sequential approach for the modeling of the human diarthrodial joints composed of three steps has been proposed. At each step the role of some anatomical structures is considered. Starting from a limited number of structures, the model gets more and more sophisticated until all the components, both passive (articular surfaces, ligaments and tendons) and active (muscles), are incorporated in the final model. According to this procedure, the behavior of the human ankle during passive motion (no loads applied) has been previously modeled by a one degree of freedom 5-5 fully parallel mechanism. Starting from this model, the kinetostatic model of the human ankle joint that replicates its behavior when external loads are applied is developed. The anatomical and mechanical characteristics and the role of the passive structures are considered; a multifiber model is developed and an optimization criteria based on experimental data is proposed. Finally an application of the developed model to an amputated ankle is presented, together with the results obtained from the optimization of the geometrical and mechanical Parameters. Although some improvements can be achieved, the model is satisfactorily able to replicate the behavior of the human ankle subject to the anterior drawer and the inversion clinical tests applied in the neutral position.

Synthesis and characterization of hydroxyapatite modified with (9r)-9-hydroxystearic acid

(9R)-9-hydroxystearic acid (9R-HSA) has been proven to have antitumoral activity because it is shown to inhibit histone deacetylase 1, an enzyme which activates DNA replication, and the (R)-enantiomer has been shown to be more active than the (S)-enantiomer both in vitro and by molecular docking. Hydroxyapatite is the main mineral component of bone and teeth and has been used for over 20 years in prostheses and their coating because it is biocompatible and bioactive. The goal of incorporating 9R-HSA into hydroxyapatite is to have a material that combines the bioactivity of HA with the antitumoral properties of 9R-HSA. In this work, 9R-HSA and its potassium salt were synthesized and the latter was also incorporated into hydroxyapatite. The content of (R)-9-hydroxystearate ion incorporated into the apatitic structure was shown to be a function of its concentration in solution and can reach values higher than 8.5%. (9R)-9-hydroxystearic acid modified hydroxyapatite was extensively characterized to determine the effect of the incorporation of the organic molecule. This incorporation does not significantly alter the unit cell but reduces the size of both the crystals as well as the coherent domains, mainly along the a-axis of hydroxyapatite. This is believed to be due to the coordination of the negatively charged carboxylate group to the calcium ions which are more exposed on the (100) face of the crystal, therefore limiting the growth mainly in this direction. Further analyses showed that the material becomes hydrophobic and more negatively charged with the addition of 9R-HSA but both of these properties reach a plateau at less than 5% wt of 9R-HSA.

Development of an instrumented customizable total knee prosthesis for experimental tests.

Total knee arthroplasty (TKA) has revolutionized the life of millions of patients and it is the most efficient treatment in cases of osteoarthritis. The increase in life expectancy has lowered the average age of the patient, which requires a more enduring and performing prosthesis. To improve the design of implants and satisfying the patient's needs, a deep understanding of the knee Biomechanics is needed. To overcome the uncertainties of numerical models, recently instrumented knee prostheses are spreading. The aim of the thesis was to design and manifacture a new prototype of instrumented implant, able to measure kinetics and kinematics (in terms of medial and lateral forces and patellofemoral forces) of different interchangeable designs of prosthesis during experiments tests within a research laboratory, on robotic knee simulator. Unlike previous prototypes it was not aimed for industrial applications, but purely focusing on research. After a careful study of the literature, and a preliminary analytic study, the device was created modifying the structure of a commercial prosthesis and transforming it in a load cell. For monitoring the kinematics of the femoral component a three-layers, piezoelettric position sensor was manifactured using a Velostat foil. This sensor has responded well to pilot test. Once completed, such device can be used to validate existing numerical models of the knee and of TKA and create new ones, more accurate.It can lead to refinement of surgical techniques, to enhancement of prosthetic designs and, once validated, and if properly modified, it can be used also intraoperatively.