Lucian Capitanu

HYBRID FORCE-POSITION SYSTEMS WITH VIBRATION CONTROL FOR IMPROVMENT OF HIP IMPLANT STABILITY

The paper presents an innovative method in which the hip implant is done by a robot system, integrated into an IT platform, which allows the surgeon control through tele-operation. To this end a hybrid position-force control is accomplished which allows the tightly held stem to follow the trajectory intended by the surgeon. The main goal of the research is developing a control system for the robot end-effector with good mechanical stability during surgery and an optimum level of mounting through pressing, avoiding bone fracture and failures in positioning the stem. An intraoperative evaluation of cementless hip implant stability is done simultaneously. Recent approaches attempt to deal with this problem by developing vibration analysis equipment which would assist the surgeon in reaching an optimum compromise [1]. The frequency response of the stem-bone system is studied, being generated through a piezo-electric excitator, connected to the stem, and measured by an accelerometer attached to the femur, in order to differentiate between stable and quasi-stable implants. For vibration control is proposed the method of piezo-dynamic inversion. The control system architecture of the robots trajectory with high tracking precision through reducing and compensating the dynamic vibrations induced by the system’s motion is presented in fig.1 and is based on the method of inversion of the system’s dynamic [2]. In addition, to the robot control systems which ensure control in a desired output trajectory, having as input signals the speed, position and possibly force [3]. There is introduced an active dampening force which is to lead to a fast diminishing of the structural vibrations of the mechanical system. The active dampening force is obtained by summing the error signal ui, resulted from the position control through the known methods, with the inverse input trajectory ud which realizes the desired output trajectory yd in conformity with the method of inversing the system’s dynamic. In concept, the method entails finding the inputs which ensure exactly the desired trajectory outputs. As can be seen in Figure 1, vibration analysis has been introduced with a double goal: 1. Controlling the level of vibration which, together with hybrid position-force control, leads to avoiding intraoperative femur fractures and ensuring a long term stable implant. 2. Analyzing the frequency response for evaluating the stem’s intra-operative stability. It is worth mentioning that two of the most important issues with cementless hip implants is correctly positioning the stem in the femur and the level of pressure applied during the stem’s insertion. Excessive pressure can lead to intra-operative fractures of the femur with a likelihood of 3-28% after cementless hip replacements [4].

Damage estimator, a possible way to predict the failure of hip and knee endoprosthesis

Purpose The purpose of this study was to realize finite element simulation in order to dynamically determine the area of the contact, the contact pressure and the strain energy density (identified as a damage function) for three different activities – normal walking, ascending stairs and descending stairs – that could be considered to define the level of the activity of the patient. Design/methodology/approach The finite element model uses a modern contact mechanism that includes friction between the metallic femoral condyles or femoral head (considered rigid) and the tibial polyethylene insert or acetabular cup (considering a non-linear behaviour). Findings For all three activities, the finite element analyses were performed, and a damage score was computed. Finally, a cumulative damage score (that accounts for all three activities) was determined, and the areas where the fatigue wear is likely to occur were identified. Originality/value A closer look at the distribution of the damage score reveals that the maximum damage is likely to occur not at the contact surface, but in the subsurface.

A Novel Engineering Spherical Bearing, with Potential Application for a Hip Implant

It is a research which could enter into contradiction with the current trend concerning the hip implants. It is known as sliding friction, which is characteristic of present artificial hip joints, is higher than in the case of rolling friction. The paper reports the studies of the functioning mode of a novel spherical bearing MoM (metal on metal) with rolling friction, with potential application for an artificial hip joint obtained by introducing a number of balls between the femoral head and the acetabular cup. After over 15 years of research upon the functional principle and constructive solution, a version that offered a coefficient of minimum friction in the hip joint came to light. This version was based on a constructive solution of motion with lower friction, “Omnitrack movement solutions”, which has been modified and rebuilt to be used as a joint of a total hip prosthesis—MOMJ. The joint was built entirely in stainless steel, SS316L medical grade. Tests have been carried out on the experimental laboratory devices that showed very low values of the coefficient of friction (μ = 0.0225). For validation, the prosthesis had to be put through tests for 500,000 cycles, in terms of physiological motion and dynamic loading, according to ISO 14242-3. Testing was conducted on a multiaxial dynamics machine, MTS Bionix, equipped with system for hip implant testing. The testing results of this total hip prosthesis with rolling friction have been successful in signing up for a friction moment of 0.525 kNmm which means a coefficient of friction μ = 0.0143, for a joint with femoral head diameter 28 mm.

Conical Tip Wear in Transition Deformation – Wear Map Construction under Cyclic Impact of Hardness Ceramic Materials

Penetrator tip wear was investigated on an apparatus testing the impact on hard ceramic materials. The experimental tests were done on a vertical impact computerized test bench of original conception. Tapered diamond and carbide-metal pins were used as penetrators. It was noticed that in some situations the tip of the penetrator remained stuck in its target, acting as a Morse cone, due to the friction force between the penetrator and its target being higher than the reaction force of the collision between the two objects. This has led us to choose an angle of 1060 at the tip of the penetrator cone. For most tests we used a maximum strength of 40 N and a sintered carbide impact pin with a 1150 HV 30/15 hardness. For the study of the penetrator tip wear we have used a granite tile that owing to its high hardnes has enabled a noticeable wear of the tip. We have established a method and an algorithm for calculating the tip wear, based on images captured under a microscope after testing. A general finding is that in all tests, besides the cyclical impact deformation and the wear of coating that were tested, the wear of the tip of the pin which applies the impacts is also noticeable regardless of its shape (spherical, conical, etc), so much so that for a large number of cyclical impacts the pin tip wear must be taken into account. Variation curves are presented in the volume of wear material and of the maximum pressure p0 at the tip of the impactor, depending on the number of impacts. From the variation curve of the wear volume of material according to the number of impacts we’ve established an analytical relationship for the wear of the tip. A logarithmic formula allows for a relative assessment of the extent to which the tip is worn, as a function of the total number of incurred impacts.

About an Experimental Modeling of Steel Surfaces Wear in Injection of Plastic Composites with Short Glass Fibres

Consequences of the wear processes at screws and barrels of injection machines and extruders for processing of plastics reinforced with short glass fibers, arise after a longer time (10,000 cycles). Because of this they are working that through appropriate modeling attempts on the profits to be obtained in a short time as much information relating to both the mechanisms of wear and its size, so that the effect can be easily confuted in practice. Starting from the idea that the greatest pressure and velocity of composite melted is in the die nozzle, was an experimental nozzle with wear samples of sizes and weights which can be measured with precision as good. For a larger accuracy of measurements, we used a method for radiometric measuring, extremely accurate. Different nitriding steels have been studied as nitriding treatments, as well as some special steels and alloyed steels. Besides these, there have been preliminary attempts made to describe and checking corrosive action of thermoplasts on metals. This was another way of checking the wear testing of the samples semi-cylindrical wear samples, which served to simulate the wear of the top layer of the barrel and screw. The first results showed that wear increases depending on the increased pressure. One of the findings is that in similar conditions of injection, with the same content of glass fiber, polyamide 6.6 produces a wear greater than polycarbonate. It was also found that increasing the concentration of glass fibres increase the surfaces wear of barrels and screws.

ABOUT FAILURE UNCEMENTED HIP REPLACEMENTS FROM FRACTURE STEM PROSTHESIS

This research refers to a prosthesis made of CoCr with porous areaMadrepore macro �, recovered during a revision surgical intervention due to femoral ste ms fracture. Such a fracture appears to prosthesis distally well fixed, but proximally mobile, leading to fracture through stems middle or proximal third. In the case of this prosthesis, object of the present study, the goal is to find the cause leading to the implants failure. The macroscopic observations showed that the stems fracture occurred on a transversal plan, proximally close to the distal area of the porous zone, due to bending efforts, not preceded by a plastic strain. Based upon microscopic observations, this research shows that this process manifested itself as a fragile fracture with highlight on a pronounced trans -crystallization. In the fracture area and in its adjacency there were detected

Wear Prediction of Total Hip Prostheses Due to Common Activities

The wear prediction of artificial joints is a very difficult task due to several factors. First, one could notice a large domain of joint loading due to the wide spectrum of common activities. Secondary, to account for the evolution of contact conditions due to wearing could imply a high level of nonlinearity and time-consuming algorithms in order to solve. The authors tried to overcome all these difficulties by using a complex predictive model that combines statistical evaluation, nonlinear mechanical analyses of load transfer by the contact interface and tribological estimations of the wear characteristics. After a theoretical description of the predictive model, one could notice an application for an artificial Total Hip Prosthesis — a frictional CoCr alloy on ultra high molecular weight polyethylene (UHMWPE) couple. Several loading regimes are considered as characteristic for the common activity of the patient (normal walking, stair ascending and descending). For every regime a dynamic Finite Element simulation of the frictional contact was performed establishing the contact traces and the contact pressure distribution. Those characteristics combined with the frequencies of the activities considered are input data for computing a special point function which distribution over the contact surface could be a good measure of the wearing regime, qualitatively as quantitatively.© 2007 ASME