Numerical Analysis of Stapes Prosthesis Constraining in the Case of Otosclerosis.

In the case of otosclerosis, it has been noticed that even though there are a variety of different prostheses on the market, due to the anatomical characteristics, it is not always possible to restore excellent mobility to the ossicles and the prosthesis. On the one hand, this happens because the incus long process and the prosthesis create difficult angles. On the other hand, incus necrosis is among the most common causes of the loss of stability to the prosthesis and stapedectomy failure. The aim of this research is to suggest an improvement for stapes prosthesis stability and numerically evaluate the impact of the prosthesis constraining to its dynamical behavior. Numerical 3D models of a standard as well as a modified (adjustable angled) stapes prosthesis were created in order to achieve this aim. Consequently, the modal analysis has been performed to evaluate the mechanical behavior of the prosthesis, assuming that the piston (thick part) would be made of Teflon, and the thin part, fixated on the incus long process, would be made from titanium alloy. Finally, the numerical analysis has been conducted by changing the boundary conditions in respect of the prosthesis constraining, where the attached stapes prosthesis connects to the ossicular chain. Subsequently, there were two hypotheses raised for the prosthesis loop constraining. The first is that during the surgery, the prosthesis is perfectly crimped with certain looseness. The second is that the prosthesis is outgrown by the tissues over time and thus becomes over-constrained. Then, the analyzed standard stapes prosthesis does not fulfil its functions because of the over-constraining that develops over time. An improvement for the standard stapes prosthesis, i.e., a modified stapes prosthesis (adjustable angled), that has been proposed in this research allows avoidance of the negative effects of the over-constrained standard stapes prosthesis that appear over time. Moreover, the proposed modified prosthesis helps to regain hearing when the angle between the incus long process and prosthesis is unfavorable.

Characterization of mechanical behaviour of healthy and injured human incus by eigenfrequency evaluation.

The usage of finite element method techniques gives a possibility to replace time-consuming experiments or imitate physical process in the ear by numerical simulation. Especially, the research of spatial motion of ossicular chain in the middle ear is of high interest for the oto-surgeons and engineers. It is known that the most affected bone from the ossicular chain is the incus. After the cholesteatoma operation and tympanoplasty, the affected incus is removed or sacrificed; thus, the possibility of transducing noise lays on the stapes, new titanium or other material prosthesis. In this case, the affected incus was removed because of the cholesteatoma that was lying in front of it in the tympanic cavity. The removed incus with the affected long process passed micro-computed tomography. The computer-aided design systems allowed redesigning a 'healthy' incus with an intact long process. In this way, it was possible to evaluate the influence of damaged long process of incus in the vibrational analysis. This article analyses the problems of mechanical behaviour of injured and healthy human incus. The numerical simulation has demonstrated that the features of healthy incus and analysed injured incus do not differ significantly, especially at low (about 500 Hz) frequencies. It explains why there is no impact of cholesteatoma on hearing for a long time in the audiogram.