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About this Article
Written by: Arthur Balteria
Written on: August 30th, 2002
Tags: biomedical engineering, health & medicine
Thumbnail by: Chittka L, Brockmann/PLoS Biology
About the Author
In Fall 2002, Arthur Balteria was a senior majoring in mechanical engineering at the University of Southern California. He is originally from San Diego, California and was a member of the USC Traveler Microsatellite design team and USC Jujitsu Club.
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Volume III Issue II > The Development of Ossicular Replacement Prostheses for the Treatment of Conductive Hearing Loss
One of the continuing goals of scientists and engineers is to develop technologies which reduce the severity of physical disabilities. Hearing loss due to defects in the middle ear occur with great frequency, and the more severe cases can be treated with ossicular replacement prostheses. The main focus of this article will be on the development of these prostheses, including the various challenges engineers have encountered. The primary concern when developing these prostheses is the material to be used, since there are several criterion these materials must meet. Material availability has in turn had a major impact on the overall design of these prostheses, since the design must be based on the properties of the material used. These implants represent a major achievement in biomedical engineering, and are just one way in which engineers have had a major impact on the daily lives of many.

Introduction

It has long been the goal of scientists and engineers to develop technologies which render severe physical disabilities manageable or even negligible. To a very large extent, this has been accomplished for severe conductive hearing loss through the development of ossicular replacement prosthesis. (At the age of four, this author was diagnosed with a middle ear disease called a cholesteatoma. Despite low risk of serious damage, the disease developed in both ears, and before the condition was corrected, it had managed to inflict substantial damage to the major structures of the middle ear on both sides.) As many as ten percent of all children develop conductive hearing loss [1], which is hearing loss caused by damage to the ear drum or three bones of the middle ear. (Once the disease had been corrected, this author's hearing in both ears had been reduced to about 30% of normal hearing.) However, through the use of an implant called a total ossicular replacement prostheses in both ears, these figures were improved substantially to 90% and 60% of normal hearing. Suffice it to say that this device has a major impact on the life of its recipients, as the improvement in hearing one receives is dramatic. The implant that made these improvements possible is widely used to correct conductive hearing loss, and represents a major achievement in biomedical engineering. Those devices used today represent the culmination of over 50 years worth of development, and it is this process which will be the focus of this article.

Background

The human ear is typically considered to have three regions: the outer, middle and inner ear. The outer ear serves to collect sound waves; the middle ear translates these signals into mechanical vibrations and conducts them into the inner ear, where the signal is translated into electrical signals to be interpreted by the brain. The middle ear consists of the tympanic membrane (the ear drum) and three ear bones (malleus incus and stapes, known together as the ossicle chain, and is where conductive hearing loss originates [2]). These can be seen in Figure 1.
Chittka L, Brockmann/PLoS Biology
Figure 1: Anatomy of the ear.
Conductive hearing loss occurs when any one of the three ear bones becomes dislodged or disconnected, when any one of them becomes bound in place by some inhibiting material, or when the ear bones start to degrade. There are numerous methods through which any of these events could happen. Ear infections, for example, can cause degradation of the bones or the development of scar tissue, which can bind the bones, fixing them in place [2]. Regardless of the cause, any of these defects could seriously affect a person's hearing.
Partial and total ossicular replacement prostheses (PORPs and TORPs, respectively) were conceptualized with the intent of replacing severely degraded or absent components of the ossicle chain. Most PORPs and TORPs have a relatively simple structure, with some typical designs shown in Figure 2. These devices are surgically implanted; TORPs span between the oval window and the ear drum, and PORPs between the end of an intact portion of the ossicle chain and either the eardrum or oval window. These two configurations can be seen in Figure 3.