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					Chapter Nine
Ossicular Implants
   Babatunde Olabowale
      Alex Szatmary
      Dianne Weeks
Introduction
Sound is conducted to the inner ear by the ossicles, three small bones. One in two hundred
people will eventually experience hearing loss by otosclerosis, the hardening and deterioration of
ossicles. In addition, profound hearing loss can be caused by malformation of the ossicles due to
degenerative defects. The leading treatment for conductive hearing loss due to deterioration or
malformation of the ossicles is ossicular implantation. This treatment consists of partial or total
replacements of the damaged ossicles with a synthetic replacement typically made of Plastipore,
hydroxyapatite or titanium. To date, detailed acoustic analysis and experimentation on the
implants themselves has not been performed. In this chapter, a sequence of models--starting from
raw calculations to lab bench experiments to clinical trials--are proposed to better understand the
acoustic properties of ossicular implants as part of the ear as a system.


The Human Ear
The ear is described in three parts, outer, middle, and inner (Figure 9.1). The outer ear consists
of the auricle and the ear canal.i The auricle, the immediately visible part of the ear, collects the
sound like a funnel and transmits the sound through the ear canal to the tympanic membrane in
the middle ear. The tympanic membrane vibrates as the sound hits it; the vibration is transmitted
through the middle ear space by the three bones, the malleus, incus and the stapes, or, in English,
the hammer, the anvil, and the stirrups; these are the three smallest bones in the human body, and
are called the ossicles. The Eustachian tube, connected to the middle ear, helps to maintain the
equalization of pressure between the middle ear and the outside atmosphere. When the vibration
reaches the stapes, this results in fluid waves in the cochlea in the inner ear. The cochlea, which
is a spiral chamber, is lined with fine hairs, cilia, to detect vibrations in the fluid. The cilia then
stimulate the auditory nerve, which sends the signal to the brain.




                                                                                                   9-2
                          Figure 9.1: Anatomy of ear (left) and ossicles (right)


Types of Hearing Loss
The two major types of hearing loss are conductive and sensorineural.ii Under conductive
hearing loss, sound is not transmitted efficiently from the auricle through the timpanic membrane
and ossicles. Sensorineural hearing loss results from damage to the inner ear, especially the cilia,
or the nerve pathways from it to the brain. Mixed hearing loss is a combination of conductive
and sensorineural hearing loss.


Causes of Hearing Loss
Conductive hearing loss could arise as a result of the fusion of the ossicles to other surrounding
parts of the middle ear. Otosclerosis, the hardening of the stapes in the middle ear, is a primary
cause of conductive hearing loss. Other causes of conductive hearing loss are ear infection,
trauma, impacted earwax, and birth defects. Depending on the cause, the conductive hearing loss
is easily treatable.
        Most cases of sensorineural hearing losses are inherited; they may not always be apparent
at birth, but they show up with age. One of these hereditary disorders, presbycusis, causes loss of
cilia with age. Other causes of sensorineural hearing loss include certain kinds of antibiotics
administered intravenously (e.g., gentamicin), exposure to loud noise, brain infection, and viral
infection in the inner ear, and inadequate oxygen at birth. Generally, the sensorineural hearing
loss cannot be surgically corrected, but can be overcome by the use of cochlear implant.


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Symptoms
Symptoms of hearing loss are plain; typically, the patient notices that they have trouble hearing,
turn up the volume, and ask people to repeat what they said.

Treatments
For patients with sensorineural loss, the only current treatment is cochlear implants, which pick
up sound, like a hearing aid, but rather than amplifying the signal acoustically, it is transmitted
by radio to a sensor in the ear, which stimulates the auditory nerve.
       If the hearing loss is primarily conductive, i.e. due to failure of the timpanic membrane or
auditory bones, then a hearing aid can be employed to improve hearing by direct amplification of
the sound. Hearing aids are common and well understood.
       Another major innovation in hearing aids is the Bone Anchored Hearing Aid (BAHA).iii,iv
The BAHA compensates for conductive hearing loss by channeling sound through the skull,
rather than auditory bones, to the cochlea. BAHA also works for patients with single-sided
hearing loss, transmitting sound from the deaf side of the head to the functioning cochlea,
through the skull. The BAHA is anchored to the skull with a titanium post, and has a small
electronic sound processor that picks up sound and transmits it through the bone. Neither regular
hearing aids, nor BAHA, though, work by repairing damage to the ossicles.

Ossicular Reconstruction
Several ossicular reconstruction surgeries have been devised, varying based on the severity of the
damage or malformation.v When the stapes are damaged, otologic surgeons may replace them
with the patient’s incus. In the case where the more than one ossicular bone is damaged, either an
autograft or a homograft is done. A homograft involves extracting a bone from genetically non-
identical member, while an autograft is when a bone is extracted from one part of a person’s
body and used on the same person. There are two major methods that are used during the
ossicular reconstruction. In one of the methods the ossicular bones are joined together using a
Teflon cup and a shaft. The shaft fits into the drilled holes in the homograft or the autograft incus
or malleus head; the Teflon cup is placed on the stapes capitalum, and the ossicle is placed under
the tympanic membrane. In the other method, only a Teflon shaft is used. The shaft is fitted into

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a hole in the incus or malleus head, and the base of the shaft is placed on a footplate. When the
bones are connected, the ossicle medial is placed onto the tympanic membrane.
       Over a decade ago, best choice the choice for correction of conductive hearing loss was
bone reconstruction, but because of the problems associated with them, they are no longer
commonly used. The common problem with bone reconstruction is that when the bones are
removed at the time of revision surgery, erosion and thinning of the bone occurs. If the thinning
and erosion continues, the amount of sound transmitted to the cochlea will reduce. In the case
where a homograft is done, there is concern with disease transmission and rejection. To avoid
these transmissions, an autograft can be done, but the patient would have to endure the pain from
the extraction point and the ear surgery; also, surgery is lengthened and is thus more risky.


Ossicular Implants
Research and use of ossicular implants as a means to treat severe conductive hearing loss has
been in development for over fifty years. These devices have gone through many stages of
improvement and alteration as new designs, materials and functional results became available. It
has been critical to researchers and surgeons to find a suitable material and method for ossicular
reconstruction because the ossicles are key to a functional middle ear and because almost half of
middle ear disease affects the ossicles directly. State-of-the-art ossicular implants have proven to
drastically improve the hearing of patients at low risk of negative side effects or long-term
complications. The structure of ossicular implants, pertinent development phases of ossicular
prosthesis, the current state of research, and future areas of advancement will be discussed.


PORP and TORP
Ossicular reconstruction was conceptualized as a means to repair the ossicular chain of the
middle ear where the ossicles are located between the eardrum and oval window. The two basic
structures used for ossicle reconstruction are the Partial Ossicular Reconstruction Prosthesis and
Total Ossicular Reconstruction Prostheses (PORPs and TORPs respectively). These small
devices are surgically implanted by the otosurgeon. The PORP is placed between the remaining
working part of the ossicle and the eardrum whereas the TORP is located between the oval
window and the eardrum. Figure XX shows an implanted titanium PORP (right) and TORP

                                                                                                9-5
(left). The PORP is located between the stapes and the eardrum in this procedure. A computer
aided design image of a modern hydroxyapatite PORP and TORP are shown in Figure 9.2.




    Figure 9.2: a) Partial Ossicular Replacement Prostheses, b) Total Ossicular Replacement Prostheses




                    Figure 9.3: Modern Hydroxyapatite TORP (left) and PORP (right)


       As the images show, the structure of these devices is relatively simple and similar despite
the use of different materials. In general the prosthesis has two main components; the shaft and
head. These implants are designed to fit in a small space, conduct sound effectively and be
implanted relatively easily. In terms of physical structure, there are a few major criteria needed
for an implant to be successful. The prostheses must be rigid enough for sound transmission, but
flexible enough for maneuverability by the surgeon during the implantation procedure as well as
for movement with the ear drum during normal operation. Furthermore, it must maintain its
structure over time.


Development and Material Selection
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Although the structure and geometry is relatively consistent, the choice of material has changed
many times over the years and has posed the biggest challenge. vi There are many concerns when
choosing the correct material for ossicular implants such as the stability, functional results,
biocompatibility and acoustic performance. Throughout the years, many new materials have
been used clinically only to be removed from the market once the long term results were reported
while others are standing the test of time.
       Ossicular implants were first attempted in the 1950’s using an alloplastic material (Vinyl-
acryl) inserted between the malleus and oval window, but it was quickly abandoned when the
body rejected it. This problem set the trend for choosing better materials and design changes.
Polyethylene, polytetrafluoroethylene (PTFE) and stainless steel were used next in
tympanoplasty surgery after they were successfully used for stapes surgery, but they were also
rejected by the tympanic membrane. In the 1960’s, research into bioactive/bioinert materials was
prevalent.vii,viii Otologists attempted to use human bone or cartilage from the ossicular site to
repair any damage, as was discussed in the previous section on ossicular reconstruction. In time
these methods were not maintained.
       Alloplastic materials were once again revisited in the 1970’s and 1980’s with Proplast,
Plastipore and Ceravital. The first two are composites of PTFE and vitreous carbon while the
latter is a glass ceramic material. Proplast and Plastipore showed promise as they were designed
to promote growth of the tissue in the porous structure of the prostheses, but eventually these
prostheses brought to surface another major concern for ossicular implants: extrusion. This
phenomenon occurs when the implant becomes exposed over time. Different methods of bonding
the implant to the tissue were researched to minimize this incident. Using autografted tissue at
the head of the Plastipore prostheses prevents extrusion.
       Eventually, the alloplastic material hydroxyapatite was used for its biocompatible nature
and rigidity.ix It allows for the ingrowth of blood vessels and the complete assimilation of the
artificial bone into the individual's middle ear and presents good acoustical properties. Titanium
was also introduced as a good choice for its sturdy, lightweight and biocompatible
properties.x,xi,xii,xiii Currently the most popular choices for ossicular transplants are Plastipore,
hydroxyapatite and titanium. Figure 9.4 shows a general timeline of the material selection for the
prosthetics.

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                         Figure 9.4: Timeline of materials used for ossicular implant


Future Development
The ossicular implants have been very successful thus far but more research is needed to
optimize their performance. This includes creating devices that are easier to customize during
operation and that mimic the ossicle group more accurately. Full restoration of hearing has not
been possible yet, but with more advanced technology and improved designs, this may become a
reality in the future.


Proposed Experiments
Ossicular implants have been shown to be effective in treating conductive hearing loss.
Hydroxyapatite, titanium, Plastipore, and stainless steel have all been successfully used in
ossicular implants; these materials are all biocompatible and alloplastic. However, to date,
detailed analysis and experimentation on the acoustic properties of the implants themselves has
not been performed.




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Accoustic Efficiency
The speed of sound provides a reasonable estimate of acoustic effectiveness. As shown in Table
9.1, the speed of sound is highest in metals and lowest in polymers. This suggests that it is
reasonable to insert a stainless steel rod into a polymer shaft as has been done before, but the
speed of sound only gives a rough measure of effectiveness. The calculation neither considers
the geometry of the material nor does it predict with certainty the loss in sound intensity as sound
travels through the medium.


                           Table 9.1: Speed of Sound for Implant Materials
              Material                   Young’s Modulus             Density       Speed of Sound
                                                (GPa)                (kg/m3)               (m/s)
Hydroxyapatite                                       10                3000                1800
Stainless Steel                                  200                   7000                5345
High Density Polyethylene (HDPE)                0.2-0.4                1000           447-2000
Titanium                                         116                   4500                5000


       Microphones provide a direct measurement of sound intensity; the higher the amplitude
of the acoustic signal, the higher the volume. Transmitting a sound through a sample of material
and then comparing the intensity at the outlet to the intensity inlet as a wave travels through a
material can measure loss of sound intensity. This ratio will here be referred to as acoustic
efficiency. Acoustic efficiency would vary with input frequency. This would first be used to
evaluate acoustic efficiencies for the ossicular implant materials as a better indicator of acoustic
effectiveness than speed of sound. If tested with generic blocks of each material, this would
provide a relative indicator of acoustic effectiveness, but would still ignore geometry.
       Acoustic efficiency can also be evaluated for specific ossicular implant designs. This
allows designers to determine the acoustic benefits of alternative geometries and material
combinations, without performing costly clinical trials. As mentioned before, sometimes
stainless steel rods are inserted into Plastipore shafts, with the intent of improving sound
conduction; the actual effects of this have not been determined. However, this accoustic
efficiency measurement would indicate whether it is worthwhile to use a stainless steel core.
                                                                                                   9-9
Implant Integration into an Artificial Ear
The efficacy of the implants, as measured in terms of ABG, varies widely from patient to patient,
and from study to study. Because engineered parts, like ossicular implants, tend to have uniform
acoustic properties, this indicates that the problem is not so much with the implants themselves,
but in how they interact and integrate with the rest of the ear. As a result, modeling the
mechanics of the whole ear in a lab bench setting, to see how the whole system processes and
filters the sound, is necessary to understand the actual issues impacting ossicular implants in
vivo. Thus, the experiments above would be repeated, but, instead of applying the sound at one
end of the ossicular implant, and measuring intensity at the other, the implant would be tested as
part of an artificial ear. This artificial ear would have a membrane at one end designed to mimic
the elastic properties of the eardrum, and at the other end would be a fluid reservoir designed to
imitate the cochlea. The ossicular implant to be tested would be placed between the membrane
and the reservoir. This artificial ear would be designed, not necessarily to have the same
geometry as the human ear, but to have similar acoustic properties. Acoustic efficiency across
the assembly would be measured as an indicator of how well the ossicular implant functions as
part of a system. In particular, different measurements should be taken with slightly different
orientations of the implant, to estimate the effect of misalignment of the implant in surgery.


Osseointegration
Osseointegration and biocompatibility have been the main material selection issues to date in
ossicular implant design, which is appropriate, because, while many materials conduct sound
easily, far fewer materials are safe and effective for implantation in the body. However,
biocompatibility of ossicular implant materials has primarily been studied with animal models or
in clinical trials; this is very expensive and risky. Instead, biocompatibility and osseointegration
can be estimated in vitro; this has already been done with hydroxyapatite. xiv While it is known
that hydroxyapatite, titanium, and Plastipore are all biocompatible and alloplastic, the extent to
which they are, in relation to each other, is not well established.
       When ossicular implants are implanted, they are not simply bioinert; new bone and
cartilage actually grows on and around them. The acoustic properties of this new bone coupled

                                                                                                 9-10
with the implant ought to be investigated. As in the biocompatibility study performed by Xu, the
implants would be placed in a solution with osteoblasts. If new bone can be grown on the
implants, in vitro, this would give samples for testing acoustic efficiency, without performing
clinical studies. These ossicular implants, with bone culture, would then be placed in the "fake
ear" to evaluate how the gestalt of the system works. Now, of course, bone would not grow in the
exact same way in vitro as in vivo, but this would give some indication of how osseointegration
would impact the acoustics of the implant, before actually doing clinical trials.


Clinical Trials
It is not known which materials are best, from a biocompatibility and ossointegration point of
view, based on clinical data. In the past, almost all trials only considered a single material, in a
single design. Experimenter bias, lack of control, and variation in evaluation of results render
inconclusive the knowledge of whether one implant design or another is better, when comparing
one study to another, except in the most extreme cases. As a result, a variety of materials are
used in ossiculoplasty, without any scientific motivation for doing so. Instead, long-term, large
scale, randomized trials should be performed, with the explicit purpose of determining which
materials are most effective in ossicular implants. These trials should systematically test different
combinations of ossicular implant materials for the head and shaft, evaluating success according
to uniform criteria. Because the materials, manufacturing process, chemical composition, and
sterilization method used here are the same as the ones already on the market, the FDA is not
expected to cause problems for these tests.xv


Homework Problems
Problem 9.1
In order to determine the loudness of sound in a room, a student had to determine the change in
pressure in the room, knowing that the speed of sound in air is 340 m/s, the air density is
1.21kg/m3, the angular velocity is 20rad/s and the average displacement of a molecule in air is
2.0m. Sound can be measure with an increment of 20dB.

Problem 9.2
Find the speed of sound in hydroxyapatite, a material commonly used in ossicular implants.

Problem 9.3


                                                                                                9-11
     Given that the threshold of pain for sound is 100 dB, calculate the corresponding pressure level.
     Would this level of pressure have an impact on bones in the ear?

     Problem 9.4
     How challenging is it to select a biocompatible material for ossicular implants?

     Problem 9.5
     Jamie’s grandfather used to be a fan of rock and roll music when he was younger (less than 50
     years old). When Jamie plays his guitar wildly in his room, his grandfather gets angry because he
     says the sound irritates his ear. Why would the music cause his grandfather discomfort?

     Problem 9.6
     Is there an alternative hearing aid or device for people who cannot use traditional aids and how is
     it effective?




     Problem Solutions
     Solution 9.1
                              p  Vs
                              p  340m / s  1.21kg / m 3  20rad / s  2.0m
                              p  4.216Pa
                              Loudness  20 log(4.216)
                              Loudness  84.3dB

     Solution 9.2
     The speed of sound, c, is given by:
                                                      E
                                                  c
                                                      
     The modulus of elasticity for hydroxyapatite is about 10 GPa, while its density is 3000 kg/m 3.
     The resulting speed of sound is 1800 m/s, about six times the speed of sound in air. Notice that
     the properties of hydroxyapatite vary with composition, which varies somewhat in practice.
                                        
     Solution 9.3
     Decibels are a relative measure; when used to analyze pressure intensity, the reference pressure
     is pref  20 10 6 Pa. Recall, from the definition of the decibel,
                                                              p 
                                               L p  20log10  .
                                                             p 
                                                              ref 
   In the case of the threshold of pain, 100 dB,


                                                                                                 9-12
                                                     p 
                                    100  20log10             
                                                    20 106 
                                                  p        
                                       5  log10        6 
                                                 20 10 
                                                    p
                                         105 
                                                20 106
                                     p 10  20 106  2Pa
                                           5
                             
This is insignificant compared to yielding stresses, on the scale of MPa. As a result, ossicular
implants do not need to be designed to be strong enough to carry a soundwave; this is trivial.
                               
Solution 9.4              
Many wear-resistant, biocompatible materials are available to act as ossicular implants. Because
ossicular implants do not need to bear a load, this greatly broadens the available materials.
Hydroxyapatite is a good choice, because its biocompatibility is well understood, as is titanium.

Solution 9.5
When we are young, the muscles in the inner ear twist when a loud noise enters the eardrum. The
twisting of the muscle reduces the intensity of the sound to about 20db, which minimizes the
affects the sound has on the individual. This twisting takes 150ms for the reaction to occur.
When we get older, the flexibility of the muscles decreases so the muscles twist less and
therefore the ear reacts less to protect itself from loud noises. For this reason, Jamie’s
grandfather’s ears are irritated by the loud music.

Solution 9.6
For patients with chronic ear infections or conductive hearing loss, the Bone Anchored Hearing
Aid (BAHA) is a surgically implantable system for treatment of hearing loss that works through
direct bone conduction. It conducts sound through the bone, not the middle ear like conventional
hearing aids. The implant is placed in the skull usually behind the ear. The BAHA consists of a
titanium implant, an external abutment and a sound processor. The soundwaves are transmitted
to the inner ear through the bone conduction that occurs when sound vibrations are sent through
the titanium implant through the skull and into the inner ear.


i
     Atlas of Human Anatomy, 2001.
ii
      Mayo Clinic, “Hearing Loss,” http://www.mayoclinic.com/health/hearing-loss/DS00172, 2007.
iii
  Cochlear, “Introduction to Baha,” <http://www.cochlearamericas.com/Products/1972.asp>,
accessed 2008.
iv
 University of Maryland Medical Center, "Bone anchored hearing aid,"
<http://www.umm.edu/otolaryngology/baha.htm> (2002).



                                                                                            9-13
v
 Lang, J, AG Kerr, and GDL Smyth, "Long-term viability of transplanted ossicles," Journal of
Laryngology and Otology 100:741-747 (1986).
vi
 Yung, MW, "Literature review of alloplastic materials in ossiculoplasty," Journal of
Laryngology and Otology, 117:6:431-6 (2003).
vii
  Farrier JB, "Ossicular Repositioning and Ossicular Prosthesis in Tympanoplasty," Archives of
Otolaryngology 443-449 (1960).
viii
  Shea, JJ, and JR Emmet, "Biocompatible Ossicular Implants," Archives of Otolaryngology,
104:4:191-6 (1978).
ix
 Goldenberg, RA and JR Emmet, "Current use of Implants in Middle Ear Surgery," Otology &
Neurotology, 22:2:145-52 (2001).
x
 Vassbotn, FS, P Moller, and J Silvola, "Short-term results using Kurz titanium ossicular
implants," European Archives of Otolaryngology, 264:21-25 (2007).
xi
   Schmerber, S, J Troussier, G Dumas, JP Lavieille, and DQ Nguyen, "Hearing results with
titanium ossicular replacement prostheses," European Archives of Otolaryngology, 263:347-354
(2006).
xii
  Dalchow, CV, D Grun, and HF Stupp, "Reconstruction of the ossicular chain with titanium
implants," Otolaryngology, 125:6:628-30 (2001).
xiii
  Ho, S, RA Battista, and RJ Wiet, "Early Results With Titanium Ossicular Implants," Otology
& Neurotology 24:149-152 (2003).
xiv
   Xu, HHK, Simon, CG, "Fast setting calcium phosphate–chitosan scaffold: mechanical
properties and biocompatibility," Biomaterials, 26:1337-1348 (2005).
xv
 FDA, “Required Biocompatibility Training and Toxicology Profiles for Evaluation of Medical
Devices,” <http://www.fda.gov/cdrh/g87-1.html> (1987).




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