Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Sound from ultrasound wikipedia , lookup
Hearing loss wikipedia , lookup
Sound localization wikipedia , lookup
Noise-induced hearing loss wikipedia , lookup
Audiology and hearing health professionals in developed and developing countries wikipedia , lookup
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. 9-3 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 nonidentical 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 9-4 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 9-6 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 (Vinylacryl) 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. 9-7 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. 9-8 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 Hydroxyapatite Stainless Steel High Density Polyethylene (HDPE) Titanium Young’s Modulus Density Speed of Sound (GPa) (kg/m3) (m/s) 3000 1800 200 7000 5345 0.2-0.4 1000 447-2000 116 4500 5000 10 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 Vs 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: c E 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 20 106 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 20 106 p 5 log10 20 106 p 105 20 106 5 p 10 20 106 2 Pa 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. Cochlear, “Introduction to Baha,” <http://www.cochlearamericas.com/Products/1972.asp>, accessed 2008. iii 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). FDA, “Required Biocompatibility Training and Toxicology Profiles for Evaluation of Medical Devices,” <http://www.fda.gov/cdrh/g87-1.html> (1987). xv 9-14