BIOMATERIALS AND COCHLEAR IMPLANTS

Transcription

1 BIOMATERIALS AND COCHLEAR IMPLANTS Attia Qamar & Paolo Sabatini 1

2 EAR ANATOMY & MECHANICS vestibule Bony Labyrinth Coclear Duct CN VIII Vibration Tympanic Membrane Middle Ear Perilymph From oval window External Ear To round window Cochlea Inner Ear Vibrations transmitted through the perilymph in the bony labyrinth will be transmitted to the endolymph in the cochlear duct 2

3 From oval window Perilymph in scala vestibuli Vestibular membrane Tectorial Membrane Endolymph in Cochlear Duct /scala media Basilar membrane Vibrations conveyed to Organ of Corti Organ of Corti To round window Perilymph in scala tympani Vibrations of Tectorial Membrane Stimulation of cilia on hair cells Tension of Basilar membrane dampens vibration Transmission of stimulus along CN VIII 3

4 Low Frequency High Frequency The Basilar Membrane in Action: tm 4

5 Hearing Impairment Two types of hearing impairment: Conductive hearing loss: : Damaged apparatus that transmits sound energy to cochlea. (Eg( Eg/ / Damaged eardrum, bone tissue encrusted ossicles) Treatment: Hearing Aid or Surgery Sensorineural hearing loss: : Damaged/destroyed hair cells within organ of Corti or destroyed auditory nerve (can be caused by a tumor). Treatment: Cochlear implant in case of viable auditory nerve, direct stimulation of sound centre in brain required otherwise. 5

6 Main Causes of Hearing Loss Heredity Infections: eg.. rubella, bacterial meningitis Acoustic Trauma: due to acute or chronic exposure to loud sounds Prescription drugs (streptomycin. (streptomycin. tobramycin, cisplatin a chemotherapy agent) Presbycusis: due to old age, acoustic trauma and age hardening of microscopic blood vessels in inner ear. Presbycusis 6

7 Candidate Criteria Profound sensorineural hearing loss of cochlea site. Extremely poor word recognition in open-set paradigms. A patent cochlea and viable auditory nerve. Good health to withstand anesthesia and surgery. 7

8 Cochlear Implant Mechanics BRIEF OVERVIEW OF THE IMPLANT: 8

9 Cochlear Implant Processing Strategies Processing Strategy Electrode Type Max. # of Channels Feature extraction Longitudinal bipolar 21 Bipolar 15 Compressed analog (Simultaneous Stimulation) Monopolar 4 Interleaved pulsatile Radial bipolar 8 (Digital) Longitudinal bipolar 21 9

10 Multichannel vs. Single Channel (Prosthesis Audio Samples) Speech: increasing channels 1, 2, 4, 8, 16 (more tolerant of degradation) Music (instrumental, complex): increasing channels 4, 8, 16, 32 (requires fine processing to achieve melody recognition) 10

11 Goals Transmit coded frequency intensities through electrode array to CNS (Central Nervous System) via multi- channel electrical stimulation of hearing nerves. Insert electrode array into correct location in cochlea, to minimize tissue and auditory nerve damage. Non toxic and biocompatible materials to prevent injury to cochlea and auditory nerve. Minimize current and charge density build up to prevent damage to surrounding tissue. Proper electrode geometry for localizing current to separate nerve fibers. Proper implant procedure to prevent infections from spreading to inner ear from middle ear (eg( eg.. meningitis) Sealed electronics to prevent corrosive body fluids from entering and causing device failure Durable enough to not result in damage due to head impact or result in fatigue or fracture due to acute movements. 11

12 Biomaterials External Components Microphone - directional Speech Processor Platinum (strong, light, one quartz-crystal oscillator) Transmitter Internal Components Receiver Stimulator Electrode Array Electrodes 12

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14 Receiver/ Stimulator Package (side by side placement, cylindrical) Requirement Impervious to body fluids Robust No corrosion No leaks Material Sintered Ceramics Seal (AlO 2, SiO 2, MgO) Titanium case (bonds with ceramic material) Seal container halves by welding (Ti to Ti) Helium 14

15 Receiver specifically Requirement Efficient Signal Transmission Robust No Migration Material coil wrapped around rare earth magnet Titanium capsule Seal container halves by welding (Ti to Ti) 15

16 Electrode Array Simulation of contour insertion Requirement No Migration Easy insertion/ limited trauma Not an infection carrier Osseointegration Insulative Material Silastic MDX (Graded stiffness) Fascia homograft, titanium collar, with hydroxyapatitie coat Silastic type A (biocompatible glue) 16

17 Electrode Contacts Requirement Conductive No electrolysis Minimum toxic electrochemical reactions Material Platinum Iridium A. Deaf, Stimulated B. Deaf, Unstimulated 17

18 Biocompatibilty Issue Solution Channel Interaction (conductive fluid in cochlea) Charge density buildup (tissue and implant damage) Spread of infections (meningitis and streptococous pneumonia) Sequential Stimulation (Interleaving) Increase contact surface area (mushroom shape) Graftin fascia about electrode entry point, Prevent seeding bacteria at time of surgery 18

19 Migration Fixation and retrospective magnet Head growth and electrode extraction Electrode junction week MRI (implant damage) Additional mm loop in lead wire and fixing array to floor of mastoid atrum Stress relief via spiral electrodes Non magnetic version 19

20 Surgery Precautions: micro-claw claw protocol to prevent seeding bacteria Flap < 6mm Bed depth approx 5mm, round Emplanted electrode array Complications: Hemorrhage : mastoid emissary vein; control with diamond burr, bone wax, crushed muscle, Gelfoam Facial Nerve and Chorda Tympani Injury Perilymph Gusher 20

21 Postoperative Complications Wound Breakdown Pain: infection, extracochlear spread of current, excessively strong magnets Infection Labyrinthitis and Meningitis 21

22 The Manufactures Advanced Bionics - Clarion Cochlear Corporation - Nucleus MED-EL EL - Combi

23 The Future For those with destroyed auditory nerves - Implantation of electrodes into cochlear nucleus - Implantation of Electrodes into auditory cortex 23

24 Refrences arshana.pdf Cochlear Implants ; Fundamentals and Applications, Graeme Clark Cochlear Implants, Schindler & M. Merzenich Cochlear Implants, Roger F. Gray Cochlear Implants in Adults and Children, NIH Consensus Development Conference, May 15-17, 17, 1995 Practical Hearing Aid Selection and Fitting, Henry Tobin 24

25 The END 25