WO2015161175A1

WO2015161175A1 - Methods of chemical cochleostomy

Info

Publication number: WO2015161175A1
Application number: PCT/US2015/026335
Authority: WO
Inventors: Anthony J. Ricci; Markus E. HUTH
Original assignee: The Board Of Trustees Of The Leland Stanford Junior University
Priority date: 2014-04-17
Filing date: 2015-04-17
Publication date: 2015-10-22

Abstract

Methods are provided for a traumatic, hearing preservation cochleostomy by local administration of a decalcifying agent is applied to the exposed otic capsule of the cochlea, for a time sufficient to resorb or substantially thin the bony capsule. Where the capsule is thinned, a final step may be performed manually to remove the thinned bone.

Description

METHODS OF CHEMICAL COCHLEOSTOMY

GOVERNMENT RIGHTS

[0001] This invention was made with Government support under contract nos. DC003896 and DC010363 awarded by the National Institutes of Health. The Government has certain rights in the invention.

BACKGROUND OF THE I NVENTION

[0002] The cochlea remains of the few sites in the body that is non-accessible to surgery.

This is largely due to it being encased in bone, while also being mechanically sensitive, which limits access by drilling to anything but a completely deaf ear. However, with advances in regenerative medicine, mechanical hearing aids and pharmacologic approaches, there is an increasing need to access the cochlea. As such, current challenges to inner ear medicine include limiting the potential damage to functional portions of the inner ear during procedures such as cochlear implantation. The creation of an atraumatic, hearing-preservation cochleostomy is integral to the future of minimally-invasive inner ear surgery.

[0003] An increasing number of patients with low-frequency residual hearing are fitted with a cochlear implant. A particular challenge is to optimize cochlear implant device properties and develop atraumatic surgical techniques to preserve residual hearing. As such, it is an object of the present invention to provide a method for cochleostomy without significant loss of hearing.

SUMMARY OF THE I NVENTION

[0004] Compositions and methods are provided for performing a chemical cochleostomy. In the methods of the invention, an effective dose of a decalcifying agent is applied to an exposed basal turn of the cochlea, for a time sufficient to resorb or substantially thin the bony capsule. The appropriate thinness can be observed as the point at which the remaining bone is sufficiently translucent that the underlying blood vessels can be observed. At this point, gentle palpitations of the region result in bowing of the remaining bone. Where the capsule is thinned, a final step may be performed manually to remove the thinned bone.

[0005] The resulting cochleostomy allows access to the inner ear for treatment, including without limitation implantation of a device, while preserving hearing. Devices include, without limitation, multichannel cochlear implants, electric-acoustic stimulation devices, etc. Delivery of therapeutic and diagnostic formulations and devices can be performed. The access to the inner ear can be utilized in methods of stapedectomy, stapedotomy, etc. Bony resorption can be utilized as part of otosclerosis surgery, and for safe resoprtion of bone within the middle ear for non-cochleostomy applications. The access to the inner ear can also be utilized in methods of skull base surgery when the inner ear is opened to provide more extensive surgical approaches to the cranial vault.

[0006] In some embodiments of the invention the decalcifying agent is an acid. In other embodiments the decalcifying agent is a chelating agent. Agents suitable for this purpose include, without limitation, phosphoric acid, EDTA, maleic acid, citric acid, lactic acid, formic acid, trichloroacetic acid, etc. In some embodiments the agent is phosphoric acid. In some embodiments the effective dose of a decalcifying agent is provided in a semi-solid formulation, e.g. a lotion, gel, paste, etc. In some embodiments the effective dose of a decalcifying agent is provided in a patch, preferably in a size, geometry and formulation suitable for the purposes of the invention. Patches may also include components such as an adhesive layer, impermeable backing membrane, release liner, and the like.

[0007] In one embodiment, the invention comprises a method of (i) surgically exposing the basal turn of the cochlea; (ii) contacting the exposed basal turn with an effective dose of a decalcifying agent, e.g. provided in a gel, patch, etc. for a period of time sufficient to resorb or substantially thin the bone; and (iii) neutralizing or removing the decalcifying agent. The method may further comprise (iv) manual removal of the thinned bone to expose the inner ear. In some embodiments, the method further comprises surgically accessing the inner ear, e.g. implanting a hearing device, therapeutic agents; etc.

[0008] Another aspect of the invention relates to the use of a decalcifying agent in the manufacture of a medicament for cochleostomy, wherein the medicament is administered to a the basal turn of the cochlea of an individual for a period of time and dose sufficient to effect a resorption or substantial thinning of the bone.

[0009] Still another aspect of the present invention provides a kit for chemical cochleostomy.

The kit includes a formulation that provides for an effective dose of a decalcifying agent, e.g. in the form of a gel, lotion, patch, and the like. The kit may also comprise instructions for use.

[0010] It is another object of the invention that the compositions and methods of use disclosed herein can be used to aid in cochlear implantation with hearing preservation, by preventing damage to the inner ear during cochlear implantation. The method of the invention provide for reproducible local bone removal, and cochlear access in the absence of mechanical drilling.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figures 1A-F A. Opened bulla (dashes), tympanic membrane (TM), basal turn (BT), round window (RW), vestibular system (VS). B. PAG on cochlea. C. Cochlea thinned using PAG. D. PAG Cochleostomy. E. Micro— pick cochleostomy. F. Combined PAG/micro— pick cochleostomy.

[0012] Figures 2A and 2B. Representative hematoxylin and eosin sections of cochlea thinned by PAG (A) and a control cochlea (B). Arrows delineate otic capsule thinning and arrowhead demonstrates where the otic capsule is broached, leaving underlying endosteal membrane intact.

[0013] Figures 3A and 3B ABR thresholds following bulla opening (A) and thinning of the cochlea with two separate five— minute applications of phosphoric acid gel (B). No significant threshold shifts were observed.

[0014] Figures 4A and 4B ABR thresholds after chemical cochleostomy (A) and micro-pick manual cochleostomy (B). Significant threshold shifts (^*) were observed at 10 (p=0.048) and 16 kHz (p=0.0013) following cochleostomy using phosphoric acid, and at 16 kHz using manual cochleostomy (p=0.028).

[0015] Figures 5A and 5B ABR thresholds following combined PAG/micro-pick cochleostomy, with initial chemical thinning and subsequent manual removal of the last osseous layer. No significant shifts were seen following cochleostomy (A), or on postoperative day 2 or 9 (B).

[0016] Figure 6. PAG cochleostomy with endosteal veins demonstrating color change from red to brown.

[0017] Figures 7A and B Individual and average ABR thresholds at baseline (A) and following manual micro— pick cochleostomy (B) for five subjects. A statistically significant threshold shift was found at 16 kHz (p=0.028).

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0018] In the methods of the invention, an effective dose of a decalcifying agent is applied to an exposed basal turn of the cochlea, for a time sufficient to resorb or substantially thin the bony capsule. Where the capsule is thinned, a final step may be performed manually to remove the thinned bone. The resulting cochleostomy allows access to the inner ear for treatment, including without limitation implantation of a device, while preserving hearing.

[0019] These and other features of the present teachings will become more apparent from the description herein. While the present teachings are described in conjunction with various embodiments, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications, and equivalents, as will be appreciated by those of skill in the art.

[0020] Most of the words used in this specification have the meaning that would be attributed to those words by one skilled in the art. Words specifically defined in the specification have the meaning provided in the context of the present teachings as a whole, and as are typically understood by those skilled in the art. In the event that a conflict arises between an art- understood definition of a word or phrase and a definition of the word or phrase as specifically taught in this specification, the specification shall control.

[0021] It must be noted that, as used in the specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0022] Cochlea. The two human cochleae are mirror-shaped, fluid-filled, coiled, fairly symmetrical bony tubes (3.2-4.2 cm long) situated in the petrous pyramids of the temporal bones. Perilymph; the fluid inside the scalae vestibuli and tympani communicates with the CSF via the cochlear aqueduct. It is surrounded by a compact bony structure; the otic capsule. It is the hardest bone in the body with a trilamellar arrangement with islands of modified cartilage and high-mineral content, which increases the stiffness of the bony labyrinth. Vibrations of fluid in the cochlea are reflected and not absorbed by the temporal bone. However, the mechanosensitivity of tissues within the inner ear make it difficult to drill this bone without loss of hearing.

[0023] Surgically, the basal end of the cochlea is of great interest. It curves in three dimensions, resembling a "fish hook." Here, a cochleostomy is made, and it is the site of the round window. The hook anatomy varies between individuals, which makes it difficult for the surgeon to optimally place the cochleostomy and reach scala tympani (ST) without destroying any inner ear structures. The large variations in cochlear lengths, angles between turns, and position in the skull base can influence the straightforwardness for the insertion of a device, particularly passing the first turn.

[0024] Decalcifying agent. As used herein, the term refers to an agent that, when applied to a bone, will remove calcium from bone, causing the bone to resorb and thin. Many agents are known for this purpose in the art, and one or a combination of agents can be formulated for use in the methods of the invention. Preferred agents act on the bone within a period of time suitable for surgery, for example such that the end point is reached within about 30 minutes, within about 25 minutes, within about 20 minutes, within about 15 minutes, within about 10 minutes, within about 5 minutes.

[0025] Decalcifying agents of particular interest include acids. Acids used for this purpose include, without limitation, phosphoric acid, maleic acid, citric acid, lactic acid, trichloroactetic acid, formic acid, etc. Phosphoric acid is of particular interest, for example in a 30-40% gel formulation. [0026] Decalcifying acids are usually used at a pH of not more than about 2, not more than about 1.75, not more than about 1.5, not more than about 1 .25, not more than about 1 ; and may be in a pH range of from about 0.1 to about 2, from about 0.1 to 1.5, from about 0.1 to about 1.

[0027] The formulation of a decalcifying agent desirably contains the acid in the region of bone for contact, e.g. in a lotion, gel, paste, patch, etc.

[0028] Neutralizing agent. It may be desirable to remove or neutralize the decalcifying agent.

Particularly where the agent is an acid, neutralization can be accomplished by contacting the treated surface with an appropriate buffer to raise the pH to appropriately neutral levels, e.g. to at least about pH 5, at least about pH 6, and may be up to or above pH 7. Prior to addition of a neutralizing agent, the decalcifying agent may be suctioned off to remove excess; or where a film or patch it used, the film or patch is physically removed.

[0029] Neutralizing agents, which may be combined with physical barriers, may also find use in surrounding the decalcifying agent at the time of application in order to protect the surrounding tissue. Alternatively a slow release neutralizing agent is added to the decalcifying agent, such the acid is neutralized by the desired endpoint of the decalcification.

[0030] The term "effective dose" or "therapeutically effective dose" refers to the amount of an agent that is sufficient to effect the desired results. The dose of decalcifying agent is typically the amount, which may be administered one, two, three, four or more times on the site, that is sufficient to cover the surface of the bony surface. Where the formulation is provided as a gel, paste, lotion, etc., the volume may be from about 10 μΙ, from about 25 μΙ, from about 50 μΙ, from about 75 μΙ, from about 100 μΙ, from about 125 μΙ, and up to about 1 ml, up to about 750 μΙ, up to about 500 μΙ, up to about 250 μΙ, up to about 100 μΙ. Where the formulation is provided as a patch, the surface area can range from about 1 mm² to about 100 mm², for example around about 5 mm², 10 mm², 25 mm², 35 mm², 50 mm², 75 mm², etc.

[0031] The specific dose will vary depending on the particular agent chosen, the dosing regimen to be followed, whether it is administered in combination with other compounds, timing of administration, and the physical delivery system in which it is carried.

[0032] "Suitable conditions" shall have a meaning dependent on the context in which this term is used. That is, when used in connection with an antibody, the term shall mean conditions that permit an antibody to bind to its corresponding antigen. When used in connection with contacting an agent to a cell, this term shall mean conditions that permit an agent capable of doing so to enter a cell and perform its intended function. In one embodiment, the term "suitable conditions" as used herein means physiological conditions. [0033] Formulations. In some embodiments of the invention, the decalcifying agent is formulated in a gel, paste or lotion composition. The compositions of the invention include a pharmaceutically acceptable vehicle to act as a dilutant, dispersant or carrier, so as to facilitate its distribution and uptake when the composition is applied to the bone. Vehicles other than or in addition to water can include liquid or solid emollients, solvents, humectants, thickeners and powders.

[0034] The therapeutically acceptable vehicle will usually be from about 5%, from about 10%, from about 20%, from about 30%, from about 40%, from about 50%, from about 60%, from about 70% up to about 99.9%, up to about 95%, up to about 90%, up to about 80%, up to about 70%, up to about 60%, up to about 50% of the composition, and can, in the absence of other adjuncts, form the balance of the composition.

[0035] The compositions may be in the form of aqueous, aqueous/alcoholic or oily solutions; dispersions of the lotion or serum type; anhydrous or lipophilic gels; emulsions of liquid or semi-liquid consistency, which are obtained by dispersion of a fatty phase in an aqueous phase (O/W) or conversely (W/O); or suspensions or emulsions of smooth, semi-solid or solid consistency of the cream or gel type. These compositions are formulated according to the usual techniques as are well known to this art.

[0036] Emulsifiers which may be used include glyceryl stearate, polysorbate 60, PEG-6/PEG- 32/glycol stearate mixture, etc. Solvents which may be used include the lower alcohols, in particular ethanol and isopropanol, and propylene glycol.

[0037] Hydrophilic gelling agents include silica gel, carboxyvinyl polymers (carbomer), acrylic copolymers such as acrylate/alkylacrylate copolymers, polyacrylamides, polysaccharides, such as hydroxypropylcellulose, natural gums and clays, and, as lipophilic gelling agents, representative are the modified clays such as bentones, fatty acid metal salts such as aluminum stearates and hydrophobic silica, or ethylcellulose and polyethylene.

[0038] In use, a quantity of the composition, for example from 1 to 100 ml, is applied to a site of interest from a suitable container or applicator and, if necessary, it is then spread over and/or rubbed into the site using the hand or fingers or a suitable device. The product may be specifically formulated for use as a treatment for a specific area.

[0039] The lotion or gel composition of the invention can be formulated in any form suitable for application to the site of interest. The composition can be packaged in any suitable container to suit its viscosity and intended use. The invention accordingly also provides a closed container containing a therapeutically acceptable composition as herein defined, for example a pre-loaded syringe containing a unit dose of the decalcifying agent. [0040] Medical dressings or patches suitable for use in the methods of the present invention for contacting a bone with a decalcifying agent can be any material that is biologically acceptable and stable to the acid. In exemplary embodiments, the patch can be a semi-solid film or gel, and may further comprise a support of a woven or non-woven fabric of synthetic or non-synthetic fibers, or any combination thereof. Such a film or gel can be provided in a unit dose suitable for the resorption of the otic capsule.

[0041] The terms "subject," "individual," and "patient" are used interchangeably herein to refer to a vertebrate, preferably a mammal, more preferably a human. Mammalian species that provide samples for analysis include canines; felines; equines; bovines; ovines; etc. and primates, particularly humans. Animal models, particularly small mammals, e.g. murine, lagomorpha, etc. can be used for experimental investigations.

[0042] The terms "therapeutic agent", "therapeutic capable agent" or "treatment agent" are used interchangeably and refer to a molecule or compound that confers some beneficial effect upon administration to a subject. The beneficial effect includes enablement of diagnostic determinations; amelioration of a disease, symptom, disorder, or pathological condition; reducing or preventing the onset of a disease, symptom, disorder or condition; and generally counteracting a disease, symptom, disorder or pathological condition.

[0043] As used herein, "treatment" or "treating," or "palliating" or "ameliorating" are used interchangeably. These terms refer to an approach for obtaining beneficial or desired results including but not limited to a therapeutic benefit and/or a prophylactic benefit. By therapeutic benefit is meant any therapeutically relevant improvement in or effect on one or more diseases, conditions, or symptoms under treatment. For prophylactic benefit, the compositions may be administered to a subject at risk of developing a particular disease, condition, or symptom, or to a subject reporting one or more of the physiological symptoms of a disease, even though the disease, condition, or symptom may not have yet been manifested.

[0044] As used herein, the terms, contact, administer, deliver are synonymous and mean the transfer of the composition being referred to from one reservoir or repository to a tissue, cell, or part of an organ, tissue, fluid or space.

[0045] Implantable device. The methods of the invention find use in combination with a variety of implantable devices and drug delivery vehicles for the inner ear. Many such devices are known and used in the art, and need not be described in detail herein.

[0046] Generally, some types of conductive hearing loss and many types of sensorineural hearing loss are treated surgically, and utilize cochleostomy methods. For example, hearing loss can arise from the absence or the destruction of the hair cells in the cochlea which then no longer transduce acoustic signals into auditory nerve impulses. To overcome sensorineural deafness, cochlear implant systems, or cochlear prostheses, have been developed that can bypass the hair cells located in the cochlea by presenting electrical stimulation directly to the auditory nerve fibers. This leads to the perception of sound in the brain and provides at least partial restoration of hearing function. Most of these cochlear prosthesis systems treat sensorineural deficit by stimulating the ganglion cells in the cochlea directly using an implanted lead that has an electrode array. Thus, a cochlear prosthesis operates by directly stimulating the auditory nerve cells, bypassing the defective cochlear hair cells that normally transduce acoustic energy into electrical activity to the connected auditory nerve cells.

[0047] A cochlear implant system typically comprises both an external unit that receives and processes ambient sound waves and an implanted processor/cochlear lead that receives data from the external unit and uses that data to directly stimulate the auditory nerve. The cochlear lead includes an electrode array that is implanted within one of the cochlear ducts, such as the scala tympani. To minimize damage to sensitive tissues within the patient's cochlea, it can be desirable for the electrode array to be accurately placed within the cochlea using a minimum amount of insertion force. The cochlear implant should be designed so that the insertion forces do not kink or otherwise damage the delicate wires and electrodes contained within the implant.

[0048] Revision surgery can be used to explant a cochlear electrode array from the cochlea and replace it with a new electrode array. During revision surgery it is sometimes difficult to insert a compliant pre-curved or lateral electrode array due to the presence of scar tissue/ossification that has formed around the previous electrode array. For a partially ossified cochlea or for a cochlea with fibrous tissue growth, a stiffer electrode can be used to ensure insertion to the full depth of the electrode.

[0049] Alternatively, a number of drug delivery devices for the inner ear are known in the art, e.g. see any one of U.S. Patent nos. 8,507,525; 8,486,052; 8,404,654; 8,333,726; 8,303,990; 8,268,866; 8,197,461 ; 8,192,488; 8,126,572; 7,840,260; 7,815,615; 7,589,1 10; 7,498,360; 7,387,614; 7,220,431 ; 7,206,639; etc. The methods of the invention find use for providing access to the inner ear and can be used in combination with such implants and devices.

Methods of use

[0050] The methods of the invention provide a means for atraumatic cochleostomy, that reliably can be performed without loss of hearing. Such methods find use in a wide variety of situations where access to the inner ear is desired for application of therapeutic agents, implantation of devices, access to the cranial vault, and the like, where maintenance of residual hearing is desired. [0051] The methods initially involve surgical access to the optic capsule, by opening and exposing the skin, and the soft tissue overlying the auditory bulla. The bony auditory bulla is removed to the point that exposes the basal turn of the cochlea.

[0052] The exposed basal turn is contacted with an effective dose of a decalcifying agent in a formulation as described herein. In some embodiments, multiple applications are required, e.g. two, three, four or more. The decalcification process is allowed to continue until the otic capsule is substantially thinned, e.g. where the remaining bone is less than about 0.25 mm thick. The appropriate thinness can be observed as the point at which the remaining bone is sufficiently translucent that the underlying blood vessels can be observed. At this point, gentle palpitations of the region result in bowing of the remaining bone. At the desired endpoint for decalcification, the active agent is removed or otherwise neutralized with a neutralizing agent, rinsed with buffer, etc.

[0053] The remaining thinned bone is readily removed surgically by any convenient method, e.g. a right angle pick or forceps.

[0054] Following completion of the cochleostomy, the inner ear is made accessible, and the surgery may further comprise implantation of a device, drug delivery, surgical access, and the like.

Kits

[0055] Embodiments of the invention include the provision of agents for use in the methods of the invention, preferably in surgical packs, e.g. sterilized and in a unit dose. The unit dose of the decalcifying agent may be the dose required to reach the desired endpoint, or may be a single dose where multiple doses are required to reach the desired endpoint. Such a unit dose may be provided as a pre-loaded syringe; as a film or gel; in a container suitable for storage of an acid and accompanied by a delivery device; and the like.

[0056] The kit may further comprise a neutralizing agent, e.g. a buffer appropriate for the acid that is used. The neutralizing agent can be packaged with the decalcifying agent, e.g. as provided on the border of a patch or film, co-formulated as a slow release for mixing with the acid; etc., or can be separately packaged, e.g. a unit dose in a syringe, container, gel, film, etc.

[0057] Kits may further comprise additional dressings, surgical tools, instructions for use and the like.

[0058] While preferred embodiments of the present invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.

Experimental

Chemical Cochleostomy

[0059] Atraumatic, hearing preservation cochleostomy is performed in the guinea pig, which serves as a relevant animal model for treatment of humans. The methods of the invention are used to safely resorb and thin the bony otic capsule of the cochlea, without damage to the underlying membranous labyrinth and Organ of Corti.

[0060] The cochleostomy is performed by anesthetizing 250-800 gm Hartley guinea pigs using intraperitoneal injection of ketamine and xylazine. Hair is removed postauricularly along the planned incision site using electric razor and Nair. A local anesthetic of 1 % lidocaine with 1 :100,000 epinephrine is infiltrated into the planned incision site. The skin is incised using a scalpel, and the soft tissue overlying the auditory bulla is incised using a hand held heat cautery.

[0061] The bony auditory bulla is exposed, entered, and the bone removed as far medially as the vestibular system, anterolateral^ as the tympanic membrane, and posteriorly as the neck. This exposes the basal turn of the cochlea. Approximately 15 μΙ of 34% phosphoric acid gel (PAG) is applied to the basal turn of the cochlea, just distal to the round window, using a blunt 25 gauge needle. After 5 minutes, the PAG is suctioned off. It is reapplied twice more for 5 minutes each. After about three five minute applications, the bony capsule is resorbed, resulting in cochleostomy. The soft tissues closed over the bulla can be sutured to perform postoperative audiograms. Following the last application of PAG, the cochlear surface is then rinsed with a buffered solution.

[0062] This method leaves a very thin layer of bone is left covering the membranous labyrinth.

The thin layer of bone is then manually removed using a right angle pick, completing the cochleostomy.

[0063] In a comparison of H&E stained paraffin sections from a cochlea that had PA cochleostomy performed, and a control it can be seen that the otic capsule is thinned. The endosteal membrane appears grossly intact in this section. Fluorescence angiography was used to view patency of blood vessels found on the inner surface of or nearby the cochleostomy.

[0064] In the animal model, audiograms were used to test the hearing of animal following treatment with the methods of the invention. With a full chemical cochleostomy, averaging audiograms shows statistically significant shift 10 and 16 kHz, and a trend towards a threshold shift in the lower frequencies following chemical cochleostomy. Similarly, a manual cochleostomy resulting in a profound hearing loss for a number of the animals. After a partial chemical cochleostomy, leaving PA acid on the bone for a reduced period of time, there is no significant threshold shift, and combining this with a manual removal of the remaining thinned bone reliably left animals with no statistically significant hearing loss.

Example 2

Development and Characterization of Chemical Cochleostomy in the Guinea Pig

[0065] Creation of an atraumatic, hearing-preservation cochleostomy is integral to the future of minimally-invasive inner ear surgery. The goal of this study was to develop and characterize a novel chemical approach to cochleostomy.

[0066] Methods: Experimental animal study in which phosphoric acid gel (PAG) was used to decalcify the otic capsule in Hartley guinea pigs. Five animals in each of five surgical groups were studied: (1 ) mechanically opening the auditory bulla alone, (2) PAG thinning of the basal turn otic capsule, leaving endosteum covered by a layer of bone, (3) micropick manual cochleostomy, (4) PAG chemical cochleostomy, exposing the endosteum, and (5) combined PAG/micropick cochleostomy, with initial chemical thinning and subsequent manual removal of the last osseous layer. Preoperative and postoperative auditory brainstem responses and otoacoustic emissions were obtained at 2, 6, 10 and 16 kHz. Hematoxylin and eosin stained paraffin sections were compared.

[0067] Results: Surgical and histologic findings confirmed that application of PAG provided reproducible local bone removal, and cochlear access was enabled. Statistically significant auditory threshold shifts were observed at 10 (p=0.048) and 16 kHz (p=0.0013) following cochleostomy using PAG alone (group 4), and at 16 kHz using manual cochleostomy (group Page 3 of 21 3) (p=0.028). No statistically significant postoperative auditory threshold shifts were observed in the other groups, including PAG thinning with manual completion cochleostomy (group 5).

[0068] Conclusion: Hearing preservation cochleostomy can be performed in an animal model using a novel technique of thinning cochlear bone with PAG and manually completing cochleostomy.

[0069] Minimally traumatic, hearing preservation cochleostomy will be integral to the future of inner ear surgery. Although most cochleostomies performed today are in patients who have severe or profound sensorineural hearing loss for the purposes of cochlear implantation, with the advent of electric acoustic stimulation devices, preservation of residual hearing has become increasingly important. Minimally traumatic cochleostomy is also important for furthering in vivo hearing research. The ability to access the inner ear without perturbation is critical to ensuring that studies of the cochlea are reliable and accurate.

[0070] Current cochleostomy techniques generally fall into one of two categories: mechanical with a drill, or thermal with a laser. We introduce the technique of chemical cochleostomy, which represents a novel approach using phosphoric acid gel (PAG). In a manner similar to decalcification protocols for histology, the acidic environment is thought to increase the solubility of the calcium and phosphate crystalline salts that are the building blocks of bone.

[0071] We hypothesized that using an acid gel would allow for resorption of the otic capsule in a localized area. Potential advantages of this approach include avoiding exposing the inner ear to mechanical injury from the drill, the introduction of free bone fragments within the scalae, the potential for uncontrolled fracture of the otic capsulei , thermal injury generated by high speed drills and Iasers2, noise-induced injury photo-acoustic injury secondary to pressure waves generated by laser use, and mechanical or thermal injury to vulnerable adjacent surrounding structures. Application of chemical agents could also potentially be performed at locations and from angles not normally accessible using a large instrument like a drill.

Methods

[0072] Animal preparation All studies were performed in accordance with methods approved by the Stanford University Administrative Panel on Laboratory Animal Care. 25 female Hartley guinea pigs were studied, with five animals in each of five groups: (1 ) mechanically opening the auditory bulla alone, (2) PAG thinning of the basal turn otic capsule, leaving endosteum covered by a layer of bone, (3) micro-pick manual cochleostomy, (4) PAG chemical cochleostomy, exposing the endosteum, and (5) combined PAG/micro-pick cochleostomy, with initial chemical thinning and subsequent manual removal of the last osseous layer. Anesthesia was achieved using intraperitoneal injections of ketamine (40 mg/kg) and xylazine (5 mg/kg). While anesthetized, temperature, heart rate, oxygen saturation and breathing pattern were monitored (Oxypulse, Oxivet MD300m, Phoenix, AZ, USA). Following conclusion of procedures and audiometry, animals were euthanized under anesthesia by decapitation.

[0073] For Group 5, two animals were sacrificed immediately following postoperative audiometry for purposes of histologic sectioning. The remaining three animals were studied longitudinally, with audiometric measurements performed on postoperative days 2 and 9 under anesthesia. The skin and soft tissue overlying the auditory bulla was closed using two layers of simple interrupted suture. No closure of the bony bulla or cochlea was performed. These animals were given carprofen (2.5 mg/kg) postoperatively for analgesia. [0074] Surgery. Surgical sites were shaved and prepped with betadine. 1 % lidocaine with 1 :100,000 epinephrine was infiltrated subcutaneously for local anesthesia. A 1.5 cm postauricular incision was made overlying the auditory bulla. The dorsolateral surface of the auditory bulla was opened using a pick and cupped forceps as far anteriorly as the tympanic membrane, as far posteriorly as the neck, and as far medially as the vestibular system (see Figure 1 ). Anatomic landmarks were identified, including the incudostapedial joint, round window, and basal turn of the cochlea (see Figure 1A). All cochleostomies were performed along the basal turn of the cochlea, overlying the scala tympani. For those animals undergoing mechanical cochleostomy, this was achieved using a right— angle pick. Chemical cochleostomy was achieved by applying approximately 15 μΙ_ of 34% PAG (Dentsply, Philadelphia, PA) to the basal turn of the cochlea for three separate five— minute applications using a blunt 25 gauge needle (see Figure 1 B). After each application, PAG was removed using suction, and rinsing with artificial perilymph (145 mmol/L NaCI, 2.7mmol/L KCI, 2.0mmol/LMgSO₄, 1 .2mmol/L CaCI₂, 5.0 mmol/L HEPES buffer, pH 7.4).

[0075] Combined PAG/micro-pick cochleostomy, with initial chemical thinning and subsequent manual removal of the last osseous layer was performed by applying PAG for two separate five-minute applications, then using a right-angle pick to complete cochleostomy. Those animals that underwent PAG thinning of the basal turn otic capsule, leaving endosteum covered by a layer of bone similarly had applications of PAG for two separate five-minute periods.

[0076] Audiometry. Audiometric measurements were performed in a sound-proofed booth.

Auditory evoked brainstem responses (ABRs) were obtained at 2, 6, 10 and 16 kHz using custom hardware and MATLAB software described previously. Briefly, needle electrodes were placed subcutaneously at the vertex and behind the ipsilateral ear for measurement, and on a hind limb as a ground electrode. For those animals undergoing cochleostomies, the cochleostomy site was left open during audiometric measurements. The sound intensity level was raised in 10 dB steps from 10 to 80 dB SPL, and 500 responses at each sound level were recorded and averaged. The peak value of the ABR was measured and the threshold at each frequency was calculated to be when this value was five standard deviations above the noise floor.

[0077] Histology. After isolating the temporal bones, cochleae were removed by opening the auditory bulla and removing the ossicles. Cochleae were then fixed in 4% paraformaldehyde solution for 4-8 hours. Decalcification was performed in 0.12M EDTA in 0.1 M phosphate buffered solution for 5-7 days until bones were soft on manual palpation. Cochleae were then embedded in paraffin, sectioned with a microtome, and stained with hematoxylin and eosin. Sections were assessed for the degree of bony resorption. [0078] Statistical analysis. Statistical analyses were performed using Excel (Microsoft, Seattle, WA). Comparisons of averaged data were performed using one— way ANOVA and student's t-tests. P values <0.05 were considered statistically significant. Sample size calculations were based on an a of 0.05 and power value of 0.8.

Results

[0079] Power calculations revealed that with five animals in each cohort, the study was powered to detect a threshold shift of 14 dB. To detect a shift of 10 dB, nine subjects would be required in each cohort. For each of the five cochleae tested in group 4, PAG provided reproducible local bone removal, and cochlear access was enabled as seen on both microscopic observations and histopathology (see Figure 2). Lack of bony covering could be confirmed both visually and via gentle palpation under microscopic observation. Based on microscopic observation, when PAG was used to thin the cochlear wall without breach of the endosteum, veins within the endosteum directly under the cochleostomy site remained patent. When PAG was used to breach the cochlear wall, endosteal vessels appeared coagulated. This was indicated by a change in color from red to brown in each of the five cochleae in group 4 (see Figure 6).

[0080] In preliminary studies, vessels undergoing such a color change demonstrated diminished or absent bleeding when mechanically broached using a micro-pick. Conversely, vessels of those animals in group 5, which underwent completion micro-pick cochleostomy after thinning the cochlea with PAG, each demonstrated no color change, and had expected bleeding from endosteal vessels when broached. No statistically significant auditory threshold shifts were observed after opening of the auditory bulla alone (group 1 ) (see Figure 3A), or after thinning of the basal turn of the otic capsule with PAG, leaving endosteum covered by a layer of bone (group 2) (see Figure 3B).

[0081] Statistically significant auditory threshold shifts were observed at 16 kHz using manual cochleostomy (group 3) (p=0.028), and at 10 (p=0.048) and 16 kHz (p=0.0013) following cochleostomy using PAG alone (group 4) (see Figure 4). No statistically significant postoperative auditory threshold shifts were observed when cochleostomy was performed by first thinning the otic capsule with PAG (group 5) (see Figure 5A). For the three animals in group 5 who were followed longitudinally for 9 days, no threshold shifts were observed at postoperative days 2 or 9 (see Figure 5B).

[0082] The ability to perform minimally traumatic cochleostomy will be integral to the future of inner ear surgery. Current approaches to the cochlea are largely mechanical, using a drill to penetrate the otic capsule, or a micro-pick to open the round window. The inner ear can also be accessed thermally using a laser, as is common during stapes surgery. Numerous studies have examined methods to preserve hearing following cochleostomy and cochlear implantation. One meta-analysis found that better hearing preservation was associated with the use of postoperative systemic steroids, with the use of soft tissue rather than fibrin glue for cochleostomy seal, and with cochleostomy rather than round window approaches9. The use of topical steroids, application of hyaluronic acid, and variation in cochleostomy site have also been investigated. Lasers of various wavelengths (carbon dioxide, erbium:yttrium-aluminum- garnet, and potassium-titanyl-phosphate, for example) and piezoelectric devices have been used as alternatives to diamond burrs.

[0083] In this study we describe a novel approach using PAG to perform chemical cochleostomy. One concern of using PAG is its corrosive strength, and thus potential for damaging nearby structures, vasculature, or the cochlea itself. In the same way that dentists take care not to allow the acid to come into contact with mucosal surfaces, we were careful to allow the PAG contact with only the intended cochleostomy site. Following each application, we suctioned all visible gel, and rinsed it away with a buffered solution to ensure that it would not spread to unintended sites.

[0084] We were able to completely resorb the otic capsule over the basal turn of the guinea pig cochlea with phosphoric acid alone. However, this technique resulted in statistically significant threshold shifts at the higher frequencies, with only a trend towards so at lower frequencies. It is reasonable that the higher frequencies were more severely affected, as our cochleostomy site lay over the basal turn, and this area corresponds to the higher frequencies on a tonotopic map of the guinea pig cochlea. Steps intermediate to full chemical cochleostomy resulted in improved preserved hearing. Manually opening the bulla with or without thinning the otic capsule with PAG resulted in no statistically significant threshold shifts (see Figure 3).

[0085] Fully manual micro-pick cochleostomy resulted in a statistically significant threshold shift at 16 kHz (see Figure 4B), with great variability between ears. Some ears had preserved hearing following cochleostomy, however others resulted in maximal hearing loss (see Figure 7). It is possible to perform manual cochleostomy in the guinea pig without significant hearing loss. However, learning to do so reliably can be challenging and time-consuming due to the small size and delicate nature of the cochlea.

[0086] Development of techniques that are both easy to learn and easy for multiple operators to reproduce would be useful. Accordingly, application of PAG is simple and straightforward, and stray gel can easily be suctioned or wiped off of unwanted surfaces. Thinning the cochlear wall first allows for a more controlled manual cochleostomy that requires less force to perform than fully manual cochleostomy, and thus is easier to complete. This combined approach consisting of PAG thinning the cochlea, followed by manual completion micro-pick cochleostomy proved to be a minimally traumatic, hearing preservation method (see Figure 5A). Hearing preservation persisted up to 9 days following surgery (see Figure 5B).

[0087] Damage to vascular structures has been hypothesized to be a contributor to hearing loss during cochlear implantation. In a guinea pig study, intrascalar blood has been shown to cause both transient and long-term hearing. In this study, because all cochleostomies were performed along the basal turn of the cochlea just proximal to the round window, endosteal veins overlying the scala tympani in this area were inevitably encountered. Endosteal vessels were consistently observed at our cochleostomy site, though their exact location and orientation was variable (see Figures 1 D and Figure 6).

[0088] In the case of fully chemical cochleostomy, we observed coagulation of these endosteal vessels. Whether this vascular effect or direct PAG entry into the scala tympani was a larger contributor to the observed high frequency hearing loss was not determined. User experience has been shown to significantly affect surgical results in numerous otolaryngologic procedures Thus, when comparing techniques, the order in which surgeries are performed is relevant. One might hypothesize that those surgeries performed later in sequence might result in better hearing results simply due to the surgeon acquiring more experience. In the case of this study, the surgeries performed for group 5 (combined PAG/micro— pick cochleostomy), were performed prior to four of the five fully manual surgeries performed in group 3. In other words, combined PAG/micro-pick cochleostomy had superior hearing results to fully manual cochleostomy despite being performed when the surgeon had less experience.

[0089] Modifications for humans may include an increase the time that needed to resorb the thicker human otic capsule, or to increase the strength of the PAG by compounding it with a stronger acid. Another tactic might be to start the cochleostomy using a drill, follow with PAG, then finish manually with a micro-pick. To increase safety, a neutralizing agent or buffer could be prophylactically applied as a physical barrier surrounding the intended cochleostomy site to prevent spread to unintended sites.

[0090] We describe a novel technique using PAG to perform chemical cochleostomy in a guinea pig model. When PAG is used to first thin the cochlear wall followed by manual completion cochleostomy, this technique consistently preserved normal hearing. Developing easy-to-learn, reproducible cochleostomy techniques is critical for hearing research, and for future application in otologic surgery. These data demonstrate that alternative means of accessing the cochlea is possible.

Claims

What is claimed is:

1. An atraumatic, hearing preservation cochleostomy method, comprising:

(i) surgically exposing the basal turn of the cochlea;

(ii) contacting the exposed basal turn with an effective dose of a decalcifying agent for a period of time sufficient to resorb or substantially thin the otic capsule; and

(iii) neutralizing or removing the decalcifying agent.

2. The method of Claim 1 , further comprising:

(iv) manual removal of the thinned bone to expose the inner ear.

3. The method of Claim 2, further comprising implanting a device or therapeutic agent in the inner ear exposed by the cochleostomy.

4. The method of Claim 1 , wherein the decalcifying agent is an acid.

5. The method of Claim 4, wherein the acid is phosphoric acid.

6. The method of Claim 5, wherein the phosphoric acid is provided at a concentration of from 30-40%.

7. The method of Claim 6, wherein the period of time is from about 10 to about 30 minutes.

8. The method of Claim 1 , wherein the decalcifying agent is provided in a gel.

9. The method of Claim 1 , wherein the decalcifying agent is provided as a film or patch.

10. A kit for use in performing atraumatic, hearing preservation cochleostomy, comprising a unit dose of a decalcifying agent sufficient to resorb or substantially thin the otic capsule.

1 1 . The kit of Claim 10, further comprising a neutralizing agent.

12. The kit of Claim 10 or 1 1 , further comprising instructions for use.