CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation of U.S. Ser. No. 12/496,697, filed Jul. 2, 2009, which claims the benefit of U.S. Provisional Application No. 61/078,929, filed Jul. 8, 2008, which are hereby incorporated by reference herein in their entireties.
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OF THE INVENTION
1. Field of the Invention
This invention relates broadly to prostheses. More particularly, this invention relates to prostheses for the total or partial replacement of ossicles in the middle ear.
2. State of the Art
Hearing is facilitated by the tympanic membrane transforming sound in the form of acoustic sound waves within the outer ear into mechanical vibrations through the chain of ossicular bones (malleus, incus, stapes) in the middle ear. These vibrations are transmitted through the ossicular bones to the footplate of the stapes where micro or macro motion of this structure results in compression waves within the fluid of the inner ear. These compression waves lead to vibrations of the cilia (hair cells) located within the cochlear where they are translated into nerve impulses. The nerve impulses are sent to the brain via the cochlear nerve and are interpreted in the brain as sound.
Hearing efficiency can be lost to erosion of the ossicular bones. Various combinations or portions of the bones can be replaced. For example, all of the ossicles between the tympanic membrane and the stapes footplates can be replaced using a total ossicular replacement prosthesis, or TORP. Alternatively, the malleus and incus can be replaced leaving all or a portion of the stapes intact. The prosthesis for such a procedure is a partial ossicular replacement prosthesis, or PORP.
Depending on the ossicular replacement, various different configurations of prostheses can be used. For example, a TORP generally extends from the tympanic membrane to the footplate of the stapes, and distributes force from its head end at the tympanic membrane to its distal end (shoe) positioned on the footplate. A PORP generally extends from the tympanic membrane to the capitulum and/or junction of the crura of the stapes. The proximal end of the PORP includes a head that distributes force across the tympanic membrane and the distal end includes a bell or cup that seats over the capitulum and crura of the stapes.
For each type of ossicular prosthesis, several lengths must be provided given the natural differences in anatomical distances between middle ear structures in different patients. This requires that a device company manufacture, and that a surgeon (or medical facility) inventory, various sized prosthesis to accommodate the variations in dimensions across the anatomy of patients.
Moreover, due to ambient or dynamic changes in pressure within the middle ear after implantation, e.g., by sneezing or high sound pressure levels (SPL) caused by an intense noise, the distance between prosthesis coupling points can change. This may situation may result in dislodgement of the prosthesis or otherwise lead to poor sound conduction along the ossicular chain. Further, post-operative scarring down can lead to the implanted device being too long, possibly resulting in a negative effective on sound conduction. Spring elements have been considered to accommodate the change in distance that occurs during pressure changes. Bornitz, Design Considerations for Length Variable Prostheses Finite Element Model Simulations, Middle Ear Mechanics in Research and Otology: 153-160 (2004), states that good sound conduction is provided by prostheses with stiff springs, but that such springs provide only very small amounts of compression (≦0.02 mm under a static load of 5 mN), which is insufficient to accommodate the change in distance under pressure. Bornitz also determined that a soft spring can provide a suitable change in compression (up to 0.53 mm under a static load of 5 mN force), but has unacceptably poor sound transfer characteristics.
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OF THE INVENTION
An adjustable ossicular replacement prosthesis includes first and second engagement structures for engaging middle ear structures, a spring assembly that biases the first and second engagement structures longitudinally apart when under compression, and a piston assembly that facilitates longitudinal movement of the first and second engagement structures and facilitates the conduction of sound waves through the prosthesis.
In various embodiments, the adjustable prosthesis contacts or engages a membrane (with or without ossicle) at one end and contacts or engages the stapes or footplate at the other end, and even after permanent adjustment to a correct length for the patient, remains compressible and expandable along that length when implanted (in vivo).
The spring assembly preferably includes a coil spring extending between a spring stabilizer at the first engagement structure and a spring platform fixed relative to a rod of the piston assembly. The piston assembly includes an axially movable rod that is slidably disposed within a hollow body of the spring stabilizer at the second engagement structure.
Relative axial pressure on the first and second engagement structures causes compression of the spring which results in movement of the rod into the hollow body (or shoe) to compress the length of the prosthesis to accommodate changes in anatomical distance as occurs under changes in pressure. The system accommodates at least 0.25 mm and preferably 5 mm of length change.
According to one exemplar embodiment, the prosthesis is a PORP and the first engagement structure is a flanged cup for placement on the stapes, and the second engagement structure is a flat head for placement against the tympanic membrane and an adjoining open hook for engagement of the long process of the malleus (when present). The spring platform in both such embodiments is provided at the underside of the flat head. Further according to this embodiment, the spring platform is fixed on the rod and the spring is located distal of the spring platform (i.e., toward the cup). The maximum length of the rod (and prosthesis) can be permanently adjusted for a particular patient by moving the head to adjust the effective length of the rod between the first and second engagement structures, and then removing the additional protruding length (above the head), e.g., with a cutter. The lower portion of the rod will only travel through the receptacle as anatomically permitted. Thus, the replacement prosthesis is permanently adjustable in length to accommodate different patient anatomies, and the spring assembly remains capable of the full range of movement, both expansion and compression, even after the prosthesis is so adjusted in length.
According to another exemplar embodiment, the prosthesis is a TORP and the first engagement structure is a shoe for placement on the stapes footplate, and the second engagement structure is a head for placement against the tympanic membrane. The prosthesis is similarly adjustable, both permanently by the physician and post-implantation under stresses encountered in vivo.
The prostheses of the invention have very good sound transmission characteristics. The displacement of the system at 100 dB SPL across a significant audible spectrum substantially approximates an intact ossicular chain.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.
BRIEF DESCRIPTION OF THE DRAWINGS
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FIG. 1 is a side elevation of a first embodiment of an ossicular prosthesis for a partial ossicular replacement procedure according to the invention, shown in an uncompressed configuration.
FIG. 2 is similar to FIG. 1, with portions shown in broken and partial sections to illustrate an inner mechanism of the ossicular prosthesis.
FIG. 3 is a side elevation of a second embodiment of an ossicular prosthesis for a partial ossicular replacement procedure according to the invention, shown in an uncompressed configuration and prior to length adjustment.
FIGS. 4 and 5 are side elevations similar to FIG. 3 showing the prosthesis being adjusted in length.
FIG. 6 is a side elevation of the prosthesis of FIG. 3 after length adjustment and in a compressed configuration.
FIG. 7 is a perspective side view of a third embodiment of an ossicular prosthesis for a total ossicular replacement procedure according to the invention, shown in an uncompressed configuration.
FIG. 8 illustrates the prosthesis of FIG. 7 shown implanted in the middle ear.
FIG. 9 graphs displacement versus frequency in response to a 100 dB SPL sound stimulus for laser doppler vibrometry measurements at the stapes for the implanted third embodiment of the invention as compared to measurements of the intact ossicular chain, in a fresh frozen human temporal bone.
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OF THE PREFERRED EMBODIMENTS