Abstract:
Ossicular replacement prostheses are manufactured from a non-biodegradable shape memory polymer. Such prostheses can include TORPs, PORPs, and incudo-stapedial joints (ISJs). The prostheses are reshaped upon application of a stimulus to capture a portion of one or more ossicles. The force of capture of a reshaped polymeric prosthesis is less than a comparable reshaped shape memory alloy prosthesis and thereby prevents recipient discomfort and/or pressure induced necrosis of the bone. In addition, biocompatible shape memory polymers can be designed with recoverable strain that is orders of magnitude higher than shape memory alloys. Furthermore, prostheses can be manufactured in a single size and easily trimmed or otherwise modified by the surgeon during the implantation procedure to tailor the prosthesis in size and/or shape for the particular anatomy. Such modification does not negatively effect the ability of the prostheses to engage the ossicle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional No. 60/823,914, filed Aug. 30, 2006, which is hereby incorporated by reference herein in its entirety. 
     
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates broadly to implantable prostheses. More particularly, this invention relates to prostheses for total or partial replacement of the ossicular bones or joints of such bones. 
         [0004]    2. State of the Art 
         [0005]    Hearing is facilitated by the tympanic membrane transforming sound in the form of acoustic sound waves within the outer ear into mechanical vibrations within the ossicular chain of bones in the middle ear. The mechanical vibrations are transmitted to the oval window where pressure on the oval window membrane causes compression waves within the fluid of the inner ear. The 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. 
         [0006]    Due to disease, trauma, or congenital malformation, the ossicles of the middle ear are sometimes damaged. Hearing efficiency can be lost to erosion of the ossicular bones: maleus, incus, and stapes. These bones can be completely replaced by a prosthesis (total ossicular replacement prosthesis, or TORP) or replaced in part (partial ossicular replacement prosthesis, or PORP). 
         [0007]    In addition, the delicate joint between the incus and the stapes is termed the incudo-stapedial joint (ISJ). The ISJ is a cartilaginous joint having a tendency to ossify in older humans. When the joint is interrupted due to erosion of the joint or the incus itself, vibrations can no longer be transmitted from the incus to the stapes. The result is a conductive hearing loss related to the disrupted ossicular chain. 
       SUMMARY OF THE INVENTION 
       [0008]    Ossicular replacement prostheses are manufactured from shape memory polymers. Prostheses are provided for TORPS, PORPS, including but not limited to pistons, as well as the incudo-stapedial joint (ISJ). The shape memory polymers can be either thermoplastic or thermoelastic, with low toxicity. In various embodiments, the prostheses are reshaped upon application of a stimulus to capture a portion of one or more ossicles. The force of capture of a reshaped polymeric prosthesis is less than a comparable reshaped shape memory alloy prosthesis and thereby prevents recipient discomfort and/or pressure induced necrosis of the bone. In addition, the shape memory material has relative low mass which means it is can be more easily acted upon. It also has no nickel which could otherwise cause a negative reaction in sensitive individuals. Moreover, it is recognized that the shape memory polymer it relatively MRI inactive, in distinction from metals, which are often used in ossicular prostheses. 
         [0009]    Furthermore, the prostheses for replacement of the various ossicles and joints can be manufactured in a single size and easily trimmed or otherwise modified by the surgeon during the implantation procedure to tailor the prosthesis in size and/or shape for the particular anatomy. Such modification does not negatively effect, and may improve, the ability of the prostheses to engage a portion of the ossicle. Moreover, the ability to tailor size and shape permits a limited inventory of prostheses. 
         [0010]    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 
         [0011]      FIGS. 1 and 2  are side elevation views of a total ossicular replacement prosthesis (TORP) made from shape memory polymer according to the invention, shown at a first length and stimulated second length; 
           [0012]      FIG. 3  is a side elevation view of another embodiment of a TORP made from a shape memory polymer according to the invention; 
           [0013]      FIG. 4  is a side elevation view of a stapedial prosthesis, more specifically a piston, made from a shape memory polymer according to the invention; 
           [0014]      FIG. 5  shows the stapedial prosthesis of  FIG. 4  with a central opening mechanically enlarged after molding and heat setting; 
           [0015]      FIG. 6  shows the stapedial prosthesis of  FIGS. 4 and 5  after application of stimulus to cause shape change; 
           [0016]      FIG. 7  is a side elevation view of a incudo-stapedial joint (ISJ) prosthesis made from a shape memory polymer according to the invention; 
           [0017]      FIG. 8  is a top view of the ISJ prosthesis of  FIG. 7 ; 
           [0018]      FIG. 9  is a section view across line  9 - 9  in  FIG. 8 ; and 
           [0019]      FIG. 10  is a bottom view of the ISJ prosthesis of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    In accord with the invention, an otological prosthesis such as a total or partial ossicular replacement prosthesis (TORP or PORP) or an incudo-stapedial joint (ISJ) prosthesis is manufactured from a shape memory polymer. Shape memory polymers may be thermoelastic or thermoplastic. For otological prostheses it is important that the shape memory polymer, while being biocompatible also be non-biodegradable and thus suitable for long-term implantation. One suitable class of polymers includes acrylics of methacrylate. More particularly, one preferred shape memory polymer suitable for biomedical applications, and specifically for fabrication of an ossicular prosthesis, is synthesized by photopolymerization from a tert-Butyl acrylate monomer with a diethyleneglycol-dimethacrylate crosslinker. A shape memory polymer as discussed above is available from MedShape Solutions, Inc. of Atlanta, Ga. It is appreciated that other shape memory polymers, such as Calo.MER™ shape memory thermoplastic available from The Polymer Technology Group of Berkeley, Calif. also may be suitable. Other biocompatible shape memory polymers suitable for long term implantation can also be used. 
         [0021]    A preferred shape memory polymer has a density of approximately 0.043 lbs/in 3 , approximately one-fifth the density of the nickel titanium shape memory alloy, Nitinol, (0.235 lbs/in 3 ), which is another material used in ossicular prostheses. The significantly less dense shape memory polymer material results in an implant of reduced mass. Such implant is easier for the remaining otological structure to act upon and move to effect auditory function. In particular, low mass is an important attribute of an ossicular prosthesis because it correlates directly to the force that is required for the tympanic membrane to overcome the inertia of the device and create compression waves at the oval window. Also, the recoverable strain of the shape memory polymer is orders of magnitude higher than nickel titanium shape memory alloy. 
         [0022]    In addition, some otologists are concerned about the potential for toxicity and patient sensitivity related to the diffusion of nickel ions from nickel titanium alloy prosthesis. In distinction from nickel titanium alloy prostheses, the shape memory polymer prostheses do not include a concentration of nickel that can elute therefrom into the surrounding tissues. 
         [0023]    Moreover, it is recognized that the use of any metal ossicular prosthesis can result in issues with magnetic resonance imaging (MRI) compatibility. MRI is an increasingly useful diagnostic tool. However, the use of MRI can induce movement of a prosthesis. If the prosthesis is moved excessively it can be displaced causing injury and/or loss of efficacy. In order to visualize the delicate structure of the ear and the surrounding brain increasingly more powerful MRI scanners are being used. Enhanced MRI compatibility is an important attribute for an ossicular prosthesis. The shape memory polymer prostheses according the invention are not affected by MRI. 
         [0024]    By way of example, and not by limitation, the following embodiments of otological prostheses fabricated at least in part from a shape memory polymer are now described. 
         [0025]    Referring to  FIG. 1 , a total ossicular replacement prosthesis (TORP)  10  replaces the maleus, incus and stapes. The TORP includes a shaft  12  having at an inferior end thereof a cylinder  14  for contacting the oval window and at a superior end thereof a disc  16  for contacting a tympanic membrane (ear drum). An exemplar TORP  10  is described in detail in U.S. Pat. No. 6,168,625 to Prescott, which is hereby incorporated by reference herein in its entirety. In accord with the invention, the TORP is fabricated from a shape memory polymer. The fabricated length is longer than the traditional length of a TORP, e.g., 3.0-7.0 mm. The TORP is then heated above the glass transition temperature (Tg), e.g., 75° C., and a load is applied such that the shaft  12  compresses to a length L 1  that is shorter than a (or no longer than the shortest) traditional TORP. The TORP is then cooled below Tg; i.e., quenched. 
         [0026]    Referring to  FIG. 2 , prior to positioning the TORP  10  (or other ossicular prosthesis) in situ, the surgeon will determine a distance between two otological structures and then position between such structures a prosthesis having a length shorter than the measured distance. The surgeon then applies an appropriate stimulus, such as heat, to effect shape change to the TORP. Generally, within approximately 20 to 40 seconds, the device lengthens to L 2  until it makes contact with the two otologic structure (oval window, capitulum or footplate of the stapes, tympanic membrane, incus, malleus) with a predetermined contact pressure, preferably in the range of 1 to 5 MPa. The prosthesis implant will fit tight, but not too tight, so as to cause no discomfort to the recipient. The use of single size prosthesis which expands in length to accommodate all patients permits the maintenance of a reduced inventory. 
         [0027]    Referring to  FIG. 3 , it is also appreciated that length adjustment of a TORP  20  can also be accomplished by designing one or more bends  22  into the shaft  24  of the TORP and compressing the TORP about the bends  22  during heating at or above the glass transition temperature (Tg). The bend(s) may be angular or curved. The prosthesis is then quenched. Similarly, a helical coil (also shown by  22 ) can also be fabricated into the shaft. Then, when heated at a predetermined temperature upon implantation, e.g., 50° C., the shaft changes shape at the feature (bend, coil, twist, etc.) to effect lengthening of the shaft and contact with the appropriate anatomy. 
         [0028]    A partial ossicular replacement prosthesis, or PORP, replaces a subset of the ossicular bones (one or more, but not all). A shape memory polymer can be used for fabrication of a PORP. Turning now to  FIGS. 4 through 6 , by way of example, a Causse-type piston prosthesis  100  for replacement of the stapes is shown. The prosthesis  100  includes a shaft  102  and a circular head  104 . The head  104  is machined with a hole  106  having a first diameter D 1 , such that the hole is uninterrupted about its circumference. The hole may optionally be interrupted with a radial slot  108  shown in dotted lines. After fabrication, the prosthesis is heated above the glass transition (Tg) temperature of the shape memory polymer, e.g., 75° C., and a mandrel having an outer diameter D 2  is inserted into the hole. The prosthesis is then quenched and the mandrel is removed to maintain the diameter D 2  of the hole  106 . Then, during implantation, when the surgeon applies a heat stimulus to the prosthesis at, e.g., 50° C., the inner diameter D 3  of the hole decreases to an intermediate diameter between D 2  and D 1  as it surrounds and engages a portion of the incus and couples thereto. The force of capture of the implant is less than a shape memory alloy bight and thereby prevents recipient discomfort and/or pressure induced necrosis of the bone. 
         [0029]    Turning now to  FIGS. 7 through 10 , a shape memory polymer incudo-stapedial joint (ISJ) prosthesis  200  obviates both a large inventory of different sized products as well as a complex product. The prosthesis  200  includes an incus slot  202  for receiving the long crus of the incus, and a tubular portion  204  for receiving the capitulum of the stapes. The slot  202  and tubular portion  204  are substantially transversely arranged at joint  206  so as to form a generally L-shaped structure. 
         [0030]    More particularly, the incus slot  202  is defined from a tube  208  having a longitudinal slit  210  along an inner portion thereof (a lower surface) and opposing lateral intersecting slits  212 ,  214  adjacent the joint  206 . Such slits  210 ,  212 ,  214  enable flaps  216 ,  218  of the tube  208  to be folded open to form the open slot  202 . In manufacture, the flaps  216 ,  218  are folded open after the prosthesis is heated above the glass transition temperature and then once opened quenched to maintain such configuration until again heated above a predetermined temperature which activates the material during surgery. 
         [0031]    The tubular portion  204  includes longitudinal slots  220 ,  222  for receiving the arches of the stapes through the tubular portion at any vertical location. The inner diameter of the tubular portion  204  is heated above the glass transition temperature and expanded with a mandrel to expand its size to accept practically any capitulum and then quenched to maintain such configuration until again heated above a predetermined temperature which activates the polymer during surgery. 
         [0032]    In use, the surgeon determines the appropriate length of each of the incus slot  202  and stapedial tubular portion  204 , depending the vertical and horizontal displacement of the incus and stapes. Both the slot  202  and tubular portion  204  are deep enough to accommodate vertical ‘Y’-height differences between the capitulum of the stapes and the shaft of the incus and for horizontal ‘X’-length differences. The additional polymer material, if any, at each of the portions  202 ,  204  can be cut and trimmed to custom fit the prosthesis to the bones. The ISJ prosthesis is then positioned to receive the capitulum with the tubular portion  204  with the arch of the stapes extending out of the slots  220 ,  222 , and the long crus of the incus extending within the slot  202 . Heat, e.g., 50° C., is then applied to the prosthesis causing the shape memory polymer to activate fixation. That is, the stapedial tubular portion  204  shrinks to secure the capitulum and the flaps  216 ,  218  close about the incus (re-forming a tube) to capture the incus. Only an appropriate length of each ossicle is clamped, and only with appropriate force, to prevent occluding the blood supply to the bones, and to prevent significantly limiting the bone motion and the transference of motion necessary for hearing. 
         [0033]    There have been described and illustrated herein several embodiments of shape memory polymer otological prostheses. While particular embodiments of the invention have been described, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. Thus, while TORPs, PORPs and ISJ prostheses have been generally described with reference to exemplar embodiments, it is appreciated that the invention is not limited thereto and is applicable to any suitable design of a TORP, PORP, and ISJ prosthesis, and even to any otological prosthesis. Moreover, while the exemplar embodiments are solely fabricated from a shape memory polymer, it is also appreciated that only a portion of an otological prosthesis may be fabricated from a shape memory polymer, and that other portions thereof may be fabricated from a conventional material including a metal, metal alloy, ceramic, or non-shape memory polymer. This permits the advantage of the beneficial characteristics of the various materials for different portions of the prosthesis when a non-uniform construct is desired. Also, while heat has been described in detail as the activating stimulus, it is appreciated that an activating stimulus that does not significantly alter the temperature of the polymer material can also be used, given an appropriate polymer. For example, light at an appropriate wavelength may be used to activate shape change in a stable biocompatible polymer for otological implants. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its spirit and scope as claimed.