Concave gase tissue expander
BACKGROUND OF THE INVENTION 
     Tissue expansion devices have been used by plastic surgeons for over thirty years for the generation of skin flaps in reconstructive surgery. Skin expansion is commonly observed, for example, in pregnant women. For the overlying skin to remain healthy, the expansion must take place gradually as in the case of the developing fetus. Additionally, the underlying pressure on the skin must be distributed uniformly to prevent necrosis of the thin overlying tissue. These considerations have guided the development of the tissue expansion devices currently in use. 
     It is particularly desirable to generate a donor skin flap on a portion of the body contiguous with the recipient area. In this way, skin texture and abundance of hair in the grafted area can be made to more or less blend with the surrounding skin giving a more esthetically pleasing result. To accomplish this, the surgeon makes a pocket under the skin and subcutaneous tissue adjacent to the area to be grafted. A tissue expander is placed in the pocket and gradually inflated over a period of weeks or months until the device is fully inflated. Since the base of most tissue expanders is thicker and less flexible than the cover, the envelope presses upward during expansion stretching the overlying skin. As the skin stretches, it becomes thinner and more vulnerable to trauma. In particular, if the underlying pressure on such distended skin is excessive, circulation may be impaired with resulting necrosis and death of cells. 
     Due to the tension on the periphery of the base of conventional tissue expanders caused by the upward pull of the expanded flexible cover, the edges of the base can curl up when the device is fully expanded or over-expanded. If the base curls up, it can cause excessive pressure on the thin overlying skin which can result in necrosis and erosion. It is the object of the present invention to provide a tissue expander with a base that protects the overlying tissue against damage associated with base pressure observed with fully inflated or over-inflated current art devices. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide a tissue expander device which, when fully inflated, reduces or eliminates damage to the overlying skin due to excessive pressure caused by curling of the base up into the thin overlying skin. The feature of the present invention which accomplishes this is the concave base. The operation of this feature can be best understood by reference to the following description taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic perspective view of the tissue expander of the present invention before inflation. 
     FIG. 2 is a schematic perspective view of the tissue expander of the present invention when fully inflated. 
     FIG. 3 (a) is a cross-sectional view along A--A&#39; (FIG. 2) of the implanted tissue expander device of the present invention prior to inflation. 
     FIG. 3 (b) is a corresponding cross-sectional view of FIG. 3 (a) for a conventional flat based tissue expander. 
     FIG. 4 (a) is the cross-sectional view of FIG. 3 (a) when the tissue expander is fully inflated. 
     FIG. 4 (b) is the cross-sectional view of FIG. 3 (b) when the conventional flat-based tissue expander is fully inflated. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following specification taken together with the drawings sets forth the preferred embodiment of the present invention. The embodiment of the invention disclosed herein is the best mode contemplated by the inventors for carrying out their invention in a commercial environment. It should be understood that various modifications can be accomplished within the scope of the present invention. 
     The tissue expander device (3) for surgical implantation beneath the skin and subcutaneous layer includes a flexible concave base (1), a thin expandable cover (2) of biocompatible elastic material attached to the base to form a continuous fluid-tight envelope with a chamber therein, and an injection reservoir (5) in fluid communication with the chamber comprising means for admitting fluid to the chamber with a hypodermic needle without puncturing the cover or base and means for substantially leak-proof sealing of the injection reservoir after the needle is withdrawn. The injection reservoir (5) is connected to the envelope by means of a fill tube (4) and includes an outer wall having a substantially dome-shaped sealing member affixed to the outer wall with the sealing member being self-sealing to punctures by the hypodermic needle. 
     Turning now to FIG. 1, the inflatable cover (2) is a thin expandable membrane of medical grade silicone elastomer approximately 0.015&#34; thick. The cover can be made by dipping a formed mandrel (not shown) into a silicone dispersion to the correct thickness, curing and removing from the mandrel. The base (1) of thickness approximately 0.060&#34; is cut to shape from raw silicone and inserted into the interior of the cover through the hole (not shown) previously cut to remove the cover from the mandrel. The base is positioned within the cover, and vulcanized to the cover by pressing over a substantially cylindrical heated platen of correct curvature (not shown) to effect the desired concavity. The fill tube (4) and injection reservoir (5) may be affixed to the cover (2) either by cementing or during the dipping process by insertion of the fill tube (4) into a receiving hole in the mandrel prior to dipping. 
     The fully expanded device is shown in FIG. 2. Since the equilibrium position of the base is concave downward, as the device is inflated there is tension created in the base (21) which tends to pull the edges of the base downward to resist the upward pull of the expanded cover (22). This opposing force exerted by the tension created in the base substantially prevents the edges of the base from turning upward during inflation. Since the base (21) and the cover (22) are of different thickness and stiffness, a ridge (23) is created at the periphery of the base. Owing to the concave shape of the base, the ridge is substantially prevented from turning upward to press on the thin overlying tissue. The flexible concave base (21) becomes substantially planar when the tissue expander device is fully inflated as shown in FIG. 2. 
     The effect of the concave base will be appreciated by turning now to FIGS. 3 and 4. FIG. 3 (a) shows the device (33) of the present invention in position under the skin (31) and subcutaneous tissue (32) prior to inflation. FIG. 3 (b) depicts a flat-based device (34) of the current art in a corresponding position. FIG. 4 (a) shows the implanted device of the present invention fully inflated. The ridge at the border of the base (43) is constrained from turning upward whereas the flat-based device (FIG. 4b) will present a raised ridge (44) to the thin overlying skin (41) and subcutaneous tissue (42) with resulting trauma. 
     This invention is not be limited by the embodiments shown in the drawings and described in the specification which is given by way of example and not of limitations, but only in accordance with the scope of the appended claims.