Prosthesis Formation Having Solid and Porous Polymeric Components

The present invention can be used in connection with a broad range of prosthetic devices of which the total hip prosthesis shown in the accompanying drawing is merely illustrative. Other prosthetic devices, not shown, are within the scope of the teachings of this invention, as will be apparent from the following description and examples of the preferred embodiment of the invention. In the accompanying drawings:

FIG. 1 is a side sectional view of a typical hip prosthesis embodying the present invention; and

FIG. 2 is a sectional view taken along the line 2--2 of FIG. 1.

As shown in FIGS. 1 and 2, a hip prosthesis 10 includes a ball head 11 (sized to fit into a matching acetabular socket) carried on a neck 12. The neck 12 extends from a platform 13 carried by a primary load-bearing member in the form of a stem 14. The stem 14 may be formed of titanium, or other suitable metal, and may have various cross-sectional configurations, here shown as square (FIG. 2). The structure so far described will be recognized as conventional.

The structure differs from the conventional and achieves the objects of the present invention in that the stem, or that portion of it adjacent bone tissue 16, is provided with a non-porous, encapsulating coating, or over-layer 17, which is formed of a high-density polymer, preferably UHMWPE, applied by compression molding around the stem component. This solid coating preferably has a thickness of approximately one and one-half millimeters. Because the coating 17 tightly encloses the cross section of stem 14, the attachment between the stem and the coating is mechanically strong.

Overlying the encapsulating coating 17 is a somewhat resilient mat of polymer fibers 18 which provide a field for bone growth intrusion. The contiguous fiber areas are bonded to each other and to the coating 17. The fiber mat 18 tightly fills the space in the prepared bone cavity between the coating 17 and the adjacent cavity wall. The depth or thickness of the fiber layer preferably is approximately one and one-half millimeters, and the layer is of generally uniform density across its thickness.

As previously mentioned, the high-density polymer preferred for forming the prosthesis is commercially available high-density polyethylene or medical grade UHMWPE. The fibers can be formed by shredding solid bars of the polymer and the touching points or areas of the fiber bonded to each other and to the encapsulating coating simultaneously in a sintering operation. Satisfactory temperature parameters for the sintering cycle have been found to be in the range of 250.degree. F. to 400.degree. F. for a time period dependent upon the size and form of the device. The sintered fiber outer material 18 can be shaped to tightly engage the adjacent bone cavity walls to minimize the formation of collagenous tissue at the bone-prosthesis interface and to maximize bone growth intrusion into the open pores of the fiber mat. Fiber lengths should be between one centimeter and two-tenths centimeters to permit convenient handling of the fibers and provide, when sintered, a satisfactory mat of fibrous consistency. A satisfactory cross-section diameter for the fibers has been found to be in the range of one-tenth to five-tenths millimeters. A porosity range for the sintered mat can be 25% to 75 %, with, 50% porosity a preferred value.

To achieve the desired biomechanical properties for the porous component, it may be desirable to control the fiber aspect ratio (the ratio of length to mean diameter of the fibers) and the sintering conditions based on the material selected. The fiber size and aspect ratio is believed to determine in part the pore size and therefore the elasticity and tensile strength. The optimum aspect ratio appears to be about 20 for fibers of commercially available medical grade UHMWPE.

As indicated, the sintering conditions are also important to achieve the desired properties. Satisfactory sintering of the fibers to the underlying non-porous layer of polymer can be accomplished by placing a mass of fibers around the article to be coated and inserting the article into a metal mold having an interior configuration matching the dimensions of the intended article. The mold is then heated to a temperature greater than the glass transistion temperature, T.sub.g, of the polymer employed. This sintering temperature is held constant, or nearly so, for a time sufficient for permanent bonds to form at junctures of the fibers, and between the fibers and the underlying non-porous polymeric substrate.

Essentially no pressure is applied during the sintering process other than the pressure of initial compaction of the fibers to conform with the interior configuration of the mold and that induced by the differential thermal expansion of the materials within the mold. The pressure of initial compaction disappears during the sintering process due largely to the creep of the polymer of the fibers at the elevated temperature.

To prepare a fibrous mat for use in the sintering process, a round bar of Hercules #1900 UHMWPE of about 2 inches in diameter is shredded by subjecting the bar to the shaving action in a lathe of a square-faced cutter, the bar rotating at a speed of approximately 500 r.p.m The fibers thus formed are observed to be continuous in length and a cross-section diameter of from 0.1 mm to 0.5 mm. The long fibers are then cut to a length of about 1/2 cm.

The shredded fiber is randomly arranged into a loose mass. The loose mass is then subjected to compression to approximately 25% of full density for about 30 minutes. On inspection, the mat now appears as a medium-density felt with a thickness of about 0.3 cm. On microscopic examination, the fibers of the mat appear to assume a tortous path defining therebetween a porous network of spaces.

A section of the mat is then cut and placed to conform as a single layer around a prosthesis as shown in FIGS. 1 and 2 which has previously had a non-porous UHMWPE coating bonded thereto. The dimensions of the wrapped section of the prothesis corresponds roughly to the desired finished dimensions. The fiber-covered prosthesis is placed in a mold of either stainless steel or aluminum having interior dimension closely matching the prosthesis. The mold is then heated for about 65 minutes at a temperature of between 280.degree.-340.degree. F.

The sintered fibrous polymeric coating thus produced is observed to achieve the objects of the present invention. The typical pore size of the coating was approximately 500 microns, while the pore volume is approximately 50%.

The fibrous mat, as described above, has substantial compliance, that is, deformation under compressive stress and, although the surface area presented to the body's environment is much greater than would be the case with a solid implant, there are no corrosion product problems with UHMWPE as might occur with a fibrous metal outer layer with the same extended surface area. If required, the prosthesis may be fixed in place for the short term, that is, for the time interval required to accomplish strengthening bone intrusion, by utilizing a conventional methyl-methacrylate bone cement applied, preferably, to an area of the bone-prosthesis interface which has no polymeric coating interposed.

While the invention has been shown in detail in the drawings and described in the foregoing description and example, the same is considered to be illustrative and not restrictive in character, it being understood that only the presently preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention as claimed are desired to be protected.