The present invention relates to implantable medical devices.  More particularly, this invention is directed to medical devices useful as prosthetic articular cartilage and in vivo scaffolds for regeneration of articular cartilagenous tissue, and to methods for fabricating such devices. 
Articular cartilage covers the ends of all bones that form articulating joints in humans and animals.  The cartilage acts in the joint as a mechanism for force distribution and as a lubricant in the area of contact between the bones.  Without articular cartilage, stress concentration and friction would occur to the degree that the joint would not permit ease of motion.  Loss of the articular cartilage usually leads to painful arthritis and decreased joint motion. 
It is known that chondrocytes, the cells that produce articular cartilage, have the ability to migrate into a defect filled with a fibrin clot and form tissue substantially similar to natural articular cartilage (Arnoczky et al. ( 1985) Arthroscopy 1:247).  Additionally, it has been shown that articular chondrocytes in tissue culture are capable of cell division and matrix synthesis (Weber et al. ( 1985) J. Ortho.  Res.  3(1):36).  However, the amount of cartilage formed by these procedures is generally not adequate to replace severely damaged joint surfaces in vivo. 
Prior art treatment of injured or diseased cartilage has generally been both by attempts at surgical repair, replacement, or by excision.  With repair or excision, regeneration of tissue may occur, although the tissue is usually temporary and inadequate to withstand the normal joint forces. 
Replacement of articular cartilage usually has been by allografting (Sengupta et al. ( 1974) J. Bone Surg.  56B(1):167-177; Rodrigo et al. ( 1978) Clin.  Orthop.  134:342-349) by periosteal grafts (see, e.g., Engkvist (1979) Scan.  J. Plast.  Reconstr.  Surg.  13:361-369; Rubak (1982) Acta Orthop.  Scan.  53:181-186) or with metal and/or plastic components (Rubash et al.,  eds. ( 1991) Clin.  Orth.  Rel.  Res.  271:2-96).  Allografting dead cartilage tissue has been tried for years with minimal success.  This approach has been only partially successful over the long term due to the host's immunologic response to the graft, failures in the cryopreservation process, and failures of the attachment sites.  Replacement of an entire joint surface with metal and plastic components has met excellent success for the older, more sedentary patients, but is generally considered insufficient for tolerating the impact of athletic activities, and has not been shown to restore normal joint mechanics. 
In alternative prior art approaches, articular cartilage has been replaced with prostheses composed of bone and/or artificial materials.  For example, U.S. Pat.  No.  4,627,853 describes the utilization of demineralized allogenic or xenogenic bone segments as replacements.  The proper functioning of these replacements depends on the differential demineralization of the bone segments.  U.S. Pat.  No.  3,551,560 describes a process of reconstructing cartilage including the use of a gel prepared from a colloidal solution of human or animal tendon.  U.S. Pat.  No.  4,620,327 describes a method for treating implants with soluble bone protein to stimulate new cartilage.  U.S. Pat.  No.  4,846,835 describes a grafting technique for transplantation of chondrocytes to promote healing of lesions in articular cartilage.  U.S. Pat.  No.  4,642,120 describes the use of gel-like compositions containing embryonal chondrocytes. 
Despite these developments, the replacement of cartilage tissue with structures consisting of permanent artificial materials generally has been less than satisfactory, principally because the opposing articular cartilage of human and animal joints is so fragile.  The articular cartilage in the joints will not withstand abrasive interfaces nor compliance variances from normal which eventually result from the implantation of prior art artificial cartilage.  Additionally, joint forces are multiples of body weight which, in the case of the knee and hip, are typically encountered over a million cycles per year.  Thus far, prior art permanent artificial cartilage have not been composed of materials having natural articular cartilage properties, nor have they been able to be positioned securely enough to withstand such routine forces. 
Repair of other tissues such as skin and nerve has been attempted using both synthetic and natural materials.  For example, endodermal implants and artificial epidermis have been fashioned out of natural collagen and glycosaminoglycans (U.S. Pat.  No.  4,060,081).  Synthetic resorbable polyesters have been used for peripheral nerve regeneration applications (Nyiles et al. ( 1983) (Trans.  Am.  Soc.  Artif.  Intern.  Organs 29:307-312), and collagen conduits have been used as a scaffold for nerve regeneration (U.S. Pat.  No.  4,963,146). 
Despite the availability Of the foregoing technologies, a structure suitable as a prosthetic articular cartilage and constructed from natural resorbable materials, or analogs thereof, has not been developed.  Thus, what is needed is an improved prosthetic scaffold for articular cartilage which is biocompatible, soft, lubricating, and durable. 
Accordingly, it is an object of this invention to provide an improved articular cartilage prosthesis which allows for normal joint motion, is biomechanically able to withstand normal joint forces, and functions at those loads to protect the surrounding cartilage. 
Another object is to provide a resorbable prosthesis which acts as a temporary in vivo scaffold for articular chondrocyte infiltration and cartilage regeneration. 
Yet another object is to provide an articular cartilage prosthesis which is composed of biocompatible materials having an organization similar to that of the normal articular cartilage. 
Still another object is to provide an articular cartilage prosthesis which is adapted for implantation by standard operative techniques. 
A further object is to provide a method of regenerating articular cartilage tissue in vivo. 
Another object is to provide a method by which such prosthetic articular cartilage scaffolds can be fabricated.