Articular cartilage repair: are the intrinsic biological constraints undermining this process insuperable?

This article reviews the experimental and clinical strategies currently in use or under development for the treatment of articular cartilage lesions. The vast majority of protocols under investigation pertain to the treatment of full-thickness defects (i.e., those which penetrate the subchondral bone and trabecular-bone spaces) rather than partial-thickness ones (i.e., those which are confined to the substance of articular cartilage tissue itself). This bias probably reflects the circumstance that partial-thickness defects do not heal spontaneously whereas full-thickness ones below a critical size do, albeit transiently. And it is, of course, a seemingly easier task to manipulate a process which is readily set in train than it is to overcome an induction-problem which Nature herself has not solved. Indeed, the reasons for this inert state of partial-thickness defects have only recently been elucidated, and these are briefly discussed. However, the main body of this review deals with the various transplantation concepts implemented for the repair of full-thickness defects. These fall into two broad categories: tissue-based (entailing the grafting of perichondrial, periosteal, cartilage or bone-cartilage material) and cell-based (utilizing chondroblasts, chondrocytes, periochondrial cells or mesenchymal stem cells). Cell-based systems are further subdivided according to whether cells are transplanted within a matrix (biodegradable, non-biodegradable or synthetic) or free in suspension. Thus far, the application of cell suspensions has always been combined with the grafting of a periosteal flap. The strengths and weaknesses of each concept are discussed.