Is There Such Thing as Spontaneous Cartilage Repair in the Horse?

It was William Hunter in 1743 who stated that “Cartilage is a troublesome tissue ....” and “When destroyed is never recovered ....”, sadly this statement still seems to stand today. Articular cartilage responds differently to injury than other tissues. Unlike other tissues, cartilage has no direct blood supply, lymphatic drainage, or innervation. In addition, articular cartilage has an extremely slow regeneration rate, and although matrix flow is described as an intrinsic cartilage repair process for small defects, chondrocytes can’t really migrate within the matrix to the site of injury over any significant distance. The more recent discovery of progenitor cells in the surface layers of articular cartilage holds great promise and challenges the traditional concept of intrinsic and extrinsic cartilage repair. But so far, sadly, in partial-thickness cartilage injury, the repair response remains limited or non-existent, probably related to the lack of a vascular supply preventing a true inflammatory or cellular response. In articular cartilage defects that penetrate the subchondral bone, a reparative response is generated. Initial granulation tissue is converted into fibrocartilage or a mixture of hyaline cartilage and fibrocartilage at best. Fibrocartilage is not as effective in maintaining joint function as hyaline cartilage. It has weaker mechanical properties, cannot distribute forces as well, and, over time, is prone to fibrillation and breakdown. Therefore, damage to articular cartilage, with or without damage to the underlying subchondral bone, has long-lasting consequences.

Cartilage repair techniques are being used clinically to address cartilage defects but so far are only successful in creating a reparative tissue similarly to spontaneous cartilage repair fibrocartilage. More recent developments in the realm of regenerative medicine have brought about new technologies that hold the promise of growing higher quality tissue that more closely approximates normal, hyaline cartilage. Ideally, the goal of cartilage repair is to generate a repair tissue that fills the defect, integrates peripherally and with the subchondral bone, and exhibits the same mechanical properties and durability as normal hyaline cartilage.  [...]