Cartilage Repair: an Update

Introduction
The horse is used for its locomotion capacities; therefore, it is not surprising that disorders of the musculoskeletal system are the most common cause of wastage in horses. Of these disorders, joint- related ailments are the most frequent and clinical interventions, such as arthroscopy, are as common in equine practice as they are in human medicine. Like in man, cartilage injury is a major problem and many horses have to be retired from their athletic careers, or even have to be euthanized, because of unrepairable damage and subsequent osteoarthritis of the articular cartilage.
Non-specific treatments, analgesic or non-steroidal anti- inflammatory drugs, are commonly used for joint disease. Viscosupplementation, intra-articular injections of high molecular weight hyaluronic acid exhibiting viscoelastic properties close to those of synovial fluid, may also be proposed for joint disease. However, such procedures, even though alleviating symptoms, only afford a slowdown of the degenerative process.
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 chondrocytes can’t migrate within the matrix to the site of injury as do cells in other tissues. In partial-thickness cartilage injury, the response is limited or nonexistent. Probably related to the extremely long turnover of the collagen framework and the lack of a vascular supply preventing a true inflammatory or cellular response. Thus, the defect is not repaired. 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. 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 hyaline cartilage, with or without damage to the underlying subchondral bone, has long-lasting consequences.