An animal model for the study of articular cartilage regeneration in-vivo facilitated by stress-shielding is introduced. The object of the model is to test the hypothesis that some form of cartilaginous tissue will grow upon a large joint surface in vivo with the joint in normal motion. The model utilizes the known capability of immature cells to differentiate. The source of cells is bleeding subchondral bone. In addition, the model provides a mechanically shielded environment in which cell differentiation and maturation can occur. The study showed that a substantial amount of tissue will grow in the animal model only when the new tissue is relieved of the normal joint stresses. The characteristics of the new tissue were observed after 12 weeks of growth. Gross observation showed that the new tissue grew to completely surround the shielding devices and covered the entire articular surface. The new tissue grew to the height of the shielded area (2 to 3mm.). Histologic evidence indicated the new growth was largely fibrous in nature but with some areas of newly differentiated chondrocytes. Biomechanical analyses quantified the tissue as being a soft, permeable neocartilage: biochemical evaluations demonstrated increased hydration with small amounts of proteoglycans. These characteristics are inferior to normal cartilage. Never the less, the tissue quality is as good or better than that obtained in other models and it grew to cover a significantly larger articulating surface than all other experimental models. Material obtained in this experiment provides a baseline of data for future experiments designed to manipulate the new tissue using tissue engineering methods and to learn how the new tissue will tolerate exposure to reintroduced normal stress
