High-level force applied to articular surfaces can cause articular cartilage ruptures or tears that do not extend into the underlying bone (Fig. 6).

Figure 6 Arthroscopic image of a partial-thickness lesion of the articular cartilage of the patella. (Reproduced from Boden BP, Pearsall AW, GarrettWE Jr, Feagin JA: Patellofemoral instability: Evaluation and management. JAm Acad Orthop Surg 1997;5:47-57.)

Articular cartilage injuries that do not cross the tidemark generally do not heal.19 Why do superficial injuries behave this way? First, these lesions do not cause hemorrhage or initiate an inflammatory response. Also, fibrin clots rarely form on exposed surfaces of normal cartilage. Chondrocytes near the injury may proliferate and synthesize new matrix, but they do not migrate into the lesion. The new matrix they produce remains in the immediate region of the chondrocytes, and their proliferative and synthetic activity fails to provide new tissue to repair the damage. By increasing the load on adjacent tissue, large articular surface defects may also cause degeneration of previously normal regions of the articular surface.