Management of Knee Articular Cartilage Injuries

Articular cartilage is a unique, biologically active tissue. In the knee, it serves as the endbearing surface for the distal femur and proximal tibia, forming a diarthrodial synovial joint capable of enduring years of impact loading. Made of hyaline cartilage, the near-frictionless surface distributes load throughout motion across 6 degrees of freedom, reducing stress transmission to the underlying subchondral bone. This permits weight-bearing during both activities of daily living and high-impact athletics. In fact, forces in the knee joint may approach 8-times body weight during deep knee bends(Reilly and Martens 1972) and pressures up to 12 MPa during maximal quadriceps contraction(Huberti and Hayes 1984) (a pressure equivalent to being 3⁄4 mile under water). These biomechanical properties rely on compression and deformation, a direct result of the biphasic nature of articular cartilage consisting primarily of water and extracellular matrix. Variation in this tissue’s thickness and the joint’s radii of curvature further influence biomechanics because of compartmentalspecific loading profiles within the tibiofemoral and patellofemoral “joints” of the knee. Arthroscopic appearance of articular cartilage in the knee (Figure 1a) should display a glistening, smooth, white surface that is firm to manual or instrumented palpation. Any disruption in the smooth surface or the tactile feel is abnormal. The loss of articular cartilage is the sine qua non of osteoarthritis. Histologic examination reveals relatively hypocellular tissue that lacks a vascular supply, neural input and output, and lymphatic drainage. These features contribute to the minimal innate healing response of isolated chondral damage and also illustrate the difficulty in making a clinical diagnosis of an isolated chondral defect (Figure 1b). Further, the role of the subchondral bone and its complex interaction with the overlying layered structure of cartilage has been emphasized in recent literature, not only in defect creation and progression, but also in surgical treatments. At the current time, these surgical procedures, cartilage repair and restoration, are designed to prevent and/or delay the initiation and/or progression of osteoarthritis.