AC injury is a common disorder of the knee. It affects people of all ages and sexes. With the ever-increasing population and the active lifestyle of the older generation, incidence of AC injuries is on the rise. In the USA more than 500,000 procedures are performed for cartilage-related injuries and majority represent repeat procedures suggesting an ineffectiveness of surgical treatment [1
]. The incidence of cartilage defects has been reported to be as high as 65% in routine knee arthroscopies [2
]; however, the relevance of these defects to symptomatology is not yet clear. Hunter reported the inability of articular cartilage to regenerate in 1743 [6
]. Early chondral lesions are often not detected due to lack of nerve supply, and the absence of vascularity limits the repair potential.
AC integrity is important for various reasons. Firstly, chondral lesions may cause mechanical symptoms such as swelling and pain. Secondly the progression to osteoarthritis is accelerated as reported by Mankin and Davis [7
]. Sahlstorm has reported radiographic evidence of OA in 100% patients in Ahlbeck stage II and III lesions at 20 years [9
]. And thirdly the complexity of its structure and functional properties such as minimizing friction and increasing contact surface area to decrease wear under load bearing makes it a difficult material to repair.
AC works not only to protect the underlying subchondral bone but also serves to minimize friction and maximize load-bearing articular surface. Thus treatment of AC loss aims to restore these properties. AC is composed of chondrocytes (5–10%), water (65–80%), collagen, large negatively charged hydrophilic proteoglycans (aggrecan, hyaluronan), and smaller glycoproteins such as fibronectin and cartilage oligomeric proteins [10
]. Microscopically, from superficial to deep, four distinct zones of AC are described [11
]. The superficial zone
is composed mainly of elongated chondrocytes. The smaller diameter collagen fibers (mainly type II) run parallel to the articular surface in this layer. The transitional zone
is composed of large-diameter collagen fibers. The deep zone
has perpendicularly arranged collages fibers and high proteoglycan content, and finally there is the calcified cartilage zone
. Type II collagen is the predominant form (95%) but types VI, IX, X, and XI are also found (mostly in the calcified layer). As the chondrocytes move closer to the superficial zone, they become flatter in a fibroblastic shape. Such an arrangement of cells along with the collagen network in the superficial zone gives hyaline cartilage its resistance to shear forces, whereas in the superficial zone protein lubricant secreted by the chondrocytes reduces the coefficient of friction.
AC injury may be chondral or osteochondral if involving the underlying bone. The insult to AC can be traumatic or degenerative. Various metabolic factors such as obesity, alcohol abuse, and diabetes as well as mechanical factors like instability, trauma, and joint misalignment are implicated in its etiology. [12
]. Pure chondral injures are painless and repair poorly due to lack of vascularity. Osteochondral injures heal by fibrocartilage secondary to initial inflammatory response. Although mesenchymal cells produce type I and II collagen, repair is mostly fibrocartilagenous in nature. It lacks the orderly structural organization of the normal hyaline cartilage and results in early degradation and fragmentation [13
Both nonoperative and operative techniques have been employed in treating AC defects. The primary aim of any treatment modality is to reduce pain and restore function. Nonoperative treatments include weight loss, muscle strengthening physiotherapy, viscosupplementation with hyaluronic preparations, steroid injections, and oral chondroitin sulphate [14
]. Operative treatment is broadly classified in three categories, namely, bone marrow stimulation (BMS)
techniques, cartilage replacement
techniques, and cartilage regeneration
techniques. This paper aims to review the current concepts in the management of articular cartilage defects in the knee joint with particular emphasis on cartilage regeneration techniques.