Articular Cartilage
Assistant
Definition
By Gregory R. Waryasz, MD
The knee joint articular cartilage of the musculoskeletal system is characterized by covering the articular surfaces of the joint.
It is part of the knee joint. It consists of articular cartilage.
Its unique structural feature is that it is located on the femoral condyles, femoral trochlear groove, tibial plateau, and the posterior surface of the patella. It is primarily made up of type II collagen. The components are hydroxyproline, proteoglycans, and glycosaminoglycans arranged in layers to provide strength.
The knee joint articular cartilage as well as all other bones, muscles, and ligaments of the body are derived of mesodermal origin in the embryo.
The function of the cartilage is to provide a smooth gliding surface for joint motion by decreasing the coefficient of friction. A small layer of fluid also lubricates the articulating surfaces.
Common diseases include arthritis/degenerative joint disease (DJD), osteochondritis dissecans and cartilage tears.
Osteochondritis dissecans is a condition where a piece of cartilage and a piece of the bone beneath it become loose due to avascular necrosis of the subchondral bone.
Commonly used diagnostic procedures include clinical history, physical exam, x-ray, CT scan, and MRI.
It is usually treated with physical therapy and NSAIDs. Arthitis can also be treated with knee replacements and arthroscopy. Osteochondritis dissecans can be treated non-operatively or the fragments can be removed with surgery. Articular cartilage tears of the knee can be treated with physical therapy, microfracture surgery, allograft, or autologous cartilage transplant.
The cartilage of the knee joint covers the ends of the femoral condyles, the femoral trochlea groove, and the tibial plateau (condyles) to form a smooth gliding surface for joint motion. It has one of the lowest coefficients of friction in the body which allows for a very efficient surface for motion. This is critical in that the knee flexes and extends hundreds and thousands of times each day. This unique surface provides padding to the ends of the bones, but must be durable enough to withstand the rigors of daily activities.
The hyaline articular cartilages of the knee is found on the distal surface of the femur, the proximal surface of the tibia and the posterior surface of the patella. In addition the medial and lateral menisci are found on the tibia, superficial to the hyaline cartilage of the tibia. They are crescent shaped discs of fibrocartilage and they act as shock absorbers by diffusing the weight and forces imposed on them across the largest surface area..
The synovial membrane secretes the synovial fluid which acts as a lubricant for the joint.
There are many types of cartilage, but Type II collagen is the predominant component of articular cartilage. It has a half life of 25 years and is composed of hydroxyproline, proteoglycans, and glycosaminoglycans. These structures are arranged in different layers which provide tensile strength at the surface of the cartilage where there are tangential forces due to the gliding motion of the knee, and compressive strength in the deeper layers to provide padding that will dissipate the axial forces that are transmitted through the knee with compression.
Another unique structural feature of cartilage is the low coefficient of friction that it maintains due to the nature of the cartilage itself, as well as the thin film of fluid that lubricates its surface. Cartilage is sensitive to traumatic loads, and when the cartilage experiences a load above a certain level, the cartilage dies and the degenrative changes of arthritis begin to set in.
Common diseases include arthritis which is a misnomer since the process is not an inflammatory one. The appropriate terminology for cartilage degeneration is degenerative joint disease, or osteoarthrosis since there has been no evidnece of inflammatory cells in histological specimans taken from affected knees.
Commonly used diagnostic procedures include high quality MRI machines (3 tesla) to detect subtles changes in the cartilagenous surface. Weight Bearing plain x-rays can detect more advanced stages of arthritis in the knee. Direct visualization through an arthroscope is the easiest way to visualize articular cartilage damage.
Advanced degeneration of the cartilage is treated with total knee replacement, whereas more isolated areas of involvement can be treated with arthroscopic procedures to restore the articular cartilage through grafting procedures or through microfracture procedures where the underlying bone is punctured to stimulate the healing process.
References
Elstrom J, Virkus W, Pankovich (eds), Handbook of Fractures (3rd edition), McGraw Hill, New York, NY, 2006.
Koval K, Zuckerman J (eds), Handbook of Fractures (3rd edition), Lippincott Williams & Wilkins, Philadelphia, PA, 2006.
Lieberman J (ed), AAOS Comprehensive Orthopaedic Review, American Academy of Orthopaedic Surgeons, 2008.
Moore K, Dalley A (eds), Clinically Oriented Anatomy (5th edition), Lippincott Williams & Wilkins, Philadelphia, PA, 2006.