Copyright 2009
Hunt Udall MD, Matthew Diltz MD
Definition
The cartilage of the hip joint covers the spherical femoral head and concave acetabulum 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 unique surface provides padding to the ends of the bones, but must be durable enough to withstand the rigors of daily activities.
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 to it, and when the cartilage experience 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 hip 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
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