Introduction: In the athletic population, reproducible imaging of carti- lage damage is vital for treatment considerations. With appropriate pulse sequencing, magnetic resonance imag- ing has been shown to be an accurate noninvasive method for the evaluation of articular cartilage injuries and for evaluating postoperative changes following chondral repair. In addition, magnetic resonance imaging does not utilize ionizing radia- tion, has direct multiplanar capabilities, and allows high- resolution imaging of soft-tissue structures. The purposes of the present review are to update orthopaedic surgeons on the applications and techniques for magnetic resonance imaging of cartilage in the athletic po pulation, to define the normal magnetic resonance imaging characteristics of articular carti- lage, to illustrate the spectrum of articular cartilage lesions that are detectable with magnetic resonance imaging, and to review normal and abnormal magnetic resonance imaging findings following cartilage repair.
Educational Objectives: After reviewing this article, the reader should (1) have a basic understanding of pulse sequences and terminology for cartilage-sensitive magnetic resonance imaging, includ- ing proton-density-weighted high-resolution fast-spin-echo sequences; (2) be able to identify normal and abnormal ar- ticular cartilage in the hip, knee, elbow, shoulder, and ankle; and (3) be able to identify normal and abnormal findings on postoperative magnetic resonance images after chondral re- pair techniques.
Basic Science of Articular Cartilage: An understanding of the structure of articular cartilage is crucial in order to understand the magnetic resonance imaging appearance of normal and abnormal cartilage morphology and is also the basis for the development of new imaging techniques. Articular cartilage is a viscoelastic material composed of chondrocytes (approximately 1%) embedded in an organized extracellular matrix composed primarily of water (65% to 80%), collagen, and proteoglycan. The predominant collagen is type II (95%), although smaller amounts ofother collagen types (types IV, VI, IX, X, and XI) have been identified1. Collagen provides the structural framework and tensile strength of articular cartilage. Chondroitin and keratin sulfates are the predominant types of proteoglycan molecules that are negatively charged and attract cations and water, which provides compressive strength to the cartilage