Complex sentence: Fibrocartilage is a tough, dense, fibrous material that helps fill in the torn part of the cartilage; however, it is not an ideal replacement for the smooth, glassy articular cartilage that normally covers the surface of the knee joint.
Split into simpler sentences: Fibrocartilage is a tough, dense, fibrous material that helps fill in the torn part of the cartilage. Yet it's not an ideal replacement for the smooth, glassy articular cartilage that normally covers the surface of the knee joint.