Apart from the muscles and tendons, which connect to the knee, there are structural components of the knee. These components include soft tissue and hard tissue. Soft tissues includes tendons, ligaments, and the meniscus (semi soft). Hard tissue includes bone as well as the articulating surface of the joint itself.

Structural damage to Soft Tissue includes:

Strain: A strain is defined as a stretching or tearing of the muscle or tendon which connects the muscle to the bone and Graded 1-3.

Grade 1: Mild damage to the individual muscle fibers (less than 5%) causing minimal loss of strength and motion.
Grade 2: More extensive damage to more muscle fibers. Muscle is not completely torn or ruptured. This grade presents significant loss of strength and active motion.
Grade 3: Complete tear or rupture of a muscle or tendon. This injury might be noticeably felt and unable to move the muscle or joint. This injury will require surgery to reattach the damaged muscle and tendon.

Sprain: A sprain is a stretch or a tear in a ligament. Ligaments connect bone to bone and are also graded 1-3.