There are three grades of sprains. Grade I sprains are mild injuries in which there is a stretching or mild tearing of the ligament, yet no joint function is lost. However, there may be tenderness and slight swelling.
Grade II sprains are caused by a partial tear in the ligament. These sprains are characterized by obvious swelling, localized tenderness, pain, joint laxity, difficulty bearing weight if the injury is to a lower extremity, and reduced function of the joint.
Grade III, or third degree, sprains are caused by complete tearing of the ligament. There is severe pain, loss of joint function, widespread swelling, and the inability to bear weight if in the lower extremity. While a Grade III sprain may be very painful when it occurs, it is sometimes not painful after the injury because the ligament fibers have been completely torn and nothing is pulling on them. If this is true, the injury will be accompanied by a significant loss in joint stability.
Strains, like sprains, are also graded in three different categories. Grade I strains are considered mild. They are categorized by some localized swelling with no significant disruption of the muscle tendon unit. Stretching or contraction of the muscle may be painful.
Grade II strains indicate some disruption of the muscle tendon unit. They will often show a loss of strength and limitation in active motion, but the muscle has not been completely disrupted.
Grade III, or third degree, strains indicate a complete rupture in the muscle tendon unit. This injury is likely to be very painful and often the individual will report hearing a loud pop or snap when the injury occurred. The site of injury is often quite visible and there will be a significant defect in the muscle that can be felt with the fingers. A Grade III muscle strain will often have very serious bruising with it as well.
Whitney Lowe, The Gale Group Inc., Gale, Detroit,