What is the difference between a sprain and a strain?

Sprains are injuries to the ligament, which attaches bone to bone. Strains are injuries to muscles and tendons. The tendons connect muscles to bone. Both sprains and strains are typically diagnosed with a particular physical exam technique. The degree or the injury is often referred to by a grade. Grade 1 involves a stretch to the fibers of the ligament or tendon. Grade 2 involves partial thickness tearing of the fibers. Finally, Grade 3 is a complete tear of the muscle, tendon, or ligament. The grade typically is diagnosed on physical exam but there are times when advanced imaging such as ultrasound or MRI is needed to fully classify the injury.  There are times when a higher grade sprain or strain may cause a joint to be unstable or a muscle tendon unit to become incompetent which may require surgical fixation.