Ligaments are strong bands of fibrous tissue that connect bone to bone, and often serve to stabilize joints. A medial ankle sprain occurs when the deltoid ligament on the inside (medial side) of the ankle joint is stretched too far. Sprains are graded based on their severity, ranging from a strain (mild), to a partial tear (moderate), to a complete tear (severe). Because the deltoid ligament is so strong, medial ankle sprains occur less often than other types of strains, however, they can take longer to heal.