Avulsion fractures occur when a small piece of bone breaks away from the rest of the bone due to a sudden impact or movement, often damaging ligaments or tendons. They are common in athletes and can occur in areas where major muscles attach, such as the pelvis, knee, and ankle. Children are more susceptible to avulsion fractures caused by powerful muscle contractions, while adults are more likely to tear tendons. Treatment typically involves rest, ice, and compression, with surgery only necessary in severe cases.
An avulsion fracture is a type of bone injury in which a small piece of bone known as a fragment breaks away from the rest of the bone. The result of a fall or other impact injury or the abrupt movement of tissue from the bone, an avulsion fracture tends to damage the small bony projections to which ligaments or tendons attach. Typically seen in athletes who jump or land explosively, an example of this type of injury is a tibial tuberosity avulsion, the small swelling on the shin bone felt just below the kneecap. Sudden and violent extension or straightening of the knee joint can cause the patellar ligament, which connects the kneecap or patella to the tibial tuberosity, to pull so hard away from the bone that it ruptures the tuberosity immediately.
Most commonly in the lower body, an avulsion fracture is seen in areas such as the pelvis, knee, and ankle where the major muscles of locomotion attach. The ischial tuberosity of the hip, for example, a bony prominence in the lower pelvis on the sitbone, is a potential site of avulsion fracture. This is because the large muscles of the hamstring group on the back of the thigh attach here via strong tendons.
Because the hamstrings can be very tight, a sudden overstretch of the muscles can overwhelm the body’s stretch reflex, a neurological constraint that is placed on the muscles to prevent stretching to the point of tearing. This reflex causes them to spring back before they get too tense. In the case of very sudden and explosive movements, however, especially in the untrained individual, the stretch reflex can be overwhelmed.
The resulting force can create such a strong pull on the muscles’ attachment tendons that the tuberosity or bump to which they attach becomes detached from the rest of the bone. However, an avulsion fracture caused by a powerful muscle contraction like this is more likely in children than in adults. In adults, the tendon typically absorbs the force and it is the tendon that tears, not the bone. A child’s still developing bones may not be able to resist the pull on the tendon protrusion, and the result is an avulsion fracture.
Avulsion fractures can also be caused by the bone moving away from the bone through the connecting ligaments. At the ankle joint, for example, the shin and talus bones in the ankle are joined together by several powerful ligaments. A severe stretch of these ligaments as experienced during an ankle sprain can break a small piece of bone where the ligaments attach. Typically, this type of injury is treated like a muscle tear or ligament sprain — with rest, ice, and compression like any soft tissue injury — and then surgery isn’t necessary unless the bone is pulled in. a considerable distance from the injury site.
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