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Avulsion injuries or fractures occur where the joint capsule, ligament, tendon or muscle attachment site is pulled off from the bone, usually taking a fragment of cortical bone. Avulsion fractures are commonly distracted due to the high tensile forces involved. There are numerous sites at which these occur. Being familiar with them is important as subacute/chronic injuries can appear aggressive.
Avulsion injuries are common among those who participate in sports, in particular adolescents.
Avulsion fractures can be classified as acute, subacute or chronic. In acute avulsion fractures, there is usually a clear preceding traumatic incident. Subacute and chronic avulsion injuries can be due to delayed presentation of an acute injury or secondary to repetitive use / overuse injuries 4.
The mechanism is from either 4:
high muscle activity
forced extreme range of motion
greater tuberosity: insertion of rotator cuff
lesser tuberosity: insertion of subscapularis (rare)
medial epicondyle: apophyseal avulsion in children
see also medial epicondylar fracture
coronoid process: insertion of capsule
radial tuberosity: insertion of biceps
olecranon process: insertion of triceps
Pelvis and hip
iliac crest avulsion: anterior abdominal wall muscles
greater trochanter: hip rotator cuff
body and inferior ramus of pubic bone: thigh adductors and gracilis
see also Osgood-Schlatter disease
lateral tibial plateau: lateral capsule
medial aspect of femoral condyle: medial collateral ligament
see also: Pellegrini Stieda lesion
Ankle and foot
dorsolateral process of the calcaneum: insertion of extensor digitorum brevis muscle
avulsion fracture 5th metatarsal styloid: insertion of peroneus brevis tendon
Many avulsion fractures are apparent of plain radiographs. The avulsed bone fragment is typically displaced in the direction of the tendon, ligament or joint capsule which is attached to it 5. CT and/or MRI may be required for detection and further characterization. Appearances will vary depending on classification 4:
acute: avulsed bone fragment with donor site and typically associated soft tissue swelling / joint effusion
subacute: fracture healing results in a mixed lytic/sclerotic appearance
chronic: sclerosis and osseous hypertrophy
On MR small avulsion fractures can easily be missed, as the avulsed cortical fragment is often poorly visualized, and the bone marrow edema is absent at the site of injury 5.
Treatment and prognosis
Most avulsion injuries/fractures are treated non-operatively 4.
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