The bony pelvis is the part of the skeletal system that connects the spine to the legs through the hip joints. It is made up of the sacrum and coccyx, which are the lowest portion of the spine, and two innominate bones, which are mirror images of each other (Right, Fig. 1: Anatomy of a bony pelvis).
Each innominate bone is three bones- the ilium, ischium, and pubis, which fuse together during adolescence. These structures make a ring that protects internal organs and provides attachment points for muscles that assist with keeping the body upright, walking, and running. In
young adults and adolescents, the attachment points of these muscles consist of a special type of cartilage called an apophysis, or a secondary ossification center, that is softer than the surrounding bone. It is a special type of growth plate. During activities, a muscle that contracts with significant enough force can break the attachment and cause an avulsion fracture, but fortunately, these injuries are relatively uncommon and rarely require surgery.
Types of avulsion fractures (Fig. 2 and 3)
- Ischial tuberosity (most common): Avulsion of hamstrings caused by rapid contraction extending the hip and flexing the knee, as in gymnastics exercises, sprinting, or hurdling.
- Anterior superior iliac spine (ASIS) (Fig. 4): Avulsion of sartorius/tensor fasciae latae similar to AIIS avulsions.
- Anterior inferior iliac spine (AIIS) (Fig. 5): Avulsion of rectus femoris caused by rapid contraction of quadriceps muscles flexing the hip and extending the knee, as in kicking a ball in soccer or swinging a baseball bat.
- Iliac crest: Avulsion of abdominal muscles caused by abrupt changes in direction or in power serves, such as in tennis.
- Greater trochanter: Avulsion of hip rotators/gluteal muscles.
- Lesser trochanter: Avulsion of iliopsoas/hip flexors.
- Inferior pubic ramus: Avulsion of adductors/gracilis
Figure 4. Right-sided non-displaced ASIS avulsion fracture & Figure 5. Right-sided AIIS avulsion fracture
Pelvic avulsion fractures usually occur in adolescents or young adults during sporting activities and are often attributed to a severe muscle strain (e.g., groin pull), feeling immediate shooting pain, focal tenderness, or swelling around the site of injury. The patient may report hearing or feeling a “pop.” Sometimes they are still able to finish the activity but walk with a limp or might not even be able to tolerate walking at all. Occasionally, this type of injury may result from repetitive activities without a clear history of a traumatic event, causing decreased motion of a joint and pain that worsens over time.
A doctor will observe for swelling or bruising at the area of injury and ask whether the patient can tolerate any weight or is relying on crutches or a wheelchair. They may have to press on certain areas to pinpoint the structure involved. Joint motion will be tested but may be limited depending on the extent of pain. The child may be asked to actively contract certain muscles to stress the avulsion site; based on the anatomy involved, stretching the involved muscle or actively contracting that muscle will reproduce symptoms.
In most cases, an x-ray will be performed to confirm the diagnosis of an avulsion fracture. The x-rays include the entire pelvis and hips to rule out other injuries and check for symmetry on the opposite side of the pelvis. Often, subtle signs of soft tissue swelling, or minimally displaced fractures can be appreciated. Your doctor may order additional tests, such as CT or MRI, to further characterize the amount of displacement or look for bone marrow edema when x-rays are not able to show a non-displaced fracture. These tests are rarely required to make the diagnosis, though.
Pelvic avulsion fractures are treated based on the location and alignment of the fracture. Surgery is rarely needed, typically only if the fracture is displaced greater than 3 centimeters from its attachment site or if it fails to heal after several months (nonunion) (Fig. 6), continuing to cause pain and disability. Initial non-operative treatment focuses on rest with the hip in a position to relax the tension on the muscle groups involved and avoiding activities that use those muscle groups. Crutches are recommended for at least two weeks with a gradual return to unassisted walking and sports-related activity. Rehabilitation should be guided by clinical improvement (resolution of pain when contracting the muscle) and healing on an x-ray. Premature return to activity may cause re-injury and prolong the original recovery time of about 6 weeks to several months. Once healed, it is important to participate in appropriate stretching to avoid repeat injury.
Figure 6. a) Ischial tuberosity avulsion fracture, b) painful avulsion fracture with lack of healing, c) post-surgical fixation of avulsion fracture
Most patients recover well without surgery and are cleared to return to activities after 6-8 weeks. It is important for patients and families to understand that even though they may be released to activities after that time, it may take several months until they are back at their pre-injury level. Especially for high-level athletes, although a quick return to play may be desired, careful and cautious rehabilitation is necessary to obtain full healing and avoid chronic problems that could lead to surgery.
Furthermore, these injuries are unique to adolescents and young adults whose growth plates at the tendon attachment sites of the pelvis are still open, and thus management by a pediatric orthopaedic surgeon is recommended to increase the likelihood of a good outcome.