Anterior Inferior Iliac Spine Avulsion

Etiology

AIIS avulsion fractures typically result from a sudden, forceful rectus femoris contraction, commonly experienced by athletes engaging in activities requiring explosive lower-limb movements. Muscle pull on the developing pelvis produces this injury, posing a heightened risk to adolescents with still-maturing growth plates.[3] The rectus femoris, a quadriceps component originating from the AIIS and extending to the extensor mechanism distally, is integral in activities involving rapid hip flexion and knee extension, like kicking or sprinting, where its activation can exert excessive AIIS tension and potentially fracture the bone.

Epidemiology

AIIS avulsion is relatively rare. However, the condition exhibits specific epidemiological trends. These fractures predominantly affect adolescents and young adults, with a peak incidence during the adolescent growth spurt. The vulnerability is attributed to the open growth plates in this age group. Using the Risser classification to grade skeletal maturity, older patients are more likely to sustain iliac apophysis (anterior superior iliac spine and iliac crest) injuries. In contrast, younger patients are more likely to develop AIIS or ischial tuberosity injuries. Complete pelvic apophyseal ossification timing accounts for these differences, with the AIIS secondary ossification center closing first and the iliac apophyses closing last.[4]

Identifying risk factors, such as previous injuries or muscle imbalances, can aid in preventive strategies. Sports-related activities, particularly those involving forceful lower limb movements such as soccer, football, and track and field, account for a significant proportion of AIIS avulsion fractures.[5] Boys are more commonly affected than girls due to muscle mass and strength differences. The incidence also varies based on cultural factors, with certain sports being more popular and predisposing individuals to this injury in specific geographic regions. Seasonal variation influences the condition's incidence, with certain sports being more frequently played during particular times of the year.[6] 

Pathophysiology

AIIS avulsion's underlying mechanism involves the rectus femoris' biomechanics during powerful eccentric contractions, illustrating the intricate interplay between physical activity, anatomical structure, and physiological development.[7] Essentially a traction phenomenon, this injury mechanism produces conditions ranging from minor avulsions to complete fractures. The mechanism is triggered by activities necessitating sudden hip flexion and knee extension, such as kicking or sprinting. Adolescents with open growth plates face heightened risk due to the pelvis' developmental state, particularly vulnerable at the tendon-bone interface where the yet-to-ossify apophysis is prone to separation under stress (see Image. Hip Bone Development).[8]

Additionally, reduced trabecular bone density in the anterior and posterior pelvic rims, as noted by Cunningham and Black, renders these areas more susceptible to injury. Growth plates are also relatively more delicate than musculotendinous units, which experience a hormonal surge in strength during growth spurts. Thus, intense muscle contractions can easily fracture developing pelvic areas by avulsion.[9]

History and Physical

A comprehensive history should identify injury mechanism details, predisposing factors, and previous hip or pelvic trauma or overuse injuries. Patients with AIIS injuries typically present with acute groin or anterior hip pain following an activity involving forceful hip flexion combined with knee extension, such as kicking, sprinting, or rapid acceleration-deceleration movements. The pain is sharp and localized to the anterior pelvic region, exacerbated by hip flexion when walking, running, or stair-climbing. Patients may report an inability to bear weight on the affected limb. Inquiries about the patient's physical activity level, sports participation, and recent changes in training routines can provide insights into potential contributing factors.

On physical examination, palpation often reveals point tenderness at the AIIS. Range of motion testing may demonstrate limited and painful hip flexion. Additional signs, such as groin or anterior hip swelling and bruising, may be present, suggesting a traumatic injury. Clinicians should perform a complete musculoskeletal and neurological examination to identify any associated injuries or deficits.[10]

Evaluation

AIIS avulsion evaluation typically involves a stepwise approach aided by imaging studies. An anteroposterior pelvis x-ray is often the initial step, followed by an oblique pelvis view if needed to enhance the detection of subtle injuries. Magnetic resonance imaging (MRI) may be used to accurately assess suspected avulsion injuries and any associated soft tissue damage. MRI is particularly helpful in cases where x-ray results are inconclusive, as in many skeletally immature patients. In rare cases, computed tomography (CT) scans may be used to further evaluate bony anatomy and avulsed fragment size and displacement.[11]

Treatment / Management

Treatment selection is often based on fracture severity, displacement, and patient characteristics. No national or international guidelines specifically address AIIS avulsion management, but general apophyseal avulsion fracture treatment principles may be applied to AIIS injuries. The American Academy of Orthopaedic Surgeons and the British Orthopaedic Association recommend nonoperative management for minimally displaced fractures, with surgical intervention reserved for injuries with significant displacement.[12](B3)

Nonoperative ManagementNonoperative management is frequently the initial approach for AIIS avulsion injuries, especially in younger individuals with open growth plates.[13][14] This strategy emphasizes rest, activity modification, and physical therapy as its core components. The 1985 Metzmaker and Pappas protocol established a 5-stage rehabilitation program applicable to all pelvic and hip apophyseal fractures.[15] While this framework provides a general timeline, contemporary protocols emphasize individualized approaches specifically tailored to AIIS injuries.[16] These protocols involve the following:(B3)

• Stage 1 (0-7 days): Ice application, analgesics, and protected weight-bearing with crutches
• Stage 2 (7-14 days but may extend to 20 days): Gentle active and passive range-of-motion exercises with continued weight-bearing assistance
• Stage 3 (4-20 days, but may extend to 30 days): Progression to resistance exercises once 75% movement is regained, gradually increasing weight-bearing
• Stage 4 (30-60 days): Focus on stretching and strengthening exercises specific to the patient's sport
• Stage 5 (after 8 weeks): Potential return to sports participation, contingent on pain-free movement and appropriate functional recovery

Operative Management

Surgical intervention may be warranted when nonoperative methods fall short or significant displacement occurs. Rajaesekhar et al suggest surgery for cases with a large fragment displacement (>2 cm) or bony exostosis formation following the original injury.[17] McKinney et al proposed a hip-and-pelvic apophyseal fracture classification system based on displacement (see Table. Classification of Hip and Pelvic Apophyseal Fractures).[18] Type 3 and type 4 injuries warrant surgery. (B3)

Table. Classification of Hip and Pelvic Apophyseal Fractures

Surgical intervention typically involves reattaching the avulsed fragment to the AIIS. Open reduction internal fixation with cortical, cannulated, or resorbable screws are all well-documented surgical techniques for AIIS avulsion fractures.[19][20][21] Arthroscopic methods may also be employed in select cases. Ultimately, the choice of surgical approach hinges on the specific fracture characteristics and the surgeon's expertise. Deciding between operative and nonoperative AIIS avulsion management requires careful consideration of the individual's age, activity level, fracture characteristics, and initial treatment response. (B3)

Differential Diagnosis

Differentiating between AIIS avulsion injuries and other conditions presenting with anterior hip or groin pain is crucial for appropriate management. This table contains common conditions presenting similarly to AIIS avulsion and their distinguishing features:

Prognosis

The prognosis for AIIS avulsion injuries is generally favorable with prompt and appropriate management, often involving conservative treatments such as rest, activity modification, and physical therapy.[22] These injuries of the growing apophysis typically heal well. However, surgery may be considered for cases with significant displacement or failed conservative measures. Recent surgical advancements, particularly minimally invasive techniques like arthroscopy, have greatly improved AIIS injury outcomes by focusing on reattaching avulsed fragments, restoring normal anatomy, and enhancing hip flexor muscle strength and flexibility. These modalities' benefits include smaller incisions, reduced infection risks, and faster recovery.[23]

Months following surgery, patients usually experience substantial pain relief, functional recovery, and a return to preinjury activities, including sports. However, surgical procedures entail risks such as heterotopic ossification, potentially causing stiffness and pain, which may be managed with NSAIDs or radiation therapy as preventive measures. Another risk is lateral femoral cutaneous nerve damage during a direct anterior approach to the hip, necessitating careful nerve localization and protection during surgery to avoid thigh sensory disturbances.[24]

Complications

Complications from AIIS avulsion injuries are relatively rare. However, pain or discomfort may continue to be triggered during activities requiring hip flexion, affecting the patient's quality of life. Neglected injuries may result in nonunion or hypertrophic nonunion. Nonunion occurs when bone ends fail to fuse. Hypertrophic nonunion entails excessive bone growth without achieving union.[25] Identifying bony bridges across the epiphyses is crucial, as these structures can impede growth through the ossification center.[26]

Significant fragment displacement may lead to nonunion if a fibrous bridge fails to form, resulting in persistent pain and functional limitations.[27] Excessive callus formation and heterotopic ossification pose additional serious complications, causing pain and restricted range of motion. Prevention involves avoiding excessive strain on the healing growth plate, including repetitive hip extension and knee flexion, until callus formation bridges the gap. Subsequently, the gradual introduction of forces can enhance strength without promoting excessive callus formation.[28]

Malunion is another potential complication that can change the hip joint's biomechanics, potentially leading to residual weakness or muscle imbalances.[29][30] A more specialized treatment approach may be required if AIIS avulsion cooccurs with other conditions impacting the pelvis or hip joint, such as labral tears or FAI.[31]