Athletic injuries are a common cause of avulsion injuries in the pelvis, apophyseal or otherwise. Early and accurate imaging diagnosis of this group of injuries is necessary for appropriate management. Using the classical understanding of the muscle-tendon-bone unit, injuries occur at variable locations that can be predicted based on patient age and risk factors. Injuries in most adults typically occur within the tendon due to chronic tendinopathy and degeneration. In younger adults, injuries typically occur at the myotendinous junction, which usually presents as a muscle strain pattern. The injury pattern in the pediatric population is unique, as the weakest link in the chain is the unfused apophysis.
Avulsion injuries most often occur with either concentric or eccentric muscle contraction. Excessive passive stretching has also been described as a mechanism for avulsions. Activities requiring running and kicking, as well as rapid directional change, can cause apophyseal avulsions. Patients often describe a “pop” at the time of injury, followed by pain, swelling, and muscle weakness.1 On physical examination, patients often have point tenderness at the site of the apophyseal avulsion.2
Radiographic imaging is usually adequate for the diagnosis of an apophyseal avulsion if there is at least partial ossification of the involved apophysis. MR imaging is often required in suspected cases of avulsion when radiographs are inconclusive or when the apophysis has yet to ossify. Ultrasound can be technically challenging, requiring a skilled operator, but can be particularly useful for assessing myotendinous injuries.
The predominant locations of apophyseal avulsion injuries in the pelvis and, therefore the focus of this review article, include the ischial tuberosity, anterior inferior iliac spine (AIIS), anterior superior iliac spine (ASIS), and pubic symphysis, in descending order of frequency (Figure 1).3 Iliac crest avulsions can also occur, but are rare.4
The ischial tuberosity is the site of origin of the hamstring muscles and adductor magnus (Figure 2) and represents the most common location for avulsion injuries in the pelvis.3 The tendinous attachments of the hamstrings include the semimembranosus and the conjoined tendon of the semitendinosus and biceps femoris. Ossification of the ischial apophysis is variable. It usually begins to ossify at 14-16 years of age and fully ossifies by 22-24 years of age. Following a hamstring avulsion injury, patients describe buttock pain and weakness. Pain is often exacerbated by walking, and patients may have an antalgic gait.
Avulsion at the ischial tuberosity often occurs due to forceful flexion of the hip with the knee extended, or due to eccentric overload of the hamstring muscles with forceful contraction against resistance, which places strain at the hamstring origins.
Accurate diagnosis can be made with radiographs if the apophysis is ossified. Typically with an acute injury, a curved osseous fragment is seen adjacent to the ischial tuberosity, morphologically similar to the ischial tuberosity (Figure 3). MRI can be used when radiographs are inconclusive to evaluate for myotendinous injury or avulsion of a nonossified apophysis. Apophyseal avulsions displaced < 2 cm often heal with conservative management.1 Avulsion fragments displaced > 2 cm may not fuse and can predispose to a fibrous union4 (Figure 4). Exuberant callus formation may cause impingement on adjacent structures, including the sciatic nerve. Injuries are more commonly unilateral, and therefore, comparison with the asymptomatic side is helpful, particularly in subtle cases (Figure 5).
The direct head of the rectus femoris originates at the anterior inferior iliac spine (AIIS) (Figure 6). Ossification begins between 13-14 years of age, with complete ossification by 16-18 years of age. Similar to the other avulsion injuries, patients often describe a “pop” followed by pain and weakness, particularly with flexion. Injury typically occurs from eccentric contraction of the rectus femoris. This most commonly occurs with forceful extension of the hip while the knee is flexed,5 a pattern often seen in sprinters and athletes involved in kicking.2
On radiographs, an osseous fragment is seen adjacent to the AIIS (Figure 7). The fragment is usually unilateral with a corresponding donor site evident on the underlying iliac bone (Figure 8). MRI is useful in cases of a nonossified apophysis or when the diagnosis is unclear based on radiographs. On MRI (Figure 9) the avulsed fragment is identified with varying levels of associated soft-tissue edema.
The reflected head of the rectus femoris originates at the superior acetabular ridge. Avulsion injuries can occur at this location; however, they are uncommon. A superior acetabular ridge avulsion has similar findings to an AIIS avulsion, including an avulsed fragment, which can be best seen on the frog-leg lateral view (Figure 10). Apophyseal injuries of the AIIS are usually managed conservatively.
The anterior superior iliac spine (ASIS) is an osseous protuberance at the anterior margin of the iliac crest (Figure 11). The sartorius and portions of the tensor fascia lata (TFL) originate at the ASIS and are the primary muscles that can cause avulsion injuries at the ASIS. The sartorius courses medially in the anterior compartment to insert on the proximal medial tibia at the pes anserine aponeurosis. This oblique course of the muscle allows for hip and knee flexion as well as hip abduction and external rotation.
The ASIS begins to ossify between 13-15 years of age and fuses with the underlying anterior margin of the iliac crest in the early 20s. The ASIS is the third most common site of apophyseal avulsion fractures in the pelvis. Apophyseal avulsion of the ASIS results from sudden forceful contraction of the sartorius with the hip in extension and the knee flexed.2 Therefore, this is most common in running athletes. Injuries may also occur with kicking.2
An avulsion injury of an ossified ASIS apophysis is seen typically as a small osseous fragment adjacent to the ASIS (Figures 12, 13). However, in rare circumstances, severe avulsion of the anterior superior iliac spine (ASIS) apophysis may cause the apophysis to displace inferiorly, projecting over the AIIS, simulating avulsion of the AIIS (Figure 13). Evaluating the underlying AIIS and ASIS for a possible donor site can be helpful. Morphologic changes to the underlying iliac bone are often seen at the avulsion site. These injuries are often unilateral, and therefore, comparison with the asymptomatic side is recommended.
The majority of ASIS avulsion injuries heal with conservative management. It is imperative that these injuries be diagnosed appropriately with medical imaging, as the high osteoblastic activity on biopsy specimens may be mistaken for osteosarcoma.
The primary musculotendinous origins at the symphysis pubis include the rectus abdominis, gracilis, adductor brevis and adductor longus (Figure 14). Avulsion injuries at the symphysis pubis are uncommon and are usually from chronic overuse, although acute avulsions do occur rarely. Acute avulsions in young athletes usually occur with forceful contraction against resistance.2 Avulsion injuries at the symphysis pubis are usually seen within the spectrum of athletic pubalgia.
As discrete osseous fragments are rarely seen, particularly in acute cases, radiographs may be negative. However, secondary changes may be seen at the symphysis pubis including widening, lysis and cortical irregularity, particularly in chronic cases (Figure 15). These injuries can be better evaluated by MRI, which can allow for identification of specific muscular involvement (Figure 16). Although the rectus abdominus also inserts at the pubic symphysis, frank avulsion is rare. More commonly, partial avulsions or tears of the rectus-adductor aponeurosis are seen with fluid undermining the attachment site. This injury is commonly seen with athletic pubalgia, and is also known as a “sports hernia.”6,7
Avulsions of the iliac crest are rare. The iliac crest is where components of the abdominal wall musculature insert. Radiographs are typically adequate to make the diagnosis and findings include irregularity of the apophysis of the iliac crest (Figure 17). MRI can confirm findings, which would show corresponding edema on the fluid-sensitive sequences.
Apophyseal avulsion injuries in the pelvis are not uncommon in clinical practice, especially in the setting of athletic injuries. Since the clinical presentation of avulsion injuries overlaps with numerous additional musculoskeletal injuries, imaging plays a key role in establishing the correct diagnosis. Understanding the underlying myotendinous and bony anatomy, mechanisms of injury, and characteristic imaging features associated with avulsion injuries will allow for prompt diagnosis and treatment, ultimately improving outcomes. Although radiographs often allow for confident diagnosis, MR imaging is helpful in cases when radiographs are inconclusive or not definitive.
Shah R, Shelat N, El-Khoury GY, Bennett DL. Avulsion Injuries of the Pelvis. J Am Osteopath Coll Radiol. 2016;5(4):5-11.