With the increasing popularity of sports and physical activity, musculoskeletal injuries are becoming more prevalent, particularly among young athletes. These injuries can have significant implications for performance and long-term musculoskeletal health, making early and accurate diagnosis crucial.[1]
The term "apophysis" refers to a secondary ossification center that serves as an attachment site for muscles and tendons. The prefix "apo-" comes from Greek, meaning away from, separate, or off. It conveys the idea of detachment or something being derived from a primary structure. The root "physis" means growth, nature, or development. In anatomy, it refers to the growth regions of bones, such as the physis (growth plate). Thus, "apophysis" literally means "a growth away from" the main part of the bone, which aligns with its anatomical function as a secondary ossification center that protrudes from the main bone and serves as an attachment site for tendons and ligaments.
Unlike epiphyses, which contribute to longitudinal bone growth, apophyses are primarily involved in muscle attachment and are subjected to significant tensile forces. Apophyseal avulsions are commonly seen in adolescents engaged in high-intensity sports, such as soccer, gymnastics, and sprinting.[2]
Avulsion injuries occur when extreme, often eccentric muscular contractions place excessive stress on the bone, leading to a fracture at the site of tendon or ligament attachment. The biomechanical vulnerability of these sites, particularly in young athletes and those with immature skeletal structures, makes apophyseal avulsions a frequently encountered pathology in musculoskeletal radiology.[3]
The pelvis is particularly prone to these injuries, with frequent involvement of the ischial tuberosity, anterior inferior iliac spine (AIIS), anterior superior iliac spine (ASIS), and pubic symphysis.[4] Below we added a 3D model of the pelvis that illustrates the most common sites of apophyseal avulsion injuries in adolescents. Interestingly, the most common sites of avulsion injuries happen at the origin of the muscles, depicted in blue, way more common than the muscular insertions, depicted in red.
The pathophysiology involves repetitive microtrauma or acute overloading forces that exceed the mechanical limits of the developing physis, resulting in apophyseal stress injury or complete avulsion2.
From an imaging perspective, radiographs are often the first-line modality, especially for detecting displaced avulsions. The avulsed fragment is typically displaced in the direction of the attached tendon or ligament.[5] Radiographs have proven to be indispensable, even superior to MR in small avulsive bony lesions. MRI is the preferred modality for assessing stress-related apophyseal injuries, particularly in cases where subtle physeal widening, bone marrow edema, and soft tissue changes are the only indicators. CT can provide additional detail in chronic cases, particularly for evaluating nonunion or osseous hypertrophy. [2]
A crucial distinction in terminology is between "apophysitis" and "apophyseal stress injury." The suffix "-itis" implies an inflammatory etiology, which does not accurately reflect the overuse mechanism underlying these injuries. Instead, the term "apophysiolysis" has been proposed to describe incomplete apophyseal fusion due to mechanical overload. This terminology is particularly relevant in cases such as acromial apophysiolysis, a condition seen in young throwing athletes. Imaging findings in these patients include physeal widening, bone marrow edema, and sclerosis, which can mimic other shoulder pathologies such as os acromiale or rotator cuff-related impingement.[6]
Avulsion injuries can be categorized based on their chronicity:[1][7]
- Acute injuries: Characterized by a clearly avulsed bone fragment, soft tissue swelling, and joint effusion.
- Subacute injuries: Display mixed lytic and sclerotic changes due to evolving healing responses.
- Chronic injuries: Present with sclerosis, hypertrophy, and potential nonunion, sometimes mimicking neoplastic or infectious conditions.
A thorough clinical examination and detailed patient history are essential in evaluating avulsion injuries. The mechanism of injury, predisposing factors such as prior pain or overuse, and sport-specific biomechanics should be carefully assessed, and should help differentiate avulsion lesions from other conditions, including stress fractures, myotendinous injuries, and even neoplastic or infectious processes. Imaging should always be interpreted in correlation with clinical findings to avoid misdiagnosis and guide appropriate management.
Bony avulsion injuries are more prevalent in adolescents due to the unique characteristics of their developing musculoskeletal system. During this growth phase, the apophysis serves as the weakest link in the muscle–tendon–bone complex. This vulnerability arises because the cartilaginous growth plate of the apophysis is less resistant to tensile forces compared to the surrounding musculature and tendons. Consequently, sudden, forceful, or eccentric muscle contractions can lead to avulsion fractures at these sites.
In contrast, adults possess fully ossified and consolidated bones, rendering the tendons or ligaments as the more susceptible components in the biomechanical chain. As a result, adults are more prone to injuries within the tendinous regions, such as tendon ruptures, rather than bony avulsions.
The increasing participation of people of all ages in sports has led to a rise in these injuries, necessitating improved awareness among radiologists to facilitate early diagnosis and prevent long-term complications.