Mechanism and localization (skier's thumb syndrome)
A classic example of an avulsion fracture is an injury in the area of the metacarpophalangeal joint of the thumb. It involves the ulnar collateral ligament (ulnar collateral ligament).
Most commonly, the injury occurs during sudden hyperextension (hyperextension) and twisting. In clinical practice, this is often referred to as skier's thumb syndrome (skier's thumb), as the injury is typical for skiers: during a fall, the thumb gets caught in the ski pole strap, taking the brunt of the destructive load.
From a pathomorphological standpoint, the injury may present as an isolated soft tissue injury (tear of the ligament itself). However, it is not uncommon for a strong ligament to withstand the tension but pull off a bone fragment from the base of the proximal phalanx. On an anteroposterior radiograph, this bone fragment is clearly visualized along the ulnar surface of the base of the phalanx. For a more detailed assessment of soft tissue status, ultrasound (ultrasound) or magnetic resonance imaging (MRI) is used.
Specific patient groups
Avulsion fractures and similar stress injuries strongly depend on the patient's age, physical activity level, and bone tissue condition. Several key risk groups are distinguished:
- Athletes and individuals with an immature skeleton. In this group, tendons and ligaments are often stronger than the bone growth plates. As a result of sudden muscle contraction, avulsion fractures occur in the area of the apophyses (sites of tendon attachment to the pelvic bones and other segments).
- Extreme athletes and recruits. Prone to fatigue (stress) injuries at tendon attachment sites. Such microinjuries are not always visible on X-rays, making MRI the gold standard for their diagnosis.
- Elderly patients. Suffer from fractures against the background of bone tissue insufficiency (insufficiency fractures) caused by osteoporosis or osteomalacia. Vulnerable zones include the sacrum and the posterior pelvic ring. The risk of such fractures increases manifold after undergoing radiation therapy for ongynecological diseases.
Anatomy and stability of the pelvic ring
To understand the biomechanics of avulsion and other types of fractures, it is necessary to know the structure of the pelvic ring. It consists of the sacrum (sacrum) and the innominate bones (innominate bones). Each hip bone is the result of the fusion of three structures: the ilium (ilium), ischium (ischium), and pubis (pubis).
A key factor in pelvic stability is not only the bones but also the ligamentous apparatus. Pelvic instability can occur even without a single fracture, solely due to ligament rupture. The main stabilizing structures:
- Sacroiliac joints (SIJ): Held together by anterior and posterior sacroiliac ligaments. Here, the posterior ligaments are the strongest in this region.
- Central part: The sacrospinous (sacrospinous) and sacrotuberous (sacrotuberous) ligaments strictly limit the external rotation of the pelvic bones.
- Anterior compartment: The ligaments of the pubic symphysis (symphyseal ligaments) hold the anterior ring, preventing its diastase (separation).
Injury biomechanics and classification
The pelvic ring breaks according to certain biomechanical laws. The anterior ring is the weakest point of the structure. Therefore, any injury to the pelvic ring initially leads to damage to the pubic rami or rupture of the pubic symphysis. Disruption of the posterior ring structures occurs only after the anterior compartment has been damaged.
In traumatology, the strict "Ring Rule" applies: given the closed ring-like configuration of the pelvis, more than one point of destruction is required for displacement. Multiple fractures or a combination of a fracture with ligament rupture are considered the norm here.
Pelvic and acetabular fractures in young patients are the result of high-energy trauma (high-energy injuries). They account for 8–9% of all blunt injuries. They are dangerous due to a high mortality rate from the risk of life-threatening hemorrhage (life-threatening haemorrhage) and visceral injury (visceral injury). The severity of ring damage directly correlates with the probability of these complications. Recognizing the injury pattern determines further tactics—correct application of corrective forces or internal osteosynthesis.
Historical names (Malgaigne fracture, "open book") have now been replaced by the Young and Burgess classification, based on the vector of applied force:
- APC (Anterior-posterior compression) — anterior-posterior compression.
- LC (Lateral compression) — lateral compression.
- VS (Vertical shear) — vertical shear.
- Complex — combined (hybrid) injuries.
Anterior-posterior compression (APC type 1)
Anterior-posterior compression injuries most commonly occur in head-on collisions (head-on collision). Forces in this case act strictly in the anteroposterior plane.
The mildest variant is type APC 1 (AP type 1). It includes:
- Symphyseal diastasis less than 2.5 cm (symphyseal diastasis <2.5 cm);
- OR vertical fractures of the pubic rami.
It is this vertical pattern of pubic rami fracture that allows this type of injury to be easily distinguished from other injuries when analyzing a radiograph in the standard anteroposterior projection.
Briefly: answers to topical questions
Which muscles attach to the anterior superior iliac spine and can cause its avulsion fracture?
The sartorius muscle attaches to the anterior superior iliac spine, the avulsion of which leads to a corresponding fracture.
- Sartorius muscle (sartorius) — crosses two joints, causing isometric contractions to place excessive load on the tendon attachment site.
Such injuries are most characteristic of children and adolescents involved in sports. This is due to the immaturity and relative weakness of the apophyseal growth plate zone.
What subtypes is the lateral compression (LC) pattern divided into in the Young and Burgess classification?
The lateral compression (LC) pattern in the Young and Burgess classification is divided into three severity types.
- Type 1 (LC type 1) — the least severe type, including oblique or comminuted fractures of the pubic rami, less commonly symphyseal rupture.
- Type 2 (LC type 2) — occurs with continued force application when the innominate bone (os coxae) rotates internally. The LC type 2a subtype with ligament rupture is distinguished.
- Type 3 (LC type 3) — severe injury, often occurring in a rollover injury.
Which specific internal organs are most frequently damaged in severe pelvic ring fractures?
The provided sources do not specify a complete list of organs most frequently damaged specifically in severe pelvic ring fractures. Urogenital tract injuries are indicated:
- Urethra — injuries occur in approximately 10% of major pelvic fractures and usually involve the proximal (posterior) urethra.
- Bladder — the majority of patients with bladder trauma have associated pelvic fractures; rarely, the bladder can be entrapped in a displaced pelvic fracture.
To memorize the sequence of pelvic destruction, use the phrase: "The facade breaks first, then the back wall collapses" (the anterior ring always suffers first, the posterior one only after it).
Class and exam questions
What is the essence of "skier's thumb" syndrome?
This is an avulsion fracture or a tear of the ulnar collateral ligament of the thumb. It occurs with sudden hyperextension, often when the thumb gets caught in the ski pole strap.
Can pelvic instability occur without bone fractures?
Yes, instability can develop solely due to disruption of the ligamentous apparatus (e.g., ligaments of the pubic symphysis and sacroiliac joints).
What is the "ring rule" in pelvic biomechanics?
Due to the ring-like structure of the pelvis, displacement of its elements always requires more than one point of failure (a combination of multiple fractures or fractures with ligamentous disruptions).
How to distinguish an APC 1 injury on a radiograph?
In APC type 1 (anteroposterior compression), an anteroposterior pelvic radiograph visualizes a vertical fracture pattern of the pubic rami or a pubic symphysis diastasis of less than 2.5 cm.
What else to study on this topic
- Clinical presentation and surgical management of "skier's thumb".
- Apophyseal stress fractures: MRI visualization in young athletes.
- Osteoporosis and radiation therapy as triggers for sacral insufficiency fractures.
- Detailed analysis of lateral compression (LC) and vertical shear (VS) according to the Young and Burgess classification.
- Protocols for life-threatening hemorrhage control in severe pelvic ring fractures.
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