Causes / typical injury mechanism: The pediatric triplane fracture is an acute, multiplanar ankle fracture of the distal tibia typically caused by supination-external rotation force. This injury uniquely occurs during the "transitional" period of partial physeal (ie, growth plate) closure. Although variations exist that do not perfectly fit the Salter-Harris classification for physeal injury, it is generally considered a Salter-Harris IV fracture due to the combination of Salter-Harris II and III fractures affecting the physis in different planes. Classically, these fractures occur through the metaphysis in the coronal plane, the epiphysis in the sagittal plane, and the physis in the axial plane. This results in 3 classically described fracture fragments of the distal tibia: (1) the anterolateral epiphysis, (2) the anterior and posterior epiphysis, and (3) the distal metaphysis and tibial shaft.

Classic history and presentation: An adolescent patient typically presents with immediate localized ankle pain, swelling, deformity, and inability to bear weight following a twisting / rotational injury incurred during a sports-related activity. Lateral triplane fractures are most common, caused by a supination-external rotation ankle injury, but medial triplane fractures may also occur. Indeed, many variations and presentations exist. For example, 2-, 3-, and 4-fragment fractures are all considered triplane fractures. By definition, as long as the fracture meets a minimum 3-plane criterion with fractures in the sagittal, transverse, and frontal planes, the fracture may be classified as triplane.

Prevalence:

• Age – 12-15 years (girls 12-14, boys 13-15). This fracture typically does not occur in patients younger than 10 years or in patients older than 16.7 years. This is due to the fact that skeletal growth continues until age 14 in females and age 16 in males.
• Sex / gender – Higher incidence in males.
• Triplane fractures constitute 5%-15% of pediatric ankle fractures.
• Of all pediatric intra-articular ankle injuries, the triplane ankle fracture contributes roughly 5%-10%.

Risk factors:

• Sports or athletic activities, in particular, soccer, basketball, football, ice skating, inline skating, roller skating, skateboarding, and snowboarding (of note, snowboarders using a soft boot have demonstrated a higher frequency than adolescents using harder boots).
• Limited ankle dorsiflexion of less than 10 degrees (predisposes to pediatric ankle injury during sports).
• Larger body mass index (BMI).

Pathophysiology: Pediatric triplane fractures are termed "transitional fractures" because they most often occur in the patient who is transitioning to skeletal maturity. Following a rotational or twisting injury, the fracture configuration is determined by both the magnitude of force and stage of ossification. Thus, these injuries typically only occur at the end of growth, prior to closure of the distal tibial physis. Medial closure of the distal tibial physis begins first, followed by lateral closure over the next 18 months. The unfused portions of the physis are weaker. This window of asymmetry from closure of the medial physis predisposes to the distal tibial fracture. Because the lateral aspect of the physis is the last to close, lateral triplane fractures occur more frequently than medial triplane fractures.

Grade / classification system: By convention, fractures involving the physis of a long bone are classified according to the Salter-Harris system. The triplane fracture of the distal tibia circumvents the simple Salter-Harris classification system of most epiphyseal fractures. It is generally considered a Salter-Harris IV fracture due to a combination of Salter-Harris II and III fractures affecting the physis in different planes: a Salter-Harris III fracture is visible anteroposterior imaging, while a Salter-Harris II fracture is seen on lateral views. Given the variability in fracture presentation, the triplane fracture is more aptly classified either by pattern or by parts.

Pattern

• Lateral triplane
• Medial triplane
• Intramalleolar (variant)
  • Type I: intra-articular fracture at the cross-section of the tibial plafond
  • Type II: intra-articular fracture outside of the tibial plafond weight-bearing zone
  • Type III: extra-articular

Parts

• 2-part
  • Anterolateral and posterior epiphysis with posterior metaphyseal fragment
  • Anteromedial epiphysis with the distal tibia
• 3-part
  • Anterolateral epiphysis
  • Posterior epiphysis and metaphyseal fragment
  • Anteromedial epiphysis with the distal tibia
• 4-part
  • Comminuted