Teaching Case

A 13-year-old boy presented to the emergency department of our hospital with a painful, swollen left knee after high jumping in gym class. During take-off, he felt a sudden snap in his knee, after landing he was unable to bear weight. Clinical examination revealed the impossibility to extend the knee.

Lateral radiograph revealed soft tissue swelling and an enlargement of the growth plate of the secondary ossification centre of the tibial tubercle (Fig. 1).
Subsequent MRI was performed for further evaluation of the extent of the lesion. MRI confirmed the presence of an avulsion fracture of the secondary ossification centre of the tibial tuberosity. The maximum gap of the growth plate was located anterolaterally and measured seven millimetres. In the ventral part of the proximal tibial epiphysis, bone marrow oedema was visualized in keeping with lesion extension in the femoro-tibial joint space (Fig. 2). Haemorrhagic effusion of the deep tibial tubercle bursa and the infrapatellar subcutaneous bursa was seen as well.

An avulsion fracture is caused by a high energy tensile force on a ligament or tendon which exceeds the strength of the bony tissue. In adolescents the growth plate is the weakest link, in comparison to adults where a tensile force will cause injuries of the musculotendinous junctions [1]. Typically avulsion fractures occur in boys between the age of 13-17 years because of the cartilage features of the growth plate. Avulsion fractures of the knee are uncommon compared to avulsion fractures of the hip or the pelvis [2]. Fractures of the apophysis of the proximal tibia comprise 0.5% - 3% of all fractures [3].
The mechanism of injury is thought to be a high energy extension of the knee against a fixed leg, such as in jumping sports [4].
Avulsion fractures of the tibial tubercle (TT) are classified according to the modified Watson-Jones classification (Fig. 3). The original classification of Watson and Jones described three categories of avulsion fractures of the tibial tubercle [5]. Ryu and Debenham introduced an additional fourth category in which the fracture extends into the proximal tibial epiphysis. The extent of the injury is related to the stage of fusion of the epiphysis [6].
Plain radiographs are the first step to define the displacement of the fracture, the degree of comminution and the extent of the injury [1].
Although MRI is not always mandatory, it may be of additional value to define the precise extent of the bone and soft tissue abnormalities, and allows a more accurate classification of the lesion. In our case, the plain radiograph showed an enlargement of the growth plate of the TT. In addition, MRI revealed bone marrow oedema in the ventral part of the proximal tibial epiphysis indicating lesion extension in the articular surface of the tibia. After MRI the lesion was classified as a type II fracture.
The treatment, either conservative versus surgical, depends on correct staging of the lesion. If displacement is marked or if the physis is comminuted, surgical approach is recommended [7]. In our case the maximum gap of the growth plate measured seven millimetres, therefore surgical repair, consisting of open reduction and internal fixation with cortical screws, was performed, followed by six weeks of immobilization in a plaster cast.
Further follow-up was uneventful five months after trauma.

TAKE HOME MESSAGE
Although rare, avulsion fracture of the TT should be considered in sportive adolescents particularly in jumping. MRI may add in correct staging of the lesion.