Pediatric Musculoskeletal Radiology

1

How does growing bone respond to trauma, and how is this different from mature bone?

The cartilaginous physis separates the epiphysis from the metaphysis. Pediatric ligaments and tendons are relatively stronger than growing bone (in contrast to adults). Given an equivalent force applied to growing versus mature bone, the growing bone has a higher likelihood of fracture. In addition, immature bone has a propensity to bow instead of break, which may cause buckles in one side of the cortex (torus/buckle fractures) or greenstick fractures (fracture of one cortex and bowing of the other). Fractures can also occur through an open physis. These patterns are not seen in mature bone.

2

What is the significance of fractures of the physis?

The cartilaginous physis is vulnerable to injury, especially at its attachment to the metaphysis. Disruption of the physis may result in slower growth and premature fusion, leading to limb length discrepancy.

3

How are fractures of the physis classified?

Physeal injuries are classified in the Salter-Harris classification ( Figure 90-1 ), increasing in severity from I to V. Type I is a fracture through the physis. The fracture line in type II includes the metaphysis and physis. Type III fracture includes the epiphysis and the physis. Type IV fracture involves the metaphysis, physis, and epiphysis. Type V fracture is a crush injury of the physis. Follow-up for Salter-Harris fractures may include magnetic resonance imaging (MRI), which can delineate an abnormally fused physis in the healing phase that may need to be disrupted to allow future osseous growth.

4

What are secondary ossification centers?

Secondary ossification centers appear and then fuse later with the primary ossification center as seen on radiographs at predictable times during skeletal maturation. For example, multiple secondary ossification centers are seen around the elbow, which appear at different ages. Their usual sequence can be remembered by the mnemonic CRITOE: C apitellum (1 year), R adial head (3 years), I nternal (medial) epicondyle (5 years), T rochlea (7 years), O lecranon (9 years), and E xternal (lateral) epicondyle (11 years).

5

Why are secondary ossification centers particularly important to understand in the setting of elbow trauma?

One important reason to understand this sequence is that a type I Salter-Harris fracture through the physis of the medial epicondyle may cause displacement of this ossification center into the region of the trochlea. This displacement might create the false impression that the trochlear ossification center is present, whereas the medial epicondylar ossification center has not yet appeared. Knowledge of this sequence allows one to identify this appearance appropriately as a displaced fracture.

6

What are the most common fractures of the wrist/forearm in children?

Distal radius fractures are the most common fractures in children. Scaphoid fractures are the most common fractures in adolescents that are skeletally mature.

7

What is gymnast wrist?

Gymnast wrist is an overuse injury (from chronic pressure on joints). Physeal widening, metaphyseal irregularity, and sclerosis can be seen. Early physeal bridging and radius growth disturbance may occur if not treated.

8

What is the equivalent of gymnast wrist in the shoulder?

Little leaguer's shoulder is an overuse injury in throwers. Subtle findings, similar to gymnast wrist, may be seen on MRI.

9

How may subtle supracondylar fractures of the elbow be diagnosed?

Pediatric elbow fractures often occur in the supracondylar region, where the humerus is relatively flat. The anterior humeral line, drawn on the lateral view of the elbow along the anterior humerus, normally intersects the middle third of the capitellum. This intersection is likely to be disrupted in supracondylar fractures. The presence of a joint effusion (hemarthrosis) is also extremely helpful and can be assessed by the presence of an elevated posterior fat pad, which is displaced and visible on the lateral view if there is blood in the joint. Displacement of the anterior fat pad (“sail” sign) may also be seen with hemarthrosis, but this finding is less specific.

10

Describe nursemaid's elbow.

Nursemaid's elbow is caused by radial head subluxation through the annular ligament of the elbow, resulting in abnormal positioning of the ligament between the radial head and capitellum. It is often caused by sudden traction on the forearm in a child 1 to 3 years old. Radiographs may appear normal but are obtained to exclude fractures.

11

List risk factors for developmental dysplasia of the hip (DDH).

• Caucasian race.

• Female gender.

• Torticollis.

• Clubfoot.

• Breech birth.

12

When is DDH suspected clinically?

It is difficult to diagnose DDH in newborns 4 weeks old or younger because of normal joint laxity, but this condition is suspected in infants with leg length discrepancy and asymmetric thigh creases. The Barlow maneuver on physical examination dislocates the femoral head rearward when DDH is present, and the Ortolani maneuver reduces the recently dislocated hip, often with a resultant “clunk.”