Soft tissue abnormality is often the only evidence of bone injury
An awareness of elbow development is essential when considering paediatric elbow injuries
Order of elbow ossification centre development
C - Capitulum (or Capitellum)
R - Radial head
I - Internal epicondyle (or medial epicondyle)
T- Trochlea
O - Olecranon
L - Lateral (or external epicondyle)
Mnemonic = C R I T O L
An awareness of normal X-ray appearances of the elbow is essential for the identification of elbow injuries. Elbow injuries often have characteristic radiological appearances, which may only be detected by the presence of soft tissue abnormalities.
There are important considerations when dealing with elbow injuries in children.
Elbow development
Bone structures at the elbow develop within multiple cartilaginous ossification centres. Typically there is ossification in the following order - Capitulum (C), Radial head (R), Internal epicondyle (I), Trochlea (T), Olecranon (O) and External/Lateral epicondyle (L).
These centres of ossification become visible from 6 months to 12 years of age and in early adulthood fuse to the humerus, radius or ulna.
The red ring shows the position of the External or 'Lateral' epicondyle (L) which has not yet ossified
All the other centres of ossification are visible
C = Capitulum
R = Radial head
I = Internal epicondyle
T = Trochlea
O = Olecranon
Normal elbow X-ray appearances
On the lateral image there is often a visible triangle of low density lying anterior to the humerus. This is the anterior fat pad which lies within the elbow joint capsule. This is a normal structure.
Anterior humerus line
A line extending from the anterior edge of the humerus should pass through the capitulum with at least one third of the capitulum seen anterior to it.
Radiocapitellar line
A line taken through the centre of the radius should extend so it passes through the centre of the capitulum.
The Trochlea (T) has not yet ossified (Red ring = predicted position)
IMPORTANT RULE: Suspect avulsion of the internal epicondyle if it is absent and there is ossification at the site of the trochlea
Raised fat pad sign
If the anterior fat pad is raised away from the humerus, or if a posterior fat pad is visible between triceps and the posterior humerus, then this indicates a joint effusion. In the setting of trauma this is due to haemarthrosis (blood in the joint) secondary to a bone fracture. This is often the only X-ray sign of a bone injury.
A post-traumatic effusion without a visible bone fracture usually indicates a radial head fracture in an adult, and a supracondylar fracture of the distal humerus in a child.
If there is a joint effusion but no history of trauma, an inflammatory cause should be considered.
A joint effusion (haemarthrosis) raises the fat pads away from the humerus
The powerful triceps muscle posteriorly displaces the ulna - taking the capitulum (C) with it
The capitulum therefore lies well behind the anterior humerus line
At least one third of the capitulum should lie in front of the anterior humerus line
Elbow dislocation
The radial head may dislocate from the capitulum of the humerus on its own or in combination with dislocation of the ulna from the trochlea. The latter is usually straightforward to identify, but radial head dislocations may be more subtle. The rule to remember is that the midline of the radial shaft, the radiocapitellar line, should pass through the middle of the capitulum.
