“A child is not a small adult …” This truism is particularly important in relation to paediatric bone injuries1 There are three major differences between a child’s and an adult’s skeleton2–4. 1. Children have growth plates that: □ have the consistency of hard rubber and act, in part, as shock absorbers □ protect the joint surface from sustaining a comminuted fracture 2. Children have a thick periosteum that: □ is not only thick but very, very, strong □ acts as a hinge, which inhibits displacement when a fracture occurs. 3. Children’s bones have an inherently different structure to adults’ bones, being: □ flexible, elastic, and plastic, allowing an injured bone to bend or to buckle. The growth plate (ie the physis) is a site of weakness. You need to be familiar with the normal radiographic appearance of the ends of the long bones in children. This will help you to detect the important injuries and will also protect you from labelling a normal developmental appearance as being abnormal. A child’s long bone grows in length initially by forming layers of cartilage and gradually converting that cartilage into bone. This layering process takes place at the end of the bone at the site of the physis (also called the epiphyseal plate or the growth plate). The physis is made of cartilage and lies between the epiphysis and the diaphysis. Cartilage is lucent on a radiograph. The cartilaginous physis remains as a radiographic lucency until the child reaches skeletal maturity and stops growing. At that time the lucent physis fuses to the metaphysis (and also to the epiphysis). When fusion occurs the linear lucency that was the physis disappears. An epiphysis is a secondary centre of ossification at the end of a long bone. Each epiphysis is initially composed solely of cartilage. As a consequence it looks as though nothing is there (take a look at a new born baby’s elbow region). Of course each of the epiphyses is present, but present as a radiolucent blob of cartilage. These invisible blobs enlarge slowly with age. Eventually they begin to ossify at their centres and become visible. Finally, these blobs of bone will fuse to the physis at maturity. “The key to accurate diagnosis is a precisely accurate assessment of the radiographs.”2 The anatomical regions in a long bone. The growth plate (the physis) is a very vulnerable structure. The joint capsule, the surrounding ligaments and the muscle tendons are all much stronger than the cartilaginous physis. A shearing or avulsion force applied to a joint is most likely to result in an injury at the weakest point, ie a fracture through the growth plate. Most growth plate injuries will heal well without any resultant deformity. However, in a few patients, failure to recognise a growth plate injury may result in suboptimal treatment with a risk of premature fusion resulting in limb shortening. If only a part of the growth plate is injured, unequal growth may lead to deformity and disability. The cartilaginous growth plate (the physis) is a weak and vulnerable site in a long bone. Injuries affecting the physis are common. This classification links the radiographic appearance of the fracture to the clinical importance of the fracture. A Salter–Harris type 1 injury has a good prognosis whereas a Salter–Harris type 5 injury has a poor prognosis. Salter–Harris fractures. Salter–Harris fractures How to remember the Salter–Harris (SH) classification Type 1. Fracture restricted to the growth plate. In this patient the epiphysis is displaced posteriorly. Note: Many type 1 fractures do not show displacement of the epiphysis, making it impossible to detect the injury to the growth plate from the radiographs. Prognosis in these cases is invariably very good. Type 2. Fracture through the metaphysis that extends into the growth plate. When a child’s long bone is subjected to a longitudinal compression force (such as a fall on an outstretched hand), this can result in two common but different types of injury in the region of the metaphysis and proximal diaphysis. Torus fracture. Results from a longitudinal compression force with little or no angulation. The axial loading is distributed evenly across the metaphysis (right). There are microfractures of the trabeculae at the injured site. The commonest sites for a torus fracture are the distal radius and/or ulna. The fracture is often subtle and appears as a ripple, a wave, an indent or a slight bump/bulge in the cortex. The bulge may be seen at both cortices or at one cortex only. Greenstick fracture. A Greenstick fracture results from an angulation force. There is a break in the cortex on one side of the bone. The opposite cortex remains intact. This occurs because of the very thick and elastic periosteum. There is usually some angulation at the fracture site, although this can be slight and subtle.
Particular paediatric points
Bones in children are different1–4
Child versus adult
The end of a long bone in a child
A persistent lucent line is normal
Parts of the skeleton are not missing
Fracture sites1,3
Epiphyseal–metaphyseal (Salter–Harris) fractures
The Salter–Harris classification
Type
Relative frequency
Prognosis for normal growth4,5
Upper limb
Lower limb
1
8%
Satisfactory
The likelihood of a growth complication is higher for all types of Salter–Harris injury as compared with the upper limb.
2
73%
Satisfactory
3
6%
Satisfactory
4
12%
Guarded
5
1%
Poor
SH 1
= S
is Separated (a widened physis)
SH 2
= A
is Above the growth plate
SH 3
= L
is beLow the growth plate
SH 4
= T
is Through the growth plate
= E
is for nothing!
SH 5
= R
has a Rammed together growth plate
Salter–Harris fracture: five types
Metaphyseal–diaphyseal fractures
Particular paediatric points
2
