Key principles

Introduction

Patients with traumatic injuries can be placed into one of three major groups. The imaging approach will differ between these groups.

Polytrauma (in which one injury may be life threatening)

▪ Imaging:

Strict local protocols and algorithms utilising early ultrasound (US) and/or multidetector computed tomography (CT). The use of plain film radiology in the Emergency Department (ED) is generally limited ¹^–⁴.

Multiple injuries (none of which is life threatening)

▪ Imaging:

Plain film radiology is utilised in the ED.

Single injury (not life threatening)

▪ Imaging:

Plain film radiology is the principal imaging investigation.

This book describes the assessment and interpretation of the plain radiographs that are customarily obtained in patients who have not sustained a life threatening injury.

Basic radiology

The radiographic image

The tissues that lie in the path of the X-ray beam absorb (ie attenuate) X-rays to differing degrees. These differences account for the radiographic image.

Radiograph of a chicken leg (bone) partially submerged in a layer of vegetable oil (fat) floating on water (soft tissue). Note the difference in the blackening of the X-ray film due to absorption by the different tissues.

Fracture lines: usually black, but sometimes white

When a fracture results in separation of bone fragments, the X-ray beam that passes through the gap is not absorbed by bone. This results in a black (ie lucent) line on the radiograph.

On the other hand, bone fragments may overlap or impact into each other. The resultant increased thickness of bone absorbs more of the X-ray beam and so results in a white (ie sclerotic or denser) area on the radiograph.

Three fractures. On the left the fragments are distracted and the fracture shows as a dark black line. In the centre the fragments overlap resulting in a dense region on the radiograph. On the right the fragments are impacted, also producing a dense region.

Fat pads and fluid levels

There are radiological soft tissue signs which can provide a clue that a fracture is likely. These include displacement of the elbow fat pads (see pp. 97 and 102), or the presence of a fat–fluid level at the knee joint (see pp. 248–249).

The principle of two views

‘One view only is one view too few’

Many fractures and dislocations are not detectable on a single view. Consequently, it is normal practice to obtain two standard projections, usually at right angles to each other. The example below shows two views of an injured finger.

At sites where fractures are known to be exceptionally difficult to detect (for example a suspected scaphoid fracture), it is routine practice to obtain more than two views.

Injured finger.

The true extent of the injury is only evident on the lateral view.

Important information: patient position

Knowledge of the patient's position during radiography is essential. A radiograph obtained with the patient lying supine may produce a very different appearance when compared with the image acquired with the patient erect.

Example 1.

Injured knee. Patient supine. A fat–fluid level in the suprapatellar bursa (p. 249) will only be seen when the radiograph is obtained with a horizontal X-ray beam. A vertical beam radiograph will not demonstrate the fat–fluid level.

Example 2.

A small pneumothorax will usually be detectable at the apex of the lung on an erect chest X-ray (CXR). On a supine CXR you need to look much lower down, ie around the heart, the diaphragm, and at the costophrenic angle ⁵.

Assessing the radiographs: discipline is essential

Missed injuries are common following trauma⁶^–⁹. Detection of a fracture, and the components of a complex injury, depends on adherence to three cardinal rules:

▪ Rule 1. Always analyse both views.

▪ Rule 2. Develop a systematic step-by-step checking process for each radiograph even if a single abnormality is obvious. Two associated abnormalities often occur. The major danger: you can be seduced by the satisfaction of search phenomenon (“Yes—I have found the abnormality!” ), and consequently a second important abnormality is overlooked.

▪ Rule 3. Check whether radiographs from the past exist. A change in appearance will often assist you in recognising an important abnormality. Similarly, an unchanged appearance may stop you from erroneously diagnosing a new injury or fracture ¹⁰.

Describing injuries

Fractures of the long bones

The radiographic appearance of a fracture needs to be described in a consistent style using accepted terminology. Imagine that you are describing a fracture of a long bone to the surgeon over the telephone ¹¹^–¹³. These are the features the surgeon will want you to describe—simply and accurately:

▪ Site.

▪ Closed or open.

▪ Fragments.

▪ Direction of the fracture.

▪ Articular surface involvement.

▪ Position of the two major fragments.

▪ Angulation.

▪ Rotational deformity.

Site.

Specify which bone and which part of the bone. The shaft of a long bone is divided into thirds:

▪ Proximal.

▪ Middle.

▪ Distal.

Left: Fracture of distal third.

Right: Fracture at junction of proximal and middle thirds.

Closed or open.

▪ Closed: the fracture has not punctured the skin; no open wound (left).

▪ Open: the bone has pierced the skin (right). Also referred to as a compound fracture.

Fragments.

▪ Comminution: more than two fragments (left).

▪ Impaction: one fragment is driven into the other (right).

Direction of the fracture.

▪ Transverse: at right angles to the long axis of the bone (left).

▪ Oblique: at an angle of less than 90° to the long axis of the bone (middle).

▪ Spiral: curving and twisting along the bone (right).

Articular surface involvement.

If a joint surface is involved the fracture is intra-articular.

Position of the two major fragments.

Displaced or undisplaced.

Displacement is always described in relation to the distal fragment.

▪ Lateral displacement (left)

▪ No displacement (middle two images)

▪ Posterior displacement (right)

Angulation.

Describe ¹¹^,¹⁴ in relation to the distal fragment. Refer either to the direction in which the apex of the fracture points or to the direction of tilt of the distal fragment. Describing the direction of tilt may be easiest, as follows:

▪ Lateral tilt (left)

▪ Medial tilt (middle)

▪ Anterior tilt (right)