Imaging of the scaphoid

The scaphoid is the second largest of the carpal bones and is named due to its boat-like shape (derived from the Greek word scaphon). It is situated laterally in the proximal carpal row, articulating with the distal radius proximally and forms joints with the lunate, capitate, trapezoid and trapezium. The scaphoid can be divided into proximal, mid and distal thirds for ease of description. The middle third is often referred to as the ‘waist’. On the palmar surface of the distal pole is the small scaphoid tuberosity where the flexor retinaculum and flexor carpi radialis tendon attaches. The scaphoid is the most frequently injured carpal bone, usually resulting from a fall onto an outstretched hand in active young patients (peak incidence in the second and third decades). The bone itself can fracture or the scapholunate ligament can rupture, leading to a rotatory subluxation of the scaphoid. Up to 80% of scaphoid fractures occur through the waist of the bone, with proximal pole fractures more common than distal pole. The single-vessel retrograde arterial blood supply to the scaphoid bone (from distal pole to proximal pole) ensures that the distal pole has excellent blood supply, whereas the supply to the proximal pole is more tenuous and is prone to avascular necrosis. In addition, as 80% of the bone surface is covered in articular cartilage, there is limited capacity for periosteal healing, thus delayed union and nonunion is often seen. Early confirmation of the presence of a fracture is important in order to ensure adequate immobilisation and referral to orthopaedic/hand surgery teams and to avoid unnecessary prolonged immobilisation in patients without fractures. Many hospitals have protocols for the investigation of suspected scaphoid fractures, beginning with radiographs and utilising cross-sectional imaging if a diagnosis is not made initially. There have been some recent studies showing that early use of MRI scanning significantly alters patient management in the majority of cases. In our hospital the imaging of scaphoid injuries is carried out according to the following algorithm that was developed between the emergency department and the imaging department (see diagram 1). We use CT as the modality of choice for investigation of occult fractures at the second Emergency Department review. All imaging modalities are further discussed below. Radiological evaluation of scaphoid fractures Plain radiographs A routine scaphoid series consists of four views of the wrist joint and scaphoid, the PA, lateral, semi-pronated oblique and semi-supinated oblique (figure 1). However, even with a dedicated scaphoid series of radiographs, up to a third of scaphoid fractures remain occult. Traditionally, patients with clinical suspicion of a scaphoid fracture but negative radiographs are immobilised in plaster pending a repeat series of radiographs in 7-14 days, at which time the fracture is often visualised due to disuse osteoporosis and surrounding hyperaemia. To avoid prolonged immobilisation following equivocal imaging, scaphoid fractures can also be further investigated using a wide variety of imaging modalities. These are usually performed where clinical suspicion remains despite negative radiographs, but may also be utilised when assessment of complications and/or assessment of healing is required. Secondary investigations Radionuclide scintigraphy has been used historically to detect injuries as it has high sensitivity for the identification of acute fractures. However, the specificity and spatial resolution of scintigraphy is limited, plus the comparatively long acquisition times have meant it is becoming less frequently used in practice. FIGURE 1 Standard scaphoid series of plain radiographic projections in a 23-year-old male following a fall, demonstrating an acute fracture through the waist of the scaphoid. DIAGRAM 1 The ideal management of scaphoid fractures. Presentation to ED