Radiographs were always closely associated with the assessment of the locomotor apparatus and its disorders. It is emblematic that the first radiographic study ever made was the image of a hand, the left hand of the wife of Wilhelm Röntgen, the discoverer of the X-rays, in 1895. In its classic form, currently named “conventional” radiography, a radiographic film is placed inside a cassette with ­intensifying screens and exposed to X-rays. The anatomic structure under study is interposed between the source of the radiation and this cassette. The resulting image translates the different densities of the several body tissues: denser tissues, such as the bone, appear whiter as they absorb the radiation in a higher degree, therefore limiting the amount that effectively reaches the film. Similarly, less dense tissues, such as fat, appear darker. Even though digital radiographs – either obtained with computed radiography (CR) or digital radiography (DR) – take advantage of the same physical principles, they do not rely on X-ray films to produce medical images. On the contrary, a sensor receives the emitted radiation and transmits the electronically obtained information. The generated image is, in fact, a computer file, which can be viewed on the screen of a workstation, archived in many types of digital media, or printed (either on paper or on a medical imaging film). Despite the advantages of the digital modalities over the conventional technique (wider dynamic range, less exposure to radiation, telemedicine capabilities – Fig. 1.1), the training and the experience of the observer are far more important than the technology involved.

The notion that X-rays became outdated after the advent of modern imaging methods has no grounds. Radiographs remain essential and consist in the first line of investigation for patients with articular disorders, so that they must be requested before soliciting any additional imaging study (Fig. 1.2). They are inexpensive and widely available, are not operator dependent, and present good reproducibility. Correct interpretation of imaging studies like ultrasonography and magnetic resonance imaging relies largely on correlation with radiographs. Two or more views are usually obtained, making it easier to determine the location of abnormal findings. Serial radiographs may be used to monitor the course of articular disease and are employed as imaging criteria in several staging and scoring systems (Fig. 1.2). In arthritic patients, for example, radiographs are very suitable for demonstration of established osteoarticular damage, such as cortical erosions, abnormal joint alignment, joint space narrowing, and ankylosis (Fig. 1.3). Moreover, radiographs are often problem-solving by themselves, as several ­musculoskeletal disorders can be readily diagnosed on X-rays (e.g., bone tumors, fractures, osteomyelitis, ­congenital ­malformations, and developmental abnormalities – Figs. 1.4 and 1.5). Disadvantages include the use of ionizing radiation (which is especially deleterious for the immature organism), low sensitivity for the early stages of articular ­diseases, and limited usefulness in the assessment of the soft tissues and anatomically complex joints. Conventional arthrograms and linear tomography (­planigraphy) are radiographic ­techniques that were popular in the past but became obsolete after the advent of cross-sectional imaging, given the higher diagnostic accuracy and the noninvasive nature of the latter (Fig. 1.6).

Ultrasonography (US) has become widely accepted for joint assessment, notably after the introduction of high-frequency transducers. In US, an array of piezoelectric crystals generates a beam of ultrasonic waves that is directed to a given structure, which will reflect them in different degrees, depending on the composition of its ­tissues. For example, the synovial fluid is a very good conductor of ultrasonic waves, while the cortical bone reflects them almost entirely. The reflections of the ultrasonic beam are transmitted to a ­computer that will process the obtained data and produce ­real-time images in a screen, therefore providing a unique dynamic characteristic to US. Although excellent for the evaluation of large joints and superficial soft tissues (Figs. 1.7 and 1.8), this study presents limitations in the assessment of deeply seated structures or small/deformed joints, to which the transducer may not fit satisfactorily. In addition, the ultrasonic beam relies on the existence of acoustic “windows” to reach the region of interest. Superficially located osseous abnormalities may occasionally be detected on US, but essentially no information is gathered about the innermost portions of the bones. Doppler US is a ­technique that detects and measures blood flow in vascular structures, being very useful in the investigation of hyperemic joint diseases and soft-tissue masses and fluid collections (Figs. 1.9 and 1.10).

US is largely used in pediatric patients because it is painless, inexpensive, and widely available; can be ­performed at the bedside; and does not require special or uncomfortable positioning. The absence of ionizing radiation must also be taken into account in this age group, as serial studies can be performed without risk of damage to the growing organism. It is well tolerated by children, and several joints can be examined expeditiously in a single session. Joint effusion and synovial thickening are readily apparent on US, as well as extension of the inflammatory process to adjacent bursae, synovial sheaths, or cysts (Figs. 1.7, 1.9, and 1.11). The articular cartilage is particularly well seen in large joints (Fig. 1.11). In arthritic patients, the degree of hyperemia can be estimated with Doppler US and monitored in sequential studies, serving as an indicator of disease activity and a marker of treatment response. The dynamic properties of US allow for assessment of a given structure in different spatial planes (Fig. 1.7), evaluation of articular relationships (such as in newborns with suspected developmental dysplasia of the hip – Fig. 1.12), and real-time demonstration of ­abnormalities that would go unnoticed with other imaging methods, such as tendon dislocations or muscular hernias (Fig. 1.8). Additionally, US is very useful to guide invasive diagnostic and/or therapeutic procedures.