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Artefacts are numerous and can be induced by the operator or as a result of the patient's anatomy or injury(ies). Scanning off incidence to bone surfaces can result in the false appearance of irregular surface margination. At entheses, the probe must be perpendicular to the tendon or ligament otherwise a hypoechoic area is created due to off incident scanning of an anisotropic structure. Avulsion fragments, when present, will result in hard shadowing that precludes evaluation of structures deep to (or behind) the fragment. Fractures which involve bone surfaces that normally hold tendons or ligaments in tension will result in relaxation of the tendon or ligament. Relaxation artefact on ultrasound has a characteristic but unusual appearance and can provide indirect evidence for fracture. When there is an avulsion fracture there can be a lack of tension in part or all of a ligament, and sequential assessment can help determine relative osseous and ligamentous contributions.

Figure 5.5 Ultrasonographic evaluation of an accessory carpal bone fracture. (a) Oblique transverse image with a linear transducer demonstrates a displaced fragment (yellow arrow) contacting the lateral margin of the deep digital flexor tendon (DDFT). (b) Transverse ultrasound of the same patient with the limb partially flexed and using a micro‐convex transducer provides a clear identification of the fracture impinging the DDFT. During dynamic assessment, the extent of the resulting laceration was possible. Palmaromedial is to the top of both images.

Nutrient foramina and other vascular canals through the bone surface interrupt cortical acoustic shadows. Knowledge of their location and expected ultrasonographic appearance differentiates them from fractures. Awareness of the normal appearances of physes at different ages, amphiarthroses and ossification fronts in juvenile patients are also essential to avoid misinterpretation.