Fractures in the Horse. Группа авторов

months, the callus should have remodelled with an appearance close to the bone's original conformation [44]. The time frames will vary according to intrinsic factors, e.g. degree of osseous compromise and patient age, and extrinsic factors, e.g. external coaptation and loading.

      A delayed union is a clinical rather than a radiographic diagnosis since the radiographic features mirror those of second intention healing. Appearance of callus in non‐union fractures provides the radiographic descriptors, hypertrophic, oligotrophic or atrophic (Chapter 6).

Ultrasonography

      General Principles

      The advantages of ultrasonography over other imaging modalities include the practicality of being a patient side tool, it does not involve ionizing radiation, the acquisition is real time and it can be used in a dynamic manner.

      Bone surfaces reflect approximately two‐thirds of incident acoustic waves, and the other one‐third is absorbed. Reflection is caused by the large difference in acoustic impedance between bone and surrounding soft tissues. The surface of compact bone creates a smooth, hyperechoic, continuous contour with strong acoustic shadowing artefact. The latter ordinarily produces a ‘clean’ shadow as absorption of the incident ultrasound beam at the bone surface is larger than the beam width [45, 46]. Discontinuity in compact bone is necessary for positive fracture identification. However, in the acute phase, secondary signs of bone trauma such as soft tissue swelling, fluid accumulation around the cortex/periosteum and haematoma formation are also useful findings. In the subacute phase identification of periosteal callus and entheseous new bone can also be helpful.

      Ultrasound is commonly used to identify suspected pelvic (Chapter 33) and rib (Chapter 35) fractures and other locations not amenable to radiography. It is also utilized to assess concomitant injuries to soft tissues and/or synovial cavities.

      Technical Considerations

      Transducers

The ultrasound transducer used is defined by the area to be evaluated. For long bones and flat bones, a linear transducer (7.5–13.0 MHz) is optimal. The elongated flat contact footprint and high frequency optimize the resolution of superficial structures. Although the long axis view is used, in some instances it can be technically easier to dynamically survey in short axis then use oblique and longitudinal views to build up information once the area has been localized. Rotating from long to short axis also helps to discriminate the bone cortex from other echogenic structures. Long axis evaluation can also assess angular and step displacement. Irrespectively, a second orthogonal plane is routinely used to complete fracture evaluation. Axial and most proximal appendicular structures can be assessed using alcohol/spirit contact. However, when assessing superficial, acute injuries, the probe should be placed gently using ultrasound coupling gel to minimize patient discomfort. Depending on the degree of soft tissue swelling, a stand‐off may be contributory, but this may be offset by patient sensitivity since the increased pressure used to produce reasonable contact may not be tolerated.

      A convex low‐frequency (2.0–6.0 MHz) transducer is employed for deeper structures or if a wider field of view is required. There is a loss of axial resolution, but this does not usually inhibit fracture identification. When surveying ribs, a convex probe can be used first. The wide field of view enables more than one rib to be imaged which makes it easier to discern specific rib numbers. Once abnormalities are located, a linear probe with improved resolution can then be employed to assess displacement and/or callus formation.

Other transducers should be used as needed to evaluate specific structures. A micro‐convex transducer (4.0–10.0 MHz) may be required for assessment of the deep digital flexor tendon in horses that have sustained an accessory carpal bone fracture [47] (Figure 5.5) while, a linear rectal transducer (8.0–12.0 MHz) is used for transrectal evaluation of the pelvis, sacrum and caudal lumbar spine.

      Artefacts and Other Misleading Features

      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.

      Limitations

The physics of ultrasound confine definitive fracture identification to points of discontinuity in ultrasonographically accessible cortices. Secondary evidence of fracture or stress remodelling such as periosteal proliferation or abnormal contours changes with the evolution of the underlying pathology: a single time point ultrasound study may thus be misleading. Examinations at multiple time points may be needed to monitor changes (or lack thereof) and ascribe significance.

      Principles of Interpretation

      With careful probe placement and beam incidence, discontinuities or buckling of the bone's accessible surfaces are readily identified. This may present as a small discontinuity in the normally continuous hyperechoic contour or overt displacement and step formation (Figure 33.4) with or without the presence of haemorrhage (adjacent hypoechoic area) (Figure 33.5a). Variable hyperechoic deposits, contiguous with the bone surface, consistent with periosteal new bone or callus (woven bone) formation, may be present in stress fractures. Assessment of adjacent soft tissues for evidence of concurrent injury to an enthesis, muscle, joint capsule or the articular cartilage should be routine.

      Entheses

      Evaluation of entheses should include the bone surface as well as the tendon or ligament at and adjacent to its attachment. A straight, on incident image of the soft tissue structure in question as it attaches to the bone surface optimizes identification of disruption in the bone surface, particularly if the avulsion fragment is small or the avulsion fracture is partial.

      The suspensory apparatus entheses are frequently affected by fractures that include a mixture of avulsion and fatigue injuries. Unicortical proximal palmar metacarpal