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 (fatigue) fractures, or proximal third metacarpal or metatarsal avulsion fractures, usually involve only part of the enthesis. Ultrasonographic features of the former include accumulation of hypoechoic tissue between the fracture and the dorsal aspect of the suspensory ligament with or without subtle changes to the osseous reflection of the third metacarpal bone. Avulsions of the suspensory ligament origin are demonstrated well ultrasonographically. This can also assess the amount of enthesis affected, degree of fragment displacement and quantify accompanying desmitis.
The suspensory ligament branches are also affected by acute injuries including avulsion fractures or become compromised by fractures of the proximal sesamoid bones. Ultrasound can assess the amount of enthesis involved, the degree of associated desmopathy and consequent athletic potential (Figure 5.6). Utilization of colour flow doppler is useful to assess potential vascular compromise prior to considering arthrodesis in biaxial mid‐body fractures of the proximal sesamoid bones.
Similar principles apply to fractures and fragmentation of the bases of the proximal sesamoid bones and associated distal sesamoidean ligament entheses which can also be impacted by fragmentation associated with chronic enthesopathy (Chapter 20).
Secondary Features
In acute phase assessment, haemorrhage or haematoma formation may be recognized as swirling echogenic fluid in actively haemorrhaging sites or as loculated cavities with thin dividing septa. In reparative phases, neovascularization can be identified with colour flow Doppler. Later hyperechoic periosteal new bone or callus formation can present with a spectrum of hyperechoic intensity and range, determined by the stage of healing, from irregular and interrupted to smooth and continuous.
Displaced fractures of the accessory carpal bone have been demonstrated to cause impingement and laceration of the adjacent deep digital flexor tendon [47] (Figure 5.5). Ultrasonographic evaluation of the carpal sheath and its contents is necessary to direct appropriate case management (Chapter 24).
Monitoring Fracture Healing
Serial ultrasound examinations can assess developing displacement, osseous resorption and callus formation and maturation. At entheses, serial ultrasound helps to distinguish between structural disruption and temporary distortion following haemorrhage. Following removal of apical or abaxial fracture fragments from proximal sesamoid bones, the formation and stability of granulation tissue between the fracture bed and amputated suspensory ligament branch can be monitored and rehabilitation tailored according to healing (Chapter 20). Both percutaneous and, in applicable cases, transrectal ultrasonographic monitoring of pelvic fractures is routinely performed.
Nuclear Scintigraphy
General Principles
Nuclear scintigraphy provides both physiological and metabolic activity information [31, 48, 49], aids in the diagnosis of occult and stress fractures which can precede identifiable structural bone changes and can be used to monitor healing [22, 34, 36, 37,50–62]. In man, sensitivity is better than radiography for detection of both traumatic and stress fractures with few false positives or negatives [28, 51]. In contrast to radiographs that rely on a significant decrease in mineral content of bone, nuclear scintigraphy is relatively independent of calcium homeostasis [63]. Following a negative radiographic examination in human patients, and provided it is not contraindicated [17], stress fracture diagnosis has now moved to MRI regardless of location [64]. However, in the equine patient, nuclear scintigraphy remains the ‘gold standard’ for identification of fractures that have not been localized by other techniques. The objectives are to locate lesions, evaluate their extent and phase of evolution and determine the presence of multiple lesions rather than define cause [49, 63] (Figure 5.7).
Figure 5.6 Abaxial fracture (arrows) of a left hind medial proximal sesamoid bone. (a) Dorsolateral–plantaromedial oblique radiograph. (b) Longitudinal ultrasound image of the medial suspensory ligament branch (proximal to the left). An abaxial avulsion fracture is evident with fragment displacement and resultant loss of tension in the associated ligament. The proximodistal length of the injury and degree of compromise of the suspensory ligament branch can be assessed. (c) Transverse ultrasound image (dorsal to left) enables the dorsoplantar location of the fracture to be assessed and thus directs the surgical approach/technique.
The physical decay characteristics of technetium 99m (99mTc) make this currently the radiopharmaceutical of choice for equine diagnostic imaging. For the purposes of bone evaluation, it is linked to a tracer phosphorous complex whose biodistribution favours localization in the skeleton [65]. In man, methylene diphosphonate (MDP) initially became the tracer of choice due to high skeletal uptake and fast blood clearance [32, 63, 66, 67]. MDP, disodium oxidronate (HDP) and methylene hydroxydiphosphonate (MHDP) have all been used in equine scintigraphy for their selective localization in bones. MDP historically has been used most and will be referred to in this chapter. Technetium 99m‐MDP (99mTc‐MDP) is administered intravenously, is rapidly distributed throughout the extracellular fluid and accumulates in the skeleton by simulating the movement of one or more of the inorganic components of bone, principally the hydroxyapatite crystal [53, 63, 68, 69]. Accumulation is thought to be by both chemical adsorption onto the surface and incorporation into the crystalline structure of hydroxyapatite [70, 71] and is greatest where the body is depositing calcium phosphate. Blood flow, bone metabolic activity, capillary permeability and local extracellular volume govern
