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 this exchange process [63, 65, 68]. At normal and subnormal rates of blood flow to healthy bone, uptake appears proportional to blood flow; at higher rates, uptake is determined by the available crystal area [72, 73]. A low pH is also reported to be a factor [71]. Increased osteocyte activity in an area of bone trauma/fracture exposes more of the mineral face of the hydroxyapatite crystals, leading to increased adsorption. At a cellular level, locally increased deposition of 99mTc‐MDP correlates histologically with the presence of osteoid in early stages of mineralization [32, 66, 67].
Figure 5.7 Forelimb scintigram of a two‐year‐old Thoroughbred racehorse with reported loss of action. Visible physes are active and symmetrical. Note multiple abnormal areas of increased radiopharmaceutical uptake in the radial carpal bones, third carpal bones and dorsodistal aspect of the third metacarpal bones. To enable complete assessment, the physes should be masked during post‐processing to eliminate the effects of count capture.
Skeletal uptake of 99mTc‐MDP starts immediately after administration, reaches approximately 50% by one hour [48] and is effectively complete within two hours of administration [65]. Most imaging is delayed until between two and three hours post injection, depending on patient size, to allow 99mTc‐MDP not localized in the bone to be excreted in urine. This reduces non‐skeletal activity and improves osseous image quality. The timing of acquisition is therefore called the delayed or bone phase. However, 2–4% of the dose is retained in the renal parenchyma that images the kidneys [63] and may obscure rib and thoracolumbar lesions.
Assessment is made with a gamma camera that utilizes the gamma photon sensitivity of sodium iodide crystals. The 99mTc