in stress fractures can vary from a focally marked fusiform area of cortical IRU to an area less intense or well defined which can represent the pathophysiological continuum between fracture and stress reaction. Asymptomatic foci can reflect prodromal change, active remodelling or healing. Alternative imaging may be needed to determine the significance of findings.
Dorsal Cortex of the Third Metacarpal Bone
The stress continuum in the dorsal metacarpus and metatarsus in racing and non‐racing horses has been studied [58, 109], and a grading scheme of one to four [109] suggested. Scintigraphy exhibited excellent sensitivity, but false positives with clinically normal limbs having IRU [109]. Interpretation is further complicated by cross over between dorsal metacarpal disease and cortical stress fractures as one process maybe superimposed on the other [110]. Nuclear scintigraphy has been utilized in Thoroughbreds to differentiate between dorsal metacarpal disease, defined as uniform diffuse IRU in the dorsal cortex relative to the palmar cortex and metaphyses, and cortical stress fractures, defined as focal intense IRU in the dorsal cortex [58]. In this location, the focal nature of the IRU has been considered more significant than intensity [61].
Enostosis‐like Lesions
These lesions are identified scintigraphically by IRU located within the trabecular bone determined on two tangential projections. Although reported to be found close to nutrient foramina [111], this is not consistent. No definitive aetiology has been established, but one proposal is that they are trabecular microfractures caused by cyclical stress [112, 113]. The degree of IRU uptake can vary from mild to marked.
Monitoring Fracture Healing
Nuclear scintigraphy has been used in man to monitor healing in both monotonic and stress fractures [19, 28, 77, 114]. In the first (acute) phase, there is a diffuse area of IRU due to increased blood flow around the fracture site. This is greater than the morphological fracture and persists for two to four weeks after injury. The second (subacute) stage has the most intense well‐defined IRU which corresponds more accurately with the anatomical fracture and lasts for 8–12 weeks (Figure 5.8b). Over the coming weeks and months as callus remodels during the third (reparative) stage, there is a more localized area of IRU with greater separation between normal and abnormal tissues followed by a gradual reduction in activity. The time of scintigraphic normalization is greater than that identified clinically or radiographically due to ongoing bone remodelling. In man, monotonic fractures can take up to 24 months [77] and stress fractures between four to six months [28]. In stress fractures, severity was a major determinant of time to resolution, and patients who failed to rest and had continuing pain had persistent unresolved lesions [28].
Figure 5.10 Four‐year‐old Thoroughbred racehorse with acute severe right hindlimb lameness. (a) Caudocranial radiograph of the right tibia on the day of presentation. No abnormalities detected. (b) Lateral and caudal scintigrams of the right tibia. Linear IRU is present in the distal tibial metaphysis and diaphysis compatible with a propagating tibial fracture. (c) Radiographs taken at two, four and eight weeks post injury. Progressive osseous resorption permits identification of sharply marginated radiolucent fracture lines (black arrows). Areas of increased radiopacity are consistent with formation of trabecular and cortical callus (white arrows), which gradually bridges the fracture. Note also the distal lateral fracture line eight weeks post injury that is slow to become radiographically apparent.
It has been suggested that horses with evidence of stress fracture undergo scintigraphic review before they return to work [106]. This is not routinely practised in the UK where financial constraints and well‐accepted stress fracture management regimes have precluded longitudinal studies. Horses in training that have undergone nuclear scintigraphy in subsequent seasons have demonstrated subtle uptake at previous fracture sites. The degree and distribution of the 99mTc‐MDP uptake is usually mild, ill‐defined and compatible with bone remodelling.
In a study of equine distal phalangeal fractures, activity was reported to persist for >25 months. This was ascribed to a fibrocartilaginous union, fracture instability, osteolysis and osteoid formation [115].
A study of dorsal cortical fractures of the third metacarpal bone reported correlation between persistence of a radiographically evident fracture line with less intense scintigraphic uptake and individuals who did not heal and required surgical intervention [58]. The supposition made was that the degree of 99mTc‐MDP uptake was directly correlated with osteogenesis and rate of repair, thus diminished uptake in the absence of radiographic resolution indicated either a delayed or non‐union.
Sequential evaluations in the days and weeks following surgery were reported in three horses (four year old, yearling and foal) that had sustained a variety of traumatic fractures to the third metacarpal or metatarsal bones. Two cases developed photopenic regions less than six days post‐operatively, one was described as extensive and at necropsy this correlated with osteomyelitis and sequestration [116].
Computed Tomography
General Principles
CT is a high‐resolution, X‐ray based, quantitative, cross‐sectional imaging technique. It has for some time been integral to fracture diagnosis and management in man, and application in horses has recently evolved rapidly. Like radiography, it measures tissue attenuation of penetrating photons; however, the X‐ray source rotates around the patient. Multidetector row CT affords excellent spatial resolution and thin and overlapping slices, which approach isotropic, allow for multi‐planar reformatted (MPR) images that can be reconstructed in any chosen plane. The MPR reconstruction and thin slices both optimize fracture identification. Articular surfaces can be assessed [117, 118], and the superior bone detail produced by CT enhances identification and mapping of fissures, subchondral bone fractures, unicortical fractures and other articular fractures. Three‐dimensional surface rendering details the topographical aspects of the fracture configuration and with segmentation permits selective removal of overlying tissues in order to visualize the complexity of a fracture.
Cone beam CT (CB‐CT) has recently been introduced to equine use. It requires markedly different image reconstruction, does not provide quantitative information about tissue density and hosts a new complement of imaging artefacts that can detract from diagnostic accuracy.
Technical Considerations
CT requires precise and relatively rapid movement of the patient relative to the photon source and detectors (gantry). Moving gantry CT scanners allow the horse to be supported by a surgery table, and the gantry itself is responsible for movement accuracy. Equipment for CT in the standing horse is now possible using both conventional and CB‐CT scanners for the head, cervical spine and distal limbs.
CT provides quantitative imaging information with high spatial resolution. Each pixel is assigned a value described as a CT or Hounsfield unit (HU). This is a measure of each pixel's density with respect to pure water which is arbitrarily designated a value of zero HU. Pixel size is determined by the field of view (set at the time of image acquisition or reconstruction) and the pixel matrix of the image; it is often sub‐millimetre size. HUs are based on X‐ray attenuation in tissue. Gas is generally −1000 HU, fat is approximately −120 HU, soft tissues 100–200 HU, cancellous bone 400–600 HU and cortical bone in the range of 1500–2000 HU; dental enamel is higher than cortical bone. Slice thickness can be varied
