Fractures in the Horse. Группа авторов

immature patients to compare growth plates, apophyses, subchondral development, etc.

Figure 5.4 Images of a right metacarpophalangeal joint. (a) Flexed dorsopalmar radiograph (lateral to left). (b) Altering limb position (reduced flexion) and beam angle (dorsal 20°distal–palmaroproximal oblique) reveals a small radiolucent fissure in the palmar lateral condyle. (c and d) Sagittal and dorsal plane reformatted CT images illustrating lesion location in the condyle.

      Fractures of the cerebral and visceral cranium are often difficult to assess, and in such cases the clinical assessment and secondary features, e.g. gas lucency in the subcutis and soft tissue swelling, can assist in directing optimal obliquity for the incident X‐ray photon beam. Opposing oblique views allow for comparison between sides and are always recommended even when trauma is sided.

      Artefacts and Other Misleading Features

      Numerous artefacts can masquerade as fractures, and knowledge of these can avoid erroneous diagnosis.

      Mach lines describe enhanced edge perception: a dark edge appears darker and a light edge lighter than expected from optical density alone. They are thought to be caused by lateral inhibition of retinal receptors.

      The Uberschwinger artefact (overshoot or rebound effect) is an artefact of DR. The basis of digital image processing involves mathematical manipulation of the image. Unsharp masking is used to obtain pixel values that are closer together when faced with structures with large differences in density, such as orthopaedic devices and bone, so they both can be viewed with one lookup table. The processing causes edge enhancement that gives the radiograph high contrast and edge definition. However, fine detail around metallic implants is lost, and noise increases creating a stripe of reduced density parallel to the interface between the two dissimilar densities [5, 6]. Failing to understand this can lead to incorrect conclusions of osteolysis, implant loosening or infection.

      Fascial planes are more radiolucent than muscle, and if superimposed on bones can create an artefact but it is usually clear that this extends beyond the bone margins.

Both poor and absence of packing of solar frog clefts and sulci can be detrimental to interpretation. Radiolucent lines should be scrutinized carefully to determine if they remain within the bone or extend beyond osseous margins. If this is inconclusive, then radiographs can be repeated following repacking and/or without packing. Alternatively, differing degrees of obliquity of the incident X‐ray photon beam will either project the artefact away from the bone or confirm that a radiolucent line remains with the bone.

      New bone production can create the appearance of a relative decreased opacity in the adjacent bone.

      A range of normal anatomic features can be mistaken for fractures. The fibrocartilage between the distal lateral radius and lateral styloid process (phylogenetic ulna) which is radiolucent for an inconstant period is a common example. Physes can also be variable in appearance, but usually they are bilaterally symmetric. Nutrient foramina are tunnels in the cortices of long bones which house blood vessels coursing to and from the medullary cavity [7, 8]. Position can be variable but size, uniformity of the adjacent bone and trajectory help to distinguish from a fracture (Figure 5.3).

      Prominent parallel‐sided bone trabeculae can, on first assessment, give the illusion of a fracture. Careful scrutiny and magnification of the image will demonstrate a slightly meandering course and no interruption of the trabecular lines.

      Distinguishing suture lines in the cerebral and visceral cranium from fractures can be challenging. Good anatomic knowledge and reference to an anatomic specimen are important. The spheno‐occipital suture can pose difficulties. It remains visible up to five years of age [9], can be up to three times wider ventrally than dorsally [10] and when a fracture is present there may be limited displacement leading to a false negative.

      Limitations

The principal limitations of radiography are that it is a two‐dimensional representation of a three‐dimensional object and that there can be a delay between injury and identification of structural change [11] (see Figures 5.12a and d and 5.13b). In the absence of displacement or distraction, fracture identification requires approximately parallel alignment of the osseous discontinuity and incident X‐ray beam. If there is trabecular injury only, intact overlying cortex may efface the fracture [12]. Recently formed, thin, woven bone (periosteal callus) is insufficiently mineralized for radiographic visualization [13] and can take two to three weeks to become apparent [14, 15]. It has been reported that for acute lytic lesions 30–50% bone loss is necessary for radiographic identification [15, 16]. In the digital era, more subtle changes can be identified, but, in basic terms, if the sum of the osteoclastic and osteoblastic processes is not sufficiently out of balance to change the recognizable radiographic density, a lesion may remain radiographically silent.

      Principles of Interpretation

      Depending upon time frame and aetiopathogenesis, fractures can produce differing appearances in compacta (cortical and subchondral) and trabecular (spongiosa or cancellous) bone. Radiographic findings are also dependent on the individual bone and location. In acute fractures, the presence of a radiolucent line, cortical discontinuity or altered contour or impacted or displaced bone fragments may be identified. In contrast, in incomplete fractures there may be only a subtle cortical lucency followed by periosteal reaction and endosteal callus formation. Fractures of trabecular bone may exhibit only faint increased radiopacity (sclerosis) due to microcallus formation [17]. A line of sclerosis perpendicular to the trabeculae can also be representative of a fracture [18]. Fractures that occur secondary to progressive bone failure may have evidence of plastic deformation, mild to extensive periosteal and/or capsular new bone formation or subchondral opacification, demineralization or a combination thereof which precede the development of a discrete fracture line.

      Fracture Types

      Monotonic Fractures

      Monotonic fractures generally present with gross cortical and trabecular disruption and can usually be regionalized following clinical examination. If there is severe guarding by the patient, lesions involving the axial skeleton and ribs may prove more difficult to isolate but, within minutes to hours, following the development of muscular swelling or a sympathetic cutaneous response the target area can usually be identified.

      The initial radiographic study should aim to establish the precise location and configuration of the fracture and indicate the possibility of accompanying injuries. This information will direct appropriate management.

      Stress Fractures

      It had been suggested that the term stress fracture be restricted to cases of osseous structural failure detected radiographically by a fracture line and that the term stress reaction be used to describe the series of changes in bone pathophysiology associated with repetitive loading [19]. As identification of a discrete fracture line is temporally and modality dependent, an accurate description is fundamental to interpretation.

Initial findings in cortical bone can include a subtle radiolucent zone or faint intracortical radiolucent striations followed by periosteal and endosteal new bone and in some cases the appearance of a delicate fracture line [20]. Further periosteal