Читать книгу Fractures in the Horse - Группа авторов - Страница 91

Following clinical localization, a standardized approach to image acquisition is usually the most rewarding and strongly recommended. For adequate assessment of common distal limb fractures, a minimum of four orthogonal projections in addition to lesion‐oriented oblique projections are recommended. This not only enables identification and mapping of the fracture but also detection of additional factors that may affect case management. As a two‐dimensional representation of a three‐dimensional object, small adjustments from the standard projections may be required to produce parallel alignment between the X‐ray photon beam and the fracture plane (Figure 5.3). When this occurs, the resultant lack of attenuation by the fracture results in a relative increase in energy to the imaging plate and a radiolucent line on the processed radiograph.

Figure 5.3 Parasagittal fracture of a right forelimb proximal phalanx. (a) Dorsopalmar radiograph (lateral to the left). Two fine linear radiolucencies (white arrows) can be appreciated in the proximal third of the bone corresponding to the fractures in dorsal and palmar cortices. (b) Dorsal 10° lateral–palmaromedial oblique of the same limb. The dorsal and palmar fracture lines and X‐ray photon beam are aligned. A discrete continuous fracture line is now evident (white arrows) extending from the metacarpophalangeal joint to the distal aspect of the medullary cavity. The nutrient foramen is identified by a yellow arrow.

The shape of the structure being imaged also requires consideration. For example, the distal aspect of the equine third metacarpal/metatarsal bone differs both between medial and lateral condyles and the dorsal and palmar/plantar articular surfaces which have different shapes and radii [3]. Optimal identification of lesions in the distal palmar/plantar surface therefore requires a projection that is both tangential to the region of interest and has minimal superimposition of other osseous structures [3, 4]. This can require several projections altering the degree of fetlock flexion and/or incident X‐ray photon beam angle to highlight different areas and effectively evaluate the condylar surfaces (Figure 5.4).

Select radiographic views of the contralateral limb are often helpful. Examples include cases when there may be bilateral lesions such as stress fracture predilection sites, in exercise‐related fractures, when radiographic evidence is weak and in skeletally 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.