Читать книгу Fractures in the Horse - Группа авторов - Страница 136

As with other modalities, the diagnosis of fracture requires evidence of osseous discontinuity. Osseous trauma on MRI is associated with other changes in tissue composition, most importantly, the presence of bone marrow signal alteration (fluid) that can result from injury even in the absence of a visible fracture. Histological evidence suggests that less severe trauma can cause marrow oedema without obvious injury to the cellular elements, while more severe trauma causes microfracture and haemorrhage [12]. In man, T1W SE and STIR sequences consistently demonstrate prominent signal abnormalities at fracture sites including patients with subtle radiographic signs [147]. The high sensitivity of MRI for recent fractures is due to the fracture line being highlighted by intra‐osseous fluid accumulation [148]. The pattern of intra‐osseous fluid accumulation has been described as like a footprint left by the injury [149] (Figure 5.14).

Figure 5.13 Six‐year‐old eventer with acute onset moderate right forelimb lameness with a positive response to local analgesia of the medial and lateral palmar metacarpal nerves at a proximal metacarpal level. (a) T2*W GRE transverse plane sMRI image at the level of the proximal metacarpus. A large triangular zone of high fluid signal is present in the palmar medial aspect of the third metacarpal bone. The zone of high fluid is demarcated by phase cancellation artefact. (b) Radiograph taken six weeks post injury. A linear radiolucent fracture line is evident in the palmar medial cortex of the third metacarpal bone. No abnormalities were detected on radiographs taken two weeks post injury.

An acute non‐displaced trabecular fracture may present as a discrete hypointense linear, solid or broken lesion in T1W images [150] surrounded by intra‐osseous fluid accumulation, i.e. STIR hyperintensity [151]. Where a fracture gap is present, there is a hyperintense line on T1W, T2*W and STIR sequences in compact and/or trabecular bone along with decreased T1W signal intensity and increased T2*W and STIR signal intensity in the trabecular bone. Occult fractures have been variably described, ranging from diffuse trabecular intra‐osseous fluid accumulation, intra‐osseous speckled or linear regions of low signal intensity on T1W images to irregular areas of high signal intensity in corresponding areas on fluid‐sensitive sequences [132]. Compression fractures of trabecular bone can present simply as a zone of intra‐osseous fluid accumulation.

Figure 5.14 T2* GRE dorsal plane sMRI image of a metatarsophalangeal joint. The phase cancellation artefact delineating the fluid signal associated with a lateral condylar fracture leaves a ‘footprint’.

Pathological changes in the bone surrounding fractures can include sclerosis (detected as reduced signal intensity on all sequences), BML (increased signal intensity on fat supressed images) or bone resorption (most typically detected as increased signal intensity on all sequences). The fracture plane itself can vary in appearance depending on the sequence, fracture configuration, width and location [145].