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Letters to the Editor |
Department of Radiology, University of Oklahoma Health Science Center, 1200 North Everett Drive, Room 1606, Oklahoma City, OK 73190. e-mail: william-yuh@ouhsc.edu
Editor:
We read with great interest the article by Dr Baur and colleagues in the December issue of Radiology (1) on the potential value of the "fluid sign" at magnetic resonance (MR) imaging for the differentiation of malignant and osteoporotic fractures. The authors described the fluid sign as being located adjacent to the end plate, associated with osteoporotic fracture, related to the severity of the fracture, and reported in rare cases of avascular necrosis of the vertebral body (1,2).
Although there was diagnostic confirmation of pathologic findings and the authors proposed plausible causes for the MR findings, a definite cause of the fluid sign remains to be determined. We therefore propose one other cause, acute vertebral ischemia, which might be worthy of consideration as a possibility for the underlying pathophysiology that produces the MR findings described by the authors as the fluid sign. We believe the findings described in the article of Dr Baur and colleagues (1) have characteristics of acute ischemic changes of the vertebral body on the basis of the location (adjacent to the end plate) and configuration (linear) of the sign and the vascular supply of the vertebral body (1,3).
Acute and severe fractures will likely cause an acute interruption of both the end plate arterial blood supply to the vertebral body and a disruption of the draining venous plexus. The area surrounding the end plate is a linear strip of bone marrow that has sparse blood supply (terminal branches) and is therefore more susceptible to ischemic injury (3). In vertebral ischemia, a characteristic linear abnormality has been reported (3) in the area surrounding the end plate that is similar to the finding reported in the current article (1). When compared with well-vascularized metastatic bone marrow, the aged osteoporotic bone is characterized by increased fatty infiltration of the bone marrow and an overall decrease in blood supply, thereby making the vertebral body more susceptible to ischemic injury. Well-vascularized metastatic bone marrow tends to have adequate blood supply that may sustain tumor growth in patients with metastatic disease. In other words, blood supply and vascularity in the metastatic bone are likely to be more abundant than those in the aged osteoporotic bone.
In addition, malignant fracture usually occurs after most or all of the bone marrow cavity is infiltrated with tumor (4), and the region surrounding the end plate is usually the first place metastatic seeding occurs (5). The blood supply may actually be increased in the region surrounding the end plate that is infiltrated by tumor when compared with that in osteoporotic bone and therefore will offer more resistance to ischemic injury. This mechanism would also provide an explanation for why the fluid sign is seen significantly more often in benign compression fractures than in neoplasia-induced fractures. We therefore believe that acute ischemia may be a possible explanation for the fluid sign on the basis of the characteristic vascular distribution of the vertebral body, location of initial metastatic seeding (infiltration), and difference in blood supply between well-vascularized metastatic marrow and yellow marrow.
REFERENCES
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