Tuesday, September 13, 2011

MRI and Scaphoid Viability

Determining the viability of scaphoid fracture fragments is important in surgical planning, with some surgeons electing vascularized bone grafts when a non-viable fragment is suspected. Contrast-enhanced MRI would, at first glance, seem to be a good way of assessing viability prior to surgery; however, there are conflicting data on this matter.

A recent study by Donati and colleagues in Radiology looked at the performance of MRI in detecting non-viable scaphoid poles in 28 patients. They defined viability based on intraoperative findings: if the surface of the proximal scaphoid pole remained pale after débridement, with lack of surface bleeding and no bleeding after removal of the surgical tourniquet, the proximal pole was considered to be nonviable.

They found that while 90% of viable scaphoid poles had enhancement, approximately half of non-viable scaphoid poles had at least some enhancement. The authors found that subjective enhancement by itself was not an accurate predictor of scaphoid viability. Objective assessment of viability with dynamic enhanced MR imaging fared even worse for detection of scaphoid non-viability.

They attribute the enhancement in non-viable fragments to the presence of fibrovascular tissue in the setting of avascular necrosis, as well as patchy areas of vascularized bone.

Some have advanced the use of non-contrast signal characteristics for determining viability, with the finding of low signal on T1- and T2-weighted images as suggestive of poor vascularity. Donati and colleagues, however, found that the vast majority of both viable and non-viable scaphoid poles has low or intermediate signal on T1-weighted images, likely related to callus formation in both groups of patients. They also found that the vast majority of both viable and non-viable scaphoid poles had hyperintense signal on fluid-sensitive sequences, possibly due to immature callus, blood vessels, bone marrow edema, and/or bone marrow fibrosis and bleeding.

Overall, Donati and colleagues found sensitivities in the range of 60%, negative predictive values in the range of 70% - 74%, and a specificity of ~90% for detection of scaphoid necrosis when using pre- and post-contrast images.

While contrast-enhanced MRI is the best we can do at this point for assessing scaphoid viability, we should keep its low sensitivity and low negative predictive value in mind when interpreting images. The high specificity of contrast-enhanced MRI, on the other hand, means that we can count on positive findings of low signal intensity on T1- and T2-weighted images and absent or small-volume enhancement as evidence of non-viability.


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