Clinical History: A 28 year-old male with a history of scaphoid fracture presents with persistent wrist pain despite four months of casting. A T1-weighted coronal image (1a) is provided. What are the findings? What is your diagnosis?
1a
Figure 1:
(1a) A T1-weighted coronal image
Findings
2a
Figure 2:
(2a) Despite immobilization, a fracture line through the scaphoid waist (arrow) remains visible. The proximal pole of the scaphoid demonstrates abnormal, diffusely low signal intensity (arrowhead).
Diagnosis
Delayed union of a scaphoid waist fracture with possible avascular necrosis of the proximal pole.
Discussion
The scaphoid is the most commonly fractured carpal bone, typically being injured after a fall on an outstretched hand in a young or middle-aged adult. Approximately 65% of scaphoid fractures involve the waist, with the next most common location (15%) being the proximal pole.1 The initial step in treatment of scaphoid fractures involves the accurate recognition of the fracture. While physical exam and plain films are often effective, non-displaced or minimally displaced fractures are frequently radiographically occult, and the presence of associated soft-tissue injuries and swelling can make the clinical diagnosis challenging. In such cases, MR is of great benefit in revealing the correct diagnosis (3a).
3a
Figure 3:
(3a) A T1-weighted coronal image of a high school basketball player injured after a fall reveals a non-displaced fracture (arrow) of the scaphoid waist. Plain films were normal in this patient.
Incomplete scaphoid waist fractures or complete, non-displaced fractures are usually treated with casting. Complex or unstable fractures, and fractures with associated ligamentous tears and carpal instability are more likely to require operative repair. Even with appropriate conservative therapy, up to 15% of patients may experience delayed union or non-union of their scaphoid fracture. Union is felt to be delayed if the fracture remains ununited after four months of cast immobilization.2 Non-union may be defined as the presence of a clearly defined fracture line or gap one year following initial injury.
If a scaphoid non-union is diagnosed within the first year, it may still be amenable to treatment via immobilization, often with the adjunctive use of bone growth stimulator therapy. However, many surgeons prefer operative fixation and/or bone graft placement for ununited fractures, and the feasibility and outcome of any such surgery is dramatically affected by the vascular status of the proximal fragment.3 Successful operative fixation of ununited fractures, with or without a bone graft, is much more likely if vascularity within the proximal pole of the scaphoid is maintained. If avascular necrosis or severe ischemia of the proximal pole is felt to be present, either vascularized grafts or the use of a palliative fusion procedure are recommended.
In order to understand the propensity for avascular necrosis of the proximal pole following scaphoid waist fracture, it is important to understand the unique nature of the blood supply to the scaphoid. The scaphoid derives its blood supply from branches of the radial artery, with the primary supply arising from a dorsal branch that enters the scaphoid at the dorsal aspect of the waist. A separate, more distal branch is also typically present, and supplies the distal pole.4 It is easy to see, therefore, that when a fracture of the scaphoid waist occurs, the blood supply to the larger proximal pole may become severely compromised, whereas the smaller distal pole will remain vascularized (4a).
4a
Figure 4:
(4a) A depiction of the typical blood supply to the scaphoid demonstrates the large dorsal branch (arrow) which enters at the scaphoid waist and supplies the larger proximal pole. A small branch (arrowhead) provides a distinct vascular supply to the distal pole. The blue line illustrates how a scaphoid waist fracture would thus disrupt the blood supply to much of the proximal pole, while leaving distal blood supply largely unaffected.
In cases of scaphoid delayed union or non-union, the gold standard for detection of an intact blood supply to the proximal pole is the presence of punctate bleeding at surgery. However, in light of the heavy influence of proximal pole vascular status upon treatment choices, an accurate pre-operative assessment is of course preferred. Classically, sclerosis on plain films or CT have been used to suggest avascular necrosis of the proximal pole, but such findings have been found to have a poor correlation with true scaphoid vascularity.5 Bone scans may be used to detect early avascular necrosis, but the poor resolution and lack of specificity of nuclear scintigraphy are problematic, as a number of factors other than avascular necrosis may result in increased uptake on bone scans. MR’s superior resolution and specificity have thus resulted in it becoming the preferred imaging test for the diagnosis and evaluation of patients with scaphoid fracture and suspected avascular necrosis.
The classic MR findings in patients with scaphoid waist fracture and avascular necrosis of the proximal pole are the presence of diffusely low signal intensity within the necrotic region on both T1- and T2-weighted images (5a,5b). Conversely, preservation of high signal intensity on T1-weighted images and the presence of edema on T2-weighted scans are felt to be indicative of retained vascularity within the proximal pole (6a,7a).
5a
5b
Figure 5:
In a patient with an ununited scaphoid waist fracture, the proximal pole (arrows) demonstrates diffusely low signal intensity on both a (5a) T2*-weighted gradient echo and (5b) T1-weighted coronal images. The subtle collapse of the proximal pole is also indicative of AVN.
6a
Figure 6:
In another patient with an ununited scaphoid fracture, the proximal pole (arrow) remains high in signal intensity on (6a) the T1-weighted sagittal image.
7a
Figure 7:
On (7a) the fat-suppressed proton density-weighted coronal view, the proximal pole (arrow) is diffusely edematous. The findings are both suggestive of retained vascularity within the proximal pole.
Unfortunately, the classic non-contrasted MR findings in patients with scaphoid non-union are at times unreliable. Even with necrosis, mummified fat may lead to preserved high signal intensity on T1-weighted images. Low signal intensity on T1-weighted images suggests necrosis, but may also be present simply due to ischemia. In patients with or without avascular necrosis, T2-weighted signal characteristics are frequently heterogeneous, with mixed high and low signal. Finally, a common pattern encountered is one in which T1 and T2-weighted data disagree (7a,8a), revealing low signal on T1-weighted images (suggesting avascular necrosis) and high signal intensity on T2-weighted images (suggesting preserved vascularity).
The use of intravenous paramagnetic contrast has been shown to improve MR’s ability to evaluate proximal pole vascularity in scaphoid non-union.7,8 The technique is most effective when fat-suppressed T1-weighted images are obtained immediately following contrast administration. The amount and rate of enhancement is proportionate to the degree of retained vascularity within the proximal pole. In general, the presence of significant enhancement within the proximal pole on post-contrast images is indicative of viable bone (10a). Enhancement of the proximal pole has been found to correlate not only with the presence of punctuate bleeding at surgery, but also with the likelihood of a successful healing following bone graft placement.
8a
Figure 8:
Disagreement of T1- and T2-weighted data. (8a) The T1-weighted coronal image reveals low signal intensity within the proximal pole (arrow), worrisome for avascular necrosis.
9a
Figure 9:
On (9a) the fat-suppressed proton density-weighted coronal view, high signal intensity throughout the scaphoid, including the proximal pole (arrow), is suggestive of retained vascularity.
10a
Figure 10:
In the same patient, a (10a) fat-suppressed T1-weighed coronal image obtained following contrast administration demonstrates enhancement throughout the scaphoid (arrows), providing strong presumptive evidence for retained vascularity within the proximal pole.
Conclusion
Scaphoid fractures are a common clinical problem in young and middle-aged adults. MR is of great value in the detection of occult scaphoid fractures. Delayed complications of scaphoid fracture including non-union and avascular necrosis of the proximal pole are well visualized with MR. Factors such as fracture displacement, associated ligamentous injuries, and avascular necrosis of the proximal pole are all critical in treatment decisions regarding scaphoid fractures, and all of these determinants can be evaluated with MR. In patients with possible avascular necrosis, contrast-enhanced MR provides the most accurate non-invasive means with which to evaluate the status of vascularity within the proximal pole.
References
1 Cooney WP, Linscheid RL, Dobyns JH. Fractures and dislocations of the wrist. In: Rockwood CA, Jr, Green DP, eds. Fractures in adults. 4th ed. Philadelphia, Pa: Lippincott, 1996; 745-867.
2 Amadio PC, Berquist TH, Smith DK, et al: Scaphoid malunion. J Hand Surg [Am] 1989 Jul; 14(4): 679-87
3 Fernandez DL. Scaphoid non-union: current approach to management. In: Nakamura R, Linscheid RL, Miura T, eds. Wrist disorders. Tokyo: Springer-Verlag, 1992:153-164
4 Gelberman RH, Menon J. The vascularity of the scaphoid bone. J Hand Surg Am 1980;5A:508-513
5 Sakuma M, Nakamura R, Imaeda T. Analysis of proximal fragment sclerosis and surgical outcome of scaphoid non-union by magnetic resonance imaging. J Hand Surg Br 1995;20B:201-205
6 Desser TS, McCarthy S, Trumble T. Scaphoid fractures and Kienbock’s disease of the lunate: MR imaging with histopathologic correlation. Magn Reson Imaging 1990;8:357-361
7 Cerezal L, Abascal F, Canga A, Garc?a-Valtuille R, Bustamante M, del Pifial F. Usefulness of gadolinium-enhanced MR imaging in the evaluation of the vascularity of scaphoid nonunions. AJR 2000;174:141 “149
8 Munk PL and Lee MJ. Gadolinium-enhanced MR imaging of scaphoid nonunions. AJR 2000; 175:1184-1185.