This separate trunk giving rise to the splenic and left gastric arteries is called the lienogastric trunk. ("lien-" latin for "spleen" - why the sudden switch to latin from greek is not clear, "splenogastric" would seem to be a more etymologically consistent term).
This lienogastric trunk is most common when the common hepatic artery is replaced to the superior mesenteric artery (Michels variation IX, 4.5%), so theoretically, one could see a lienogastric trunk in about 1 in 20 patients.
Another more rare variation with a lienogastric trunk would be a separate origin of the SMA, CHA, and the lienogastric trunk. The CHA and lienogastric trunk would then both arise directly off the aorta (below). This variant is much less common and represents only about 0.4% of cases.
|In this patient receiving surveillance for ocular melanoma, a separate common hepatic artery from the aorta (green arrow) and lienogastric artery (red arrow) were noted.|
As the image above shows, there can be a wide spectrum of variant anatomy in the hepatic and splenic arterial circulation, including both separate lienogastric trunks and separate splenic trunks... so although classification systems like Michels' are useful for a conceptual framework of hepatic vascular variation and their probabilities, there are so many possible varieties of hepatic arterial circulation that it's important not to let your mind get boxed into the categories and to describe exactly what you see.
1. "Gastrointestinal Angiography" Reuter, Redman, and Cho. 3rd ed. (1986)
2. Sahani D, Mehta A, Blake M, et al. "Preoperative Hepatic Vascular Evaluation with CT and MR Angiography: Implications for Surgery" September 2004 RadioGraphics, 24, 1367-1380.