Size discrepancy in microvascular anastomosis is a common issue in free flap transfer. The sudden change of caliber may cause turbulence to the blood flow and predisposes to platelet aggregation. Discrepancies in the cut end diameters have been managed by many geometrical methods in order to reduce the risk to thrombosis. In this paper, we pretended to summarize the techniques described and published about the management of size discrepancies in microvascular anastomosis, from the simple mechanical expansion with the jeweler's forceps to the sophisticated hardware such as devices or laser. Advantages and disadvantages are analyzed for many geometrical designs of anastomosis. We may conclude there is not an ideal technique to manage every size discrepancy, rather to question for the best method with the less complications, we should search for the best procedure to fit a specific case. A small caliber discrepancy may be well managed only with mechanical expansion. In traumatic or ischemic leg reconstruction, an end to side anastomosis will offer the continuity of the blood flow to both the vessel and the flap. In head and neck reconstruction, when a great discrepancy may be anticipated and the upstream donor vessel is smaller than the recipient one, a sleeve anastomosis can be performed. In the clinical set of a gap between the vessel ends, a graft must be used. Other geometrical designs (fish mouth or oblique cut), devices, glues or adhesives and laser helped anastomosis should be considered according the surgeon experience.
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