Computational modeling of microarterial anastomoses with size discrepancy (small-to-large).

OBJECTIVE: Among causes of anastomotic failure in microvascular surgery is vessel size mismatch. Where the option of an end-to-side anastomosis is unavailable, an end-to-end construct must be used. Several end-to-end techniques are described to deal with size mismatch. The aim of this study was to numerically model arterial flow patterns and wall shear stresses in four idealized end-to-end anastomoses, where the upstream or recipient artery is smaller. The four techniques modeled were: an invaginating anastomosis; a fish-mouth incision of the smaller vessel; an oblique section of the smaller vessel; and a wedge excision of the larger vessel. MATERIALS AND METHODS: Flow rate in the right femoral artery of a single outbred male Wistar rat was recorded by transit time ultrasound. Initially, upstream vessel diameter in the models was set at 1 mm, and downstream at 2 mm. The wedge technique was further modeled using a shorter wedge, and using a downstream vessel diameter of 3 mm. Walls were deemed noncompliant. Flow was modeled by the finite volume method using the commercially available computational fluid dynamics code Fluent (Fluent Inc., Lebanon, NH; http://www.fluent.com). RESULTS: Ring vortices were seen in the invagination and fish-mouth models and showed similar characteristics, although they were less pronounced in the fish-mouth model. The oblique section model demonstrated complex, spiral, counter-rotating vortices that dissipated downstream. Flow separation was least in the first wedge model, with centralization of flow during high but decelerating flow rate. Shortening the wedge length or increasing the downstream vessel diameter to 3 mm led to flow separation. Wall shear stresses were broadly similar for all constructs. CONCLUSION: Of those modeled, excision of a wedge of the larger vessel proved the best construct. Where a vessel diameter ratio is 1:2, wedge length should be twice the diameter of the larger vessel. A vessel ratio of 1:3 leads to flow separation when using the wedge technique.