Natural and surgically imposed anastomoses of the circle of Willis.

ABSTRACT

The efficacy of the circle of Willis as a flow equalizer is well known. Most cerebral macrovasculatures also contain other natural anastomoses which are activated in times of stenotic stress. For the past several decades, neurosurgeons have surgically augmented the cerebral network with additional vessels which further increase the flow of blood to a defrauded region of the brain. It is desirable to know quantitatively what role these anastomoses play in the delivery of blood. Apart from computer simulation, such knowledge remains out of reach to the medical community but with modern simulation techniques, a wealth of information can be made available. This paper presents both time-dependent and period-averaged results of a detailed study of cerebral anastomoses. Four different models of the macrovasculature in the circle of Willis vicinity have been developed, two of which contain an extracranial-intracranial (EC-IC) anastomosis. Five cases were developed to show how the amount of blood flow is related to the sizes of the anastomoses. Since the EC-IC bypass is only marginally beneficial in those patients whose cerebral circulations are well-equipped with naturally occurring anastomotic vessels, procedures should be developed to screen for their presence or absence. The fluid mechanics associated with the EC-IC bypass operation dictate a beneficial result. Since the surgical procedures fail to consistently show reduction in risk even when good grafts have been made, there is an enigma in the study group results.