By Ajay K Wakhloo
With the development of the Surpass FD, we engineered the porosity and mesh density of braided tubular implants to optimise the effect of intra-aneurysmal flow reduction for a given size of the parent artery harbouring the aneurysm, writes Ajay K Wakhloo.
Coil embolisation has proven to be safer than standard surgical clipping for brain aneurysms and therefore is replacing the latter. However, the occlusion rate of endovascular treatment is highly dependent on aneurysm morphology and location, techniques of coiling and operator’s skills. Large, giant and wide-neck aneurysms, fusiform and blister aneurysms as well as aneurysms associated with segmental artery disease remain an endovascular treatment challenge because of the significant risk of coil herniation into the parent artery, a subtotal aneurysm occlusion or coil compaction with aneurysm recanalisation in 20–80% of cases and aneurysm regrowth. But to prevent (re)rupture, the goal of treatment remains a complete exclusion of the aneurysm from the circulation that demands a complete endothelial coverage of the aneurysm neck which is not provided by coiling in most instances. Adjunctive techniques such as balloon-assisted or stent-assisted coiling have been introduced to enable endovascular treatment of giant and wide-neck aneurysm and reduce recanalisation rates. However, recanalisation requiring intervention persists to be a challenge.
Based on our in vitro and in vivo experimental studies focused on haemodynamics of aneurysm/parent artery complex that were performed during more than two decades, we proposed the use of fine mesh stent-like tubular structures, which we branded as flow diverters, to overcome the aforementioned issues. A flow diverter is placed within the parent artery and bridges the aneurysm or diseased vessel segment harbouring the aneurysm. Our studies showed that the work mechanism of these devices is to reduce blood flow within the aneurysm and create a stable aneurysmal thrombus while maintaining patency of vital perforating branches that supply normal brain tissue, depended on the porosity (metal/metal-free area) and mesh density (pores/mm²) of the implant. With the development of the Surpass FD, we engineered the porosity and mesh density of braided tubular implants to optimise the effect of intra-aneurysmal flow reduction for a given size of the parent artery harbouring the aneurysm. To maintain a homogenous distribution of number and size of pores/mm² across the aneurysm neck, the number and the thickness of wires of the implant were modified with the diameter of the flow diverter. Blood enters the aneurysm due to acceleration and deceleration of flow during the pulse cycle, while blood flow through adjacent perforators is facilitated by the pressure differential between the right atrium and the left ventricle.
By bridging the aneurysm neck, the mesh of the Surpass FD generates sufficient impedance to the flow entering the aneurysm for a blood stasis which initiates the formation of an aneurysmal thrombus.
As we demonstrated in experimental studies, the clot gradually transforms to scar tissue while the Surpass implant, which serves as a scaffold for tissue growth, is covered progressively by endothelium.
Our clinical studies show excellent durable complete occlusion rates at six months of >80% and progressing to >90% at one-year angiographic follow-up and a high safety profile (< 5% permanent morbidity and mortality) in otherwise surgically or endovascularly difficult to treat brain aneurysms. Surpass FD received CE mark in 2011 and since then has been successfully used in a variety of aneurysms of the anterior and posterior circulation including aneurysms distal to Circle of Willis originating from the anterior and middle as well as posterior cerebral arteries. Some early data in the treatment of middle cerebral and anterior communicating artery bifurcation aneurysms are promising, but longer-term follow-up studies are mandatory. The Surpass FD comes pre-mounted on a delivery system that tracks over a 0.014” microwire and is placed under fluoroscopy through a percutaneous access to the desired location. Surpass FD is available in various diameters (2.5–5.3mm) and lengths (15–50mm) to cover most of brain aneurysms with one single device (Figure). Flow diversion represents a paradigm shift in endovascular treatment of brain aneurysms as we are moving away from saccular coil embolisation to endovascular repair/bypass of the diseased arterial segment for a definite cure.
Ajay K Wakhloo is a professor of Radiology, Neurology and Neurosurgery, and Chief, Division of Neuroimaging and Intervention and New England Center for Stroke Research, University of Massachusetts Medical School, Worcester, USA. Wakhloo is a paid consultant for Stryker, which since last November owns Surpass