Reimplantation

Reimplantation is the technical equivalent of the STA-MCA bypass because it uses an end-to-side anastomosis to join an efferent branch deliberately occluded and detached from an aneurysm to an adjacent artery with robust donor flow. However, the donor–recipient relationship is reversed with reimplantation: the reimplanted artery is the revascularized recipient in the IC-IC bypass, whereas the reimplanted scalp artery is the revascularizing donor in the EC-IC bypass. The convergent STA-MCA bypass adds extracranial flow to the anastomosis, whereas the divergent reimplantation draws intracranial flow from the anastomosis. Reimplantation pairs the detached efferent artery with a nearby donor, such as an adjacent bystander or a preserved efferent branch from the aneurysm. These two partners must have the mobility and proximity to join within the surgical field. Requirements for reimplantation are met in four locations: the Sylvian fissure, callosal cistern, crural and ambient cisterns, and lateral cerebellomedullary cistern. Therefore, four aneurysm groups (MCA, ACA, PCA/SCA, and PICA) are amenable to reimplantation, and their arterial participants and working arenas are contained within four key anatomic triangles.

Microsurgical Anatomy

Sylvian Triangle

The Sylvian triangle is bordered by the Sylvian surface of the temporal lobe inferiorly (superior temporal gyrus), the Sylvian surface of the frontal lobe superiorly (inferior frontal gyrus), and reflected dura against the pterion or orbit anteriorly (Fig. 16.1). This triangle is opened by aggressively drilling away the pterion and lesser wing of the sphenoid bone during pterional and orbitozygomatic approaches, and by widely separating the frontal and temporal lobes. The skull base limits the anterior boundary of the triangle, but a thorough Sylvian split into the operculum expands its superior and inferior borders. The Sylvian triangle is stocked with arteries that can serve as donors for reimplantation, including the superior, middle, and inferior trunks of MCA bi- or trifurcations, M2 insular stem arteries, M3 opercular branches, temporopolar artery, and anterior temporal artery. The M1 MCA is not a practical donor because lenticulostriates on this segment limit its mobility and tolerance to temporary occlusion. M2 segments branch into unnamed stem arteries (eight per hemisphere, on average) and offer many donors for reimplantation distal to the MCA bifurcation with muscular walls and excellent caliber (up to 2 mm diameter). Early MCA branches (i.e., ATA and TPA) originate proximal to pathology at the MCA bifurcation and remain viable after aneurysm trapping. The ATA has a proximal diameter of 1.4 mm at its origin, a distal diameter of 1.1 mm as it exits the Sylvian triangle at the M3-4 junction, and a mean cisternal length of 34 mm. The TPA is smaller and not as reliable, with a proximal diameter of 1.3 mm at its origin, a distal diameter of 1.0 mm as it exits the Sylvian triangle, and a mean cisternal length of 37 mm. The arterial candelabra in the Sylvian triangle brings together donors and recipients in close proximity, with stem and early arteries that are often mobile and tortuous. The M3 opercular segments leave the Sylvian triangle with a perpendicular course that makes them less suitable for reimplantation. Cortical branches (orbitofrontal, prefrontal, precentral, central, anterior parietal, posterior parietal, middle temporal, posterior temporal, temporo-occipital, and angular arteries) are outside the Sylvian triangle, small in size, and too distally located to participate in reimplantations.

Falco-Frontal Triangle

The falco-frontal triangle is the natural corridor of the anterior interhemispheric approach and is opened by the separation of the medial frontal lobes from the falx down to the corpus callosum (Fig. 16.2). This triangle is defined by the falx and the contralateral frontal lobe medially and the ipsilateral frontal lobe laterally. The falco-frontal triangle is a potential space opened with subarachnoid dissection down into the callosal cistern and retraction of the frontal lobe, either with fixed retraction in the nose-up head position or with gravity retraction in the midline-horizontal head position. The midline falco-frontal triangle has large, bilateral ACA trunks running in close proximity. Eight cortical branches radiate from each ACA trunk along its course around the corpus callosum (orbitofrontal, frontopolar, callosomarginal, anterior internal frontal, middle internal frontal, posterior internal frontal, paracentral, and pericallosal artery), offering 16 potential doors within the triangle. The OrbFrA is the first cortical branch of the infracallosal A2 ACA, arising just distal to the ACoA and coursing forward to the anterior cranial fossa floor. The FrPolA is the second A2 ACA branch and originates 1 cm distal to the OrbFrA. The internal frontal arteries are alternative donors that do not require interruption of perfusion in mainline ACA trunks during cross-clamping. The AIFA originates from the A2 ACA adjacent to the rostrum; the MIFA originates from the precallosal A3 ACA adjacent to the genu, or from the CmaA; and the PIFA originates from the supracallosal A4 ACA segment or the PcaA, adjacent to the body of the corpus callosum. The paracentral artery arises from the A4 segment or the CmaA. Left-right pairings of donor and recipient are natural because participants are usually “kissing.” Unilateral superior-inferior pairings can also be constructed between the PcaA and the CmaA, which are separated only by the width of the cingulate gyrus and can be mobilized together.

Fig. 16.1 Anatomy of the Sylvian triangle. The Sylvian triangle is opened by aggressively drilling the pterion and widely splitting the Sylvian fissure. This triangle is bordered by the Sylvian surface of the superior temporal gyrus, the Sylvian surface of the inferior frontal gyrus, and dura reflected against the pterion and orbit anteriorly. Arteries within the Sylvian triangle that can serve as donors for reimplantation include the superior, middle, and inferior trunks of the MCA bi- or trifurcations; the M2 insular stem arteries in the candelabra; the M3 opercular branches; and early arteries from the M1 MCA (temporopolar artery and anterior temporal artery).

Fig. 16.2 Anatomy of the falco-frontal triangle. This triangle is the surgical corridor of the anterior interhemispheric fissure, as seen in the coronal plane. The falco-frontal triangle is bordered by the falx and contralateral frontal lobe in the midline and the ipsilateral frontal lobe laterally, extending to the corpus callosum inferiorly and opening superiorly without a border. Splitting the anterior interhemispheric fissure, dissecting into the callosal cistern, and retracting the ipsilateral medial frontal lobe open this triangle. Arterial donors for reimplantation within the falco-frontal triangle include the ACA trunks (the A2 and A3 ACA), the pericallosal and callosomarginal arteries, and the cortical branches radiating from the ACA, like with the internal frontal arteries. The triangle’s midline location enables left–right reimplantations as well as superior–inferior reimplantations.

Tentorial-Oculomotor Triangle

The carotid-oculomotor triangle is the working window for clipping basilar apex aneurysms because most aneurysms are viewed within this triangle from a transsylvian perspective. It also provides a larger working space than the carotidoptic and supracarotid triangles, and is unobstructed by hypophyseal or lenticulostriate arteries. However, the carotid-oculomotor triangle is not favorable for bypasses because its route runs deep to the sensitive oculomotor nerve to the P1 PCA segment, which harbors thalamoperforators that are intolerant of temporary occlusion during anastomosis. Instead, the tentorial-oculomotor triangle’s route runs lateral and posterior to the oculomotor nerve where arterial segments are more superficial and less populated with critical perforators (Fig. 16.3). The oculomotor nerve superomedially, the tentorial incisura inferolaterally, and the temporal pole posteriorly border this tentorial-oculomotor triangle. The triangle is reached through a transsylvian/pretemporal corridor. Small temporal bridging veins can be divided to open the pretemporal route with the Sylvian venous systems that drain posteriorly and/or superiorly, but large temporal bridging veins with anteriorly draining Sylvian venous systems must be preserved and can restrict access. Anterior venous drainage forces the exposure into the Sylvian fissure or requires mobilization of the temporal bridging vein and sphenoparietal sinus laterally. A wide Sylvian fissure split with dissection along the AChA’s cisternal segment releases the temporal lobe’s medial attachments to the frontal lobe. Temporal lobe retraction in a posterolateral direction is needed to expand the triangle posteriorly. The triangle’s inferolateral border can be expanded by dividing and tacking the tentorium behind the entrance of the trochlear nerve’s cisternal segment into its tentorial dural sleeve.

The P2A PCA runs over the oculomotor nerve to enter the triangle and has an average diameter of 2.1 mm. The SCA’s s1 and s2 segments run through the triangle and have average diameters of 1.7 and 1.5 mm, respectively, giving the SCA slightly more caliber in the pretemporal tentorial-oculomotor triangle than in the traditional subtemporal corridor. Duplications of the SCA are not uncommon and reduce the caliber of each rostral and caudal trunk. The close proximity of the PCA and the SCA creates a natural pairing for reimplantation. No other donors or recipients inhabit this triangle, but the ATA and the TPA have sufficient cisternal length to transpose after distal transection down into the triangle for donor reimplantation, thereby donating flow to an occluded SCA or PCA. The ATA and the TPA can be repurposed as donor arteries because they supply the non-eloquent anterior temporal lobe and an ensuing infarction would be limited, clinically silent, and tolerable.

Vago-Accessory Triangle

The vago-accessory triangle is the natural window for the far lateral approach (Fig. 16.4) and an established triangle that I use to clarify dissection routes to PICA aneurysms for clipping, to access the pontomedullary sulcus as an entry zone for central pontine cavernous malformations, and to transpose dolichoectatic vertebrobasilar arteries with symptomatic brainstem or cranial nerve compression syndromes (“macrovascular” decompression). This same triangle is used for PICA reimplantation. The vagoaccessory triangle is defined by the vagus nerve superiorly, the accessory nerve laterally, and the medulla medially. The hypoglossal nerve divides the vagoaccessory triangle into two smaller triangles: the suprahypoglossal triangle above the hypoglossal nerve between CN X, XI, and XII; and the infrahypoglossal triangle below the hypoglossal nerve between CN XI and XII and the medulla. The glossopharyngeal, vagus, and accessory nerves originate from the retro-olivary sulcus and course to the jugular foramen, whereas the hypoglossal nerve originates from the preolivary sulcus and courses to the hypoglossal foramen. Consequently, the course and depth of the hypoglossal nerve differ from that of the vagus and accessory nerves. As a result, supra- and infrahypoglossal areas are not simple two-dimensional triangles, but rather are three-dimensional corridors. With PICA reimplantation, the PICA is transected from a complex aneurysm and transposed from the suprahypoglossal triangle to the infrahypoglossal triangle, thereby moving it to a more superficial site in the vagoaccessory triangle less entangled in lower cranial nerves. The V4 VA segment is most accessible at its origin inferiorly where it pierces the dura and becomes less accessible distally along its superomedial course to the vertebrobasilar junction. Only PICA and V4 VA inhabit the vagoaccessory triangle, but the contralateral p3 PICA’s caudal loop lies in the posterior midline just outside of the triangle, as does the ipsilateral AICA above the glossopharyngeal nerve, making them both potential donors for reimplantation.

Fig. 16.3 (opposite) Anatomy of the tentorial-oculomotor triangle. The triangle is reached through a transsylvian/pretemporal corridor, requiring a wide Sylvian fissure split, dissection along the AChA’s cisternal segment, release of the temporal lobe’s medial attachments to frontal lobe, and division of the small temporal bridging veins. Posterolateral retraction on the temporal pole is mandatory to expand the triangle posteriorly. The triangle is bordered by the oculomotor nerve superomedially, the tentorial incisura inferolaterally, and the retracted temporal pole posteriorly, and it lies adjacent to the carotid-oculomotor triangle. Arterial donors for reimplantation include the P2A PCA running over oculomotor nerve and the SCA’s s1 and s2 segments running under oculomotor nerve. These arterial segments are more superficial and less populated with critical perforators. Arterial donors inhabiting this triangle are limited, but the ATA and the TPA have sufficient cisternal length to transpose down into the triangle after distal transection.

Fig. 16.4 Anatomy of the vago-accessory triangle. This triangle is the natural window of the far lateral approach and is opened with dissection into the cerebellomedullary cistern and tonsillar elevation. The triangle is defined by the vagus nerve superiorly, the accessory nerve laterally, and the medulla medially. The hypoglossal nerve divides it into two smaller triangles: the suprahypoglossal triangle above the hypoglossal nerve between CN X, XI, and XII; and the infrahypoglossal triangle below the hypoglossal nerve between CN XI and XII and the medulla. The V4 VA is the only arterial donor within this triangle available for reimplantation, but the PICA’s redundancy enables transposition from the suprahypoglossal to the infrahypoglossal triangle, moving it to a more superficial site in the vagoaccessory triangle. The contralateral p3 PICA’s caudal loop and the ipsilateral AICA both lie just outside of the triangle, but can sometimes be reached for reimplantation.

Donor and Recipient Dissection

Reimplantation revascularizes a single efferent artery and therefore is best suited for a fusiform aneurysm or pathological arterial segment, or a bifurcation aneurysm whose clipping preserves only one efferent branch. In these cases, the affected efferent is reimplanted on an adjacent bystander artery, on the preserved branch, or even the parent artery. The key aspects of reimplantation technique are proximity to a nearby donor within the bypass triangle and mobility of the recipient to reach that donor. Recipient arteries mobilize by releasing arachnoidal trabeculations and adhesions, straightening curves and tortuosities, and occasionally dividing a branching twig to adjacent non-eloquent cortex. Sacrificing a critical perforator to eloquent brain, such as from the PICA to the medulla, is not acceptable, but an insular perforator or a small branch to the temporal pole is expendable.

A transected efferent artery loses length and mobility. Proximal mobilization for reimplantation on the parent artery increases tension on the arterial stumps and dooms an end-to-side anastomosis, but distal mobilization for reimplantation onto an uninvolved artery decreases tension on the efferent artery and compensates for lost length. Distal release and downstream mobilization of the recipient artery is a critical step in uniting it with an appropriate donor. Therefore, the search for donor arteries should be directed distal to the aneurysm.

Reimplantation was performed most frequently with PICA aneurysms, and the PICA’s looping and recurring anatomy reverses these relationships (Fig. 16.5). A transected PICA mobilizes best to a more proximal site on the parent V4 VA, as it transposes from the suprahypoglossal triangle to the infrahypoglossal triangle. This transposition away from the aneurysm actually moves it upstream on the parent artery, making the V4 VA available as a donor unlike the parent artery with other aneurysms. The PICA’s anatomy and tortuosity make PICA reimplantation onto a more proximal V4 VA site uniquely favorable and account for the heavy use with PICA aneurysms (52%), as compared to MCA (29%) and ACA aneurysms (14%).

Reimplantation Technique

Reimplantation was performed in only 4% of my overall experience (21 cases, Table 16.1), the least of the seven bypasses. Reimplantation employs an end-to-side anastomosis with a long linear arteriotomy in the donor and a fish-mouth arteriotomy in the recipient. This anastomosis is performed extraluminally with standard technique. If the recipient artery has limited mobility and cannot rotate for extraluminal suturing of both suture lines, the first suture line is sewn intraluminally with the in-situ technique.