Preoperative imaging has traditionally included MR angiography and/or venography (MRA/MRV), CT angiography and/or venography (CTA/CTV), and digital subtraction angiography (DSA) [7]. Each modality has its benefits and limitations. DSA has traditionally represented the gold standard for elucidating vascular anatomy, but it also carries radiation damage, as well as the cost and time limitations as an invasive procedure. With this methods, it is difficult to acquire the entire image of the skull base, so that the relationship between the aneurysm and the bone cannot clearly present in front of the surgeons. Conventional CTA/CTV offers higher sensitivity and specificity in detecting vascular anatomy than MR-based techniques, but these imaging techniques become more limited in detecting small structures near the skull base due to the interference from adjacent osseous structures. Furthermore, they cannot provide a 3D immersive virtual reality environment for the surgeons [8]. The use of virtual reality technology has been reported in neurosurgery in recent years. The operator can perform segmentation, obtain measurements, and simulate intraoperative viewpoints and the removal of bone and soft tissue in the workstation. But the surgeons cannot use it as a navigation tool during the operation because simulation can only be performed on the computer. Sensory haptic feedback is also different from the real clipping [9–11]. 3D printing technology has important application value in the diagnosis and treatment of aneurysm especially in preoperative surgical rehearsal and neurosurgical training [12–15]. We think its advantages can be reflected in the following aspects:
Efficient and high emulation
Manufacturing time is always a concern emphasized by researchers. Wurm etc. [12] reported that 40 h are required to prototype a biomodel and Kimura etc. [13] needed 7 days. On the other hand, with the 3D printing technology, 20 h is enough for us to collect patient-CTA data, process data into STL and fabricate the model. Less production time and convenient data source make the 3D printing model meet the demand of clinical application. It is fast enough to fabricate an aneurysm model for emergency operations on ruptured cerebral aneurysms. The 3D printed model is easy to carry, so the surgeon can bring it into the operation room as a tool for intraoperative navigation. In our study, the high simulation degree of 3D printed aneurysm model has been proved. The 3D physical model is a tool in understanding the anatomy of the aneurysm (dome size, orientation, and neck size) and the associated vascular structures from every angle.
Simulation for operation
Our study showed that a model of this nature could be helpful in judging the best surgical approach and selection of the best clip. It can help the surgeons to simulate the operation on the models, including adjusting the head position, drilling the bone, measuring anatomic numerical value and choosing the shortest path to aneurysm. The cerebrovascular model not only allows the surgeon to have a true visual experience, but also a tactile experience which two-dimensional image cannot provide. Posterior circulation aneurysm is adjacent to the brainstem and the clipping operation is performed in a small space, so the choice of operative approach is more important than other parts of the aneurysm in surgery. We have completed 4 models of the patients with verte-brobasilar artery aneurysm, and designed operative approach by grinding occipital bone. It is convenient for the operator to determine the best approach before operation. Neurovascular model also plays a crucial role in the operation of multiple intracranial aneurysms. Treatment of bilateral aneurysms via a unilateral key-hole approach is minimal invasive to patients. The reason of choosing unilateral approach depends on the aneurysm pointing direction [16]. Aneurysms pointing in lateral, anterior or posterior directions are more difficult to clip from the contralateral side, because further inferior extension of the neck may be difficult to recognize. After simulation with 12 3D printed models of multiple aneurysms, we could comprehend the anatomy of the aneurysm orientation and neck exposure. The giant aneurysm clipping is a challenging procedure that can only be mastered under the guidance of a seasoned surgeon [17]. The aneurysm clips are always replaced repeatedly during operation, which not only prolongs the operation time, but also causes the risk of premature rupture and inadvertent vessel injury. Our 3D model has a flexible aneurysm part, the aneurysmal neck can be occluded with an actual aneurysm clip after bone grinding as is done during the actual operation. Especially in the giant aneurysm with thrombus which had a wide neck, this model allowed us to estimate how close to the neck we could place the clip without occluding parent vessels and choose the best clip placement. Getting familiar with the complicated structure of the aneurysm and its associated vessels and practicing various techniques of clipping make the surgeon treating the aneurysms confidently during live surgery. Wurm etc. [12] found that tactile feedback during the simulation makes the three-dimensional printed biomodel superior to stereolithography biomodel. We also found that procedure and evaluation of the patency of parent and branching vessels was judged to be realistic to real clipping in the DLP-based 3D printed biomodel. Endovascular interventions can be practiced in hollow model that can be coiled, stented, or flow diverted. Namba etc. [18] reported the intracranial aneurysms treated with a microcatheter shape determined by 3D printer hollow aneurysm model and the pre-planned microcatheter shapes demonstrated stability in all except in 1 large aneurysm case. But we could not use the model to test microcatheter in endovascular treatment and evaluate the cerebral hemodynamic changes of aneurysms as the vessel parts of our models were solid.
Applicability
It has been reported in many articles that the 3D printed model can serve as a valuable teaching resource for trainees [12, 13, 19, 20]. The 3D printed model is also thought to have potential as counseling devices for patients and educational tool for the young doctors. The aneurysm model can create a training environment for junior surgeons to learn surgical approaching, clipping direction, selection of clip, and the shape of the aneurysm in the simulation. Such a model would also inspire neurosurgeons to get new scientific research ideas in the areas of surgical techniques and the shape of aneurysms. [21] Furthermore, training and assessment of surgical skills through the physical models will advance competency training.
Customized fabrication
We could customize the model to meet different demands of neurovascular surgery. Although we could create the aneurysm models by many methods, the fabrication is time consuming and costly. Fabrication cost and time varied with model size. For example, if we want to fabricate a model for a patient with anterior circulating aneurysm, we could remove posterior cranial fossa when duplicating the geometry of the solid skull, as it takes less time and money to fabricate a model that includes anterior cranial fossa and the circle of Willis arteries than a whole skull model. The modeled hollow vessels without skull could also be used to simulate clipping of a giant or complex aneurysm [22, 23]. When we generate the 3D rotational angiography data, we found that we could fabricate a 3D printed model including skull, vessels, nerve fiber bundle, aneurysm and brain functional areas by advantage integration of different data sources, such as CTA and functional MRI.
Limitations of our models
In the study, we found some limitations of our 3D printed aneurysm model : 1. The data source came from CTA image, so the clarity of vascular enhancement will affect the quality of the model. 2. The accuracy of printing still needs to be improved, especially replica parts of small perforating vessels and terminal blood vessel is still unsatisfactory. 3. Materials are the limiting factor for further refinement of the models today. There are some gaps between the vascular model and the real human blood vessel. 4. The cost of model are expensive, so the high price make it better to be used in the complicated patients. How to improve materials and printing accuracy are the efforts of our ongoing study on 3D printing model.