1 Training Models in Neurosurgery

1.1 Introduction

•Neurological surgery is one of the most technically demanding medical specialties, with a steep learning curve.

•Neurosurgery has benefted from several technical advancements in the last 3 decades that involved visualization, instrumentation, and approaches.

•In recent years there has been the development of an increasing number of training models and courses to augment the training of new generations of surgeons and in an attempt to lessen the steep learning curve mentioned earlier.

•Changes in regulations regarding working hours for neurosurgical trainees have challenged the neurosurgical community to develop methods to maintain high quality standards without compromising the neurosurgical training, which is a necessary component of each residency. This is extremely important when dealing with highly complex lesions that are not routinely encountered such as deep-seated tumors and vascular lesions.

•To augment the clinical exposure to the pathology, an alternative option of reliable training that simulates life-like conditions is needed.

•Laboratory training is usually advocated to master the intricate anatomy of the brain and micro-neurosurgical technique.

•Both physical and computer-simulated training models are becoming increasingly available. These include models based on synthetic materials, animals, and human cadaver.

•Human cadaveric models are still deemed the most benefcial as they resemble life-like anatomical and technical challenges however there is a lack of blood circulation.

•Excellent spinal and cranial models have been created, focusing on performing surgical approaches with normal surgical anatomy. Some expose the residents to anatomy altered by pathological processes.

wing artery (Fig. 1.1), human and bovine placental vessels, and more recently three-dimensional printing.

•Live models provide the most realistic method for many types of training with pulsatile blood fow, natural viscosity and coagulation. Utilizing these models, however, has become more difcult as institutional protocols and review boards continue to impose necessary but signifcant constraints regarding the use of live animals.

1.2.2 Virtual Reality

•Computer generated graphics along with CT and MRI data have allowed for the re-creation of human anatomy in a virtual space. This has become particularly useful for understanding the complex anatomy of the central nervous system and the spatial relationship between anatomical structures in three dimensions.

•Currently there are three main types of virtual simulators: they are simplifed, augmented, and immersive.

○Simplifed VR systems are the most basic consisting of only a computer-user interface with no sensory interaction by the user.

○Augmented VR systems allow for more interaction and manipulation by the user with the use of external props. The ‘Robo-Sim-Endoscopic neurosurgical simulator’ is an example.

○Immersive VR systems are the most technologically advanced and involve creating a physical presence in a virtual world utilizing primary sensory input/output and haptic and kinesthetic modalities.

•Three-dimensional printing for neurosurgical training is a more recent advancement, which produces a multi-texture reconstruction and allows for the planning and training of specifc and complex operative procedures (Fig. 1.2).

1.2 Simulation Models in

Neurosurgery

•There have been widespread advancements over recent years when it comes to simulation in neurosurgery.

•The majority of simulators are divided into either physical or virtual reality (VR) subtypes.

1.2.1 Physical Simulators

•The physical simulators include cadaver models, live animal models, and synthetic models. Despite many recent advancements, the cadaver model has remained the most efective and most commonly used method for neurosurgical training.

•Cadavers and mannequins are particularly useful for the practice of basic skills such as drilling techniques, neuroendoscopy, spinal decompression and instrumentation.

•Microsurgical training is among the most practiced type of simulation, and it can involve the use of synthetic models such as silicon tubes, dead tissue models such as the chicken

Fig. 1.2 Implantation of a 3D printed aneurysm in a right sided opercular artery. (Copyright © 2015 Arnau Benet et al. Reproduced from Benet A, Plata-Bello J, Abla AA, AcevedoBolton G, Saloner D, Lawton MT. Implantation of 3D-printed patient-s ysm models into cadaveric specimens:

a new training paradigm to allow for improvements in cerebrovascular surgery and research. BioMed Res Int 2015;2015:939387.)

Abbreviations: D = dura; MCAA = middle cerebral artery aneurysm; R = retractor; RFL = right frontal lobe; RTL = right temporal lobe; SC = surgical clips.

1.3 Training Models in Vascular

Neurosurgery

1.3.1 Chicken Wing Artery

•The brachial artery is harvested from a chicken wing and can be used to practice end-to-end, end-to-side, or side-to-side anastomosis under a microscope (Fig. 1.1).

•Shapes of the arteriotomy, type of sutures as well as incident angle of the bypass graft may vary between surgeons; however, the basic yet crucial principles of microsuturing are exercised in this model.

•The distance the sutures are placed from the vessel edge is precise and the needle must penetrate the entire thickness of the vessel wall without touching the intima. It is important to keep the vessel and material moist throughout the anastomosis to prevent the structures from drying out.

•The integrity of the anastomosis can be evaluated by injecting water into the vessel and looking for leaks. In addition, the technique can be evaluated by cutting the artery, placing it under a microscope, and looking at the intimal appearance surrounding the anastomosis in search of kinks or strangulation.

•One of the drawbacks of this model is the variability in vessel diameter across diferent wings. Also, many of the wings suffer from freezing artifact after thawing which creates many difculties when attempting to harvest the arteries.

1.3.2 Human and Bovine Placental

Vessels

•Recently, the utility of both human and bovine placental vessels in microsurgical training has been explored and validated as an efective way to train neurosurgeons in low-fow and high-fow neurosurgical bypass techniques.

•Human placental arteries have wall thicknesses, amounts of connective tissue and diameters that closely resemble those in the human brain. Bovine placenta can be readily used if human specimens are not available as acquisition it does not require institutional review board or Institutional Animal Care and Use Committee approval, which makes it an easy replacement.

•The two umbilical arteries and the vein are cannulated and colored normal saline is incorporated into a fow circuit

at around 100 to 180 mmHg. This allows for the trainee to encounter “bleeding” if a vessel is damaged and practice hemostatic techniques using bipolar coagulation or ligation.

•The placenta is placed into a skull with a previously created bone window to simulate depth and the restrictions of a craniotomy. Vessels with diameters of 0.8 to 1.5 mm and 1.0 to 2.0 mm are used to mimic the human middle cerebral artery

(MCA) and superfcial temporal artery (STA), respectively.

Vessels with diameters of 8.0 mm to 9.0 mm and 2.0 to 3.0 mm are used to model the human ICA and radial artery (RA), respectively.

•End-to-side, end-to-end, and side-to-side bypasses with a long interposition graft are several of the microvascular techniques that can be practiced using this model.

•Some of the limitations of this model include the viability of placentas, the exposure to potential infectious agents using human placentas and the absence of adjacent soft-tissue that would need to be dissected in a real case.

1.3.3 Three-Dimensional Printing

•Recent advancements in endovascular surgery have changed the surgical indications for aneurysm clipping such that aneurysms not amenable to endovascular approaches are most often highly complex and challenging lesions. This decrease in the number of aneurysms considered amenable for surgery has led to an increase in the level of expertise required to handle those that need to be clipped. It is important that new methods are available for future neurosurgeons to practice and refne the techniques necessary to address these during their careers.

•In this model, 3D models of aneurysms are printed based on real patient data and implanted in human cadavers at the same anatomical region as the modeled patient (Fig. 1.2).

•The cadavers are connected to two liquid reservoirs inside pressure bags to simulate surgical bleeding during the simulation.

•Benet et al demonstrated this method by printing both patient specifc middle cerebral artery and basilar apex aneurysms, placing them in human cadavers using standard surgical techniques and evaluating their utility in the operative training as well as case management and planning.

○It was found that the fexibility of the neck and branches of the aneurysm were similar to those of the modeled patient.

○The aneurysm domes were also sufciently rigid to produce mass efect without compromising the aneurysm’s integrity.

○To simulate mass efect, a Foley catheter was introduced beforehand and progressively infated to allow room for implantation of the modeled giant aneurysm.

•This 3D aneurysm implantation model may help supplement the declining number of aneurysms treated surgically by incorporating hands-on training for patient specifc aneurysms considered for surgery.

•Currently 3D printers can only produce aneurysm models at least 1 mm in size. Patients with aneurysms smaller than this cannot be represented with this model. There is also a limit to how closely the 3D printed aneurysm will represent the source image in the patient, as MRI does not always pick up perforators, especially ones coming of the aneurysm dome.

1.4 Training Models In

Neuro-Oncology

1.4.1 Injectable Tumor Model

•In 2010, Gragnaniello et al proposed an injectable skull based tumor model, using Stratathane resin ST-504 polymer (SRSP), that could be used to train future neurosurgeons for the delicate dissection of a tumor from surrounding neural and vascular structures (Fig. 1.3).

•Stratathane resin ST-504 polymer (SRSP) is used for its characteristics to emulate brain tumors diferent in consistency ranging from rubbery meningioma to suckable pituitary adenomas. The polymer, developed in cooperation with the scientists of the Nanotechnology Center, refects most of the properties of real extra-axial central nervous system (CNS) tumors.

•It causes displacement of surrounding neural and vascular structure, and because of its non-adhesive nature, it afords us a dissection plane very similar to the one described in the aforementioned tumors. It also has a consistency similar to these tumors; it can be incised by micro-instruments, but it cannot be suctioned like a jelly, which helps in imitating the exact conditions of live surgery.

•The polymer has a distinct characteristic on T2-weighted MR images that enables its superb delineation and further assists the preoperative planning of the procedure.

•The pressurized heads consisted of cadavers in whom an artifcial circulation was established in the existing vasculature using a red solution imitating real blood infused via a mechanical pumping device.

1.4.2 Intraventricular Tumor Model

•As endoscopic approaches to intraventricular tumors become more common, there is an increasing demand for appropriate laboratory training models.

•Ashour et al describe a model in which the polymer mentioned above is injected into the lateral ventricle of formalin-

fxated, latex injected cadaveric heads under direct endoscopic and neuro-navigated guidance (Fig. 1.4).

Fig. 1.3 Tumor dissection through the optic-carotid window (A and B). Abbreviations: ICA = internal carotid artery; R = retractor; RFL = right frontal lobe;

RON = right optic nerve; T = tumor.

Fig. 1.4 Intraventricular tumor model. (A) Injection system of the intraventricular tumor model. (B, C) Endoscopic view of

right lateral ventricle depicting tumor model injected into the ventricular atrium in two separate cases.

Abbreviations: CH = cadaver head;

CN = caudate nucleus; CP = choroid plexus; opy monitor; IS = injection

system; MF = Monro foramen; SP = septum pellucidum; T = tumor; XRM = X-ray machine.

•This model allows the trainee to refne endoscopic techniques required for the resection of a solid intraventricular lesion in the presence of pathology-distorted anatomy. In addition, diluting the polymer at diferent ratios allows for the simulation of infltrative and non-infltrative tumor behavior toward the ependymal and surrounding critical structures.

1.5 Models To Simulate Blood

Flow To The Brain

1.5.1 Garrett’s Model

•In 2001 Garrett developed the frst model to simulate circulation to the brain using a very neat and elegant design.

•Permico, a commercially available solvent, is used to create an arterial tree using two or three remote sites. An arterialarterial circulation, which isolates the section of arterial tree under investigation, can be established, achieving infow and outfow through catheters inserted at remote sites.

•Infow is established through a large bore end hole catheter with balloon occlusion (Meditech OB, Boston Scientifc,

Natick, Mass) that is placed through a peripheral access site.

Outfow is established through a variety of catheters placed distally to the area of interest.

•A variety of pumps and tubing may be used, including those used for cardiopulmonary bypass grafting. Pumps may produce pulsatile or non-pulsatile fow.

•Diferent solutions may be used to simulate blood, but a crystalloid solution colored with red dye is an inexpensive substitute.

1.5.2 Aboud’s Model

•Without any doubt, the most powerful tool to simulate live conditions into cadaver heads is the one developed by Aboud et al at the University of Arkansas for Medical Sciences.

•Cadaver models injected with colored silicone, gelatin, or any other congealed material lack pulsation and vascular flling, which allow manipulation of vessels, hemostasis, clipping, and suturing. On the other hand, live anesthetized animals do not represent true human anatomy.

•In this model, in which vessels in a cadaveric head are flled with colored fuid under pulsating pressure for arteries and under static pressure for veins, the capability of bleeding, pulsation, vascular flling, and softness of the vascular tree makes it very close to live surgical condition.

•This model allows a trainee to perform many surgical procedures, among other, opening the Sylvian fssure, suturing vessels, practicing anastomosis, dissecting and clipping artifcial aneurysms, and practicing surgical approaches.

References

1.Aboud E, Al-Mefty O, Yaşargil MG. New laboratory model

for neurosurgical training that simulates live surgery. J Neurosurg 2002; 97(6):1367–1372

2.Ashour AM, Elbabaa SK, Caputy AJ, Gragnaniello C. Navigation-guided endoscopic intraventricular injectable tumor model: cadaveric tumor resection model for neurosurgical training. World Neurosurg 2016; 96:261–266

3.Belykh E, Lei T, Safavi-Abbasi S, et al. Low-fow and high-fow neurosurgical bypass and anastomosis training models using human and bovine placental vessels: a histological analysis and validation study. J Neurosurg 2016; 125(4):915–928

4.Benet A, Plata-Bello J, Abla AA, Acevedo-Bolton G, Saloner D,

Lawton MT. Implantation of 3D-printed patient-specifc aneurysm models into cadaveric specimens: a new training paradigm to allow for improvements in cerebrovascular surgery and research. BioMed Res Int 2015; 2015:939387

5.Nader R, van Doormaal T, et al. Skull base tumor model. J Neurosurg 2010; 113(5):1106–1111

6.Hino A. Training in microvascular surgery using a chicken wing artery. Neurosurgery 2003; 52(6):1495–1497, discussion 1497–1498