Neurosurgery in 4D: A Meditation on the Future of 3D Modeling Technologies for Clinical Neurosciences

Throughout its history, neurosurgery has developed a uniquely symbiotic relationship with emerging technologies, largely attributable to the potential for such innovations to provide a practical bridge across the tremendous gaps in applied neuroscience. There is a singular boldness to operating on the nervous system, particularly given that many aspects of both normal function and disease remain incompletely understood. In spite of this, a fundamental pillar of neurosurgery is the mandate to use all available resources in rendering treatments as safe, effective and gentle as possible.

Within this context, technology has accordingly provided some of the most impactful avenues for improving neurosurgical care, and our operating room has evolved into one of the most critical spaces for the development and advancement of novel tools and techniques. In many ways, the history of neurosurgery can be told through its tools. Imaging modalities like pneumoencephalography, computed tomography and magnetic resonance have each transformed practice. Advanced visualization aids such as the operating microscope and endoscope, indocyanine green or 5-ALA and intra-operative stereotaxis have improved the safety and quality of innumerable operations. New devices, ranging from deep brain stimulators to endovascular devices to spinal instrumentation, have wildly expanded the scope of what can be treated.

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Neurosurgical innovations are often most impactful when a technological advance can be readily integrated into patient care. At present, neurosurgery is rapidly approaching just such an inflexion point with the rise of 3D modeling.

Broadly defined, 3D modeling is the creation of digital objects in a manner that can be mapped, mathematically and physically, to Cartesian space in three dimensions. As such, 3D models contrast with traditional forms of representing 3D space using 2D techniques such as vanishing point perspective in that the digital objects also encode volumetric information. Informally, this creates the framework for objects to interact with one another and with users in a way that very closely approximates a variety of real-world environments.

Although 3D models can be generated using a variety of techniques, the most common clinically salient strategy uses cross-sectional imaging to build triangular mesh matrices in a process called “segmentation.” Once 3D parts have been defined using the triangular mesh frameworks, they can be further modified by 3D rendering software, much like a physical object can be sculpted, divided, scaled or decorated. The final 3D objects can then be exported to a variety of 3D platforms, including simple viewing devices based on browsers or mobile devices, virtual or augmented reality (VR/AR) devices that might require a wearable headset, or an additive manufacturing process for 3D printing.

Medical 3D models are by no means novel, particularly within the context of teaching clinical anatomy; however, the increasingly widespread availability of software and hardware that support the development, testing and implementation of 3D models across a range of clinical spaces has dramatically accelerated their utility and consequent approval in practice. At present, most innovations in medical 3D modeling are clustered into three domains: education, clinical care and research.

Education Applications

Perhaps the most obvious space for the deployment of high-fidelity 3D models across a range of platforms is simulation and training. Neurosurgery residency curricula are already beginning to integrate resources built on VR/AR or 3D printing technologies, such as approach selection trainers; simulators for aneurysm clipping, pedicle screw placement, ICA blow-out injury management or temporal bone drilling; and guides for cadaveric dissection, among others.^8–10,13,16 Still other applications are under development, such as fully immersive angiography suite simulators, which will provide both practical training on capital equipment, as well as example cases for rehearsal, and radiation safety education modules. As these resources become more sophisticated and high-fidelity, they will provide a vital supplement for early neurosurgical training. Accelerating the learning curve by integrating 3D models and simulators is an area of critical need, particularly given the overarching trends in duty hour restrictions and the shifting medical-legal context.

Clinical Applications

Although some clinical applications of 3D modeling are further out in the development pipeline than educational tools, a large number are already in early testing and development. Some of these are patient-facing and passive, such as VR-based educational content for informed consent or postoperative care.^1,18,25 Others have a more interactive or treatment-adjunctive dimension, such as postoperative physical therapy instructions delivered by AR or VR-based distraction modules during painful procedures, such as lumbar drain placement or spine injections.^3,17,21 Finally, many other applications are surgeon-facing and aimed at enhancing various phases of care. Prominent examples under development include preoperative approach planning with VR or 3D printed models of patient-specific anatomy and pathology in situ, and intra-operative treatment enhancement via AR projections to assist with patient positioning or placement of an EVD, pedicle screw or other device, among many other proposed applications.^{2,7,12,14,19,22,24} The most integrated applications in contemporary neurosurgical practice are those related to cranial reconstruction, including cutting guides for cranial vault remodeling in pediatric craniosynostosis or bone flap replacement with personalized 3D-printed cranioplasties.^{4–6,11,15,20,23}

Research Applications

The academic applications of 3D modeling are both forward-looking and retrospective. At once, neurosurgeons are engaged in developing novel and innovative applications of these emergent technologies, as well as ensuring their safety in preclinical and clinical assessments. Much of the work to date in 3D modeling medical research has been technical and focused specifically on developing the new tools or more efficient techniques, that have collectively given 3D modeling a meaningful avenue towards clinical implementation.^5,10,13,16 In parallel, other efforts have shown preliminary efficacy data in a variety of parallel spaces, including trainee education, patient counseling and treatment enhancement. These efforts highlight one of the most important routes forward for the study of 3D modeling in neurosurgery, as the field evolves from answering more simple questions like “Can we achieve this?” to the more nuanced assessments of “Does incorporating these new tools result in a significant improvement in outcome or quality as compared to standard-of-care, sufficient to justify the additional time, cost or risk?”

Conclusion

In many ways, the pursuit of surgical cures for neurologic disorders is an inherently aspirational endeavor. From a practical perspective, the tools available to remove, restore, regenerate or replace tissue within the central nervous system are crude at best and evolving slowly, edging forward in tandem with the limited pace of neuroscience and bioengineering. Although still in its clinical infancy, 3D modeling stands poised to influence essentially all aspects of neurosurgical practice and training, whether through additive manufacturing, immersive technologies, or other yet-to-be-developed  tools at this exciting frontier in translational science.

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