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AANS Neurosurgeon | Volume 28, Number 2, 2019


The Development of Functional Neurosurgery

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Editor’s Note: One obvious way to think about the world of neurosurgery is that it is the culmination of all the care delivered across the subspecialties. Just a few decades ago, subspecialization (and the whole world of CAST, enfolded fellowships, post-residency fellowships, etc.) was the exception, limited to just a small number of academics. Today it plays a much larger role across the neurosurgical spectrum. In a series of pieces, AANS Neurosurgeon explores each of the subspecialties in neurosurgery’s orbit. 

When appreciating the marvels and innovations of modern medicine, functional neurosurgery cannot be overlooked. For centuries, neuroanatomists, such as Francis Gall, endeavored to understand the localization of cerebral activity. Many followed and built upon Gall’s theory, in order to gain a complete understanding of the human brain and its function. However, it is the work of Sir Victor Horsley and Robert Clarke in London that developed the field of stereotaxy, leading to the birth of functional neurosurgery.1

The Essential Stereotaxic Frame

The development of stereotaxy occurred over the course of a century, incorporating years of innovation, adoption and adaptation. In the early 1900s, Horsley and Clarke invented the first stereotaxic frame. This head mount allowed for the precise and accurate placement of probes, needles or blades into an animal’s brain while minimizing damage. The mount’s ability to accurately predict a defined coordinate space utilizing the Cartesian coordinate system, allowed for smaller structures of the brain to be identified. The surgical space was co-registered with bony landmarks along with an atlas of anatomical brain specimens. This frame became the prototype that scientists and neuroanatomists later used to create other stereotaxic devices.1,2

Although successful in animals, due to design limitations, this frame was not used in humans. However, the concept achieved clinical utility when an external guiding device in combination with X-ray imaging was developed and used in 1947 by Ernest Spiegel and Henry Wycis on humans in the operating room. Specifically, they conducted pneumoencephalography to delineate the loci of the foramen of Monro and pineal gland; following this, they developed stereotactic plans in relationship to these structures. These data informed the coordinates of the frame. Building on the work of Spiegel and Wycis, Lars Leksell developed a prototype rectilinear coordinate system in conjunction with a semicircular sliding arc frame, allowing for target localization and trajectory planning. Advancements of imaging and refined coordinate systems enabled the contributions and insights of many others. One such example is the Todd-Wells frame, which, in the 1950s, used the arc-quadrant principle. Although the patient’s head was fixed, the holder could be moved to set an intracranial target at a deep focal point. Another breakthrough was that of Leksell’s spiral diagram, which dealt with radiologic parallax; it allowed x-ray compatible indicators to turn planar imaging into proximal and distal X, Y, Z coordinates. These innovations in frames, simplification of the co-registration process and radiographic elements allowed for the adoption of stereotaxy by surgeons across the world.1-3

Advanced Imaging and Computing 

The development of efficient imaging techniques further advanced the field; advances in both cat scans (CT) and magnetic resonance imaging (MRI) enabled visualization of brain structures and abnormalities and development of stereotactic neuronavigation. In the early 1980s, a localizer system was added to frames to define targets in CT space; this was commercialized in the Brown-Roberts-Wells (BRW) frame. The BRW frame eliminated the need for structural landmarks and reference points as computer-based systems now generated intracranial landmark locations and target coordinates.1 Following this, stereotactic methods were used to perform guided craniotomies, radiosurgery and tumor resections. Initially, surgeons relied on their own calculations of coordinates; however, by the mid-1980s, computers were able to calculate transformations utilizing perioperative and intraoperative imaging. The burden of calculations has greatly improved with the advent of neuronavigation platforms, which have become increasingly user-friendly over the last 20 years.1,2

Frameless Systems

Although its foundation is set on frame-based stereotactic procedures, functional neurosurgery has made strides in frameless navigation technology. Due to its ease of use and added patient comfort, these techniques yield great results with enhanced targeting accuracy.4 Navigation systems now include:

  • Video sensors;
  • Intraoperative monitor;
  • Reference frames; and
  • Light reflecting ball tabs marking the frame and instrument.

These components, along with preoperative CT or MRI images, can generate three dimensional maps of the patient’s cranial anatomy using external and internal landmarks.5 An intermediary step between a stereotactic frame and frameless stereotaxy are burr hole mounted frames. By utilizing fiducial marker bone screws and preoperative CT images, burr holes and corresponding trajectories to targets can be planned.6 Both “off-the-shelf” and  patient-specific customized frames allow for neuronavigation-guided trajectories to be planned.7,8 Stereotactic robotic arms have been introduced and shown great promise for stereoelectroencephalography in particular.

On the Horizon

As neurosurgeons look into the future, it is anticipated that stereotactic and functional applications will continue to evolve to include stereotactic delivery of treatments, such as laser interstitial therapy, deep brain stimulation and ablation as well as gene- and cell-based therapies. These new technologies coupled with those mentioned above and a deeper understanding of brain physiology should also open the door to new indications for intervention. Some of the most exciting functional research right now encompasses this and includes:

  • Restoring post-stroke function
  • Reversing spinal cord injury
  • Impacting medically refractory psychiatric conditions
  • Enhancing sensory functions (notably, hearing and vision)

While it may be one of the younger neurosurgical subspecialties, it may be the most rapidly growing and changing. 


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1. Brown, J. A., Pilitsis, J. G., & Schulder, M. (2020). Functional Neurosurgery: The Essentials. New York: Thieme Medical Publishers.

2. Raslan, A. M., & Burchiel, K. (2019). Functional neurosurgery and neuromodulation. St. Louis, MO: Elsevier.

3. Iskandar, B. J., & Nashold, B. S. (1995). History of Functional Neurosurgery. Neurosurgery Clinics of North America, 6(1), 1–25. doi: 10.1016/s1042-3680(18)30474-1

4. Atteya, M. M. E. (2019). Frameless stereotaxy: It is all about precision. Childs Nervous System, 36(1), 179–187. doi: 10.1007/s00381-019-04390-y

5. Idris, Z., Hallaert, G., Vanhauwaert, D., Kalala, J., Dewaele, F., Baert, E., … Caemaert, J. (2008). Frameless Neuronavigation-Guided Endoscopic Total En-Bloc Removal of a Third Ventricular Colloid Cyst: A Case Report on Surgical Technique. Min – Minimally Invasive Neurosurgery, 51(3), 173–177. doi: 10.1055/s-2008-1073133

6. Fukaya, C., Sumi, K., Otaka, T., Obuchi, T., Kano, T., Kobayashi, K., … Katayama, Y. (2010). Nexframe Frameless Stereotaxy with Multitract Microrecording: Accuracy Evaluated by Frame-Based Stereotactic X-Ray. Stereotactic and Functional Neurosurgery, 88(3), 163–168. doi: 10.1159/000313868

7. Bot, M., Munckhof, P. V. D., Bakay, R., Sierens, D., Stebbins, G., & Metman, L. V. (2015). Analysis of Stereotactic Accuracy in Patients Undergoing Deep Brain Stimulation Using Nexframe and the Leksell Frame. Stereotactic and Functional Neurosurgery, 93(5), 316–325. doi: 10.1159/000375178

8. D’Haese, P.-F., Pallavaram, S., Konrad, P. E., Neimat, J., Fitzpatrick, J. M., & Dawant, B. M. (2010). Clinical Accuracy of a Customized Stereotactic Platform for Deep Brain Stimulation after Accounting for Brain Shift. Stereotactic and Functional Neurosurgery, 88(2), 81–87. doi: 10.1159/000271823


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