Navigation in surgery is made up by a group of techniques that solve problems with precise anatomical localisation and targeting of anatomical structures. This is especially important when operating on the brain, as it contains a lot of sensitive areas that must be avoided at all costs and there is a very low tolerance for error in neurosurgery. The intraoperative view of the target area is often constrained and lacks anatomical landmarks for orientation. All these factors add up into navigation being a critical part of neurosurgery.
The two main approaches to navigation in neurosurgery are framed and frameless navigation. We discuss frameless navigation in this page.
Advantages over frame-based navigation
Frame-based navigation has been used for better precision of neurosurgery since 1950's [1], but it has severe limitations and disadvantages. The major one is that only burr-hole procedures can be performed with a frame-based navigation. Another one is patient discomfort before and during the operation and a very limited view of the surgical field through the frame's burr-hole. This leads to possibly not being aware of complications occurring during the surgery.[[1]]
Framelessly navigated surgery work-flow
An operation using frameless navigation has several steps
- MRI is performed to get the brain structure data, with markers placed on the skin
- algorithms/clinicians segment the brain, mark critical structures, devise the best point-of-entry and the surgical route
- fiducials (markers) are placed on the skin
- the head is stabilized to avoid movement - a three-point Mayfield clamp is used to immobilize the head (for optical navigation)
- the head is registered to pre-operative data (see our page on registration)
- the instruments are registered into the same coordinate system
- the surgeon is shown the point-of-entry and the directions by an AR system (see our page on AR)
- possibly, intra-operative images are taken and used instead of the pre-operative data, to compensate for brain-shift
- instruments can be tracked in real-time, overlaid over on a 3D model of pre-op or intra-op data
- target area is highlighted, risk-areas are highlighted, the surgeon can operate with higher confidence
figure: A Mayfield clamp. [2]
Electromagnetic (EM) Navigation
A magnetic field is generated encompassing the surgical area. Devices containing miniaturized coils can then be located within the field. Several EM fiducials are placed on the patient and these are used to calculate the head position in real-time. Tools for EM navigation also contain coils and so they can be tracked without the need for visual contact.
This has several advantages over optical based navigation, as there is no need for the head to be fixed, and instruments' tip can be tracked directly, instead of optically tracking the handle of the instrument and calculating the position of the tip, which may be slightly off due to the instrument bending.
This approach was found to be superior to other technologies especially in cases where freedom of head movement was useful. EM neuronavigation is very well suited to CSF diversion procedures (shunt placement), awake craniotomies, and cases in which rigid head fixation was undesirable, such as in pediatric cases. This technology improves the application of neuronavigation to shunt placements and ventricular catheter placements in patients with traumatic brain injury. [3]
This has been proven (albeit with a relatively small sample size) by Hayhurst [4], as shunt failure rates were significantly decreased with shunts placed using EM navigation compared to anatomical-landmark-based navigation.
Bibliography
1) Mezger et al. (2013) Navigation in surgery, Langenbecks Arch Surg. 398(4): 501–514.
2) Olusum surgical instruments, http://www.olusumcerrahi.com/en/urun_detay.asp?ProductsId=6174 (access 17/07/17)
3) Hayhurst et al. (2009) Application of electromagnetic technology to neuronavigation: a revolution in image-guided neurosurgery
4) Hayhurst et al. (2012) Effect of electromagnetic-navigated shunt placement on failure rates: a prospective multicenter study