It is composed of a central ring (R), on the perimeter of which four planar laser diodes (D) have been installed at right angles to each other. Two radio-opaque cables (C) attached to weights (W) are attached to the ring just behind two opposing diodes. Cross-hairs on a bombsight (B) and a power compartment (P) complete the device.

Watch a brief narrated slideshow about the components of the Localizer.

Image reproduced by permission from Journal of Neurosurgery: Spine.

Reliance solely on palpation of anatomical landmarks is prone to inaccuracy, often requiring extension of the skin incision or working awkwardly through an angled trajectory. In our study, surgeon’s initial estimate of the target site based on palpation of anatomical landmarks proved to be inaccurate in 23.7% of the cases. Not surprisingly, accuracy of the surgeon’s estimate was inversely associated with body mass index, thickness of subcutaneous fat under the incision, and presence of transitional anatomy (J Neurosurg Spine 10:145-153, 2009). At worst, such inaccuracy may result in wrong-level surgery (J Neurosurg Spine 7:467-472, 2007).

Some surgeons insert a needle into the spine before or after skin prep and obtain a lateral x-ray before making the incision. This technique has three shortcomings: invasiveness, inaccuracy, and illegibility. If the needle is inserted before the skin prep, the risk of infection may be increased. If it is inserted after the site is prepped and draped, the iliac crest and sacrum may not be readily palpable under the drapes, increasing inaccuracy. Thin needles are difficult to visualize on xrays, particularly in obese patients. Thick needles produce subcutaneous and intramuscular bleeding, interfering with a clean microsurgical exposure. Localization accuracy suffers if the needle is inserted too superficially or at angle in obese patients. Risk of dural puncture exists if the needle is inserted too deeply in thin patients.

For pedicle screw insertion, fluoroscopy is indispensable after the skin incision has been made. However, fluoroscopy is relatively inaccurate when used to mark the location of the pedicles on the skin before the incision is made. This inaccuracy arises from potential misalignment of the radio-opaque marker (which by necessity is outside the body) with the spine target, the x-ray source, and the image intensifier. This alignment error can be significant if the procedure is being performed via a percutaneous or mini-open approach.

For a detailed description and calculation of alignment errors, see Slideshow #2.

For a detailed description of the alignment errors that are avoided by use of the localizer, please view Narrated Slideshow #2.

Figure reproduced by permission from Journal of Neurosurgery: Spine.

2. The localizer’s bombsight is centered on the skin mark.

3. The two sagittal beams are aligned with the midline of the spine (placing the two axial beams prependicular to the spine).

4. The two radio-opaque cables are aligned with the two axial beams.

5. the crosshairs on the x-ray source (machine) are aligned with one axial laser beam.

6. The x-ray film is centered on the opposite axial laser beam.

7. An x-ray is obtained. The two cables are nearly superimposed on the x-ray, traversing through a disc space. If this is not the desired disc space, the localization can be repeated or the correct disc space can be counted above or below the localized segment.

To view an animation of the localization steps and intraoperative photos, watch Narrated Slideshow #3.

Illustrated and copyrighted by Delilah Cohn; reproduced by permission here.

Figure is reproduced with permission from Journal of Neurosurgery: Spine.

Black arrows depict laser diodes; white arrows depict cables; arrowheads depict laser beams.
Image reproduced with permission from Journal of Neurosurgery:spine.

1. The location of the target is estimated based on palpation of anatomical landmarks and marked on the skin.

2. The localizer’s bombsight is centered on the skin mark.

3. The two sagittal beams are aligned with the midline of the spine (placing the two axial beams prependicular to the spine).

4. The two radio-opaque cables are aligned with the two axial beams.

5. the crosshairs on the x-ray source (machine) are aligned with one axial laser beam.

6. The x-ray film is centered on the opposite axial laser beam.

7. An x-ray is obtained. The two cables are nearly superimposed on the x-ray, traversing through a disc space. If this is not the desired disc space, the localization can be repeated or the correct disc space can be counted above or below the localized segment.

Illustrated and copyrighted by Delilah Cohn ; reproduced by permission here.