Methods to determine Pedicle Screw
Placement Accuracy in Spine Surgery
Spine surgeons frequently utilize spinal fusion and pedicle screw fixation, which have been demonstrated to have significant advantages. Despite their proven benefits, spinal fusion and pedicle screw fixation do come with certain drawbacks, including pseudo arthritis, adjacent segmental degeneration, screw loosening, and incorrect screw placement.
Incorrect screw placement has the potential to cause a range of issues for patients, varying in severity from minor to major consequences. The consequences of incorrect screw placement may include neurological impairments, such as the onset of new radicular pain, sensory loss, or weakness, and can be as severe as paralysis.
While these risks are legitimate, they are typically minimal when surgeries are performed by qualified and experienced spine surgeons. Achieving precise screw placement is essential and a crucial factor in the success of the surgery.
In certain situations, incorrect screw placement may lead to significant vascular damage, including aortic perforation. Surgeons usually obtain post-surgery images to evaluate and verify the precise positioning of pedicle screws. At present, there is no universally accepted or definitive method for evaluating the accuracy of pedicle screw placement.
Numerous studies in the literature have utilized both the 2 mm increments and the in-and-out grading systems. It is evident that the majority of surgeons will employ imaging to evaluate the positioning of pedicle screws during and after the operation. Upon reviewing the literature, it was found that CT imaging is the most commonly employed technique for post-operative imaging.
The standard parameters of CT imaging, such as the thickness of the slices and the gantry tilt, have an impact on both the precision of imaging measurements and the degree of radiation exposure experienced by the patient. While gantry tilt is seldom altered, adjustments to the thickness of the imaging slices are more frequently made. To balance precision and patient safety by reducing radiation exposure, imaging with a thickness of 2mm is frequently employed.
The grading system that is most commonly utilized involves measuring the breach of pedicle screws in increments of 2mm. If the breach of the pedicle is less than 2mm, it is considered to be safe. While this grading system is deemed acceptable, it is important to take into account the location of the breach, particularly when determining the safety of a screw.
The in or out classification is the second most frequently utilized method for evaluating the placement of pedicle screws. While a small breach can be easily identified via imaging, it is important to distinguish its clinical significance from that of a larger breach. Therefore, this method is less effective in identifying whether a screw is positioned in a safe zone versus an unsafe one. What is crucial for precision and safety is the clinical significance of the breach.
The accuracy of screw placement is notably higher when a navigated technique is utilized in comparison to a free-hand technique. Screw placement accuracy varies depending on the grading system employed (2mm increment vs. in or out) and the region of the spine being operated on (thoracic vs. lumbar).
When a free-hand technique is used, the accuracy of screw placement is considerably greater with the 2mm grading system than with the “in” or “out” classification system. Employing a navigation technique and the 2mm increment grading system results in a markedly enhanced accuracy of screw placement, particularly in the thoracic region.
The greater precision of the 2mm classification system in detecting improperly positioned screws might help account for these findings. Additionally, since the placement of screws in the thoracic region is typically more difficult, it is not surprising that discrepancies were found between thoracic and lumbar screws depending on the grading system and surgical approach used.
Most of these grading systems are flawed because they do not take into account the direction of the breach. To illustrate, a breach categorized as grade B should be more worrisome if it is located medially and within the canal, compared to a similar graded screw that is laterally positioned and outside the facet joint or canal.
Moreover, there are cases where screws are deliberately positioned outside of the pedicle when there is a possibility of bone loss or compromise of the pedicle during surgery. To put it differently, a patient who presents with symptoms and an imaging-documented breach is more concerning in actual clinical practice than a larger breach that is visible on imaging but does not cause any symptoms in the patient.
The accuracy of screw placement is significantly higher when using a navigated technique as compared to the standard free-hand technique. Improved positioning of screws using imaging may result in enhanced patient outcomes, although it is uncertain if the improvement would have a significant impact in clinical practice.
Navigation may be an option to ensure secure screw placement, but it is essential to take into account several limitations associated with navigation, such as longer operation time, a steeper learning curve, increased expenses, and more complicated equipment. Furthermore, while the increase in accuracy achieved through navigation is statistically significant, its cost-effectiveness needs to be evaluated through a thorough cost-benefit analysis.
At present, the grading system for assessing pedicle screw accuracy that is most commonly used and acknowledged is the 2 mm increment grading system that relies on CT imaging. The majority of evaluation techniques rely entirely on imaging. Nonetheless, there is still no universally accepted and definitive method to determine the precision of pedicle screw placement in the realm of spinal surgery.
An all-encompassing grading system ought to comprise three elements:
- the degree of deviation measured through imaging
- the position of the deviation concerning critical anatomical landmarks
- whether or not any associated clinical symptoms are present.
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I am Vedant Vaksha, Fellowship trained Spine, Sports and Arthroscopic Surgeon at Complete Orthopedics. I take care of patients with ailments of the neck, back, shoulder, knee, elbow and ankle. I personally approve this content and have written most of it myself.
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