Case Report

Enhanced Precision in Navigated Spine Instrumentation: A Novel Accuracy-Validation Technique Utilizing ‘Accuracy Check Points’

Kolahi K*, Lindvall E and Miller JD
Department of Orthopaedic Surgery, UCSF Fresno, USA


*Corresponding author: Kolahi K, Department of Orthopaedic Surgery, UCSF Fresno Community Regional Medical Center, 2823 Fresno Street, Fresno CA 93721, USA


Published: 27 Jun 2016
Cite this article as : Kolahi K, Lindvall E, Miller JD. Enhanced Precision in Navigated Spine Instrumentation: A Novel AccuracyValidation Technique Utilizing ‘Accuracy Check Points’. Clin Surg. 2016; 1: 1039.

Abstract

Background: The advancement of computer-assisted navigation provides potentially improved accuracy in spine instrumentation. Relying solely on the navigation system, however, may lead to inaccurately placed screws, and potentially devastating outcomes. This concern is of particular importance in the trauma setting, where fracture fragments may easily move intraoperatively. A surgeon would benefit from a reliable surgical technique, validating the accuracy of the navigation system.

Purpose: We describe a novel technique to validate the accuracy of computer navigation systems intraoperatively using “accuracy check points” (ACP). Small metallic markers are placed in bone such that the position of each mobile segment can be quickly verified.

Study Design: Case report.

Methods: A case report of an 80 year old female with a C1 posterior arch and odontoid fracture,instrumented with computer-assisted navigation. A novel technique using APC’s was employed intraoperatively.

Results: The patient’s cervical fractures were instrumented and stabilized without cortical breach or neurologic injury.

Conclusion: We suggest the use of accuracy check points to validate accuracy during computernavigated surgery.
Keywords: Computer; Navigation; Spine; Accuracy; Checkpoint; Validation; Instrumentation; Screw; Fracture; Trauma

Introduction

Computer-assisted navigation allows for potentially improved accuracy in spine instrumentation, as compared to conventional methods [1-3]. However, navigated spine instrumentation, particularly in trauma settings, may prove difficult, as patient anatomy may shift during the course of a procedure, thereby rendering the displayed image of digitized anatomy erroneous [4,5]. Techniques to validate the accuracy of navigation systems have previously been described. One method involves obtaining intraoperative 2D fluoroscopic images of the spine and comparing the bony anatomy to the navigated image [6]. Unfortunately, this method is both subjective and cumbersome. In addition, spinal segments may shift in the time required to evaluate 2D images and remove the C-arm. Another technique involves placing the pointed tip of a tracked tool against the bony anatomy, and confirming its appropriate location on the navigated display [6,7]. This method is quick and does not require additional imaging. However, it is best employed with a pinpoint bony landmark that can be indisputably identified in navigated imaging. Such pinpoint anatomy often cannot be reliably found, particularly in multiple segments.

Ideally, surgeons would be able to employ a simple and reliable surgical technique to validate accuracy, which could be done moments before instrumentation. In this paper, we discuss a method of placing ‘accuracy check points’ (ACP) at every reasonably mobile segment of bone. This involves placing small metallic screws at each desired segment, such that accuracy can be swiftly and confidently confirmed.

Figure 1

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Figure 1
Axial CT image showing a C1 posterior arch fracture.

Figure 1B

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Figure 1B
Coronal CT image showing an odontoid fracture.

Figure 2

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Figure 2
Patient reference tracker attached to Mayfield frame.

Figure 3

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Figure 3
Base of reference tracker covered with a sterile towel

Case Presentation

AA is a generally healthy 80 year-old woman who suffered a ground-level fall in her home, and who presented to our emergency room with neck pain. A thorough neurologic examination demonstrated that her sensory and motor systems were intact. Plain radiographs and a Computerized Tomography (CT) scan of the cervical spine demonstrated a minimally-displaced C1 posterior arch fracture and an Anderson and D'Alonzo type-III odontoid fracture (Figure 1A and B). Our planned treatment included posterior C1- C2 instrumentation and fusion, utilizing intraoperative CT based
navigation.

Figure 4

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Figure 4
Accuracy check point screw in C2. 4B: Screw in occiput. 4C: C1 posterior arch.Second Stage.

Figure 5

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Figure 5
Tool registration validation procedure.

Figure 5B

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Figure 5B
Position markers remain in line of sight of the position sensor camera.

The patient was positioned in the prone position and secured to a 3-pronged Mayfield clamp. A reference frame, in the form of a tracker secured to the patient, which allows instrument navigation relative to anatomy, was attached to the Mayfield clamp (Figure 2A and B). We chose to mount our reference tracker in this fashion because of the inherent rigidity of the construct. Another mounting option involves securing the reference tracker to an exposed spinous process, by using a toothed clamp [7]. After the patient was prepped and draped, the sterile reference tracker was attached to a mounting jig already secured to the frame. The sterile to non-sterile interface was covered with a sterile blue towel (Figure 3).

Dissection was carried out down to the exposed occiput and posterior elements of C1 and C2. At this time ACP markers were inserted. 1.5 mm titanium screws were inserted into the occiput and spinous process of C2 (Figure 4A and B). In this case, a mobile, disassociated posterior C1 arch precluded the insertion of an accuracy-point screw at that level, which normally would have been done.

The patient’s anatomy was then covered with sterile draping, allowing the reference tracker to remain exposed. The mobile O-arm was positioned over the desired anatomic structures, and a CT scan
was performed. A tracker built into the O-arm, along with the patient reference tracker, needs to be in the line of sight of a position sensor camera. Infrared light emitted from the position sensor is reflected back to the camera by reflective markers on each tracker. This allows the position sensor camera to triangulate the spatial locations of each tracker, and automatically register the scanned image to the reference frame tracker [6].

Figure 6

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Figure 6
Validating accuracy check points at 6a-occiput and 6b-C2.

Figure 7

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Figure 7
Measuring axial length of lateral mass.

Figure 8

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Figure 8
Navigated image showing orientation of drill guide 8a without and 8b- with projection of drill length.

The O-arm, along with the cover drapes, was subsequently removed. From this point forward, we were careful not to exert excessive force in the operative field, as the patient’s anatomy could shift relative to the reference frame. Prior to instrumentation, a series of validation steps were performed. Firstly, the calibration of any tracked instrument was validated. Surgical tools are equipped with trackers, each of which has a unique configuration of reflective markers, allowing for spatiotemporal tracking relative to the patient reference frame. The tool offset of its tip, relative to the tracker, is typically pre-calibrated. However, this calibration may be altered by tool warping or a malpositioned tracker. Therefore, prior to any instrument use, the tool tip should be inserted into a special divot on the patient reference frame (Figure 5A and B). Both the tool tracker and the patient reference tracker are directed towards the position sensor camera, allowing the computer to confirm that the tool offset matches its pre-calculated calibration [6].

Next, we ensured that the position of the patient’s anatomy matched the displayed CT image. We utilized a tracked tool with a pointed tool tip. The tool tip was placed directly on the head of each 1.5 mm titanium screw. The displayed image automatically shows multiple orthogonal CT views at the tool tip’s location. Once the tip was placed over the screw, we confirmed that each CT cut displayed our tool tip as directly over the screw head, constituting an active ACP (Figure 6A and B). Checking each ACP allows for 3D validation that the displayed image matches the patient’s real-time anatomy. This procedure is rapid, and was repeated prior to each lateral-mass screw insertion.

Although we measured our screw lengths and diameters with
pre-operative imaging, we also confirmed our measurements with intraoperative navigation. A pointed tool tip was positioned over the desired screw projection, and real-time measurements were taken, utilizing multiple CT cuts of the lateral mass (Figure 7).

Figure 9

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Figure 9
Surgeon’s view of drill guide placement.

Figure 9B

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Figure 9B
Drilling to set length.

Figure 10

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Figure 10
With screw projection, accuracy check points validated on 10aocciput and 10b-c2.

Figure 11

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Figure 11
Monitoring screw orientation throughout insertion.

After our lateral mass screw measurements were confirmed, we positioned a tracked drill guide over our desired screw position. A projection of our pre-determined screw length could then be added to the displayed image (Figure 8A and B). We confirmed that our drill position and orientation was centered over a safe area of bonevia multiple CT cuts. The drill was set to a pre-determined length and we drilled to a stop, making sure that our orientation did not alter along the course by monitoring a display (Figure 9A and B). A ball tipped probe was used to palpate the walls of the drill hole as an additional measure of accuracy confirmation. Screw taps and screwdrivers could
also be tracked. Therefore, prior to tapping or screw insertion, we again checked accuracy points by placing these tools over the 1.5 mm screw heads (Figure 10A and B). A screw projection could be added to the display, and screw orientation could thus be monitored during insertion (Figure 11A and B).

Figure 12

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Figure 12
The surgeon is able to re-image the patient after instrumentation to confirm proper placement of screws.

Figure 13

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Figure 13
Final construct

After all four lateral mass screws were inserted, we repeated the intraoperative CT imaging, as before. We confirmed that each screw was in a safe location (Figure 12). If any adjustments had been required, we would have been able to do so prior to closure. After we confirmed the desired placement of instrumentation, linkage bars were attached, and the wound was closed in routine fashion (Figure 13).


Discussion

Intraoperative navigation has been shown to decrease the risk of perforation and neurologic injury during spine instrumentation, as compared to conventional methods [1-3,8]. In this study, we have demonstrated a surgical technique allowing for potentially increased accuracy with the addition of ACP’s.

The limitations of intraoperative CT navigation include the increased cost, radiation exposure to the patient, and surgical time [4,5,8]. Our accuracy validation technique adds to surgical time, and approximately 5-10 seconds are required for each instance in which an ACP is visualized. The surgeon is assured, however, that her image display matches the real-time anatomy at the penultimate step of instrumentation.


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