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Maalouly et al. Mini-invasive Surg 2021;5:35 https://dx.doi.org/10.20517/2574-1225.2021.57 Page 3 of 12

Biplanar fluoroscopy was used to check the position of the screws after placement and any unacceptable
screws were revised immediately and noted. Postoperatively erect radiographs were taken to check on the
construct. Follow up was planned using our standard protocol with X-ray radiographs at one, three, and
twelve months postoperatively and computed tomography (CT) cans at six months postoperatively of
instrumented levels for all patients.

Analysis
The accuracy of pedicle screw placement was determined as grades (Grade A, 0 mm; Grade B, 0-2 mm;
Grade C, 2-4 mm; Grade D, 4-6 mm; and Grade E, > 6 mm) based on the 2 mm incremental system
according to CT scans at six months developed by Gertzbein and Robbins . The number of screw breaches
[5]
was recorded for each group. All revised screws were considered as inaccurate. All other screws were
considered for analysis irrespective of intraoperative change of trajectory or freehand placement of screws
using the ExcelsiusGPS navigation system. The number of acceptable screws divided by the number of total
screws placed with robotic navigation resulted in an accuracy percentage for this study.

Surgical technique
Patients were positioned prone or lateral. The position was determined based on the surgeon’s preference
and the surgical procedure planned. The patients who underwent posterior only instrumentation was
positioned prone on the Jackson table. The surgical field was prepped and draped in the usual sterile fashion
allowing access to bilateral posterior superior iliac spine (PSIS) in all cases. The ExcelsiusGPS® fiducial
marker [called the dynamic reference base (DRB)] was placed in the right PSIS via a small stab incision
during prone position and in the left PSIS in the single lateral position. In cases where an open incision was
utilized, the DRB was mounted on the spinous process clamp attached to the lower most spinous process
not considered in the construct. The intraoperative CT fixture (ICT) was attached to the fiducial marker, in
a plane parallel to the floor and just above the patient’s skin over the area of interest.

Patients undergoing OLIF procedure in conjunction with posterior instrumentation were positioned in the
left lateral decubitus position over a flat top table and taped in the usual fashion to allow tilting of the table
without allowing any patient movement intraoperatively. The surgical field was prepped and draped widely
allowing access to the left Iliac crest and PSIS. The DRB was placed on the left PSIS, while the surveillance
marker was placed a few inches anterior to the DRB along the crest. The DRB post was tilted caudally to
avoid any interference between the camera and the reference frame. The intraoperative CT fixture (ICT
frame) was attached on the DRB post and placed flat over the area of interest on the patient’s skin.

Imaging protocol
The ExcelsiusGPS robotic navigation platform supports three imaging protocols: preoperative CT scan,
intraoperative CT scan, and intraoperative fluoroscopy.

Preoperative CT scan
The preoperative CT scan was utilized in patients undergoing instrumentation for more than five levels.
The CT scan was carried out using a special protocol involving a scan in supine position with 1 mm image
cuts for integration with the robotic navigation platform. In patients with preoperative CT scan, validation
of landmarks was done using serial orthogonal radiographs for each level to be instrumented. The
radiographs were then transferred to the robot navigation system for screw trajectory planning. The
validation and verification system of the ExcelsiusGPS robot system provided alternate ghosting images of
the radiographs and the CT scan during the planning phase.