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Page 4 of 12 Maalouly et al. Mini-invasive Surg 2021;5:35 https://dx.doi.org/10.20517/2574-1225.2021.57

Intraoperative CT protocol
Patients undergoing less than four-level instrumentation or lateral position surgery were subjected to the
intraoperative CT protocol. The O-arm-2 (Medtronic Sofamore Danek Inc, Memphis, TN) was used to
acquire the three-dimensional (3D) CT scan. The surgical table was raised to a height of 110 cm from the
floor for easy maneuvering of the O-arm doughnut without major adjustments needed for the CT spin. The
surgical field was draped circumferentially in order to maintain sterility [Figure 1]. The scan was then
transferred to the ExcelsiusGPS navigation system for pedicle screw trajectory planning, which included
entry point, trajectory, screw length, and screw width.

During the planning phase, axial, sagittal, coronal, and 3D reconstruction views were available for the
surgeon on the navigation screen for planning of the screw trajectories. Care was taken to use the widest
and longest screws possible for each pedicle [Figure 2A].

The trajectories were matched to allow as small an incision as possible. The plan was also adjusted by the
surgeon for the best possible way to allow easy rod insertion and reduction of deformity based on the
surgeon’s experience [Figure 2B].

Time taken for planning each screw was noted. The planning time also included sterile draping of the robot
arm by the assistant done simultaneously. On validation of landmarks on the DRB with those seen on the
navigation screen, the reference frame ICT was then removed from the field taking care not to disturb the
DRB. The robot was then wheeled into the surgical field. The robot was docked securely to the floor once all
trajectories could be reached by the end effector arm. The navigated instruments, including a position
tracker, drill, and navigated screw guide, were registered in the system previously by the scrub nurse. The
surgeon utilized a foot pedal to bring the robotic arm to the planned screw trajectory. With the end effector
in position, a stab incision was made, and a power drill was first used to cannulate the pedicle, and the screw
was then placed through the stable, rigid end effector. Real-time visualization of the screw trajectory and
indicators of excessive skiving force were available to the surgeon through the process of screw insertion.
Once all screws were placed, the robot was undocked and removed from the surgical field and screw
placement was checked on orthogonal radiographs. Any unacceptable screws were revised immediately
before advancing to the next step. Rods were inserted and the construct completed. The time taken for rod
insertion was excluded from the calculation. Final A-P and lateral radiographs were taken to check the
construct.

Radiation exposure: The dose imparted at each scan was calculated from the dose report generated by the
O-arm and converted to mSv using a uniform tissue factor of 0.015 for uniformity in calculations. Similarly,
all fluoroscopy exposures were measured in milliseconds based on the readings from the C-arm.

RESULTS
The ExcelsiusGPS Spine robot guidance system was successfully used in 41 of 43 consecutive cases between
April 2019 and February 2020. Two cases were abandoned due to technical reasons where the robot could
not connect with the intraoperative image acquisition system. Those cases were excluded from the study.
Statistical analysis was done using Microsoft Excel (Microsoft Corporation, Redmond, Washington, United
States). Fixation was done across 86 motion segments over the course of the study period. The mean age of
patients was 70.9 ± 10.5 years. Thirty (60%) patients were female and 20 (40%) were male [Table 1]. The
mean BMI was 29.2. Of the 41 patients, 17 patients were operated in the single lateral position; 8 patients
were operated in the lateral position and then repositioned in prone position for posterior fixation; and 16
patients were operated in prone position only. Out of the 41 lumbar fusion patients, 33 had interbody fusion