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optical tracking system. Vertebrae are segmented from volumetric CT data and
visualized in situ. A surgical drilling device was virtually extended with a mirror for
intuitive planning of the drill canal, control of drill direction and insertion depth.
The system was evaluated using realistic replica of lumbar vertebrae against the
classical, monitor-based navigation system providing three orthogonal slice views
on the operation site. Outcome was assessed according to the procedure time and
accuracy measurements recorded based on the post-procedure CT images of the
drilled vertebral models.
Given the fastening strength estimates, we believe it is not critical to employ a
complex
finite element model under hypothetical loading conditions, especially
given the current formulation suits any loading condition and provides a surrogate
measure for the screw holding power. This approach provides consistent trends
with implant dimension within the limits de
ned by the clinical standard implan-
tation procedures conducted with no computer assistance, chosen as gold standard
for assessment of the plans. Claims that the proposed planning approach would lead
to superior outcome compared to the clinical standard would be very dif
cult to
make, as they would invalidate the quality of health care currently delivered in the
clinic via the standard axial CT image-based planning approach. However, we
claim that the proposed approach provides objective measures for planning (i.e., the
effect of implant dimension and trajectory combined into the Fastening Strength
metric), and enables planning prior to the procedure and, if desirable, outside the
OR, potentially leading to shorter procedure and anesthesia time.
The virtual templating and planning can be completed any time before the
procedure, once the patient CT image dataset is available. Following data importing
into the surgery planning platform, the actual planning task requires 30
60 min of
-
effort (5
10 min for each vertebral level), depending on the complexity of the case.
The planning output consists of an automatically generated report that lists each
instrumented vertebral level, selected implant dimensions and trajectory, accom-
panied by orthogonal views and 3D volume rendered representations. Following the
addition of the Fastening Strength feature to the platform, measures such as dis-
placed bone volume, mean voxel intensity and Fastening Strength can also be made
available in the report; however their knowledge is more bene
-
cial during the
actual planning process, when selecting different size implants and assessing their
optimal trajectories to optimize holding power. Lastly, if a full scale physical spine
model is required, additional processing time (1 h) is necessary to generate a stereo-
lithography (STL)
file containing the surface model information; the 3D printing
process may require up to 24 h to complete, but once set up, the printing is mostly
automated.
One limitation of the current study, besides the small sample size available for
analysis, is the comparison of the virtual planning outcome to traditionally con-
ducted procedures, therefore making it dif
cult to account for any potential devi-
ations from the plan that could have occurred during the intervention. In addition,
the retrospective virtual planning was performed by a (less experienced) fellow,
while the actual procedures were performed by a staff surgeon, which explains the
fellow
