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4.2 Assessment of Fastening Strength Estimates
We conducted an initial assessment of the developed platform using retrospective
clinical data from patients who underwent spine surgery that involved the
implantation of pedicle screws. For this study, we have limited our analysis to four
lumbar cases, consisting of a total of 28 pedicle screws implanted in the lumbar
spine of four patients. Procedures were performed by a staff surgeon following
standard planning in the OR using 2D axial images from the pre-operative CT scans
for implant size estimates, with no use of the virtual planning platform. All implants
were deemed successful with no revisions. Post-operative CT scans routinely
ordered for follow-up purposes were used as ground truth (clinical gold standard)
for our assessment.
Several weeks after each procedure, a spine surgery fellow used the virtual
planning platform to plan each procedure. The plans yielded a total of 30 virtual
lumbar implants being suggested across all four cases. Although a total of 28
pedicle screws were implanted during the procedures, and the retrospective plans
suggested 30 implants (i.e., one extra vertebral level suggested for instrumentation
in one patient), only 26 implants were homologous
implanted/suggested at the
same vertebral level in both procedures and plans, respectively. Therefore, for the
sake of a consistent paired assessment, only the 26 homologous implants (same
vertebral level in both plan and procedure) were considered in the analysis.
—
4.3 Implant Dimension Assessment
The dimensions of the implants selected using the virtual planning platform are
automatically generated in the output report. However, to determine the dimensions
of the screws implanted during the procedure, we used the virtual platform to
“
reverse plan
”
the post-operative CT such that the
“
reverse planned screws
”
match
those visible in the post-operative CT images (Fig.
8
). Following reverse planning,
the implant dimensions were included in the automatically generated output report.
This approach was chosen to eliminate any uncertainty associated with the user-
conducted measurements caused by the partial volume effects and beam hardening
artifacts induced by the presence of the metallic screws in the post-operative CT
images.
4.3.1 Implant Fastening Strength Assessment
The Fastening Strength of the planned implants was directly assessed by estimating
the intensity
area product across the pedicle volume displaced by the virtually
implanted screws. To compute the Fastening Strength for the post-operative
assessment, each
—
“
reverse planned
”
vertebra was registered to its homologous
