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images are reviewed and critical vertebrae are identi
ed. The length of the vertebra
is measured from the pedicles to the anterior surface of the vertebral body.
Moreover, the width of the bone at the narrowest point of the pedicle is measured to
ensure selection of screws which will not penetrate into the spinal column. The
angle of approach is determined by an estimated deviation from the spinous pro-
cess. Consistent with current clinical practice, the proposed screws and angles of
insertion are documented, by hand, on a planning form. Nevertheless, the planning
is limited to the review of the 2D axial slices of the anatomy. Often, the axial views
cannot provide true measurements of the vertebral body or pedicle width and depth,
which may in turn lead to inadequate decisions with regards to the implant size and
trajectory.
In response to these challenges and driven by the motivation and insight of our
orthopedic surgery collaborators, we have developed a clinician-friendly applica-
tion that provides full 3D visualization for superior surgical planning. This appli-
cation uses routine 3D CT or MR image data to generate detailed models and
templates for better planning of pedicle screw instrumentation procedures.
The initial iteration of our spine surgery planning platform was developed
simultaneously with the iPlan platform developed simultaneously by BrainLab [
11
]
and features similar capabilities, including selection of virtual implants, trajectory
planning via virtual templating, as well image segmentation, registration and
overlay of multiple datasets into the templating workflow. Our platform was
employed extensively within the Division of Orthopaedic Surgery at Mayo Clinic
for a wide variety of non-routine clinical cases, especially pediatric cases of spine
deformity correction procedures.
In this work we also describe how the initial platform is augmented by introducing
and additional metric to optimize planning
the Fastening Strength. This metric is a
surrogate measure of the screw holding power, as described in Sect.
3.2.3
, and
combines implant dimension, trajectory and bone strength into a single metric that
enables pre-procedural assessment of each implanted screw. While the screw holding
power is a well-known concept to the mechanical engineering design community, to
our knowledge, this is its
—
first application to implant placement and trajectory
planning for surgical use. As further illustrated, its main bene
t is the feasibility to
also consider bone strength (i.e., density) when making decisions with respect to the
implant size and trajectory, by providing a consistent relative measure in response to
the dimensions, trajectory and bone mineral density characteristics of each implanted
pedicle screw.
Herein we describe the platform infrastructure and capabilities, present pre-
liminary studies conducted to assess impact on typical instrumentation procedures,
and present the formulation of the Fastening Strength metric, its integration into the
platform, as well, as its assessment against retrospective pre-procedural plans and
post-procedural outcome in several cases. Lastly, we share our initial clinical expe-
rience in employing the proposed tool for the planning of several complicated spinal
correction procedures for which the traditional planning approaches proved
insuf
cient.
