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Development of Anatomically Accurate Pelvis Bone Geometries for Finite Element Analysis
+1Stockwell K; 1Kesler, N M; 1Morrison, J; 2Petrak M. and 2Burnell C.
+1University of Manitoba, Winnipeg, Manitoba,
2
Concordia Hospital, Winnipeg, Manitoba
Senior author [email protected]
ABSTRACT INTRODUCTION:
Increasing popularity of alternative bearing surfaces for Total Hip
Arthroplasy (THA) has lead to the prevalence of implanted thin-wall
designs and modular press-fit acetabular components (i.e., cups) in
younger adults with higher bone density. Thin-wall systems may
deflect and deform more than conventional THA systems that use
metal on polyethylene bearing articulations during implantation. Metalon-Metal (MOM) resurfacing used to treat osteoarthritis in younger
more active patients is a thin-wall system and is drastically different in
cup design than in conventional THA. This difference in design
philosophy has led to 2D and 3D finite element studies that model the
surgical implantation of both MOM and THA acetabular cups. The
dominant trend in accurate bone model development for finite element
analysis is to use CT scanning and image segmentation. Recently laser
scanning has been shown to be a less expensive alternative to CT
technologies. The purpose of this study is to provide a detailed and
improved methodology for the creation of 2D and 3D CAD solid
models of bone via laser scanning and CAD work for the development
of cadaver bone libraries. Our primary goal is to improve previous
methodologies (Phillips et al. 2006) by eliminating user specified
cross-sectioning, thereby making the process more automatic. Having
established an accurate method for 3D solid models we introduce a
methodology for the creation of 2D axisymmetric models that are
directly comparable in geometry to their 3D counterpart thereby
ensuring that their finite element analyses will also be comparable.
The motivation for studying 2D models is well established in the
literature, with recent work (Yew et al. 2006) recognizing the
importance of relating 2D and 3D models. While 3D models have the
appeal of geometric accuracy, the geometric complexity and necessary
size of the models is daunting. Extensive progress has been made with
studies ongoing in modeling the boundary constraints (e.g. ligament
“fixation” within the body), the variations in cortical and subchondral
thickness and the contact between bone and cup during implantation.
Alternatively 2D axisymmetric modeling enables fast computation over
a 3D model (minutes vs. hours or days), and yields important estimates
on difficulty to implant, typical forces required, the effect of friction
and amount of polar gap remaining.
METHODS:
The methodology presented in this paper for laser scanning,
converting the measurements into a 3D model and subsequently creating
2D models is suitable for use on cadaveric bones, reproductions and
anthropological samples. The method was tested using NextEngine
Desktop 3D scanner on 32 innominates, 17 rights and 15 lefts and using
ShapeGrabber gantry on a Sawbones reproduction. Our technique
recommends attaching 14 fiducial markers on the 14 landmarks that are
placed on the innominate to render reconstruction simply and accurately.
The landmarks are placed on the following areas of the innominate;
three on the lateral/posterior side of the ilium, one on the posterior side
of the ischium, one each on the lateral side of the pubis and ischium, two
on the medial/anterior side of the ilium, one on the medial side of the
acetabular fossa, one each on the medial side of the ischium and pubis,
one each on the inferior end of the pubis and ischium, and two on the
anterior and posterior ends of the iliac crest.
Each innominate is scanned with the iliac crest most superior and the
acetabulum orthogonal to the viewing plane of the scanner, the upright
position. The innominate is rotated 360 degrees and six scans, one per
60 degree rotation, are taken in the upright position at standard speed
and a wide precision(75 DPI). The NextEngine ScanStudio software is
then utilized to compare scans against one another for the presence and
marking of common fiducial markers. The scans are aligned to a
tolerance of 0.254 mm (0.01 in), often achieving a smaller tolerance.
Additional scans are required to fill holes in the reconstructed surface.
Common areas where holes are present include the iliac crest, the
inferior regions on the acetabulum, the ischium and the pubis, and the
superior and inferior borders of the obturator foramen. The innominate is
repositioned with the problem area orthogonal to the viewing plane of
the scanner and a scan is taken. Fiducial markers common to both scans
are then used to align the new data to a tolerance of 0.254 mm (0.01 in).
The model is fused, generating a single surface model, when all scans
have been aligned and the model is free of areas with large holes. Small
holes on the surface are filled and the triangular mesh is simplified to a
tolerance of 0.127mm (0.005 in).
With the exterior of the bone surface defined the interior cancellous
bone is defined by offsetting a specified uniform thickness (e.g. 1mm) of
cortical bone, thereby eliminating the manual cross-sectioning and
related operations employed in previous methodologies (Phillips et al.
2006). In areas where the bone is less than two cortical thicknesses
there should be no surface and the offset surface will self-intersect. The
software is then used to remove the self-intersecting facets of the mesh
and complete the resulting boundary of the cancellous bone.
A sphere is defined in the acetabulum and used to simulate the
surgical reaming of the acetabulum. Twelve clusters of three points are
evenly placed around the lunate surface of the cancellous (offset) model
of the innominate and one cluster of four points is placed on the floor of
the acetabulum of the cortical model (for medial positioning). These
point clusters define a sphere, which represents a reamed acetabulum in
which all subchondral bone has been removed from the lunate surface
and the acetabular fossa has not been disturbed.
The 2D models of two previous experimenters (Spears et al. 1999 and
Yew et al. 2006) are derived from an earlier model (Pederson et al.
1982) that is based on “a mid-frontal cross section … in a plane inclined
superiorly about 15° posteriorly, showing reasonable inferior and
superior symmetry”. Our method defines a cutting plane with the crosssection including a central axis of the acetabulum, which passes through
the center of the acetabulum and is orthogonal to a plane fit to the
acetabular rim. A plane is fit to the acetabular rim by placing six
clusters of four points evenly around the rim. The orthogonal of the
fitted plane is passed through the center of the reamed acetabulum to
produce the acetabular axis. Two cross sections of the acetabulum are
prepared using the acetabular axis and another point selected on the
innominate to generate a cutting plane. The first cutting plane is defined
using the acetabular axis and a point located midway between the greater
sciatic notch and the most protruded point of the ischial tuberosity. This
cutting plane yields the first cross section which has been chosen to be
similar to the cup model of a previous study (Yew et al. 2006). The
second cutting plane and cross section is generated using the acetabular
axis and a point located midway between the anterior superior iliac spine
and the posterior superior iliac spine for similarities to another previous
model (Pederson et al. 1982). The cross sections are split by the
acetabular axis and one half of each is used to produce a 2D axisymetric
model for further finite element analysis.
RESULTS:
The use of 3D offset to define the cancellous bone over the arbitrary
sectioning eliminates a possibly serious source of error in computing
models. The offsetting of 2D serial sections (Phillips et al. 2006) and
subsequent remeshing the 3D surface resulted in observable mistakes in
thickness and shape of cortical layer. This is demonstrable in any region
where the normal to the innominate does not lie in the section and the
resulting offset surface in such regions is a Tcos(Θ) where T is the
desired offset and Θ is the angle between the section and the normal.
The method for creating 2D axisymmetric models is based on the
central axis, which ensures that the acetabular diameter and fossa are
represented in the model. Specifically the models produced by our
method have much thinner acetabular fossa (e.g. 1.5mm vs. 12.3mm).
DISCUSSION:
Utilization of a 3D offset requires hand cleaning of the thin regions,
which result in self-intersection once offset. This limitation involves
intensive work to remove the self-intersections but removes the error
and user specificity in the operation. The work also provides a detailed
methodology for creating the 2D axisymmetric models that are located
by landmarks. While these models will have cups at an anteversion and
inclination equal to the acetabular rim, this is a limitation of the model
type and has been incorporated into the methodology.
Poster No. 2396 • 55th Annual Meeting of the Orthopaedic Research Society