ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

The ScanIP wizard creates the new implanted femurs using the CT data, the neutrally
implanted model and positions created by the +CAD wizard. First, it searches for
misplaced implants i.e. outside femur cortex boundaries, by applying an ‘‘Out-of-
Boundaries Removing’’ tool. A reference volume or mask is created to filter out all
misplaced implants. It is also possible to use a filter that allows the user to specify a
critical threshold that all new implant positions should not exceed, defined as a
percentage of the reference volume. For each selected position, the implant mask is
substracted from the reference mask. Finally each model is exported to a third ScanIPbased
wizard to automatically generate a 3D 4-noded tetrahedral mesh, see Figs. 2a and
2b. For the neutrally implanted femur, the bone (implant) consisted of 63541 (58504)
nodes and 244085 (246161) elements. The bone-implant mesh is deliberately refined to
better capture micromovements at the interface. The bone apparent density of the bone
( ) was assumed to be linearly related to the Hounsfield unit from the CT scan. For the
calibration between elastic modulus of the bone ( E ) and , the relationship
1.52
was used [4], see Fig. 2c. For frictional contact analysis, a pair of
E 7281
surfaces is automatically defined with a coefficient of friction of 0.3. The boundary
conditions are fixed distal end of the bone. To simulate normal walking mode, joint and
abductor loads are applied at a single node at the center of the top surface of the head of
the implant and on the greater trochanter, respectively.
(a) (b) (c)
Fig. 2: FE mesh of implanted femur (a); View of refined internal mesh (b); distribution
of Young’s modulus (MPa) in bone (c).
4. RESULTS AND DISCUSSIONS
Initially, the neutrally implanted femur is considered and micromovements are recorded
at the entire interface between the bone cavity and the external surface of the implant,
see Fig. 3. It is assumed that this position corresponds to zero translations and rotations
around the implant local coordinate system. A maximum implant micromotion of
28.57µm was found near the distal tip of the stem. It is just above the reported
admissible micromotion level (28µm) for successful bone ingrowth [5]. The highest
micromotions were found laterally on the posterior side. The neutral position suggests
that bone osseointegration will not be prevented and not cause the formation of
undesirable fibrous tissues around the implant. Note that the average micromotion is
only 2.35µm. Next, using the automatic positioning wizard, an efficient DOE is
conducted, based on a Latin Hypercube Sampling technique. For each of the 36 new
implant positions considered, micromotions are computed at the femur-implant
interface. Fig. 4 displays the considered implant translations and rotations and the
resulting variability in both average and maximum micromotions.