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From MR images to generic models
Generic models of the male and female hips have been created from
MR images. These models are animatable, i.e. with the kinematic (hip
joint centre, range of motions) and topological information in order
to perform the motion simulation of the articulated skeletal. The
topological modeling tool possesses interactive functions to achieve
the positioning and the orientation of the 3D reconstructed
surfaces. It allows the definition of anatomical and mechanical frames
together with the corresponding parameters involved in the motion
analysis.
As the femur head is not a perfect sphere (it is in fact close to a
3D conchoid), the resulting joint articulation cannot be modeled as an
ideal ball and socket joint. In order to correctly assess the range of
motion of a patient's hip, the medical doctor cannot proceed with such
an approximation. An automatic computation methodology of the hip
joint center (HJC) location has been designed and implemented in the
topological modeler. According to orthopedic surgeon's experience, the
HJC is defined as the point inside the femoral head that remains fixed
during motion of the joint for a restricted range of motions. We
augment the accuracy of the HJC location which could be occurring from
3D model based method. We proposed a HJC calculation method by fitting
a conchoid shape to the acetabular rim on MR Images for the
improvement of accuracy. Based on the complete anatomical
reconstructed models, the pelvis and femur coordinates systems are
implemented following the ISB recommendation. Hence, the HJC, used for
calculating the hip joint ranges of motions, approximates the center
of a ball and socket joint. For the calculation of the extreme range
of motions, the femur moves with respect to the fixed pelvis until the
first collision occurs between two bones. The measurement of the
ranges of basic motions is expressed using the defined joint
coordinate system. As the hip joint model only takes bones into
account, the movements can exceed the real maximum range of motion of
the patient. These values are then used to initialize the kinematical
joint model and provide the surgeon with basic information. Analyzing
where impingements occur allows surgeons to plan the reshaping of the
femoral head in order to avoid or reduce impingements. The HJC
location process is validated with experimental data from
cadavers.
Cross-validation of the hip joint center HJC evaluation
It has been carried out using optical motion capture with the same
volunteers as for MRI recordings. Several methods for the computation
of the HJC from optical trajectories have been evaluated. We found
that simple linear methods give unsatisfactory results. The least
square computation proved to be more reliable for that purpose. The
location of HJC as obtained using this technique is comparable to the
HJC calculated in our previous work.
Pre-assessment of kinematical information according to femoral head
reshaping
By reshaping of the hip bone through surgery, patient can avoid
abnormal impingements and get greater range of motions. The shape of
the femoral head affects the kinematical information of the hip joint
and especially its range of motions. In this study, we will assess the
relationship between the femoral head shape and the kinematical
information of the hip joint. By changing the shape of the femoral
head by a computer-assisted operation planning system, it is possible
to pre-estimate the gain of hip range of motions. Based on this
result, an optimal surgery planning is possible.
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