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Peter Widhalm: Robuste Beinachsenbestimmung mittels Acitve Shape Models

DA-Abschlussvortrag: Automatic Assessment of the Knee Alignment Angles on Full-limb Radiographs

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  • Presentation
When Jan 20, 2009
from 04:00 pm to 04:00 pm
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The knee alignment angle, defined by the mechanical axes of the femur and
the tibia, is of high importance in orthopedics and traumatology, in
particular for pre-operative planning and post-operative follow-up
assessment. It serves as a predictor for pre-arthritis and the
post-operative angulation is known to be associated with the clinical
outcome.

So far, only manual and semi-automated methods of measuring the alignment
angle exist. The most serious drawback of these techniques is that they lack
reproducibility. The points in the anatomical structure, which define the
angle to be measured, are only vaguely defined and cannot clearly and
precisely be identified. The resulting variability between repeated
measurements precludes the detection of small changes. A fully automatic
measurement method of axis alignment that provides a consistent definition
of anatomical landmarks would eliminate inter and intra reader variabilities
caused by human interpretation and lead to more accurate and reproducible
results.

In the course of this thesis, a novel method for the automatic measurement
of alignment angles is developed and prospectively tested. It allows a fully
automatic assessment of knee alignment angles in full-limb radiographs with
high precision.

Based on the coarse position estimates of the bones, their contours are
delineated
by Active Shape Models, controlled by ongoing estimates of the
reliability of the model. The regions around the joints are refined using
submodels. Landmarks are identified by their index and can be matched
between different instances of a shape. Hence, the defining points of the
axes can be located in a straightforward and repeatable way, when they are
directly represented by landmarks on the contour and annotated manually in
the training phase on any instance of the bone.

Overlapping structures and the compound nature of the large radiography
acquisition which results in partially missing data and changing intensities
let standard ASMs fail. For this reason, a search procedure is intruduced,
which is controlled by ongoing estimates of the fit confidence during the
search, leading to an improved result robustness, even if the spatial
initialization is poor and the structures of interest are partially cropped
or occluded.

Experimental results show that the automatic assessment of the knee
alignment angle allows for an accurate and observer independent
quantification with high precision and improves the detection of small
changes.
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