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Morphological study of the proximal femur: a new method of geometrical assessment using 3-dimensional reverse engineering

This study presents a new method of using computerized tomography images combined with reverse engineering technique to obtain and analyze the three-dimensional inner and outer geometry of the proximal cadaveric femur. Three-dimensional models were reconstructed from the computerized tomography images and approximated with 2D and 3D fitting algorithms based on reverse engineering methods.

Fig. 1: Two different thresholding values were applied for the optimized segmentation of the outer and inner cortical boundaries.

Methods:
After scanning, the CT dataset was imported into Mimics which is used in this study to interface between CT data and commonly used CAD systems. A 3D graphic model of each femur was developed by a thresholding and region growing technique to extract the boundaries of the proximal femur from the CT dataset. In order to optimize the geometry of both outer and inner cortical surfaces, two thresholding values were applied: a lower threshold was applied to extract the outer cortical surface including femoral head, femoral neck, trochanteric region and femoral shaft; a higher threshold was applied to extract the inner cortical surface of the intramedullary canal (Fig. 1).

Fig. 2: The IGES inner and outer contours on the 3D graphic model.

The resulting optimized inner and outer contours were then exported into IGES format (Fig. 2).

Fig. 3(a): Geometric approximation of the proximal femur with different entities.

The IGES contours of each proximal femur were then imported into a reverse engineering CAD software and were displayed as point clouds instead of polylines. The shape of specific portions of the proximal femur was then approximated with a geometric configuration such as circle, ellipse and sphere that have the best fit to the real geometry.

Fig. 3(b): Geometric approximation of the proximal femur with different entities.

The following parameters were calculated for each femur: femoral head diameter, femoral neck axis, femoral shaft axis, anteversion angle and neck-shaft angle. These data represent the geometry of the studied proximal femur and can be used for the design of proper size and shape of femoral and trochanteric nail systems.

References

Banchong Mahaisavariya ^d, Kriskrai Sitthiseripratip ^a, Trongtum Tongdee^c, Erik L.J. Bohez ^a, Jos Vander Sloten ^b, Philip Oris ^b

^a Industrial Systems Engineering, Asian Institute of Technology, Pathumtani, Thailand
^b Biomechanics and Engineering Design, Katholieke Universiteit Leuven, Leuven, Belgium
^c Department of Radiology, Faculty of Medicine, Siriraj Hospital, Mahidol University, Bangkok, Thailand
^d Department of Orthopaedic Surgery, Faculty of Medicine, Siriraj Hospital, Mahidol University, Bangkok, Thailand

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