Formetric 3D Features
An optical system for spinal and pelvis analysis
One way to meet the objective of developing and producing a radiation-free, fast, and reliable device to complement x-ray measurement systems is to use the combination of 3D-shape measurements and biomechanical modeling to reconstruct and display the spine structure and calculate the key spinal and pelvic parameters, as shown in Figures 1 and 2.
Figure 1.
An example of key spinal and pelvic parameters.
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Figure 2.
Reconstruction and display of the spine structure.
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Capturing and measuring the dorsal profile (back shape) — Rasterstereography
White Light Raster Line Triangulation (WLRT) enables the scanning of objects in 3D by projecting raster lines on its surface and by capturing these lines under a known and fixed angle with a camera. Figure 3. Based on triangulation algorithms, spatial coordinates of all raster points are calculated, resulting in a dense point cloud of randomly distributed points describing the measured surface. These data points are transformed to a regular grid by using interpolation, which will simplify further calculations. In this way the system captures and analyses a body shape, statically or dynamically.
Figure 3.
Projecting raster lines on back shape.
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Rasterstereography excels by its high precision (error ≤ 0.1mm) and allows a radiation-free representation of the profile. For angular data, the reproducibility of an individual rasterstereographic shot is 2.8°. The speed of the measurement is fast at 0.04 seconds and the total dorsal surface is registered simultaneously.
The recognition of the anatomical structure through the automatic identification of anatomical landmarks on the body surface provides the basis for a reconstruction of the 3-dimensional profile of the dorsal surface.
Mathematical construction and display of the spine structure
The aim of capturing, measuring and analyzing the back shape (dorsal surface) is to obtain information about the 3-dimensional shape of the vertebral column.
It is well documented that the vertebral rotation is correlated to the surface rotation and this allows establishing the relationship between the back shape and the shape of the spinal midline. In our case, the surface rotation is measured by the angle of the surface normal (horizontal component). To do this, the high sampling density and resolution provided by rasterstereography is essential.
In the Turner-Smith (Turner-Smith et al.) model, the lateral coordinate of the vertebral bodies can be calculated from the line of the spinous processes by adding an extra displacement produced by vertebral rotation. In this model, the vertebral rotation is assumed to be proportional to the mean surface angle in the central region (K=2.5).
Figure 4.
Construction of the spinal midline from surface data.
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In Figure 4 the vertebra is assumed to be undistorted. This is generally justified in minor to medium curves. The model assumes only that the surface normal points to the vertebral body centre. Therefore the model is virtually independent of vertebral distortion.
The calculated construction of the spinal midline requires three inputs:
1. The line of the spinous processes;
2. The surface rotation at the locus of the line of the spinal process;
3. The anatomical landmarks needed for reference to the underlying skeletal structure.
Curvature analysis is combined with an algorithm for data smoothing and calculation of the surface normals. As a by-product, the original measurement points are transformed into a regular square grid over the frontal (x-y) plane. The result of this procedure is presented in Figure 4.
1. The line of the spinous process
The line of the spinous processes is estimated by the symmetry line of the back. The symmetry line (solid line in Figure 5) is composed of the symmetry points of the horizontal profiles. A symmetry point, in turn, is defined by that point which divides the profile into two halves with minimum lateral symmetry (with respect to surface curvature). For the model, we assume the symmetry line to be a representation of the line of the spinous processes and it is a generalization of the medial sagittal profile.
Figure 5.
Reconstructed back with symmetry line, surface rotation, and anatomical landmarks.
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2. Surface rotation
As mentioned above, we measure surface rotation by the horizontal component of the direction of the surface normal. From any grid point in Figure 5, components of the surface normal are known from curvature analysis. On the symmetry line these values are calculated by interpolation. In Figure 5, the surface normals are represented by bars erected on the symmetry line. As the results show, it is reasonable to assume that the horizontal component of the normal angle is equal to vertebral rotation.
3. Anatomical landmarks
An automatic recognition of four anatomical landmarks (vertebra prominens (VP), sacrum point (SP), right crista iliaca posterior superior (DR), and left crista iliaca posterior superior (DL) by means of the connected software provides the basis for a reconstruction of the three-dimensional profile of the dorsal surface.
The landmarks are used for skeletal reference of the surface data. In particular, the vertebra prominens landmark is used as the origin of a body-fixed coordinate system both for radiography and for surface measurements. Furthermore, trunk length, trunk imbalance, pelvis inclination, and similar parameters may be determined from these landmarks. In Figure 5 the landmarks are represented by black dots.
Figure 6.
A typical report printed with the selected view and parameters; other views and parameters can be selected.
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This well proven system is an ideal tool for chiropractors as it allows multiple scans of your patients to produce key information on the spine and pelvis complex of your patients. With the Formetric you can first produce an evaluation and decide on your treatment, then you can monitor and document the progress of your patient as shown in Figure 7. You can inform your patient with a document the patient can keep.