Quantitative morphometry and its comparison with the semiquantitative methods

Quantitative morphometric assessment of vertebral deformity was introduced in order to obtain an objective and reproducible measurement, using rigorously defined point placement and well-defined algorithms for fracture defin-

Fig.4 Example of six-point placement in quantitative vertebral morphometry

ition [3, 42]. Typically six points are used to derive the anterior height (ha), the central (middle or middle-vertebral, hm) height, and the posterior height (hp; Fig. 4). This exclusively quantitative approach has, however, a number of drawbacks including projectional effects that significantly influence the reliability of these measures performed in isolation.

In general, a substantial number of mild deformities detected by visual reading are missed by the quantitative technique when applying the common threshold values for reduction in vertebral heights such as 15-20% or 3 SD decrease. Furthermore, a significant number of false positives are found with quantitative techniques. The choice of point placement in the quantitative technique, but especially the choice of the threshold for defining vertebral deformity, gives results that vary in specificity and sensitivity. Most of the moderate to severe deformities are detected by both techniques. However, only expert visual evaluation can detect mild and subtle deformities, as well as appreciate anatomical, pathological and technical issues that bear on the evaluation of fracture detection.

The strength of a semiquantitative approach is that it makes use of the entire spectrum of visible features that are helpful in identifying deformities [15, 49]. The visual interpretation, when performed by the expert eye, also separates true deformities from normal or anomalous vertebrae. In addition to changes in dimension, vertebral deformities are generally detected visually by the presence of endplate deformities, the lack of parallelism of the endplates, and the general altered appearance compared with neighboring vertebrae. Some of these visual characteristics are not captured by the six-digitization points used in quantitative techniques; this can cause some deformities to remain undetected. For example, only an experienced observer can make the subtle distinctions between a fractured endplate and wedge shaped appearance caused by the remodeling of the vertebral bodies in degenerative disc disease (Fig. 3). This is often interpreted as a wedge fracture in quantitative studies.

In the absence of distinct characteristics of a fracture, however, a reader using a visual approach could rather arbitrarily consider a mild wedge deformity normal, anomalous, or fractured; in such a case, a well-defined quantitative criterion could be useful. Even here, however, with borderline wedge deformity, small subjective difference in joint placement could result in considerable variation in fracture/nonfracture discrimination of sequential films or even on the same film.

Most incident fractures, as with prevalent fractures, are easily identifiable visually on sequential radiographs. The unavoidable variation in position and parallax may result in differences in point placement on follow-up radiographs. This can result in the morphometric detection of an incident fracture that would be interpreted visually as simply an alteration in projection. These sources of false-positive or false-negative interpretation are especially common when parallax problems due to radiographic technique or patient positioning are encountered.

Intraobserver variability for a semi-quantitative approach depends on experience and training. The same however, is true for digitizing techniques: an experienced observer is more consistent in the placement of the points for digitization.

A number of comparative studies have evaluated the relative performance of the quantitative morphometric and the semiquantitative methods and moderate correlations were found in most of them [1, 17, 29, 52]. The concordance was high for fractures defined as moderate or severe by semiquantitative reading. There was, however, a significant discordance for fractures defined as mild in the semiquantitative reading. Additionally, the interobserver agreement was demonstrated to be better for the visual semiquantitative approach. The authors of these studies concluded that quantitative morphometry should not be performed in isolation, particularly when applying highly sensitive morphometric criteria at low threshold levels without visual assessment to confirm the detected prevalent or incident vertebral deformities as probable fractures.

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