Biology Reference
In-Depth Information
O'Neill and Ruff (2004) compared two different methods for analyzing the biplanar
radiographs:thee ipsemodelmethod(EMM)andthelatexcastmethod(LCM).
EMM, which is simply the standard biplanar method described above, uses the cortical
width measured from each of the biplanar radiographs, one radiograph in the AP direc-
tion and the other taken perpendicular in the ML direction. LCM also uses biplanar
radiographs, but in conjunction with a latex cast taken of the external surface of the
bone. With EMM, an assumption is made that the cross-section is elliptical, the general
assumption of biplanar radiography. EMM appears to have systematic bias by overesti-
mating the cross-sectional properties when compared to actual cut cross-sections (abso-
lute errors averaging 5
e
12%) (
O'Neill and Ruff, 2004
). LCM has a much better
correspondence with the actual shape properties from the cut bone section (absolute esti-
mation errors between 3 and 8%) and is thus preferred. The authors applied regression
equations to correct for the systematic bias, and were able to improve the accuracy of
the EMM on an independent sample (absolute errors reduced to less than 4%) (
O'Neill
and Ruff, 2004
).
Bloom and colleagues conducted radiographic scans using DEXA (which essentially
provides a low resolution X-ray) of living individuals and is discussed later in this chapter.
They applied a method that combines the cortical thickness measurement from both the
medial and lateral thicknesses, which they called “combined cortical thickness” (CCT)
(
Bloom and Laws, 1970
). Ben-Itzhak and colleagues followed the same model to estimate
the CCT by using biplanar radiographs to estimate cross-sectional area of the humerus in
Neandertals and humans (
Ben-Itzhak et al., 1988
).
Densitometry
There are four ways to clinically determine bone density: (1) dual energy X-ray absorpti-
ometry (DEXA), (2) quantitative computed tomography (QCT), (3) peripheral quantitative
computed tomography (pQCT), and (4) ultrasound. The DEXA scanner is designed
to have the patient lie on a table while the arm of the machine with a small pencil beam
X-ray passes over the subject, much like the laser image scanner in your home office. The
advantages of DEXA are that it looks at integral bone mass and areal density and that it is
relatively low radiation, so it is not detrimental to a living patient and it is not limited to
the periphery of the bone, as is the case with pQCT, which will be discussed further in
this chapter. DEXA exposes the patient to no more radiation than an international flight.
DEXA is relatively sensitive to subtle changes in bone density and body composition (the
proportion of fat and lean tissue mass).
The disadvantages of DEXA are that it does not determine volumetric density, and can
provide only a summary measure of density across a scan path. The DEXA image is a flat-
tened two-dimensional image, like a traditional radiograph, and thus must make assump-
tions about a three-dimensional object to calculate the bone mineral density (BMD),
similar to the analogy of the doughnut from above. Like EMM biplanar radiography, it inac-
curately assumes a cylindrical cross-section. It is also unable to distinguish trabeculae from
cortex. A volumetric density, as interpreted by the CT scanner, will provide a more accurate
density than a two-dimensional interpretation.
