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CHAPTER
24
Changes in Bone Density during
Development
Frank Rauch
Shriners Hospital for Children, Montreal, QC, Canada
INTRODUCTION
used bone densitometric parameter is areal bone min-
eral density (BMD), which represents the ratio between
bone mineral content and the area of the bone projection
area in the plane of measurement. Results are given in
the unit of g/cm
2
. An important methodological issue
in the use of this parameter is that it depends on bone
size (
Figure 24.1
). A larger bone will have a larger areal
BMD, because the radiation beam emitted by the DXA
device travels a longer distance through it, and there-
fore more of it is absorbed than when it travels through
a smaller bone. As bone size scales to some extent with
body height, this means that, all other things being equal,
children with short stature will have lower areal BMD
results than children whose stature is within the per-
centile curves. This is an important consideration when
interpreting results in subjects with OI, as short stature is
very common in this population.
Areal BMD results are often expressed as z-scores
which provide a measure of how much a patient's
result deviates from the average value in healthy chil-
dren of the same age and sex. It is calculated as the dif-
ference between the patient's result and the mean value
of healthy children, divided by the standard deviation
within a group of healthy children. A result of 0 means
that the value is exactly on the average level for healthy
children. As areal BMD is a size-dependent measure, so
is the z-score. Thus, an areal BMD z-score of −2.5 may be
normal for a short child, whereas an areal BMD z-score
of −1.5 can be indicative of a bone problem in a tall child.
In contrast to areal BMD, “volumetric BMD” is size-
independent. Volumetric BMD represents the ratio
between bone mineral content and bone volume. As DXA
measurements are based on two-dimensional projection
images, this technique cannot determine volumetric BMD
Over the past two decades, the quantification of bone
mass and density with bone densitometric equipment
has considerably gained importance in pediatrics.
1-4
These methods originally were developed for the diagno-
sis and follow-up of postmenopausal osteoporosis. With
more widespread availability of densitometric devices, an
increasing number of children and adults with osteogen-
esis imperfect (OI) are undergoing such examinations.
5
There is currently no proof that the information yielded
by densitometric devices leads to improved clinical man-
agement and outcome in pediatrics. Nevertheless, most
clinicians who follow children and adolescents with OI
would probably agree that bone densitometry should
be included in a work-up of children and adolescents
who have sustained fractures without adequate trauma
or who have radiological evidence for inadequate bone
mass.
6
This will apply to most children who are evaluated
for OI. Densitometry is also used to document the effect
of drugs that have an effect on bone.
METHO
DOLOGICAL CONSIDERA
TIONS
Bone densitometry in growing subjects poses specific
problems.
7-9
This section highlights some of the issues
that should be taken into account when bone densito-
metry is performed in subjects with OI, focusing on dual-
energy X-ray absorptiometry (DXA), which is the most
widely used technique for the determination of bone
mass and density.
The absolute mass of mineral within a bone or part of
a bone is called bone mineral content. The most widely
