Skeletal Anatomy in Densitometry


Characterizing the Skeleton in Densitometry

The Spine in Densitometry

The Proximal Femur in Densitometry

The Forearm in Densitometry

The Metacarpals, Phalanges, and Calcaneus

Bone Physiology


Densitometry is primarily a quantitative measurement technique rather than a skeletal imaging technique. Nevertheless, there are unique aspects of skeletal anatomy in densitometry that must be appreciated to properly utilize the technology and interpret the quantitative results as well as the skeletal images.

characterizing the skeleton in densitometry

The bones of the skeleton can be described by four characteristics, one of which is unique to densitometry. The characterizations are important, as this often determines which skeletal site is the most desirable to measure in a given clinical situation. A skeletal site may be described as axial or appendicular, weight-bearing or nonweight-bearing, central or peripheral, and predominantly cortical or trabecular.

The Axial and Appendicular Skeleton

The axial skeleton includes the skull, ribs, sternum, and spine (1), as shown in Fig. 2-1. In densitometry, the phrase "axial skeleton" or "axial bone density study" has traditionally referred to the lumbar spine and PA lumbar spine bone density studies. This limited use is no longer appropriate because the lumbar spine can also be studied in the lateral projection and the thoracic spine can be measured as well. The skull and the ribs are quantified only as part of a total body bone density study and as a consequence, the phrase "axial bone density study" has never implied a study of those bones, although they are part of the axial skeleton. The appendicular skeleton includes the extremities and the limb girdles as shown in Fig. 2-1. The scapulae and the pelvis are therefore part of the appendicular skeleton. The proximal femur is also obviously part of the appendicular skeleton, although it is often mistakenly described as being part of the axial skeleton. Contributing to this confusion is the current practice of including dual energy X-ray bone density studies of the proximal femur and pelvis under the same Current Procedural

Fig. 2-1. The axial and appendicular skeleton. The darker shaded bones comprise the axial skeleton. The lighter shaded bones comprise the appendicular skeleton. Image adapted from EclectiCollections™.

Technology (CPT) code2 of 76075 used for DXA spine bone density studies in which the studies are described as studies of the axial skeleton (2).

The Weight-Bearing and Nonweight-Bearing Skeleton

Regions of the skeleton are also characterized as weight bearing or nonweight-bearing. This division is obvious but not without clinical significance. The cervical, thoracic, and lumbar spine and lower extremities are weight-bearing regions of the skeleton. Portions of the pelvis are weight-bearing. The small calcaneus is also part of the weight-bearing skeleton and is perhaps the most sensitive of all the bones to the effects of weight-bearing forces. The remainder of the skeleton is nonweight-bearing.

The Central and Peripheral Skeleton

Skeletal sites may also be characterized as central or peripheral. This classification is unique to densitometry. The spine, in either the PA or lateral projection, is considered a

1 See Appendix VII for a list of CPT codes used in bone densitometry.

Fig. 2-2. (A,B) The central and peripheral skeleton. The darker shaded bones in (A) comprise the central skeleton. The darker shaded bones in (B) comprise the peripheral skeleton. Images adapted from EclectiCollections™.

central site. The proximal femur is also a central site even though it is not part of the axial skeleton like the spine. The calcaneus and the various forearm sites are all peripheral sites although the calcaneus is a weight-bearing site, whereas the forearm sites are not. As an extension of this terminology, bone densitometers that are used to measure bone density in the spine, hip, or both are called "central" machines even though the machines may also have software that allows them to be used to measure a peripheral site like the forearm. The description of a bone densitometer as a central machine is generally reserved for DXA and older DPA devices. Although QCT is used clinically to measure bone density in the spine, as a matter of convention, QCT is rarely described as a central technique, although it would be appropriate to do so. Densitometers that can only be used to measure the distal appendicular skeleton like the forearm or calcaneus are called "peripheral" machines. Because there is no current application of QUS for the central skeleton, it is understood that QUS devices are peripheral devices. As a consequence, it is not necessary to distinguish between central and peripheral QUS devices so the term peripheral is not used in conjunction with QUS devices. Figure 2-2 illustrates the central and peripheral skeleton.

The Trabecular/Cortical Composition of the Skeleton

The skeleton is composed of two types of bone: cortical bone and trabecular bone. Cortical bone is also called compact bone or haversian bone. It is typically found in the shafts of long bones and the vertebral endplates. Trabecular bone is also called cancellous bone or spongious bone and is primarily found in the vertebral bodies, pelvis, and distal ends of long bones. Trabecular bone contains hematopoietic or fatty marrow. Eighty percent of the skeleton is cortical bone. The remaining 20% is trabecular bone. Trabecular bone consists of plates, arches, and struts with marrow occupying the spaces between these structures. Cortical bone is a more solid structure forming the outer casing of the bones (1).

Skeletal metabolism as a whole is roughly equally distributed between the two types of bone even though the skeleton is 80% cortical bone. This is because trabecular bone has a higher metabolic rate per unit of volume than cortical bone (3). In any one bone however, rates of change in bone density may be greater at sites that are predominantly trabecular in composition compared to sites that are predominantly cortical. Rates of change are also greater in axial trabecular bone than in appendicular trabecular bone. If a patient is being followed over time to look for changes in the BMD from a disease process or therapeutic intervention, the greatest magnitude of change will generally be seen at a site that is predominantly trabecular bone. There are certain disease processes, however, that seem to have a predilection for sites that are predominantly cortical in composition. Hyperparathyroidism, for example, may cause demineralization at predominantly cortical sites, like the femoral neck or 33% radial site. Conversely, Cushing's disease may preferentially destroy the trabecular bone of the axial skeleton. In a disease like acromegaly, hypogonadism may cause a profound decrease in the trabecular bone of the spine, while excessive growth hormone causes an increase in the density of the cortical bone of the appendicular skeleton.2

Forearm Composition

The exact percentage of trabecular and cortical bone of many of the sites used in densitometry remains controversial. In a classic study, Schlenker and VonSeggen (4) quantified the average percentage of cortical and trabecular bone along the length of the radius and ulna in four cadaveric female forearms. The forearms were taken from women aged 21, 43, 63, and 85 years. The distribution and percentage of trabecular bone in the radius and ulna were similar. The maximum percentage of trabecular bone was seen in the first two centimeters proximal to the radial and ulnar styloids. The percentage of trabecular bone then dropped precipitously in both bones in a transitional region that lay between 2 and 3 cm proximal to either styloid and remained very small throughout the remainder of the proximal radius and ulna. The percentage of trabecular bone in the four subjects in the most distal 10% of the radius ranged from 50 to 67%, whereas in the region that represented 30 to 40% of the total length measured from the styloid tip, the percentage of trabecular bone ranged from only 0.6% to 6.8%.

Vertebral Composition

The composition of whole vertebra or the isolated vertebral body remains in dispute. The traditional view is that 55 to 75% of the calcium content of the whole vertebra is in trabecular bone. These figures are largely derived from early anatomic studies in which

2 See Chapter 6 for a discussion of the effects of diseases on bone density.

the methods used to arrive as such conclusions were poorly described (5,6). The traditional view was challenged in 1987 by Nottestad et al. (7) who performed anatomic dissections of 24 vertebrae taken from 14 normal individuals, 10 of whom were women with an average age of 72 years and 4 of whom were men with an average age of 63 years. The vertebrae were ashed and the calcium content was assayed using atomic absorption spectrophotometry. Nottestad et al. found that trabecular bone accounted for only 24.4% of the calcium content of whole female vertebrae. Trabecular calcium accounted for 41.8% of the calcium content in the vertebral body. The percentages were less in men, averaging 18.8 and 33.5%, respectively. Eastell et al. (8) refuted this finding based on anatomic dissections of L2 from 13 individuals, 6 men whose average age was 38.5 years and 7 women whose average age was 40.9. In this study, cortical and trabecular contributions to calcium content were determined by microdensitometry and by dissection and ashing. They reported that the whole vertebra was 72% trabecular bone in women and 80% trabecular bone in men. Adjusting these figures to compensate for the expected difference between the two-dimensional measurements that were actually performed and the three-dimensional structure of whole vertebrae, the percentages of trabecular bone in whole vertebrae dropped slightly to 69% in women and 77% in men.

Femoral Composition

The composition of the commonly measured sites in the proximal femur was briefly studied by Baumel (9) using anatomic dissection of the upper end of the femur in six cadavers (age at death 49 to 79 years). In this small study, the percentage of trabecular bone in the femoral neck was 36.45% (±3.85%) and in the trochanter, 39.06% (±3.79%).

All Sites

Despite these controversies, clinically useful characterizations of the composition of densitometry sites can be made. Table 2-1 lists the most commonly assessed skeletal sites and their relative proportions of trabecular and cortical bone (10). Note that the spine, when measured with QCT, is described as 100% trabecular bone. This is because the three-dimensional, volumetric measure that is obtained with QCT allows the center of the vertebral body to be isolated from its cortical shell and the highly cortical posterior elements. The two-dimensional areal measurement employed in DPA and DXA measurements of the spine cannot do this. Although the posterior elements are eliminated from the scan path on a lateral spine study performed with DXA, elements of the cortical shell remain. Therefore, although the measurement of the spine in the lateral projection with DXA is a highly trabecular measurement of bone density, the measurement is not a measure of 100% trabecular bone.

the spine in densitometry

Studies of the lumbar spine performed with DPA or DXA are generally acquired by the passage of photon energy from the posterior to anterior direction. They are properly characterized as PA spine studies. Nevertheless, these studies are often called AP spine studies, probably because plain films of the lumbar spine are acquired in the AP projection. The Lunar Expert, a fan-array scanner, actually does acquire lumbar spine bone density images in the AP direction. Compared to plain radiography, however, the beam direction in a DXA study of the spine has less influence on the appearance of the image and little if any influence on the measured BMC or BMD. Studies of the lumbar spine may

Table 2-1

Percentage of Trabecular Bone at Central and Peripheral Sites

Table 2-1

Percentage of Trabecular Bone at Central and Peripheral Sites

PA Spinea


Lateral Spineb


Femoral Neck






Total Body




33% Radius or Ulnac


10% Radius or Ulnad


8-mm Radius or Ulnae


5-mm Radius or Ulnae


4-5% Radius or Ulna'




a These percentages are for DXA PA spine studies only. A volumetric measurement of 100% trabecular bone could be obtained with QCT.

b These sites are considered to be highly trabecular but the exact percentage of trabecular bone is not known.

c This site is often called the "proximal" site. d This site is often called the "distal" site. e Distance in mm indicates the separation distance between the radius and ulna at the site in question.

f This site is often called the ultradistal site, but may be called simply "distal" as well.

a These percentages are for DXA PA spine studies only. A volumetric measurement of 100% trabecular bone could be obtained with QCT.

b These sites are considered to be highly trabecular but the exact percentage of trabecular bone is not known.

c This site is often called the "proximal" site. d This site is often called the "distal" site. e Distance in mm indicates the separation distance between the radius and ulna at the site in question.

f This site is often called the ultradistal site, but may be called simply "distal" as well.

also be acquired in the lateral projection using DXA. Such studies may be performed with the patient in the supine or left lateral decubitus position, depending on the type of DXA unit employed.

Vertebral Anatomy

The whole vertebra can be divided into two major components: the body and the posterior elements. The posterior elements consist of the pedicles, the lamina, the spinous process, the transverse processes, and the inferior and superior articulating surfaces. The appearance of the image of the spine on an AP or PA spine study is predominantly determined by the relative density of the various elements that make up the entire vertebra. Figure 2-3A is a photograph of a posterior view of the lumbar spine with the intervertebral discs removed. Figures 2-3B and 2-3C demonstrate the appearance of the spine as first the transverse processes and then the vertebral bodies are removed from the photograph. What remains in Fig. 2-3C is characteristic of the appearance of the lumbar spine on a PA DXA lumbar spine study and consists largely of the posterior elements. The posterior elements form the basis of the DXA lumbar spine image seen in Fig. 2-4.

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