The entire bone mass of the human is composed of 80% cortex and 20% spongiosa. The spongiosa has by far the larger surface and consequently reacts faster to metabolic changes. Physiologic remodeling takes place in small functional units in which osteoclasts and osteoblasts interact.

Primary Osteoporosis (Idiopathic):

1. Postmenopausal osteoporosis (Type I osteoporosis):
   
   
   
   • Between age 50 and 65 years. Estrogen deficiency induces high bone turnover and leads to accelerated trabecular resorption and perforation, especially within the first five postmenopausal years.
     
   • Fractures predominantly of the spongiosa and primarily affecting the vertebral bodies.
     
   • Peripheral fractures (e.g., femoral neck) are less frequent.
     
   • Gradual transition to senile osteoporosis is possible.
   
   
2. Senile osteoporosis (Type II osteoporosis):
   
   
   
   • Decreased bone turnover, inadequate dietary calcium, and age-related decreased synthesis of calcitriol can cause a high bone turnover by means of hyperparathyroidism, even in old age.
     
   • General loss of bone mass in the spongiosa and cortex of bone.
     
   • Fractures tend to involve the peripheral skeleton: femoral neck, distal radius, proximal humerus, tibia, and pelvis.

Secondary Osteoporosis:

Fig. 6.2 Senile osteoporosis with thin rod-like trabecule, reduced number of trabecule, and decreased trabecular connections. 3-D CT of block specimen (4 × 4 mm³) with ultra-high resolution.

Fig. 6.3 Postmenopausal osteoporosis with marked longitudinal striation caused by accentuated vertical trabecule. Sharply outlined vertebral bodies (‘picture-framing’) are also seen.

Fig. 6.4 Wedge-shaped thoracic vertebral body in osteoporosis.

Fig. 6.5 Lumbar ‘codfish’ vertebral bodies with biconcave indentation. Extreme accentuation of the vertebral outline is seen.

Osteoporosis is characterized by decreased bone density, rarefied trabecular structure, cortical thinning and fractures.

All types of osteoporosis involve the spine more frequently than other skeletal sites.

Decrease in bone mass corresponds to a decrease in radiographically detected calcium with an increase in the radiolucency of bone.

Radiography does not show decreased bone density until approximately 30% of the bone mass is lost. Consequently, radiography is not appropriate for the early detection of demineralization.

Caution:

• The radiographic appearance of osteopenia depends on technical factors (kV/mAS, hardening of the x-ray spectrum, etc.), and standardization is necessary to avoid misdiagnoses.
  
• Interpretation of osteopenia depends on the subjective assessment of the examiner.
  
• Digital radiography is not suitable for assessing osteopenia since the adjustment of the optical densities by histogram equalization compensates for any increased radiolucency.

Osteoporosis can produce the following findings in the cortex:

• increased endosteal resorption (especially with the increased bone turnover seen in postmenopausal osteoporosis) resulting in cortical thinning due to endosteal scalloping,
  
• increased intracortical resorption with tunneling (longitudinal striation) and trabecular transformation. Such changes are also seen in the context of primary hyperparathyroidism, renal osteodystrophy, and reflex sympathetic dystrophy (Sudeck atrophy).

An osteoporotic appearance that is practically indistinguishable from genuine osteoporosis can be observed in:

Fig. 6.7 Severe patchy osteoporosis of the right hand due to longstanding immobilization in hemiplegia.

Fig. 6.8 Moth-eaten osteoporosis in the femoral stump following amputation for osteosarcoma.

Fig. 6.9 Senile osteoporosis. Cortical thinning with corresponding enlargement of the marrow space.

Many methods have been proposed for measuring the extent of demineralization with radiography. Indices that consider both severity of vertebral deformity and the number of affected vertebral bodies can be helpful to monitor therapy. However, they are all unsuitable for the early diagnosis of osteoporosis.

Bone Densitometry: It is the goal of bone densitometry to determine the fracture risk before fractures have occurred. It is used in subjects at risk of osteoporosis or suspected of having increased bone loss.

Bone densitometry basically measures bone mineral density and content. The two requirements for this measurement are a representative site and a reliable, reproducible method. Preferred sites are the lumbar spine, proximal femur, distal radius, and calcaneus. For the assessment of fracture risk, the lumbar spine should be measured, since measurements of the peripheral skeleton do not always predict the fracture risk in the axial skeleton.

Planar methods (area density in g/cm²):

• Single x-ray absorptiometry (SXA),
  
• Dual x-ray absorptiometry (DEXA).

Volumetric methods (quantitative computerized tomography [QCT], true bone density in mg/ml):

• Single energy QCT (SE-QCT),
  
• Dual energy QCT (DE-QCT),
  
• Peripheral QCT (pQCT)

Quantitative sonography (QUS)

Quantitative MRI (QMR)

The major criteria for determining the quality of any measurement technique are absolute precision and reproducibility. Which measurement error is more critical depends on the goal of the examination. If an individual measurement is obtained to determine the mineral content of the patient relative to subjects of the same age and sex (early detection, risk analysis), the reliability rests primarily on absolute precision. If the rate of loss is the most important measurement in a specific patient (e.g., for monitoring therapy), reproducibility is the most crucial criterion. To determine a loss rate of 2.5%, the reproducibility error of the method should not exceed 1%.

Absorptiometry and computed tomography are the most common methods in widespread use and play a practical role. Both of these are based on the attenuation of the radiation beam passing through the skeleton.

Single x-ray Absorptiometry (SXA)

This measures the attenuation of an x-ray beam at a peripheral site, most commonly the radius. It is relatively simple and inexpensive but has been abandoned because of suboptimal precision and reproducibility.

Dual x-ray Absorptiometry (DEXA)

The advantage of quantitative CT is the selective measurement of a small volume of spongiosa of bone and the simultaneous display of its structure (microcallus, micro calcifications). Extraneous calcifications, such as aortic calcifications, do not present a problem in measuring the bone density. The disadvantage is that the radiation dose delivered is higher than that of DEXA.

Practical remark: The report of a QCT should read as follows:

The bone mineral density was calculated from representative axial sections of three lumbar vertebral bodies… The absolute bone mineral density averaged over the three vertebral bodies is… mg/ml. This value is… standard deviations below the average in a population of the patient’s age and sex (Z score) and… standard deviations below the peak bone mass of a young adult (T score).

Conclusion: No osteoporosis/osteopenia/osteoporosis/manifest osteoporosis.’

(Note: One standard deviation corresponds to 30 mg/ml; consequently, a calcium content of less than 85 mg of CaHA corresponds to osteoporosis.)

Peripheral Quantitative CT

A small, specially designed unit measures the bone density of the distal radius using the principle of quantitative CT. This method enables clinical follow-up and requires an experienced technologist. Moreover, this method can provide information about the biomechanical property of the bone but alone is unsuitable for establishing the initial diagnosis of osteoporosis.

Quantitative Sonography and Quantitative MRI

Both of these methods are currently being investigated and are not yet used in clinical practice.

Fig. 6.10 Disuse osteoporosis of the knee after trauma. a Femoral condyles, b tibial plateau. The fat-equivalent density in the region of the trabecular resorption is indicative of osteoporosis.

Fig. 6.11 Schematic drawing of a DEXA unit.

Calcium metabolism