Bone mineral density measurements of the axial and appendicular skeleton are an important part of the diagnosis and follow-up of metabolic bone disease. This is particularly true for osteoporosis, a disorder characterized by compromised bone strength. The reduction in density and quality of bone leads to an increased risk of fracture.
The incidence of osteoporotic fractures increases with age. Osteoporosis could, consequently, become a significant social/healthcare problem in developed countries where the population is aging.
An early diagnosis of osteoporosis is fundamental to preventing complications and unnecessary suffering. This is now possible using precise imaging methods. The two main aims of imaging are to identify the presence of osteoporosis, and to calculate bone mass using semiquantitative or quantitative methods. Conventional radiography is typically used for the former, densitometric techniques for the latter.
The radiological appearance of osteoporosis is essentially always the same, irrespective of the cause. Osteoporosis is still most commonly diagnosed on conventional radiography, a modality that is widely available. This is despite the advent of newer, highly accurate and precise quantitative diagnostic imaging techniques, such as dual x-ray absorptiometry (DXA) and quantitative CT (QCT).
Conventional radiography is useful, both alone and in conjunction with CT or MRI, when detecting complications of osteopenia (e.g., fractures), for the differential diagnosis of osteopenia, or for follow-up examinations in specific clinical settings, such as progression of soft tissue calcifications, or signs of secondary hyperparathyroidism and osteomalacia in renal osteodystrophy. It is relatively insensitive to the detection of early disease, though. A substantial amount of bone loss (~30%) must occur before it can be detected on x-ray images. Variations in radiographic exposure factors, film development, and patients' soft tissue thickness can also make it difficult to diagnose early signs of osteoporosis.
Compression fractures can occur anywhere in the spine, although T4-L4 is the most common site. A vertebral fracture appears as an alteration in the shape and size of the vertebral body. A wedge is seen when the anterior vertebral body height is reduced in relation to posterior height. Reduced midstructure height in relation to posterior height is visualized as an endplate (mono- or biconcave), and when all heights of a vertebra are reduced in relation to the dimensions of adjacent vertebrae, this is seen as a crush, or collapse, vertebral deformity.
The posterior height of the thoracic vertebrae is normally 1 to 3 mm greater than the anterior height. A height loss of more than 4 mm is considered a true vertebral fracture deformity. This criterion is also used for lumbar vertebrae, although differences between anterior and posterior heights are not as large in lumbar vertebrae as they are in the thoracic spine. The shapes of vertebrae and their dimensions, as well as their posterior border, should be compared with those of adjacent vertebrae when defining a fracture.2
