1 Introduction to spine osteoporosis severity

There are two components of osteoporosis severity in relationship to the spine: the degree of vertebral compression and the underlying bone fragility. Unlike traumatic fractures, the vertebral compression fractures of osteoporosis are asymptomatic in 60% of the cases. Furthermore, there is variation of vertebral shapes in the normal population (see Fig 5.5–1). Several investigators have measured vertebral dimensions in large populations, including the anterior height, the posterior height, the midpoint height, and the area of the vertebra.

2 Identifying osteoporosis vertebral fractures

Approaches to identification of a vertebral fracture, have been described in detail by Szulc [1], Many investigators use the cut-off of 3 standard deviations to define a vertebral fracture (as shown in the above graph). The prevalence rates vary depending on the reference range and the methodology, and there is still debate about the best way to define these fractures. It is also difficult to apply these methods when there are anatomic irregularities such as scoliosis, osteoarthritis, or Scheuermann’s disease.

Accurate measurements require precise and specific radiological techniques. These measurements are tedious and expensive, and Genant has proposed a semi-quantitative method which involves subjective screening of the vertebra based on a graded set of figures. The vertebra which appear to have a fracture can then be measured. Each vertebra (from T4 to L4) could have a fracture from mild (grade 1) to severe (grade 3). A sum of the grades gives an overall index. The International Osteoporosis Foundation has adopted this approach and have developed an educational training program available on their website [2].

A study of the inter-observer variability of identifying fractures was done using standardized x-rays of 2451 patients with osteoporosis (T-score <-2.5) who were enrolled in an international study. As shown in the graph, in 20% of the women, local radiologists disagreed with radiologists in a central reading facility about whether there was a fracture. An expert radiologist (HKG) felt that 66% of the disputed xrays actually had fractures. As stated by the authors, “there will always be some discrepancies between individuals with no fractures and those with mild fractures” (Fig 5.5–2).

Vertebral fractures are related to clinically important outcomes, but the symptoms vary, and the degree of compression is not necessarily related to the amount of pain. It is probable that some of the fractures occurred gradually and therefore did not cause acute pain. Michael Nevitt, et al [3] took spine x-rays in 7223 older women and repeated the x-rays 3.7 years later. During that time 371 women had a new vertebral fracture. Women were asked carefully about pain and disability symptoms. Increased back pain was experienced in 22% of women who did not have a new fracture and 38% of women who did have a new fracture. The fractures, however, are harbingers for future fractures and disability. Accumulated fractures result in height loss, kyphosis, disfigurement, decreased pulmonary function, and esophageal reflux. Even asymptomatic fractures have been found to be associated with decreased quality of life.

Vertebral fractures are ignored in about half of the radiology reports from chest x-rays. Even moderate to severe fractures are frequently not mentioned on the reports, as shown in three recent studies [4–6]. This is especially unfortunate because the presence of a vertebral fracture (even a mild or asymptomatic one) is an important risk factor for future fractures, and treatment could prevent many of these fractures.

The EPOS study [7] found that overall the risk of a new vertebral fracture was 6.1 times more likely if there was a prevalent fracture. This varied with the shape of the deformities: there was no increased risk with loss of posterior height, but a 6-fold increased risk with loss of anterior plus middle height. The risk also increased with number of deformities:

These findings are similar to those in clinical trials, in which new vertebral fractures are found by xrays but only a minority of patients were aware of the occurrence of the fracture [8–14].

In a clinical trial of women with osteoporosis (Crans GC Bone 2005) [15], the spine deformity index at baseline (which measures the sum of the fractures graded according to severity) was related to the incidence of a vertebral fracture during the 3 years of the study. Those with the most severe initial fractures had the highest risk of getting a new fracture. In this study, even mild fractures increased the risk of subsequent fractures. These fractures all showed a height reduction of at least 20%. Therefore, even a mild vertebral compression fracture is an indication of underlying bone fragility. Indeed, osteoporosis has been defined as a disease of decreased bone density and increased bone fragility leading to an increased risk of fractures. The fact that a prevalent vertebral fracture is a much stronger predictor for future fractures than bone density is not generally appreciated.

3 Defining osteoporosis severity

Bone density

The disease severity scale that is used most often to describe osteoporosis is based on the classification proposed by the World Health Organization. This is based on the bone density.

The WHO definitions are based on bone density of young populations at peak bone density (25 years old):

Normal	above—1SD below the mean
Osteopenia	between—and -2.5 SD below the mean
Osteoporosis	lower than -2.5 SD below the mean
Severe osteoporosis	osteoporosis plus the presence of a fragility fracture

This scheme presents several problems. It applies only to white women and is heavily dependent on the young population used to define a standard deviation. The original definition did not specify the method used to measure bone density, which is usually measured by dual energy xray absorptiometry (DEXA). Other methods, such as quantitative computed tomography or ultrasound do not give the same fracture prediction per standard deviation from the young mean score. Even with DEXA the relative risks for a future fracture are different if the spine is measured than if the proximal femur is measured. The risk of a fracture depends on other factors, especially age, so that an 80 year old woman with osteopenia will have a risk of fracture about 5 times higher than a woman age 50 with the same bone density. Furthermore, there is no WHO category for women with osteopenia who have a fragility fracture—even though the majority of osteoporotic fractures occur in these women. The risk of a fracture is greater when bone density is in the osteoporosis range, but there are many more women in the osteopenia range. As discussed above, these women have a high risk of future fracture and should be in a different category from other women with osteopenia.

These problems with the WHO categories have led to a new approach which estimates the risk of a future fracture using a combination of bone density and clinical risk factors (including occurrence of a fracture after the age of 50). The final models are still being developed but important risk factors include age, race, gender, history of fracture, weight, smoking, muscle strength and parenteral history of hip fracture. These risk factors include some that relate to the bone fragility and others that relate to the risk of falling. Treatment based on the risk of fracture will be more clinically meaningful than based on bone density alone.

3 References