Assessment of femoral bone fragility and fracture risk by ultrasonic measurements at the calcaneus

Age and Ageing, March, 1998 by Steven Boonen, Patrick Nicholson

Keywords: bone fragility, calcaneus, hip fracture risk, osteoporosis, quantitative ultrasound

Introduction

Fracture of the proximal femur in elderly people is associated with appreciable morbidity and mortality [1]. The life-time risk of a hip fracture is 15.6-17.5% in women and 5.2-6.0% in men, depending on the method of analysis [2, 3]. Ultimately, 19% of hip fracture victims will require long-term institutional care because of the fracture [4]. Mortality at 1 year in patients sustaining a proximal femoral fracture is 22-33% [5].

To allow targeting of preventive care, there is considerable interest in predicting the risk of hip fracture. Bone densitometry is an established method to detect incipient bone fragility and to determine the risk of developing fractures. However, numerous in vitro studies have indicated that bone strength is only partially related to bone density. Densitometry performance has improved to the point where the inability of bone mass density readings to account completely for mechanical failure can no longer be attributed to measurement variance. It is clear, therefore, that factors other than reduced bone density contribute to fragility fractures.

The importance of fragility arising from non-mass factors is currently being evaluated by the use of quantitative ultrasound (QUS). Ultrasound utilizes portable equipment and does not involve ionizing radiation. These advantages could make ultrasound an adequate screening tool if it proves to be an accurate indicator of fracture risk. Here, we review evidence of certain limitations in existing radiographic bone densitometry techniques and describe the fundamental background to ultrasonic methods of bone assessment. We then deal with the use of calcaneal ultrasound to predict density, strength and fracture risk at the proximal femur.

Limitations of hip fracture prediction by bone mineral density (BMD) measurements

Bone densitometry is currently used to assess skeletal status and predict fracture risk [6]. The most commonly used method for measuring bone mass is dual-energy X-ray absorptiometry (DXA), which is a precise method for the determination of BMD at the proximal femur. Many studies have shown that bone mass declines with age in men as well as in women, with the density of the proximal femur decreasing at a rate of 0.6-1% per year after age 50 [7-9]. This decrease in bone mass is the most important cause of diminishing bone strength with ageing [10] and, therefore, one of the major determinants of fracture risk. Cross-sectional studies have indicated significant differences in femoral BMD between (cervical or trochanteric) hip fracture patients and age- and sex-matched controls [11-13]. Moreover, several longitudinal studies have confirmed the relationship between axial as well as appendicular bone mass and osteoporotic fracture risk, including the risk of femoral fractures [14-16]. Although measurement of bone mass at specific sites can predict the risk of fractures at any site [14], the best prediction is made for the site where the measurement is performed [Z 15]. Low femoral bone density is a stronger predictor for hip fracture than measurements at the spine or radius. In women aged 65 years or more, each standard deviation (SD) decrease in proximal femoral BMD is associated with a 2.7-fold increase in hip fracture risk (95% confidence interval 2.0-3.6) after adjustment for age, implying that a woman whose hip density is 1 SD below the mean for her age is [2.7.sup.2] times more likely to have a hip fracture than a woman whose bone density is 1 SD above the mean [15]. All measurements of femoral bone density are related to the risk of both cervical and trochanteric fracture. Even in women aged 80 years and over, BMD continues to be strongly associated with both overall fracture risk and the risk of femoral fractures [17].

However, the increase in hip fractures with age is not fully accounted for by the decline in bone density of the hip, suggesting that age has an effect on fracture incidence that is independent of bone mass [18]. Even after adjustment for bone density, each decade increase in age is still associated with other age-related factors are of pathogenic significance. Furthermore, virtually every study that has contrasted BMD in patients with and without fragility fractures has found overlap between the two populations [19]. This overlap of BMD becomes a particularly compelling problem in older individuals. No less than' 54% of women aged 50 years and over have osteopaenia--defined as a BMD more than 1 SD below the young normal mean [20]--at one or more sites [21]. Finally, even elderly women with very low BMD of the proximal femur ([is less than] 0.6 g/[cm.sup.2]) have only about a 2%, annual risk of hip fracture [ 18].

Other factors must therefore increase the susceptibility to hip fractures. These factors contribute to the risk of hip fracture in other ways, presumably by affecting the incidence or impact of falls or by influencing fragility-related characteristics of bone other than bone density. In fact, bone strength depends not only upon bone mass but also upon a variety of qualitative aspects of bone, some of which are not directly associated with BMD [22]. Aspects of bone quality, such as accumulation of microdamage [23], subclinical osteomalacia [24], bone microstructure [25] and geometric characteristics ]26], have been implicated in the pathogenesis of fractures of the proximal femur, although the extent to which most of these phenomena act independently of bone mass remains uncertain.