Hip axis length and osteoporotic fractures

Recent studies have shown that hip axis length is related to hip fracture risk in elderly women. (’,’) The hip axis length, defined as the distance along the femoral neck axis from the base of the trochanter to the pelvic brim, is associated with hip fracture risk independent of age, height, weight, and femoral bone density. Each 0.5 cm increase in hip axis length increases the risk for hip fracture by a factor of 2. However, the mechanism of this interaction is unclear. The concept of the hip axis length representing the femoral moment arm during a fall is intuitively appealing. From basic biomechanics, we know that the bending strength of a beam is related, among other things, to the perpendicular distance between an applied force and the base of the beam (the moment arm). The longer the moment arm, the less force required to bend or fracture the beam. However, the true femoral moment arm may or may not bear any relation to the hip axis length as it is defined. Indeed, there is evidence specifically indicating that the hip axis length is related to femoral strength via other mechanisms. First, the true femoral moment arm during a fall will depend greatly on the circumstances of the fall itself. It will depend on the impact site and angle of impact, both of which can be expected to vary greatly. In addition, the true moment arm should be measured to the center of the femoral head and not include the acetabulum. Although impact at or near the hip is a highly significant risk factor for hip our data show that the types of falls varied greatly among the women in the Study of Osteoporotic Fractures. (s) Thus the association between the hip axis length and the true moment arm should be weak at best. Second, some evidence exists that the femoral portion of the hip axis length is not the important measurement. Detailed measurements of hip axis length from pelvic films indicate that it is the acetabular thickness, and not the femoral neck length (i.e., the distance along the neck axis from the top of the femoral head to the base of the trochanter), which is predictive of fracture. (6) The hip axis length may be a surrogate marker of the acetabular width, as it includes both the femur and the acetabulum. However, the pelvic film measurements failed to find a significant association between acetabular width and trochanteric fractures, while hip axis length was significantly associated with both femoral neck and trochanteric fractures. Thus the significant predictive association between hip axis length and hip fracture cannot be attributed to pelvic structure alone. Third, we reported a weak but significant correlation between femoral neck width and hip axis length. ‘’) Those with increased hip axis length tended to have wider femoral necks. Although the bending strength of a cylindrical beam is directly related to the moment arm, it is also related to the moment of inertia, which is a function of the width to the fourth power. Thus a very small increase in width will greatly increase the strength of the beam and the femoral neck as well. Although the mechanism of the hip axis length may not be purely related to the femoral moment arm, it is possible that the hip axis length is a marker for the ability of the hip to absorb the energy of a fall. Alternatively, the hip axis length may be related to the degree that the femur protrudes beyond the pelvis, increasing the chances of a direct impact on the trochanter. If this is true, then the hip axis length would be a specific marker for hip fractures and should show no association with other osteoporotic fractures. If the hip axis length is associated with other types of fractures, then it may reflect some general body characteristic leading to an increase risk of falling or fracture. Recently a subset of women from the Study of Osteoporotic Fractures were investigated to determine if the hip axis length is associated with fractures other than those of the hip. The Study of Osteoporotic Fractures is a prospective investigation of fracture incidence in women aged 65 years and older, recruited from four metropolitan areas in the United States. From 1988 to 1990,8074 women participated in visit 2 of the study, which included a dual X-ray absorptiometry (DXA) examination of the hip. These participants were then monitored for the occurrence of all types of fractures. Cases of wrist and humerus fractures were verified by the radiologist’s report, while vertebral fractures (defined as a 15% reduction in anterior, posterior, or midvertebral height) were detected from digitized measurements of spinal films. From these fracture groups, 50 wrist, 50 humerus, and 75 Vertebral fracture cases were randomly selected for analysis. At the time of our analysis, this represented 13% of the total number of wrist fracture cases, 24% of the humerus fracture cases, and 15% of the vertebral fracture cases. For comparison, a cohort of 100 controls were randomly chosen from all participants. With this sample size, differences of hip axis length of less than 0.2 cm (one-third of the population standard deviation) between the fracture and control groups can be detected with more than 95% confidence. The characteristics of these subjects are shown in Table 1 . Hip axis length was measured from the femoral DXA scan by the same observer using previously published tech-