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American Journal of Epidemiology Advance Access originally published online on January 4, 2006
American Journal of Epidemiology 2006 163(5):441-449; doi:10.1093/aje/kwj055
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American Journal of Epidemiology Copyright © 2006 by the Johns Hopkins Bloomberg School of Public Health All rights reserved; printed in U.S.A.

Original Contribution

Longitudinal Changes in Forearm Bone Mineral Density in Women and Men Aged 45–84 Years: The Tromsø Study, a Population-based Study

N. Emaus1, G. K. R. Berntsen1, R. Joakimsen2 and V. Fonnebø1

1 Institute of Community Medicine, Faculty of Medicine, University of Tromsø, Tromsø, Norway
2 University Hospital of Tromsø, Tromsø, Norway

Correspondence to Nina Emaus, Institute of Community Medicine, Faculty of Medicine, University of Tromsø, Tromsø NO-9037, Norway (e-mail: nina.emaus{at}ism.uit.no).

Received for publication February 8, 2005. Accepted for publication October 7, 2005.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
The aim of this study was to describe changes in bone mineral density in Norwegian women and men aged 45–84 years in a population-based, longitudinal study. Bone mineral density (g/cm2) was measured at distal and ultradistal forearm sites with single x-ray absorptiometric devices in 3,169 women and 2,197 men at baseline in 1994–1995 and at follow-up in 2001 (standard deviation, 0.4 years). The mean annual bone loss was –0.5% and –0.4% in men and –0.9% and –0.8% in women not using hormone replacement therapy at the distal and ultradistal sites, respectively. In men, age was a negative predictor of bone mineral density change at both sites. Women not using hormone replacement therapy had the highest bone loss at the ultradistal site 1–5 years after menopause. The correlation between the two measurements was high: r = 0.93 and r = 0.90 in women and r = 0.96 and r = 0.93 in men for the distal and ultradistal sites, respectively. More than 70% kept their quartile positions, indicating a high degree of tracking of bone mineral density measurements. Although the study population live above the polar circle, the rate of bone loss was not higher at the distal and ultradistal forearm sites compared with that of other cohorts.

bone density; densitometry; follow-up studies; forearm; longitudinal studies; men; women

Abbreviations: HRT, hormone replacement therapy; SD, standard deviation; TROST, Tromsø Osteoporosis Study


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Osteoporotic fractures in both sexes constitute a major health problem with substantial morbidity and cost (1Go, 2Go). The causation of fracture is complex, but bone fragility is an important contributor to fracture risk (3Go). Bone mineral density is a good surrogate measure of bone strength, predicting 60–70 percent of its variation (4Go). A strong relation between bone mineral density level and the probability of fracture has been documented (5Go). Bone mineral density in the elderly is a function of the amount of bone gained during growth and the amount of bone lost during aging (6Go, 7Go). Bone loss estimates derived from cross-sectional studies may be subject to cohort effects, and longitudinal studies provide the best foundation for precise estimations of bone loss (8Go, 9Go).

Bone mineral density changes in women through menopause (10Go–14Go) and in old age (15Go–22Go) have been described through longitudinal, population-based surveys. These changes in men are, however, not extensively explored longitudinally in population-based samples (23Go–25Go). Studies, based on representative samples, comprising both sexes from the same population are even more rare, and those existing are from elderly populations (26Go–30Go). Longitudinal, population-based studies describing bone mineral density changes in both sexes from middle age into old age are therefore still lacking.

The Tromsø Osteoporosis Study (TROST) is part of the Tromsø Study in northern Norway. With a follow-up of more than 6 years, TROST has obtained repeated bone mineral density measurements from the distal and ultradistal forearm sites of 3,169 women and 2,197 men aged 45–84 years. The aim of this study was to describe and compare variations in bone mineral density changes in women and men from middle into old age. With the long follow-up, we also wanted to study the degree of tracking of bone mineral density measurements by assessing how well the second measurement was predicted by the first.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
Study participants
The Tromsø Study is a longitudinal, population-based, multipurpose study that focuses on lifestyle-related diseases (31Go). It was initiated in 1974 (Tromsø I), with the surveys repeated in 1979–1980, 1986–1987, and 1994–1995; the fifth survey was performed in 2001 (Tromsø V). In 1994–1995 (Tromsø IV), TROST had bone density measured on a total of 7,311 subjects (4Go,162 women and 3,149 men) aged from 45 to 84 years. These numbers corresponded to 80 and 79 percent of the invited women and men, respectively. In 2001, the 6,755 persons still alive and still living in Tromsø were invited for another examination. Bone densitometry was performed on a total of 5,366 subjects (3Go,169 women and 2,197 men), which corresponds to 80 and 78 percent of the invited women and men, respectively. The follow-up examination therefore included 61 and 55 percent of the women and men originally invited in 1994. The mean age at baseline in 1994 was 60 (standard deviation (SD), 7.4) years and 61 (SD, 7.2) years for the participating women and men, respectively. The mean follow-up time was 6.5 (SD, 0.4) years. The participants signed a declaration of consent prior to both examinations. The Regional Committee of Research Ethics recommended the study, with approval by the Norwegian Data Inspectorate.

Comparison of responders with nonresponders
Data for the first study, Tromsø IV, were compared for nonresponders, partial responders, and full responders. For nonresponders, we had only age and sex data; for partial responders, we had data from the first part of the examination in addition to one or two questionnaires; and for full responders, we had a complete data set. The analysis gave no indication for any differences among the groups (32Go). After Tromsø V, we could use baseline characteristics from Tromsø IV to compare participants who attended both studies with those lost to follow-up, because either they missed participating for unknown reasons or they were ineligible (deceased or moved out of town). Comparisons of the three groups are displayed in table 1. Participants attending both studies were younger and taller, had a lower body mass index (women), and had better self-perceived health. They also smoked less and had a higher bone mineral density at both forearm sites.


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  		TABLE 1. Comparison of participants lost to follow-up (participating in Tromsø IV only) with those who participated in both the Tromsø IV (1994–1995) and Tromsø V (2001) longitudinal studies, Norway*

 
Measurements
Bone densitometry was performed in both surveys at the distal and ultradistal sites of the forearm with two single x-ray absorptiometric devices (DTX-100; Osteometer MediTech, Inc., Hawthorne, California). The distal site includes both the radius and the ulna from the 8-mm point (point where the ulna and radius are separated by 8 mm) and 24 mm proximally. The ultradistal site includes only the radius and stretches from the 8-mm point up to the radial endplate. The nondominant arm was measured except when it was ineligible because of wounds, plaster casts, and so on.

In both studies, by use of the same protocol, participants were allocated to the two densitometers depending on accessibility. Quality control with respect to precision and correction of artifacts in Tromsø IV was reported previously (33Go, 34Go). In the second survey, Tromsø V, one of the two densitometers had a major repair, and the x-ray tube had to be replaced on both densitometers during the survey. Quality control routines, using the European forearm phantom (QRM GmbH, Möhrendorf, Germany), revealed that one of the machines measured at a higher bone mineral density level before the x-ray tube replacement than the other one did, the mean difference being 0.005 g/cm2 (35Go). The European forearm phantom data were used to adjust the differences in densitometer measurement level. The internal variation of each machine was studied by both coefficient of variation, which is equal to the standard deviation/mean x 100, and comparison of the European forearm phantom measurement levels at different time periods and was found to be satisfactory, with a mean coefficient of variation of 0.9 percent measured with the European forearm phantom (35Go).

All scans were reviewed and reanalyzed, and the results from Tromsø IV have been described previously (34Go). The scans from Tromsø V were analyzed by four technicians, one of whom also did the analysis in Tromsø IV. To test for reliability, we obtained three intraobserver tests (each technician compared with him/herself) and three interobserver tests (each technician compared with the other technicians). Each pair corrected a minimum of 27 and a maximum of 127 similar scans. We missed one intraobserver test and one interobserver test possibility, with one technician reviewing 273 (5 percent) of the scans. At the distal site, there were no significant differences among the technicians with respect to bone mineral density, either in intraobserver or in interobserver testing. At the ultradistal site, there were significant differences among the technicians with respect to bone mineral density in two of the three intraobserver tests and in two of the three interobserver tests. From these tests, we could derive that one of the technician's measurements was approximately 0.001 g/cm2 lower than the others. This would entail an effect of less than 1 percent on the annual bone loss estimates (g/cm2) and reduce the estimates of percentage of change by 0.02 percentage points. We also compared annual change estimates (g/cm2), and they were not technician influenced (p > 0.29) at any sites (analysis of variance). We therefore decided not to do any correction of the data. After exclusion of invalid scans, which were due mostly to excessive movement artifacts, there remained 3,093 and 3,060 repeated measurements for women and 2,150 and 2,160 repeated measurements for men at the distal and ultradistal sites, respectively.

Other measurements
Height and weight were measured to the nearest centimeter and half kilogram. The participants wore light clothing without shoes. Body mass index was calculated as weight in kilograms divided by the square of the height in meters.

Questionnaires
Two self-administered questionnaires were filled in by the participants in Tromsø IV, one before entering the study and the other during the study, in which the participants provided data on different lifestyle variables at baseline. We used data on smoking status and self-perceived health to assess possible selection bias. Women's menstrual status at baseline was also derived from answers to the questionnaires. Women using hormone replacement therapy (HRT) were classified as "HRT users." Women who were aged more than 44 years, were not using HRT, and were either pregnant or had a time from the last menstrual period of less than 180 days were classified as "premenopausal." Women who were aged more than 44 years, were not using HRT, were not pregnant, and had a time from the last menstrual period of between 180 and 364 days were classified as "perimenopausal." Women who were aged more than 44 years, were not using HRT, and had a time from the last menstrual period of 1 year or more were classified as "postmenopausal." When information about menstruation was lacking completely and menstrual status could not be determined, menstruation status was defined as "missing." For further classification of HRT use in the period of follow-up, we have used information provided from questionnaires in Tromsø IV.

Statistical analysis
Bone mineral density measurements from intra- and interobserver testing were compared by use of a one-sample paired t test. Change in bone density was estimated by calculating the difference between measurements from Tromsø V and Tromsø IV. This total estimate was divided by the length of each participant's follow-up time to calculate the annual changes that are presented by 5-year age groups as mg/cm2 and percent, with 95 percent confidence intervals. Regression analysis was used to investigate how age and sex predicted changes in bone mineral density. The difference in annual bone loss rates in women according to reported HRT use in the follow-up period and years since menopause was analyzed by use of analysis of variance, applying the Bonferroni correction.

To investigate the variation of changes in bone mineral density and to identify possible "fast losers," we used the annual loss estimates to categorize the participants into groups of "losers," "nonlosers," and "gainers" through calculation of the minimal difference, which represents the true biologic change with 95 percent certainty (95 percent detection limit). It is theoretically given by the following formula: {Delta} percent = z x coefficient of variation x {surd}2 (36Go). The median coefficient of variation estimated on our material was for an intermediate term between two measurements of 1.25 and 1.86 percent at the distal and ultradistal sites, respectively (33Go). Persons with an annual loss or gain of more than ±3.46 percent were categorized as true "gainers/losers" at the distal site. At the ultradistal site, the equivalent 95 percent detection limit was ±5.14 percent.

Tracking was assessed by use of Pearson's correlation coefficient and correlation with ranking of the variables. We divided values for bone mineral density measured at baseline and at follow-up into four quartiles, the highest quartile being categorized as position 1 and the lowest quartile being categorized as position 4. The values from both studies were categorized, respectively, and each participant's positions in both studies were compared.

The statistical analysis was performed by use of SPSS, version 11, software (SPSS, Inc., Chicago, Illinois). A p value of less than 0.05 was regarded as statistically significant.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
Changes in bone mineral density by age
Annual changes in bone mineral density according to 5-year age groups in men and in women reporting no HRT use in the follow-up period are displayed in table 2 and in figure 1. In men, the rate of bone mineral density loss was associated with age at both sites (p < 0.001), with an increase in the rate of loss of approximately 0.2 percent per 10-year increase in age (beta, –0.02). In women, a smaller bone mineral density loss rate in the age group 45–49 years compared with the other age groups indicated a possible nonlinear association at both sites. The test of linear interaction between age and sex was therefore not assessed.


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  		TABLE 2. Annual bone mineral density changes in mg/cm2 and percentage (%) with 95% confidence intervals in men and women (not using hormone replacement therapy), according to 5-year age group, the Tromsø IV (1994–1995) and Tromsø V (2001) longitudinal studies, Norway

 

Figure 1
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  		FIGURE 1. Distal (A) and ultradistal (B) sites: annual percentage of change in bone mineral density (BMD) by age in men and women (not using hormone replacement therapy) in the longitudinal Tromsø IV (1994–1995) and Tromsø V (2001) studies, Norway. Black bars, women; white bars, men.

 
Bone mineral density changes in women
The highest rate of bone loss was seen in women who were not using HRT and in women who had stopped using HRT during the follow-up period (table 3). The differences between the groups also remained significant (p < 0.001) at both sites after adjustment for age. Among postmenopausal women not using HRT, the highest bone loss rates were seen in the period 1–3 years after menopause at the ultradistal site (table 4) (p > 0.001) and also, when adjusting for age, with the same trend at the distal site (p = 0.065). Women reporting to be premenopausal at baseline and not using HRT in the period of follow-up had bone mineral density loss rates at the ultadistal site that were not significantly different from those of women 1–3 and 4–5 years after menopause. At the distal site, their bone mineral density loss rates were not significantly different from those of any other group (table 4).


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  		TABLE 3. Annual bone mineral density changes in mg/cm2 and percentage (%) with 95% confidence intervals in women, according to reported hormone replacement therapy use in the follow-up period, the Tromsø IV (1994–1995) and Tromsø V (2001) longitudinal studies, Norway

 

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  		TABLE 4. Bone mineral density changes in mg/cm2 and percentage (%) with 95% confidence intervals in women not using hormone replacement therapy who were classified according to menopausal status and years since menopause, the Tromsø IV (1994–1995) and Tromsø V (2001) longitudinal studies, Norway

 
"Fast losers"
Among women not using HRT, 1 percent (n = 16) were losing more than –3.6 percent annually at the distal site. Their mean age was 62.0 (SD, 8.6) years. Nine of these women were in the lowest bone mineral density quartile at baseline, and in the second survey they were all in the lowest bone mineral density quartile. Only three men lost more than –3.6 percent annually at the distal site. At the ultradistal site, only three women lost more than –5.14 percent annually.

Tracking of bone mineral density measurements
The correlations in the measurements between the two studies are significant (p < 0.001) and high at the distal and ultradistal sites, respectively, in men (r = 0.96 and r = 0.95) and in all women (r = 0.93 and r = 0.90). Including only women reporting no HRT use, the correlation coefficient is r = 0.94 and 0.91 at the distal and ultradistal sites, respectively. The ranked correlation is slightly less but also high.

Among men, 79 and 75 percent keep their quartile position from the first to the second survey, and 10 and 12 percent either lose or gain one position, from all quartiles, at the distal and ultradistal sites, respectively. Among all the women, 74 and 70 percent keep their quartile position, whereas 12 and 14 percent either lose or gain one or two quartile positions at the distal and ultradistal sites, respectively. A similar pattern is seen also when only the women not using HRT are included in the analyses; 75 and 69 percent of the women keep their quartile position, 14 and 16 percent lose, and 10 and 11 percent gain one position at the distal and ultradistal sites, respectively. In both sexes, at both sites, the changes are all from original quartile positions.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
The main findings from this population-based survey are that the mean annual bone mineral density loss in men aged 45–84 years is less than –0.5 and 0.4 percent, negatively predicted by age, at the distal and ultradistal sites, respectively. In women not using HRT, the equivalent bone mineral density changes are –0.9 and –0.8 percent. There is a high degree of tracking in bone mineral density measurements.

Two of the strengths of this study are its long follow-up and a high attendance rate of more than 78 percent in both studies. The single x-ray absorptiometric measurement of the distal forearm is thought to be one of the most precise densitometric methods (33Go, 37Go–39Go), and we had densitometer performance strictly controlled in both studies. Although fracture risk is best predicted by bone mineral density measurements from the same anatomic site, no site is superior with respect to prediction of all types of fragility fractures (5Go). When central dual x-ray absorptiometry is not available, peripheral bone mineral density measurement can be used to assess fracture risk at both peripheral and central sites (5Go, 40Go, 41Go), and they still constitute a valuable tool for the diagnosis of osteoporosis (42Go).

Irrespective of high response rates, nonresponse may generate selection bias. As displayed in table 1, participants lost for follow-up in general seem to be less healthy or having a less healthy lifestyle than those who participated in both studies. As smoking status is associated with greater bone loss rates (43Go) and low self-perceived health might indicate a greater degree of comorbidity (44Go), we possibly have some "healthy selection bias" in the material. Similar findings are observed in other longitudinal studies within the field. In a prospective osteoporosis study in Rochester, Minnesota, nonrespondents were less healthy than were full respondents (45Go). In the Framingham Osteoporosis Study, cohort members without longitudinal data were more likely to be older, to have a lower mean baseline bone mineral density, and to have lower physical activity scores, and they were less likely as participants to have reported good health (30Go). The Rotterdam Study also reported selection in favor of the more mobile and healthy population with probably lower rates of bone loss, and loss to follow-up was most likely related to illness, so that true progression was probably underestimated (28Go). Despite some possible selection bias, with the high attendance rates, we do feel confident that the results from our study are comparable to other population-based studies in the field.

At the forearm site, we have the possibility of comparing age-related changes of both trabecular and cortical bone, as the distal site contains mainly cortical and the ultradistal site contains mainly trabecular bone (46Go). We have compared our results with findings from other longitudinal, population-based studies on bone mineral density changes, limited to studies with data from the distal and ultradistal radius.

Annual percentages of decline of approximately 1.0 percent were seen at the distal and proximal radius in previous studies of 1,000 Japanese-American postmenopausal women aged 55–74 years (15Go, 16Go, 18Go) and of 271 White women aged 55–80 years (17Go). The loss rates in both of these studies are slightly higher at the distal site than that in our cohort for the concurrent age groups. In men, we observed an increasing rate of bone loss at the distal site with increasing age, from about –0.30 percent per year at ages 45–59 years to 0.75 percent per year at ages 70–74 years. Similar trends were seen in a large study of Japanese-American men aged 51–82 years (23Go, 24Go) and in the Mayo Clinic study of the Rochester, Minnesota, population (23Go, 24Go). The Framingham longitudinal study reported annual loss rates of –1.2 and –0.9 percent at the distal radius and of –1.0 and –0.8 percent at the ultradistal site in elderly women and men (aged 67–95 years) (30Go). These rates are slightly higher than those in our cohort at similar age groups. In summary, despite difficulties in comparing studies, the population of Tromsø living above the Arctic Circle does not seem to have higher bone loss rates than do other comparable populations.

Because of the different environments of the bone cells, decline in trabecular bone mass is thought to begin earlier than that in cortical bone mass, which is thought to occur increasingly after the age of 40 years and to be mainly age related (47Go). Our findings of bone mineral density development in the age group 45–84 years are supportive of this concept. In men, with age being a negative predictor of bone mineral density changes at both sites, the loss rates are higher at the distal than at the ultradistal site. In women not using HRT, the ultradistal site bone loss rates decrease from –1.3 percent in the age group 50–54 years to –0.6 percent in the age group 65–69 years, indicating that the most dramatic trabecular bone loss in women had occurred before that age. This is also supported by the findings of highest bone loss rates in women 1–5 years after menopause, findings which are comparable to those of Guthrie et al. (12Go) and Ahlborg et al. (48Go), who studied bone loss in relation to menopause in a longitudinal study of more than 16 years (healthy volunteers).

Tracking of a characteristic is defined as the ability to maintain the same position within a distribution over time (49Go, 50Go) or as the ability to predict future values from earlier measurements (51Go). As such, the term "tracking" is used to describe the extent of predictability or relative constancy that a measurable characteristic may have in a group of individuals over repeated observations (52Go). A number of methods may be used (53Go), and we used both the Pearson correlation coefficient and the comparison of quartile position between the two studies. Our findings are comparable to the findings of Sowers et al. (17Go) and Ahlborg et al. (48Go) and, therefore, supportive of those of Gilsanz and Nelson (54Go), who indicate that the morphologic traits that contribute to the strength of bone track throughout life, with values remaining in the same position relative to population percentiles. The high degree of tracking also indicates that one bone mineral density measure expresses a person's bone mineral density level and, as such, supports the notion that, except for patients with expected rapid bone loss or on bone mass treatment, there are rarely indications for frequent repeated bone mass measurements (55Go–59Go).

Notwithstanding the high degree of tracking, there is interindividual variation in bone loss estimates illustrated through both the confidence intervals and the distribution of participants into different "loss groups." As we used the notion of "minimal detectable difference," persons losing more than –3.14 percent annually at the distal site were identified as "fast losers," 1 percent of the women not using HRT. In the study of Sowers et al. (17Go), 30 percent of women aged 55–80 years lost at least 2 percent annually; the equivalent rate in our study would be 12.5 percent. We found, however, as did Sowers et al. and Nguyen et al. (60Go), that the rates of bone loss were not generally associated with baseline bone mineral density (or quartile positions).

In conclusion, our study is one of the first to describe bone mineral density changes in a longitudinal, population-based study comprising both sexes from the age of 45 years to well above 80 years. The frequency of fractures appears to be increasing in many countries (61Go), but the incidence of fractures varies (62Go). The Scandinavian countries, together with North America, have the highest incidence of hip and forearm fractures in the world (63Go, 64Go). Even if the study represents a northern population, the observed bone loss rates are not greater than those observed in other comparable populations.


   ACKNOWLEDGMENTS
 
Conflict of interest: none declared.


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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