BONE MINERAL DENSITY OF LUMBAR SPINE IN BRAZILIAN CHILDREN AND ADOLESCENTS AGED FROM 6 TO 14 YEARS 

Antonio Sergio Macedo Fonseca 1

Vera Lucia Szejnfeld2

Maria Teresa Terreri3

José Goldenberg2

Marcos Bosi Ferraz2

Maria Odete Esteves Hilário 4

 

1.      Assistant Professor, Department of Pediatrics – Universidade Federal do Rio Grande do Norte

2.      Associate Professor, Division of Rheumatology, Department of Medicine - UNIFESP/EPM

3.      Pediatric Rheumatologist, Division of Allergy, Immunology and Rheumatology, Department of Pediatrics - UNIFESP/EPM

4.      Associate Professor, Division of Allergy, Immunology and Rheumatology, Department of Pediatrics - UNIFESP/EPM

 

Division of Allergy, Immunology and Rheumatology, Department of Pediatrics and  Division of Rheumatology, Department of Medicine - UNIFESP/EPM

Address for correspondence:

Maria Teresa Terreri

Rua Loefgreen 2381- ap.141

CEP:04040-004

São Paulo – SP  -  Brasil

Telephone number: (011) 576-4426

Fax number: (011) 5579-1590

E-mail address: terreri@uninet.com.br

 

Main subject of the paper: bone mineral density in normal Brazilian children and adolescents

The paper is intended to be a full original paper in the Clinical Investigation Section

The authors wish that their names act as referees. 

Running title: bone mineral density in normal Brazilian children and adolescents

BONE MINERAL DENSITY OF LUMBAR SPINE IN BRAZILIAN CHILDREN AND ADOLESCENTS AGED FROM 6 TO 14 YEARS

 

ABSTRACT; INTRODUCTION; MATERIAL AND METHODS; RESULTS; DISCUSSION; REFERENCES

Abstract

The authors performed a study of bone mass in eutrophic Brazilian children and adolescents using DXA, in order to obtain curves of  bone mineral content (BMC) and bone mineral density (BMD) by chronological age and correlate these values with weight and height. Health Caucasian children and adolescents, 120 boys and 135 girls, aged from 6 to 14, residents in  São Paulo, Brazil, were selected from the Pediatric Department, outpatient clinic of Hospital São Paulo (Universidade Federal de São Paulo). The measurement of bone mineral content (BMC), bone mineral density (BMD) and the area of the vertebral body of the L2-L4 segment were obtained through a unit of DXA. BMC and BMD values for the lumbar spine (L2-L4) showed a progressive increase between 6 and 14 years of age, in both sexes, presenting a distribution that fitted an exponential curve. We identified an increase of mineral gain in female patients older than 11 years which  was maintained until 13 years of age, when there was a new decrease in the velocity of bone mineralization. Male patients presented a period of accelerated gain of bone mass after 11 years of age that was maintained until 14 years of age. At 14 years of age the BMD mean values for boys and girls were 0.984 g/cm2 and 1.017 g/cm2, respectively. In a stepwise multiple regression analysis, the results of pairs of variables showed that the pair “vertebral area-age” was the most significant in the determination of the BMD values and the introduction of a third variable (weight or height), did not significantly raise the coefficient of determination.  

Key-Words: Bone mineral density - Lumbar spine densitometry - Children – Adolescents

 

INTRODUCTION

The increase of the life-span in developing countries has turned osteoporosis into a major health issue, as it had already been observed in developed countries1.

The problems concerning the control of loss of bone mass after adulthood, which is a characteristic of this disease, has motived an increased interest regarding prophylactic procedures, such as the accomplishment of an adequate peak of bone mass2,3, since the bone mass that is present at any time during adulthood is the ratio between the amount obtained during adulthood and the loss caused by the aging process4.

Despite the different opinions about the period when this peak of bone is reached5,6,7,8 the authors agree that the peak of the mineral bone mass is acquired up to the end of puberty9,10,11, which has caused the rise of interest in the study of bone mineral content during childhood and adolescence.

The dual-energy X-ray absorptiometry (DXA), with its low radiation dose and high level of accuracy and precision, has been extensively employed to measure bone mass in children and adolescents.

The authors performed a study of bone mass in eutrophic children and adolescents using DXA, in order to obtain curves of  bone mineral content (BMC) and bone mineral density (BMD) by chronological age which can allow later comparisons with groups of children and adolescents with diseases or other osteopenizing conditions and correlate these values with anthropometric data.

 

MATERIAL AND METHODS

Health Caucasian eutrophic children and adolescents, 120 boys and 135 girls, aged from 6 to 14, residents in São Paulo, Brazil, were selected from the Pediatric Department of Hospital São Paulo (Universidade Federal de São Paulo), from May 1995 to January 1998, and invited to take part in this study. The exclusion criteria were: history of endocrinopathy, nephropathy, gastroenteropathy, or rheumatic diseases, bronquial asthma, malnutrition, short stature (below 10 percentile), obesity (over 120% of weight for height), premature birth, prolonged immobility period (more than two weeks), history of two or more accidental fractures or one pathological bone fracture, treatment with corticosteroids or other drugs that affect bone metabolism.

All children were clinically assessed by a pediatrician (ASMF). Weight and height were measured by using electronic scale and wall anthropometer.

The measurement of bone mineral content (BMC), expressed as g; bone mineral density (BMD), expressed as g/cm2; and the area of the vertebral body of the L2-L4 segment, expressed as cm2; were obtained through a unit of DXA available commercially (DPX, Lunar Radiation Corp., Madison, WI, USA) using medium mode scan. The BMD was only studied at lumbar spine, since the evaluation of the femoral neck may be biased due to the presence of growth cartilage as well as to the technical difficulties related to the posicioning in this age group.

Statistical analysis: The Kruskal-Wallis test was employed to compare the values of BMD, BMC, vertebral area, weight and height between genders. The analyses of multiple linear regression, which tested BMD, BMC, vertebral area (L2-L4), weight, height and age with BMC and BMD as dependent variables were performed through the stepwise, forward and backward methods. The statistical package SPSS/PC was used for processing the data. The significant level was set as 0.05.

 

RESULTS

The mean values of  BMC, BMD, vertebral area (L2-L4), weight and height for age and gender with their standard deviations are shown on tables 1 and 2.

The BMC and BMD results for the lumbar spine (L2-L4) showed a progressive increase between 6 and 14 years of age, in both sexes, presenting a distribution that fitted an exponential curve (figure 1 and 2). This distribution showed that the increase of bone mass was not steady, being observed periods with distinct velocities of  bone mineralization.

Female children between 6 and 10 years of age presented a period of slight increase of bone mass. We identified an increase of mineral gain in female patients older than 10 years which  was maintained until 13 years of age, when there was a new decrease in the velocity of bone mineralization.

Male children also presented a primary period of less intense mineralization that continued until 11 years of age, followed by a period of accelerated gain of bone mass that was maintained until 14 years of age.

It was observed, in both sexes, a statistically significant difference between the BMD at 6 years and BMD in children 11 years old and older (H observed=98.1 and 62.7 for girls and boys, respectively; H critical=15.5; p<0.05).

The values of BMD during the interval between 6 and 14 years of age increased 54% and 59% in boys and girls, respectively. At 14 years of age the BMD mean values were 0.984 g/cm2 and 1.017 g/cm2, which corresponds to 81% and 85% of the expected bone density for Brazilian Caucasian men and women between 35 and 40 years of age, respectively12.

The analysis of the relationship between the weight and height and bone mass showed a striking correlation between BMC and the following variables: vertebral area (r2= 0.91 and 0.87 for females and males, respectively), weight (r2 = 0.73 and 0.69, respectively) and height (r2 = 0.70 and 0.76, respectively). However, after vertebral area and age adjusts, weight and height did not present  additional influence in the determination of the BMC measurements, as demonstrated by the analysis of multiple regression and for this reason the regression equation, that considers the vertebral area and children age was chosen (BMC = -17.468 + 1.246 VA + 0.618 Ag, r2= 0.92 for girls and BMC = -12.102 + 1.059 VA + 0.423 Ag, r2= 0.87 for boys).

Regarding BMD, we observed a less intense correlation when compared to BMC, although it was equally important: vertebral area (r2 = 0.68 and 0.52 for females and males respectively), weight (r2 = 0.62 and 0.45, respectively) and height (r2 = 0.59 and 0.49, respectively). During the stepwise multiple regression analysis, the results of pairs of variables showed again that the pair “vertebral area-age” was the most significant. The introduction of a third variable (weight or height), as previously observed with BMC, did not raise the coefficient of determination (r2) significantly and for this reason the regression equation, that considers the vertebral area and children age was chosen (BMD = 0.225 + 0.012 VA + 0.025 Ag, r2= 0.72 for girls and BMD = 0.336 + 0.099 VA + 0.017 Ag, r2= 0.55 for boys).

 

DISCUSSION

 The mean values of BMD in the lumbar spine per age obtained in our study were quite equivalent to those obtained in studies performed with a smaller group of Finnish children13, using a similar equipment (Lunar DPX, Medium mode).

The values of BMC and BMD in the lumbar spine showed a progressive increase with age, a kind of distribution that approached an exponential function, as observed by other authors14. This arching relationship was determined by the variation in the velocity of mineral gain that happened at different ages. After an initial period with slight increases of the values of BMC and BMD, we observed a period of  rapid growth and accumulation of bone mass in the lumbar spine, especially striking after 10 and 11 years of age in the female and male sex, respectively. This acceleration of bone mass gain between 10 and 14 years of age, later in the male sex, has been extensively documented in the literature2,7,10,11,15,16,17,18 and it seems to be associated with pubertal growth. Unfortunately, evaluation of Tanner puberal state could not been done which could have partially explained the increase in bone mass.

The striking increase of BMC and BMD, reaching values that are similar to those expected for a young Brazilian adult12 demonstrates the importance of this period to attainment an adequate peak of  bone mass. However, this increase of bone mass variables does not only reflect the real increase of bone mineral density, but also the increase of bone volume that happens with the growth process.

In DXA, such as in other projection methods, BMD is calculated from the BMC, expressed as g and the projected area of the region of interest (in this case, vertebral body) without taking the bone thickness into account. Thus the increase of bone size causes an unreal increase of BMD obtained by this method. Since the vertebras are complex structures that grow in all dimensions during childhood and adolescence, it is justifiable to suppose that this method underestimates the BMD value of smaller children and overestimates that of bigger ones. In diseases states where disturbs of growth occur, this systematic error can produce distortions in the results, making the mineral bone mass evaluation by DXA cumbersome19.

In order to minimize this error, Katzman et al8 and Kröger et al10 introduced some equations which objective was to obtain “corrected” or “volumetric” BMD (vBMD), expressed as g/cm3. Although these “corrected” values still present distortions, the vBMD curves obtained by these authors showed an increase with age that was less intense than those obtained from BMD without correction or “areal” BMD. Those results confirm the limited data of vBMD in lumbar spine utilizing quantitative computed tomography (QCT), that did not show a significant increase with age20 or showed a little expansion of bone mass only in puberal period 21.

In  our series, we observed a strong correlation between the values of BMC and BMD and the variables weight and height, in both sexes. This close relationship between the bone mass and anthropometric variables, had already been identified in the early studies of Mazess and Cameron22 and later confirmed in studies performed in bone from the appendicular bone structure19,23,24,25,26,27,28 and from the lumbar spine10,11,15,16,29,30.

For the analysis of stepwise multiple regression we decided to use additionally the variable vertebral area (L2-L4), in a like manner as  used by De Priester et al.26, who considered the values of bone width, obtained by screening, in the regression analysis, when elaborating a predictive equation for BMC values of the forearm obtained by single photon absorptiometry (SPA).

Indeed, during multiple regression by the stepwise method,  vertebral area (cm2), chronological age (years), weight (kg), and height (cm) were used as independent variable, with BMC (g) and BMD (g/cm2) as dependent variables. Borderline p values were reached while only the vertebral area and age were considered. The use of other variables (weight and height), after the vertebral area and age adjustments did not prove to be significant to predict BMC or BMD.

These results support the idea that correlations between the BMD and the anthropometric variables weight and height are in great part due the increase of bone dimension during childhood and adolescence, and confirm that BMD depends on age and growth in this period.

Although the present study has obtained equations for the predictive values of BMC and BMD for children and adolescents between 6 and 14 years of age with statistical significance and high coefficients of correlation, which permit comparisons with groups of sick children31, the distinctive characteristics of our series call for additional studies in order to establish the validation of its application from other parts of the country.

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