Elsevier

Bone

Volume 48, Issue 4, 1 April 2011, Pages 786-791
Bone

Exercise loading and cortical bone distribution at the tibial shaft

https://doi.org/10.1016/j.bone.2010.11.013Get rights and content

Abstract

Cortical bone is not a uniform tissue, and its apparent density [cortical volumetric density (vBMD)] varies around the bone cross-section as well as along the axial length of the bone. It is not yet known, whether the varying vBMD distribution is attributable to modulation in the predominant loads affecting bone. The aim of the present study was to compare the cortical bone mass distribution through the bone cortex (radial distribution) and around the center of mass (polar distribution) among 221 premenopausal women aged 17–40 years representing athletes involved in high impact, odd impact, high magnitude, repetitive low impact, repetitive non-impact sports and leisure time physical activity (referent controls). Bone cross-sections at the tibial mid-diaphysis were assessed with pQCT. Radial and polar vBMD distributions were analyzed in three concentric cortical divisions within the cortical envelope and in four cortical sectors originating from the center of the bone cross-section. MANCOVA, including age as a covariate, revealed no significant group by division/sector interaction in either radial or polar distribution, but the mean vBMD values differed between groups (P < 0.001). The high and odd-impact groups had 1.2 to 2.6% (P < 0.05) lower cortical vBMD than referents, in all analyzed sectors/divisions. The repetitive, low-impact group had 0.4 to 1.0% lower (P < 0.05) vBMD at the mid and outer cortical regions and at the anterior sector of the tibia. The high magnitude group had 1.2% lower BMD at the lateral sector (P < 0.05). The present results generate a hypothesis that the radial and polar cortical bone vBMD distributions within the tibial mid-shaft are not modulated by exercise loading but the mean vBMD level is slightly affected.

Research Highlights

►Cortical bone is not a uniform tissue, and its density varies. ►It is not yet known, whether the varying vBMD distribution is loading dependent. ►No loading dependency was observed in vBMD distribution between loading groups. ►Cortical bone vBMD distributions are not modulated by exercise loading.

Introduction

The strength of a whole bone is an important predictor of fracture risk, and is determined by the material and architectural properties of bone [1], [2], [3]. It is well known that bones adapt to prevalent imposed loads and muscle forces mostly via changing their mineral mass and architecture, rather than altering the material properties [4], [5]. Currently there are a number of X-ray based bone imaging techniques that permit a reasonable estimation of the apparent volumetric BMD (vBMD) of the cortex, which reflects both the porosity of bone tissue, mineralization and other bone material properties [6], [7], [8]. Further, it has been reported that about 70% of the age-related reduction in cortical vBMD is attributable to increased porosity [6], which suggests that apparent vBMD of the cortex would provide an adequate surrogate of cortical porosity [9].

It is well established that cortical vBMD within long bones is not uniform, and that there are differences between individuals both around the bone cross-section [10], [11], [12], [13], [14], [15], [16] and along its axial length [13], [14], [15]. These differences observed in the distribution of cortical vBMD between different sectors around the center of mass or neutral axis (polar distribution), and within the circumferential layers within the cortex (radial distribution), during growth [10], [11], [17], and aging [12], [13], [16] have raised an important question as to whether loading plays an important role in modulating the distribution of cortical bone density [10], [11]. To date, such changes have been observed in a single randomized controlled trial (RCT) investigating both the polar and radial distribution in response to exercise and hormone replacement therapy in postmenopausal women [18].

In our previous analyses of athletes' bone data, the long-term specific exercise loading was clearly associated with direction-specific structural adaptations at the tibial midshaft [19], [20], [21], [22]. To extend this geometric analysis to apparent material properties of cortical bone, the present study was carried out to evaluate whether cortical vBMD distribution within the cortical envelope (i.e. radial distribution) or in different anatomic directions around the cortex (i.e. polar distribution) is associated with long-term exercise loading, and whether they are related to specific loading patterns in athletes involved in a diverse range of sporting activities.

Section snippets

Methods

A convenience sample of data from 180 premenopausal women representing athletes with a long history of participation in different sports and 41 physically active, non-athletic referents (Table 1) was used in the present study [19], [21], [22], [23]. The age at which the athletes started their competitive career in sports was obtained from a questionnaire. Based on each athlete's sport, they were divided into five near-distinct types of exercise loading as previously reported [19], [21], [23]:

Results

Descriptive characteristics of the study groups are provided in Table 1. There were no amenorrheic women in any group. The proportion of women using hormonal contraceptives varied from 38% to 60% among the different exercise loading groups, and it was 56% among the referents. ANOVA indicated that the proportion of hormonal contraceptives users did not differ between groups (F = 0.512, P = 0.767).

Discussion

Present findings indicate that there was no detectable interaction between the type of exercise loading and the general pattern of radial or polar distribution in cortical vBMD at the tibial mid-shaft. It is, however, noted that we found significantly lower cortical vBMD between some exercise loading groups and referents at specific radial segments and polar sectors. In particular, we found that cortical vBMD in athletes representing high-impact (triple jumpers, high jumpers and hurdlers) and

Acknowledgments

This study was financially supported by Competitive Research Funding of the Pirkanmaa Hospital District, Tampere University Hospital (Grant 9K121), the Finnish Ministry of Education and the Päivikki and Sakari Sohlberg Foundation. Associate Professor Robin Daly is supported by a National Health and Medical Research Council (NHMRC) Career Development Award (ID 425849).

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