Introduction

Osteoporotic fractures pose a serious public health problem given their high prevalence and enormous impact in terms of morbidity, mortality and economic cost¹. Hence there is considerable interest in understanding the underlying pathophysiology of bone fragility, which, from a mechanical standpoint, is determined by bone strength. Bone resistance, in turn, comes from the integration of bone mineral quantity, bone architecture, and the material properties of bone.

The mineral quantity of the bone is usually measured by bone densitometry (DXA), the most commonly used, standardized method for assessing bone mass and fracture risk². Bone architecture, both at the micro- and macroscopic level, is examined using different imaging techniques, including high-resolution peripheral quantitative tomography, bone magnetic resonance and the more accessible Trabecular Bone Score³. However, the material properties of bone are difficult to assess due to its high complexity, reflected in its multiple constituents including non-collagenous proteins, crystallinity, hydration of bone tissue, and the characteristics of mineralization and collagen, among others^4,5. Furthermore, as researchers need bone tissue samples for analysis, the study of these properties has traditionally been restricted to a few centers specialized in bio-mechanics.

Microindentation has been developed as a technique to measure the material properties of bone easily and non-invasively. However, the property specifically measured has not yet been determined, so for the time being, the mechanical strength of the bone is evaluated globally. This technique involves measuring the penetration distance of a needle in the cortical bone to gauge its mechanical resistance⁶. The procedure is usually carried out on the anteromedial side of the tibia in a practical, safe and painless way⁷. There are currently two types of clinical microindentation: the cyclic microindentation, using the BioDent^® instrument (Active Life Scientific Inc., Santa Barbara, USA). The other is impact microindentation (IMI), carried out with the OsteoProbe^®. Several clinical studies can provide relevant information on bone strength and risk of fracture with both types^6,8. However, given its greater manageability, OsteoProbe^® has replaced BioDent^® in clinical studies. Despite its increasing use, there are still many unresolved basic issues surrounding IMI implementation including the effect of age on bony material properties. We examined the influence of age on bone tissue mechanical strength in a cohort of healthy men and women.

Material and methods

Results

For our study, 69 women and 19 men of Caucasian origin were recruited. The participants’ characteristics and the BMSi measurements are shown in table 1. The coefficient of inter-operator variation was less than 5%.

Table 1 Characteristics of the participants 

Linear regression analyzes showed that BMSI does not correlate with age in women (R²=0.076, p=0.25) nor in men (R²=0.0016, p=0.74) (Figure 1). Likewise, no significant associations were detected between the BMSi and the age in the ANOVA analysis adjusted for BMI neither in women (p=0.73) nor in men (p=0.78). Finally, no significant differences were observed in the BMSi between the subgroup of women aged 20-39 years and those older than 60 years (p=0.8) (Figure 2).

Figure 1 Linear regression analysis performed to evaluate the correlation between the Bone Material Strength index (BMSi) and age in healthy men and women 

Figure 2 T-test carried out to compare the BMSi between women aged 20-39 years and women >60 years of age as an indirect measure of pre and postmenopausal status,respectively 

Discussion

In the present study, the influence of age on the mechanical resistance of bone tissue measured by IMI in a cohort of healthy men and women was evaluated. The results indicate that bone tissue resistance is not determined by age in women or men, and that therefore, it is not affected by aging. Furthermore, no BMSi differences were found between the subset of younger women versus the subset of older women which would indicate that the depletion of estrogen that accompanies menopause does not exert a significant effect on the mechanical resistance of the bone tissue.

Bone microindentation has emerged as a promising new tool to evaluate bone mechanical resistance in living individuals^6-8. Although it is still unclear which physical properties are specifically measured, several clinical studies reveal that this technique has a good discriminant capacity between patients with and without fragility fractures^9-11, although studies in geriatric populations with osteoporotic fractures show discrepancies¹².

The measurements that result in an altered BMSi seem to be especially informative in those conditions associated with an increased fracture risk that are not explained by abnormal BMD values^13-16.

Given the increasing use of IMI as a complementary technique for assessing bone health in clinical research and its potential future role in clinical practice, it is imperative to clarify the possible effects of physiological factors, such as age, on the mechanical resistance of the bone tissue.

Our study results indicate that BMSi is not significantly affected by aging or estrogenic depletion. Mirzaali MJ, et al. observed through micromechanical studies in cadaver bone that the properties of microindentation in the elderly were constant with age¹⁷ which concurs with our study. On the contrary, aging and estrogenic depletion reportedly exert a negative effect on BMD and bone architecture^18,19. This reinforces the notion that the microindentation technique measures a very specific characteristic of bone strength or bone quality completely different from other techniques available to date. Microindentation may cause the separation of the protein-based "glue" proteins, which hold together the mineralized collagen fibrils, a property that would constitute the first barrier of resistance to fracture²⁰.

Currently, bone tissue resistance is considered to be genetically determined, but, at the same time, clinical studies show that it can be negatively influenced by non-genetic factors, such as a deficient glycemic control^13,15,21, an excess of tissue local adipose¹⁶, treatment with glucocorticoids²², excess growth hormone²³, chronic kidney disease^24,25 and HIV infection²⁶. In addition, alterations in certain signaling pathways and in the intracellular storage of lipids also seem to affect the mechanical resistance of bone tissue as has been observed in studies on the diseases of Camuratti-Englemann²⁷ and Gaucher type 1²⁸, respectively.

Our study has several limitations. First, the small number of subjects included in the study limits the generalization of our findings. Another limitation is that the factors that can affect the mechanical resistance of the bone tissue are not yet fully known. Therefore, these factors have not been introduced as co-variates in the statistical model for the adjustment of the confounding factor. This has been tried to compensate by means of the strict exclusion criteria used at the moment of the recruitment of the individuals and thus control the possible heterogeneity of the studied cohort.

Another limitation lies in the fact that this technique is performed exclusively on the cortical bone of the anteromedial tibia, so the generalization of BMSi results to other skeletal sites is debatable. However, we believe that the values obtained by microindentation in the tibia reflect the mechanical strength of the bone globally, since clinical studies have shown an inverse correlation between BMSI values and the incidence of osteoporotic fractures in other skeletal locations such as hip, and even in bones with a greater trabecular component, such as the vertebrae^10,11. Finally, the data on the menstrual status were not collected, thus limiting the evaluation of the effects of menopause on the mechanical resistance of the bone tissue. This problem was counteracted by categorizing the subgroup of younger women as premenopausal and the subset of older women as postmenopausal.

In conclusion, the mechanical resistance of the bone tissue does not seem to be affected by aging and estrogen-related depletion related to menopause. Additional studies are needed to corroborate these findings in order to facilitate the implementation of the IMI in research and clinical practice.