Sex differences in long bone fatigue using a rat model

Abstract

Stress fractures can occur because of prolonged exercise and are associated with cyclic loading. Fatigue is the accumulated damage that results from cyclic loading and bone fatigue damage is of special concern for athletes and army recruits. Existing literature shows that the rates of stress fracture for female athletes and female army recruits are higher than their male counterparts. In this study, we used an ex vivo rat model to investigate the fatigue response of female and male bones. We determined the strain versus number of cycles to failure (S/N) for each sex and found that for a certain initial strain (5,000-7,000 me) female bones have shorter fatigue life. To further characterize the bone response to fatigue, we also determined the creep that occurred during the fatigue test. From the creep data, for a certain strain range, female bones accumulated greater residual strains and reached the critical strain at a faster rate. In summary, this study demonstrates that female rat bones have a lower resistance to fatigue in the absence of a physiological response such as muscle fatigue or osteogenic adaptation. From these results, we hypothesized that creep was the underlying mechanism that accounted for the fast deterioration of female bones during fatigue.

The fatigue resistance of rabbit tibiae varies with age from youth to middle age

“…Some studies suggest that bone fatigue life is dependent upon the stress applied and time and independent of loading frequency and number of cycles [32]. Creep strain has been suggested to be the determinant of failure, particularly under higher stresses and strains [18]. Our older adult female group had both much lower creep rates and damage index rates indicating that both creep and damage mechanisms were modified by skeletal maturation.…”

“…In fact, an approximate 10-fold difference due to aging was highly detectable in this study; therefore, differences between the young and skeletally mature as well as between the sexes must be much smaller. Sex differences previously reported in the fatigue resistance of tibiae from mature rats may have been due to methodological differences/problems or may be intrinsic to the rat model [18]. Certainly, rodent bone is microstructurally different from that of rabbits and larger mammals due to their lack of osteonal remodeling.…”

“…This is the site of maximum tensile strain in threepoint bending. Load levels were determined to achieve initial peak-to-peak strain levels of 3,000, 3,500, 4,000, 4,500, 5,000, 5,500, and 6,000 με as done previously [18,25,26], and the tests were run under load control. When two tibiae from the same rabbit were fatigue tested, they were not tested at the same strain/stress level.…”

“…To our knowledge, only one study has investigated sex differences in the ex vivo fatigue behavior of cortical bone. Moreno et al [18] used tibiae from 16-week-old rats and demonstrated whole-bone fatigue occurred faster in females than males and seemed to depend on the rate of creep strain accumulation. Unfortunately, rodent bones are microstructurally quite different from those of human and other higher mammals due to a lack of osteonal remodeling.…”

Detection of fatigue microdamage in whole rat femora using contrast-enhanced micro-computed tomography

“…Initial flexural strains were approximated as e¼6dd/L 2 (Turner and Burr, 1993;Moreno et al, 2006), where d is the specimen diameter, d is the vertical deflection at the midspan, and L is the support span. The secant modulus degradation during fatigue loading was calculated as the percent reduction in instantaneous secant modulus at a given number of loading cycles relative to the initial secant modulus.…”