Osteons are the main sites of cortical bone remodeling. Osteons were formed either within or at the periosteum on existing bone. Primary osteons were generated during appositional bone growth, whereas secondary osteons are generated during internal bone remodeling. Since 1999, Bell et al. (
1999) have focused their studies on the diameter of the central canals of osteons that constitute the femoral neck cortex, and they have found that fractures are likely to occur in regions where osteons with relatively large central canals are distributed, leading to the concept of “cortical porosity.” They have also introduced the concept of “cortical bone trabecularization,” based on the findings of a morphological analysis of the central canals of osteons. They observed that cortical porosity tended to occur in regions where the proportion of secondary osteons was high—regions undergoing bone remodeling.
Variations in the layering of the collagen fibers that constitute the lamellar structure of osteons have been observed using a circularly polarized light microscope. Gebhardt (
1906) described images of the collagen fiber structure in osteons obtained using circularly polarized light microscopy, and Ascenzi and Bounucci (
1967,
1968,
1972) later observed the microstructure of the human femur and proposed three different osteon morphotype (OM) classifications, including longitudinal-type (dark-type) osteons, in which the collagen fibers are aligned parallel to the longitudinal axis of the femoral diaphysis and are resistant to tension, and transversal type (bright-type) osteons, in which the collagen fibers are aligned in a spiral transverse to the axis of the femoral diaphysis and therefore resist compression forces. Numerous studies in recent years have examined osteon structure using circularly polarized light microscopy. Bromage et al. (
2003) classified osteons into three morphotypes (bright, dark, and alternating) and Bigley et al. (
2006) classified them into four groups, adding a hooped type. Beraudi et al. (
2010) modified the OM classification system proposed by Bigley et al. (
2006) and evaluated images of osteons from the diaphysis of the human fibula. Also, numerous studies have found that osteon size decreases with age, while the overall osteon population density (OPD) in cortical bone tissue increases with age (Britz et al.
2009; Currey
1964; Kerley
1965; Maat et al.
2006; Martin et al.
1980). Currey (
1964) reported that the haversian system shrinks with age, and Martin et al. (
1980) stated that osteons may decrease in size with age because of diminishing osteoclast activity. In addition, Britz et al. (
2009) reported that age, sex, and body weight may also influence osteon size. The high level of bone remodeling observed in regions with elevated osteon density (Miszkiewicz
2016) may be associated with mineral homeostasis maintenance, bone microdamage repair, and adaptations to changes in the mechanical burden (Burr
2002). The femur is affected by load stress during both standing and walking, and the muscles attached to the femur continuously exert mechanical forces in
various directions (Duda et al.
1997; Rybicki et al.
1972). However, muscle mass and resiliency slowly decline with age (Burr
1997; Frost
1997). These factors give rise to qualitative and quantitative changes in the femoral cortical bone. The aim of the study reported in the present paper was therefore to explore interindividual differences in the age-related fragility of femoral cortical bones by evaluating both the population density of secondary osteons and the structural patterns of secondary osteon morphotypes.