Surface microcracks signal osteoblasts to regulate alignment and bone formation

Abstract

Microcracks are present in bone and can result from fatigue damage due to repeated, cyclically applied stresses. From a mechanical point, microcracks can dissipate strain energy at the advancing tip of a crack to improve overall bone toughness. Physiologically, microcracks are thought to trigger bone remodeling. Here, we examine the effect of microcracks specifically on osteoblasts, which are bone-forming cells, by comparing cell responses on microcracked versus non-microcracked hydroxyapatite (HA) specimens. Osteoblast attachment was found to be greater on microcracked HA specimens (p<0.05). More importantly, we identified the preferential alignment of osteoblasts in the direction of the microcracks on HA. Cells also displayed a preferential attachment that was 75 to 90 μm away from the microcrack indent. After 21 days of culture, osteoblast maturation was notably enhanced on the HA with microcracks, as indicated by increased alkaline phosphatase activity and gene expression. Furthermore, examination of bone deposition by confocal laser scanning microscope indicated preferential mineralization at microcrack indentation sites. Dissolution studies indicate that the microcracks increase calcium release, which could contribute to osteoblast responses. Our findings suggest that microcracks signal osteoblast attachment and bone formation/healing.

Mechanical and biological evaluation of a hydroxyapatite‐reinforced scaffold for bone regeneration

“…Additionally, the presence of surface deformities such as crevices and micro‐cracks provided attachment points and topological cues for the cells. Micro‐cracks can enhance osteoblast attachment and alignment along their direction . On day 7, fluorescence microscopy showed MC3T3‐E1 cells attaching and aligning along a crack on the HAp surface (Fig.…”

“…Samples sintered at 1200°C for 5 hr exhibited surface deformities and micro‐crack formation, which have been reported to have both mechanical and biological benefits. Mechanically, micro‐cracks can help dissipate strain energy, thereby, increasing fracture toughness—a material property that is characteristically low for Hap . Additionally, Shu et al has reported that osteoblast attachment was significantly greater on HAp with induced micro‐cracks compared to HAp without the presence of micro‐cracks .…”

“…10,23 Additionally, Shu et al has reported that osteoblast attachment was significantly greater on HAp with induced micro-cracks compared to HAp without the presence of micro-cracks. 23 However, a large number of micro-cracks could prove to be problematic as the application of force could cause them to grow and combine to form larger cracks, leading to mechanical failure. It was determined that HAp packed under 44 MPa of pressure for 10 min and sintered at 1200 C for 5 hr were sufficient conditions for maximizing column mechanical properties.…”

Self‐healing materials: what can nature teach us?

“…The bone matrix itself may play a role: a recent experiment by Shu et al bridges the gap between our work and that of Kennedy et al : they grew cells in vitro on the surface of a sample of hydroxyapatite (the mineral phase of bone). They found that introducing microcracks into the mineral enhanced osteoblast activity, possibly because those cells were sensing the calcium released during cracking . Further clues as to how this calcium release might stimulate altered cell behaviour come from recent in vitro work on notched bone samples …”

“…They found that introducing microcracks into the mineral enhanced osteoblast activity, possibly because those cells were sensing the calcium released during cracking. 45 Further clues as to how this calcium release might stimulate altered cell behaviour come from recent in vitro work on notched bone samples. 46 It seems clear that the osteocyte network is acting as a mechanical transducer, sensitive to local changes caused by the presence of microcracks.…”