Stimulating angiogenesis mitigates the unloading-induced reduction in osteogenesis in early-stage bone repair in rats

Abstract

Accelerating fracture healing during bed rest allows early mobilization and avoids prolonged fracture healing times. We tested the hypothesis that stimulating angiogenesis with deferoxamine (DFO) mitigates the unloading-induced reduction in early-stage bone repair. Rats aged 12 weeks were subjected to cortical drilling on their tibial diaphysis under anesthesia and treated with hindlimb unloading (HU), HU and DFO administration (DFOHU), or weight bearing (WB) for 5 or 10 days (HU5/10, DFOHU5/10, WB5/10; n = 8 per groups) until sacrifice for vascular casting with a zirconium dioxide-based contrast agent. Taking advantage of its absorption discontinuity at the K-absorption edge, vascular and bone images in the drill-hole defects were acquired by synchrotron radiation subtraction CT. Bone repair was reduced in HU rats. The bone volume fraction (B.Vf) was 88% smaller in HU5 and 42% smaller in HU10 than in WB5/10. The bone segment densities (B.Seg) were 97% smaller in HU5 and 141% larger in HU10 than in WB5/10, and bone thickness (B.Th) was 38% smaller in HU10 than in WB10. The vascular volume fraction (V.Vf) was 35% and the mean vessel diameter (V.D) was 13% smaller in HU10 than in WB10. When compared according to categorized vessel sizes, V.Vf in the diameter ranges 20–30, 30–40, and >40 μm were smaller in HU10 than in WB10, and V.Seg in the diameter range >40 μm was smaller in HU10 than in WB10. In contrast, there was no difference in B.Vf between DFOHU5/10 and WB5/10 and in V.Vf between DFOHU10 and WB10, though B.Seg remained 86% smaller in DFOHU5 and 94% larger in DFOHU10 than in WB5/10, and B.Th and V.D were 23% and 14% lower in DFOHU10 than in WB10. Vessel size-specific V.Vf in the diameter ranges 10–20 and 20–30 μm was larger in DFOHU5 than in HU5. In conclusion, the enhanced angiogenic ingrowth mitigates the reduction in bone repair during mechanical unloading.

Osteophytes and fracture calluses share developmental milestones and are diminished by unloading

“…Fracture callus and osteophyte development have both been shown to be sensitive to local biomechanics after injury . Previous studies have observed decreased osteophyte formation after unloading in a guinea pig model of chemically induced OA, and decreased fracture callus growth in a rat tibial fracture model . Our results agree with these studies, and suggest that loading from normal ambulation influences both osteophyte and fracture callus formation.…”

“…9,14,39,[41][42][43][44] Previous studies have observed decreased osteophyte formation after unloading in a guinea pig model of chemically induced OA, and decreased fracture callus growth in a rat tibial fracture model. 6,7,24 Our results agree with these studies, and suggest that loading from normal ambulation influences both osteophyte and fracture callus formation. Tail suspension has been shown to decrease blood perfusion to hindlimbs, and angiogenic factors have been shown to combat the HLU-induced reduction in osteogenesis which may indirectly impact inflammation in hindlimbs, consequently affecting fracture callus and osteophyte formation.…”

“…Our results agree with these studies, and suggest that loading from normal ambulation influences both osteophyte and fracture callus formation. Tail suspension has been shown to decrease blood perfusion to hindlimbs, and angiogenic factors have been shown to combat the HLU‐induced reduction in osteogenesis which may indirectly impact inflammation in hindlimbs, consequently affecting fracture callus and osteophyte formation . While our FRI results do suggest that HLU leads to decreased protease activity and inflammation after both fracture and ACL rupture, the exact mechanism may differ by injury type.…”

“…These results followed a similar time course to bone restabilization by callus formation, during which full restabilization has been found to occur after 8 weeks in rats and at least 28 days in mice . Furthermore, fracture callus formation has been shown to depend on normal loading during ambulation, with unloaded fractures leading to smaller calluses . Mechanisms of fracture healing have been much more thoroughly characterized than osteophyte formation, with a plethora of studies investigating mechanical and biological factors in callus formation .…”

“…22,23 Furthermore, fracture callus formation has been shown to depend on normal loading during ambulation, with unloaded fractures leading to smaller calluses. 21,24 Mechanisms of fracture healing have been much more thoroughly characterized than osteophyte formation, with a plethora of studies investigating mechanical and biological factors in callus formation. 21 Therefore, identifying similarities and differences between osteophyte development and fracture healing may lead to revelations regarding mechanisms of osteophyte formation and their clinical treatment.…”