Effect Of Athletic Fatigue Damage And The Associated Bone Targeted Remodeling In Rat Ulna

Backgrounds:Bones are important load bearing organs with mechano-sensitivity.Fatigue damage of long bones are prevalent in running athletes and military recruits,characterized as generation and growth of microcracks,and decrease of mechanical properties,which are harmful in influencing athletic performance and military combat effectiveness.Researches on bone fatigue damage and the associated bone targeted remodeling have been widely developed.Those earlier were emphasized on basic fatigue experiments,microcrack observations and cellular response assays,while those in recent years were focused on computational simulations of microcrack germination and bone targeted remodeling based on the progressive marking,staining,imaging or modeling technologies.However,there are still problems or limitations in existing studies,say,diversity of experimental subjects and detecting conditions,or one-sideness of mechanism discussions.We,therefore,carried out a multi-level study on bone fatigue damage and the targeted remodeling based on a basic view that bones are dynamic responding systems of load bearing capacity,mechano-sensitivity and bioactivity,and our findings would be theoretically and clinically significant.Objective:To construct a bone fatigue damage model of rat ulna based on mechanical property tests in vitro and the Finite Element Analysis（FEA）of solid model of cortical bone,and obtain a systematic understanding of bone fatigue damage,responding mechanisms and processes of bone targeted remodeling.Method:（1）Mechanical properties of rat ulnae were tested in vitro on an Instron5865 under axial compression,and classic index of static mechanical properties(Elastic Modulus,E;Compressive Strength,σ_b;Peak strain,ε_max;Fracture Strength,σ_f;Fracture Energy,H_f)and fatigue properties（stress-strain（σ-ε）loops;Cyclic Secant Modulus,E_s;Cyclic Energy Dissipation,H_c;Fatigue E-N curves）were evaluated;（2）A 3D Solid model of the cortical bone with 9 osteons included was build up in Pro\Engineer and imported in ANSYS.Static simulations were developed to verify the stress concentrating effect of osteons and the Fatigue postprocessing module of ANSYS was used to compute the fatigue life and fatigue partial usage（‰）of the model under certain fatigue loadings;（3）The athletic fatigue damage model of rat ulna was constructed and verified by athletic fatigue loading（1.5 Hz;3000με;8000 cycles（or 1.5 h）/2 day）andσ-εdata sampling on an Instron 5865.Serum TRAP-5b and BGP were detected to determine the successive responses of osteoclasts and osteoblasts,and HE stained non-calcified sections of rat ulnae were observed with optical microscope to monitor microcrack germination and osteocyte apoptosis,and mechanical properties were tested to evaluated changes of static and fatigue mechanical properties of bone under athletic fatigue loading.Results:（1）Mechanical property tests showed that axial compressive E andσ_b were604 MPa and 8.81 MPa,respectively.Ulnae subjected to 30000 cycles fatigue loading didn’t experience stress fracture,but faster decrease of E_s in early 3000 cycles and obvious increases of average H_c and terminal fatigue degree were detected with loading frequency and macro peak strain.For athletic fatigue loading condition（1.5 Hz;3000με）,7000/12000 cycles were needed respectively to result in 20%/25%E_s decrease;（2）FEA of the 3D Solid model of cortical bone was done in ANSYS.Static simulation results verified the stress concentrating effect of osteons,while the fatigue simulations guaranteed bone safety（Fatigue Partial Usage 0.01‰,1500με,8000 cycles）under physiological exercises and warned risks of fatigue fracture（Fatigue Partial Usage 9.15‰,3000με,8000 cycles）under high intensity execises or trainings;（3）Based on the athletic fatigue damage model of rat ulna,we carried out multi-level tests and observations,which showed that microcrack generation and fatigue mechanical property decrease appeared right after fatigue loading,while the influences on material and static mechanical property of bone arose as fatigue damage accumulated.Bone targeted remodeling was activated and proceeded to repair fatigue damage to the full in order of stable osteocyte apoptosis,slowing bone resorption of osteoclasts and enhancing bone formation of osteoblasts.Conclusion:（1）FEA of the 3D Solid model of cortical bone by ANSYS verified the stress concentrating effect of osteons;（2）The athletic fatigue damage model of rat ulna we constructed could cause bone fatigue damage and induce bone targeted remodeling;（3）Microcracks in cortical bone under the athletic fatigue loading resulted in osteocyte apoptosis around,which activated bone targeted remodeling to repair fatigue damage continuously;（4）There are limitations of bone targeted remodeling in fatigue damage repair,beyond which,say in unscientific intensive training,bones would probably be exposed to high risks of stress fractures for rapid damage accumulation or excessive microcrack growth.