Research On The Counter-effect Of Constrained Dynamic Loading On Osteoporosis In Ovariectomized Mice

Backgrounds: Osteoporosis (OP) especially postmenopausal OP, characterized asestrogen deficiency induced deterioration of bone microstructure, lower BMD and higherfracture risk, is common among middle age and old age women. Rodent models havebeen playing important roles in studying and curing osteoporosis for their effectiveapplying in bony physiology, and researchers have attached increasing importance to OPmodel of mice. Also, researches in mechanobiology have gradually discovered thedelicate connections between stress/strain and osteo-operation which provide a newdirection for prevention and treatment of OP.Objective: Build up OP model of mice by ovariectomy and study the counter-effectof dynamic loading on OP in ovariectomized (OVX) mice.Method:(1) Image data of mice ulna obtained from Micro-CT were remodeled inMimics and imported in Ansys12.0, and the integral strain–diaphyseal strain curve andthe load-diaphyseal strain curve under limiting material constants using finite elementanalysis (FEA) were determined for the final loading conditions;(2) Female KM mice(2-month-old) were randomly divided into2groups: sham operation group (SHAM) andovariectomized group (OVX). The former were sham ovariectomized while the latterovariectomized, whereafter only right ulnas of the OVX mice were applied constraineddynamic loading (2500μ,15Hz)15min every3rd day over a period of4weeks.Finally, blood and all of the ulnas being divided into3groups (SHAM, OVX+Loadingand OVX) were harvested, then three-piont bending mechanical property, trabecularmicrostructure index, tissue alkaline phosphatase (AKP) and serum bonegamma-carboxyglutamic-acid-containing proteins (BGPs) content were tested andanalyzed.Results:(1) In FEA, we detected that under small strain conditions (with local strain<5000μ), fine linearity existed between integral strain and diaphyseal strain, as well asa definite relation between load and diaphyseal strain, contributing to the determinationof loading condition. Verification of the loading condition showed that, distribution of local strain in ulna under peak load experienced centeral tendency with a major valuebetween200-3000μ. For the high loading frequency (15Hz), smaller strain range from50-200μ could also be beneficial for bone formation.(2) Vaginal smears in the first5postoperative days showed that mice in the SHAM group had a normal life with a regularestrous cycle, while mice in the OVX group experienced dioestrum. An increasingweighting in the OVX mice suggested declined regulation of estrogen. A significantdecrease in serum BGP in the OVX mice might reflect lower activity of osteoblasts. Andthe worse mechanical properties of the ulna in the OVX mice4weeks after ovariectomyadded a further proof to the successful upbuilt of mice OP model.(3) HE-stainednon-decalcified slices and Micro-CT resuls of ulna trabecular bone qualitatively showedthat obvious deterioration or breakage of trabecular bone existed in the OVX group v.s.the SHAM group. The ulnar osteogenic activities in the OVX+Loading group weresignificantly enhanced, with higher levels of tissue AKP and serum BGP compared withthe OVX group, which provided molecular biological evidence for the counter-effect ofmechanical loading on OP. The microstructure of the trabecular bone in theOVX+Loading group improved with a higher B.Ar/T.Ar, thicker Tb.Th and smallerTb.Sp when compared with the OVX group. And better mechanical properties were alsoobserved in the OVX+Loading group with higher fracture displacement, fracture strain,fracture load and fracture energy when compared with the OVX group.Conclusion:(1) Using FEA, we determined rational loading conditions withpredictable strain distribution.(2)4weeks after ovariotomy, OP model in OVX mice wassuccessfully upbuilt.(3) The constrained dynamic loading could facilitate osteogenesis,improve trabecular microstructure as well as mechanical property and then effectivelyresist osteoporosis.