tTlKIN512Trebacz,H.2001."Disuse-inducedDeteriorationofBoneStrengthisnot Stopped AfterFreeRemobilizationinYoungAdult Rats". JournalofBiomechanics,34,1631-1636.SummaryThe aimofthe present article wastoexamine theeffectsofaperiodofunilateralhindlimbimmobilization, followed by a periodoffreeremobilizationonfemorainrats.Afteraperiodofimmobilization bonereachesa newsteadystate,andmetabolismre-stabilizesatalevelequivalenttothedecreasedbonemassandloadlevels.However,it hadpreviouslynot beeni-i’Uii-f’ten’determinedwhetherextine4onofimmobilization priortotheonsetofanewsteady statewouldstopthelossofbone,which was a goalofthisstudy.Also inquestion waswhethertheosteopeniainduced by immobilizationistemporary.orpermanent, andwhethercompleterecoveryofthe boneispossible.Forty adultmale rats wererandomlydividedintotwocontrol andtwoexperimentalgroupslabeledCO,C4, E0,andE4,respectively.Posterior rightlegsofrats in bothexperimentalgroupswereimmobilizedbysecuringthe legtothe body with bandagesandpaddedtape,whilecontrol rats were allowedtomovearoundunhindered. The rats were housedandfedinthismannerfor a periodof2weeks, after whichoneofthecontrolgroupsand oneoftheexperimentalgroupsCOandEOwerekilled.Thelegsofrats in the second experimentalgroup wereremobilized,andthe rats were allowedtomove freely abouttheircages.After fourweeksoflreeremobilization, the remaining controlandexperimental groupsC4andE4werekilled.Bothposterior femoraofallratswere removedandcleanedoftheirsofttissue priortoanalysis.2The femoraweremeasuredforlength,minimumdiameter,andmaximumdiameter,andtotalcross-sectionalarea was subsequentlycalculated.Both femora were subjectedtomechanicalthree-pointbendingtests,with a1mmdistancebetweenthetwosupports,andadeformationrateof2,mm1min.Aload-deformationcurved was developed,fromwhich theresearcherwasabletodeterminethe maximum bending moment, theyieldpoint, theamountofworktofailure, andthe stifihessofthebone.Theresearcherremovedmarrowfromthe brokenfemora,after which thefemorawere dried in heat for 24-hoursandweighed. The boneswereashedinafurnace,weighed again, and thepercentageofmineralization wascalculated.Varioustypesoft-testswereusedtotestmeandifferencesbetween values in the correspondinggroupsCOwascomparedtoEO,andC4toE4.TheresearcheralsoconductedmultipletestsoffemoralpropertiesamongstcontrolCOvs.C4andamongstexperimentalBOv.E4oups,alonwit7M‘wi’tt-wtcQftca-ecomparisonsofrightvs.leftfemorainbothgroups.Alpha levels foralltests weresetat.05.,Vçf1itAlthoughthe rats were adults at theonsetofthe experiment,controlrats grewsteadilythroughoutthestudy.Experimentalrats lost weightduringimmobilization;afterremobilizationthey began rapid weightgain.TestsbetweenCOandEOshowed thattwoweeksofimmobilizationresulted in a lower bone density inlothrear femora for the experimentalrats.However,therewerenosignificantdifferencesbetween thesegroupsinmass,size,mineralization,or mechanical parametersofthefemora.After fourweeksofremobilization, thedensityoftheright femora in theE4group wasstillsignificantlylower comparedtothatofthecontrolgroupC4,andmineralizationwasalsosignificantlylowerfortheE4gloup.1IkfLgAdditionally,statistically significantdeficitsmmechanicalparameterswerepresen,frTheE4ratshadalowermaximumbendingmoment, alower yieldpoint,and lessstiffness than theC4rats.Afterremobilization,despiterapidbody weightgainandanincrease inmass,lengthanddensity3ofbothrear femora, theE4group did notexhibitachangeinmechanical parametersoftheright,previouslyimmobilizedfemora, while theirleftfemora improvedontheseparameters.CritiqueAtfirstglance,theresultsofthisstudy seemsomewhat confounding.Itwouldseemlogicalthatbone would be weakerafterimmobilization,andregainstrengthaftermobilization.Theresultsfromthisstudyarejustthe.opposite,.butareexplained throughafewsubtledifferencesinbonecharacteristics.The author pointsoutthat themedullarycavity in the rightfemoraoftheE4group increased in size after remobilization.Additionally,mineralizationdidnotriseinthisgroup.The authorsuggeststhattheseeffectsarerelatedtoanincreasedlevelofresorptionduetoimmobilization, andthatthe developmentofbone inexperimental rats waseitherblunted,or could not equal the rateofrapid weightgainthat theE4group exhibited afterremobilization.The change in medullary sizeandlowermineralizationare thoughttobe thereasonsfordecreasedmechanical parameters exhibitedduringthree-pointbending.Thepresentstudyreportssomefindingsthatarecontradictorytosomeofthepreviousresultsinthearea,which arepresentedinTable5ofthearticle.Kaneps,Stover,andLane1997conductedastudyona groupof20mongreldogs.Utilizing a similarstudydesign,twoexperimentalgroups,ofseven dogs each, had theirright forelimbs immobilized, while the othersixdogsweredivided intotwocontrol groups: After16weeks, one experimentalgroupandonecontrolgroupwere euthanized. The remainingexperimentalgroup was remobilizedfreelyfor16weeks,after which the dogscompletedanadditional 16-weektreadmill exerciseprogram.Resultsfromthisstudyshowed anearlycomplete recoveryofbonemechanicalproperties, withonlythecorticalbonefailurepointb6injhiglierincontrols.ThisrecoveryofmechanicalpropertiesisindirectcontrasttothefindingsofTrebacz2001.Also contrasting the current_7Jtrct4LC14resultswas the fact that the bonesofdogsthatwere euthanized after16weekshadsignificantlylowervaluesfornearlyallmeasures,includingyieldpoint,stiffness,andbone mineraldensity.Thesecontrastingresultsaremostlikelyduetothedifferencesin durationofthestudies.Sincethedogswereimmobilizedforaperiodof16weeks, they most likely reachedanewsteadystateofbone metabolism,causing thedecreasedstrengthat this point. Also, the dogshada32-weekperiod torehabilitateafter remobilization, including a16-weektreadmillprogram.Itwouldhave been interestingtoseetheeffectsofarehabilitationprogramonanadditionalgroupofexperimentalratsin theTrebacz2001study.Thiswould provide a more thoroughinvestigationofthe healingpropertiesassociatedwith immobilization andremobilizationbeforeanewsteadystateofbone metabolismisreached.One weaknessofthestudyisthe lackofdescription about ratbehaviorfollowingremobilization.The author makes many comparisonsofthe right and left rear femoraoftheexperimentalrats after remobilization, showingthatthe left,undisturbedfemur underwentnormal growth,andshowed normal mechanicalparameters, while theright femur lagged behind.However,the only description givenisthatwithinone dayofremobilizationexperimentalrats"usedalllegswhilemoving"p.1632.Thisleavesspeculationastothe extentthat the rats