An Experimental Study On The Stress-Shielding Effects Of Locking Compression Plate

BackgroudIt is well known that the stress tranfer between the different device whenyoung’s moduli of the devices are different;this is defined as stress-shielding.Assuch,a stiffer material will have a higher elastic modulus and undertake muchload.In orthopedic application,implant materials result in bone loss due tostress-shielding, and is followed by necrosis,which induces porosis, delayedbone healing and other complications. Two processes that affect the bone afterplating became apparent. One is a short-term process, possibly caused by theinterference of the plate with the blood supply to the cortex.The secondindependent process is caused the effect of stress-shielding.Two effects is relatedwith the areas of contact with plate and the rigidity of the plate.With theupgrading of material of internal fixators and the developing of plate designed toreduce of the region of contact with bone.As a―internal fixator‖,LCP have the feature of external fixation,which contact minimally with bone,moreoverpossess the advantage of small distance between bone and implant.Biomechanically,they can be far stiffer than external fxators,becausesubcutaneous plates are located much closer to the bone surface than externalfxator bars. Additionally, the unique feature of the desigened,can adapt toaccomplish the satisfactory fixation with techninique of MIPO and avoid toincrease the risk of infection and other wound and fracture healingcomplications. As a newer-designed internal fixtor, Locking plate technologycombined with techninique of MIPO，how about the degree of stress-shielding?It is reported rarely in recent years. With LCP presenting to the public andapplying extensively in clinical,studying the property of Biomechanics becomethe focus and want to acquire more clear index of the effects or explore the basisof improvement.1. Preparaing the animal models, obtaining materials from samples andanalyzing the histology.Objective:To prepare the animal models that is applied with two differentplates to fix the intact femora.One side is fixed with LCP,the other with LC-DCPin the same dog. To understand the difference of BMD through DEXA and findthe pathological changes by radiologic examination in different periods.Thenexecute,obtain materials from samples and explore whether there are themarked differences in Microcosmic cell level by histologic slices in6^thweek,12^thweek,24^thweek. Methods：Twenty-four adult dogs are chosen for theexperiment. Adopting the technique of MIPO, we fix bilateral intact femorawith adult’s forearm plate made from pure titanium KangHui. The left femora isfixed with LCP and the right femora is fixed with LC-DCP in the same dog.Plates with two different contact areas are fixed to bilateral femora in periods of the6^th、12^thand24^thweek, and then the dogs are executed to obtainthe materials. After removing plates, the changes of bones fixed with differentplates can be assessed through the radiographically, histologically,histomorpho-metrically,light-microscopy,DEXA. Result:The side plated withLC-DCP has marked callus formation through x-ray exmanition,probably thestimulating reaction of the cortex due to contacting tightly with the surface ofbones and interference from the plate.Results of histologically,DEXA reveal thatchanges of cell morphous,degree of osteoporosis and BMD of bones in side ofLCP have obvious advantage compared with the side of LC-DCP.Conclusion:The consequence of histological and DEXA show that plates fixedwith LCP have conspicuous advantages compared with LC-DCP. Theseadvantages lie in reducing effectively the degree of porosis from infer-platebone. That is to say that plates fixed with LCP can decrease obviously theeffects of stress-shielding for areas of plated-bones.2. The analysis of three-dimensional MicroCT image with two differentplated-bones.Objective: Applying the machine of MicroCT made from U.S.A GEexplorelocus SP,we observe the changes of image reconstructed exactly in the bonecross-sectiongal areas,the cortex,regeneration bone of pin-track under the plateand caculate the TMC,TMD of specimens at different time through softwareABA for two different samples of plated-bones.By analyzing the statistics,itreflects the effects of stress-shielding from two different plates according to thechanges of quantities. Methods：Apply the MicroCT of U.S.A GEexplore locus SPto acquire images,reconstruct exactly three-dimensional images with software ofMicview V2.1.2,analyze the statistics of skeleton with software ABA.Result:The appearance of images show that the reconstruction images in bone cross-sectional areas and the degree of puffing in the cortex at the side ofLC-DCP have marked compared with the side of LCP,on the contray,theregeneration bones of pin-tack at the side of LCP exceed that of the side ofLC-DCP.We analyze the specimens of TMCand TMD with the statistics at thedifferent time and discover that there are significant differences on the TMDbetween both sides in statistics（P<0.05）,while there is no significant differenceson the TMC in the6^thweek in statistics（P>0.05）, and they are differentobviously in the12^th,24^thweek.The TMD and TMC of LCP side is more than theside of LC-DCP, but the bone mass decreases greatly,especially the side ofLC-DCP. Conclusion:The degree of bone loss caused by the effects ofstress-shielding of LCP is lower than that of the side of LC-DCP,that is to saythe effects of stress-shielding of LCP on plated-bones is smaller than that ofLC-DCP side.3. The analysis of biomechanics with two different plated-bones.Objective:Applying the machine of INSTRON made from England,we measurethe specimens of the properties of biomachanics at different time with two differentsamples of plated-bones. By analyzing the statistics,it reflects the effects ofstress-shielding from two different plates according to the changes of properties ofbiomachanics. Methods：Apply the INSTRON of England to measure the specimens of theaverage bending strength and average compressive strength through the test of three-pointbending and the test of compression. Apply matched t-test to analyze the statisticsindividually. Result: We analyze the specimens of the average bending strength andaverage compressive strength with the statistics at the different time and discoverthat there are significant differences between both sides in statistics（P<0.05）.Theaverage bending strength and average compressive strength of LCP side is more than theside of LC-DCP, and the properties of biomachanics of specimens in24^thweek have improved obviously compared with the specimens in12^thweek. Conclusion:The specimens of the properties of biomachanics of LCP side is stronger than that ofthe side of LC-DCP,thereby to show the effects of stress-shielding of LCP onplated-bones is smaller than that of LC-DCP side.