Establishment And Biocompatibility Evaluation Of Ectopic Prefabricated Vascularized Porous Polyethylene (Medpor(?))

Background and ObjectivesIn reconstructive medicine,biomaterials are used for the augmentation or replacement of tissues.If autologous transplant materials are not sufficiently available,reconstructive options for bone and cartilage tissue defects include the use of Porous Polyethylene(Medpor?)implants.Medpor is manufactured from high-density pure polyethylene that is sintered to create a series of consistent interconnecting pores with an average pore size of 240 μm while maintaining a flexible framework.This porous character permits ingrowth of vascularity,bone,and soft tissue to reduce the incidence of infection while increasing the strength of the implant.This highly stable and flexible alloplast has been approved for use in humans and is available in rectangular blocks or preformed anatomical shapes for specific craniofacial applications.It is believed that well-vascularized porous implants can reduce the incidence of implant migration and extrusion.Fibrovascular ingrowth into porous implants provides biologic anchorage,which limits migration and extrusion,and is perceived to lower the incidence of infection.Moreover,it is well known that enhanced fibrovascular ingrowth reduces the incidence of implant exposure.If fibrovascular ingrowth begins,strong resistance to infection often follows.They hypothesized that the porosity of polyethylene initially offers a greater surface area to harbor bacteria,but the subsequent vascular ingrowth protects the implant from infection.Once vascularization into the pore is complete,porous polyethylene implants have resistance to infection.A rapid and sufficient vascularization is essential for the survival and functional integration of tissue constructs after implantation.Interference with or delay of these events promotes implant infection and dislocation,which ultimately might result in implant rejection.Microtia reconstruction to recreate a malformed ear remains a surgical challenge.The classic and still most popular technique requires grafting using several autologous cartilaginous ribs.Good cosmetic results can be obtained using autologous rib cartilage grafts for ear reconstruction,with limited incidence of resorption or extrusion of the grafted cartilage.This,however,involves a major additional procedure and creates a permanent defect at the donor site,as the rib cartilage does not regenerate.An alternative technique utilizes a Porous Polyethylene(Medpor)auricular framework.However there are applications which are less favorable for implant integration,such asreconstructive ear surgery and rhinoplasty,where blood supply is weak.Various strategies have been investigated for improving the vascularization of biomaterials,including coating of implants with autologous material,such as chondrocytes,adipocytes,adipose tissue derived microvascular fragments,or a cellular dermis as well as with specific growth factors,e.g.basic fibroblast growth factor(bFGF)or vascular endothelial growth factor(VEGF)and novel surgical approaches to enhance the nature of the vascular networks formed.In prefabrication approaches,the scaffolds are implanted in a “donor” tissue location that would promote greater neovascularization than the defect location.After a period of prefabrication,the scaffolds with an extensive vascular network would then be transferred to the recipient site.Its cellularization with autologous human cells will be most desirable,as it would avoid immuno-rejection.Medpor implants were implanted into abdomen of rats for establishment of ectopic prefabricated vascularization.The implants were harvested at 1,2,4 weeks.We decided the best time for ectopic prefabricated vascularization based on the degree tissue grew in.Then Medpor implants were implanted into abdomen of 20 rats subcutaneously for establishment of ectopic prefabricated vascularization and harvested at the best time based on the result before.Medpor implants with or without ectopic prefabricated vascularization were randomly implanted into subcutaneous pockets of the right and left in the back.After 2 weeks,the samples were excised for histological analyses.MethodsPart 1Establishment of ectopic prefabricated vascularized Medpor1.We used sheets of Medpor?(DIM 38 x 50 x 0.85 mm)for our experiments,which were carefully cut into small pieces of 10 x 10 x 0.85 mm with a scalpel.For subcutaneous implantation,SD rats were anaesthetized by intraperitoneal injection(i.p.)of pentobarbital sodium.The ventral side of each rat was shaved and cleaned with betadine solution and then sterile saline.Two incisions of 1 cm in length were made and dissected to create two subcutaneous sacs into which two Medpor? sheets were allocated randomly and inserted,respectively.The wounds were then closed with silk sutures.2.The SD rats were sacrificed on week1,2 and 4 post surgery and implants were collected for histological analysis.3.For histology,implants were fixed in 4% paraformaldehyde for 24 h,dehydrated and embedded in paraffin blocks.Sections of 5 mm in thickness were cut,deparaffinized,stained using standard hematoxylin-eosin staining and visualized by microscopy,the inflammatory reaction and fibrovascular ingrowth were observed.Photographs were taken with a light microscope.Part 2Biocompatibility evaluation of ectopic prefabricated vascularized Medpor?1.SD rats were anaesthetized by intraperitoneal injection(i.p.)of pentobarbital sodium.The ventral and dorsal sides of each rat were shaved and cleaned with betadine solutionand then sterile saline.Prefabricated vascularized Medpor? were extract for reserving.Two incisions of 1 cm in length were made and dissected to create two subcutaneous sacs into which Medpor sheets with or without ectopic prefabricated vascularization were allocated randomly and inserted,respectively.The wounds were then closed with silk sutures.The SD rats were sacrificed after 2 weeks and implants were collected for histological analysis.The 20 Medpor? sheets were as the control and the 20 prefabricated vascularized Medpor? were as experimental group.2.For histology,implants were fixed in 4% paraformaldehyde for 24 h,dehydrated and embedded in paraffin blocks.Sections of 5 mm in thickness were cut,deparaffinized,stained using standard hematoxylin-eosin staining,Masson trichrome staining and immunohistochemical detection of CD31 and visualized by microscopy.Results1.Gross Examination1.1 Establishment of ectopic prefabricated vascularized Medpor?The implants harvested after 1 week showed external surface of the implant were surrounded by transparent fibrous film.Vessel within the peripheral fibrovascular tissue could barely be seen inside the implants.The implants harvested after 2 weeks showed partial fibrovascularization extended toward the center of the implant and fibrovascular tissue surrounded the external surface.Vessel within the peripheral tissue could be seen.The implants harvested after 4 weeks showed fibrovascular tissue surrounding the external surface of the implants were dense.Fibrovascularization extended to the center of the implant(100%).The implants were fixed in the surrounding tissue stably.1.2 Biocompatibility evaluation of ectopic prefabricated vascularized Medpor?Two groups of SD rats recovered well after surgery.There were no complications such as infection.All the SD rats tolerated the implants well.The experimental group showed dense fibrovascular tissue surrounded the external surface of the implants and the intertrabecular spaces were filled with it.The implants were fixed in the surrounding tissue stably.The intertrabecular spaces were partially filled with peripheral fibrovascular tissue in control group.2.HE stained2.1 Establishment of ectopic prefabricated vascularized Medpor?(1)At week 1 after implantation,histological examinations showed that the implants were surrounded by vascularized granulation tissue,the in growth depth of peripheral fibrovascular tissue(from the external surface of the implant edge toward the center)was about 0.1mm.(2)At week 2 after implantation,histological examinations showed that fibrovascularization of the intertrabecular spaces appeared to occur in stepwise increments,the implants were surrounded by vascularized granulation tissue,the ingrowth depth of peripheral fibrovascular tissue was about 0.6mm.(3)At week 4 after implantation,histological examinations showed that vascularized granulation tissue filled the pores of the polyethylene biomaterial.100% fibrovascularization was noted.Mature fibrous capsule could be found in peripheral.2.2 Biocompatibility evaluation of ectopic prefabricated vascularized Medpor?The 20 prefabricated vascularized Medpor? pieces were as experimental group while the 20 Medpor? pieces were as the control.Full-thickness fibrovascular ingrowth into the implants was observed in the experimental group.Plenty of blood vessels could be detected in the fibrovascular tissue surrounding the external surface of the implant.The ingrowth depth of peripheral tissue in control group was about 1/3.All surfaces showed a fibroblast layer in contact with the surface.This layer increased slightly at the edges of the implant as would be expected due to mild edge irritation.There is little or no indication of any inflammatory cell infiltration on any of the surfaces.For all implants,collagen appeared external to the cell layer,away from the implant consistent with a mild fibrotic response.3.Masson Stained3.1 Biocompatibility evaluation of ectopic prefabricated vascularized Medpor?The external surface of the implants was surrounded by fibrovascular tissue and collagen appeared external to the cell layer in both groups.The depth of fibrovascular tissue growth into the implant in was 0.85mm(100%)in the experimental group while 0.6mm(67%)in the control group.4.Immunohistochemical Stained4.1Biocompatibility evaluation of ectopic prefabricated vascularized Medpor?The expression of CD31 in the experimental group significantly higher than in control group.An increased microvessel density(MVD)of prefabricated vascularized Medpor compared with Medpor was demonstrated by immunohistochemical detection of CD31 as a marker of endothelial cells.Conclusions1.At week 1 and week 2 after implantation,the ingrowth depth of peripheral fibrovascular tissue(from the external surface of the implant edge toward the center)was about 0.1mm and 0.6mm.The histological examinations showed that fibrovascularization of the intertrabecular spaces appeared to occur in stepwise increments.At week 4 after implantation,histological examinations showed that vascularized granulation tissue filled the pores of the polyethylene biomaterial.100% fibrovascularization was noted.Mature fibrovascular tissue could be found in peripheral.Accordingly,4 weeks is best for prefabricated vascularization.2.The modeling of an ectopic prefabricated vascularized Medpor? is available in SD rats.3.The depth of fibrovascular tissue grown into the implants in experimental group was deeper than that in control group.4.An increased microvessel density(MVD)in experimental group compared with in control group was demonstrated by immunohistochemical detection of CD31 as a marker of endothelial cells.5.The method is instructive for auricle reconstruction with Medpor? in clinical.