Preliminary Study Of The Biomechanical Behavior And Physical Characteristics Of Tantalum (Ta)-coated Prosthesis

The ideal joint replacement prosthesis which coated with coating materials should possess not only high mechanical strength of whole prosthesis, but also porous structure at certain size on the surface of prosthesis, to facilitate the bone tissue adhering to and growing into the micropores on prosthetic surface to achieve the postoperative prosthetic stability. With deepening of the study on adhesion between bone and prosthetic surface, the treatment measures of prosthetic surface have been increasingly diversified.In the recent years, bioceramic and metallic coating has become the major choices for the surface treatment of prosthesis. Hydroxyapatite (HA, Ca10[PO4]6[OH]2) and titanium (Ti) coatings have represented the porous biological ceramic coating and the metal coating respectively. Most of the prostheses are made by coating HA or Ti onto the surface of stainless steel or Ti alloy substrates through plasma spraying or thermal sintering process. Such coatings provide a porous structure on prosthetic surface with the functions of bone induction and conduction to improve the biological connection between prosthesis and human tissues, overcomes the drawbacks of former prosthesis in comprehensive performance such as mechanical strength, biocompatibility, etc. and becomes the ideal material for bone reconstruction in recent years[1‐5].However, with the increase in joint revision surgeries post arthroplasty, defects of HA- and Ti coated prosthesis have been gradually observed. It has been reported that the decohesion and degradation of HA coating and the migration of HA particles eventually lead to the prosthesis loosening. In addition, defects of HA-coated prosthesis in mechanical strength cause coating decohesion and fatigue fracture during the surgical procedure and under postoperative loading, to a certain extent, exacerbating the gap formation at prosthesis-coating and bone-prosthesis interfaces and facilitating the canal formation for HA particle migration. Following the gradual canal formation and the migration of degraded HA particles, prosthetic stability is severely affected.Although Ti coated prosthesis has some advantages in surface mechanical strength, corrosion resistance and stability of prosthesis coating, but there is still a gap between Ti and HA coated prosthesis in bone induction and bone ingrowth at early stage of implantation. Long-term clinical observation also found that there are prosthesis loosening caused by coating decohesion and shedding in some cases after prosthetic replacement which applied Ti coated prosthesis.With the diversifying in the methodology of processing prosthetic surface and the deepening in understanding the adhesion between the bone and the prosthetic surface, finding and researching coating materials that have good biocompatibility and can ensure the long-term postoperative prosthetic stability remain one of the main directions in the field of articular prosthesis study.Since the middle of last century, Tantalum(Ta) biomaterial has been used in medical applications. The advantages of Ta as coating material become evident gradually after the application of porous Ta sheath components on the prosthetic surface. The good biocompatibility and superior chemical stability of Ta material in comparison to Ti and other metals provide a theoretical basis for its possible application as prosthetic coating. But as a a novel prosthetic coating, whether Ta materials possess good biocompatibility and other necessary physical properties such as becomes our focus of the present study. Since biocompatibility and stability are the keys determining whether a new prosthetic coating can be used in clinic, so this study has preliminarily investigated the physical characteristics, biocompatibility, and biomechanical behavior of Ta as coating material on the prosthetic surface, aiming to provide experimental basis for its future clinical application.In this study we investigated the general physical characteristics, biocompatibility, and the relationship between biological behavior and physical characteristics of Ta coating prosthesis through three research parts. The first part is a research related to the general physical characteristics of Ta coating. The second part is a research about the biological behavior of Ta coating prosthesis. The third part is a research on the relationship between biological behavior and physical characteristics of Ta coating prosthesis.Ⅰ.The general characteristics of Ta coated material.1. The general physical characteristics of Ta coating interface.Objective: Investigat the general physical characteristics of Ta coating interface, such as the adhesive strength of coating , mechanical strength of coating surface, the chemical composition of coating and provid an experimental data for clinical application of the Ta coating material. Methods:(1)The Ta-coated testing specimens (with 25~35% of two-dimensional [2D] porosity and a coating thickness of 150μm) was prepared by sand blasting and plasma spraying process, The morphous of coating surface, the metallographical structure and the coating’s chemical composition were investigated by the metallurgical microscope, the scanning electron microscope(SEM) observation, the energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction analysis (XRD). (2) Medical Ti-6Al-4V substrate was used to make Ta-coated interface mechanical testing specimens (with 25~35% of 2D porosity and a coating thickness of 150μm) by sand blasting and plasma spraying process. The same substrates and specifications were applied to the control with Ti and HA coating .biomechanical testing. The coating adhesive strength and the surface hardness of coating were tested using WS-2005 coating adhesion automatic scratch tester and HV-120 Vickers hardness tester. Results: (1) SEM observations show that there are obvious roughness and clear pore structure on Ta coating surface. The coating has a higher density and bonds closely to substrate. No internal delamination and cracks have found in Ta coating. (2) The Vickers surface hardness (HV) of Ta coating (423.57±7.41) is significantly higher (P < 0.01) than that of Ti coating (202.86±4.35) and HA coating (123.29±3.99); The coating adhesive strength for Ta coating to peel is statistically higher than that for Ti coating and HA coating (P < 0.01). Conclusions: There is a stable pore structure on the surface of Ta coating material. The interface of coating and substrate is bond closely. Ta coating has a higher coating-substrate adhesive strength and surface mechanical strength, hereby is resistant to the damages resulted from the repeated loading changes and the relatively high shear stress, which has significance for maintaining the prosthetic surface porosity and characteristics of biomechanics.2.A study of the tribological properties of Ta coated biomaterialsObjective: To evaluate the tribological properties and the biotribological properties of Ta coated biomaterials for its future clinical application. Methods(:1)medical 316 stainless steel (0Cr17Ni12Mo2) materials were used as the base material for both the experimental group(group A)and the control group. The base material was pretreated with sand blasting, and Tantalum coating was prepared by plasma spraying system (Sulzer & Metco, USA). The system forms Ta coating with 25-35% 2D porosity and a coating thickness of 400μm. For the control groups (Groups B and C), base materials of stainless steel were coated with HA and Ti, respectively with the same specifications as in Group Ta). 0Cr17Ni12Mo2 rods were used as grinding materials, The test was done with a MMW-2 type computer-controlled abrasive wear test machine. The Wear morphous of coating surface and the amount of wear were observed and analyzed. In the meantime, the metallographical structure and the coating’s chemical composition were investigated by the metallurgical microscope observation and the energy-dispersive X-ray spectroscopy (EDX).(2)Ta-, HA- and Ti-coated test pieces with 316 stainless steel as substrates were used as the experimental group (group A)and the control group(Groups B and C) respectively. All coating with 25-30% 2D porosity and a coating thickness of 400μm. The pig femur were used as grinding materials, processed in accordance with standard wear test specimen preparation. The test was done with a MMW-2 type computer-controlled abrasive wear test machine. The Wear morphous of coating surface, the wear amount of test peices, the coating-bone friction coefficient, and the impact of bone-coating friction under load were observed and analyzed. In the meantime, the metallographical structure and the chemical composition on surface of test pieces were investigated by the metallurgical microscope observation and the energy-dispersive X-ray spectroscopy (EDX) respectively. Results: (1) The failure mechanism of Ta coating was mainly fatigue spalling. Compared with HA and Ti coatings, Ta surface coating suffered the least weight loss, and its surface wear resistance was higher than that of Ti coating and HA coating (P <0.01). (2) There was no significant reduction in the mass of the metal test pieces in metal-bone wear experiment. With time of testing, the friction coefficients gradually decreased, and finally settled around 0.40, and bone debris gradually deposited and accumulated in the porous surface of the coatings, forming a smooth film of bone debris which prevent further damage on bone grinding materials. Conclusions: (1) The failure mechanism of Ta coating was mainly fatigue spalling. Ta coating has good abrasion wear resistance and fatigue resistance, and has a superior mechanical characteristics than HA and Ti coatings for biological materials (prosthesis). (2) The bone-coating friction does not damage Ta coating on the prosthesis due to the formation and maintenance of a solid lubrication interface.Ⅱ.The biological characteristics of Ta coated material.1. A study of the cytotoxicity and biocompatibility of Ta coating prosthesis.Objective: To preliminary assess the cytotoxicity and biocompatibility of Ta coated biomaterials for its future clinical application. Methods:(1)Ta-, HA- and Ti-coated test pieces with Medical Ti alloy(Ti-6Al-4V)material as substrates were prepared for experiment of the cytotoxicity, and the leaching liquor of these test pieces were prepared for cell culture respectively(.2)The L929 cell were cultured and subcultured for the further cytotoxicity experiment.(3)The L929 cell were subcultured using the different leaching liquor of test pieces as culture solution, and the cytotoxicity grade of test pieces were evaluated through the MTT colorimetric test for cell culture respectively. Results: There are no significantly difference in the Relevant Growth Rate (RGR) and cytotoxicity among the Ta coating, HA coating and Ti coating (P >0.05), the cytotoxicity grade of Ta-coated test piece was 0 grade. Conclusion: Ta coating material has a good biocompatibility and toxicological characteristics for the human body.2. A study of periprosthetic osteogenesis on Ta coated prostheses.Objective: To observate the periprosthetic osteogenesis of Ta coated prostheses in vivo at early stage after its implantation, so as to provide evidences for the application of Ta coated biomaterials. Methods:(1)Ta coated prostheses were prepared for experiment of implantation. In the meantime, Ti coated prostheses was prepared as control group (2) Prostheses were implanted the femurs epicondyle of the New Zealand white rabbits, and histopathological observation, histomorphometric analysis, SEM observation, EDX analysis were employed to evaluate the periprosthetic osteogenesis of prostheses. Results: (1) observation of histopathology show that At Early Stage (4W) after the implantation, there are significant bone formation around the the Ta and Ti coated implants, and new bone tissues bond closely and stably to these all coatings. With prolonged time after the implantation, thickness of new bone tissues and number of trabecular bones around the two coatings increase continuously. 9W and 15W after the implantation, new bone tissues bond closely and stably to Ta- and Ti-coated implants. (2) Histomorphometric analysis of the bone tissue show that Ta- and Ti-coated implants has an obvious trends in periprosthetic osteogenesis At Early Stage after the implantation, though there are no significantly difference in periprosthetic osteogenesis among Ta- and Ti-coated implants(P﹥0.05). (3) SEM observation of the new tissues on prosthetic surface shows the attachment of a thin layer of new bony tissues to the surface of Ta- and Ti-coated prosthesis with regular, uniformed, and clear trabecular structure. (4) EDX analysis reveals that major component elements of the new tissues on surfaces of the two coatings are C, O, Ca, and P. Conclusion: Just like other metal-coated prostheses which have used in the recent clinic orthopedics, Ta coating has a good comprehensive performance on the stability and periprosthetic osteogenesis after its implantation.Ⅲ.The stability of Ta-coated prosthesis in vivo at early stage of implantation.Objective: To evaluate the stability Ta-coated prosthesis in vivo at early stage of implantation and provide evidences for the application of Ta coated prosthesis. Methods:(1)Ta coated prostheses were prepared as experimental group, Ti coated prostheses as a control group.(2)Prostheses were implanted the femurs epicondyle of the New Zealand white rabbits, and biomechanical testing (pull-out testing), histopathological observation, histomorphometric analysis, EDX analysis were employed to evaluate the ultimate shear strength of prostheses and the amount of osteogenesis around prostheses, both which effect the stability of HA-coated prosthesis in vivo. Results: (1) At Early Stage after the implantation, there are no significantly difference in periprosthetic osteogenesis among Ta- and Ti-coated implants(P >0.05). (2) There are no significantly difference in the ultimate shear strength between Ta- and Ti-coated implants (P >0.05). In the meanwhile, the ultimate shear strength of all kinds of coated prosthesis at 16 weeks is significant higher than that at 4 weeks after implantation. (3)Histological observation and EDX analysis after pull-out testing show that the periprosthetic bone tissues are evident on the surface of prostheses coating at early stages after implantation. Push-out test fractures the prosthesis and bone interface around Ta- and Ti-coated implants, and no coating residues can be observed on fractured bone section. (4) According to EDX analysis of the prosthetic surface elements before and after push-out test. It was found that the ultimate shear stress is not able to cause Ta and Ti coating damaged. Conclusion:(1) The periprosthetic bone ingrowth and the bone formation is not only factor that ensure stability of coating prostheses after its implantation. The adhesive strength and porosity of coating are also important factors for the stability of coating prosthesis in vivo. (2)Ta-coated prosthesis exhibits good biocompatibility with the bone tissue which is comparable with the current clinically metal-coated prosthesis. (3)Ta coating has a higher coating-substrate adhesive strength and hereby is resistant to the damages resulted from the repeated loading changes and the relatively high shear stress, which promotes the postoperative prosthetic stability and prolongs the life of prosthesis in vivo.