| Objective:(1) To design and establish cemented femoral stem prostheses model with intramedullary plug technique for rabbits.(2) To investigate the physical and mechanical properties of bone cement with the different weight proportion addition of n-HA/PA66,and to select a proper composition proportion for clinical application.(3) To evaluate the n-HA/PA66-PMMA and intramedullary plug in primary hip replacement, using a rabbit model, to investigate the osteogenesis of n-HA/PA66 at bone-cement interfaces and in the PMMA,and to measure the bonding strengths of the interfaces.Methods:(1) Femoral stem prostheses and nanometer-hydroxyapatite/polyamide66 (n-HA/PA66) intramedullary plugs were designed,and produced for rabbit cemented femoral stem prosthesis replacements. The prosthesis was fixed by polymethylmethacrylate (PMMA). In group BC (n = 8) the intramedullary plug was unused while in group PBC (n = 8) the plug was used. The pressure of distal femoral medulary cavity, the function of joint, the bonding strengths of bone-cement interfaces and prosthesis-cement interfaces were tested and measured. X-rays of the hips were performed to observe the location of prosthesis and scanning electron microscope (SEM) was performed to observe the structural feature of the bone-cement interface.(2) n-HA/PA66-PMMA were prepared by mixing polymethylmethacrylate powder with 120～380μm diametered n-HA/PA66 particles, they were divided into 5 groups according to there weight proportion of 0:10,1:10,2:10,3:10 and 4:10.The doughing time,Setting time, polymerization temperature, compressive strength and bending strength of different n-HA/PA66 - PMMA samples were measured respectively, there cross-sections were microphotographed and observed by SEM. The hemiprosthetic hips were fixed with different n-HA/PA66-PMMA in rabbit femurs, the push-out tests and pull-out tests were conducted to measure the bonding strengths of the bone-prosthesis interfaces and bone-cement interfaces.(3) Eighty rabbits were divided into four groups, i.e., BC group, HBC group, PBC group and PHBC group, with 20 rabbits in each group. Then the femoral heads were replaced with prostheses, which were fixed with PMMA in group BC and PBC, and with 3:10 n-HA/PA66-PMMA in group HBC and PHBC. In group BC and HBC the intramedullary plug was unused while in group PBC and PHBC the plug was used. The samples of bone were taken 2, 4, 8, 12, or 24 weeks after operation respectively. The new bone formation and osteogenesis process on the interface were observed by using the X-rays, sclerous tissues light microscope, interfacial scanning electron microscope,tetracycline labeling fluorescent microscope, IGF-1 histochemical and interfacial vitodynamics test.Results:(1) The pressure of distal cavitas medullaris increased after the cavitas medullaris of shaft of femur was blocked by bone cement. The pressure increasing of group BC was obviously superior to that of group PBC (P <0.05). Both groups had got good artificial joint location and function after replacement. In group BC the bone cement had been deeped into the cavitas medullaris at the distal end of stem of prostheses, while in group PBC, no migration of the plug and no cement leakage through the plug were identified on postoperative X-rays. The bonding strength of cement-prosthesis interface had no significant deviation between group BC and group PBC (P> 0.05). However, the bonding strength of cement-bone interface in group BC was obvious inferior to that in group PBC (P <0.05). The results of scanning electron microscope showed that there had tight bonding between bone and cement in group PBC; however, there were gaps in group BC and with many pores being in bone cement.(2) With increasing the additions of n-HA/PA66, Doughing time was significantly shortened and viscosity increased, setting time of the cement was significantly elongated and polymerization temperature decreased, when the n-HA/PA66 - PMMA weight proportions no more than 3:10, they were all better than the ISO 5833 standard. When the n-HA/PA66-PMMA weight proportions no more than 3:10, the n-HA/PA66 particles were well connected with BC in the compounds, compressive strength and bending strength significantly increased; whereas, as the weight proportion increased to 4:10, they decreased significantly, and there were many irregular gaps between PMMA and particles,interval porosity increased. The push-out tests and pull-out tests showed that the hemiprosthetic stems could be well fixed in rabbit femurs with all the n-HA/PA66-PMMA but the 4:10 weight proportion.(3) The X-rays showed that the cement mantles cannot fill the proximal cavitas medullaris very well at 2-4 weeks postoperatively in group BC and HBC, and radiolucent line between the cement and the cortical bone was observed, whereas, the cement mantles were well-distributed and the bone and cement were tightly binded in group PBC and PHBC, and no radiolucent line was observed. The bone density of the femurs were increased gradually at 8-24 weeks postoperatively in group PHBC and HBC, and no loosening signs of bone-cement interfaces, even the interfaces could not be differentiated at 24 weeks; whereas, prostheses loosening, dislocation and irregularly bone resorption were observed in group PBC and BC.Histological observation: In group PHBC and HBC, the interfaces were visible 4 weeks after operation, and osteogensis was active, there were abundant osteoblasts, mesenchymal cell hyperplasia and incursion on the surface of osteoid, the osteoid turned into woven bone through calcification 8-12 weeks after operation and new trabecular bone replaced the n-HA/PA66 particles and connected by bone-bridge 24 weeks after operation. On the contrary, a fibrous layer was consistently observed between PMMA cement and bone, no bony tissues could be observed in PMMA cement, and the loosening signs might be observed in some of the bone-cement interfaces. Osseous cells could growth into internal of the n-HA/PA66 intramedullary plugs, osteogenesis process was only on the surface of the plug at 2-12 weeks, new trabecular bone replaced part of the n-HA/PA66 until 24 weeks, but the main part of the material remained their original structure and cannot be degraded.Bone formation ability observation: Green fluorescent light of new bone tetracycline labeling was deeped into PMMA gradually with the n-HA/PA66 degradation, and there was only a little labeling near the PMMA in group BC and PBC. Many IGF-1 expressions in osteoblasts were found in the regenerated bones at 2-24 weeks in group HBC and PHBC, but it only at 2-4 weeks in group BC and PBC.SEM showed that the intimate contact between the n-HA/PA66-PMMA and cortical bone well interdigitated with the cement through the active bone ingrowth in group HBC and PHBC. Whereas, the pure PMMA will degenerate after long term fixing at 24 weeks, some of the interfaces would be broken in group BC and PBC.The pull-out tests showed that the shear strengths of prosthesis-cement interfaces at 24 weeks were all significantly decreased compared to that at 2 weeks(P<0.05). The push-out tests showed that the shear strengths of cement-bone interfaces using the n-HA/PA66 intramedullary plugs were better than no using at 2 weeks. The shearing strengths of cement-bone interfaces using n-HA/PA66 particles were increased gradually from 8 weeks to 24 weeks, while no using it has no improvements but slightly decrease.Conclusions:(1) Fixing the rabbit's femoral stem prosthesis by bone cement and applying intramedullary plug technique can improved cement penetration of proximal cavitas medullaris and relieve the intramedullary high pressure of distal cavitas medullaris. The normal joint function can be substitute after replacement. Having a stable fixation and being easy reproducible, this experimental model can simulate clinical artificial joint replacement.(2) When the n-HA/PA66-PMMA weight proportions no more than 3:10, n-HA/PA66-PMMA were demonstrated better physical properties and higher mechanical strength than the reference bone cement, could well fix the hemiprosthetic stems of the rabbits and bear early weight loading.(3) By sealing the femoral cavity, distal cement leakage is prevented and the proximal intramedullary pressure is increased. Thus the ability of the cement to interdigitate with bone is enhanced, the shear strength of the bone-cement interface is strengthened at 2-8 weeks, but it is not strengthened at 12-14 weeks. The osteogenesis of n-HA/PA66 intramedullary plug in cavitas medullar is at 24 weeks, but the main part of the material remained their original structure and cannot be degraded. The osteogenesis of 3:10 n-HA/PA66 particle can bring the new bone formation in PMMA, thus its mechanical properties could be well maintained. 3:10 n-HA/PA66 particles impregnated bone cement fixing the rabbit's prosthesis can achieve both the initial fixation offered by the bone cement and long term fixation offered by the bony tissue ingrowth. |
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