Optimal Selection And Experimental Study For The Retention Of Expandable Implant In Osteoporosis

Background: Implant technology is a milestone in the development of oral medicine, and prosthetics with implant have showed good function and aesthetic results compared with the traditional dentures. Implant prosthetics were also recognized as the first choice for restoration of missing teeth with a reputation of the third denture evolution, and more and more people have been benefiting from it. The perfect function of an implant is depended on ideal osseointegration, Whether an implant will be successfully osseointegrated or not is determined by implant design, implant surface treatment, the quality and quantity of jaw bone, surgical techniques and load conditions, among which implant design and bone quality are extremely important. A large number of studies have shown that the quality and quantity of jaw bone reduction will lead to prolonged healing time, and even lead to implant failures. The reduction of bone mass often occurs in elders who suffer from a retrogression process called osteoporosis, which is a systemic disorder of bone metabolism, including bone microstructure damage, bone mineral proportion and bone matrix decrease, as well as cortical thinning and trabecular bone reducing, so to resulting in bone fragility and fracture risk increased. The bone defects caused by OP will weaken the support of the bone tissue to implant and extending implant healing time, thus affecting the initial implant stability and long-term bone osseointegration,whereas would easily lead to implant loosening and falling. In order to improve the initial and long-term retention of implants, researchers have attempt a variety of optimizations, and these methods have significantly improved the implant early and longer retentions and increased bone-implant contact in normal bone, and simultaneously, shortened the osseointegration time. Nevertheless, due to the specificity of OP, the poor bone properties still affect the bone-implant osseointegration unfavorably. Thus, it has important clinical significance to explore effective way of correcting adverse results to bone-implant healing in osteoporosis.Objectives: This study aims to design an expandable implant (EI), and to evaluate its effects in osteoporosis.i) The design of the contour and internal structures for EI. Design and produce an implant which would be expanded at its apical part, then construct its models with computer software, and made the optimal selections of the implant shape and expansion ratios for EI.ii) Fabricating with EI according to the optimizations, then following the in vitro implant inserting and removing experiment and implant intensity measurement,so as to evaluate the mechanical properties of the EI.iii) The establishment of osteoporotic animal models. Female sheep were ovariectomized (OVX) one year before implantation take place. Measuring the bone density in both spine and mandibular angle in the period of pre-OVX and one year after OVX to assure the animal were osteoporosis and prepare for the further experiment.iv) Investigation of retention experiment in animals for EI. Evaluate the results of EI after comparing with the control group in osteoporotic sheep implantation. Materials and Methods:I) Design of structures for EI, and establish the adaptively three dimensional finite element models of mandible bone assembled with EI. Design with EI according to literature reports and that was regularly used in orthopedics, trying to make it in simple structures with efficient expansions. Establish models of EI and mandible bone using Pro/E software for frame assembles and Ansys Workbench software for two-way parameter seamlessly transfer functions in finite element analysis. Then the accuracy examination by simulating mechanic loads was followed.II) Optimal expansion angle and expansion length rate selections.Set expansion angle (A) and expansion length ratio (R) as variances. The ranges of A and R were respectively 0°to 4°and 1/6 to 5/6. Simulated loads of 100N and 30N were applied along the implant axial and buccolingual directions. The maximum equivalent stress (Max EQV stress) in jaw bones and the implant-abutment complex, and the maximum displacement (Max displacement) in implant-abutment complex were evaluated. The combination of A and R was selected when the Max EQV stress and Max displacement were the relatively low.III) Manufacturing of EI and the implantation in vitro bone study with biomechanical test.EI was fabricated with A in 2°and R in 3/6 rate, a relatively better combination. The length and diameter of the implant were 13mm and 4.3mm. The implant could be expanded by a core in its inner structures. The mechanical intensity was tested by cyclical fatigue test and three-point bending test. The tests were carried out in vitro jawbone of aged sheep, and the inserting, expanding and turning out tests were also carried out in order to evaluate the actual application of EI.IV) The establishment of osteoporotic animal models.Twelve female sheep age in 2.5 years with the weight about 50Kg were selected, and they were bi-ovariectomized (OVX) to simulate the postmenopausal animals. The animals were fed with lower calcium contained foods as well as being injected with hormones (Methylprednisolone, 0.45mg/kg/d,for nine months). All animals were tested the bone mineral density by Dual energy X-ray absorptiometry (DXA) in the lumbar spine and mandibles before bi-OVX and 12 months after that. Simultaneously, with the DXA, the Micro-CT evaluations for trabecular bone was carried out to analysis the osteoporotic animal were achieved.V) The study and results analysis about inserting EI in the osteoporotic bone by animal experiment.24 EI were inserted in the bi-mandibular angles of the sheep with the same amount of normal implant as the control group. The samples were divided into two different groups according to the sample harvested time (3-month and 6-month). The bone formation around the implants were observed by X-rays, three-dimensional CT reconstructions, pull-out tests, implant stability quotient, histomorphlogy and micro-CT analysis. These results were used to evaluate the retention of the implants and to compare the effects of EI with normal implant. Results:I) After studying the outer contour and the internal structure of EI, an expandable implant which could be expanded by screwing its inner core to the root part was preliminarily designed. By using computer software, three dimensional finite element models of osteoporotic mandible bone assembled with EI was established.II) In the Ansys Workbench DesignXplorer environment, the expansion angle (A) and expansion length ratio (R) were selected as optimal variable parameters. Results showed that A and R favored stress distribution in jaw bones under axial load and buccolingual load, respectively. A between 1.5°to 2.5°and R between 2/6 to 3/6 were the combinations with optimal biomechanical properties for expandable implant in the osteoporotic mandible, for which made the lowest Max EVQ stress and the best implant stability.III) The EI was manufactured with parameters according to the optimal selection results, and in this experiment, a combination of 2°for A and 3/6 for R was chosen. Then followed the implant in vitro bone study with biomechanical test. In the test, EI could endure loadings of 200-250N for 150,000 times in the cyclic fatigue test, there was no curve and collapse in EI with a load of 1.05±0.45KN in the Three-point bending test, and the in vitro bone study manifested that EI could insert and effectively expanded with larger remove torques.IV) The osteoporotic sheep models were established after one year of the bi-OVX. The BMD in the spine of the sheep were 1.066±0.90 g/m3 and 0.752±0.05 g/m3 before and after one year of the OVX, respectively. The BMD of the mandibular angles were 0.846±0.82 g/m3 and 0.524±0.09 g/m3 , respectively. These data demonstrated a significant results in statistics with P values <0.01. The micro-CT images of the cervical femoris also showed bone trabecula decrease in the different periods, and this was a further verification for the osteoporotic sheep model.V) Results in animal experiment: The ISQ values in EI and the control groups were respectively 63.7±4.5 and 55.2±5.8 after the 3-month of implant insertion; that were 83.5±6.3 and 78.3±7.6 after 6-month of implant insertion, respectively. The pull-out test showed that the Max pull-out force in EI and the control groups were respectively 490.6±72.7 and 394.5±54.5 after the 3-month of implant insertion; that were 587.6±65.3 and 448.5±78.6 after 6-month of implant insertion. These results in Max pull-out force were statisticly significant. Quantitative histological analysis in the stained histological sections manifested significantly higher BIC results in EI than in the control group, they were respectively 64.57±4.88 and 50.46±5.32 after 3-month of insertion,68.31±5.79 and 56.85±5.04 after 6-month of insertion. Micro-CT analysis demonstrated that the Tb.N,Bv/Tv and Tb.Sp around the implant were significantly better in the EI group than the control group.Conclusion:I) This design of EI had simple structures and easy applcations. Using CAD/CAE technique, the principal variance were optimally selected,which played an important role in evaluating of biomechnical parameters and choosing of dimensions for EI.II) This design and manufacture of EI could withstand a certain degree of fatigue test and stress test, showing a certain mechanical strength.III) The method of establishing osteoporotic sheep models by OVX and hormone injection was highly reliable, for the BMD decrease met the requirements, and the large animal model were convenient for experimental study.IV) The EI showed better biomechanical properties, higher bone-implant-contact, and larger peri-implant osseo quantities. Hoping that EI would be a useful implant structure improvement in osteoporosis.