| Background: The theory of osseointegration, which was proposed by professor Br?nemark, was regarded as a milestone in the history of dental implantology. Osseointegration was originally defined as a direct structural and functional connection between ordered living bone and the surface of a load-bearing implant. This definition has two meanings. One is that no scar tissue, cartilage or ligament fibers are present between the bone and implant surface, and the other is that the status of osseointegration could be maintained under functional loading.Application of loads to implants at the time of surgical placement or shortly thereafter is called immediate implant loading, and this loading protocol is becoming more common with acceptable success rates. Immediate loading has been the focus of much research. In addition to the establishment of the theory of osseointegration, Br?nemark set some recommendations ensuring durable osseointegration of dental implants. The most important recommendation was using a stress-free healing period of 3?6 months before loading, which was also considered as the delayed loading protocol. Early /immediate loading was identified as the dominating risk factor for osseointegration by Br?nemark et al. The rationale for such a long delayed loading period was that premature loading may lead to fibrous tissue encapsulation instead of direct bone apposition. The main disadvantage of this delayed loading protocol is that the treatment time is lengthened. The discomfort, inconvenience, and anxiety associated with waiting period remain challenges to both patients and clinicians.However, the necessity of waiting to load an implant was not scientifically but rather clinically based. It is therefore justifiable to question whether this healing period is an absolute prerequisite for obtaining osseointegration. In this background, immediate loading of dental implants was put forward, and received good results. The protocol, under which the prosthesis is attached to the implants the same day the implants are placed, is defined as immediate loading. The use of such protocols has obvious advantages for the patient, because, for example, treatment time and the number of surgical interventions are reduced.According to most researchers, primary implant stability is the most important determining factor on the success of immediate loading, which is related to bone qualities and quantities, implant dimensions/designs, and surgical techniques. It is thought that the success of immediate loading, to a great extent, is dependent on clinicians'ability to detect and monitor implant stability. Presently, various methods have been suggested to define implant stability: tapping test (percussion of the implant with a mirror handle), insertion torque analysis, removal torque analysis, and the Periotest method. However, these methods have many disadvantages. The tapping test is empirical and not sensitive enough to monitor different degrees of implant stability. But the insertion torque analysis can only be used during implant placement. For the removal torque analysis, non-physiologic forces usually are used. So it is not suitable for long-term clinical stability assessing. On the other hand, these two methods also have the disadvantages of invasiveness and inaccuracy. Periotest is less invasive and more accurate, but many clinical studies indicated that lack of resolution, poor sensitivity, and susceptibility to operator variables limited the use of Periotest in measuring implant stability.The method of resonance frequency analysis (RFA) has been introduced into the oral field to quantitatively monitor dental implants'stability and is considered as an ideal technique. It has been shown that RFA is able to measure the changes in implant stability over time, and can discriminate between successful implants and clinical failures. In vitro and in vivo investigations have revealed that the RFA technique is non-invasive, easy to use, and capable of analyzing the degree of osseointegration. According to previous studies, RFA values of dental implants are mainly associated with the stiffness of the implant-bone system (bone qualities, stiffness of implant components), and the effective length (the distance from the upper bone surface to the shoulder of the implant + the length of the abutment). However, effects of bi-cortical anchorages, implant dimensions and transducer types on the RFA values remain unknown.Objectives:?To generate 3-dimensional models of dental implants for finite element analysis (FEA) by computer.?To caculate the effects of bi-cortical anchorages, implant dimensions and transducer types on RFA values by modal analysis.Methods:?Using the ANSYS software, 3-D models of dental implants and alveolar bone segments were generated, which had 3 different bone qualities (D2, D3 and D4). Then the models were meshed, and material properties were defined. The interfacial contact of bone and the implants was simulated as a frictional contact.?In fact, the resonance frequency of an implant is equal to its natural frequency. Therefore this study used the modal analysis to caculate implants'natural frequency values (NF values). By the modal analysis, NF values of implant-bone complexes were computed and the effects of bi-cortical anchorages, implant dimensions and transducer types on NF values were analyzed. Two types of bi-cortical anchorages were simulated: buccal and apical type. In the two bi-cortical anchorage models, different implant diameters (3.75mm, 4.0mm, 4.5mm, 5.0mm and 5.5mm) and lengths (10.0mm, 11.0mm, 12.0mm, 13.0mm and 13.5mm) were modelled and effects of these dimension parameters on NF values were analyzed. The L-shaped transducer and the aluminum peg transducer were modelled and their effects on NF values were analyzed. The L-shaped transducer is of titanium poperty, which is connected to the implant by the fixing screw. But the peg transducer is made of aluminum, which is connected to the implant by its tip screw. The peg transducer is in cylindrical shape, but the L-shaped transducer is designed as a L-shaped cantilever.?When studying the effects of bi-cortical anchorages, NF values of two vibrating modes were computed and they were the bending mode (BM) and axial mode (AM). The BM values are same to measurements of the RFA device.Results:?3-D FEA models of implant-bone complexes were successfully created on the computer. Different degrees of buccal bi-cortical anchorages were simulated by buccally displacing implants to contact compact bone. The results showed that buccal bi-cortical anchorages significantly enhanced bending and axial NF values. The increasing rates resulting from 0.5mm engagement ranged from 10.5 to 42.3%, with a mean of 24.3%. From 0 to 0.5mm engagement, the NF values maintained an increasing trend, and from 0.5 to 1.0mm engagement, the values levelled off or even decreased. In 0.5 and 1.0mm engagement models, increasing implant diameter resulted in small increases of NF values. In the control and 0mm engagement models, increasing implant diameter resulted in small fluctuations of NF values.?3-D FEA models of the apical type of bi-cortical anchorages were produced. In the D2 bone models, 13.0 and 13.5mm impants were bi-cortically anchoraged, but in D3 and D4 models, only implants of 13.5mm length were bi-cortically stabilized. Regardless of implant diameter, increasing implant length resulted in increases of BM values. Compared with the 10.0mm implants, BM values of 11.0mm implants had a mean increasing rate of 2.4%. However, the increasing rates of 12.0, 13.0, and 13.5mm implants were notable, which ranged from 29.0 to 30.1% under the D2 bone quality, and from 14.9 to 17.5% under the D3 bone quality. In the D4 models, the increasing rates of 13.0 and 13.5mm implants ranged from 83.0 to 86.7% with a mean of 85.0%, and the increasing rates of 12.0mm implants were 22.6% (3.75mm diameter) and 16.2% (5.5mm diamter). In most cases, 5.5mm diameter implants had slightly lower BM and AM values compared to 3.75mm diameter implants, with the mean decreasing rates were 8.6% (BM) and 3.3%( AM).?3-D FEA implant-bone models of the two transducers were generated, and only BM values of implants were computed in this section. In general, BM values of implants connected with the L-shaped transducer had a range of from 3763 to 4464 Hz, which was much less than the range of from 9192 to 10002 Hz generated in implants connected with the aluminum peg transducer. When bone quality increased from D4 to D2, BM values of the L-shaped transducer models went to higher levels with increasing rates ranging from 12.7 to 16.7% (mean: 14.9%), however BM values of the aluminum peg transducer models increased by a range of from 7.4 to 8.5% (mean: 7.9%). In the L-shaped transducer models, increasing implant length resulted in higher BM values, with a mean increasing rate of 3.2%. In the aluminum peg transducer models, increasing implant length brought about higher BM values only when bone quality was D3. And in D3 bone quality models, a linear correlation was found between BM values of the two transducers models (r = 0.996,P=0.004). In D3 and D4 models, linear correlations were also found between BM values of the two transducers models, but without no statistical significance.Conclusions:?Buccal bi-cortical anchorages could significantly increase both bending and axial NF values of dental implants, but extra-buccal cortical bone engagement could not produce considerable incremental increases of NF values as anticipated. Increasing implant diameter could result in limited increases of NF values in case of implants being bi-cortically anchored.?An increase of the implant length could enhance BM values, and moreover, closer approach of fixture ends to the sinus floor and bi-cortical anchorages, resulted from using longer implants, could remarkably enhance BM values. Implant diameter has slightly negative effects on BM and AM values.?BM values of the L-shaped transducer models were much less than those of the aluminum peg transducer models. The L-shaped transducer seems to be more sensitive to bone quality changes than the aluminum peg transducer. Only in D3 bone quality models, the two transducers measure smilar varying trends of implant BM values. |
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