CHAPTER 5 CONCEPTUAL DESIGN
5.2 Concepts
5.2.4 Concept 4
This concept is similar to concept 3 except the disengagement mechanism, as shown in Fig. 5.2-8. The disengagement mechanism is composed of a bottom plate, a connector, two rotational cylinders, and two springs, as shown in Fig. 5.2-9. It is installed on the lower tray by fixing the bottom plate and connected to adjustor by the connector. The rotational cylinder is assembled with a spring to install to the bottom plate, and there are two cylinder sets in this concept. The stop pad lies on the bottom plate between the two rotational cylinders, it is used to stop the rotation of the rotational cylinder and the spring in a specific direction. The two rotational cylinders are designed to rotate in the same direction that allows the installation of the spring with a preload. When the disengagement occurs, the connector disengages from one rotational cylinder. It means that if the connector disengages from the anterior rotational cylinder at one side, then it must disengage from the posterior rotational cylinder at another side. In this concept, the magnitude of the force for disengagement also can be adjusted by changing different spring and setting different preload to meet the requirement. However, the manufacture of disengagement mechanism seems to be difficult on this concept.
Fig. 5.2-8 Assembly view of concept 4
Fig. 5.2-9 Structure of mechanism in concept 4
In this section, four concepts are developed and described according to their construction, motion, and characteristic respectively. In next chapter, Finite Element Analysis (FEA) is introduced to estimate the conditions under force applied and the decision matrix will be used for concept evaluation. Finally, a concept will be selected for further design works.
CHAPTER 6
SIMULATION AND ANALYSIS
After four concepts are generated, the performances have to be evaluated. Preliminary studies for comparing the strength between different designs of mechanism are proposed by using the Finite Element Analysis method. The results of the Finite Element Analysis can provide valuable information for realizing the stress distribution in material and predicting the failure will arise or not. All the conditions, such as load conditions, boundary conditions, and material properties are required to be considered carefully to get more accurate results. The packaged software CATIA® is selected as the tool for establishing the finite element models and proceeding finite element analysis. The analysis of four aforementioned conceptual designs and one commercial product are going to be discussed below.
6.1 Analysis Conditions
6.1.1 Load and Boundary Condition
The load and boundary conditions should be set up according to the real situations and constraints. The MAD suffers from failure caused by sleep bruxism at present. Sleep bruxism is defined as a stereotyped movement disorder characterized by grinding or clenching of the teeth during sleep [54]. The behavior of grinding applies force in horizontal direction and the clenching applies in vertical direction. The grinding force has not been presented in literatures until present. A study mentioned that an axial load of 100N is simulated to indicate as bruxism in their finite element analysis [55]. Therefore, in this study, the force of 100N in vertical direction is simulated as the load condition on the MAD.
The fracture morphologies in the MAD are the detachment of the bonding interface between the resin and fixed portion of the mechanism, and the fracture of resin itself that the
occurs on the tray or the interface of tray and resin, the scope of analysis in this study is only to simulate and compare the strength of resin and the bonding interface for each finite element model. The finite element model is set to fix at the interface between resin and tray. The bonding interfaces are simulated as perfect bond to calculate the stresses on those contact surfaces. The load with a value 100N is applied on the connector to meet to the real condition.
6.1.2 Material Properties
The material selection is limited to the materials for medical usage.
Polymethylmethacrylate (PMMA) is a kind of self-curing resin that is used extensively for the fixation of prosthesis and orthodontic device in dentistry or other medical applications. It is a homogeneous and isotropic material which often fractures in the brittle manner [56]. In the manufacture of the MAD, PMMA resin is used to fix the mechanism on the tray. The SUS 316L stainless steel, one kind of material for medical usage, is selected as the constructed material for the mechanism of conceptual designs. The commercial product TAP-T is made of titanium alloy (Ti-6Al-4V). All of the materials mentioned above have different properties that are listed Table 6.1-1 for using in FEA process.
Table 6.1-1 Material properties for FEA
PMMA
6.1.3 Failure Criterion
It is necessary to introduce a suitable failure criterion for FEA results to judge whether the finite element model suffers from failure or not. In this study, two failure criteria are needed for the bonding interface and the PMMA resin respectively. The strength of bonding interface between PMMA and metal have been proposed in several studies in terms of shear bonding strength. The shear bonding strength for PMMA to bond with the stainless steel and titanium alloy are 25.24 MPa [63] and 34.7 MPa [64] respectively. This value is going to compare with the principal shear stress in FEA results.
PMMA resin often fractures in brittle manner, not in ductile. Therefore, the generally used criterion, Von Mises stress, is not appropriate for judging the fracture of PMMA. A study had presented the results of a test for the failure of PMMA under stresses, and shown the results behave following the Coulomb-Mohr criterion [65]. The Coulomb-Mohr criterion judges failure by maximum principal stress σ1 and minimum principal stress σ3, as shown in Fig. 6.1-1 [66]. The failure strengths relate to the equation 6-1.1 to equation 6-1.3, where St
and Sc mean the tensile strength and compressive strength.
0
The design factor nd is introduced as an index to compare the strength between all finite element models. Design factor is a factor of safety that is calculated by the stress and the failure strength, as shown in equation 6.1-4. The larger value of design factor represents the more safety design. All of the stress values in the results of FEA are going to be translated into design factors that the smallest one is considered as the design factor of whole finite element model. Finally, the relative strength between each model is obvious by comparing the smallest design factor value in each model.
Fig. 6.1-1 Coulomb-Mohr criterion
6.2 Analysis Results
6.2.1 Case I – The Results of Concept 1
Fig. 6.2-1 to Fig. 6.2-4 show the simulation results of bonding interface and resin in the upper portion of concept 1. The maximum principal shear stress on bonding interface is 3.04 MPa. The design factor is 8.29 for the bonding interface and 8.39 for the resin. This means that the strength of bonding interface and resin is almost the same.
Fig. 6.2-5 to Fig. 6.2-8 show the simulation results of bonding interface and resin in the lower portion of concept 1. The maximum principal shear stress on bonding interface is 0.76 MPa. The design factor is 33.03 for the bonding interface and 32.56 for the resin. This means that the resin is weaker than bonding interface. Furthermore, the upper portion is weaker than the lower portion in this concept.
Fig. 6.2-1 The principal shear stress distribution on bonding interface of concept 1
Fig. 6.2-2 The principal stresses distribution in resin of concept 1
Fig. 6.2-4 The minimum principal stress distribution in resin of concept 1
Fig. 6.2-5 The principal shear stress distribution on bonding interface of concept 1
Fig. 6.2-6 The principal stresses distribution in resin of concept 1
Fig. 6.2-8 The minimum principal stress distribution in resin of concept 1
6.2.2 Case II – The Results of Concept 2
Fig. 6.2-9 to Fig. 6.2-12 show the simulation results of bonding interface and resin in the upper portion of concept 2. The maximum principal shear stress on bonding interface is 24.3 MPa. The design factor is 1.04 for the bonding interface and 1.18 for the resin. The bonding strength is on the edge of failure.
Fig. 6.2-13 to Fig. 6.2-16 show the simulation results of bonding interface and resin in the lower portion of concept 2. The maximum principal shear stress on bonding interface is 1.7 MPa. The design factor is 14.88 for the bonding interface and 13.41 for the resin. This means that resin is weaker than the bonding interface. Furthermore, the upper portion is weaker than the lower portion in this concept.
Fig. 6.2-9 The principal shear stress distribution on bonding interface of concept 2
Fig. 6.2-11 The maximum principal stress distribution in resin of concept 2
Fig. 6.2-13 The principal shear stress distribution on bonding interface of concept 2
Fig. 6.2-15 The maximum principal stress distribution in resin of concept 2
Fig. 6.2-16 The minimum principal stress distribution in resin of concept 2
6.2.3 Case III – The Results of Concept 3
Fig. 6.2-17 to Fig. 6.2-20 show the simulation results of bonding interface and resin in the upper portion of concept 3. The maximum principal shear stress on bonding interface is 9.91 MPa. The design factor is 2.55 for the bonding interface and 1.95 for the resin. This means that resin is weaker than the bonding interface.
Fig. 6.2-21 to Fig. 6.2-24 show the simulation results of bonding interface and resin in the lower portion of concept 3. The maximum principal shear stress on bonding interface is 15.3 MPa. The design factor is 1.66 for the bonding interface and 1.46 for the resin. This means that resin is weaker than the bonding interface. Furthermore, the lower portion is weaker than the upper portion in this concept.
Fig. 6.2-17 The principal shear stress distribution on bonding interface of concept 3
Fig. 6.2-18 The principal stresses distribution in resin of concept 3
Fig. 6.2-19 The maximum principal stress distribution in resin of concept 3
Fig. 6.2-20 The minimum principal stress distribution in resin of concept 3
Fig. 6.2-22 The principal stresses distribution in resin of concept 3
Fig. 6.2-24 The minimum principal stress distribution in resin of concept 3
6.2.4 Case IV – The Results of Concept 4
The upper portion in concept 4 is the same with the concept 3, as shown in Fig. 6.2-17 to Fig. 6.2-20.
Fig. 6.2-25 to Fig. 6.2-28 show the simulation results of bonding interface and resin in the lower portion of concept 3. The maximum principal shear stress on bonding interface is 18.9 MPa. The design factor is 1.33 for the bonding interface and 1.39 for the resin. This means that the strength of bonding interface and resin is almost the same. Furthermore, the lower portion is weaker than the upper portion in this concept.
Fig. 6.2-25 The principal shear stress distribution on bonding interface of concept 4
Fig. 6.2-27 The maximum principal stress distribution in resin of concept 4
Fig. 6.2-28 The minimum principal stress distribution in resin of concept 4
6.2.5 Case V – The Results of Commercial Product TAP-T
Fig. 6.2-29 to Fig. 6.2-32 show the simulation result of bonding interface and resin in the upper portion of TAP-T. The maximum principal shear stress on bonding interface is 3.68 MPa. The design factor is 9.44 for the bonding interface and 4.61 for the resin. This means that resin is much weaker than the bonding interface.
Fig. 6.2-33 to Fig. 6.2-36 show the simulation result of bonding interface and resin in the lower portion of TAP-T. The maximum principal shear stress on bonding interface is 6.84 MPa. The design factor is 5.07 for the bonding interface and 1.31 for the resin. This means that resin is much weaker than the bonding interface. Furthermore, the lower portion is weaker than the upper portion in this concept. If the material of TAP-T is changed to SUS 316L stainless steel, the design factor of bonding interface will down to 6.87 for upper portion and 3.68 for lower portion.
Fig. 6.2-29 The principal shear stress distribution on bonding interface of TAP-T
Fig. 6.2-30 The principal stresses distribution in resin of TAP-T
Fig. 6.2-31 The maximum principal stress distribution in resin of TAP-T
Fig. 6.2-32 The minimum principal stress distribution in resin of TAP-T
Fig. 6.2-33 The principal shear stress distribution on bonding interface of TAP-T
Fig. 6.2-34 The principal stresses distribution in resin of TAP-T
Fig. 6.2-35 The maximum principal stress distribution in resin of TAP-T
Fig. 6.2-36 The minimum principal stress distribution in resin of TAP-T
Table 6.2-1 Comparison of all finite element models
nd for bonding interface nd for resin
upper lower upper lower
Concept 1 8.29 33.03 8.39 32.56
Concept 2 1.04 14.88 1.18 13.41
Concept 3 2.55 1.66 1.95 1.46
Concept 4 2.55 1.33 1.95 1.39
TAP-T 9.44 5.07 4.61 1.31
TAP-T (316L) 8.28 4.6 5.37 1.35
Table 6.2-1 shows the comparisons of all finite element models based on the index of design factor. The results may be varied by adjusting the setting of dimensions for concepts.
As the preliminary results shown, the strength of concept 1 is better than the commercial product TAP-T which made of SUS 316L stainless steel. The strength of concept 3 and concept 4 is almost the same or slightly better than TAP-T. However, a new function, the disengagement mechanism, and a mechanism which combined adjustment and lateral movement are proposed in these two concepts. All of the results from FEA are going to be considered as an important reference for concept evaluation process in the next section.
6.3 Concept Evaluation
The evaluation bases on the decision-matrix method [51]. In the decision matrix, several items should be included, such as criteria, importance, alternatives, evaluation of each alternative using criterion, and the final score for each alternative. The criteria choose from the customer requirements in QFD. The importance item in the decision matrix refers to the importance of customer requirements in QFD to calculate the weighting for each criterion by binary-matrix method [52]. The alternatives here are the four concepts described above.
Before starting to evaluate, choose one concept as a datum for comparison. All other concepts are compared with the datum by judging each criterion, resulted in superior, the same, or inferior to the datum, and represented by symbol “＋”, “S”, and “－” respectively. Finally, compute the sum of the plus scores and minus scores which have been multiplied by the importance weighting.
After the evaluation of the decision-matrix, the concept 3 is presented as the best design which bases on the customers’ requirements. The result exhibits that concept 3 is good at the lateral movement which confirms to the real condition, the small height of whole assembly, the functions to promote comfort, the easy operation process, and so on.
Table 6.3-1 Decision matrix for concept evaluation.
6.4 Comparisons
In last section, the final design is produced by the decision-matrix method. The QFD target which developed in section 4.2 is introduced to compare with the specifications of the final design to estimate the achievement. The QFD target is defined considerately in accordance with the patent analysis and market survey. The results of comparison are going to be an essential reference for the future works.
As the Table 6.4-1 shown, the specification of the final design is listed in the “Achieved”
row. The specification achieves several QFD targets which listed in the “QFD Target” row.
The adjustable range is less than the target, but larger than commercial product TAP-T. In the lateral mobility, the height of the inner wall of tray, and separate height of mechanism are better than the target. Those specifications provide the larger movable range, prevent the inappropriate touch with gums, and avoid the jaw opening exaggeratedly that all of them are referred to the comfort during using time. The contact area is referring to the area of the fixation portion on the tray. In the final design, the shape of fixation portion is just a preliminary design. It is required to be modified for obtaining the higher strength of fixation on the tray in the future. The large volume of the final design seems to be the drawbacks.
Maybe the tolerable volume of mechanism for patients can be tested by clinical experiments in the future. At the same time, the durable time of the device and time for adaptation by patient are required to be tested under real using situation.
Table 6.4-1 Comparison between the QFD target and the achieved target
Adjustable range (Front-rear) Adjustable range (Up-down) Pitch of adjustment Jaw rotate downwardly Lateral movability Tray Dentition-fitted tray Contact area Even contact Steps to adjust Height of the inner wall of tray
mm mm mm y/n mm y/n y/n mm2 y/n # mm
TAP-T 7 0 0 y 7.5 y y 55 y 1 9
QFD Target 3-8 0 0 y 8 y y 50 y 1 9
Achieved 3-7.5 0 0 y 9.8 y y 23.5 y 1 8
Table 6.4-1 Comparison between the QFD target and the achieved target (cond.)
Separate height of mechanism Time of using MAD Steps to wear Disengage mechanism Depth of mechanism Biocompatible material Steps to clean Time for adaptation # of componenets Durable time Maximum load
mm hr # y/n mm y/n # week # yr N
TAP-T 7 > 8 3 n 12 y 3 1 6 3 －
QFD Target 8 > 8 3 y 14 y 3 1 10 3 100
Achieved 6.3 － 3 y 16 y 3 － 12 － 100
CHAPTER 7
CONCLUSIONS AND FUTURE WORKS
7.1 Conclusions
Snoring is not only a very prevalent phenomenon during sleeping time but also an extremely prevalent disorder that influence the health of snorer. The obstruction of breathing leads to many symptoms in nocturnal and daytime that causes the variation of physical condition, the reduction of work efficiency, and even the happening of an accident. Therefore, the treatment of snoring is important. Based on the reviews of medical literatures, the MAD has been approved as an effective therapy for snoring and mild to moderate OSA. In this study, several new designs of the MAD are proposed and some conclusions can be made as follows:
1. The patent analyses and commercial products surveys assist the realization of the techniques and developments of the MAD which provide plenty information for consulting in the later procedures of design.
2. The QFD method is applied to clarify the customers’ requirements, the engineering specifications, the evaluation and specifications of commercial products, and then defines the target specifications for designing a competitive product.
3. In the conceptual design phase, four concepts are proposed in the end of the conceptual design procedure. Moreover, the disengagement function and a combination design which integrates adjustment and lateral movement functions are proposed.
4. The results of finite element analysis are translated into the design factors to be considered as an index of strength, and compared between each concept and one commercial product.
5. The concept 3 is evaluated as the best concept between all of the concepts by proceeding the decision-matrix method.
7.2 Future Works
About the future works of this study, some points can be described as below:
1. Search for the information about the grinding force in sleep bruxism to make the strength simulations more accurate and similar to the real loading situation.
2. Dimension optimization is recommended for the final design to reduce the size and to obtain the optimum strength.
3. The prototype of the final design can be manufactured after all of the dimensions are decided.
4. The clinical experiments are required to verify the efficacy of therapy by using the MAD of the final design.
5. Apply for the 510(k) notification from FDA before the product promoting to the market.
REFERENCES
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[3] Elaine N. Marieb, Jon Mallatt, Human anatomy, The Benjamin/Cummings Publishing Company, Inc., Redwood City, California,1992.
[4] David N. F. Fairbanks, Samuel A. Mickelson, and B. Tucker Woodson, Snoring and
[4] David N. F. Fairbanks, Samuel A. Mickelson, and B. Tucker Woodson, Snoring and