QFD for MAD

In this section, the QFD for MAD will be established step-by-step and the engineering targets for the design of MAD will be generated in the end, as shown in Fig. 4.2-1.

z Step 1: Identify the customers (Who)

The customers are widely defined as all personnel relating to this product, including consumers, manufacturing personnel, sales staff, service personnel, and so forth. In this case, doctors, patients, and sales staff are considered as the customers of MAD.

z Step 2: Determine the customers’ requirements (What)

After the customers have been identified, the next step is to determine what is to be designed and what the customers want. Many requirements can be observed from customers who are using the existing products. Furthermore, surveys are usually used to gather information. For example, the literatures, the questionnaires, the face-to-face interviews, or any other ways can gather the opinions from people. All of the requirements can be classified into several types, such as functional performances, human factors, manufacturing, life-cycle concerns, and etc. In the design of MAD, the requirements are developed by the above mentioned ways and the results are shown in Table 4.2-1.

z Step 3: Determine relative importance of the requirements (Who vs. What)

The third step of the QFD method is to evaluate the relative importance of each customers’ requirement. The elimination of the least important requirements is proceeded by customers before ranking all items. Referring to the results listed in Fig. 4.2-1, the smaller number represents the more important requirement. The results can be used to generate a weighting factor for each requirement.

Table 4.2-1 List of customers’ requirements for MAD

Distribute force caused by bruxism

Easy to adjust

Easy to wear into mouth Easy to remove from mouth

Avoid exaggeration of opening of jaw Use for a long time

Comfortable to use

Period of Using Manufacturing

Low cost

Structure of parts are simple Parts are easy to be manufactured Product is easy to be manufactured Others

Close mouth completely

Simple operational steps to wear Opening mouth is allowed Breathing through mouth

No Impingement of tongue space Not irritate oral tissues

Easy to clean

Difficult to disengage from dentition Without side effects

Long life Tiny size

Difficult to break

Good-looking appearance z Step 4: Identify and evaluate the competition (Now)

The goal in this step is to determine the competition’s ability of existing products for each of the requirements. The results can bring out what already exists and that is something can be improved on what already exists. For each customer’s requirement, the existing design can be rate on a scale of 1 to 5 as follows:

1 ＝ The product does not meet the requirement at all.

2 ＝ The product meets the requirement slightly.

3 ＝ The product meets the requirement somewhat.

5 ＝ The product satisfies the requirement completely.

In this study, the most popular products in the market, TAP-T, is selected as the competition benchmark, as shown in Fig. 4.2-1.

z Step 5: Generate engineering specifications (How)

In this step, the customers’ requirements will be translated into a set of measurable engineering specifications which are the parameters for design, as shown in Fig. 4.2-1. A set of units is associated with each of the measures. Furthermore, the direction of improvement, more is better (↑) or less is better (↓), is also developed here.

z Step 6: Relate customers’ requirements to engineering specifications (What vs.

How)

The center portion of the house of quality represents the relationships between engineering specifications and customers’ requirements. Each cell of the portion will be mentioned according to the strength of relationship. Different numbers are filled in cells to refer to different strength as follows:

9 ＝ strong relationship 3 ＝ medium relationship 1 ＝ weak relationship

Blank ＝ no relationship at all

The results in this step are presented in Fig. 4.2-1.

z Step 7: Identify relationships between engineering requirements (How vs. How) Engineering specifications may be dependent on each other. Realize the dependency during design process can help us to know the work for meet one specification may cause positive or negative effects on others. The results are presented in Fig. 4.2-1 by referring to several typically used symbols as follows:

＃ ＝ Negative

¯ ＝ Strong Negative

€ ＝ Strong Positive { ＝ Positive

z Step 8: Set engineering target (How much)

The final step in the QFD is to determine a target value for each engineering specification. Comparing to the specifications of competition products can establish the target for the new product. All the target values for the design of MAD are given in Fig.

4.2-1.

After the QFD house has been established, the design problem is fully understanding, and the product specifications are also determined. In the following of the product design process, conceptual designs can be obtained.

CHAPTER 5

CONCEPTUAL DESIGN

5.1 Design Method

Based on the results generated from QFD, the conceptual design phase is proceeding to innovate and create concepts for new designs. A general conceptual design process can be described as follows:

z Problem formulation: make sure the problem required to be solved.

z Overall function: generate the main function of the product.

z Functional decomposition: divide the overall function into several sub-functions.

z Concept generation: generate concepts to achieve each sub-function respectively.

z Concept combination: combine the concepts from sub-functions to form various complete conceptual designs for the new product.

There are still some problems existing in the commercial products. The most significant one is the failure of the MAD during using. The failure of the MAD will lead to the ineffective treatment and the extraneous expenses for repairing the device. Therefore, the problem here is to design a new MAD which will not break easily during the using time. The main function of the MAD is to make the jaw move forward to achieve the purpose of treating snoring and OSA. Thus the overall function in designing the MAD can be defined as:

maintain the jaw position advancement. In according to the overall function, the functional decomposition proceeded to identify all the sub-functions, and the result is shown in Fig.

5.1-1.

The overall function is divided into several sub-functions by the consideration of the functions which should be included. The functional decomposition leads to a better understanding of the design problem. In the MAD, six primary sub-functions promote the effective work on treating snoring and OSA, which are the fixation of the MAD, the connection between both fixers, the adjustability of jaw advancement, the lateral movement of the jaw, the prevention of the device from breaking, and the force directions acting on the jaw.

After all sub-functions have been developed, the next goal is to generate as many concepts as possible for each sub-function. A popular method, brainstorming, is selected to generate concepts because of its advantage of gathering ideas from each group member in their own viewpoint. The group of idea sources is formed by the members in my laboratory.

As the brainstorming method proceeding, all of the members should follow the four rules below [51]:

z Record all the ideas generated.

z Generate as many ideas as possible, and then verbalize these ideas.

z Think wild. Silly or impossible ideas sometimes lead to useful ideas.

z Do not allow evaluation of these ideas, just the generation of them.

The result of concept generation is a list of concepts generated for each function. The next step is to combine the individual concepts into complete conceptual designs by using the method that is to select one concept for each function and combine those selected into single design. Finally, various conceptual designs will be generated by accomplishing the conceptual design process.

5.2 Concepts

There are many combinations can be generated by combining individual concepts of each sub-function which described in the above section. However, some of them are impossible to be assembled together. Among the useful combinations, the relation between each individual concept should be good for arranging in pairs and without incompatible. In addition, some combinations are similar or almost the same with existent design or commercial products. At last, four complete concepts are selected and going to be described individually bellow.

5.2.1 Concept 1

Fig. 5.2-1 shows the conventional assembly of the MAD which includes an upper tray, a lower tray, and a mechanism to maintain the jaw advancement. The upper and lower tray are made by the acrylic resin, one kind of thermoplastic material, which can provide a good fit with dentition to make the well-fixed MAD. All of the concepts are going to use this method to perform the function of fixation. The mechanism of concept 1 is composed of an upper plate, a bottom plate, a sliding plate, and two screws, as shown in Fig. 5.2-2.

Fig. 5.2-2 Structure of mechanism in concept 1

The upper plate is fixed on the upper tray. Within the upper plate, the space provides a moveable region for sliding plate to allow the lateral movement of the lower jaw. The “S”

shape is designed to form the side wall of the upper plate. When the biting force applies on the bottom surface of the upper plate, the “S” shape makes less torque and tensile force applying to the fixed portion to avoid the failure of the MAD. The bottom plate fixes on the lower tray and assembles two screws together. These screws connect to the sliding plate with nuts to adjust its position and against the bottom plate moving backward.

This concept is workable for moving the lower jaw forward with an adjustable amount.

Furthermore, the proper height of the mechanism makes the lower jaw opening and downward slightly that causes the advancement more smooth to prevent TMJ from soreness.

However, adjustor is composed of two screws. It means that each adjustment process has to make twice efforts.

5.2.2 Concept 2

This concept includes the disengagement function to prevent the device from failure. Fig.

5.2-3 shows the whole assembly of concept 2. In this concept, the mechanism is composed of an upper plate, a bottom plate, and a screw, as shown in Fig. 5.2-4. The grooves on the upper and bottom plates are used to fix them more secure to the tray respectively. The upper plate provides a nut to connect with one end of the screw. On the other end, the root of the notch head tracks within a slot which is provided by the bottom plate.

The screw between the upper and bottom plate can adjust the amount of the jaw advancement and limit the moveable region of the lower jaw in the lateral direction. As the lower jaw move more widely to keep in touch with the side wall of the slot, the notch head receives the force applied from the border of the slot and starts to deform and shrink until the notch head disengages from the slot. The amount of the force for disengagement should less than the force which can make the device failure. However, the only method about how to adjust the disengagement force is the replacement of different materials. Maybe this is difficult to achieve and become a disadvantage of this concept.

Fig. 5.2-3 Assembly view of concept 2

Fig. 5.2-4 Structure of mechanism in concept 2

5.2.3 Concept 3

This concept includes the disengagement function to prevent the device from failure. A concept of the adj-slider integrated the adjustment function. Therefore, lateral movement is provided and combined in this concept. The adj-slider is installed in the upper tray opposite to the disengagement mechanism which is installed in the lower tray, as shown in Fig. 5.2-5. The adj-slider is composed of a sliding base, a sliding post, a screw set, a telescopic tube, and an upper plate, as shown in Fig. 5.2-6. The upper plate and the two ends of the sliding base are fixed to the upper tray to form a triangle. The sliding post slides along the slot of the sliding base at one end, and connects with the telescopic tube at another end. Then, the telescopic tube pivot to the upper plate to form a telescopic sliding mechanism in the triangle. The screw set is installed coaxial to the sliding post to receive the retractive force from the lower jaw and transfer to the sliding base during using time. As the screw set elongates, not only pushes the lower jaw forward but also limits it to a narrower range for lateral movement. The result of moveable region conforms to the border movement of the mandible recorded in the horizontal plane [53].

The disengagement mechanism is composed of a bottom plate, a fixed plate, a rotational plate, a pin, a spring, and a connector. It is fixed to the lower tray at the bottom plate. The bottom plate fixes the fixed plate together to hold a pin to be the rotational axis of the rotational plate. In Fig. 5.2-7, a spring installed between the bottom plate and the rotational plate provides a restoring force which is used to hold the connector to connect with the adjustor. As the device works to reach the limitation of the sliding base, the continuous applying forces lead to the deformation of the spring and the disengagement of connector. The magnitude of the force for disengagement can be adjusted by designing different spring constant to meet the requirement.

Fig. 5.2-5 Assembly view of concept 3

Fig. 5.2-6 Structure of mechanism in concept 3

Fig. 5.2-7 The spring installed in concept 3

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

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