Home Prosthodonsia Application of Biomechanical Principles to Design A Removable Partial Denture

Application of Biomechanical Principles to Design A Removable Partial Denture

Overview

Objectives of Prosthodontic Treatment:
- the elimination of oral disease to the greatest extent possible;
- the preservation of the health and relationships of the teeth and the health of oral and paraoral structures, which will enhance the removable partial denture design;
- the restoration of oral functions that are comfortable, are esthetically pleasing, and do not interfere with the patient’s speech
→ The goal is to provide useful, functional removable partial denture prostheses by striving to understand how to maximize every opportunity for providing and maintaining a stable prosthesis (Mc Cracken, 2011).
- Because removable partial dentures are not rigidly attached to teeth, the control of potential movement under functional load is critical to providing the best chance for stability and patient accommodation.
- The consequence of prosthesis movement under load is an application of stress to the teeth and tissue that are contacting the prosthesis. It is important that the stress not exceed the level of physiologic tolerance, which is a range of mechanical stimulus that a system can resist without disruption or traumatic consequences.
Menurut Maxfield
- Kemampuan benda hidup untuk mentoleransi kekuatan atau beban yang diterimanya, bergantung pada besar atau intensitas kekuatan/beban tersebut. Struktur anatomi penyangga GTSL (gigi penyangga dan residual ridges) merupakan benda hidup yang terpapar oleh beban.
Kemampuan struktur anatomi jaringan penyangga untuk menahan beban, bergantung pada :
1. Macam kekuatan/beban yang ditahan
2. Durasi, frekuensi, intensitas kekuatan/beban yang ditahan
3. Kemampuan gigi dan/atau mukosa yang harus menahan kekuatan/beban
4. Pengaruh pemilihan material GT dan penggunaannya terhadap resistensi gigi dan/atau mukosa penyangga dalam menahan kekuatan/beban
5. Adanya perubahan pada kemampuan resistensi gigi-mukosa penyangga terhadap kekuatan/beban
Penyebab Kegagalan Perawatan GTSL
- Diagnosis dan Rencana Perawatan
  
  Diagnosis yang kurang tepat, perencanaan desain GTSL tidak menggunakan surveyor
- Prosedur persiapan rongga mulut
  
  Prosedur persiapan rongga mulut tidak dilakukan secara sistematis, persiapan rongga mulut yang inadekuat, gagal untuk mengembalikan kondisi kesehatan jaringan penyangga secara optimal sebelum prosedur pencetakan, hasil cetakan jaringan keras dan lunak yang kurang akurat.
- Desain kerangka GTSL
  
  Gagal untuk menggunakan rest secara tepat atau salah menempatkan rest, salah menempatkan konektor minor dan mayor, penggunaan desain klamer yang kurang tepat, penggunaan klamer tuang terlampau menutupi permukaan gigi penyangga sehingga mengganggu estetik pasien.
- Prosedur Lab GTSL
  
  Problema saat pembuatan model kerja akibat hasil cetakan yang kurang akurat, pemilihan material cetak atau gypsum yang kurang tepat, teknisi lab kurang mendapatkan informasi yang lengkap terkait desain, teknisi lab tidak mengikuti petunjuk order lab atau instruksi tertulis
- Penyangga basis GTSL
  
  Basis GTSL tidak menutupi seluruh permukaan struktur anatomi penyangga GTSL, struktur anatomi penyangga GSTL kurang tercetak secara akurat.
- Oklusi
  
  Gagal untuk membuat oklusi yang harmonis, tidak menggunakan material yang tepat terutama pada permukaan oklusal gigi artifisial GTSL yang akan berhadapan dengan permukaan oklusal antagonis.
- Hubungan Dokter Gigi dengan Pasien
  
  Gagal memberikan KIE terkait perawatan kesehatan gigi dan mulut pasien, kurang tepat dalam cara pemeliharaan GT nya, pasien tidak melakukan kontrol periodik atau melakukan instruksi dokter gigi untuk memelihara kesehatan gigi dan mulutnya

Types of Forces on RPD

Kenapa penting?
- Removable partial dentures by design are intended to be placed into and removed from the mouth. Because of this, they cannot be rigidly connected to the teeth or tissue. This makes them subject to movement in response to functional loads, such as those created by mastication.
- It is important for clinicians who provide removable partial denture service to understand the possible movements in response to function and to be able to logically design the component parts of the removable partial denture to help control these movements.
- "RPD has to be in a state of equilibrium" (Singla, 2006)
Singla S G, Lal J. Removable partial dentures designing: Forces as primary concern. J Indian Prosthodont Soc 2006;6:179-84

Forces Acting on Masticatory Apparatus (Singla, 2006):

VERTICAL FORCES
- Bodily movement of saddle toward tissue
  - Definisi
    - When vertical forces are applied, any compression that occurs is uniform over the entire seat. Difference in transmission of load is due to difference in resiliency of periodontal ligament and denture bearing mucosa.
  - Solusi
    
    This problem can be tackled with applying philosophy of design:
    - Broad stress distribution
    - Physiologic basing
    - Stress equalization.
- Rotational movement of saddle along horizontal axis
  - Definisi
    - Periodontal ligament is better able to resist masticatory forces as compared to residual ridge. Therefore, latter is likely to get the larger brunt of load than tooth. Gradually with passage of time, resorption occurs, especially at the distal end. There is sinking of the saddle at the distal end, which may ultimately contribute towards rotational action along transverse axis
- Movement of saddle away from the tissue
  - Definisi
    
    This means total lifting of the saddle away from the base rather than lift at one end. This may be the case in any type of saddle.
  - Major forces of retention, i.e., physical forces, as in complete denture must be employed with
    - Intimate contact
    - Wider area coverage
    - Posterior seal
    - Polished surfaces
  - Then we have the powerful Direct retainer. The use of direct retainer depending on the:
    - Number of saddles
    - Size of saddles
    - Location of saddles
    - "Don't be too miser or too liberal, be judicious and have a simple design" Always remember the fundamental principle of placing the retaining element nearer to the saddle.
HORIZONTAL FORCES
- Orofacial musculature and tongue muscle cause lateral movement
  - Definisi
    - Masticatory forces and forces exerted by orofacial musculature including tongue can cause these movements, i.e., lateral bodily movement and anteroposterior movement.
  - Lateral bodily movement
    - Definisi
      
      The morphology of supporting tissues and the clasp design provide sufficient bracing. In addition, resistance will be provided by rigid parts of a denture i.e., saddle and major connector resting on the slopes in the stippled areas (gambar bawah) against the forces whose directions are shown by arrows.
    - Lateral bodily movement is more problematic in resorbed ridges and weak abutments, wherein alternatives are:
      - Use of more number of rigid bracing elements so as to distribute the forces over wider areas.
      - Reduction in cusp height to reduce lateral forces Orofacial musculature and tongue
- Anteroposterior movement (A-P movement)
  - Definisi
    - Horizontal forces can also cause anteroposterior movement, which can be forward or backward.
  - Macam
    - In case of Bounded saddles
      
      contact of saddle itself with abutment teeth resists A-P movement.
    - In case of free end saddle; (distal extension cases).
      - Forward movement is prevented by:
        
        Extension of major connector on anterior part of palate
        
        Using a Linguoplate major connector in case of mandibular arch.
      - Backward movement is prevented by:
        
        Coverage of pear shaped retromolar pad
        
        Minor connector contacting the mesiolingual surface of abutment tooth
        
        By encircling more than 180º of the abutment with the clasps.
Rotational movement
- Anteroposterior rotation in free end saddle along transverse axis
  - Definisi
    - Rotational forces cause rotational movement, which can be anteroposterior rotation or lateral rotation.
    - Anteroposterior rotation in free end saddle (gambar atas)
    - Rotation around transverse axis can be away from the tissues or towards the tissues
- Lift at the heel
  - Definisi
    
    A-P rotation away from the tissues (Lift at the heel)
    - This movement should be distinguished from bodily lift of saddle away from the basal seat, i.e., complete loss of retention.
    - Loss of retention in this case is only at the distal end with the direct retainers still firmly in place.
    - This movement cannot be prevented, because we can not put a stop to the dislodging forces acting on the saddle. However, definitely, certain measures can be taken to minimize the deleterious effects this movement will have on the remaining supporting structures.
  - Prinsip FIsika
    - Consider a bar with a single support than behaves like a sea-saw rotating around the single fixed axis (gambar) This rotational movement can be prevented by tying the bar from above or providing additional solid support
    - If this situation is simulated to that of an RPD, of course it cannot be tied to the maxilla from above so as to prevent the A-P rotation. The only alternative in this case is to provide a solid support on the other hand as far as possible from the fulcrum line in the form of a rest (indirect retainer). This limits the sinking at the anterior end and consequently, lift at the heel is prevented (gambar samping)
    - In the process, this additional support at the anterior end is subjected to load, which might be deleterious to the said tooth. We must aim at minimizing this load or distributing it widely.
    - Devan Statement
      - Removable partial dentures (RPD) are objects that move or are allowed to move when placed in function. Since, nature demands equilibrium for every object, living or non-living, RPD has to be in a state of equilibrium, i.e., a state in which opposing forces or influences are balanced.
      - Keeping in mind Devan's statement 'to preserve that remains,' forces should be given major consideration while designing a partial denture so as to ensure the dynamics of these appliances without deleterious effects to the supporting structures.

Biomechanics of RPD

Definisi
- In the terminology of engineering mechanics, the prosthesis induces stress in the tissue equal to the force applied across the area of contact with the teeth and/or tissue. This same stress acts to produce strain in the supporting tissue, which results in load displacement in the teeth and tissue.
- The understanding of how these mechanical phenomena act within a biological environment that is unique to each patient can be discussed in terms of biomechanics.
- In the design of removable partial dentures, with a focus on the goal of providing and maintaining stable prostheses, consideration of basic biomechanical principles associated with the unique features of each mouth is essential. Oral hygiene and appropriate prosthesis maintenance procedures are required for continued benefit of optimum biomechanical principles.
- Unquestionably, the design of removable partial dentures necessitates mechanical and biological considerations.
- Most dentists are capable of applying simple mechanical principles to the design of a removable partial denture.For example, the lid of a paint can is more easily removed with a screwdriver than with a half dollar. The longer the handle, the less effort (force) it takes.
- This is a simple application of the mechanics of leverage. By the same token, a lever system represented by a distal extension removable partial denture could magnify the applied force of occlusion to the terminal abutments, which would be undesirable
Pemilihan komponen
- The strategy of selecting component parts for a partial denture to help control movement of the prosthesis under functional load has been highlighted as a method to be considered for logical partial denture design.
RPD’s Component
- MAJOR CONNECTOR
  - unification
  - load distribution
  - rotation check
- MINOR CONNECTOR
  - unification
  - stabilization
  - load transmission to abutment
- DIRECT RETAINER
  - retention
  - bracing and recoprocation
  - support
- INDIRECT RETAINER
  - for checking antero-posterior rotation
- DENTURE BASE
- ARTIFICIAL TEETH
  - mastication
  - esthetics
- Saddle
  - main component
  - distributes load
Pertimbangan pemilihan desain GTL
- Direct Retainer
  - Support:
    - Tooth Borne
    - Tooth Mucosa Borne
  - Penutupan yang minimal dari Gigi dan Gingiva
  - Letak Garis Suvey
  - Bilateral bracing
- Indirect Retainer
  - Efektifitasnya tergantung jarak terhadap garis fulcrum
  - Guiding Plane memberikan retensi indirek yang efektif
  - Memerlukan positif vertical stop seperti : Rest seat yang sdh dipreparasi
- Major Connector
  - Jumlah gigi sisa
  - Letak Diastema
  - Ada/tidak Torus
  - Comfort
  - Pertimbangan anatomi dan inklinasi gigi sisa
  - Estetik
  - Kebiasaan Pasien
- Minor Conector
  - Letak sesuai dengan daerah gigi yang hilang
  - Jumlah tergantung letak dan banyaknya gigi yang hilang
Supported by?
- TOOTH-SUPPORTED PROSTHESIS
  - Definisi
    - GTSL yang disangga oleh gigi-gigi penyangga (tooth-supported) memiliki pergerakan yang minimal ketika berfungsi karena gigigigi penyangga mampu menahan beban fungsional.
  - Pertimbangan untuk menentukan gigi penyangga meliputi:
    - Kondisi struktur jaringan tulang penyangga
    - Rasio mahkota – akar
    - Anatomi dan morfologi mahkota dan akar
    - Jumlah gigi asli yang tersisa
    - Posisi gigi geligi pada rahang
- TOOTH-TISSUE SUPPORTED PROSTHESIS
  - Definisi
    
    GTSL yang disangga oleh gigi dan mukosa penyangga (toothtissue supported), dimana kondisi residual ridgenya bervariasi untuk dapat dijadikan penyangga gigi tiruan.
    - Variasi tersebut akibat respon tulang alveolar terhadap kondisi kehilangan gigi dimana bentuk tulang alveolar pada residual ridge pasca ekstraksi bervariasi dan akan terus mengalami perubahan. Selain itu jaringan lunak penyangga juga berpotensi mengalami perubahan akibat inflamasi yang dipicu oleh tekanan berlebih.
    - Oleh karena itu, saat menentukan desain GTSL dokter gigi harus memahami potensi timbulnya beban fungsional yang berasal dari sisi rahang berlawanan serta mempertimbangkan variasi respon jaringan residual ridge terhadap beban fungsional tersebut yang dapat mempengaruhi pergerakan GTSL.
Class I Lever/ FIrst Class Lever
- Definisi
  - The greater the mechanical advantage is, the lesser is the force required to lift the bar.
- Prinsip
  - In an RPD, by shifting the resistance anteriorly, mechanical advantage can be reduced and consequently forces transmitted at the anterior end are reduced. We can also widely distribute this load byinvolving more number of teeth for providing indirect retention.
- Rotation towards the tissues (Sinking at the distal end)
  
  With passage of time, ridge resorption, especially at the distal end leads to sinking of the saddle and rotational movement toward the basal seat. In such a situation an RPD can behave like an extraction forcep, with consequent damage to the supporting structures
- Solution to this problem are:
  - 'repeated rebasing' which is not practically feasible for the dentist as well as for the patient
  - use of mesial rests instead of distal rests, which permits more even distribution of load and less stress on abutment teeth.
- Disto-occlusal rest
  
  When a disto-occlusal rest is used, fulcrum 'F', lies near the distal marginal ridge (DMR) and as the vertical loading occurs, denture sinks at the distal end. The clasp terminal moves up to engage the undercut, hence constituting the resistance arm in accordance with Class I lever and thereby exerts tipping forces on the abutment.
- Mesio-occlusal rest
  
  In case of mesio-occlusal (MO) rest, when effort 'E' is applied, fulcrum F shifts to mesial marginal ridge (MMR) and clasp terminal rotates downward and mesially. At the same time, remaining part of clasp, i.e., shoulder portion (that lies above the survey line) provides resistance R to downward sinking. Thus, a Class II lever situation is constituted which is beneficial.
- Since this resistance R is situated closer to the rotational center F than the clasp terminal, these forces are well resisted as in case of a pole embedded in sand.
- We can also extend the occlusal rest to the adjacent tooth for wider distribution of load
- With the use of a MO rest, there is an increase in length of lever arm, which makes rotational action more vertical in gingival area of abutment toot
- RPI system is one such system designed to incorporate MO rest and allows vertical rotation of saddle towards mucosa without damaging the supporting structures of abutment tooth.
- An additional retainer anteriorly if permissible (esthetically) can prevent upward rotation at the anterior end of RPD, hence minimizing the sinking at the distal end of the saddle
- Lateral rotation in free end saddle
  - Lateral rotation along sagittal axis
  - This can be checked by Cross arch bracing, i.e., rigid major connector is extended onto the opposite side of the arch as far posteriorly as possible and a retainer unit is provided at its distal most end
DIFFERENTIATION BETWEEN TWO MAIN TYPES OF REMOVABLE PARTIAL DENTURES
- Kennedy Class I and Class II types of partial dentures on the one hand and the Class III type of partial denture on the other.
- Pertimbangan
  - Manner in which each is supported.
    - The Class I type and the distal extension side of the Class II type derive their primary support from tissues underlying the base and secondary support from the abutment teeth (Figure 10-1, A and Figure 10-2).
    - The Class III type derives all of its support from the abutment teeth (Figure 10- 1,B and Figure 10-2).
  - For reasons directly related to the manner of support
    
    the method of impression registration and the jaw record required for each type will vary.
  - the need for some kind of indirect retention exists in the distal extension type of partial denture
    
    whereas in the tooth-supported, Class III type, no extension base is present to lift away from the supporting tissues because of the action of sticky foods and the movements of tissues of the mouth against the borders of the denture. This is so because each end of each denture base is secured by a direct retainer on an abutment tooth. Therefore the tooth-supported partial denture does not rotate about a fulcrum, as does the distal extension partial denture.
  - Pemilihan material basis yang dapat direline
    - akrilik, kalau kerangka logam sulit→ harus dibuat ulang
    the manner in which the distal extension type of partial denture is supported often necessitates the use of a base material that can be relined to compensate for tissue changes. Acrylic-resin is generally used as a base material for distal extension bases. The Class III partial denture, on the other hand, which is entirely tooth supported, does not require relining except when it is advisable to eliminate an unhygienic, unesthetic, or uncomfortable condition resulting from loss of tissue contact. Metal bases therefore are more frequently used in tooth-supported restorations, because relining is not as likelyto be necessary with them.
DIFFERENCES IN SUPPORT
- The distal extension partial denture derives its major support from the residual ridge with its fibrous connective tissue covering.
- The length and contour of the residual ridge significantly influence the amount of available support and stability (Figure 10-3).
- Some areas of this residual ridge are firm, with limited displaceability, whereas other areas are displaceable, depending on the thickness and structural character of the tissues overlying the residual alveolar bone.
- The movement of the base under function, determines the occlusal efficiency of the partial denture and also the degree to which the abutment teeth are subjected to torque and tipping stresses
Impression Registration
- Dicetak secara akurat
  - The anatomic form and the relationship of the remaining teeth in the dental arch, as well as the surrounding soft tissues, must be recorded accurately so the denture will not exert pressure on those structures beyond their physiologic limits.
  - A type of impression material that can be removed from undercut areas without permanent distortion must be used to fulfill this requirement. Elastic impression materials such as irreversible hydrocolloid (alginate), mercaptan rubber base (Thiokol), silicone impression materials (both condensation and addition reaction), and the polyethers are best suited for this purpose.
- Distal extension base harus dicetak
  - The supporting form of the soft tissues underlying the distal extension base of the partial denture should be recorded so firm areas are used as primary stress–bearing areas and readily displaceable tissues are not overloaded. Only in this way can maximum support of the partial denture base be obtained.
  - An impression material capable of displacing tissue sufficiently to register the supporting form of the ridge will fulfill this second requirement. A fluid mouth-temperature wax or any of the readily flowing impression materials (rubber base, the silicones, or the polyethers in an individual, corrected tray) may be employed for registering the supporting form. Zinc oxide–eugenol impression paste can also be used when only the extension base area is being impressed (see Chapter 15).
- Hal yang perlu diperhatikan
  - HOWEVER…
  - No single impression material can satisfactorily fulfill both of the previously mentioned requirements.
  - Recording the anatomic form of both teeth and supporting tissues will result in inadequate support for the distal extension base. This is so because the cast will not represent the optimum coordinating forms, which require that the ridge must be related to the teeth in a supportive form. This coordination of support maximizes the support capacity for the arch and minimizes movement of the partial denture under function.
DIFFERENCES IN CLASP DESIGN
- Apakah membutuhkan direct retention?
- A fifth point of difference between the two main types of removable partial dentures lies in their requirements for direct retention.
- Tooth -supported
  
  pada klamer yang terletak di ujung masing edentulous ridge→ menjaga mudah melewati tanpa halangan
  - The tooth-supported partial denture, which is totally supported by abutment teeth, is retained and stabilized by a clasp at each end of each edentulous space. Because this type of prosthesis does not move under function (other than within the physiologic limitations of tooth support units), the only requirement for such clasps is that they flex sufficiently during placement and removal of the denture to pass over the height of contour of the teeth in approaching or escaping from an undercut area. While in its terminal position on the tooth, a retentive clasp should be passive and should not flex except when one is engaging the undercut area of the tooth for resisting a vertical dislodging force.
  - Cast retentive arms are generally used for this purpose. These may be of the
    - circumferential type
      
      arising from the body of the clasp and approaching the undercut from an occlusal direction, or of the
    - bar type
      
      arising from the base of the denture and approaching the undercut area from a gingival direction. Each of these two types of cast clasps has its advantages and disadvantages.
- combination tooth and tissue–supported RPD
  - because of the anticipated functional movement of the distal extension base, the direct retainer adjacent to the distal extension base must perform still another function, in addition to resisting vertical displacement. Because of the lack of tooth support distally, the denture base will move tissue-ward under function proportionate to the quality (displaceability) of the supporting soft tissues, the accuracy of the denture base, and the total occlusal load applied. Because of this tissue-ward movement, those elements of a clasp that lie in an undercut area mesial to the fulcrum for a distal extension (as is often seen with a distal rest) must be able to flex sufficiently to dissipate stresses that otherwise would be transmitted directly to the abutment tooth as leverage.
  - On the other hand, a clasp used in conjunction with a mesial rest may not transmit as much stress to the abutment tooth because of the reduction in leverage forces that results from a change in the fulcrum position. This serves the purpose of reducing or “breaking” the stress, hence the term stressbreakers, and is a strategy that is often incorporated into partial denture designs through various means. Some dentists strongly believe that a stress-breaker is the best means of preventing leverage from being transmitted to the abutment teeth. Others believe just as strongly that a wrought-wire or bar-type retentive arm more effectively accomplishes this purpose with greater simplicity and ease of application. A retentive clasp arm made of wrought wire can flex more readily in all directions than can the cast half-round clasp arm. Thereby, it may more effectively dissipate those stresses that would otherwise be transmitted to the abutment tooth. A discussion of the limitations of stress-breakers has been presented in Chapter 9
  - Only the retentive arm of the circumferential clasp, however, should be made of wrought metal. Reciprocation and stabilization against lateral and torquing movement must be obtained through use of the rigid cast elements that make up the remainder of the clasp. This is called a combination clasp because it is a combination of cast and wrought materials incorporated into one direct retainer. It is frequently used on the terminal abutment for the distal extension partial denture and is indicated where a mesiobuccal but no distobuccal undercut exists, or where a gross tissue undercut, cervical and buccal to the abutment tooth, exists.
  - It must always be remembered that the factors of length and material contribute to the flexibility of clasp arms. From a materials physical property standpoint, a short wrought-wire arm may be a destructive element because of its reduced ability to flex compared with a longer wrought-wire arm. However, in addition to its greater flexibility compared with the cast circumferential clasp, the combination clasp offers the advantages of adjustability, minimum tooth contact, and better esthetics, which justify its occasional use in tooth-supported designs.
- The amount of stress transferred to the supporting edentulous ridge(s) and the abutment teeth will depend on:
  - the direction and magnitude of the force;
  - the length of the denture base lever arm(s);
  - the quality of resistance (support from the edentulous ridges and remaining natural teeth);
  - the design characteristics of the partial denture: the location of the rest, the design of the minor connector as it relates to its corresponding guiding plane, and the location of the retentive arm are all factors that influence how a clasp system functions. The greater the surface area contact of each minor connector to its corresponding guiding plane, the more horizontal the distribution of force (Figure 10-4).

Steps of Designing RPD

Imajinasi, survey

When planning treatment for partially edentulous patients, the dentist is confronted with myriad combinations of edentulous spaces and remaining teeth. It is up to the dentist to understand the functions of parts and to select the ones that will counter various forces generated around fulcrum lines by levers or inclined planes. When a patient comes, view the diagnostic models, outline the saddle and try to imagine the forces to which it can be subjected and movements it can make. After this, make judicious use of various components without complicating the design. Just remember, You are to prescribe and lab is to execute and not the opposite. RPD is a Tertiary prevention aid. Without mechanical and biological consideration, an RPD can be and often is unknowingly designed as a destructive machine.
Garis fulkrum
- Kelas I : Bipodal
- Kelas II : Tripodal
- Kelas III : Quadrilateral
- Kelas IV : Quadrilateral
Garis fulkrum (fulcrum line)/garis rotasi: garis imajiner yang ditarik melalui rest oklusalyang ada pada gigi penyangga utama
Desain GTSL

Kekuatan/beban yang berpotensi untuk merusak struktur penyangga GTSL harus dikontrol melalui penentuan desain GTSL yang adekuat. Desain GTSL harus memenuhi persyaratan berikut:
- Support
  
  Kemampuan untuk menahan kekuatan yang menekan struktur penyangga GTSL melalui penggunaan rest, konektor mayor dan basis GT.
- Retensi
  
  Kemampuan untuk menahan kekuatan yang akan melepas GTSL melalui penggunaan retainer direk dan indirek.
- Stabilisasi (Bracing)
  
  Kemampuan untuk menahan kekuatan horisontal, lateral atau torsi melalui penggunaan konektor minor, proximal atau lingual plates, bagian rigid dari klamer, basis GT dan rest
- Factors related to the opposing arch tooth position, the existence and nature of prosthesis support in the opposing arch, and the potential for establishing a harmonious occlusion can greatly influence the partial denture design.
- Opposing tooth positions that apply forces outside the primary support of the prosthesis can introduce leverage forces that act to dislodge the prosthesis. Such an effect is variable and is based on the nature of the opposing occlusion, because the forces of occlusion differ between natural teeth, removable partial dentures, and complete dentures.
- In general, removable partial dentures opposing natural teeth will require greater support and stabilization over time because of the greater functional load demands. Therefore, occlusal relationships at maximum intercuspation should be broadly dissipated to the supporting units.
ESSENTIALS OF PARTIAL DENTURE DESIGN
- The design of the partial denture framework should be systematically developed and outlined on an accurate diagnostic cast based on the following prosthesis concepts: where the prosthesis is supported, how the support is connected, how the prosthesis is retained, how the retention and support are connected, and how edentulous base support is connected.
- In developing the design, it is first necessary to determine how the partial denture is to be supported. In an entirely tooth-supported partial denture, the most ideal location for the support units (rests) is on prepared rest seats on the occlusal, cingulum, or incisal surface of the abutment adjacent to each edentulous space (see Figure 10-1, B). The type of rest and amount of support required must be based on interpretation of the diagnostic data collected from the patient.
- In evaluating the potential support that an abutment tooth can provide,consideration should be given to
  - periodontal health;
  - crown and root morphologies;
  - crown-to-root ratio;
  - bone index area (how tooth has responded to previous stress);
  - location of the tooth in the arch;
  - relationship of the tooth to other support units (length of edentulous span); and
  - the opposing dentition. (For a more in-depth understanding of these considerations, review Chapters 6and 12.)
- In a tooth and tissue–supported partial denture, attention to these same considerations must be given to the abutment teeth. However, equitable support must come from the edentulous ridge areas.
- In evaluating the potential support available from edentulous ridge areas, consideration must be given to
  - the quality of the residual ridge, which includes contour and quality of the supporting bone (how the bone has responded to previous stress) and quality of the supporting mucosa;
  - the extent to which the residual ridge will be covered by the denture base; (3) the type and accuracy of the impression registration;
  - the accuracy of the denture base;
  - the design characteristics of the component parts of the partial denture framework; and
  - the anticipated occlusal load. A full explanation of tissue support for extension base partial dentures is found in Chapter 16.
  - Denture base areas adjacent to abutment teeth are primarily tooth supported. As one proceeds away from the abutment teeth, they become more tissue supported. Therefore it is necessary to incorporate characteristics in the partial denture design that will distribute the functional load equitably between the abutment teeth and the supporting tissues of the edentulous ridge. Locating tooth support units (rests) on the principal abutment teeth and designing the minor connectors that are adjacent to the edentulous areas to contact the guiding planes in such a manner that the functional load is dispersed equitably between the available tooth and tissue supporting units will provide designs with controlled distribution of support (see Figure 10-4).
- The second step in systematic development of the design for any removable partial denture is to connect the tooth and tissue support units. This connection is facilitated by designing and locating major and minor connectors in compliance with the basic principles and concepts presented in Chapter 5. Major connectors must be rigid so that forces applied to any portion of the denture can be effectively distributed to the supporting structures. Minor connectors arising from the major connector make it possible to transfer functional stress to each abutment tooth through its connection to the corresponding rest and also to transfer the effects of the retainers, rests, and stabilizing components to the remainder of the denture and throughout the dental arch.
- The third step is to determine how the removable partial denture is to be retained. The retention must be sufficient to resist reasonable dislodging forces. As was stated in Chapter 7, retention is accomplished by placement of mechanical retaining elements (clasps) on the abutment teeth and by the intimate relationship of the denture bases and major connectors (maxillary) with the underlying tissues.
- Algoritma yang dibuat dengan tujuan membantu klinisi dalam menentukan rencana perawatan dan desain GT (clinical decision making) pada case-based reasoning process untuk GTSL
- DESIGNING RPD’s TREATMENT SEQUENCES
  
  The design process, which is a series of steps that lead toward a solution of the problem, includes identifying a need, defining the problem, setting design objectives, searching for background information and data, developing a design rationale, devising and evaluating alternative solutions, and providing the solution (i.e., decision making and communication of solutions) Designing a removable partial denture characterized by being open ended and ill structured.
Sequence
- DIAGNOSTIC ASSESSMENT
- PRELIMINARY IMPRESSIONS AND MAKING THE DIAGNOSTIC CAST
- SURVEYING THE DIAGNOSTIC CAST AND DETERMINE THE MOST ADVANTAGEOUS POSITION OF THE DESIGNED RPD’s PATH OF INSERTION AND WITHDRAWAL
- MOUNTING THE DIAGNOSTIC CAST IN CENTRIC RELATION POSITION (ARTICULATOR)
- DRAW THE IDEAL RPD DESIGN WHICH ACCOMODATE BASIC BIOMECHANICAL PRINCIPLES
- REVISE AND FINALIZE THE RPD DESIGN
DESIGNING RPD’s TO DO LIST
- Survey the diagnostic cast in a classical manner.
- Outline the supporting rests based on these rules
  - The position of rest seats is most often dictated by the classification. For Class III and IV arches, the quadrilateral configuration is indicated. The tripod clasping is used for Class II arches. For a Class I arch, a bilateral configuration is required.
  - In the tooth supported edentulous spaces, the rests should be placed next to the edentulous spaces (Fig.1)
  - In the distal extension areas, the rest seats must be located far from the edentulous ridge (e.g. RPI design)(Fig.1)
- Outline the proximal plates and minor connectors based on these rules
  - All the rests next to the tooth-supported areas must be connected to proximal plates (Fig.2)
  - In the distal extension areas, the proximal plates are not connected to the rests. Mesial rest must be connected to a minor connector.
  - Figure 2: Outlining proximal plates and minor connectors that are connected to rests
- Outline the major connector on the cast. The type of major connector is selected based on the support and rigidity needed for preservation of tissues, and anatomic limitations
- Outline the retention mesh for the denture base resin considering these rules:
  - a. In the tooth-supported areas, metal bases should be used [Figure 3]. However, in long span areas, unhealing extraction sites and severe resorption of residual ridge, using acrylic resin bases is necessary.
  - b. In the maxillary distal extension base, the retentive meshwork must be extended to hamular notch [Figure 3]; but in the mandible, these elements should cover only two-thirds of ridge length.
- Outline the direct retainers considering these rules:
  - In tooth supported edentulous spaces, the type of clasp selected is not critical. Tooth and tissue contours and esthetics should be considered, then the simplest possible clasp selected.
    
    NOTE: In the case of mesial abutment of the modification area of a Class II mod 1 situation, wrought wire should be used in a mesiobuccal undercut and half-T or I-bar clasp must be used in a distobuccal undercut[Fig. 5].
  - In the abutment tooth next to the distal extension area:
    - If a distobuccal undercut is present, the vertical projection retentive clasp is preferred.
    - If a mesiobuccal undercut is present, a wrought wire clasp should be indicated [Fig.5].
  - Each retentive element must be opposed by a reciprocal or bracing element. Lingual plating may be substituted.
- Outline the indirect retainers as far anterior to the fulcrum line as possible.
  
  In some instances, the primary rest (draw in step 2) might act as an indirect retainer, too
This simple and systematic designing procedure consists of six major steps, that must be remembered and followed in this manner:
- rests,
- minor connectors and proximal plates,
- retentive meshwork
- major connector,
- direct retainers,
- indirect retainers.
- In each step, there are 2-3 rules that must be considered.
Sumber:
- Mosharraf R. A systematic method for designing removable partial denture framework. J Indian Prosthodont Soc 2008;8:192-4
- Mc Cracken
- Stewart’s Clinical
- Arthur