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Table of Content

• Introduction
• Terminologies
• Methods of retention
• Types of flanges
• Cobalt chromium designs of major connectors
• Legal aspects
• Ethical aspects
• References

Introduction

Prosthetics is the science related to substituting the missing parts of the body with artificial ones and when seen in relation to dentistry, it is known as prosthodontics. When the prosthetics are designed in such a way that they replace the missing hard and soft tissues and are also capable of being removed by the wearer, the term removable prosthodontics is used.¹

While delivering the dental prosthesis, it is imperative to ensure that, it eradicates related oral pathology as much as possible, conserves the relation of the remaining teeth with each other and the paraoral tissues which in turn supports the prosthesis and reinstates the esthetics and at the same time does not prove as a hindrance to one’s speech.²

Terminologies used in Removable Partial Denture

Abutment – tooth, portion of tooth or implant serving as prothesis support

Tooth supported partial denture – type of prosthesis when the edentulous space has teeth on either sides and the prosthesis derives support from them

Tooth tissue supported partial denture – type of prosthesis when there is natural tooth present on one side for support and tissue on the other side and the prosthesis derives support from them

Distal extension partial denture – this is a type of tooth tissue supported partial denture that extends posteriorly on the ridge and there is no posterior support from a natural tooth.

Cast – a positive replica of the maxillary or mandibular arch

Model – a positive replica of the maxillary or mandibular arch for demonstration purpose

Indications for RPD

1. Loss of multiple teeth: such cases cannot be treated by fixed prosthesis. However, RPD provides additional support and stability from teeth and tissues by distribution of forces.
2. If there is no supporting tooth distal to the edentulous area, FPD is not an option as the unbalanced forces may produce harm to the tissues.
3. Bone loss of residual ridge: in such cases both the teeth and some portion of the ridge needs to be replaced by the RPD
4. Cross arch stabilization: RPD provides both sideward and forward backward stabilization as they as bilateral prosthesis.
5. Esthetics: RPD may give a better esthetic result compared to an FPD
6. Compromised periodontal support: when the remaining teeth have reduced bony support and are mobile, RPD may derive support both from the teeth and tissue and provide some amount of reinforcement to the teeth.

Kennedy’s classification for partially edentulous arch

Kennedy Class I arch: Characterized by bilateral edentulous areas located posterior to the remaining natural teeth

Kennedy Class II arch: Displays a unilateral edentulous area located posterior to the remaining natural teeth

Kennedy Class III arch: Presents a unilateral edentulous area with natural teeth both anterior and posterior to it

Kennedy Class IV arch: Displays a single, bilateral edentulous area located anterior to the remaining natural teeth. It is important to note that the edentulous space must cross the dental midline.

Modification spaces

Each Kennedy classification, except Class I, refers to a single edentulous area. In reality, additional areas of edentulism may occur within a dental arch. Kennedy referred to each additional edentulous area— not each additional missing tooth—as a modification space. Dr Kennedy included the number of modification areas in the classification (e.g., Class I, Modification 1; Class II, Modification 3).

Retention

A complete denture is said to be an ideal one when it when it supplements the oral functions and is accepted by the patient. To serve this purpose, it is vital that the prosthesis is fixed during function.³ Retention is defined as that quality inherent in the dental prosthesis acting to resist the forces of dislodgment along the path of placement. (GPT, 9^th edition) It is the property by which it opposes the forces which may lead to its displacement.⁴

Review of Literature

The first ever mention about the factors of retention was by Fish. He also described the three surfaces and stressed upon the idea that the tissue, polished and occlusal surfaces, all three contribute towards retention of complete denture. He proposed that an optimum layout of the three surfaces supports other factors in denture retention. While Fish emphasized the importance of all the three surfaces, there were other authors such as Craddock who believed that the polished surface had greater potential to accentuate the retention of the complete denture. According to him, the buccinator muscle has a retentive action on the buccal flange of the denture and others have suggested that external base contours and the position of the teeth must be decided in operation. Thus, the dislocation of the denture in use can be avoided. Schlosser and Fish held the view that a balanced functional occlusion is important for retention. The occlusal scheme may vary but there must not be any hindrances when in use. The exact position, level and slope of the teeth is important if retention has to be achieved.⁵

Authors such as have considered retention as the least important factor for a successful prosthesis. However, when the patient comfort is considered, retention is a very important factor. Any prosthesis that is not retentive will be discontinued by the patient in the long run. Stability is the property by which a denture resists lateral movement or backward and forward movement maintaining its position. This is also very important as it helps in mastication. If the patient can chew comfortably, physiological comfort is established and so is patient acceptability. Support is the property by which the prosthesis resists the vertical forces. The prosthesis has a long life if it has a good support.⁵

There are various factors that are responsible for providing retention and are as follows²:

1. Anatomical factors
2. Physiological factors
3. Mechanical factors
4. Physical factors
5. Muscular factors

Anatomical Factors

• The retention of the denture improves with increase in the amount of the area that must be covered thus, contributing to increased retention in the maxillary dentures. The average coverage of a maxillary denture is about 24 sq.cm while that of mandibular denture is 14 sq.cm.
• The displacement of the tissues below the denture will lead to its displacement when they move back to their original position. Thus, care must be taken to cause minimum displacement or provide relief for the tissues.

In case of maxillary denture, there is a posterior palatal seal. It extends between the hamular notches on the soft palate and maintains a seal during movement. Special care must be taken for prevention of over or under extension of the retrozygomatic space so as to maintain a border seal during function.²

In case of mandibular denture, retention may be compromised as compared to maxillary denture due to tongue mobility, which causes difficulty in establishing a lingual border seal, as well as absence of optimum ridge properties for retention. Establishing a close contact with the tissues and at the same time taking care to prevent over extension. Border seal can be obtained with respect to the posterior extension on the retromolar area by providing beading at the junction of pear-shaped pad and retromolar pad.⁵

Physiological Factors

• Saliva is an important factor for retention. Both thick and watery saliva can compromise retention. Hence patients with conditions like ptyalism and xerostomia may both face issues with retention. Salivary film must be intact between the tissue surface and the denture base. Reliefs should be given only where they are needed. Otherwise, they may cause air inclusion in the film and reduce retention. Another cause of air inclusion may be thermal and polymerization shrinkage that cause distortional changes in the acrylic denture base and it does not replicate the model from which it is produced. This especially becomes prominent if the patients have deep palatal vault. Here there is liable to be a space between the mucosa and fitting surface of the base in the midline; air inclusions and consequent reduced retention maybe seen in such dentures.³

Mechanical Factors

• Undercuts exist due to the elasticity of the mucosa and the submucosa over basal bone and they may enhance or compromise the retention. They improve the retention when they are unilateral and compromise when they are bilateral.
• Magnetic forces can be made use of for improving retention. For e.g. Magnets can be placed intramuscularly and on dentures to provide increased retention.
• Suction chambers and discs have also been used in the past which were placed on the palatal aspect of maxillary denture. They created a negative pressure area that increased retention. Their use has minimized for the fear of causing palatal hyperplasia.²
• Denture adhesives are commercially available in powder, liquid or cream form and may be used to enhance retention. The dentist must educate the patient regarding its utility, advantages and disadvantages. Also, they must know the indications so as to know which patients they can recommend the use of denture adhesives. It is usually applied on the tissue surface of the denture and is soluble in water. They essentially work by enhancing the adhesive and cohesive properties. The adhesives upon coming in contact with water for a thick hydrated mixture that has better retentive properties than saliva. The dislodging forces act less successfully on the dentures and thus the efficiency and patient acceptability of the dentures is improved. However, any allergic reaction must be observed.³

Physical Factors

• Adhesion is the attraction between dissimilar surfaces. Saliva spreads between the denture base and tissue and acts as an adhesive medium. This is why patients with reduced salivary flow have reduced retention. In patients suffering from xerostomia, there is inadequate saliva resulting in denture base sticking to the underlying tissues. This will not only lead to inadequate adhesion but will also cause discomfort and ulcers in the long run. In such patients, a sialagogue or salivary substitute must be advised.
• Cohesion is the attraction between similar surfaces. This exists within saliva and is more when the saliva is serous and watery compared to when saliva is thick and ropy. The more the denture bearing surface, the more will be the cohesive forces acting on it.
• Surface tension is “the tension or resistance to separation possessed by the film of liquid between two well-adapted surfaces” (GPT). This property also acts through saliva and opposes the forces causing movement of denture away from the tissues leading to its dislodgement. Presence of air near the margins is very important for this physical property to act. According to Stefan’s law,
  r-denture surface area
  k- viscosity of liquid
  h-width of space between the two surfaces
  F-surface tension

Thus, based on this formula, for maximum retention through surface tension, saliva must be thin and not viscous. There should be a perfect fit present between the base and tissue underneath, the condition must be optimum for good adhesive and cohesive forces and a large area must be covered by the denture base.

• Capillarity and capillary attraction are observed when a thin film of saliva flows between two surfaces and increases contact and thereby increasing retention.
• Atmospheric pressure – peripheral seal is the area of contact between the tissues and the denture. If adequate peripheral seal is achieved, there will be a negative pressure area between the denture base and the underlying tissues. Thus, during function there will be simultaneous movement of the denture base with the tissues. The partial vacuum create prevents the dislodgement of the denture and depends on the surface area of the denture.²

  

• Gravity adds on to the retention of the mandibular denture while it may cause dislodgement of the maxillary denture. The action of gravity is dependent on the weight of the denture. Thus, if a metal framework is used for the denture base, it will act to dislodge the maxillary denture if the other retentive forces are not balancing it or are not strong enough. On the other hand, a mandibular denture base made out of metal will improve its retention.³

Muscular Forces

A neutral zone exists between the buccinator muscle and the tongue such that the teeth arranged in this zone produce maximum retention. Thus, the artificial teeth must be arranged in this zone parallel to the occlusal plane.  Along with this, the polished surfaces must be so shaped as to support and contact the cheeks, lips, and tongue

• The facial muscles may also affect the retention of dentures. We can enhance the neuromuscular control by making some changes on the external surface of the denture. However, the authors following various schools of thought do not agree with this idea and emphasize that there is little significance of border seal and atmospheric pressure. The retention is achieved mainly by means of adhesive and cohesive forces.^2,3

Flange Designs

Labial flange

Indicated when bony undercuts are absent, lip line and lip activity are normal while the teeth are not sound periodontally and the alveolar supporting bone of the teeth is lost. It is contraindicated when undercuts are evident in the premaxillary region of the alveolar residual ridge and the results will not be pleasant due to fuller lips.⁶

Partial flange

This is a short flange extending on the labial surface of the maxillary ridge and extended further as the ridge resorption takes place. It is indicated when the denture is required to serve as a surgical splint and when there are undercuts in the labial and buccal portion of the residual ridge. It is contraindicated when the patient has an unusual active lip line and when it is not economically feasible for the patient to undergo so many corrective procedures.⁶

Flangeless dentures/socketed dentures

Patients may have bulky ridges and when a prosthesis is given on a bulky maxillary ridge, it will add onto the fullness and will give a chimpanzee like appearance to the patient. In some patients, preprosthetic surgery to improve the outcome may not be possible due to various diseases such as hypertension, diabetes, etc. other indications include undercuts in premaxilla and buccal residual ridge, a high lip line and an active lip that would produce unesthetic results with display of conventional labial flange. Use of flangeless dentures eliminates the need of surgery. Flangeless immediate dentures are contraindicated when there is extensive bone loss as a result of periodontal disease and hence making it esthetically perfect becomes tough, an anterior fixed partial denture in use has rendered an uneven contour to the anterior residual ridge.⁶

Another indication of flangeless dentures is patients with labially inclined premaxilla with undercuts. Giving conventional dentures will compromise the esthetics and preprosthetic surgery may be needed. Flangeless dentures present as a non-surgical option in this case.⁷

Lingual flange

For properly recording lingual flange, it is important to check the insertion of the lingual frenum, the position of sublingual caruncles, the size and movement of the tongue, the range of movement of genioglossus muscle and the mylohyoid muscle form. Relief is provided to the lingual frenum by placing a notch that does not cover the sublingual caruncles. Sublingual eminence is noticed between the lower border of lingual flange and the tongue base. The space is more when one pulls back the tongue and is less when one touches the tongue to the lower alveolar slopes. The space is also reduced in case of macroglossia. When one retrudes the tongue, the role of the sublingual wing becomes important as it ensures contact of denture border with the tongue. The sublingual prominence is translucent and lighter in color and can be identified from the more vascular and darker ventral surface of the tongue. This differentiation along with the location and action of muscles help in accurate placement of the lower border of sublingual extention.⁸

A suggested modification is flexible lingual flanges. A study performed to check the retention of flexible lingual flanges has suggested improved retention as compared to acrylic resin flanges. This may be attributed to the physical properties of the flexible acrylic which allowed effective engagement with the lingual pouch undercut and close adaptation to the supporting tissues. Flexible denture flanges have a close fit with the tissues and hence there is only a thin film of saliva between tissues and the denture base which further amplifies the physical properties such as adhesion. The negative pressure area under the denture adds to the retention.⁹

Flexible acrylic resin lingual flanges

Labial flange extensions would be more appropriate for conventional complete denture while tooth supported partial dentures (Class III and IV) such as partial anterior denture incorporating four teeth does not require maximal extension, since most of the support for these dentures comes from the teeth. Partial flange is most appropriate here while for single tooth cases, flangeless designs should be used.¹⁰

Whenever, there is entrapment of tissue above the undercut, it is preferred to make the flange smaller than relieving the internal surface. If relieving is done, a space will be created between the denture and ridge or the ridge slopes and this will compromise the retention.¹⁰

It is important that the bases have two finish line which do no overlap. Internal finish line is such that it is located away from the abutment teeth. With finish line, a separate resin and metal border can be created and this also avoids the problem of acrylic becoming very thin. If the acrylic becomes thin, it may fracture causing trauma to the nearby tissues. The strength of acrylic lies in bulk. The finish lines are fabricated in such a way that they produce a small undercut. This way, a bulk is created for the strength and retention of resin. External finish line is positioned lingual to lingual surface of the edentulous area. The location of the finish line is decided based on the teeth to be replaced and the closer one can get to re-establishing the natural shape of the palate.¹¹

Cobalt chromium major connectors in removable partial dentures

According to GPT, major connector is defined as that part of the removable partial denture that connects the components on one side of the arch to the components on the opposite side of the arch. They are different for use in the maxillary and mandibular arches.

Ideal requirements of major connectors are:

• The major connector should not be flexible. It should be unbending and dispense the occlusal forces in such a way that it does not undergo a change in shape on being subjected to occlusal forces.
• Provision of vertical reinforcement and guard the soft tissues.
• It should also be capable of providing indirect retention if needed.
• It should give chance for placing denture base when required
• It should be comfortable to wear
• It should be self-cleansing
• It should inhibit any food accumulation’

There are many types of maxillary major connectors. Out of all these, the most multipurpose major connector is palatal strap. It consists of a broad but thin metal plate that goes from one side of the palate to the other. Width of the strap can be adjusted depending on the number of teeth missing. The optimum width is about 8mm. The extension of this major connector involves three planes namely, the horizontal plane and right and left lateral slopes.

Indications:

1. Unilateral case of distal extension partial denture
2. Bilateral edentulous spaces present for e.g. Kennedy’s class III

Advantages:

1. The resistance gained is decent with the least volume of metal employed.
2. Due to its coverage of three planes, resistance to distortion is also good.
3. Patient comfort is amplified with use of thin metal
4. adhesion is increased and thereby increasing retention.
5. The design provides indirect retention and prevents displacement by sticky food or gravity.

Disadvantages

1. Large palatal coverage
2. Posterior border of the connector should end before the junction of hard and soft palate so as to avoid any discomfort
3. If the median suture is very prominent, this major connector cannot be placed across it.
4. Possibility of having papillary hyperplasia exists

Double lingual bar/Kennedy bar

This is a mandibular major connector which is also known as lingual bar with cingulum bar (continuous bar) retainer. The difference between double lingual bar and lingual plate is that the metal in the middle is absent. Lower portion is pear-shaped and upper portion is half oval, 2-3mm high and 1mm thick. The upper bar dips into the embrasures. In case of diastema, a step-back design is used. The two bars are joined with by a minor connector placed between the canine and the premolar. This minor connector serves the purpose of placing rests on mesial fossa of first premolar.

Indication:

1. Large interproximal embrasures needing indirect retention
2. Large diastema where metal display is to be avoided.

Advantages:

1. Indirect retention
2. Horizontal stabilization
3. Interproximal embrasures and gingival tissues are not covered which allows saliva to flow and patient to adapt better

Disadvantages:

1. Interference with the free movement of the tongue
2. Chances of food entrapment if the fit is improper¹¹

Legal Responsibilities

A dental technician is one who follows the prescription of the dentist to make custom made dental devices such as fixed or removable dental prosthesis, complete dentures, retainers, space maintainers etc. dentistry involves prevention, diagnosis and rehabilitation of the various hard and soft tissues of the oral and paraoral structures and the dental technician forms an important part of the dental team. Thus, as a part of the dental team, there are certain legal obligations of the dental technician especially the one fabricating removable partial denture. The dental technician should

• Be aware of the laws and regulations and work within the confines of the same
• Agree and obey the instructions given by the dentist. He can communicate with the dentists to suggest other options or materials available or to clarify doubts
• Register the dental laboratory, have a certificate to perform dental technology, disclose the source of his materials and participate in continuing dental education programmes.
• conform with the directions, recommendations and assistance given by the manufacturers of materials
• assume the legal responsibilities for work within the laboratory as specified by the laws and regulations of the country, state and/or governmental jurisdiction;
• provide the dentist with the required data about the laboratory work performed, as well as the materials employed; and
• protect all data i.e personal Information, any pathology diagnosed or his/her dental devices.

The dentists are also expected to provide with all the necessary information which will help the technician to provide the best quality product. The dental technician should have access to all the necessary information such as age, gender, specific guidelines and recommendations in written format which will also serve as a legal document. It is the legal obligation of the dentist to take responsibility for the treatment and the prescribed prosthesis. The dental technician should not make any kind of diagnosis of his own and should not prescribe any dental prosthesis. He/she should not be a part of any programmes which will provide any kind of certification for him to practice independently as a health care provider.¹²

Accuracy is very important in fabrication of removable partial dentures if esthetics, form and function need to be restored. For accurate fabrication it is of utmost importance that the instructions are correctly communicated to the technician. It should be in the form of written work authorization. It should be clear, brief and easy to understand. Studies carried out in several countries have pointed out that there were flaws especially in the creation of metallic removable partial dentures. The reason attributed for this could be inadequate communication or lack of training on the dentists’ part to write laboratory instructions. As a result, some countries have employed certain ethical and legal guidelines to design and convey the design features to the dentist. Neto et al observed that about 25% acrylic RPD cases and 75% cobalt chromium cases given to the technician has written instructions by the dentist. Written instructions are better than verbal as there are lesser chances of forgetting any important instruction and also that it serves as a legal document. Verbal communication is helpful for any additional clarification that the technician might require from the dentist. The study also found out that the dentists were less likely to communicate with the technician if an acrylic RPD has to be fabricated compared to a cobalt chromium RPD as the cost involved in the latter is more. The dentists should never rely solely on the technicians to design the prosthesis on their own as it is the dentists who examine, diagnose and plan the treatment. A design diagram along in the work authorization will help achieve better outcome.¹³

Another study pointed out the failure of most of the dentists in including surveyed preliminary casts. Surveying helps in identifying the ideal path of insertion and prevents unnecessary damage to the tooth structure. It should be done prior to the planning the partial dentures as the path of insertion; requirement of tooth preparation and master impressions may be identified. These findings will affect the functional as well as the financial aspect of the treatment as a poorly planned prothesis will lead to tissue damage while repeat fabrication of prosthesis leads to unnecessary financial burden and wastage of time. Another finding of this study was that there was lack of proper disinfection techniques of the impressions. The body fluids such as saliva and blood may be commonly seen on the impressions. In many cases, the technicians were unsure if the impressions sent to them had been adequately disinfected as there had been no mention of it in the written communication.  Sending such infected material via the postal system is illegal and unethical in the United Kingdom. It may cause transmission of diseases such as HIV and hepatitis from the patients to the other members of the dental team due to lack of awareness.¹⁴

Ethical Aspects

• Confidentiality: the written communication received by the technician from the dentist must include a complete medical history of the patient. the technician must preserve a copy of the patient records and should keep it confidential. A complete and elaborate account of any present or past illnesses, allergies, infections, medications must be conveyed to the technician. Any update in the medical history during subsequent appointments must also be conveyed to the dental team. Having the entire information can serve two purposes: it will ensure the technician follow universal precautions in case the patient is suffering from any infection and will avoid the particular allergen, the patient is allergic to.
• Non-maleficence: once the identity of the patient is known to the technician, he/she has a moral obligation to not discriminate against the patient on the grounds of race, religion, gender, nationality. They are obliged to do no harm to the patient. This also means that the dental technician should know his or her limitation when the relevant skills are concerned and to convey to the dentist that the prosthesis asked to be fabricated is beyond his ability. In this case, the dentist can decide the next course of action.
• Truthfulness: the technician should always try to justify the trust of the dentist and the patient. This involves clear communication, maintaining integrity and promoting reliability.
• Help in informed decision making of the treatment while suggesting all the possible options and giving the final right to the patient to decide what is best for the patient.
• Creating and maintaining a safe work environment
• Respecting the opinion and decision of the dentist in prosthesis fabrication but at the same time make polite suggestions to the dentist about any new material or alternative treatment options the dentist might have missed.
• Consent is important even for the dental technicians before they start treatment. The consent should be clear and specific. In case the patient is under age, the parents or guardian’s consent is imperative before treatment.
• Promote and maintain high standards of service. For this, the technician should regularly upgrade his or her knowledge by attending continuing dental education programmes and using only certified materials in the laboratory for processing. Any substandard material used only to save the cost may produce undesirable effect on the patients and in extreme conditions may even risk his life.
• No monetary reward should be accepted from the patients or the dentists.¹⁵

When faced with ethical dilemma, Doctors Ozar and Sokol suggested four steps to help in taking ethical decisions

• The initial step is to look for alternatives, the most suitable direction, the associated situations and communicating the prominent characteristics of the treatment to the patient.
• Second step is the professional risk. The normal course of action must be thought and then apply it to make the choice for the situation.
• Third step is thinking about all the other things at stake if alternative treatment options are considered.
• In the final step, one must decide what should be done by ranking the options, applying professional principles, morals, regulations and standards. ¹⁶

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