Firstly, a sprue is a channel that is created to give passage for molten alloy to flow through, filling a mould formed by a wax pattern. For them to be effective they must be positioned carefully. They can either be direct sprued from the casting crucible to the pattern used mostly for single units (crowns, onlays, inlays etc.) Or indirect which has a connector or reservoir bar, positioned between the pattern and crucible former. Commonly used for multiple single units or partial dentures. The alloy Ticonium is cast using a single sprue approaching from underneath the refractory cast. Both type IV gold and chrome-cobalt alloys are cast using a sprue from above. Co-Cr partial denture castings have multiple sprues, a main sprue generally 4mm in diameter attaches to the thickest part of the pattern e.g. major connector. Areas separated by a long span of latticework or meshwork have auxiliary sprues attached to disperse the flow of molten metal reaching these areas further away from the main sprue. They are slightly smaller in diameter than the main sprue curving down from the entrance to the pattern. Molten Co-Cr is sluggish to flow which is why a large diameter sprue is used compared to single units i.e.. Crowns. These also in turn act as a reservoir. Without the additional sprues miscasts will occur as Co-Cr contracts at a significant rate. Spruing gold, like Co-Cr, is sprued to the thickest part of the pattern often the cusp tips. If the sprue is attached to a thin section, this area may solidify before the metal can spread throughout the mould causing porosity or a casting which is under extended. Also, if the metal if forced to travel between thin-thick-thin areas it may cause a turbulence causing internal deformations. Smaller sprues are used with single units unlike denture framework. A 3mm sprue is enough when used together with a reservoir. These are a bulbous area built up in wax at double the size of the thickest part of the cast, they should be positioned at the hottest part of the mould (central) around 3mm from the wax pattern itself. They are used so that any contraction porosity should then occur in the reservoir and not in the casting.  Unlike chrome, which is sluggish to flow, gold is high in density flowing at a higher speed and solidifies quicker meaning it is less likely for defects to occur therefore a smaller sprue can be used.   For all types of castings and materials sprue connections should be rounded. If sharp edges are left in the pattern the refractory material may break off as the molten metal enters, carrying the fragments deep into the casting.

Acrylic lingual plates are often prescribed as a temporary solution as a cheaper alternative where the dentures life is expected to be short or where alterations will occur. As a more permanent connector it has many disadvantages. A lingual plate is used when there is insufficient vertical space for a lingual bar. This lack of space may be related to gingival recession, high muscle attachments, or high frenum attachments on the lingual aspect of the mandibular arch. Often less scope is available when designing an acrylic partial denture as it is weak and less rigid than metal alloys. This means that they are often more bulky to prevent flexing or fracture and covers more of the gingival margins. Especially when designing a lower APD the surface area is much smaller and little relief is given to the gingival margins as the major connector must cover the abutment surfaces to provide additional strength to the acrylic. These design issues cause tissue damage over time and with little to no tooth support the denture is primarily tissue supported instead. For the APD to be tooth-supported metal alloys in the form of rests and clasps are integrated into the acrylic major connector, however these connections are weak as there is no chemical bond present between stainless steel and acrylic. Considering acrylics bio-compatibility, some people suffer with allergic reactions to acrylic and are not able to have an acrylic denture. It also is a porous material suffering with a higher accumulation of plaque. When comparing to a metal major connector, its thermal conductivity is inferior. This becomes important when registering different temperatures in the mouth. Lastly acrylic resin is radiolucent, if accidentally swallowed the location of the prosthesis will not show up through x-rays making it more difficult to find its location.

·       The supporting teeth should be physiologically sound. Abutment teeth are used to support the denture and prevent it from dislodging. If the teeth are compromised the metal denture will apply pressure causing it to weaken the structure and possibly cause removal of the tooth. ·       Periodontal breakdown- Clasped teeth suffer periodontal breakdown when excessive functional forces in excess of their physiological limits are placed on them. This danger is most often witnessed in the abutment teeth of free end saddle cases. The bony structure and soft tissues covering edentulous ridges and hard palate breakdown under excess pressure, leading to resorbed ridges. Therefore itll lead to tooth mobility, possible aggravation of existing periodontal diseases or inflammation of mucous membrane.   ·       Oral Hygiene- Any component placed in the oral environment covers part of the tissue, this has an undoubted affect on oral hygiene. The component should either be self-cleansing (have space beneath, around it) or be easily removed to enable access for cleaning. If the patient is unlikely or unable to regularly clean bacteria can grow under the denture baring area which can lead to inflammation of tissue that could progress to the underlying tissue and diseases such as teeth decalcification and caries. ·       Allergies- Some (although it is less than 3%) have an allergic reaction to Co/Cr alloy used in denture construction. Other alloys or designs should be a consideration in these cases. Stainless steel, Gold etc. ·       Unnecessary- If there is no movement of surrounding teeth predicted, no increase in masticatory efficiency can be achieved, or no improvement of aesthetics then a partial denture is deemed unnecessary and will not provide any benefit to the patient. ·       If the denture has a design error within the occlusal area this can cause problems creating tooth mobility or possible muscle dysfunction damaging the TMJ. More still trauma induced by components of the metal denture can cause abrasion to the tissue, inflaming the soft tissue and possible the underlying tissue or in the worst case denture induced hyperplasia, where excess folds of connective tissue form as a protection against continuous rubbing.

Retention’s definition is “resistance to removal from the tissues or teeth”. The amount of retention provided by a clasp assembly is dependent upon many factors. These include the type of clasp used, flexibility of the retentive arm, the axial convergence of tooth surfaces apical to the height of contour. As a general rule the amount of retention designed into the RPD should always be the minimum necessary to resist reasonable dislodging forces.   Type of clasp used:         Circumferential clasp (Circle or Akers) - Ring Clasp - Embrasure Clasp (Double Akers) - C Clasp (Hair pin or reverse action)         Bar Clasp - RPI Clasp - RPA Clasp - Combination Clasp The type of clasp used determines how it’ll be placed in relation to undercut. Retentive portions of clasps enter undercuts by approaching from either gingivally (infra bulge) or occlusally (supra bulge). The gingivally approaching clasps exert a trip action which increases retention. Varying the angle at the contact with tooth increases or decreases the trip action.   Axial convergence of tooth surfaces apical to the height of contour:         Size/how far into the angle of convergence (undercut) the clasp terminal is placed         The depth of undercut engaged or the location of clasps on the undercut of tooth is determined when surveying. The greater undercut, the greater retention. An undercut too big will strain the clasp and traumatise the teeth. An undercut too small will give insufficient retention         Upon insertion and removal, the clasps arms will contact the axial surfaces of the abutments Clasp arms will flex as they pass over the greatest convexities of the abutments = Height of contour Once they pass the "height of contour" and the RPD is fully seated, the retentive arms return to its "passive" state The retentive terminus is placed in an undercut area relative to the path of insertion         Undercut is measured perpendicular to the path or insertion. The degree of undercut is measured using gauges which considers the materials MoE     Flexibility of the retentive arm: (related to length, diameter, cross sectional form and clasp material)         Length, the length of clasp can affect clasp retention, increasing the length increases the flexibility, measured from the point where the taper begins, may be increased by curving.         Diameter - Thicker the clasps, the more rigid they are. The thickness and length of clasps must be related to the MoE of the alloy used.         Cross sectional form - round more flexible         Clasp material - wrought wire has greater tensile strength than cast (can be used in smaller diameters) Materials: gold, steel,chrome all have different MoE. MoE: a greater MoE means a stiffer material. It would require more force to deflect it to the same degree as a material with a lower modulus. It would require more force to deflect it to the same degree as a material with a lower modulus.   Clasp length & material      &   Depth of Undercut engaged        Cobalt /Chromium 0.25mm (10 thou. inch)     Cast gold  (e.g. premolar clasps). 0.25 mm      Long cast gold (e.g. ring clasps) 0.5mm (20 thou. inch)       Wrought clasps (gold/ stainless steel) 0.75mm (30 thou. inch)       Long wrought clasps (may engage) 0.75mm (30 thou. inch)  

Definition: Indirect retention is a method which attempts to reduce the continual loading and unloading of tissues by preventing movement of the denture base away from the mucosal tissues. It uses an anteriorly placed component (on the hard or soft tissues) to prevent rotation occurring around a supporting unit (occlusal rest, onlay). Occlusal rests, onlays and continuous clasps, may act as indirect retainers. To carry out these functions they must be strong, rigid and fit tooth surfaces accurately. Occlusal rest- a rigid extension of a removable partial denture onto the occlusal surface of a posterior tooth for support of the prosthesis. Onlay- an occlusal rest portion of a removable partial denture that is extended to cover the entire occlusal surface of the tooth Continuous clasps- Circumferential clasp of a removable partial denture in which the body is attached to an occlusal rest and the clasp arms extend across the buccal or lingual surfaces of more than one tooth before engaging a proximal surface undercut. Principles to be considered when considering incorporating supports in partial denture design. 1. The vertical load should be distributed over as many teeth as possible. One natural tooth to one artificial tooth. Hence as a saddle length increases, so should the number of supporting teeth. 2.Wherever possible, anterior and posterior bounded saddles should be supported. 3.Support is most effective when movement of the load-bearing tooth is prevented due to the presence of adjacent teeth. 4. On an isolated standing tooth, support should be spread over both sides to prevent rotation. 5. When designing a partial denture for free end saddles (Kennedy classes 1 & 2): By moving the support anteriorly, the load taken by the supporting tooth may be decreased, causing the load on the ridge to increase.  6. Support should not interfere with occlusion or articulation. Any imbalance of the occlusal table can cause TMJ problems.  Materials: 1) Yellow or white casting golds. 2) Cobalt Chromium casting alloys.  (Rests made from this may be thinner than (1) above.) 3) Wrought stainless steel.  (These are used only temporarily since they cannot be adapted to fit tooth surfaces accurately.)

The high frequency induction casting machine- Induction systems are capable of melting alloys by passing a high-density electric current through a copper coil. This electric current produces a magnetic field around the coil. The polarity (direction) of the current is then altered rapidly from positive to negative. Placing an alloy capable of having its polarity changed (usually magnetic alloys) within the magnetic field causes the molecules of the alloy to oscillate and the molecular bonds to break down. The intensity of molecular activity produces heat and the alloy becomes molten, in effect, the heat is generated from within the alloy itself, not the coil. In typical dental technology induction systems, the coil surrounds a tapered crucible into which an alloy is placed. Gold alloys are not capable of polarising, therefore, a carbon crucible must be used so it is the crucible becomes hot and then transfers the heat to the alloy. To prevent heat from the metal and crucible damaging the induction coil, water is circulated through the copper coil to keep it cool during the melting process. Carbon arc melting is used to melt high fusing alloys. A high electric input (30 A) is used to create an electric arc at the end of two electrodes. The electrode arms are adjusted to 3-4 mm apart with the arc between them and this placed about 12 mm from the alloy to heat it. The manufacturer’s instructions must be closely followed because the arc can damage skin and eyes. The apparatus should be used in a separate room with the means to prevent access to unprotected personnel. Normally, the operator is required to wear a black apron to prevent glare and a specially designed facial visor. A foot operated control switch is generally used to allow both hands to be free during its operation. In carbon resistance melting an electric muffle (similar to a small furnace) is used to melt alloys up to 1000°C. The alloy is placed in a carbon crucible, both are then placed in a muffle (usually a part of the casting machine). The carbon crucible heats up and melts the metal, usually gold.

The lost wax process- The lost wax process is a relatively simple and inexpensive way of producing acrylic dentures and cast restorations. A wax pattern is embedded in a mould material (such as plaster or investment) and the wax is then eliminated either by melting it away with hot, running water or heating it to such high temperatures in a furnace that it is incinerated. We can then force molten metal or acrylic down a channel or directly into the cavity, filling the void left by our original wax pattern. Thus a metal substructure or acrylic denture is produced. The mould materials for casting alloy structures must be capable of withstanding very high temperatures without disintegrating. In addition, upon heating the mould must undergo sufficient expansion to counteract the contraction of the alloy on cooling to normal room temperatures in order to produce an accurate fitting metal substructure. This mould cannot be reused as it's destroyed when removing the pattern. The wax pattern for a dental alloy casting may be created in two different ways: The duplicate refractory cast technique. Here, the master model is surveyed, the path of insertion selected and undesirable undercuts waxed out. A duplicate model is produced in refractory investment material and the wax pattern produced on this duplicate model. The wax pattern is invested on the duplicate model. This technique is commonly used in the production of partial or full metal denture bases. The "lift-off" technique. The wax pattern is formed on the master cast or die, lifted off the model and then invested. This technique is only used in simple designs (like crowns and inlays) because complex patterns are likely to distort.

Electro – brightening- Electrobrightening/electropolishing is the process by which we submerge a cobalt chrome framework in an acidic solution then run an electric current through the alloy to remove the outer layer at a microscopic level. This procedure creates a high shine on the surface of the alloy. Immediately after sandblasting a Co/Cr alloy substructure it is important that we don't touch it's surface again with our bare hands. This is because the residual grease on our hands will adhere to the surface of the chrome and prevent the electrolytic action. The result would be dark "spots" on the surface of the chrome. To electrobrighten a Co/Cr substructure, suspend the chrome on an anode (here, a non-reacting cathode stainless steel hook) submerge it in the electrolytic solution, turn on the current and set the timer. How does electrobrightening work? The electrolyte and Co/Cr substructure act as anodes (+) and a non-reactive stainless steel plate submerged in the electrolyte acts as a cathode (-). When an electric current is applied to the electrolyte it acts as a conductor and allows the current to pass through it to the cathode plate, creating an electric circuit. Metal ions (atoms or molecules with a net electric charge) on the surface of the metal framework now become positively charged (+), oxidise and are attracted towards the negative cathode (-). Electric fields naturally focus on microscopic "peaks" (micro-projections) possibly because of higher current densities in these areas. This means more material is taken from the "peaks", and less from the "valleys" (micro-depressions). This is referred to as anodic levelling (see 5 & 6 in Fig 20 below). The result is a smoother, more reflective surface which appears brighter to the eye (even though as little as 5-10 microns of metal are removed). A by-product of oxidisation is the formation of oxygen bubbles along the surface of the framework which also contributes to the cleaning process by removing microscopic debris.  A current of about 2.5 Amps for 6 minutes is sufficient for the average RPD alloy framework. However, this may vary depending on the temperature and age of the electrolyte (warm electrolyte is quicker than cold and old electrolyte is less efficient). A low electric current will result in an "etched" surface and a high current rapidly reduces the thickness of the alloy. CHECK THE ELECTRIC CURRENT! An example electrolyte solution could be made up of: Ethylene glycol: 80% Sulphuric acid: 15% Water: 5% (Strananought, 1974, p215).

Every first described this type of denture in 1949, named after himself the ‘every’ denture is described as a precision plastic partial upper denture. The four main principles are: Wide embrasures & free-occlusion- An ‘Every’ denture should have wide embrasures (between contiguous standing and artificial teeth). This reduces gingival contact and reduces plaque accumulation. This principle is reinforced in the requirement to leave the gingivae uncovered wherever possible.‘Free-occlusion’ refers to the prevention of any occlusal interference, which may result in damaging lateral forces. A free occlusion has no tendency for the upper and lower cusps to interlock or hinder movement. Designed free of the immediate supporting tissues of the remaining teeth. (If the denture base is kept clear of the supporting tissues of the remaining teeth the risk of caries and cut out denture caused periodontal disease is reduced.) Palatally, the acrylic should be at least 3 mm from the gingival margins. Point contact between adjacent standing and artificial teeth & No clasps or occlusal rests (If we get rid of clasps the risk of caries and lateral thrust to the teeth is reduced and eliminates the effects of torque.) Natural teeth have a buccally placed contact with each other and this point contact is copied between adjacent standing and artificial teeth. To maintain this point contact throughout the arch, ‘distal stabilizers’ are used to contact the distal surface of the last standing tooth. These are not clasps but are used to maintain the point contact and prevent the last standing tooth from drifting distally, so maintaining contact along the arch. Maximum retention following the principles employed in complete denture construction. Retention is obtained primarily by atmospheric pressure with a modified peripheral seal. Also by the accuracy in fit between the denture base and mucosa. Finally, maximum retention is obtained by following those principles normally employed with complete denture construction. This includes extending the denture base to cover as large an area as possible. The fit of the denture should be accurate and the polished surfaces should be shaped to assist muscular forces. If we can get a large saddle area, lateral and vertical thrust to the alveolar ridge will be spread over the widest possible area and this will reduce alveolar absorption to the minimum.

Definition: Stress breakers can be designed in RPD’s by using connectors fitting between the retention unit and denture base. It is designed to relieve abutting teeth from excessive stress during chewing. Stress breakers benefits: Horizontal forces transferred to teeth are minimised. There is a balance of force between the teeth of the alveolar ridge. The intermittent pressure results in a massaging effect to the alveolar ridge. Re-lining is unnecessary and is claimed to save the abutment tooth. Splinting of mobile teeth is still possible.     Stress breakers come under two categories: -        Type 1 stress breaker- utilizes a hinge or movable joint between the direct retainer and the denture base, This joint may be in the form of hinges, ball and socket devices or sleeves and cylinders. Hinged type stress breakers allows vertical and hinge movement of the base. The hinge is usually rigid. The soft tissue absorbs a minimum of the load adjacent to the hinge, and a maximum of load toward the distal of the ridge. The base aloows movement in a vertical plane only. The movement may be unrestricted, or it may be controlled by a stop arrangement built into the device. This serves to prevent some direct transmission of the tipping forces to the abutment teeth as the base moves toward the tissue under function. The hinge type of device spares the tooth from all stresses that results from vertical movement of the base.   -        Type 2 stress breaker- utilizes a flexible connection between the direct retainer and the denture base. This design allows movement of the distal extension base. Also included in this category are those using a movable joint between two major connectors. The earliest of such connectors were double lingual bars of wrought metal, one supporting the clasp and the other components and the other supporting and connecting the distal extension bases More commonly there are: Split major connectors-A lower RPD framework with partial division of a lingual plate to achieve stress breaking action. (split is provided between the denture base area and major connector) when occlusal forces are applied they’re transferred more towards the tissue supported base and then they are transmitted to the abutment teeth. Wrought wire connectors soldered to the main major connector Clasps having a stress breaking effect Hidden lock partial dentures- two piece casting, the top half is cast with major connector supporting the direct retainers and then the bottom half (the connector between denture base) is cast to the major connector. The split between the two connectors is made by a thin oxide shell. The bottom half (connector) is two bars connected by the midline. Disjunct partial dentures- tooth borne and mucosa borne parts of denture are disjoined. Provides independent movement between the tooth supported and tissue supported parts decreases the forces on periodontally weakened remaining teeth.   Stress breakers may be indicated if the patient has well formed residual ridges and weak abutment teeth. This is because the forces applied to the abutment teeth must be minimal in order to maintain the remaining health of the teeth. Too much force and the abutment teeth may be damaged. As the ridges are still healthy the stressbreaker is able to minimize the forces applied to the abutment teeth and direct it down into the tissues. If the ridges had resorbed the tissues would be left more compressible resulting in more stress acting on the abutment teeth. Another indication is if there are distal extensions in the RPD. The longer the saddle the more torqueing force and rotation may occur, these forces will increase the stress applied to the abutment teeth. Finally if internal attachments are used to retain a distal-extension base…..

Due to the fact that both Kennedys class 1 and 2 involve free end saddles, problems occur as a result of the dentures inability to anchor itself down. Both classifications must derive support from the remaining teeth and residual ridges, however if designed with unequal forces being distributed over the RPD, rapid destruction of the periodontal tissues may happen with an additional potential of abutment loss. Rpds must provide equal distribution of forces if not, destruction of the residual ridges will decrease their height, causing rocking to the denture and needing continuous realignments of the denture. All portions of the residual ridge that can provide support should be covered by an accurately fitting denture base. Broad coverage permits favourable distribution of stresses often described as a snowshoe effect. With the free end saddles in some instances sticky foods may lift denture bases away from supporting tissues. This displacement produces rotation of the rpd on the most posterior abutment, auxiliary rests should be placed as far as is practical from the fulcrum line, rests minimize rotation and aid in retention, these are indirect retainers. Flexible direct retention is designed to flex or move into an area of greater undercut as forces are applied to the rpd, clasp design is essential as this prevents rocking movement in the denture. Class 1 must compensate for rotational forces as both saddles are free to move, they must be designed in consideration of- -        provision of optimum support for the distal extension denture bases, -        incorporation of flexible direct retention, -        and provision of indirect retention. Class 2 gains some retention from the opposite side however must be designed with- -        must include well-adapted denture base, -        properly designed direct retention, -        and appropriately positioned indirect retention.

Occlusal supports help to control stresses by directing forces within the long axis of the abutments. Whilst the periodontal ligaments can be subjected to vertical forces, they struggle under horizontal or torsional forces. Rests function to: -        prevent movement of the removable partial denture framework in a cervical direction, -        preventing trauma to the mucosa located around the abutments and beneath the framework. -        Also maintains the retentive portions of clasps in prescribed positions. -        Assist in distribution of occlusal loads of several teeth. The rest is successful in resisting movement of the denture, by securely seating into a preparation of an abutment tooth at an 90o angle or less with the proposed path of insertion to prevent its migration. Support is provided almost entirely by rests. Some support is furnished by other ridged metal located above the survey line e.g. rigid portions of retentive arms and bracing and reciprocating arms. However, they should never be relied on to provide primary support. Occlusal supports are attached to abutments. The number of abutment teeth there are, influences the amount of force that each tooth must withstand. So if there are less abutment teeth available the abutment tooth must direct a larger load causing stress onto a tooth which may cause damage or mobility. Alternatively implants can be used as a rest. In this application, the implant eliminates compression of supporting soft tissues, controls vertical movement of the denture base, eliminates or alters fulcrum lines, and serves to increase support and stability of the prosthesis. An acrylic every denture is designed with no type of rest/stops/clasps but when compared to a metal denture, if no occlusal support is given the metal could possibly rub against the soft tissues as it is a tougher material for the oral cavity. Also metal being the heavier choice, clasps are needed to maintain retention of the denture. With clasps, rests aid in keeping them in their desired location. Occlusal supports may not be needed when it comes to acrylic dentures, however when working with metal, a heavier more rigid material, supports are needed to maintain position of the framework. Also protecting the mucosa by directing occlusal forces more evenly down the long axis of the abutment teeth.