Triple trays was invented and patented by Dr. S Werrin in the 1980’s with the mantra of “Do it better, easier and faster!” Triple trays are referred to as ‘Dual Arch Trays’ and enables the dentist to capture the prepared restorative areas, the opposing arch and occlusal registration in one procedure. This saves chair time and impression material.
In theory this works, but why is it over 70% remakes in crown & bridge occur from the use of triple trays? Here is some research into this.
Impressions are key information between Dentist and Dental Labs across the world to transfer reproducible information of the patients mouth since the mid 1800’s. In my time I have found the clinical and technical work is usually of a high standard but why do we have remakes? It’s usually the impression itself.
As a commercial dental laboratory we see 1000’s of impressions weekly. We can only see the physical factors of the impression not the representation of the true I/O information as we do not have access to the I/O of the patient so we cannot judge if the impression is true or false.
Acrylic, Co/Cr & Valplast RPD Design principles
Removable partial dentures assist with mastication (chewing food), help maintain phonetics (speech) and help with aesthetically pleasing to blend in naturally when are teeth missing.
They can also help with maintaining oral health and the prevention of over eruption, titling and drifting of adjacent teeth into areas where teeth are missing.
They should be designed and constructed to minimise oral damage and is used with a high level of oral hygiene from the patient.
The underlying principles of support, retention and stability should be understood whatever type of partial denture is to be made.
With any form of dental treatment the main aim is to provide dental treatment than provides function and aesthetics that manages the patients expectations within the limitations.
Before any treatment, there are crucial clinical stages that set the foundation of designing a partial denture. Obtaining the patient history which will indicate the parameters of expectations and I/O, E/O examinations that will indicate the limitations. You will then be able to formulate a treatment plan that will manage the patient expectations within the limitations. Within the treatment it’s a good time to find if they are allergic to anything e.g latex, nickel, monomer etc.
The natural teeth should be examined, their number, position and occlusal relations noted, and evidence of caries, plaque, periodontal diseases and tooth mobility recorded. The state of the mucosa should be examined and previous dentures inspected in relation to the natural teeth and the patient’s experience. Radiographs of the teeth and supporting tissues may be necessary. Pathological conditions should be investigated and treated appropriately.
Extraction of teeth, periodontal treatment or restorative treatment of any kind should be completed prior to partial denture construction. However, it is essential that the provisional design of the dentures is made early in the treatment plan so that the most appropriate restorations are placed in any natural teeth which will act as abutments for the partial denture.
When crowns or cast restorations are to used to restore abutment teeth they should be designed to incorporate suitable features for the partial denture (e.g. guide planes, rest seats, milled ledges and suitable undercuts for clasps). Dietary advice and attention to oral hygiene should be given as appropriate.
Design principles for RPDS :
There are various resources online with information that go into detail on RPD’s. This guide simplifies the design principles with the key components that a dentist and dental laboratory will need to create a successful RPD.
After any clinical assessment, primary imps should be taken to assess the design of an RPD:
Main stages in design principles of any RPD:
Step 1: Patient expectations, parameters, limitations and future proofing
Step 2: Surveying and design of RPD within expectations, parameters and limitations
Step 4: Mouth preparation, rest seats, guide planes and retentive areas
Step 5: Construction of RPD within surveying, design, expectations, parameters and limitation
Most common types of removable partial dentures:
Cobalt Chrome (Co/ Cr, metal with acrylic saddle areas)
Valplast Flexible (Nylon flexible material)
Minimum thickness RPD’s (mm):
Acrylic: between 3 – 4 mm
Cobalt chrome: - between 1 – 2mm (can go beyond it’s limitation)
You can go beyond this minimum thickness within reason, but risk weakening the materials mechanical properties and may cause it to break.
How do you choose what material is best for RPD’s ?
There are many factors that influence this decision:
After primary imps stages:
After taking the primary impression ask the laboratory to cast study models and design a RPD based on the patient expectation and parameters.
When you have study models to hand, it allows you to see everything from all angles and will help with the design process. It allows you to evaluate the occlusion, adjacent teeth, connector design, retention and support areas and surveying a path of insertion.
The design will also highlight areas that may need preparation within the mouth e.g there may be no space for a rest seat as the occlusion is too tight as shown on the articulated models.
The main areas of preparation in the mouth are rest seats, guide planes and retentive areas.
The study model, design on the model and a secondary special tray impression should be sent for the next stages for rpd construction. A secondary impression is required for more accuracy, a primary impression is more a diagnostic tool.
Please see below a Co/Cr RPD design sheet to download in.pdf and print and send with your case!
5 Components of any RPD :
The component or components that provide the area of where the teeth are placed and provide the flange extensions in acrylic of an RPD.
The function of a component or components to provide resistance to displacement towards the oral tissues on function.
Force is transmitted to:
Mucosa via palatal areas, saddle areas.
Teeth via rest seats, occulsal, cingulum, incsical.
The function of a component or components to provide resistance to displacement away from the oral tissues on function via:
Mucosa coverage: Giving aheshion / cohesion
NB Muscular effort: Tongue, Cheeks, Lips
Clasps – C clasps, roach clasps, I bar
For clasps to be effective, the tips of the clasps must be engaged into an undercut with an ideal depth of undercut at 0.25mm.
For a chrome clasp the ideal minimum length is 14mm
For a wrought stainless steel clasp a minimum of 7mm.
Studies have shown anything below the minimum length are subjected to deformation and became non retentive. If a clasp is placed at more than 0.25mm of an undercut it will deflect beyond it’s elastic limit and will deform permanently.
Depth of undercuts
Deep undercut do provide more retention but co/cr clasps that engaged into a deeper undercut have to be more flexible to prevent permanent deformation. Stainless steel wrought clasps are more flexible and is favoured in this situation. If there is undercuts, e.g due to short teeth or conical teeth, it’s possible to create an undercut by the use of composite bonded or trim the teeth to create an undercut.
Rest seats on the RPI system on a free end saddle will have the rest seat on the mesial aspect of the tooth. On a bounded saddle it’s common to place the rest seat next to the saddle area.
A study has shown retainer designs with distal rest seats tend to move the clinical crown distally and the root mesially at the apex, resulting in horizontal forces in the bone. By placing rest seats more anteriorly provides an axis of rotation that directs applied forces in a more vertical direction.
What is the RPI system in Co/Cr ?
It’s short for rest, plate and I bar clasp. The rest seat is placed on the mesial aspect of the tooth with an I bar clasp placed on the tooth and retention mesh for acrylic.
Bracing and reciprocation
The function of a component or components to provide resistance to lateral displacement during function via extension of flanges into the full depth of the sulcus and the reciprocal arms and non retentive components of clasps.
What are guide planes?
They are prepped areas on teeth adjacent to an edentoulous area and assist with bracing and reciprocation and path of insertion.
Path of insertion:
This is obtained by placing the models onto a surveyor and assessing an ideal path of insertion and displacement whilst highlighting the ideal depth of undercuts for retention. This is a vital tool and uses a graphite rod and various tools to block out and keep undercuts for a path of insertion.
There are many types of paths of insertion for RPDS, we have three main categories, single, multiple and rotational which is determined by the tooth undercuts, infrabulge & suprabulge, guide planes, tissues and anatomy.
The connector design is based on many factors from patient expectations and preference, parameters, limitations and future proofing. Wherever possible, connectors in chrome’s should be placed at least 3mm away from the gingival margins and should be designed with hygiene and the least amount of risk to adjacent teeth in mind.
The most common types of connectors are bar and plate.
Bar – Feel bulky but are kind to the mucosa.
Plate – Covers more of the mucosa palatally and lingually and is less noticeable to the patient
Please see below our information on our laboratory services for reemovable prosthetics :
Also see guides on:
The first stage of any denture case is: Patient details & history, denture history, E/O & I/O examinations, diagnoses, prognosis, treatment options and a treatment plan.
How to take bite registration is broken down into 3 steps:
Step 1: Orientation
Step 2: Facial and smile features
Step 3: Register the bite
Tools: Fox’s bite plane, rim former, Willis gauge, wax knife, Bunsen burner, PVS dispensing gun.
Materials: Wax, bite registration paste.
Step 1: Orientation
Information from the patient facial & smile features creates the parameters for the technician to work too. As the Technician will not see the patient, we have to provide this information in a form that they will understand e.g lines scribed on a bite rim.
The retention, extension, stability and support of each base plate would be the assessed individually before continuing. If any of the above is not correct preform an imp wash inside of the base plate and have a new model made. If all of the above is sufficient, continue assessment:
Orientation: Frontal Plane & Occulsal Plane
For establishing the orientation a Fox’s Bite Plane tool is used. Fox’s Bite Plane establishes the orientation of the occlusal plane in an anteroposterior direction (Spee curve) and also the frontal plane with the interpupillary line.
With the use of a fox’s bite plane we assess the frontal plane by placing the fox plane on the bite block anteriorly and checking if it's parallel to the interpupillary line. Adjust the anterior section if one side is higher or lower until this is level and parallel to the interpupillary line. Use a heated Rim Former or carve the wax..
The occlusal plane is assessed by placing the fox plane in the same position and placing a horizontal device e.g ruler from the ala of the nose to the tragus of the ear and making sure the orientation of the fo’x bite plane is parallel to to this.
This will set the parameters of how the anterior and posterior teeth will be placed for aesthetics, phonetics, comfort, chewing efficiency and balanced occlusion.
Tissue support is determine by preference, clinical decision e.g checking the profile buccally of the previous denture. This determines how far the anterior teeth will be positioned forward and how thick the anterior flange will be.
Step 2: Facial & Smile Features
Facial features sets the aesthetic parameters for the anterior teeth in terms of mould selection, e.g width and height of anterior teeth & positioning of the teeth within the centre line & canine lines. All facial features should be scribed onto the upper bite registration.
The centre line (mid line) is dictated via the philtrum, DO NOT use the nose, as this can be give a false reading as the nose is not usually centralised with the mid line of the face and can be skewed. This allows centralisation of the upper centrals to the exact mid-line of the patient.
The canine line is dictated via the width of the nose, this is common in 95% of humans. This determines the width of the anterior teeth. The canines should not go past this line, unless other factors dictate the canine lines e.g patient preference. The body or the labial mesial body should be on or just after this line in the tooth set up, this will allow a natural alignment within the patients facial features.
Smile features work in sync with the facial features, for the creation of the anterior aesthetic parameters. This also determines how much tooth should be shown when the patients is smiling or how much is shown when the patients lip is at rest, for example when the patient smiles and shows too much gum, this would mean that the smile line should be set higher and the anterior teeth should either be set higher or more of the necks of the teeth exposed to correct the 'Gummy Look'.
The smile line is dictated from the bottom of the upper lip at the highest point (high lip line). Ask the patient to preform a big smile and scribe this on the bite block at that level (cold wax knife, never use a heated instrument). This dictates how much tooth neck and gum is shown when the patient smiles, it also can dictates the dentogenics of the smile by optimising tooth positioning.
The low lip line is dictated by many factors including age. Ask the patient or use the previous denture to dictate how much bite rim should be shown and make sure you tell the patient that this is how much tooth will be shown when the lip is relaxed.
With the combined parameters of the facial features and smile features, it creates a box, within that box is the parameters for the aesthetics of the denture which dictates, the positioning, width and height of the anterior teeth.
Step 3: Register The Bite
OVD, RVD & Free way space
Once the aesthetic parameters is set, we need to establish the functional parameters of the bite registration via vertical dimensions, jaw relationship and registering the bite.
The vertical dimension is obtained with the help of a Willis Guage. A Willis Gauge is a tool used to measure the vertical dimension in millimeters between the maxilla and mandible.
OVD (Occulsal vertical dimension) is an important factor for patients aesthetics and function. A reduced OVD will cause complaints about aesthetics and an increased OVD may lead to discomfort.
OVD indicates the measurement between the occlusal relationship of the maxilla and mandible. For dentate patients, the OVD is established by occluding the maxillary and mandibular teeth together. In edentulous patients, this is established by bite rims placed on the maxillary and mandibular ridge, this can also be obtained from the patients previous set of dentures.
RVD (Resting vertical dimension) indicates the measurement between the maxilla and mandible when the muscles are relaxed e.g not in occulsion, this is obtained by removing the lower denture and asking the patient to close their lips together without occluding.
FWS (Freeway space) Free way space is the established between the RVD & OVD measurement of the maxilla and mandible when the mandible is in its physiologic rest position. This is usually between 2 – 4 mm.
With the help of a Willis guage, the solid arm facing towards you on will be placed on the base of the patients nose and the second arm towards the border of the patients chin that slides up and down . You then lock it into position via the screw on the movable arm, this will show the overall vertical dimension (OVD) in mm.
The RVD is then obtained by removing the lower denture and asking the patient to close their lips together without occluding and check the number on the Willis gauge e.g 47mm on the Willis gauge.
Now we calculate the FWS by subtracting the RVD from the OVD, this is now the free way space. e.g RVD 47mm – OVD 45mm = 2mm FWS. Increasing or decreasing the dimensions is dependent on the situation for example if they have an old set of dentures. Make sure that the wax blocks have even bilateral contact when establishing the bite.
If there is no previous denture present, you can visually see the patients facial muscles in determining if the OVD is correct. Often, if the OVD is increased or reduced beyond it physical dimensions the facial muscles will often indicate this by facial tension.
Once the OVD is established, ask the patient to bite together several times and scribe a location mark between the posterior regions of upper bite rim and lower posterior part in a closed position, this will establish a reproducible jaw relationship. Asking the patient to roll their tongue backwards can help with creating a reproducible jaw relationship if the patient has abnormal bite patterns.
Take the bite rims out and cut location grooves into posterior regions of the upper and lower bite rims to allow space for the bite registration material to flow into this area and lock together the U/L bite rims, this will register the bite and transfer the patients jaw relationship into the bite rims and will allow the dentist and technician to verify it’s accuracy.
Place the U/L bite rims into the mouth and ask the patient to bite together once more to check if the bite is reproducible via the location mark lines. Once the bite is reproducible and verified, open or place the bite reg paste in the posterior region areas of the location groove areas and ask the patient to close and wait for it to set. Warn the patient, when they open the bite rim will be joined together. Check and verify if needed.
Remember, all of the hard work preformed will not work if a shade has not been taken.
Click the 'NEXT PART 2' button to find out how we transfer this information from the bite registration in the laboratory into a tryin stage.
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Private Dental Laboratory in London
About the author:
Kash Qureshi is a Clinical Dental Technician (Denturist) in the U.K who oversees and quality controls over 3000+ fixed and removable prosthesis including implant cases from a clinical and technical aspect monthly at Bremadent Dental Laboratory & Swissedent Denture Clinic in London.
www.swissedent.co.uk www.bremadent.co.uk email@example.com