Imani: RPD Biomechanics, Components, and Treatment Planning

What is support?

Resistance to vertical components of mastication and occlusal forces applied in the direction of the basal seat. Resistance of movement towards the tissue.

Which RPD components give support?

Rests, major connectors, denture bases

Where does movement happen in the sagittal plane?

Movement happens around the horizontal axis.

What is retention?

Resistance to removal in a direction opposite that of its insertion.

Which RPD components aid in retention?

Direct retainers (clasp arm), indirect retainers, guiding planes, and minor connectors

Where does movement happen in the frontal plane?

Movement happens on the longitudinal axis.

What is stability?

Resistance to dislodging on the opposite side of the occlusal load. Stability prevents rotation around the longitudinal axis. Stability resists bucco-lingual movement of the RPD.

Which RPD components aid in stability in the frontal plane?

Minor connectors, clasp assembly (reciprocating arm), major connectors (primarily for support), rests, denture base in long span distal extension, artificial tooth placement.

Which RPD components aid in stability in the vertical plane?

Denture base, maxillary major connector, minor connector, reciprocal arm of clasp

What is the primary stress bearing area of the maxilla?

Horizontal hard palate

What is the secondary stress bearing area of the maxilla?

Residual ridge

Which areas of the maxilla require relief?

incisive papilla, medial palatine suture, and torus

What is the primary stress bearing area of the mandible?

Buccal shelf, retromolar pad

What is the secondary stress bearing area of the mandible?

Residual ridge

What is the biggest anatomical feature of the buccal shelf?

External oblique ridge

What are the factors that influence the amount of force on the abutments that dentists can control?

The type of load and the fit of the casting. The direction of the load and ridge angle are also factors, but are dependent upon the patient's anatomy.

What about the edentulous span would cause greater leverage forces on abutment teeth?

If the edentulous span was longer, greater leverage forces would be applied to abutment teeth.

What type of lever is created in class I, II, and IV Kennedy Classifications?

Class I lever

Which type of lever is the most efficient?

Class I lever

What type of occlusal loading is seen in Kennedy Class III RPD?

Axial

What are mechanics?

The study of physical bodies subjected to forces.

What is biomechanics?

The application of mechanical principles to living organisms.

What is the goal of biomechanics in RPD?

The goal is to understand the possible movements of a RPD in response to functions such as mastication, speaking, and swallowing and to logically design a RPF so that forces resulting from function can be properly distributed between the teeth and residual ridges.

Class III RPD are entirely supported by

Teeth

Which type of tissues do primary stress bearing areas have?

Primary stress bearing areas have thicker mucosa and/or underlying cortical bone that is less prone to resorption.

Bearing areas provide better stability of a RPD when

they are parallel to the plane of occlusion or perpendicular to forces of mastication

While designing a RPD, areas of thin submucosa require what?

Areas with thin mucosa require relief.

While designing a RPD, areas where submucosa is loosely attached to the periosteum, inflamed or edematous require what?

Pre-prosthetic surgery

How much tissue is considered to be well-formed?

1 mm of firm and thick tissue. Large, well-formed ridges can support greater occlusal loads.

Which type of clasp is more flexible, wrought wire or CC?

Wrought wire is more flexible.

Should the clasp arm be active or passive at the terminal position?

Passive

When does the reciprocating arm contact the tooth during insertion?

It contacts the tooth before the retentive tip passes over the height of contour of the abutment.

Discuss the importance of the length of the clasp.

The longer a clasp is, the more flexible.

Which materials are used in clasp construction? Which is more flexible?

Gold alloy and chrome-cobalt alloy. Gold alloy is more flexible.

Which type of restoration create more friction to the abutment?

Gold

Which biomechanic considerations for RPD design control stress?

Direct retention, clasp position, clasp design, splinting of abutment teeth, indirect retention, occlusion, denture base, major connectors, minor connectors, and rests and rest seats.

How does direct retention control stress?

The retentive clasp arm, adhesion of saliva to the denture base and cohesion of salivary molecules to one another, frictional retention by the presence of guide planes, and neuromuscular control of the patient.

How does clasp position control stress in Kennedy class III RPD with modifications?

Clasp position controls stress by creating a quadrangular configuration connecting the most posteriorly placed clasps to the most anteriorly placed clasps, with the area being as wide as possible.

How does clasp position control stress in Kennedy class II RPD?

Clasp position controls stress by creating a triangular configuration with the posterior clasp placed as far posteriorly as possible and the other two clasps as far anteriorly as possible.

How does clasp position control stress in Kennedy class I RPD?

Clasp position controls stress by creating a bilateral configuration connecting the most posterior teeth.

How is rotational movement around the fulcrum prevented in Kennedy class I RPD?

Rotational movement around the fulcrum in Kennedy class I RPD is prevented by using indirect retainers.

How does clasp design control stress?

In distal extension RPDs, RPI clasp design should be used instead of CCC design.

Why should wrought wire combination clasps not be used in distal extension RPDs?

The flexibility of the wrought wire, when met with occlusal forces, causes the clasp to rotate, the minor connector to lose contact with the tooth, and the clasp arm tip to move occlusally and create distal torquing forces on the abutment tooth.

Why are RPI clasps a better choice for distal extension RPDs?

The RPI clasp design creates a more favorable fulcrum point (via the mesial rest) and increases resistance to distal displacement of the RPD. There is also less force applied to the abutment tooth.

How does the splinting of abutment teeth control stress?

Splinting of abutment teeth increases PDL area.

When is the splinting of abutment teeth indicated?

Splinting of abutment teeth is indicated in cases of short or tapered roots, lone standing abutments, and when cross arch stabilization is needed.

How does indirect retention of Kennedy classes I, II, and IV control stress?

Indirect retainers help resist rotation and/or displacement of a RPD. Indirect retainers are positioned at definite rest seats?

What type of auxillary rest can control stress?

A lingual bar auxiliary rest.

How does a smooth functioning occlusion control stress?

It minimizes the load transferred to the teeth and soft tissues.

Describe the characteristics of teeth added to a RPD that aid in proper occlusion.

These teeth require proper mesio-distal width, the correct number of teeth should be selected, the teeth should be sharp for efficient cutting and chewing, and there should not be a steep cusp incline as these create horizontal forces.

How do denture bases control stress?

Denture bases should be properly adapted to the residual ride and have maximum coverage for support and distribution of pressure over the residual ridge.

How do major connectors control stress?

Major connectors are rigid and are supported by rests.

How do minor connectors control stress?

Minor connectors are rigid, have direct contact with enamel which creates a distinct path of insertion, and have close contact with the guiding plane. This is great for stability.

How do rests and rest seats control stress?

Rests and rest seats are a ball and socket joint that have an angle less than 90 degrees to the guiding plane that allows for proper occlusion.

Which supportive RPD component can be found on an abutment?

A rest seat which sits on the occlusal third of the abutment.

Which stabilizing RPD component can be found on an abutment?

A reciprocating arm just above the junction of the middle and gingival third of a guide plate.

Which retentive RPD component can be found on an abutment?

A retentive arm that the sits 2/3 above the HOC and 1/3 below the height of contour with the terminal end being pointed occlusally.

Which tooth factors affect retention? Describe them.

The angle of gingival convergence, which is the angle formed between the analyzing rod of a surveyor and the tooth surface apical to the HOC, and the distance of the terminal end of the clasp from the height of contour.

Which prosthetic factors affect retention?

The clasp length, diameter, and cross sectional form and material affect retention.

What is reciprocation?

Reciprocation counteracts the action of the retentive tip as the RPD is being inserted and removed.

Which Kennedy classifications are Basic CCC indicated for?

All Kennedy classifications.

Which Kennedy classifications are embrasure clasps indicated for? Which teeth should not be used as abutment teeth when an embrasure clasp is indicated?

Kennedy classifications ii, iii, and iv. Pre-molars

Which suprabulge clasp designs require a 0.01" undercut to engage the retentive arm?

Basic ccc, embrasure clasps, ring clasps, and reverse action clasps

Where do rest seats sit in ring clasps?

MO and DO

How much of an undercut is needed for WW cc clasps?

0.02"

Which infrabulge clasp designs require a 0.01" undercut to engage the retentive arm?

I-bar, Y or modified T-bar

Which suprabulge clasp has a ML undercut of 0.01"?

Ring Clasp

How large is the contact area of an I-bar?

1 mm

How far away should an I-bar be from the gingival margin?

3mm

The I-bar crosses the gingival margin at a [blank] angle

perpendicular

Where does the I-bar contact the abutment?

The I-bar contacts the abutment tooth just above the undercut.

How far away should the horizontal approaching arm be from the gingival margin?

3-5mm

What are the contraindications of an I-bar?

Shallow vestibule, 0.02" undercut, and high buccal freni

How long should an I-bar be from its origin to its distal extension?

10 mm

What is the difference between an I-bar and a modified T or L-bar?

T or L-bars contact the tooth disto-bucally. The undercut still needs to be 0.01"

What takes place during phase I of treatment?

Examination and tx planning, preliminary design, and patient education and motivation

What is the most important data measure while examining the patient?

Oral hygiene and plaque control

What is the first assessment completed during examination? Describe what is being assessed.

Periodontal assessment. Each tooth should be assessed individually for mobility and attachment loss. There should be a 1:1 crown to root ratio. When examining mobility and attachment loss, keep trauma to the PDL in mind, as a tooth can be mobile without attachment loss due to trauma of the PDL.

What is the second assessment completed during examination? Describe what is being assessed.

Prosthodontic assessment. Each tooth is assessed for proper contours, fillings, and/or the presence of a crown.

Describe what measures should be taken if a tooth is not sound and pristine due to improper contours.

If a tooth does not have the proper contours for proper clasp assembly design, modifications should be made. Such modifications may include the fabrication of a survey crown or enameloplasty.

What should be done if a tooth is severely malpositioned?

The tooth should be extracted.

What should be done if decay is present during prosthodontic assessment?

If decay is present, it should be removed and the remaining tooth structure should be evaluated. If the lesion is less than 1/3 of the bucco-lingual dimension of the tooth, place a restoration. If the lesion is larger than that, place a survey crown or consider extraction if the tooth is non-restorable.

What should be done if a filling is present during prosthodontic assessment?

If a filling is present, take its size into consideration. If there is NO leaking, recurrent decay, or fracture and the filling can accomodate the rest seat, maintain it. If it cannot accomodate the rest seat, remove it.

What should you do if there is leaking, recurrent decay, or fracturing of a restoration?

Remove the decay and evaluate the remaining tooth structure. If the lesion is small and has no cusp involved, place a filling. If the lesion is large and involves a cusp, but is restorable, place a survey crown. If the lesion is large and the tooth is non-restorable, extract the tooth.

If a crown is present on a tooth, what should be evaluated?

The contour of the crown in relation to the clasp assembly should be evaluated. If the crown has the proper contours and closed margins, keep it and consider preparations for a rest seat. If the preparation does not go through the metal and the crown is still cemented, maintain the crown. If the preparation will go through the metal, a proper seal will be lost and a survey crown should be fabricated and seated.

What should be done if a crown has secondary decay or an open margin?

The crown should be removed and the tooth reassessed. If there is sufficient tooth structure for ferrule, retention and resistance form, the crown should be redone. If there is insufficient ferrule, consider crown lengthening and fabricating a new crown or extraction. If there is insufficient tooth structure, considering treating the tooth with a root canal, post and core, and crown or extraction.

Which type of evaluation is after prosthodontic assessment?

Endodontic evaluation

What is assessed during endodontic evaluation?

Previous RCT, the presence of peri-apical lesion, and the size of peri-apical lesions

What has research shown about the success and failure rates of RCT treated teeth that did not have crowns?

There was a high failure rate in teeth that were endodontically treated but did not have crowns.

How is prosthetic space evaluated?

Prosthetic space is evaluated with the fabrication and use of record bases and wax rims and recording of the present maxillo-mandibular relationship.

How is the position of the maxilla recorded?

A facebow is used to determine the position of the maxilla in space. The facebow records the relationship of the maxilla to the TMJ's and is then transferred to mount to maxillary cast.

How is the position of the mandible recorded?

The position of the mandible is recorded at the vertical dimension of occlusion in maximal intercuspal position OR in centric relation if the cast cannot be hand articulated.

When should the casts be articulated in CR?

The casts should be articulated in centric relation when there are less than 3 non-aligned pairs of teeth.

How do you fabricate a record base and wax rim?

Triad is cured, sticky wax is added, then pink wax is added, reduced, and contoured properly. The wax rims should not touch one another or the adjacent teeth. The wax is notched and registration material is added on the mandibular wax rim, then the patient is guided to close down to record occlusion.

After the cast is mounted with the correct maxillo-mandibular relationship, what about the prosthetic space is evaluated?

The anterior prosthetic space is evaluated for proper incisal guidance, the presence of an open bite, the average overbite and over-jet of the teeth, and/or a deep bite.

The posterior prosthetic space is evaluated for its mesio-distal length, the tilt or drift of the teeth that are present, adequate VDO, excessive space due to bone resorption, lack of space due to super-erupted teeth in the opposing arch, and the occlusal embrasure spaces.