retention, stability & support

Rola M. Shadid, BDS, MSc

Retention, Stability & Support

RetentionResistance to denture removal in a direction opposite that of its insertion.*

StabilityResistance of denture movement under the effect of horizontal or rotational forces.

SupportResistance of movement of denture toward the basal seat area.

The act of chewing foodsAdhesive nature of certain foodsGravity (for upper denture) *Surrounding musculaturePrematurities in occlusion

Dislodging Forces

Retention

Primary retention is from basal seat

Secondary retention by improving stability & minimizing dislodging forces

Retention

Physical means

Mechanical means

Primary Retention

Adhesion

Cohesion

Interfacial surface tension

Atmospheric pressure

Capillary attraction

Gravity

Physical Forces in Denture Retention

Adhesion & Cohesion *

Figure: retention due to pressure differential between saliva film & the air

Interfacial Surface Tension*

Relationship between the width of the buccal channel and resistance to flow of saliva: (a) wide channel, rapid flow, poor retention; (b) narrow channel, slow flow, good retention.

Interfacial Surface Tension

Atmospheric pressure *

Capillary attraction

Physical Forces in Retention

Saliva: quantity & quality *

Surface area: the larger the surface area, the greater the retention.#

Intimacy of contact: the closer the contact between the tissue and the impression surface of denture, the greater the retention.

Peripheral seal

Factors Affecting the Physical Forces

The denture border should be shaped so that the channel between it and the sulcus tissues is as small as possible.It is not possible to maintain a close approximation between the border of a denture and the mucosal reflection in the sulcus at all times because the depth of the sulcus varies during function.

Peripheral Seal

Peripheral Seal

The denture has to be constructed so that the border conforms to the shallowest point that the sulcus reflection reaches during normal function. *

It is not possible to produce a facial seal along the posterior border of the upper denture as it crosses the palate. In this area, another approach to create the smallest possible space between denture and mucosa is adopted. A groove known as a post-dam is cut into the working cast so that the posterior border of the finished denture has a raised lip which becomes embedded a little way into the palatal mucosa.

Peripheral Seal

Peripheral SealRight: denture poorly retained because the thin flanges failed to create a facial seal and the palatal coverage did not make the most of the area available. Left: the replacement denture corrected

these errors and as a result had excellent retention.

UndercutsOverdentures attachmentsImplantsMucosal insertsRubber suction discs & suction chambers (No longer used)Magnets

Mechanical Aids in Retention

Undercuts

Selection of path of insertion to improve retention by utilising undercuts: (a) single path of insertion to engage labial undercut; (b) dual path of insertion to engage unilateral undercut.

Mucosal Inserts Mucosal insert or

implant button: a nonreactive metal appliance that is affixed to the tissue-bone surface of a denture and offers added retentive qualities to the denture. It consists of a base, cervix, and head.

Rubber suction disc & suction chamber

No longer used because they cause damage of soft tissues

The surrounding musculature

Occlusal schemes

Proper patient instructions

Psychological factor

Neuromuscular control

Denture fixative

Long-term soft liners

Secondary Retentive Features

Cheek muscles LipsModiolus can cause unseating of denture in premolar regionTongueFloor of the mouthSoft palate

Surrounding Musculature*

Masseter (affects the distobuccal region of mandibular denture)

Neutral zone (zone of minimal conflict)

Surrounding Musculature

Neutral Zone *

oExplaining the central role of the tongue, lips and

cheeks in controlling the denture and giving specific advice – such as supporting the posterior border of the upper denture with the tongue when incising.

oOffering advice, for example, cutting food into smaller pieces before inserting them into the mouth.

oChewing on both sides of the dental arch simultaneously

o Starting with softer ‘easier’ foods before progressing to more challenging morsels.

Proper Patient Instructions

Neuromuscular Control As the patient incises, the upper denture is controlled by the tongue pressing against the posterior border.

An aid to retention, particularly under difficult anatomical circumstances.

Come in powder, paste or sheet form, the latter having the advantage of staying longest between the denture and mucosa.

Denture Fixative

enable free, flexible margins to extend into the anatomical defect and engage tissue undercuts.

can be constructed as an integral part of the denture base or as a separate obturator section retained on the denture base by rare earth magnets.

Long-Term Soft Liners

Stability

Intimate contact (accuracy of fit)Residual ridge size & contourResidual ridge qualityPalatal vaultNeutral zone & surrounding musculature *

Factors Affecting Stability

Flange shape & contour: correct contouring the external surface of the denture base can enhance the stabilizing potential of surrounding musculatureOcclusal factorsAbnormal ridge relationships *Direct bone anchorage #

Factors Affecting Stability/continue

Factors Affecting Stability/continue Pressure from the

bolus on the posterior part of the lower occlusal table, which overlies a sloping part of the ridge, causes the lower denture to slide forwards.*

Support

Firm resilient tissue covered by

keratinized tissue & firmly attached to underlying bone

Tissue of uniform thickness The bone be resistant to

resorption The support areas should be at

right angles to the occlusal surfaces

Requirements of Tissues Capable of Support *

All areas of denture bearing area should contribute to support except relief area

Primary stress bearing areas

Supporting Areas of Denture Foundation

Greater area of coverage puts less force per unit area (snow shoe principle)

The larger surface area of maxillary denture (22.96cm2) allows a wider stress distribution compared with mandible (12.25cm2).

Supporting Areas of Denture Foundation

Surface areaNature of supporting mucosaImpression procedureAccuracy of fitDirect bone anchorage *

Factors Affecting Support

Retentive forces > displacing forces

Adequate support Stability

References:

I. Complete Denture Prosthodontics, 1st Edition, 2006 by John Joy Manappallil, Chapter 2

II. Basker’s Prosthetic treatment of the edentulous patient. Fourth edition. Chapter 4.