BIOLOGICAL CONSIDERATION IN MANDIBULAR IMPRESSION PROCEDURES

BY SARAVANAN .T.T 1ST YEAR PG

INTRODUCTION Complete denture impression

procedures are perhaps one phase on which much has been spoken about. The literature on the subject shows a persistent disagreement ever since 1850.

Much of this confusion results from the fact that many impression procedures have been developed on empirical basis.

Many have used the available knowledge of functional and histological anatomy for the development of their procedures, but the variation in these techniques indicate a wide difference in interpretation of the foundation of dentures.

Whatever the method used it is generally agreed that good impressions are basic for the construction of a good denture.

DEFINITIONS

IMPRESSION A negative likeness or copy in reverse of the

surface of an object . – gpt 8

An impression can also be defined as an imprint of the teeth and adjacent structures for use in dentistry. - gpt 4

COMPLETE DENTURE IMPRESSION A complete denture impression is a negative registration

of the entire denture bearing, stabilizing and border seal areas present in the edentulous mouth

PRELIMINARY IMPRESSION A preliminary impression is an impression made for the

purpose of diagnosis or for the construction of a tray

MUCOUS MEMBRANE

MUCOUS MEMBRANE

The bones of the upper and lower edentulous jaws are covered with soft tissue, and the oral cavity is lined with soft tissue known as mucous membrane.

The denture bases rest on the mucous membrane, which serves as a cushion between the bases and the supporting bone.

The mucous membrane is composed of two

layers

Mucosa

Submucosa

The mucosa is formed by the stratified

squamous epithelium and a subjacent

layer of connective tissue known as the

lamina propria.

• The submucosa is formed by connective

tissue.

It may contain glandular , fat , or

muscle cells and transmits the blood

and nerve supply to mucosa.

• The thickness and consistency of

submucosa are largely responsible for the

support that the soft tissue affords the

denture, since in most instances the

submucosa makes up the bulk of the

mucous membrane.

• In a healthy mouth the submucosa is firmly

attached to the periosteum of the

underlying bone of the residual ridge and

will usually successfully withstand the

pressure of the denture.

HISTOLOGY OF THE MUCOUS MEMBRANE COVERING CREST OF THE RESIDUAL RIDGE

BONE

PERIOSTEUM

SUBMUCOSA

MUCOSA

CLASSIFICATION OF

ORAL MUCOSA:

•The oral mucosa is divided in three catogories depending on its location in the mouth •Masticatory mucosa•Lining mucosa•Specialized mucosa

The masticatory mucosa covers the

crest of the ridge

the residual attached gingiva firmly adherent to the

supporting bone

hard palate.

It is characterized by a well defined keratinized

layer on its outermost surface subject to changes in

thickness.

The specialized mucosa covers the dorsal surface

of the tongue. This mucosal covering is keratinized.

The lining mucosa is generally devoid of the

keratinized layer. It is found to cover the :

mucous membrane of lips, cheek

vestibular spaces

alveolingual sulcus

soft palate

ventral surface of the tongue and,

the unattached gingiva found on slopes of

residual ridge.

BIOLOGICAL CONSIDERATIONS

The considerations for the mandibular impressions are generally similar to that for those of maxillary impressions and yet there are many differences owing to the following facts: The basal seat of mandible is different in

size and form from the maxillary counterpart.

The submucosa in some parts of mandibular basal seat contains anatomic structures different from those in the upper jaw.

The nature of the supporting bone on the crest of residual ridge usually differs between the two jaws.

The presence of the tongue complicates the impression procedures for the lower denture.

The available area of support from an

edentulous mandible is 14 cm2 while the

same for the edentulous maxilla is 24cm2 .

The supporting and the peripheral sealing

areas will be in contact with the dentures

fitting or impression areas. The support for

the mandibular denture is derived from the

body of mandible.

The landmarks can be broadly grouped into:

Limiting structures:

Labial frenum Labial vestibule Buccal frenum Buccal vestibule Lingual frenum Alveololingual sulcus Retromolar pads Pterygomandibular raphe.

Supporting structures:

Buccal shelf area Residual alveolar ridge

Relief areas:

Crest of the residual alveolar ridge Mental foramen Genial tubercles Torus mandibularis.

BUCCAL SHELF AREA The area between the

mandibular buccal frenum

and the anterior edge of the

masseter is known as the

buccal shelf.

It is bounded medially by

the crest of the residual

ridge , anteriorly by the

buccal frenum , laterally by

the external oblique line and

distally by retromolar pad.

The buccal shelf forms the primary support for the mandibular denture as it is made primarily of cortical type of bone.

The buccal shelf area can range from 4-6 mm wide on an average mandible to 2-3 mm or less in narrow mandible.

The buccal shelf is very wide and is at right angles to the vertical forces of occlusion. For this reason it offers

excellent resistance to such forces.

Crest Of The Mandibular Ridge The crest is covered by the fibrous

connective tissue, but in many mouths the underlying bone is of cancellous type without a cortical bony plate covering .

The fibrous connective tissue is favorable for resisting the externally applied forces, such as the denture. However, with the underlying cancellous bone this advantage is lost .

Labial Frenum:

This is single narrow band but may

consist of 2 or more bands.

The activity of this area tends to be

vertical so the labial notch on the denture

should be narrow.

Buccal Frenum:

This is usually in the area of 1st pre molar.

The oral activities in these area are

horizontal as well as vertical (ex. Grinning

and puckering) thus needing wider

clearance.

The contour of the denture will be little

narrower in this area due to the activity of

depressor anguli oris muscle.

Labial Vestibule:

It is the sulcus between the buccal

frenums.

The major muscle in this area is orbicularis

oris whose fibers are mainly horizontal

thus overextension in this area should be

avoided.

Buccal Vestibule:

The buccal vestibule extends from the

buccal frenum posteriorly to the outside

back corner of the retromolar pad and

from the crest of the residual alveolar

ridge to the cheek.

Pear Shaped Pad: The retromolar pad as

described by Sicher is the soft

elevation of mucosa that lies

distal to the third molar.

It contains loose connective

tissue with an aggregation of

mucous glands and is

bounded posteriorly by the

temporalis tendon , laterally

by the buccinator, and

medially by the

pterygomandibular raphe and

the superior constrictor.

Lingual Vestibule:

It can be divided into three areas

anterior vestibule/sublingual crescent

area/ anterior sublingual fold

the middle vestibule/ mylohyoid area

the distolingual vestibule/ lateral throat

form/ retromylohyoid fossa

Anterior lingual vestibule

This extends from the lingual frenum to

where the mylohyoid ridge curves down

below the level of sulcus. Here a

depression the premylohyoid fossa can be

palpated.

This is mainly influenced by the

genioglossus muscle, lingual frenum and

some part by anterior portion of sublingual

glands .

Middle vestibule:

This is the largest area and is mainly influenced by mylohyoid muscles and somewhat by sublingual glands.

The mylohyoid muscle is the largest muscle in the floor of the mouth whose principal function occurs during swallowing. Its intra oral appearance is misleading because the membranous attachment makes the muscle appear to be horizontal when contracting.

Sears has shown that at maximum contraction the fibers are still in a downward and forward direction so that a denture can be extended below the muscle attachment along the mylohyioid ridge.

The average mylohyoid border is 4-6 mm beyond the mylohyoid ridge in fair to good ridge it is about 2-3 mm . If the ridge is flat it is often advantageous to make mylohyoid border thicker (4-5mm or more).

Distolingual vestibule:

The lateral throat form is bounded anteriorly by mylohyoid muscle, laterally by pear shaped pad, posterolaterally by superior constrictor, posteromedially by palatoglossus and medially by tongue.

The so called “s” curve of the lingual flange of the mandibular denture results from stronger intrinsic and extrinsic tongue muscles, which usually place the retromylohyoid borders more laterally and towards the retromylohyoid fossa, as the oppose weaker superior constrictor muscle.

The posterior limit of the mandibular denture is determined mainly by the palatoglossus muscle and somewhat by weaker superior constrictor muscle this is area is called posterior/ retromylohyoid curtain.

Neil described this area and noted that the

denture could have three possible lengths,

depending on the tonicity, activity, and

anatomic attachments of the adjacent

structures-

Class I throat form: The horizontal border is usually 2-3 mm thick, but a thicker border of 4-5 mm should be used for better seal if the ridge is flat. The retromylohyoid curtain area should be thinner, about 2-3 mm, and very rounded and smooth.

Class II throat form is about half as long and narrow as class I and about twice as long as class III.

Class III lateral throat form has minimum length and thickness. The border usually ends 2-3 mm below the mylohyoid ridge or sometimes just at the ridge.

BASIC REQUIREMENTS FOR IMPRESSION MAKING

Knowledge of Basic anatomy Knowledge of basic reliable technique Knowledge and understanding of

impression materials Skill Patient management

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OBJECTIVES OF IMPRESSION MAKING

1) RETENTION2) STABILITY3) SUPPORT4) ESTHETICS5) PRESERVATION OF REMAINING

STRUCTURES

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