Biological consideration in maxillary

impressions

The foundation for dentures is made up of bones covered by the

mucous membranes (mucosa and submucosa).

• Mucosa can be of three types, namely, masticatory, lining and

specialized mucosa depending on the location in the oral cavity.

• The submucosa varies in thickness and consistency and is

responsible for supporting the denture. When thin, it gets easily

traumatized and when loosely attached, it gets inflamed or

edematous.

• Ultimate support for the maxillary denture is the bone of two

maxillae and the palatine bone.

• The anatomical landmarks in maxilla are as follows (Fig. 4-7):

FIGURE 4-7 Anatomic landmarks of maxillary arch.

Supporting Structures

(i) Primary stress-bearing areas:

• Hard palate

• Posterolateral slopes of residual alveolar ridge

(ii) Secondary stress-bearing areas:

• Rugae

• Maxillary tuberosity, alveolar tubercle

Hard palate

The cortical bone in the hard palate, composed of the palatine processes

of the maxillae and the horizontal processes of the palatine bones, has been

shown to resist resorptive changes.

• A cross-section of the hard palate shows that it is covered by tissues

of varying depths.

• Therefore, it is important to employ an impression technique that

equalizes the pressure distribution.

• The submucosa in the mid-palatine suture is extremely thin and,

therefore, relief should be provided in the part of the denture

covering area.

• The horizontal portion of the hard palate lateral to the midline acts

as the primary stress-bearing area, as it resists resorption and is

covered by keratinized mucosa. The trabecular pattern in the bone

is perpendicular to the direction of force, making it capable of

withstanding any amount of force without marked resorption.

Residual ridge

• It is defined as ‘the portion of the alveolar ridge and its soft tissue

covering which remains following the removal of teeth’. (GPT 8th Ed)

• It resorbs rapidly following extractions and continues at reduced

rate throughout life.

• The submucosa over the ridge has adequate resiliency to support the

denture.

• The crest of the ridge may act as a secondary stress-bearing area.

• The posterolateral portion of the residual ridge is a primary stressbearing area.

• The remaining facial slopes of the maxillary residual ridge are not

essential in denture support.

Rugae

• These are the thick fibrous bands of tissues located in the anterior

segment of the palate.

• The rugae area acts as the secondary stress-bearing area because it is

set at an angle to the occlusion plane of the residual ridges and is

rather thinly covered by the soft tissues.

• Also, the rugae resist anterior displacement of the denture.

• Folds of the mucosa play an important role in speech.

Maxillary tuberosity

It is the bulbous extension of the residual ridge in the second and

third molar regions.

• It terminates at the hamular notch.

• The rough prominence behind the position of the last tooth is the

alveolar tubercle.

• It is considered as the secondary supporting structure.

• The posterior part of the ridge and the tuberosity are considered as

one of the most important areas of support, as these are least likely

to resorb.

Alveolar tubercle

The medial and lateral wall resists horizontal and torquing forces,

whereas the lateral wall resists the anterior movement of the denture.

Limiting structures

Labial frenum: It is a fold of mucous membrane at the median line. It is a

passive frenum, as it contains no muscle. This frenum is fan-shaped

and it converges as it inserts onto the labial aspect of the ridge. The

labial notch in the denture should not only be narrow but also be

wide enough to accommodate the labial frenum without

interference.

Labial vestibule: It extends from the buccal frenum on one side to the

other and is divided into two compartments by the labial frenum. It

is covered by the lining mucosa. This space is easily distorted

because of the presence of submucosa and, therefore, should be

completely filled to provide retention.

Orbicularis oris: It is the main muscle lying in this region. Its tone

depends on the support received from the thickness of the labial

flange and positioning of the artificial teeth. Because its fibres run

horizontally and anastomoses with fibres of buccinator, it has an

indirect ef ect on the extent of the denture base.

Buccal frenum: It lies between the labial and the buccal vestibule. It

requires more clearance than the labial frenum and the buccal notch

should be broad enough to allow its movement. Three muscles are

associated with it, namely, orbicularis oris (pulls the frenum

forward), caninus or levator anguli oris (attaches beneath the frenum

and affects its position) and buccinator (pulls it backward).

The borders of the denture should be moulded in

such a way that the depth and width of the frenum

are exactly recorded (Fig. 4-8).

Buccal vestibule: It extends from the buccal frenum anteriorly to

hamular notch or pterygomaxillary notch posteriorly. The size of

the vestibule is controlled by the following:

• The contraction of the buccinator muscle

• Position of the mandible

• The malar process of zygomatic arch

• Amount of bone lost from the maxilla

The ramus and coronoid process of the mandible and

the masseter determine the width of the buccal

vestibule. The lateral movement of the mandible alters

the shape and size of the posterior part of the

vestibule. The distal end of the flange of the

denture should be adjusted so as to avoid

interference to the coronoid process during

function.

The distobuccal border is influenced by the masseter

muscle which acts outside the buccinator muscle

during contraction and by the coronoid process

during lateral movements.

Hamular notch: It is situated distally to the tuberosity of the maxilla

and mesially to the hamulus of the medial pterygoid plate. It serves

as an anatomic guide for the distal extension of the impression in

this area.

The denture border should extend to the hamular

notch beyond tuberosity. The posterior palatal seal

(PPS) should extend through the centre of the deep

part of the hamular notch as it does not contain any

muscle attachments (Fig. 4-8). They are always

located in the soft plate and guide the location of

the posterior border of the denture.

F

I

G

U

R

E

4

-

8

L

o

c

a

tio

n

o

f la

bial v

e

s

tib

ule, b

u

c

c

al f

r

e

n

u

m

a

n

d

b

u

c

c

al v

e

s

tib

ule.

Relief areas

Relief areas are divided into three categories which are as follows:

(i) Tissues susceptible to resorption should not be loaded (e.g. some

maxillary and most mandibular ridge crests)

(ii) Areas that have thin mucosa over hard cortical bone (e.g. midpalatine raphe, tori, exostosis and lingual surface of mandible)

(iii) Areas of mucosa overlying neurovascular bundles (e.g. incisive

papilla, in some cases mental foramen)

Incisive papilla

• It is a pad of fibrous connective tissues overlying the orifice of the

nasopalatine canal.

• It is located on the line immediately behind and between the central

incisors.

• It covers incisive foramen carrying the nasopalatine nerves and

vessels.

• It may lie on the crest of the alveolar ridge and its position can vary.

• It should be relieved in every denture.

• Position of the papilla indicates the amount of bone loss (Fig. 4-7).

Mid-palatine raphe

• It extends from the incisive papilla to the distal end of the hard

palate.

• The mucosa is thin and unyielding.

• The underlying bony union is very dense.

• It is here that the palatal torus, if present, is located.

• It should be relieved to avoid tissue impingement between the

denture base and bone (Fig. 4-7).

Fovea palatini

• These are formed by coalescence of mucous glands and are located near

the midline of the palate.

• These are usually two in number and are present one on each side of

the midline, slightly posterior to the junction of the hard palate and

the soft palate.

• These are always located in the soft palate and guide the location of

the posterior border of the denture (Fig. 4-7).

Postpalatal seal

It is defined as ‘the seal area at the posterior border of a maxillary removable

dental prosthesis’.