CHAPTER 9

Biologic considerations for mandibular impressions

Biologic considerations for mandibular im- the mandible. The peripheral retention seal is
pressions are generally similar to those for max- provided by the form of the denture’s borders
illary impressions, and yet there are many dif- as determined by the macroscopic and micro-
ferences. The basal seat of the mandible is dif- scopic anatomy of the limiting structures.
ferent in size and form from the basal seat of The total area of support from the mandible
the maxillae. The submucosa in some parts of is significantly less than from the maxillae. The
the mandibular basal seat contains anatomic available denture-bearing area for an edentu-
structures that are different from those found lous mandible is 14 cm? whereas for an edentu-
in the upper jaw. In addition, the nature of the lous maxilla it is 24 cm*. This means that the
supporting bone on the crest of the residual mandible is less capable of resisting occlusal
ridge usually differs between the two jaws. forces than the maxillae are and extra care must
These variances are often sufficient to require be taken if the available support is to be used
major modifications in impression procedures to advantage.
for the mandible and the maxillae. The pres-
ence of the tongue and its individual size, SEQUELAE OF TOOTH LOSS
form, and activity complicate the impression When the teeth are removed from the man-
procedures for lower dentures and the patient's dible, the alveolar tooth sockets tend to fill
ability to learn to manage them. The clinical with new bone but the bone of the alveolar
incorporation of the biologic principles of sup- process starts resorbing. This means that the
porting and limiting structures will enable the bony foundation for a mandibular denture be-
dentist to unravel what is sometimes called the comes shorter vertically and narrower bucco-
“mystery of the lower denture” and successfully lingually. Thus the foundation for the basal seat
provide prosthodontic treatment for edentulous is less favorable as a support for the denture.
patients. The bony crest of the residual ridge becomes
The same fundamental principles are in- narrower and sharper. Often, sharp bony spi-
volved in the support of amandibular denture as cules remain and can cause tenderness when
are involved in the support of a maxillary den- pressure is applied by a denture.
ture (Chapter 7). Both the support or stress- The total width of the bony foundation (or
bearing area and the peripheral sealing area the mandibular basal seat) becomes greater in
will be in contact with the denture’s fitting or the molar region as resorption continues. The
impression surface. The denture bases must reason is that the width of the inferior border
extend as far as possible without interfering in of the mandible from side to side is greater
the health or function of the tissues, whose than the width at the alveolar process from side
support is derived from bone. The support for to side (Figs. 9-1 and 9-2).
a mandibular denture comes from the body of Other changes occur on the occlusal surface

194
 Biologic considerations for mandibular impressions 195

Fig. 9-1 After the removal of teeth, resorption of the alveolar process causes the bony
support for a mandibular denture (the residual alveolar ridge) to become more narrow
buccolingually and reduced in height. A, Crest of the residual ridge (cancellous bone); B,
the buccal shelf (compact bone); C, the genial tubercles. Notice that if the buccolingual
axes of the condyles of this mandible were extended posteriorly they would meet in the
region of the foramen magnum.

of the bone. The shrinkage ‘of the alveolar pro- tions need dentures, however, and this means
cess in the anterior region moves the residual that the impressions must be made in such a
bony ridge lingually at first. Then, as resorp- way that maximum advantage is gained from
tion continues, this foundation moves progres- each part of the basal seat.
sively further forward (see Fig. 19-5). Bone loss
MACROSCOPIC ANATOMY OF THE
frequently continues on the mandible below
SUPPORTING STRUCTURES
the level of the alveolar process.
With resorption of the alveolar process oc- Support for the lower denture is provided by
clusal contours of residual ridges often develop the mandible (the bone) and the soft tissues
that make them curved from a low level anteri- overlying it. Some parts of the mandible are
orly to a high level posteriorly (Fig. 9-3). These more favorable for this function than others,
conditions can cause severe problems of den- and pressures must be applied to the bone
ture stability, which must be considered in im- through the soft tissues according to the ability
pression making and in occlusion. of the tissues and different parts of the bone to
People who have these unfavorable condi- resist the stresses of occlusion.
196 Rehabilitation of the edentulous patient

Fig. 9-2 Depending on the shape of the mandible and the nature of the resorption,
when the alveolar ridge resorbs it often results in a mandibular basal seat (A) that be-
comes wider and larger. This change occurs because, as resorption moves the crest of
the ridge more inferiorly, the width of the mandible becomes greater than that of the al-
veolar process at the time the teeth were removed. A denotes the width of the basal seat:
B, the genial tubercules; C, the mental foramen; D, the coronoid process.

Crest of the residual ridge

known as the buccal shelf or buccal flange. It is
The crest of the residual alveolar ridge is bounded medially by the crest of the residual
covered by fibrous connective tissue, but in ridge, anteriorly by the buccal frenum, later-
many mouths the underlying bone is cancellous ally by the external oblique line, and distally
and without a good cortical bony plate covering by the retromolar pad (Fig. 9-5). The buccal
it (Fig. 9-4). The fibrous connective tissue shelf may be very wide and is at right angles to
closely attached to the bone is favorable for re- the vertical occlusal forces. For this reason it
sisting externally applied forces, such as those offers excellent resistance to such forces. Some
from a denture. However, if the underlying buccinator fibers are located under the buccal
bone is cancellous, this advantage is mostly flange because the mandibular attachment of
lost. this muscle is close to the crest of the ridge in
the molar region. This attachment is dissimilar
Buccal flange area and the buccal shelf
to other muscle insertions insofar as the fibers
The area between the mandibular buccal fre- run anteroposteriorly, paralleling the bone,
num and the anterior edge of the masseter is and the denture does not resist the contracting
 Biologic considerations for mandibular impressions 197

Fig. 9-3. Bony resorption has caused an anteroposterior curvature (A to B) in the bone
of the basal seat as viewed from the side. Such an inclination of the residual alveolar
ridge creates problems in maintaining the stability of a mandibular denture. Artificial teeth
should not be placed over the line B-A. The genial tubercles, C, mandibular foramen, D,
condyle, E, and coronoid process, F, are all indicated.

force of the muscle (Fig. 9-6). The inferior part skill with which the impressions are made will
of the buccinator is attached in the buccal shelf determine the effectiveness of the distribution
of the mandible, and thus contraction of the of pressure to selected parts of the basal seat
muscle does not lift the lower denture. The (Fig. 9-8). The requirements of mandibular im-
buccal shelf is the principal bearing surface pressions can be fulfilled by one of several se-
of the mandibular denture, and it takes the lective pressure techniques.
occlusal load off the sharp: narrow crest of
Flat mandibular ridges
the residual alveolar ridge that so many eden-
tulous mandibles present (Fig. 9-6). It is cov- On the labial surface of the anterior region of
ered with good smooth cortical bone, which is the mandible several muscles are close to the
usually at right angles to the occlusal forces crest of the ridge, especially in badly resorbed
(Fig. 9-7). ridges. This proximity accounts for the short
These relative conditions vary from patient flanges necessary in this region. These muscles
to patient, so a choice as to the best distribu- should not be impinged on, since their action is
tion of pressures on the mandibular basal seat nearly at right angles to the flange. Many
must be made. If the residual bony ridge is un- edentulous mandibles are extremely flat be-
favorable (that is, if it is sharp, spiny, or full of cause of the loss of cortical bone (Fig. 9-7). The
nutrient canals), masticatory pressures should surface is weakened and changed in form by
be transferred to the buccal shelf. Otherwise, the more rapid resorption of the cancellous
the residual ridge can help carry the load effec- portion of the mandible. The bearing surface
often becomes concave, allowing the attaching
tively. The accuracy of the diagnosis and the
198 Rehabilitation of the edentulous patient

Fig. 9-4 The crest of the residual alveolar ridge consists of spongy or cancellous bone.
Its porosity and roughness, particularly in the molar region, A, make it unsuitable as the
primary stress-bearing area for a mandibular denture. Therefore the buccal shelf, B, is
usually selected as the primary region for supporting a mandibular denture. (See Figs.
9-5 and 9-7.)
 Biologic considerations for mandibular impressions 199

Fig. 9-5 The buccal shelf (within

dotted line) in this patient's mouth ex-
tends posteriorly from the buccal fre-
num, A, to the retromolar pad, B, and
from the external oblique line on the
lateral to the crest of the residual alve-
olar ridge, C, medially.

Fig. 9-6 Cross section showing a lower

denture in place and its relation to the man-
dible, tongue, and cheeks. The polished
contours of the denture, A, aid the muscles
in pressing it to place. Notice that the at-
tachment of the buccinator, B, goes under
Mi taly \\Nv\
hes the buccal flange
;
area , to a point near the
Yi Yui, we I In ve
‘ofl ly crest of the ridge. The lingual flange, C, ex-
My yyy yusiny|) (Nip
a a int i tends away from the bony contour of the
Mi i Moll Milf)
mandible and is under the tongue; this aids
a Naa in seating the denture. D, The lingual pol-

6
ess ished surface contour provides space for
the tongue and acts to place rather than dis-
place the denture. The mylohyoid muscle, E,
contracts during swallowing and should not
be impinged on.
200 Rehabilitation of the edentulous patient

Fig. 9-7 The buccal shelf (outlined by the dotted line) consists of smooth compact
bone. By both its nature and its position at right angles to the occlusal forces, it is well
Suited to provide support for a lower denture. The crest of the residual ridge should not
be used as a primary stress-bearing area. Notice that the mental foramina, A, are on the
crest of the ridge in this badly resorbed mandible. The mental vessels and nerves must
be relieved of impingement by the denture base. Notice also the difference between the
location of the mental foramina here and their more usual position in Figs. 9-1 and 9-2.
B
is the genial tubercles.

structures, especially on the lingual side of the addition, important variations in the basal seat
ridge, to fall over onto the ridge surface. Such for a mandibular denture include the stages of
conditions require displacement of these tis- change in the mandible, a sharp mylohyoid
sues by the impression, which will gradually ridge, resorption in the area of the mental fora-
reestablish a suitable bearing surface. The crest men, insufficient space between the mandible
of greatly resorbed ridges is often at the level and the tuberosity, low mandibular ridges, the
of the mental foramina, and the nerves and direction of resorption of the ridges, and a
blood vessels are easily compressed unless the torus mandibularis.
area is palpated and relieved on the impression Stages of change in the mandible. Fig. 9-9
(Fig. 9-7). portrays the mandible at various stages of de-
velopment. The final illustration shows it fully
Bone of the basal seat
formed, with loss of the alveolar process down
The configuration of bone that forms the to a point opposite the mental foramen. As the
basal seat for a mandibular denture varies con- alveolar process is progressively lost, the at-
siderably among patients. Factors that influ- taching structures converge and thus the sup-
ence the form of the supporting bone of the porting surface of the denture becomes more
basal seat are listed in Chapter 7 (pi Sian and more limited.
 Biologic considerations for mandibular impressions 201

Fig. 9-8 Cast showing the distribution of forces in a selective pressure impression pro-
cedure. PS, Primary stress-bearing area (the buccal shelf); SS, secondary stress-bearing
area (slopes of the residual ridge); SR, secondary relief area when the crest of the ridge
is sharp and thin; V, peripheral seal area.

Mylohyoid ridge. Soft tissue usually hides the considered a desirable form of the mylohyoid
sharpness of the mylohyoid ridge, which can be ridge for an edentulous patient whereas in Fig.
found by palpation. The shape and inclination 9-12 the mylohyoid ridges are bulbous, irregu-
of the ridge vary greatly among edentulous pa- lar, and severely undercut. Extremely thin and
tients. sharp mylohyoid ridges are seen in Fig. 9-13,
In Fig. 9-10 are diagrams illustrating cross which illustrates another source of aggravation
sections of the mandible to show the inclination and soreness for edentulous patients.
of the mylohyoid ridge and the level of the my- Mental foramen area resorption. Severe re-
lohyoid muscle in the various parts of the man- sorption of bone near the mental foramina or
dible from the incisal to the third molar region. the crest of the residual ridge results in com-
Note the various levels of attachment of the pression of the mental nerves and blood vessels
mylohyoid muscle as it extends posteriorly if relief is not provided in the denture base
along the ridge from the symphysis mandibu- (Fig. 9-7). Pressure on the mental nerve can
lae. Anteriorly the muscle attaches close to the cause numbness of the lower lip.
inferior border of the mandible, and posteriorly Insufficient space between the mandible and
it may be flush with the superior surface of the the tuberosity. The maxillary sinus enlarges
residual ridge. Fig. 9-11 depicts what would be throughout life if it is not restricted by natural
 )
y Y,
li

Fig. 9-9 Four stages of development of the mandible. A, At birth the condyles are not
fully formed. B, At 8 years. C, Adulthood. D, Edentulism.

Fig. 9-10 Relationships of the mylohyoid muscle in various regions. The letters
with
prime signs denote cross sections of the designated areas: A, canine region;
B, premolar
region; C, first molar; D, third molar. In D’, notice that the mylohyoid
ridge approaches the
level of the alveolar crest. The angle of the posterior lingual flange in
the molar region is
affected by this muscle; anteriorly, only the length of the flange is affected.
 Biologic considerations for mandibular impressions 203

Fig. 9-11. An edentulous mandible with a moderate undercut beneath the mylohyoid
ridge, A. The denture base cannot extend into this relatively slight undercut (dotted line)
because the mylohyoid muscle, attaching to the ridge, moves outward and upward dur-
ing contraction. Notice the location of the genial tubercles, B, midway between the supe-
rior and inferior borders of the mandible on its lingual surface. C is the mandibular fora-
men.

Fig. 9-12 An edentulous mandible with bulbous and irregular mylohyoid ridges, A, that
are severely undercut, B, and cannot be used for retention. Notice the bony projection
anteriorly on the crest of the alveolar ridge, C, and the genial tubercles, D.
204 Rehabilitation of the edentulous patient

Fig. 9-13 An edentulous mandible with a flat residual alveolar ridge. The mylohyoid
ridges, A, are knifelike and at the crest of the alveolar ridge. The spiny roughness of the
alveolar ridge, B, cannot be detected when mucous membrane covers the ridge. The lo-
cation of such roughness is usually indicated by a band of narrow fibrous tissue along
the center of the ridge. Relief must be provided in the impression and the finished den-
ture. Notice that the genial tubercles, C, are almost at the crest of the alveolar ridge an-
teriorly, an indication of the severe resorption that has occurred.

A B A B A B A B
Fig. 9-14 Progressive resorption of the maxillary and mandibular ridges makes the
maxilla narrower and the mandible wider. The lines A-B illustrate how
this narrowing af-
fects the bone. The lines C and D represent the centers of the ridges.
Notice how the
distance between them becomes greater as the mandible and maxillae resorb.
 Biologic considerations for mandibular impressions 205

teeth or dentures, thus moving the tuberosity maxillae have been edentulous, the smaller
downward. The angle of the mandible is fre- their bearing area is likely to be. The opposite
quently made more obtuse by the early loss of is true of the mandible, which inclines outward
posterior teeth with retention of the anterior and becomes progressively wider according to
teeth. Removal of this posterior support de- its edentulous age. This progressive change of
stroys the necessary counterbalance against the mandible and maxillae in the edentulous
muscle pull at the angle of the mandible. Such state makes many patients appear prognathic
“straightening” of the mandible reduces the (Fig. 9-14).
maxillomandibular space in the posterior re- Torus mandibularis. The torus mandibularis
gion and is the cause of difficulty in obtaining is a bony prominence usually found near the
sufficient space for the teeth and denture first and second premolars, midway between
bases. The lack of space causes many denture the soft tissue of the floor of the mouth and the
failures. crest of the alveolar process. In edentulous
Low mandibular ridges. Frequently the man- mouths where considerable resorption has
dibular supporting area is depressed rather taken place, the superior border of this promi-
than elevated because of differences in the rate nence may be flush with the crest of the resid-
of resorption of cortical bone and cancellous ual ridge on the lingual side. It varies in size
bone. Lingually, on these greatly resorbed from a pea to a hazelnut (Fig. 9-15). The cause
mandibles, the bone has shrunk down to the for its occurrence is not known, but it is some-
level of the attachments of the structures in the times coincident with a bulbous torus palati-
floor of the mouth. This makes the lingual nus. The torus mandibularis is covered by an
flange of the denture more difficult to adapt. extremely thin layer of mucous membrane and
Direction of ridge resorption. The maxillae for this reason may be irritated by slight move-
resorb upward and inward to become progres- ments of the denture base. It should be re-
sively smaller because of the direction and in- moved surgically if relief cannot be provided
clination of the roots of the teeth and the alve- for it inside the denture without breaking the
olar process. Consequently, the longer the border seal.

yy)

Fig. 9-15 Tori mandibulari, X. Surgical reduction of these will be necessary before a
satisfactory seal can be developed by a mandibular denture.
206 Rehabilitation of the edentulous patient

MACROSCOPIC ANATOMY OF LIMITING pull actively across the denture borders, pol-
STRUCTURES ished surfaces, and teeth. Therefore the den-
Mandibular dentures should extend as far as ture should extend less in this region, and the
possible within the limits of health and function impression must be functionally trimmed to
of the tissues and structures that surround and have the maximum seal and yet not displace
support them. This is the same principle that the denture when the lip is moved (Fig. 9-17).
governs the extent of maxillary dentures; but it The lower lip must be supported to an ex-
is more difficult to apply to mandibular than to tent equal that provided by the natural teeth
maxillary dentures, because the structures on and their investing structures (Fig. 9-18). The
the lingual side of the mandible must be con- length and thickness of the labial flange in the
sidered as well as those around the labial and labial vestibules vary with the amount of tissue
buccal surfaces of the denture. The structures that has been lost. The tone of the skin of the
on the lingual of the mandible are more com- lip and of the orbicularis oris depends on the
plicated to control than those on the buccal and thickness of the flange and the position of the
labial. The problem is the greater range of teeth.
their movement and the speed of their actions. There is no muscle extending from the resid-
ual ridge to the lip between the two triangulari
Buccal and labial borders
(depressors anguli oris), so the labial flange can
The underlying structures around the border be extended in length and thickness to supply
of a complete denture vary according to their the necessary support for the lip (Fig. 9-18).
location. This fact is overlooked constantly and Buccal vestibule. The buccal vestibule ex-
is a reason why the best possible mandibular tends from the buccal frenum posteriorly to the
denture coverage is seldom attained. If a care- outside back corner of the retromolar pad and
ful study is made and used, the size of the from the crest of the residual alveolar ridge to
mandibular denture may be found larger than the cheek (Fig. 9-18). The buccinator, in the
would be expected. Mandibular dentures cheek, extends from the modiolus (anteriorly)
should be wide back of the buccal frenum and to the pterygomandibular raphe (posteriorly).
narrow in the anterior labial region. The man- Its lower side attaches in the molar region in
dibular labial frenum contains a band of fibrous the buccal shelf of the mandible (the part of the
connective tissue that helps attach the orbicu- bone between the residual ridge and the exter-
laris oris; therefore the frenum is quite sensi- nal oblique line). The buccinator’s action oc-
tive and active and must be carefully fitted to curs in a horizontal direction, so it cannot lift
maintain a seal without causing soreness (Fig. the lower denture, even though the buccal
9-16). flange of a properly extended denture will rest
The part of the denture that extends be- on its inferior attachment.
tween the labial frenum (labial notch) and the External oblique ridge and the buccal flange.
buccal frenum (buccal notch) is called the man- The extension of the mandibular labial and
dibular labial flange. This flange is limited in buccal flanges is governed by the same general
extension because the fibers of the orbicularis factors. The impression space in the labial ves-
oris and the incisivus labii inferioris are fairly tibule between the labial and buccal frena is
close to the crest of the ridge. determined by the turn of the mucolabial fold
The buccal frenum connects as a continuous (the line of flexure of the mucous membrane as
band through the modiolus at the corner of the it passes from the mandible to the lip and
mouth to the buccal frenum in the maxilla (see cheeks). The space is not extensive. The buccal
Fig. 19-6). These fibrous and muscular tissues flange area, which starts immediately posterior
 Biologic considerations for mandibular impressions 207

Fig. 9-16 The labial notch. In A, notice that it can be narrow or broad (7 or 2) depend-
ing on the width of the labial frenum. In B, the broadly contoured notch is correct for this
patient. If it were too narrow for function of the labial frenum, soreness would result; if too
broad, a loss of seal could occur.

ae Nae ae ae

SS RIK

Fig. 9-17 A, Typical contours of the labial and buccal borders of a mandibular denture:
mandibular
1, Broad buccal flange; 2, mandibular buccal notch for the buccal frenum; 3,
labial notch for the labial frenum. B, Proper contour of the buccal notch
labial flange; 4,
for each patient.
(arrow) of a lower complete denture. The contours will vary somewhat
 208 Rehabilitation of the edentulous patient

Fig. 9-18 Correlation of anatomic landmarks. A, In the mandibular arch: 7, Labial fre-
num; 2, labial vestibule; 3, buccal frenum; 4, buccal vestibule; 5, residual alveolar ridge;
6, retromolar pad; 7, pterygomandibular raphe; 8, retromylohyoid fossa; 9, lingual tuber-
cle; 10, alveololingual sulcus; 17, submaxillary caruncles; 12, tongue; 13, lingual frenum;
14, buccal shelf and premylohyoid eminence. B, In the mandibular final impression: 7,
Labial notch; 2, labial flange; 3, buccal notch; 4, buccal flange; 5, alveolar groove; 6, ret-
romolar fossa; 7, pterygomandibular notch; 8, retromylohyoid eminence; 9, lingual tuber-
cular fossa; 10, lingual flange; 12, inclined plane for the tongue; 13, lingual notch; 74,
buccal flange that fits on the buccal shelf; 15, premylohyoid eminence.

to the buccal frenum and extends to the ante- close to the crest of the ridge and the denture
rior portion of the masseter, swings wide into rests directly on a considerable portion of this
the cheek and is nearly at right angles to the muscle. The bearing of the denture on muscle
biting force, thus providing the lower denture fibers would not be possible except for the fact
with its greatest surface for resistance to verti- that the fibers of the buccinator and its pull
cal occlusal forces (Fig. 9-19). when in function are parallel to the border and
The external oblique ridge does not govern not at right angles to it, as the masseter fibers
the extension of the buccal flange because the are; hence its displacing action is slight. More
resistance or lack of resistance encountered in resistance is encountered in this region when
this region varies widely. The buccal flange the denture is first inserted than is manifested
may extend to the external oblique ridge, or up a few weeks after the patient has worn the
onto it, or even over it, depending on the loca- completed dentures. Thus it is possible to
tion of the mucobuccal fold. However, palpa- stretch and displace these tissues and create
tion of the external oblique ridge is a valuable this area, invaluable for biting resistance and
aid or landmark in helping to ascertain the rel- stability, that is so sorely needed when the re-
ative amount of resistance or lack of resistance sidual ridge is sharp or narrow.
of the border tissues in this region. Masseter muscle region. The distobuccal
The buccal shelf is successfully utilized, de- borders of the mandibular denture must con-
spite the fact that the buccinator fibers attach verge rapidly to avoid displacement because of
 Biologic considerations for mandibular impressions 209

cheek, 7, and
Fig. 9-19 A, The external form of a new denture (right) should permit the
flanges to help hold the denture in place. B,
tongue, 2, to rest on the buccal and lingual
new denture slopes toward the tongue (from 7 to 2) so it will
Here the lingual flange of the
action of the mylohyoi d muscle. It thus takes advanta ge of all the avail-
accommodate the
Immediately posterior to
able basal seat to increase its retention, stability, and support.
toward the cheek and provides the
the buccal notch the buccal flange extends outward
greatest surface area for resisting vertical occlusal forces.
210 Rehabilitation of the edentulous patient

contracting pressure of the masseter muscle, perpendicular surface and the origin of the
whose anterior fibers pass outside the buccina- muscle on the zygomatic arch is medialward,
tor in this region (Fig. 9-20). the muscle pulls more directly across the disto-
When the masseter contracts, it alters the buccal denture border; therefore it forces the
shape and size of the distobuccal end of the buccinator and tissues inward, reducing the
lower buccal vestibule. It pushes inward space in this region. If the opposite is true,
against the buccinator muscle and suctorial pad greater extension is allowed on the distobuccal
of the cheek. portion of the mandibular impression. One can
The distobuccal border of the mandibular register this masseter pull on the impression by
impression encounters the action of the mas- softening the compound with an alcohol flame
seter to a greater or lesser degree depending along the distobuccal border, tempering the
on the shape of the mandible and the origin of compound in warm water, and, after seating
the muscle. If the ramus of the mandible has a the impression in the patient’s mouth, exerting

Fig. 9-20 Finished rubber base impression with border outline landmarks: A, Mandibu-
lar labial notch; B, mandibular labial flange; C, mandibular buccal notch; D, buccal
flange; E, area influenced by the masseter; F, retromolar pad area; G, lingual notch; H,
premylohyoid eminence; /, retromylohyoid eminence. Notice the S curve of the lingual
flanges and also that, in the molar region, the flanges slope toward the tongue and ex-
tend below the attachment of the mylohyoid muscles on the mylohyoid ridges. The slope
of the lingual flanges allows the mylohyoid muscles to contract and raise the floor of
the
mouth without displacing the lower denture. The length of the lingual flange
in the molar
region allows it to reach the mucolingual fold of tissue in the floor of the mouth
to maintain
the seal of the lower denture. The posterior end of the lingual flange bends
laterally to-
ward the mandible to fit into the retromylohyoid fossa. This part of the denture
guides the
tongue onto the top of the lingual flange.
 Biologic considerations for mandibular impressions 211

a downward pressure by placing the index fin- der seal in this region. It contains some glandu-
gers on the impression in the second premolar lar tissue and some fibers of the temporalis ten-
region. While this downward pressure is being don, but it also has active structures working
exerted on the impression by the dentist, the through it (see Fig. 9-35). Buccinator fibers en-
patient is instructed to exert a closing force. ter it from the buccal side, and fibers of the su-
These opposing forces will cause the masseter perior pharyngeal constrictor enter it from the
to contract and trim the compound in that area lingual; the pterygomandibular raphe enters
since the relation of the mandible and maxillae the pad at its superoposterior inside corner.
causes the masseter to affect the distobuccal The actions of these structures limit the extent
border directly. The relative size of the mas- of the denture and prevent placement of extra
seter will influence its action on the buccinator: pressure on the retromolar pad during impres-
a masseter that is of smaller diameter will have sion procedures or when reducing the posterior
less influence (perhaps none) on the border. borders of the pad on the cast.

Distal extension of the mandibular Lingual borders

impression The lingual extension on mandibular impres-
The distal extent of the mandibular impres- sions has been the most abused and misunder-
sion is limited by the ramus of the mandible, stood border region in complete denture con-
by the buccinator fibers that cross from the struction. This misunderstanding is caused by
buccal to the lingual as they attach to the the peculiarities of the tissue under the tongue,
pterygomandibular raphe and the superior con- which has less direct resistance than the labial
strictor, and by the sharpness of the lateral and buccal borders do and thus will not toler-
bony boundaries of the retromolar fossa (which ate overextension of the lingual flange. Because
is formed by a continuation of the internal and of their peculiar lack of immediate resistance,
external oblique ridges ascending the ramus). these tissues are easily distorted when the im-
If the impression extends onto the ramus, the pression is being made. Such extension over a
buccinator and adjacent tissues will be com- long time will cause tissue soreness or dislodg-
pressed between the hard denture border and ment of the denture by tongue movement. The
the sharp external oblique ridge. This will not lingual border of the mandibular impression is
only cause soreness but also limit the function
of the buccinator, which is part of the kinetic
chain of swallowing.
The desirable distal extension is slightly to
the lingual of these bony prominences and in-
cludes the pear-shaped retromolar pad, which
forms a splendid soft tissue seal of the type that
is so valuable in carrying out the principles in-
volved in impression sealing.
Retromolar region and pad. The distal end of
the mandibular denture region is bounded by
the anterior border of the ramus. Thus the
denture includes the retromolar pad _posteri-
orly, which defines its posterior limit (Fig. 9-
21). The retromolar pad (a triangular soft pad of
tissue at the distal end of the lower ridge) must Fig. 9-21. The retromolar pad, x, is the posterior
be covered by the denture to perfect the bor- landmark for a mandibular denture.
212 Rehabilitation of the edentulous patient

easily carried down along the bony surface of understanding of the anatomy and function of
the mandible into the undercut below the my- the floor of the mouth. The mylohyoid muscle
lohyoid ridge, since the mylohyoid muscle is a arises from the whole length of the mylohyoid
thin sheet of fibers that in a relaxed state will line, extending from about 1 cm back of the
not resist the impression. However, extension distal end of the mylohyoid ridge to the lingual
of the lingual flange under the mylohyoid ridge anterior portion of the mandible at the sym-
cannot be tolerated in function because it will physis. Medially the fibers join those from the
displace the denture, causing soreness and lim- mylohyoid muscle of the opposite side, and
iting function, unless the flange is made to par- posteriorly they continue to the hyoid bone.
allel the mylohyoid muscle when it is con- The muscle lies deep to the sublingual gland
tracted. Although such a mechanical lock might and other structures about the region of the
seem desirable to secure additional retention, second premolar, and so does not affect den-
it cannot be tolerated because of physiologic ture borders in this region except indirectly.
factors. Therefore the border tissues in this However, the posterior part of the mylohyoid
area must be treated in a distinctly different muscle in the molar region affects the lingual
manner from one involving the usual methods impression border in swallowing and moving
and materials (Figs. 9-10 to 9-12). the tongue. Fortunately, the posterior exten-
Influence and action of the floor of the mouth. sion of the impression can go beyond the mus-
An acceptable lingual border that will result in cle’s attachment line, since the mucolingual
a stable denture can be secured with a proper fold is not in this area. Thus the impression

Fig. 9-22 Muscles of the floor of the mouth, posterior view. A, The mylohyoid: B, the
geniohyoid; C, hyoid bone. Notice that the mylohyoid muscle is positioned more
superi-
orly on the mandible as its attachment extends posteriorly on the mylohyoid ridge.
For
this reason, action of the mylohyoid affects the slope of the lingual flange of the impres-
sion in the molar region and causes the flange to slope toward the tongue.
 Biologic considerations for mandibular impressions 213

may depart from a stress-bearing area of the plete the lingual border seal in the retromylo-
lingual surface of the ridge and be suspended hyoid fossa and guide the tongue on top of the
under the tongue in soft tissue on both sides of flange (Fig. 9-20).
the mouth, thereby reaching the mucolingual Sublingual gland region. In the premolar re-
fold of soft tissue for a seal. The distance that gion on the lingual side of ridge, the sublingual
these lingual borders can be away from the gland rests above the mylohyoid muscle. When
bony areas will depend on the functional move- the floor of the mouth is raised, this gland
ments of the floor of the mouth and the amount comes quite close to the crest of the ridge and
that the residual ridge has resorbed (Figs. 9-6, reduces the vertical space available for flange
9-22, and 9-27). extension in the anterior part of the mouth
Mylohyoid muscle and mylohyoid ridge. An (Fig. 9-23).
extension of the lingual flange well beyond the The lingual frenum area is likewise rather
palpable portion of the mylohyoid ridge, but shallow, sensitive, and resistant. It should be
not into the undercut, has other advantages. registered in function because at rest the
One is that the lack of direct pressure on this height of its attachment is deceptive. In func-
sharp edge of bone will eliminate a possible tion, it often comes quite close to the crest of
source of discomfort. If the impression is made the ridge, although at rest it is much lower
with pressure on or slightly over this ridge, dis- (Figs. 9-25 and 9-28).
placement of the denture and soreness are sure Direction of the lingual flange. The extension
to result from lateral and vertical stresses. On of the lingual flange under the tongue is a con-
the other hand, if the border stops above the cept vastly different from one that has it ending
ridge, vertical forces will still cause soreness, at the mylohyoid ridges. The lower border of
and the seal will be broken easily. If the flange the lingual flange runs parallel to the lower
is properly shaped and extended, it will com- edge of the mandible from the lingual frenum

Fig. 9-23 Lingual side of the mandible showing the positions of the sublingual gland
mylohyoid
relative to the mylohyoid muscle, A, at rest and, B, in a contracted state. The
line is denoted by C.
214 Rehabilitation of the edentulous patient

f
Fig. 9-24 Dotted line marks the lower border of the lingual lange on
} the eft si

tice that the flange roughly parallels the lower border of the mandible

Fig. 9-25 Aec

VICsSs tioning material
Notice th at
al the
lt terior region
 Biologic considerations for mandibular impressions 215

to the posterior end of the denture (Fig. 9-24). If an impression is made under these con-
This makes the flange short in the anterior re- ditions, the muscle and other tissues in
gion and long in the posterior region because this region will be trapped under the
the crest of the ridge of the mandible turns up ridge and buccal to their functioning posi-
rather sharply as it approaches the ramus (Fig. tion when the tongue is placed against the
9-25). The posterior extension is bounded par- upper incisors. The sublingual gland and
tially by the action of the glossopalatine mus- submaxillary duct may be pushed down
cle, which usually is no farther back than the and laterally out of position by resistant
distal extent of the retromolar pad. impression material. This can be avoided
by shaping this part of the lingual flange
Alveololingual sulcus of the tray to slope inward toward the
The alveololingual sulcus (the space between tongue and making the final impression
the residual ridge and the tongue) extends pos- with a very soft impression material.
teriorly from the lingual frenum to the retro- When the middle of the lingual flange
mylohyoid curtain. Part of it is available for the is made to slope toward the tongue, it can
lingual flange of the denture. extend below the level of the mylohyoid
The alveololingual sulcus can be considered ridge. Otherwise, it must end at the level
in three regions. of the mylohyoid ridge. If the flange
1. The anterior region slopes toward the tongue and extends be-
This extends from the lingual frenum to low the mylohyoid ridge, the tongue can
where the mylohyoid ridge curves down rest on top of the flange and aid in stabi-
below the level of the sulcus. Here a de- lizing the lower denture on the residual
pression (the premylohyoid fossa) can be ridge. In addition, the slope of the lingual
palpated and a corresponding prominence flange in the molar region provides space
(the premylohyoid eminence, Fig. 9-20, for the floor of the mouth to be raised
H) can be seen on impressions. The pre- during function (tongue movements and
mylohyoid fossa results from the concav- swallowing) without displacing the lower
ity of the mandible (as viewed from denture. The seal of the lower denture is
above) joining the convexity of the mylo- maintained during these movements of
hyoid ridge (also from above) (Fig. 9-18). the floor of the mouth because the lingual
The lingual border of the impression in flange remains in contact with the mu-
this anterior region should extend down colingual fold in the alveololingual sulcus
to make definite contact with the mucous (Figs. 9-26 and 9-27).
membrane floor of the mouth when the 3. The posterior region
tip of the tongue touches the upper inci- This part of the alveololingual sulcus is
sors. the retromylohyoid space or fossa. It ex-
2. The middle region tends from the end of the mylohyoid
This part of the alveololingual sulcus ridge to the retromylohyoid curtain—be-
extends from the premylohyoid fossa to ing bounded on the lingual by the ante-
the distal end of the mylohyoid ridge rior tonsillar pillar (at the distal end by
(Fig. 9-18), curving medially from the the retromylohyoid curtain and superior
body of the mandible. The curvature is constrictor) and on the buccal by the my-
caused by the prominence of the mylohy- lohyoid muscle, mandibular ramus, and
oid ridge. When the mylohyoid muscle retromolar pad. The superior support for
and the tongue are relaxed, the muscle the retromylohyoid curtain is provided by
part of the superior pharyngeal constric-
drapes back under the mylohyoid ridge.
216 Rehabilitation of the edentulous patient

Fig. 9-26 The alveololingual sulcus has an S shape, starting at the midline. Notice that
the S results from the contour of the residual ridge and the prominence of the mylohyoid
ridge. This characteristic form is equally apparent on the dissected (left) and undissected
(right) sides.

Fig. 9-27 A tissue-conditioning impression illustrates the typical

S curve of the lingual
flange. This type of contour permits tissues of the
floor of the mouth to function normally.
A, The slope of the lingual flange toward the tongue
in the molar region allows the mylo-
hyoid muscle to contract and raise the floor of the
mouth without displacing the denture.
B, The distal end of the lingual flange turns buccall
y to fill the retromylohyoid fossa. This
part of the flange guides the tongue on top of the
lingual flange of the denture.
 Biologic considerations for mandibular impressions 217

tor. The actions of this muscle and of the Lingual frenum and lingual notch. The lin-
tongue (and their effects on the alveolo- gual frenum (that is, the anterior attachment of
lingual sulcus) determine the posterior the tongue) is extremely resistant and active
extent of the lingual flange. The denture and often wide (Fig. 9-18, A, 13). It forms the
border should extend posteriorly to con- lingual notch in the lower impression. The
tact the retromylohyoid curtain (the pos- denture border needs complete functional
terior limit of the alveololingual sulcus) trimming so movements of the lingual frenum
when the tip of the tongue is placed will not displace the denture or create soreness
against the front part of the upper resid- of this sensitive band of tissue (Fig. 9-28).
ual ridge. Lingual flange. The lingual flange of the den-
The attachment of the mylohyoid muscle ex- ture occupies the alveololingual sulcus (the
tends about 1 cm distal to the end of the mylo- space between the residual alveolar ridge and
hyoid ridge, which prevents the denture from the tongue). The distal end of the alveololin-
locking against the bone in this region (Fig. 9- gual sulcus ends at the retromylohyoid curtain.
26). However, two objectives are accomplished This curtain of mucous membrane is supported
when the lingual flange extends into this area. above by the superior constrictor and is pulled
First, the border seal is made continuous from forward when the tongue is thrust out.
the retromolar pad to the middle of the alve- The distal extent of the lingual flange is
ololingual sulcus. Second, this part of the partly limited by the glossopalatine arch, which
flange is so shaped that it guides the tongue on is formed by the glossopalatine muscle and the
top of the lingual flange of the denture. Such a lingual extension of the superior constrictor.
contour helps the patient control the denture Moving anteriorly, the lingual flange in the
without interfering in the functions of the soft molar region is influenced by the mylohyoid
tissues. When the lingual flange is developed muscle, which attaches to the mylohyoid ridge.
in this manner, the border of the flange has a The flange extends below and medialward from
typical S curve as viewed from the impression the mylohyoid ridge to occupy the alveololin-
surface (Fig. 9-27). gual sulcus as limited by the mucolingual fold

lingual notch is
Fig. 9-28 X denotes the lingual notch in a completed impression. The
ridge.
usually broad and often is close to the crest of the
218 Rehabilitation of the edentulous patient

(the line of flexure of the mucous membrane as there is contact between the flange and the
it passes from the tongue to the floor of the mucous membrane when the tongue is raised
mouth). This means that the buccal surface of or thrust out (Fig. 9-29). This mucolingual fold
the flange rests not on mucous membrane in is extremely flexible and mobile because of the
contact with bone but on soft tissue. The flange type of tissue it is and because of the mobility
leaves the bony attachment at the mylohyoid of the entire floor of the mouth. The border tis-
ridge and slopes inward under the tongue to fill sue on the lingual side of the residual ridge is
the alveololingual sulcus. Thus there is a space unlike the border tissue in any other part of
between the flange and the mucous membrane the mouth as regards function and resistance in
when the mylohyoid muscle is relaxed, but border molding (Fig. 9-6). It has so little resis-

Myyaa:
a
\\V |VI \\ \ \\\
\\\
\ \\\\\ Ni
fees \\\\\ [||
i MN —

J 1AN el

Fig. 9-29 A, Relationship of the mylohyoid muscle to the lingual flange of

a denture,
posterior view. Notice that the muscle attaches inferiorly to the
flange from the middle of
the mandible posteriorly through the premolar region. However,
in the molar region, it at-
taches to the mylohyoid ridge superior to the border of the flange
(circle) and its move-
ment during contraction affects the shape of the lingual flange
in the molar region. 7, Hy-
oid bone; 2, the geniohyoid muscle: 3, the mylohyoid muscle;
4, portion of the lingual
flange (circle) covering the mylohyoid muscle; 5, angle
of the mandible: 6, mandibular
foramen; 7, incisive fossa: 8, hard palate; 9, median
palatine suture: 10, pterygoid ham-
ulus; 17, lateral pterygoid plate: 12, Styloid process; 13,
nasal septum: 14, foramen mag-
num. B, Lower denture, inferior view. 1, Premylohyoid
eminence: 2, retromylohyoid emi-
nence; 3, the mylohyoid muscle (cut section covered
by the lingual flange of the denture
in the molar region); 4, space in the denture for
the mylohyoid muscle. Arrow denotes the
direction that the mylohyoid muscle moves during
contraction. Space in the lingual flange
allows movement of the muscle and overlying floor
of the mouth without displacement of
the lower denture or creation of soreness in the
floor of the mouth.
 Biologic considerations for mandibular impressions 219

tance that it is easily distorted and for this rea- scopic anatomy have been discussed. A review
son needs a special type of technique and im- of this part of Chapter 7 will be helpful at this
pression material to record its correct turn. point because the material is applicable also to
The anterior part of the lingual flange over considerations for mandibular impressions.
the sublingual gland is usually shallow because
of the mobility of the tissues that are controlled Supporting tissues
indirectly by the mylohyoid muscle. The mylo- The microscopic anatomy of the supporting
hyoid muscle in this region extends nearly to tissues of the lower impression will be de-
the inferior border of the mandible, and yet scribed for the crest of the residual ridge and
the glandular and other tissues move above it the buccal shelf.
in such a way that only a relatively short flange Crest of the residual ridge. The mucous
is usable (Figs. 9-22 and 9-23). The combina- membrane covering the crest of the lower
tion of a typical arch form of the lingual side of residual ridge is similar to that of the upper
the mandible, the projection of the mylohyoid ridge insofar as, in the healthy mouth, it is
ridge toward the tongue, and the existence of a covered by a keratinized layer and is firmly
retromylohyoid fossa at the distal end of the al- attached by its submucosa to the periosteum of
veololingual sulcus causes the border of the lin- the mandible. The extent of the attachment to
gual flange to assume its typical S shape when the bone varies considerably. In some patients
viewed from the impression surface (Fig. 9-20). the submucosa is loosely attached to the bone
Starting at the midline, the flange curves out- over the entire crest of the residual ridge, and
ward, following the lingually concave residual the soft tissue covering is quite movable. In a
ridge. At the premylohyoid fossa, which is lo- relatively few patients the submucosa is rela-
cated at the front end of the mylohyoid ridge, a tively firmly attached to the bone on both the
premylohyoid eminence forms in the flange. At crest and the slopes of the lower residual
this point the border of the lingual flange ridge. When the soft tissue is movable, it must
curves away from the body of the mandible to be carefully registered in its resting position in
accommodate the mylohyoid muscle when it is the final impression. Occasionally surgical pro-
contracted or when the tongue is raised. At the cedures are indicated to increase the amount
distal end of the mylohyoid ridge, the lingual of the “residual attached gingivae.” When
flange turns laterally toward the ramus to fill these tissues become inflamed, the submucosa
with infiltration by numerous
the retromylohyoid fossa and complete the typ- is edematous,
ical S form. The distal end of the lingual flange inflammatory cells. Obviously, the tissue must
is called the retromylohyoid eminence. Its be healthy at the time the final impression is
most prominent contour lies medial, posterior, made.
and below the level of the retromolar pad. The mucous membrane of the crest of the
lower residual ridge when securely attached to
MICROSCOPIC ANATOMY“ the underlying bone is histologically capable of
In Chapter 7 the importance of microscopic providing proper soft tissue support for the
lower denture. However, the underlying bone
anatomy to maxillary impression making, the
histologic nature of the soft tissue and bone of of the crest of the lower residual ridge is can-
the oral cavity, a classification of the oral mu- cellous, being made up of spongy trabeculae
(Fig. 9-30). Therefore the crest of the lower re-
cosa, and clinical considerations of oral micro-
sidual ridge may not be favorable as the pri-
mary stress-bearing area for a lower denture.
*We wish to acknowledge the assistance of Dr. Steve Ko- The method of incorporating space in the final
las, Professor, Department of Oral Pathology, Medical Col- impression tray before the final impression is
lege of Georgia School of Dentistry, Augusta.
 220 Rehabilitation of the edentulous patient

made ensures that proper relief will be pro- as the mucous membrane overlying the crest of
vided for the crest of the lower residual ridge the lower residual ridge. However, the bone of
during making of the final impression. the buccal shelf is covered by a layer of com-
Buccal shelf. Anatomically, the buccal shelf pact bone (with its haversian systems) (Fig. 9-
is defined as that part of the basal seat located 31). The nature of this bone, plus the horizon-
posterior to the buccal frenum and extending tal supporting surface provided by the buccal
from the crest of the lower residual ridge to the shelf, makes it the most suitable primary
external oblique ridge (see Fig. 9-5). The mu- stress-bearing area for a lower denture. The
cous membrane covering the buccal shelf is horizontal direction of the fibers of the buccina-
more loosely attached and less keratinized than tor allows the denture to rest on this part of the
the mucous membrane covering the crest of muscle without damage to the muscle or dis-
the lower residual ridge, and it contains a placement of the denture.
thicker submucosal layer. Histologically, fibers The method of forming the lower final im-
of the buccinator are found running horizon- pression tray allows additional load to be
tally in the submucosa immediately overlying placed on the buccal shelf during the making of
the bone. the final impression (Fig. 9-32). The tray comes
The mucous membrane overlying the buccal into direct contact with the mucosa of the buc-
shelf may not be as suitable histologically to cal shelf, and the soft tissue is slightly displaced
provide primary support for the lower denture as the final impression is made.

sg
: ==4 Mucosa
BS é oS
Mucosa Bvs.

Es
754:
1p
i)
Se

nO Submucosa

Submucosa

Buccinator
muscle

Spongy bone Compact bone

Fig. 9-30 Histology of the crest of the lower re- Fig. 9-31 Histology of the buccal shelf of the
sidual ridge. The submucosal layer of mucous mandible. Bone forming the buccal shelf is com-
membrane covering the crest may be of adequate pact, in contrast to the spongy bone that forms the
thickness and firmly attached to the residual ridge. crest of the lower ridge (Fig. 9-30). The nature of
However, the bone that forms the crest of the compact bone makes the buccal shelf suitable as
lower ridge is cancellous or spongy; therefore this the primary stress-bearing area for a lower den-
part of the ridge is generally not used for primary ture.
support of a lower denture.
Fig. 9-32 Buccal flanges of the final impression tray in the region of the buccal shelf
This
(arrows) are left in direct contact with the cast on both sides when the tray is made.
final
part of the tray directly contacts the mucosa of the buccal shelf during making of the
The
impression and places an additional load on the supporting tissues in this region.
rest of the tray has been relieved from the cast by a wax spacer.

palate, retromolar
Fig. 9-33 Mucous membrane lining the retromylohyoid fossa, soft
lip; RMC, retromyl ohyoid curtain (formed by the mucous
pad, and cheek. LL, Lower
ar pad; PR, pterygo mandibular
membrane covering the superior constrictor); RP, retromol
ohyoid curtain lies at the posterior
raphe; BMC, buccal mucosa of the cheek. The retromyl
posterio r boundar y of the retromyl ohyoid
end of the alveololingual sulcus and is the
of the histolog ic section in Fig. 9-34, A.
fossa. Asterisk denotes the location
 222 Rehabilitation of the edentulous patient

Compact bone

Medial pterygoid
muscle

Superior constrictor
muscle

ey Submucosa
SATE
RIE
‘anon
iy
Ba
Gow

Mucosa

is
EH

Fig. 9-34 A, Histology of the retromylohyoid curtain at the site of the asterisk in Fig.
9-33. Notice the superior constrictor muscle and, posterior to it, the medial pterygoid
muscle. Contraction of the medial pterygoid limits the space available for the posterior
part of the lingual flange in the retromylohyoid fossa. B, Relationship of the medial ptery-
goid to the superior constrictor. When the medial pterygoid contracts, it forces the supe-
rior constrictor anteriorly, thus limiting the length of the lingual flange in this region. B is
the buccinator; M, the masseter: MP, the medial pterygoid; PR, pterygomandibular ra-
phe; RM, ramus of the mandible; RMC, the retromylohyoid curtain; SC, the superior con-
Strictor. Contraction of the medial pterygoid, which lies posterior to the superior constric-
tor, causes the retromylohyoid curtain (mucosal covering of the superior constrictor)
to
move anteriorly, thus limiting the space in the retromylohyoid fossa for the retromylohyoid
eminence at the posterior end of the lingual flange of a denture.

;
—— LINGUAL BUCGA T=
2 i is i)“A M4\\inf Mucosa
} Qiy
NN
3
) .ee
: Y/at:SAoiSy
RS oe €
O
i)
Sy]
¢}
ss Fig. 9-35 Histology of the posterior part of the
AS 4)
3
CA

RK Ost retromolar pad. Notice the fibers of the buccinator

=,
> co

laterally and of the superior constrictor medially,

Pterygo- Submucosa These join to form the pterygomandibular raphe.
mandibular Fibers of the temporalis tendon lie deep to these
raphe
structures. Because of the histologic makeup
of
Superior Buccinator the retromolar pad, it should not be displaced
constrictor muscle dur-
ing making of the final impression.
muscle
Temporalis
i
SSS ne mt
SS
ae tendon

Compact bone
 Biologic considerations for mandibular impressions 223

Limiting tissues 9-34). The length and form of the lingual flange
The microscopic anatomy of the limiting tis- of the lower final impression tray must reflect
sues is described for the vestibular spaces, the the physiologic activity of these structures lest
alveololingual sulcus, and the retromolar pad. their normal movement be restricted or they
The mucous membrane lining the vestibular tend to dislodge the lower denture.
spaces and alveololingual sulcus of the lower The retromolar pad lies at the posterior end
jaw is quite similar to that lining the vestibular of the crest of the lower residual ridge. Histo-
spaces of the upper jaw. The epithelium is thin logically, its mucosa is composed of a thin,
and nonkeratinized, and the submucosa _ is nonkeratinized epithelium; and, in addition to
formed of loosely arranged connective tissue fi- loose areolar tissue, its submucosa contains
bers mixed with elastic fibers. Thus the mu- glandular tissue, fibers of the buccinator and
cous membrane lining the vestibules and the superior constrictor, the pterygomandibular ra-
alveololingual sulcus is freely movable, which phe, and the terminal part of the tendon of the
allows for the necessary movements of the lips, temporalis (Fig. 9-35). Because of its histologic
cheeks, and tongue. Anteriorly the submucosa nature, the retromolar pad should be regis-
of the mucous membrane lining the alveololin- tered in a resting position in the final lower im-
gual sulcus contains components of the sublin- pression.
gual gland and is attached to the genioglossal
Cross sections of the mandible
muscle. In the molar region the submucosa at-
taches to the mylohyoid muscle, and the mu- Cross sections of the mandible reveal the
cous membrane covering of the retromylohyoid proximity of muscle attachments and the lack of
curtain is attached by its submucosa to the su- a broad bearing surface. The bony contour nat-
perior constrictor. Posterior to the superior urally is much narrower and sharper than the
constrictor fibers, which run in a horizontal di- soft tissue contour. This fact often deceives the
rection, is found the medial pterygoid muscle dentist as to the width and contour of the bear-
running in a vertical direction (Figs. 9-33 and ing surface.