Lec.3 Prosthodontics 2019/2020 د.ايناس عبد الستار

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Anatomical Landmarks of Mandibular Arch

A. Limiting Structures 1.Labial frenum

2.Buccal frenum

3.Labial vestibule

4.Buccal vestibule

5.Lingual frenum

6. Lingual vestibule

7.Retromolar pad area

8.pterygomandibular raphe

1.Labial frenum: it is a fold of mucous membrane may be single or multiple. It is not as

pronounced as the maxillary one. It contains some of fibrous band attached to orbicularis

oris muscle therefore the frenum is active in mastication and the denture must be fitted

carefully around it to maintain a seal without causing soreness.

2.Buccal frenum: is a fold or folds of m.m extending from the buccal mucous membrane

reflection to the residual ridge crest in the region just distal to the cuspid eminence. It may

be single or double and activated in function by muscles(the fibers of buccinators are

attached to the frenum) so there must be enough space in the denture for it to prevent

denture displacement during function.

3.Labial vestibule: space extends from labial to buccal frenum on each side, it is limited

inferiorly by the m.m reflection, internally by the R.R and labially by the lower lip. The

denture flange extention in this area is limited because of the muscles that are inserted

close to R.R crest like mentalis muscle.

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4.Buccal vestibule: space extends posteriorly from the buccal frenum till the retromolar

region. It is bound by the residual alveolar ridge on one side and buccinator on the other

side. This space is influence by the action of masseter muscle and filled with buccal

flange.

5.Lingual frenum: is a fold of m.m observed when the tongue tip is elevated . It is

activated with the tongue movement therefore it must be molded well in the impression to

provide enough space for it in the denture so as to prevent denture displacement or tissue

ulceration.

6. Lingual vestibule: it extends from the lingual frenum to the retromylohyoid curtain,

bounded externally by the R.R and internally by the tongue, it is filled by the lingual

flange of the denture. It can be divided into 3 parts:

i. Anterior part (premylohyoid): extend from the lingual frenum to the 1st premolar area

(premylohyoid fossa) .

ii. Middle portion: extend from the premylohyoid fossa to the distal end of the mylohyoid

ridge

iii. Distal end (retro mylohyoid): it extends from middle portion to the retromylohyoid

curtain posteriorly.

7.Retromolar pad area: pear shaped triangular soft pad of tissue at the distal end of the

lower ridge. It is an important structure which forms the posterior seal of the mandibular

denture. The denture base should extend approximately 1/2 – 2/3 over this pad.

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8.pterygomandibular raphe

B.Supporting Structures A.Primary stress bearing area Buccal shelf area B.Secondary stress bearing area

Residual Ridge A.Primary stress bearing area Buccal shelf area: it is a very dense bony area , bounded laterally by external oblique

ridge , medially by R.R crest, mesially by the buccal frenum and distally by the anterior

edge of the masseter muscle and retromolar pad area. However, the bone of the buccal

shelf is covered by a layer of cortical bone resist the resorption plus that this area lies at

right angles to the vertical occlusal forces, makes it the most suitable primary stress-

bearing area for a lower denture.

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External oblique line: is a ridge of dense bone extended from just above the mental

foramen superiorly and distally to be continuous with the anterior border of the ramus.

This line is the site of attachment of the buccinator muscle.

B.Secondary stress bearing area

Residual Ridge: is the bony process of the arch that remains after teeth have been lost. It

is usually resorbed and its shape and size change after extraction of the natural teeth.

However, as the underlying bone often is cancellous, the crest of the R.R may not be

favorable as a primary stress- bearing area for a lower denture.

C.Relief Areas 1.Mylohyoid ridge or line

2.Mental foramen

3.Mandibular tori

4. Genial tubercles 1.Mylohyoid ridge or line: it is an irregular bony ridge on the lingual surface of the

mandible from which the mylohyoid muscle arises to form the floor of the mouth.

Anteriorly the ridge lies close to the inferior border of mandible while posteriorly; it lies

flush with the residual ridge.

It is covered by a thin mucosa which may be traumatized by denture base hence it should

be relieved

The area under the mylohyoid ridge is considered as an undercut. The lingual flange of a

lower denture must extend well beyond this ridge but not into the undercut.

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2.Mental foramen: located on the external surface of the mandible between 1st and 2

nd

premolar area. As resorption takes place, the mental foramina will come to lie closer to the

R.R crest which may lead to compression of mental nerve and blood vessels by the denture

base and may cause numbness of lower lip unless relief is provided.

3.Mandibular tori: are bony prominence or exostosis usually found bilaterally on the

lingual surface of the mandible at the premolar areas. They often need to be removed

surgically as it can be difficult to provide relief within the denture for the torus without

breaking the border seal of the denture.

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4. Genial tubercles: are bony structures located anteriorly on the lingual surface of the

mandible away from R.R crest. However, with resorption, they become increasingly

prominent so surgical correction may be needed.

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External surface of the mandible

Position of the mental foramen before and after teeth extraction