Anatomy of the Mandibular Arch

Anatomy of the Mandibular Arch

Mandibular Ridge:

• Structure: Consists of 3 layers: fibrous tissue (connective tissue), cortical bone, cancellous bone.

• Function: Supports force from dentures. Cancellous bone does not bear force.

• Note: The cortical bone is necessary to support dentures.

• Division: Anterior and posterior ridges.

Anterior Ridge:

• Location: Located between the two canines.

• Bone resorption: High bone resorption.

• Force bearing: Secondary force bearing area.

Posterior Ridge:

• Bone resorption: Less bone resorption.

• Force bearing: Primary force bearing area.

• Note: If significant resorption occurs, the teeth become the primary force bearing area.

• Buccal Shelf (Fish’s space): Area of bone located between the ridge, buccal frenulum, retromolar triangle and external oblique line.

Classification of Mandibular Ridge:

• Type 1: Little resorption, high ridge, favorable.

• Type 2: Moderate resorption, high ridge. Mucosa is flabby in some places, unfavorable.

• Type 3: Significant resorption, flat ridge, unfavorable and sometimes requires a third impression.

• Type 4: Concave ridge, unfavorable and sometimes requires a third impression.

Retromolar Triangle:

• Location: Area of mucosa bulging behind the 8th tooth.

• Relations: Anterior to the ligament, lateral to the buccinator muscle, medial to the retromolar fossa and superior constrictor muscle.

• Mucosa: Mainly adherent mucosa. 1/3 distal has loose tissue with many salivary glands.

• Boundary: 2/3 anterior lies horizontally at the level of the occlusal plane (determines occlusal plane).

• Denture: Covers the anterior 2/3 of the retromolar triangle for peripheral adaptation.

Buccal Shelf:

• Function: Bears the main force due to the thick cortical bone layer and attachment of the buccinator muscle.

• Bone resorption: Minimal bone resorption.

• Location: Lies fairly perpendicular to the occlusal plane.

• Width: Average denture: 2-3mm. Small denture: 4-6mm.

• Note: The wider and flatter, the better the force bearing. When the ridge is flat, the buccinator muscle attaches mostly to the ridge crest.

Mandibular Base Boundary:

• Note: Can extend beyond the external oblique line by 1-2mm. The shape of this line must be captured during impression taking.

Mylohyoid Ridge (Internal Oblique Line):

• Function: Attachment site for the mylohyoid muscle.

• Note: The denture base boundary should extend beyond this line by 4-6mm.

Shape of the Mandibular Base:

• Note: The external oblique line is covered by 1-2mm, the internal oblique line by 4-6mm, and the retromolar triangle by its anterior 2/3.

Lingual Frenulum:

• Location: Connects the floor of the mouth to the mucosa of the alveolar bone, lying on the genioglossus muscle.

• Note: Cannot stretch like a muscle. When opening the mouth wide, the frenulum may push the mandible upwards.

Mandibular Buccal Frenulum:

• Location: Away from the canine, closer to the first premolar.

• Note: Attaches obliquely in the anteroposterior direction. The denture periphery should be narrow in this area.

Mandibular Vestibule (Buccal Space):

• Location: Lies between the two buccal frenula and the two first premolars.

• Contains: The buccal shelf and the external oblique line.

• Width: Dependent on the buccal shelf and buccinator muscle.

• Note: If the ridge has minimal resorption, the denture in this area should be 1-2mm thick.

Genial Tubercles:

• Function: Attachment site for the genioglossus muscle and geniohyoid muscle.

• Note: Not affected by bone resorption. The denture base boundary should cover the genial tubercles, not end on the hard part to avoid pain.