ANATOMICAL LANDMARKS FOR MANDIBULAR ARCH
Anatomical landmarks of maxillary and mandibular arch
1. Anatomy
2. Clinical significance
Limiting structures
• Labial frenum
• Labial vestibule
• Buccal vestibule
• Buccal frenum
• Lingual frenum
• Alveololingual sulcus
• Retromolar pads
• Pterygomandibular raphe
Primary stress-bearing area
• Buccal shelf area
• Residual alveolar ridge
Relief area
• Mental foramen
• Genial tubercles
Labial frenum
Anatomy
• The mandibular labial frenum contains a band of fibrous connective tissue and helps to attach orbicularis oris muscle, therefore, the frenum is sensitive and active.
Clinical significance
• During final impression making, sufficient relief must be given for the labial frenum without compromising the peripheral seal.
Labial vestibule
Anatomy
• It runs from the labial frenum to buccal frenum.
• Fibres of orbicularis muscle, incisivus, and mentalis are inserted near the crest of the ridge. Mentalis muscle is particularly the active muscle.
Clinical significance
• Extent of the denture flange in this region is often limited because of muscles that are inserted close to the crest of the ridge. Thick denture flanges may cause dislodgement of dentures when patient opens the mouth wide open.
• Depth of the flange is determined by the mucolabial fold.
Buccal vestibule
Anatomy
• It extends from buccal frenum to the outside back corner of the retromolar pad.
• It is nearly at right angles to biting force.
• Extent of the vestibule is influenced by buccinators muscle, which extends from modiolus anteriorly to pterygomandibular raphe posteriorly and has its own fibres attached to buccal shelf and external oblique ridge.
• The masseter muscle contracts under heavy closing force and pushes inward against the buccinators muscle to produce a masseteric notch in the distobuccal border of the lower denture.
Clinical significance
• The distobuccal border of the lower denture should accommodate the contracting masseter muscle so that the denture does not dislodge during heavy closing force.
Buccal frenum
Anatomy
• It is a fold of mucous membrane extending from buccal mucous membrane reflection toward the slope or crest of the residual ridge in the region just distal to the cuspid eminence.
• May be single or double, broad U/V shape.
• It overlies depressor anguli oris muscle.
Clinical significance
• Relief for the buccal frenum is given in denture to avoid displacement of the denture.
Lingual frenum
Anatomy
• It is a fold of mucous membrane existing when the tip of the tongue is elevated.
• It overlies the genioglossus muscle which takes origin from the superior genial tubercle.
Clinical significance
• The relief for the lingual frenum should be registered during function.
• Relief for the lingual frenum should be provided in the denture.
Retromolar pad
Anatomy
• It is a triangular soft pear-shaped pad of tissue at the distal end of the lower ridge.
• Mucosa composed of thin, nonkeratinized epithelium and loose alveolar tissue.
• Submucosa contains glandular tissue, fibres of buccinators and superior constrictor muscle, pterygomandibular raphe and terminal part of the tendon of temporalis.
Clinical significance
• The distal end of the denture pad should cover two-thirds of the
retromolar pad.
• The retromolar pad provides the peripheral posterior seal for the lower denture.
Alveololingual sulcus
Anatomy
• It is the space between residual ridge and tongue. It extends from
lingual frenum to retromylohyoid curtain posteriorly.
• The borders are the following:
Anterior region
o Extends from lingual frenum to premylohyoid fossa where the mylohyoid ridge curves down below the level of the sulcus.
Middle region
o Extends from premylohyoid fossa to the distal end of the mylohyoid ridge curving medially from body of mandible.
Posterior region
o The flange passes into the retromylohyoid fossa. No longer influenced by mylohyoid muscle.
Clinical significance
• The lingual flange of the lower denture will be short anteriorly than posteriorly.
• The lingual flange in the middle region slopes medially towards the tongue and maintains the peripheral seal in the middle region.
• The lingual flange in the posterior region passes into the retromylohyoid fossa and turns laterally towards the ramus and forms the typical S-shaped form of the lower denture.
Retromylohyoid fossa
Anatomy
• It is the area posterior to the mylohyoid muscle and bounded by retromylohyoid curtain.
• Posterolateral portion of the retromylohyoid curtain overlies superior constrictor muscle, posteromedial the palatoglossal muscle and lateral surface of tongue and the inferior wall the submandibular gland.
• Protrusion of the tongue causes the retromylohyoid curtain to move forward.
Clinical significance
The over extension of the lingual flange in the retomylohyoid fossa area can cause soreness and dislodgement of denture during swallowing.
Pterygomandibular raphe
• It arises from the hamular process of the medial pterygoid plate and gets attached to the mylohyoid ridge.
Clinical significance
• It is very prominent in some patients where a notch-like relief may be required on the denture.
• A simple wide open visual and digital inspection is sufficient to determine the need for clearance.
Residual alveolar ridge
Anatomy
• Covered by fibrous connective tissue and underlying bone is cancellous without cortical bony plate covering it.
• Since it is cancellous (crest of the residual ridge), it may not be favourable as primary stress-bearing area.
• The slopes of the residual alveolar ridge have thin plate of cortical bone.The slopes of the ridge are at an acute angle to the occlusal forces. Hence, it is considered as a secondary stress-bearing area.
Clinical significance
• Any movable soft tissue overlying the residual alveolar ridge should not be compressed while making impression.
Buccal shelf area
Anatomy
• The area between the mandibular buccal frenum and anterior edge of the massester muscle is known as buccal shelf area.
• Bounded medially by the crest of the alveolar ridge, laterally by the external oblique ridge, distally by the anterior aspect of the retromolar pad and anteriorly by the buccal frenum.
• Buccal shelf area is covered by a layer of dense and smooth cortical bone.
• Buccal shelf area lies at right angles to the vertical occlusal forces; makes it suitable as primary stress-bearing area for the lower denture.
Mental foramen
Anatomy
• As resorption takes place, mental foramen will come to lie closer to the crest of the ridge.
• Mental nerves and vessels may be compressed by denture unless relieved.
• Pressure on the nerve will cause numbness of lower lip.
Genial tubercles
Anatomy
• These lie away from the crest of the ridge.
• With resorption, genial tubercles become increasingly prominent.
Torus mandibularis
Anatomy
• Bony prominences usually found bilaterally and lingually near the 1st and 2nd premolar midway between the soft tissues of the floorof the mouth and crest of the alveolar process.
• It should be removed surgically as it is difficult to provide relief within the denture.