Muscles of Mastication – Attachments and Actions

Diagnostic casts

A diagnostic cast should be an accurate reproduction of the teeth and adjacent tissues, poured in dental stone because of its strength and abrasion resistance.

The impression for the diagnostic cast is made with irreversible hydrocolloid (alginate) in a perforated impression tray.

Importance of DIagnostIc casts

1. Diagnostic casts permit viewing of the occlusion from the lingual and buccal aspects.
2. To analyze the existing occlusion in case of needed occlusal adjustment and occlusal reconstruction.
3. Aids in diagnostic wax-up to determine the occlusion before definitive treatment is begun.
4. Diagnostic casts help to survey the dental arch as a whole.
5. Helps in surveying the cast to determine existing soft tissue undercuts and parallelism of teeth for overdenture cases.
6. Aids in mouth preparation decisions, as to the removal of soft tissue undercuts
7. Helps to show and discuss the case with a patient regarding the treatment plans, needed corrections, and problems if any.
8. Aids to select and fabricate trays.
9. If needed can duplicate diagnostic cast in case an undercut has to be blocked.
10. Diagnostic casts may be used as a constant reference.
11. In areas where alterations are required as in soft tissue or hard tissues, a rough alteration can be mocked on the duplicate cast to assess the outcome.

Read And Learn More: Complete Dentures Question and Answers

Types Of Denture-Bearing Areas (based on function)

Primary stress-bearing areas in maxillary Dentures are

• The horizontal portion of the hard palate
• Residual alveolar ridge.

The residual ridge and most of the hard palate are the primary stress-bearing areas in the upper jaw.

The fibrous connective tissue covering the crest of the residual alveolar ridge and the compact bone made up of haversian systems aids in supporting the denture due to its firmness and position.

Primary and secondary retentive areas

1. The rugae area in the anterior part of the hard palate is at an angle to the occlusal plane of the residual ridge which helps in secondary retention provided it is not distorted in the impression procedure.
2. The glandular region the posterior part of the hard palate if covered by a denture help in the retention of the denture.

Relief areas

• Incisive papilla
• Ridge, cuspid eminence, malar bone
• Fovia palatines
• Midpalatine suture.

1. (Incisive papilla)
   • The incisive papilla covering the incisive foramen is located behind and between the central incisors. If this area is not relieved it may cause interference with the blood and nerve supply.
2. (Median palatine raphe)
   • As the mucosa in the median palatal suture is thin and attached to the periosteum, not relieving this area can cause soreness and rocking of the denture.

Other areas

1. Zygomatic process
   • In long-term edentulous patients, the zygomatic or malar process located opposite the first molar region needs to be relieved. If not can to lead soreness of the underlying tissues and affects retention.
2. Posterior palatal area
   • As the posterior palatine foramina are thickly covered by soft tissue they do not need to be relieved except in extreme cases of resorption.
3. Vibrating line of the palate
   • It is an imaginary line drawn across the palate that marks the beginning of motion when the patient says “ah”. It extends from one pterygomaxillary notch to the other and passes about 2 mm in front of the foveae palatine.
   • Ideally, the distal end of the upper denture should end 1 or 2 mm posterior to the vibrating line, covering the tuberosities and extending into the hamular notches.
   • The posterior palatal seal area is an area between the anterior and posterior vibrating lines which should be closely contacted by the denture. If a gap exists it jeopardizes the retention of the denture.

Denture-bearing area in the mandible

Relief Areas

• Crest of alveolar ridge
• Mental foramen
• Mylohyoid ridge
• Torus mandibularis.

Supporting Structure

• Primary stress-bearing area: Slopes of residual alveolar ridge.
• Secondary stress-bearing area: Rugae area.

Types of rugae.
Answer:

Rugae are raised areas of dense connective tissue radiating from the median suture in the anterior one-third of the palate. Palatal rugae in mammals are transversely running crests, which are exclusively formed by the mucosa of the hard palate except where an ossified base can be distinguished.

Types of rugae Definition: Rugae are anatomical folds or wrinkles of irregular fibrous connective tissue located on the anterior third of the palate (GPT-8).

They are also called “plica palatine” or “rugae palatine”.

Types of rugae Prosthodontic consideration: This area is a secondary stress-bearing area. It resists the anterior displacement of the denture.

Classification of Types of Rugae

Types of rugae Based on shape

• Curved
• Wavy
• Straight
• Circular.

Types of rugae Based on length

• Primary rugae: 5 to 10 mm or more
• Secondary rugae: 3-5 mm
• Fragmentary rugae: Less than 3 mm.

Based on unification

Based on unification Converge: Two rugae originate away from the center and unite towards it.

Based on unification Diverge: Those rugae which originate from the center and diverge away.

1. Diverge
2. Converge
3. Curve
4. Wavy
5. Straight
6. Circular

Based on unification Functions

• Palatal rugae have been shown to be highly individual and consistent in shape throughout life.
• The anatomical position of the rugae inside the oral cavity (surrounded by cheek, lips, tongue, and the buccal pad of fat) also gives some protection in cases of trauma or incineration.
• When identification of an individual by other methods is difficult, palatal rugae may be considered as an alternative source of information.
• It is assumed that the rugae facilitate food transport through the oral cavity, prevent loss of food from the mouth, and participate in food crushing.
• Because of the presence of tactile and gustatory receptors, rugae contribute to the perception of taste, mechanical food qualities, and tongue position.

Classification of Frenal Attachment.

Frenal attachment House classification

Class 1—Frenum is located away from the crest of the ridge.

Class 2—Frenum is located nearer to the crest of the ridge.

Class 3—Frenum encroaches on the crest of the ridge and may interfere with the denture seal.

Frenal attachment Labial frenum

• It appears as a fold of mucous membrane extending from the mucus lining of the lip to or towards the crest of the residual ridge on the labial surface
• It may be a single/multiple
• It may be narrow/broad
• It contains no muscle fibers of significance
• Attachment is of three types
  • Close to the crest of the ridge
  • Average
  • Distal to the crest of the ridge.

Frenal attachment Clinical Consideration

• Sufficient relief should be given during the final impression procedure and in the completed prosthesis because the overriding of the function of the frenum will cause pain and dislodgement of the denture.
• During the impression procedure, the lip should be stretched horizontally outwards for the proper recording of a frenum.
• If the frenum is attached close to the crest, frenectomy should be done to avoid decreased border seal.

Frenal attachment Buccal frenum

Fold or folds of mucous membrane extending from mucous membrane reflection area to or towards the slope or crest of the residual alveolar ridge.

Frenal attachment Significance: Levator anguli oris (caninus muscle) lies beneath it, and is hence influenced by other muscles of facial expression.

Frenal attachment Clinical Consideration

During the final impression procedure and in the final prosthesis sufficient relief should be given for the movement of the frenum because overriding of the function of the frenum will cause pain and dislodgement of the denture.

1. During the impression procedure, the cheek should be reflected laterally and posteriorly.
2. If the frenum is attached close to the crest of the alveolar ridge, a frenectomy is to be done.

Frenal attachment Mandibular

Frenal attachment Labial frenum

It is a fold of mucous membrane extending from the mucous lining of the mucous membrane of lips to or toward the crest of the residual alveolar ridge on the labial surface.

Frenal attachment Clinical Consideration

During the final impression procedure, the lip has to be reflected anteriorly and horizontally. During the final impression procedure and in the final prosthesis provision should be made in the form of the notch to prevent overriding of function, which may result in a laceration.

Frenal attachment Lingual frenum

• It is a mucobuccal fold that joins the alveolar mucosa to the tongue.
• significance: It overlies the genioglossus muscle, which takes its origin from the superior genial spine on the mandible.

Frenal attachment Clinical Consideration

• Sufficient relief should be given in the final impression and the final denture to prevent overriding the function of a frenum.
• During the impression procedure touch the tip of the tongue to the incisive papilla region.

Muscles of mastication.

Muscles of mastication The muscles are:

• Masseter muscle.
• Temporalis muscle.
• Medial pterygoid muscle.
• Lateral pterygoid muscle.

Muscles of mastication Masseter muscle

Muscles of mastication Shape: Rectangular muscle.

Muscles of mastication Origin: From the zygomatic arch and extends downward to the lateral aspect of the lower border of the ramus of the mandible.

Muscles of mastication Insertion: Extends from the region of the second molar at the inferior border posteriorly to include the angle of the mandible.

Muscles of mastication Made up of Two Heads

• The superior portion consists of fibers that run downward and slightly backward.
• The deep portion consists of fibers that run in the vertical direction

Muscles of mastication Action: When the fibers contract, the mandible is elevated and teeth are brought into contact.

• The muscle provides the force necessary to chew efficiently.
• A superficial portion also aids in protruding the mandible.
• When the mandible is protruded and biting force is applied, the fibers of the deep portion stabilize the condyle against the articular eminence.

Muscles of mastication Nerve supply: Masseteric branch of the anterior division of mandibular nerve.

Temporalis Muscle

Temporalis Muscle Shape: Large fan-shaped muscle.

Temporalis Muscle Origin: Originates from the temporal fossa and the lateral surface of the skull.

Temporalis Muscle Insertion: They extend downward between the zygomatic arch and the lateral surface of the skull to form a tendon that inserts on the coronoid process and anterior border of the ascending ramus.

Temporalis Muscle Can be divided into three portions according to fiber direction:

Temporalis Muscle Anterior portion: Fibers directed almost vertically.

Temporalis Muscle Middle portion: Fibers directed obliquely.

Temporalis Muscle Posterior portion: Fibers directed almost horizontally

Temporalis Muscle Action: When muscle contracts, it elevates the mandible, and teeth are brought into contact.

• If only portions contract, the mandible is moved according to the direction of those fibers that are activated.
• Because of the angulation of fibers, the muscle is capable of coordinating closing movements.
• When the anterior portion contracts, the mandible is raised vertically.
• When the middle portion contracts, the mandible elevates and protrudes.
• When the posterior portion contracts, the mandible protrudes.

Temporalis Muscle Nerve supply: Temporal branches from the anterior division of the mandibular nerve.

Medial pterygoid muscle

Medial pterygoid muscle Origin: The muscle originates from the medial aspect of the lateral pterygoid plate.

Medial pterygoid muscle Insertion: Extends downward, backward, and outward to insert along the medial surface of the mandibular angle.

Medial pterygoid muscle Action: When fibers contract, the mandible is elevated and the teeth are brought into contact.

• Muscle is active in protruding the mandible.
• Unilateral contraction brings about an intrusive movement of the mandible.

Medial pterygoid muscle Nerve supply: A branch from the trunk of the mandibular nerve.
Lateral pterygoid muscle The muscle is described as two distinct and different muscles.

Medial pterygoid muscle They are:

• Inferior lateral pterygoid muscle.
• Superior lateral pterygoid muscle.

Inferior Lateral pterygoid muscle

Inferior Lateral pterygoid muscle Origin: Originates at the outer surface of the lateral pterygoid plate and extends backward, upward, and outward.

Inferior Lateral pterygoid muscle Insertion: Inserts on the neck of the condyle.

Inferior Lateral pterygoid muscle Action: When the inferior muscle contracts, condyles pull down the articular eminences, and the mandible is protruded.

• Unilateral contraction creates intrusive movement of that condyle and causes lateral movement of the mandible to the opposite side.
• When this muscle functions with mandibular depressors, the mandible is lowered and condyles glide forward and downward on the articular eminences.

Superior Lateral pterygoid

Superior Lateral pterygoid Origin: Originates at the infratemporal surface of the greater sphenoid wing, extending almost horizontally, backward, and outward.

Superior Lateral pterygoid Insertion

• Is on the articular capsule, the disc, and the neck of the condyle.
• 60–70% of fibers attach to the neck of the condyle.
• 30–40% of fibers attach to the disc.
• Attachments are more predominant on the medial aspect than on the lateral aspect.

Superior Lateral pterygoid Action

• The muscle is active in conjunction with the elevator muscles.
• Active during the power stroke and when the teeth are held together.

Superior Lateral pterygoid Nerve supply: A branch from the anterior division of the mandibular nerve.

Muscles of facial expression.
Answer:

Muscles of facial expression Orbital group :

• Orbicularis oculi
• Procerus
• Corrugator supercilious

Muscles of facial expression Nasal group :

• Compressor naris
• Dilator naris
• Depressor septi nasi

Muscles of facial expression Oral group

Muscles of facial expression Upper set:

• Risorius
• Zygomaticus major
• Zygomaticus minor
• Levator labii superioris
• Levator anguli oris
• Levator labii superioris alaeque nasi

Muscles of facial expression Lower set :

• Depressor anguli oris
• Depressor labii inferioris
• Mentalis

Muscles of facial expression Lateral set:

Buccinator

Muscles of facial expression Circular set:

Orbicular oris

Orbicularis oris

Orbicularis oris Origin: Intrinsic part: Superior incisivus, from maxilla Extrinsic part: strata.

Orbicularis oris Insertion: Intrinsic part: Angle of mouth.

Extrinsic part: Lips and angle of the mouth.

Orbicularis oris Nerve Supply: Motor nerve supply: Buccal branch of the facial nerve.

Orbicularis oris Sensory nerve supply: Mandibular division of Trigeminal nerve.

Orbicularis oris Action: During deglutition, it seals lips/mouth.

Orbicularis oris Clinical Significance

• In the upper lip it is supported by the six maxillary teeth and in the lower lip the superior border is supported by the incisal edge of the maxillary teeth when they are in occlusion.
• When the muscles are relaxed, the lips become flaccid, it is important for the dentist to make functional movements when making an impression for the dentures.

Mentalis

Conical, fasciculus, situated at the side of the frenulum of the lower lip.

Mentalis Origin: It arises from the incisive fossa of the mandible.

Mentalis Insertion: Inserted into the skin of the chin.

action: It turns the lower lip outward and in contracting makes the lower labial vestibule shallow.

Mentalis Clinical Significance

This muscle can render the labial vestibule shallow and hence can dislodge the lower denture, particularly when the anterior ridge is highly resorbed. In certain cases, surgical repositioning of the mentalis muscle is indicated.

Buccinator

Buccinator Origin

• Upper fibers arise from the outer surface of the maxillary alveolar process in the molar region.
• Lower fibers arise from a corresponding area of the mandible
• Middle fibers arise from the pterygomandibular raphe.

Buccinator Insertion

• Upper fibers run in the upper lip to continue with the fibers to the opposite side.
• Lower fibers run in the lower lip to continue with the fibers of the opposite side.
• Middle fibers: The upper of these fibers runs in the lower lip to continue with similar fibers of the opposite side. The lower of these fibers runs in the upper lip similarly.

Buccinator Nerve Supply

• Motor nerve supply – Buccal branch of the facial nerve.
• Sensory nerve supply – Mandibular division of trigeminal nerve.

Buccinator Action in Deglutition

Push food towards teeth during mastication, which helps to close the mouth.

Buccinator Clinical Significance

• In the lower jaw, it becomes part of the denture-bearing areas.
• The fibers of the muscle contract in a line parallel with the occlusal plane so it does not directly dislodge the denture, but only when the masseter is activated it pushes the buccinator medially against the denture borders in the area of the retromolar pad.
• This is a dislodging force, and the denture border should be countered to accommodate this interaction between the buccinator and masseter muscles.
• This counter in the denture base is termed a masseteric groove, and if proper freedom is not given at the distobuccal flange it will displace the denture. In the upper jaw, the position of attachment of the muscle determines the vertical height of the distobuccal flange of the maxillary denture.
• The action of the buccinator muscle pulls the corners of the mouth laterally and posteriorly. The major function of the muscle is to keep the cheeks taut, else during closure, the muscles collapse and get in between the denture teeth, which is a major problem in senile individuals with flaccid muscles.

Buccinator Common facial expressions and the muscles producing them

1. Smiling and laughing – Zygomaticus major
2. Sadness – Levator labi superiors and levator anguli oris
3. Grief – Depressor anguli oris
4. Anger – Dialator naris and Depressor septi
5. Frowning – Corrugator supercell and Procerus
6. Horror, terror, and fright – Platysma
7. Surprise – Frontalis
8. Doubt – Mentalis
9. Grinning – Risorius
10. Contempt – Zygomaticus minor

Tmj ligaments.
Answer:

These are composed of collagenous connective tissue and act predominantly as restraints to the motion of the condyle and the disc.

Tmj ligaments Three Functional Ligaments

1. Collateral ligaments
2. Capsular ligament
3. Temporomandibular ligament.

Tmj ligaments Two Accessory Ligaments

1. Sphenomandibular ligament
2. Stylomandibular ligament.

Functional LIgaments

Functional LIgaments Collateral Ligaments

• Short, paired (medial and lateral) structures attach the disc to the lateral and medial poles of each condyle.
• Restrict movement of the disc away from the condyle, allows smooth synchronous motion of the disc condyle complex.
• Vascular, innervated, fibroblastic in nature.
• The medial collateral ligament receives fibers from the inferior head of the lateral pterygoid muscle.

Functional LIgaments Capsular Ligament

It encompasses the joint, retaining the synovial fluid.

Attachments

Attachments Superiorly: Temporal bone along the border of the fossa and the eminence

Attachments Inferiorly: Neck of the condyle along the edge of the articular facet.

The capsular ligament surrounds the joint spaces and disc attaching anteriorly, posteriorly, medially, and laterally, where it blends with the collateral ligaments. They resist medial, lateral, and inferior forces.

The capsular ligament is well-innervated and provides feedback regarding the position and movement of the joint

Attachments Temporomandibular (Lateral) Ligament

• A single structure that functions in paired fashion with corresponding ligaments on opposite TMJ.
• Can be separated into two distinct portions, which have different functions. outer oblique portion Descends from the outer aspect of the articular tubercle and zygomatic process posters inferiorly to the outer surface of the condylar neck.
• The inner horizontal portion Originates medial to the outer oblique portion of the ligament and outer surface of the articular tubercle and the zygomatic process runs horizontally backward to attach to the lateral pole of the condyle and posterior aspect of the disc.

Attachments Function

• Outer portion: Limits the amount of inferior distraction that the condyle may achieve in translatory and rotational movements.
• Inner portion: Limits posterior movements of the condyle during lateral movements (protects the methodical tissues).
• Also called as “check ligament” of TMJ.

Accessory ligaments

Accessory ligaments Sphenomandibular Ligament

• Arises from the spine of the sphenoid bone and descends into a fan-like insertion on the mandibular lingula and lower portion of the medial side of the condylar neck.
• Important landmarks during surgery as the maxillary artery, auriculotemporal nerve, and inferior alveolar nerve pass b/w the ligament and the mandibular neck. Function: Partly contributes to the translation of the mandible.

Accessory ligaments Stylomandibular Ligament

A dense thick band of deep cervical fascia extending from the styloid process to the posterior border of the angle of the mandible blends with the fascia of the medial pterygoid muscle.

Accessory ligaments Function: Similar to above, also limits excessive protrusion of the mandible.

Stability and support in the complete denture.
Answer:

Complete denture Stability

• Stability is defined as, “The quality of a denture to be firm, steady, or constant, to resist displacement by functional stresses and not to be subject to change of position when forces are applied.” –GPT.
• The stability of the denture indicates its resistance to withstand horizontal forces.

Complete denture  Factors affecting stability

1. Round, parallel ridges offer better stability than flat ridges.
2. Fibrous connective tissue that is firm aids in stability more than flabby tissue.
3. An accurate impression with maximum coverage increases stability. Impressions should record the stress-bearing and non-stress-bearing areas under stress and relief respectively.
4. A well-contoured occlusal rim with the occlusal plane oriented parallel to the ridge posteriorly and parallel to the interpupillary line anteriorly stabilizes the denture.
5. If teeth are arranged in a well-contoured occlusal rim, balanced occlusion, and neutral zone is achieved easily which again aids in the stability of the denture.
6. The polished surfaces should be contoured in regard to the Neutral zone or action of various muscles that tend to displace the denture.
   • The neutral zone is defined as, “The potential space between the lips and cheeks on one side and the tongue on the other. Natural or artificial teeth in this zone are subject to equal and opposite forces from the surrounding musculature.” –GPT

Complete denture Support

Support is defined as, “The resistance to vertical forces of mastication, occlusal forces, and other forces applied in a direction towards the denture-bearing area.”

Complete denture Factors affecting support

1. Wide and maximum coverage of denture-bearing area distributes forces over a wide area which is termed the “snowshoe” effect. This effect reduces the force per unit area.
2. If the primary stress-bearing areas are stressed and if the incisive papillae and median palatine areas are relieved, it improves the support factor in a denture.
3. The peripheral tissues need to be accurately recorded to prevent over-extension of the denture and tissue irritation.

Closed Mouth Impression

• Closed mouth techniques record the tissues in their functional position.
• In the closed mouth technique pressure is applied by closing against occlusion rims or teeth that are attached to the impression trays by the patient.
• Other muscle actions such as swallowing, grinning, or pursing the lips are also done, while the impression material molds and records the tissue surface.

Impression materials used for this technique are:

1. Impression compound.
2. Waxes that flowed at mouth temperature.
3. Soft liners.

The significance of the closed mouth technique as argued by Macmillan (1947) is that this technique is capable of trimming the lingual borders of the lower denture during tongue movement.

Closed mouth impression Disadvantages

1. Soft tissues displaced by this impression technique tend to rebound to their undisplaced position when the forces are released. This unseats the denture.
2. As the denture is constantly held in a displaced position, the pressure limits the normal blood flow.
3. Excessive bone resorption due to lack of proper blood flow
4. Inadequate retention.
5. Often the dentures made with closed-mouth impressions are overextended and must be arbitrarily trimmed.

• • Soft liners are good for refitting complete dentures and also possess the advantage of retaining compliance for many weeks.

Selective Pressure Techniques

• The selective pressure technique is a combination of extension for maximum coverage with intimate contact with the movable, loosely attached tissues and light pressure in noncompressible areas.
• Principle of impression: The mucosa over the ridge is able to withstand pressure, whereas the midline and incisive papillae are covered by a thin layer of submucosal tissue which cannot withstand pressure.

Materials Used

For Preliminary Impression

Impression compound relieved over the midline and incisive papilla areas in maxillary. In mandibular impression, the only relief provided is on the crest of the ridge when the ridge is knife-edged.

Materials Used For Final Impression

• Impression plaster or zinc oxide-eugenol impression paste as a wash impression which also records the relieved areas with minimal pressure and the ridge areas with pressure.
• Hence the midline and papilla sections of the denture will not make contact with the mucosa when the denture is not in function and when in function only applies mild pressure.
• The impression is refined with a minimum of pressure as escape holes are placed. This technique is common for maxillary arch.

Materials Used Another Technique

• A wax spacer is used to relieve the relief areas in a custom-made tray. The borders of the tray are kept 2 mm short of sulcus depth. A border extension and refining with a low-fusing impression material is done.
• After the borders have been refined, the wax spacer is removed, escape holes are placed and the final impression is made with a free-flowing zinc oxide-eugenol impression paste to record the supporting tissues in their undistorted positions.
• Minor corrections with waxes that flow at mouth temperature can be done. The impression is poured into stone immediately after the impression material sets.

Materials Used Disadvantages

1. Cannot be done on flabby ridges. This technique demands firm, healthy mucosal covering over the ridge.
2. Exceeding the movements will produce an under-extended denture.

Materials Used Advantages

1. Bone resorption is argued to be less compared to other techniques.
2. Good retention, support, and stability