DENTAL ANATOMY OF PREMOLARS

General Features of Premolars

—  They have at least two cusps.

§  one large buccal cusp,

§  Smaller lingual cusp

—   The lower second premolar may- sometimes- have two lingual cusps. 

Maxillary 1st Premolar

Maxillary 2nd Premolar

Mandibular 1st Premolar

Mandibular 2nd Premolar

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Muscles of the Facial Expression

Muscles of the Facial Expression

Muscles of Facial Expression are unique in that they migrate to their destinations about the scalp, neck, and mostly about the face from second pharyngeal arch mesenchyme and thus receive their motor innervation via the facial nerve (CN VII), the nerve of the second arch. Although most of these muscles originate on bone, most do not insert on bone; rather, they insert into the dermis of the skin and freely intermingle with muscles in their vicinity. Upon contraction, this arrangement and groupings of muscles about the orifices of the face convey movements about these orifices that we interpret as emotions.

The muscles of the face (and scalp) are derived from the second pharyngeal arch (hyoid arch) mesenchyme that migrates to its final destination.

Muscles of the Face and Scalp

Considering the origin of these muscles, it is not surprising that they receive motor innervation from branches of the facial nerve (CN VII).

Rather than inserting into bone, these muscles insert into the dermis of the skin, thus their orchestrated contractions convey various shapes to the face that we interpret as emotions. It is important to understand that fascicles of these muscles intermingle with each other, and they tend to act in groups to control the orifices around which they are grouped, such as the orbit, nose, and mouth. It is according to this grouping that they are described.

Muscles of the Face and Scalp
Muscle	Location	Origin
Scalp
Frontalis	Forehead	Procerus, corrugator, orbicularis oculi
Occipitalis	Back of the head	Mastoid process and superior nuchal line
Temporoparietalis	Temple	Temporal fascia
Ear
Auricularis anterior	Anterior to ear	Temporal fascia
Auricularis superior	Above ear	Temporal fascia
Auricularis posterior	Behind ear	Mastoid process
Nose
Procerus
Nasalis
Depressor septi
Eye
Orbicularis oculi	Around the orbit	Nasal process of frontal bone, frontal process of maxilla, medial palpebral ligament, and lacrimal bone
Corrugator	Deep to the orbicularis oculi	Medial aspect of superciliary arch
Mouth
Levator labii superioris	Upper lip	Zygoma and maxilla just above infraorbital foramen
Levator labii superioris alaque nasi	Upper lip and side of nose	Maxilla, frontal process
Levator anguli oris	Corner of mouth	Canine fossa of maxilla
Zygomaticus major	Cheek and corner of mouth	Temporal process of zygoma
Zygomaticus minor	Cheek and corner of mouth	Maxillary process of zygoma
Risorius	Cheek	Masseteric fascia
Depressor labii inferioris	Lower lip	Oblique line of mandible
Depressor anguli oris	Corner of mouth	Oblique line of mandible
Mentalis	Chin	Incisive fossa of mandible
Orbicularis oris	Circumscribes the mouth	Muscles in the vicinity, maxilla, nasal septum, mandible
Buccinator	Cheek	Pterygomandibular raphe, alveola arches of mandible and maxilla
Neck
Platysma	Neck and chin	Pectoral and deltoid fascia

Muscles of the Ear and Nose

The three external muscles of the ear are the auricularis anterior, superior, and posterior. Similarly, the three muscles of the nose are the procerus, nasalis, and depressor septi. These two groups of muscles are fairly inconsequential.

Muscles Surrounding the Orbit

Orbicularis Oculi

The orbicularis oculi muscle is composed of two parts, the palpebral portion and the orbital portion. The former originates from the medial palpebral ligament (attached to the medial aspect of the orbit) and inserts into the lateral palpebral raphe (attached to the lateral aspect of the orbit). The orbital portion of the muscle describes an oval around the orbit.

The orbicularis oculi is innervated by the temporal and zygomatic branches of the facial nerve and acts to close the eyelid completely. Forceful closure is mediated by the orbital portion, whereas the palpebral portion is responsible for light closure, as in blinking.

Corrugator

The corrugator (supercilii) muscle is located deep to the superomedial aspect of the orbicularis oculi, at the medial aspect of the eyebrow. It originates at the medial extent of the superciliary arch and inserts into the skin of the eyebrow.

It is innervated by the temporal and zygomatic branches of the facial nerve; the combined actions of the paired muscles approximate the eyebrows, producing frowns.

Muscles Surrounding the Mouth

Orbicularis Oris

The orbicularis oris completely encircles the mouth. Its fibers are positioned at various depths and angles in the two lips. Fascicles of this muscle, some of which are derived from those of neighboring muscles—especially the buccinator—freely intermingle with fascicles of other muscles acting on the lips, permitting extensive movability. Many of the fibers of the buccinator cross over each other at the angle of the mouth so the upper fibers proceed to the lower lip and the lower fibers to the upper lip. Hence, the origin of the orbicularis oris is complex and is usually considered to be from the fibers of the surrounding muscles as well as from the alveolar portion of the maxilla, the septum of the nose, and the area lateral to the incisive fossa of the mandible. Insertion is into the skin and into itself, forming an ellipse around the mouth.

Buccal branches of the facial nerve innervate this complex muscle, which closes the lips and, during stronger contraction, purses them, as in osculation and whistling.

Risorius

The risorius is a small, horizontally placed muscle that originates in the masseteric fascia and inserts in the skin of the corner of the mouth. This is the smiling muscle; it is responsible for drawing the corners of the mouth laterally. The risorius is innervated by buccal and mandibular branches of the facial nerve.

Depressors of the Lip

The depressor labii inferioris is quadrangular in shape. It originates on the medial extent of the oblique line of the mandible and inserts into the skin of the lower lip. It acts to depress the lower lip.

The depressor anguli oris (triangularis) originates on the oblique line of the mandible and inserts into the skin of the corner of the mouth and depresses it, expressing sadness.

The mentalis is a small muscle of the chin. Its origin is in the incisive fossa of the mandible, and it inserts into the skin of the chin to wrinkle it and also to protrude the lower lip, as in drinking.

The platysma was previously detailed in Chapter 7. All of the muscles of this group, except the platysma, are innervated by the buccal and mandibular branches of the facial nerve.

Elevators of the Lip

Five muscles elevate the lip and corner of the mouth. The levator labii superioris alaque nasi is the most medial of these muscles, originating from the frontal process of the maxilla passing inferiorly along the side of the nose. It then splits into a medial and a lateral portion to insert into the wing of the nose and into the upper lip. This muscle functions in dilating the nostril and raising the upper lip.

The levator labii superioris originates from the maxilla and zygoma just inferior to the orbit. Its fibers pass across the infraorbital foramen to insert into the upper lip, lateral to and intermingling with the fibers of the levator labii superioris alaque nasi. The levator labii superioris elevates and protrudes the upper lip.

The levator anguli oris lies deep to the levator labii superioris. It originates below the infraorbital foramen, from the canine fossa of the maxilla, to insert into the corner of the mouth. This muscle elevates the angle of the mouth and assists in the formation of the nasolabial furrow.

The zygomaticus minor, a slender muscle arising from the maxillary process of the zygomatic bone, inserts just lateral to the insertion of the levator labii superioris muscle. This muscle elevates the upper lip. It also assists in the formation of the nasolabial furrow.

The zygomaticus major is the lateral-most muscle of this group. It originates on the temporal process of the zygomatic bone and inserts into the corner of the mouth. This muscle elevates the corner of the mouth and pulls it laterally.

All of the five muscles acting to elevate the lips are innervated by the buccal branches of the facial nerve.

Muscle of the Cheek

The buccinator, a quadrangule-shaped muscle occupying the space between the mandible and the maxilla, is the primary muscular component of the cheek. It lies deep to the muscles of facial expression and is separated from them by the buccopharyngeal facia and the buccal fat pad. The parotid duct pierces the substance of this muscle to enter the oral vestibule.

The buccinator originates on the maxilla and mandible, specifically on the buccal surfaces of the alveolar processes in the vicinity of the three molars, and from the pterygomandibular raphe, a collagenous tendinous inscription attached to the pterygoid hamulus and the mylohyoid line of the mandible. This raphe is interposed between the buccinator and superior pharyngeal constrictor muscles.

The buccinator inserts into the fleshy corner of the lip in such a fashion that the upper fascicles and the lower fascicles decussate at the corner of the mouth and insert into the lower and upper lips, respectively, becoming fibers of the orbicularis oris. The highest and lowest fascicles, however, continue without decussation into the upper and lower lips, respectively.

The buccinator muscle acts to press the mucosa of the cheek against the teeth, thus aiding in mastication and deglutition. In addition, it assists in distending the oral vestibule and forcefully expelling air, as in blowing dust particles off a surface. The buccal branch of the facial nerve innervates this muscle.

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PTERYGOPALATINE FOSSA

PTERYGOPALATINE FOSSA

The pterygopalatine fossa—

           A small, pyramid-shaped space.

           Situated between the maxilla, sphenoid, and palatine bones.

           It communicates via canals, fissures, and foramina with various regions of the skull.

          The contents of the pterygopalatine fossa include

                     The terminal portion of the maxillary artery;

                     The pterygopalatine ganglion;

                     The maxillary division of the trigeminal nerve; and branches of these structures.

Maxillary Artery

The third, or pterygopalatine portion, of the maxillary artery enters the pterygopalatine fossa from the infratemporal fossa via the pterygomaxillary fissure

Maxillary artery and its distribution in the deep face

Branches of the pterygopalatine portion of the maxillary artery are the posterosuperior alveolar, infraorbital, greater palatine, pharyngeal, and sphenopalatine arteries as well as the artery of the pterygoid canal.

The posterior superior alveolar artery branches from the maxillary artery as that vessel enters the pterygomaxillary fissure. It travels on the maxillary tuberosity and enters the posterior superior alveolar foramen accompanied by the like-named nerve. The vessel ramifies within the maxilla to vascularize the maxillary sinus, molars, and premolars as well as the neighboring gingiva.

The infraorbital artery, a continuation of the maxillary artery, enters the orbit through the inferior orbital fissure, lies in the infraorbital groove, leaves the orbit via the infraorbital canal, and enters the face by way of the infraorbital foramen. Branches of the infraorbital artery are the orbital branches, serving the lacrimal gland and the inferior oblique and inferior rectus muscles; the anterior superior alveolar branches, which vascularize the anterior teeth and the maxillary sinus; and the facial branches.

The greater palatine artery and its branch, the lesser palatine artery, pass through the pterygopalatine canal and gain entrance to the palate via the greater palatine and lesser palatine foramina, respectively, to vascularize the hard and soft palates as well as associated structures. The pharyngeal branch passes dorsally, through the pharyngeal canal, to vascularize the auditory tube, sphenoidal sinus, and portions of the pharynx. The sphenopalatine artery leaves the pterygopalatine fossa via the sphenopalatine foramen on its medial wall to enter the nasal fossa. The distribution of this vessel and its branches is discussed later in this chapter. The small artery of the pterygoid canal passes through the posterior wall of the pterygopalatine fossa via the pterygoid canal. It supplies part of the auditory tube, pharynx, middle ear, and sphenoidal sinus.

Maxillary Nerve

The maxillary division of the trigeminal nerve enters the pterygopalatine fossa at its posterior boundary via the foramen rotundum. While in the fossa, it gives off the zygomatic nerve, which, passing into the orbit through the inferior orbital fissure, will bifurcate to form the zygomaticotemporal and zygomaticofacial nerves.

The maxillary division of the trigeminal nerve

The posterior superior alveolar nerves also branch from the maxillary nerve, exit the fossa via the pterygomaxillary fissure, and enter the maxillary tuberosity to serve the maxillary sinus, molars, and adjacent gingiva and cheek. The maxillary nerve then enters the orbit by way of the inferior orbital fissure and is referred to as the infraorbital nerve.

While in the pterygopalatine fossa, the maxillary nerve communicates with the pterygopalatine ganglion via two small trunks, the pterygopalatine nerves; however, these nerves do not bear a functional relationship with the ganglion. Postganglionic parasympathetic fibers derived from the ganglion ride along and distribute with branches of the maxillary division of the trigeminal nerve.

Pterygopalatine ganglion and associated nerves and arteries

Orbital branches are slender nerves that supply the periosteum of the orbit and the mucoperiosteum of the ethmoidal and sphenoidal sinuses. The greater palatine nerve and its branches, the lesser palatine and posterior inferior nasal branches, descend through the pterygopalatine canal to supply regions of the palate, gingiva, tonsil, and lateral wall of the nasal fossa.

Posterior superior nasal branches leave the pterygopalatine fossa via the sphenopalatine foramen to serve the posterior aspect of the nasal fossa and part of the ethmoidal sinus. Its nasopalatine branch grooves the vomer bone in its path to the incisive foramen of the anterior hard palate, which it supplies. The pharyngeal nerve traverses the pharyngeal canal to innervate part of the nasopharynx.

Pterygopalatine Ganglion

The pterygopalatine ganglion seems to be functionally associated with the maxillary division of the trigeminal nerve because it is suspended by the pterygopalatine nerves within the fossa. It is, however, a parasympathetic ganglion of the facial nerve (cranial nerve VII).

This ganglion receives its parasympathetic preganglionic root by way of the pterygoid canal, which opens onto the posterior wall of the fossa. The preganglionic parasympathetic fibers synapse with postganglionic parasympathetic cell bodies within the ganglion. Postsynaptic parasympathetic fibers leave the ganglion and distribute with branches of the maxillary division of cranial nerve V. These fibers are secretomotor in function. They provide parasympathetic flow to the lacrimal gland and mucosal glands of the nasal fossa, palate, and pharynx.

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SWELLINGS OF THE JAW

Bone Swellings

Bone swellings are lesions that characteristically present as asymptomatic hard lumps, covered by normal epithelium. Developmental disorders, benign and malignant tumors are included in this group of lesions.

•        Torus mandibularis
•        Torus palatinus
•        Multiple exostoses
•        Osteoma
•        Osteosarcoma
•         Chondrosarcoma
•         Burkitt lymphoma
•         Multiple myeloma
•         Paget disease
•       Odontogenic tumors

Torus Mandibularis

Definition and etiology Torus mandibularis is a developmental malformation of unknown etiology.

Clinical features It presents as an asymptomatic bony swelling, covered by normal mucosa. The lesion displays slow growth during the second and third decades of life. Characteristically, the lesions appear bilaterally on the lingual surface of the mandible, usually in the area adjacent to the bicuspids. The diagnosis is based on clinical criteria.

Torus mandibularis

Treatment Unnecessary unless full denture construction is required.

Torus Palatinus

Torus palatinus at the midline of the hard palate

Definition and etiology Torus palatinus is a developmental malformation of unknown etiology.

Clinical features It presents as a slow-growing, nodular, lobular or spindled, asymptomatic bony swelling covered by normal mucosa. Characteristically, the lesion appears along the midline of the hard palate.It occurs more often in women, and usually appears during the third decade of life. The diagnosis is based on the clinical findings.

Treatment Unnecessary unless full denture construction is required.

Multiple Exostoses

Multiple exostoses may occur on the buccal surface of the maxilla, and rarely on the mandible. Clinically, the lesions appear as multiple asymptomatic bony swellings. The diagnosis is based on the clinical findings.

Multiple exostoses on the maxilla.

Treatment Unnecessary unless full denture preparation is required.

Osteoma

Definition Osteoma is a benign neoplasm that consists of mature compact or cancellous bone.

Etiology Unknown.

Clinical features

It presents as an asymptomatic, slow-growing bony swelling of the jaws. The size ranges from a few millimeters to several centimeters. Multiple jaw osteomas are a common feature of Gardner syndrome.

Gardner syndrome: osteoma of the mandible.

Laboratory tests Histopathological examination, radiography.

Differential diagnosis Exostoses, osteosarcoma.

Treatment Surgical excision.

Osteosarcoma

Definition Osteosarcoma is the most common primary malignant neoplasm of bone.

Etiology Unknown.

Clinical features

The jaws are affected in 6–7% of cases, and usually during the third decade of life. Both jaws are affected equally and it is more common in men. Clinically, the lesion presents as a rapidly growing hard swelling that progressively produces facial deformity. Pain, paresthesia, tooth loosening, and nasal obstruction may also occur.

Osteosarcoma of the upper jaw, presenting as a hard swelling.

Laboratory tests Histopathological examination, radiography, CT scans.

Differential diagnosis Chondrosarcoma, Ewing sarcoma, metastatic tumors, odontogenic tumors and cysts, giant-cell tumor.

Treatment Surgical excision and supplementary radiotherapy and chemotherapy.

Chondrosarcoma

Chondrosarcoma  is more common in men than in women, between 30 and 60 years of age. Clinically, it appears as a painless hard swelling that progressively enlarges, causing extensive bone destruction with pain and loosening of the teeth.

Burkitt Lymphoma

Definition Burkitt lymphoma is a high-grade malignant B-lymphocyte lymphoma.

Etiology Epstein–Barr virus is closely associated.

Clinical features

The malignancy is prevalent in central Africa (the endemic form), and usually affects children 2–12 years of age. Cases have also been observed in other countries (the nonendemic form), and recently in patients with AIDS. The jaws are the most common site of lymphoma (60–70%). Clinically, it presents as a rapidly growing hard swelling that causes bone destruction, tooth loss, and facial deformity.Pain, paresthesia and large ulcerating or nonulcerating masses may also be seen.

Burkitt lymphoma, facial deformity.

Burkitt lymphoma, gingival mass

Burkitt lymphoma on the gingiva in a young patient with AIDS

Laboratory tests Histopathological examination, radiography.

Differential diagnosis Central giant-cell granuloma, ossifying fibroma, other non-Hodgkin lymphomas, and odontogenic tumors.

Treatment Chemotherapy, radiotherapy.

Multiple Myeloma

Definition Multiplemyeloma is a relatively rare malignant plasma-cell disorder.

Etiology Unknown.

Clinical features The malignancy is more common in men over 50 years of age, and the jaws are affected in about 30% of cases. Clinically, it presents with bone swelling, tooth mobility, pain, and paresthesia. A painless soft swelling, usually on the alveolar mucosa and gingiva, may develop as part of the overall disease spectrum.

Multiple myeloma, swelling on the gingiva

Laboratory tests Bone-marrow biopsy, radiography, serum and urine protein electrophoresis.

Differential diagnosis Plasmacytoma, non-Hodgkin lymphoma, Ewing sarcoma, leukemia, Langerhans cell histiocytosis.

Treatment Chemotherapy, radiotherapy.

Paget Disease

Definition Paget disease, or osteitis deformans, is a chronic, relatively common disorder characterized by uncoordinated bone resorption and deposition.

Etiology Unknown.

Clinical features Clinically, the signs and symptoms develop gradually and are characterized by bone pain, headache, deafness, visual disorders, dizziness, and progressive bone enlargement. Progressive expansion of the maxilla and the mandible lead to symmetrical thickening of the alveolar ridges.

Paget disease, enlarged maxilla

Edentulous patients may complain that their dentures do not fit due to alveolar enlargement.

Paget disease, alveolar enlargement

Delayed wound healing, bleeding, and osteomyelitis after tooth extraction may occur. The maxilla is more frequently affected than the mandible. Malesare more often affected than females. Two major forms of the disease are recognized: (a) the monostotic, and (b) the polyostotic. The clinical diagnosis should be confirmed by a histopathological and radiographic examination. Elevations of serum alkaline phosphatase and urinary hydroxyproline levels are common findings.

Differential diagnosis Fibrous dysplasia, osteosarcoma, multiple exostoses, fibro-osseous lesions.

Treatment Most cases require no treatment. Calcitonin and bisphosphonates may slow the pathological process.

Odontogenic Tumors

Definition Odontogenic tumors are a group of lesions that originate from odontogenic epithelium and ectomesenchyme.

Etiology Unknown. Some are neoplasms and others hamartomas.

Classification On the basis of the tissue of origin, three major varieties are recognized: (a) tumors of odontogenic epithelium, (b) tumors of odontogenic ectomesenchyme, and (c) mixedod ontogenic tumors.

Clinical features Most odontogenic tumors are usually asymptomatic for long time and are discovered only during a routine radiographic examination. However, with time they may form a usually painless slow-growing swelling or expansion of the mandible or the maxilla.

Odontogenic myxoma, expansion of the retromolar area

Extraosseous calcifying epithelial odontogenic tumor presenting as a gingival mass

The clinical signs and symptoms are not diagnostic and the final diagnosis should be made by radiographic and histopathological examinations.

Differential diagnosis Different varieties of odontogenic tumors, odontogenic cysts, osteosarcomas, chondrosarcomas, multiplemyeloma.

Treatment Surgical excision.

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