Prosthodontics Dr.Yassen H. Lecture -2- Anatomy & Physiology Related to Prosthodontics (Myology) Muscles are divided or classified into: 1. Muscles of facial expression. 2. Suprahyoid muscles. 3. Infrahyoid muscles. 4. Muscles of mastication. 5. Muscles of the tongue. 6. Muscles of the soft palate. 7. Pharyngeal muscles. 1. Muscles of facial expression: The muscles responsible for the expression seen in the lower half of the face are: Zygomaticus major Zygomaticus minor. Levator labii superioris. Levator labii superioris alaeque nasi. Levator anguli oris. Mentalis. Depressor labii inferioris 1
Depressor anguli oris. Risorius. Platysma. Incisivus superioris. Incisivus inferioris. Orbicularis oris. Buccinators. The actions of these muscles are responsible for various facial expressions including smiling, laughing and frowning. Usually they act as a group of muscles, upon the loss of teeth, the function of the entire group of muscles is seriously impaired; therefore, the restoration of the lip to its original position and contour is an extremely important function of the prosthetic appliance. Most parts of these muscles insert into the orbicularis oris and of course they are very flexible. They have origins on the skull, these perioral muscles of facial expression generally do not insert in to bone and need support from the teeth for proper function, again if these muscles are not properly supported either by natural teeth or by the artificial teeth none of the facial expression will appear as normal, such as the naso-labial sulcus, the philtrum, the commissures of the lips, as well as the mento-labial sulcus. When incorrectly positioned artificial teeth or if incorrectly contoured denture base, the normal tonicity of the muscles will be affected, as well as the facial expression. This insertion around the oral cavity opening is very important, where they partly insert into the connective tissue of the skin and partly into the mucous membrane of the lips. 2
The area which is laterally and slightly above the corner of the mouth there the location of the modiolus which is a concentration of many fibers of this group of muscles. It represents area where extrinsic perioral muscles decussate to join the intrinsic fibers of the orbicularis oris muscle. The labial flanges of the maxillary denture frequently need to be reduced in thickness in the area of the modiolus. So as not to affect the stability of the maxillary denture. When the perioral muscles are stretched during, mouth opening, the vestibular space between the muscles in the cheek and the slopes of the residual alveolar ridges is restricted. So that reduction of the bulk of the flange to accommodate this action helps to prevent the denture from being dislodged when the mouth is opened. The labial flanges of the mandibular denture base is affected similarly and should be reduced in thickness. Also in the lower denture, the mandibular premolars should be arranged on the crest of the residual ridge to avoid any interference with this modiolus. Buccinator Muscle This muscle provides support and mobility for the soft tissues of the cheek. It is a wide; rather thin muscle arising from a horse shoe shaped line along the outer surfaces of the maxillary and mandibular alveolar processes in the area of the molar teeth. In addition the buccinator originates from the pterygomandibular raphe or ligament which serves as the junction between the buccinator muscle and the superior constriction muscle of the pharynx. 3
The buccinator fibers arising from the mandible follow the external oblique line to the mesial aspect of the first molar and then ascend slightly toward the corner of the mouth. Buccinator fibers that originate from the maxillary alveolar process in the area of the molar descend slightly toward the corner of the mouth. Some of the fibers insert into the mucous membrane of the cheek in and around the modiolus. The remaining fibers enter the upper and lower lips to become a part of the orbicularis oris muscle. In the lower jaw it becomes part of the denture bearing area. It has three heads: a) Superiorly attached to the alveolar process of the maxilla in the molar area. b) Inferiorly attached to the alveolar process of the mandible in the molar area. c) Middle from the ramus. The fibers of the buccinator muscle are arranged approximately horizontally and parallel to the occlusal plane but they run perpendicular or at a right angle to the fibers of the masseter muscle. It is not a true muscle of mastication, but it aids in conjunction with the tongue in keeping the bolus of food on the occlusal table during mastication. Due to the direction of pull of this muscle fibers, by which the denture is prevented from being dislodge by having its buccal flange resting on a portion of the buccinator muscle. The buccinator muscle alone does not dislodges the lower denture directly, but when the masseter muscle is activated it pushes the buccinator medially against the denture border in the region of the retromolar pad, so its action is indirect. To avoid this dislodging action, the denture border should be 4
contoured in a manner to accommodate the interaction of the buccinator muscle and masseter muscle. This contour is called the "masseter grove". The vertical height of the distobuccal flange of the maxillary denture is limited by the position of the buccinator muscle in the upper jaw. Orbicularis Oris Muscle It is the sphincter muscle of the mouth. It has no skeletal attachments except through the attachments of the incisivus labii superioris and inferioris muscles and the nasolabialis muscles. It composed not only of intrinsic fibers but also of extrinsic fibers of many of the muscles that coverage at the modiolus. The complex arrangement of the muscle fibers of different origins facilitates the various movements of the lips. The upper lip is supported by the six maxillary anterior teeth and not the denture border. When the teeth are in occlusion, the superior border of the lower lip is supported by the incisal third of the six maxillary anterior teeth.it is important for the dentist to make sure that the actions of these muscles are recorded when making impressions for the dentures. Also, the angles of the mouth are easily irritated if the lips are stretched taut when an impression tray is inserted. 5
2. Suprahyoid muscles: They are a group of muscles, their function is the elevation of the hyoid bone, the elevation of larynx and at the same time depression of the mandible. These muscles are the digastric, stylohyoid, mylohyoid and geniohyoid muscles. Mylohyoid Muscle This muscle attached to the mylohyoid ridge and insert into the hyoid bone. It limits the extension of the lingual flange of the mandibular denture. The mylohyoid forms the muscular floor of the mouth. It elevates the hyoid bone, the tongue, and the membranous floor of the mouth during swallowing. Over extension of the lower lingual flange below the mylohyoid line will impinge on the mylohyoid muscle, thus can affect its action and can unseat the denture. Geniohyoid Muscles This muscle arises from the inferior mental spine (genial tubercle), which is located on the inner aspect of the symphysis menti just above the anterior attachment of the mylohyoid muscle. This muscle usually present no problems in complete denture construction unless there is extensive bone resorption in this area where they will be high near the crest of the ridge causes pain when pressure is applied. 6
3. Infrahyoid Muscles These group of muscles consists of sternohyoid, omohyoid, sternothyroid, and thyrohyoid muscles. They have no particular significance in complete denture construction with respect to denture borders, but having an effect on mandibular movement: by fixing the hyoid bone so that the Suprahyoid muscles can act on the mandible. 4. Muscles of Mastication: The muscles that have been designated as the muscles of mastication are the masseter, temporalis, medial pterygoid, and lateral pterygoid muscles. Their origin is from bones of the skull and they are inserted into the mandible, they are involved in the masticatory and non-masticatory movement of the mandible. These muscles function as a group along with Suprahyoid, Infrahyoid and other groups of muscles to produce the precise movements necessary for mastication, deglutition, speech and respiration. These muscles are very powerful, but only the masseter muscle can directly influence the contour of the denture base. 7
Masseter Muscle Its fibers run from the zygomatic arch downwards and backwards to the outer surface of the ramus of the mandible. The primary action of this powerful muscle is to elevate the mandible. It has got an effect on the distobuccal corner of the mandibular impression, when the masseter contracts it applies pressure against the buccinator which in turn forms the distobuccal border. Temporalis Muscle It's a fan-shaped muscle, that's originated from the temporal fossa and associated fascia on the lateral aspect of the skull. Its fibers are divided into three groups based on the orientation of the fibers. The anterior fibers are oriented vertically and form the bulk of the muscle, the middle fibers are increasingly oblique, and the posterior fibers are almost horizontal in their inclination. The temporalis inserts on the coronoid process and along the 8
anterior border of the ramus of the mandible down to the area of the third molar tooth. The anterior and middle fibers elevate the mandible, while the posterior fibers elevate as well as retract the mandible.an indentation of the temporalis muscles may be recorded on the posterior buccal flange of the maxillary impression. Medial Pterygoid Muscles It runs essentially from the medial surface of the lateral pterygoid plate, pyramidal process of the palatine bone, and the maxillary tuberosity, and inserted into the medial surface of the ramus and angle of the mandible. Its primary action is to elevate the mandible. Sometimes, indentation of this muscle will be recorded in the maxillary impression at the posterior border of the buccal flange. Lateral Pterygoid Muscle It is the fourth muscle of mastication, has no direct effect on the border of the denture. The lateral pterygoid muscle has two heads; the superior head arises from the infratemporal crest and the adjoining surface of the greater wing of the sphenoid bone. The inferior head arises from the lateral aspect of the lateral pterygoid plate. 9
The superior head is the smaller and insert in part into the anterior and medial surface of the articular disk with the bulk of its fibers joining the inferior head to insert on the anterior surface of the neck of the condylar process. The superior head appears to function during elevation of the mandible, whereas the inferior head functions during depression and protrusion of the mandible. The muscles of mastication are usually divided into two groups: (the elevator and the depressor). The elevator muscles are usually stronger than the depressors where the muscles which are responsible for protruding and moving the mandible from side to side are usually stronger than the retractors, their effect is mainly manifested through the centric jaw relation recording made with the aid of weak posterior fibers of temporalis muscle while the antagonist is the stronger lateral pterygoid. *The elevator muscles are the medial pterygoid, masseter, anterior and middle fibers of temporalis, the superior head of the lateral pterygoid muscle, the posterior fibers of the temporalis elevate and retract. *The depressor muscles are the inferior head of the lateral pterygoid which is responsible for protrusion as well. 5. Muscles of the Tongue The tongue is a powerful muscular structure composed of intrinsic and extrinsic muscle fibers. Functions of the tongue are coordinated with those of other muscles thus producing muscular movement that are necessary for various functions such as mastication, deglutition and speech. 10
The tongue is located in the floor of the mouth and it is in intimated contact with the lingual flanges of the mandibular denture. The denture flanges must be contoured to allow the tongue to have its normal wide range of functional movements. For example, the tip of the tongue is used to moisten the lips many times a day, while during mastication, the tongue takes the food from the floor of the mouth and the labial and buccal vestibule and places it on the occlusal surfaces of the teeth. The intrinsic muscles are originated and inserted within the tongue, producing changes in the shape of the tongue (responsible for the change in shape) on the other hand the extrinsic muscles are originated from structures other than the tongue and outside the tongue, they can move the tongue as well as change in its shape. They include: Genioglossus, styloglossus, hyoglossus and palatoglossus. 11
Palatoglossus Muscle It limits the distal extension of the lingual flange of the mandibular impression along with superior constrictor muscle. There are three different size of the tongue it may be either large, medium (normal), or small: *Medium size (normal): Normal size tongue can usually adapted to complete denture very easily. Provided a well extended denture flanges, properly contoured placing them in compatible positions with the tongue and cheek forming or ending up with the teeth placed in the (neutral zone). *Large size (macroglossia): Such a tongue may interfere with the impression making. *Small size (Microglossia): Such a tongue will give a reduced support interfering and lowering the amount of retention of the lower denture. According to the height of the tongue, there are three different positions: High: here the tongue is higher than the level of the residual ridge. Medium: the tongue here is leveled with the residual alveolar ridge. Low: the tongue here is lower than the level of the residual alveolar ridge. Since the tongue is located in the floor of the mouth, then it lies in contact with the lingual flanges of the lower denture, thus 12
denture flanges lingually should be shaped in a way to permit a normal tongue movement without any interference, that's why the patient should be asked to move his tongue to the right and left as well as protruding it anteriorly in muscle trimming procedures during lower impression making as well he's asked to moisten his lips. The tongue is usually forming a good landmark for successfully leveling the lower occlusal plane. The occlusal plane of the lower denture should be kept in low position allowing the lateral borders of the tongue to rest up on the occlusal surface of the teeth, thus the mouth is widely opened and increasing the lower complete denture stability. The neutral zone forms a balanced position for the posterior artificial teeth located in between the tongue lingually and the muscles of the lips and cheeks laterally and buccally contributing largely to the steadiness of the lower complete denture. 6. Muscles of the soft palate: The tensor veli palatini, levateor veli palatini, muscular uvulae, palatoglossus and palatopharangeus are group of muscles forming the movable curtain extending downwards and backwards into the pharynx. Tensor Veli Palatini They drop the soft palate; the denture restores a portion of this muscle in the posterior palatal seal area. Levator Veli Palatini Levator veli palatini elevates the soft palate. 13
7. Pharyngeal muscles Include superior constrictor, middle constrictor and inferior constrictor muscles. The superior constrictor is one of the most interest in complete denture construction, it has four sites of origin which are the posterior border of the medial pterygoid plate, the pterygomandibular raphe, the posterior end of the mylohyoid line and the side of the tongue. The action of this muscle exerts pressure against the distal extremity of the mandibular denture, so that an over extension in this area will be very painful to the patient as the denture perforate the tissue and create a painful lesion. Retromolar pad area: A pear or triangular shaped pad of tissue of on the superior aspect of the mandibular ridge just behind the last lower molar. Pterygomandibular raphe: It runs from the hamulus to the mandible in the area underlying the retromolar pad. This raphe is formed by: a) A superior constrictor- medially. b) Buccinator - laterally. Rugae: Irregularly shaped folds of firm fibrous tissues aid the tongue in speech. The anterior half of the palate forms the fatty zone while the posterior half of the palate is glandular zone. Fovea palatinae: A two small pin point depressions in the midline, in the posterior part of the vault, located usually on the soft palate, they are usually formed from a coalescence of a mucus glands 14
ducts, they considered an ideal guide for the location of the posterior border of the maxillary denture. Vibrating line: It is an imaginary line drawn across the palate, marks the binging of motion of soft palate when the individual says (Ah), it extends from one hamular notch to the other. In midline it passes about 2 mm in front of fovea palatinae, it always on the soft palate and at the junction of the movable and immovable part of the soft plate. The vibrating line is not to be confused with the junction of the hard and soft palate since the vibrating line is always on the soft palate. It should be described as an area and not a line, its direction varies according to the shape of the palate, the higher the vaults, the more abrupt and forward the vibrating line. A flat palatal vault affords a broader posterior palatal seal area; it is usually further posterior having a gradual curvature. Stress bearing area: The epithelium in these regions is firmly attached to the underlying bone, dense, thick and keratinized. The lamina propria is dense thick, collagenous and well attached to the periosteum especially in the anterior part of the palate which is called the rugae area at the area of the intermaxillary suture, the submucosa is thin and firmly attached to the underlying bone. In the area of premolars between the maxillary sutures and the residual alveolar ridge, it consists of reticular and adipose tissues. In the molar region, the submucosa is rich in glandular and adipose tissues, which is continuous with the soft palate area. 15
Temporomandibular Joints Each temporomandibular joint involves the articulation of the condylar process of mandible with the mandibular fossa (glenoid fossa) of the temporal bone. The temporomandibular joints are synovial joints but differ from most synovial joints in that the articular surfaces of the bony components are covered with dense fibrous connective tissue instead of hyaline cartilage. In addition the temporomandibular joint has a fibrous articular disc interposed between the bony articular surfaces, which add to the complexity of the normally functioning joint and to the diagnosis and treatment of the abnormally functioning joint. Movement of the mandible can be classified as masticatory and non-masticatory. The masticatory movements are necessary for introduction, grasping, crushing and grinding of food, as well as the swallowing of the triturated mass. The nonmasticatory movements include all of the other normal mandibular movements, such as those used in speech and wetting the lips and also habitual or abnormal movements such as bruxism, clenching, or tapping the teeth together. It is important to note that the time spent each day for nonmasticatory movements exceeds the time used for masticatory movements. A patient is considered suffering from "TMJ dysfunction syndrome" when one of the following symptoms is present: 1. Pain and tenderness in one or more of the muscles of mastication. 2. Sounds (clicking) during condylar movements. 3. Limitations of the mandibular movements. 16
A successful complete denture construction requires healthy TMJs with both structural and functional harmony of the osseous structures, as well as the intra-articular tissue and the capsular ligaments. The presence of unhealthy TMJs will consequently affect the process of impression making especially because of the need of a big and wide mouth opening and the ability of mandible to move from side to side and this will come true during centric jaw relation recording. Oral mucosa Depending on its location, it could be classified into: 1. The masticatory mucosa. 2. The lining Mucosa. 3. The specialized mucosa. In edentulous patients covers: The masticatory mucosa 1. The crest of the residual ridge, here it is firmly attached to the underlying supporting bone. 2. The hard palate, this masticatory mucosa is characterized by a well-defined keratinized layer on its outermost surface. The soft tissues should be recorded when or during their resting condition. Tissues tend to return to their normal form after a period of time after being displaced, thus, creating an unstable denture, so that proper relief is sometimes needed for proper recording of these tissues in their undistorted form. 17
The region of the median palatal suture exhibits a very thin submucosa firmly attached to the underlying bone, that's why the covering tissues of this suture is non resilient. Here a very little or no stress can be subjected to this area during the impression making procedure or rocking of the denture above the center of the palate will occur. The lining mucosa Found in the lips, cheeks and vestibular spaces, this type of mucosa is not firmly attached or in other words, it was loosely attached to the underlying bone, other areas include the alveololingual sulcus, the soft palate and the ventral surface of the tongue. This lining mucosa is a non-keratinized mucosa. The specialized mucosa Covers the dorsum of the tongue, this mucosa is keratinized including the papillae on the upper surface of the tongue. The soft tissue covering of the hard palate varies in thickness and in their consistency according to its location whether anteriorly located when the submucosa of the hard plate contains adipose tissues while posteriorly it contains glandular tissues. The posterior palatal seal rests on an area in between the movable and non-movable parts of the soft palate also called vibrating line or post dam area. The degrees of activity of the soft palate muscles as well as variability in the contour of the soft palate govern the contour, extent and depth of this posterior palatal seal area. 18
Salivary glands The major exocrine glands that secrete saliva are the paired parotid, submandibular and sublingual glands, in addition to these 3 major glands, numerous minor salivary glands are distributed throughout the oral cavity in the lips, cheeks, tongue and the palate. Saliva is a mixture of two types of primary secretions: 1. A serous secretion (thin, watery) containing the enzyme ptyalin for the digestion of starchy foods. 2. A mucous secretion (viscid, sticky or adhesive) for lubrication purposes The parotid gland considered to be serous. The submandibular glands are mixed but mostly serous secretions. The sublingual glands are mixed but mostly mucous secretions. The parotid gland empties its secretions into the oral cavity via a duct which opens through the buccal mucosa near the maxillary second molar tooth. 19
The submandibular and the sublingual glands are located in the floor of the mouth, also empty their secretions into the oral cavity via a duct, the position of their ducts are very important for any overextension or pressure area exerted by the lower flange on the openings of these ducts will cause blocking out of the duct, where the patient will eventually complain of swelling under the chin, this could be relieved immediately after removal of the lower dentures from the mouth. The saliva is very important to lubricate the bolus of food, in addition coats the lining of the oral cavity to protect the lining mucosa. When the food particles are coated with mucosa, they adhere to each other and slide along the epithelium with ease. Saliva also lubricates the surfaces of a denture, thereby making the denture more compatible with the movements of the lips, tongue, and cheeks. Some of the pathologic conditions that decrease salivation are atrophy of the salivary glands with aging, fibrosis of glands following irradiation therapy of head and neck tumors and diseases of the brain stem that directly depress activity in the salivatory nuclei and block salivation, other conditions include some types of encephalitis, poliomyelitis, diabetes mellitus, diabetes insipidus, diarrhea caused by bacteria of food, elevated temperature from acute infectious diseases, and vitamin deficiency particularly vitamin A, numerous drugs used to treat these and other conditions can alter the quantity and quality of saliva. Pathologic conditions that may be accompanied by increased salivation are digestive tract irritants and painful lesions of the oral cavity. These may be due to a vitamin deficiency, surgical trauma, an ill-fitting denture, or inadequate 20
inter-occlusal distance when artificial dentures are made. Saliva is considered a major factor in evaluating the physical influences that contributes to denture retention. The physical forces in which saliva is involved are adhesion, cohesion, capillarity and atmospheric pressure. Some patients may have excessive salivary secretions, affecting the process of impression making and even may cause a gagging reflex. Excessive salivation, particularly by the submandibular and sublingual glands, presents a problem in impression making. When this problem exists, appropriate drugs can be administered orally before making the impression. Excessive secretions of mucus from the palatal glands may distort the impression material in the posterior two thirds of the palate. To counteract this problem the palate may be massaged to encourage the glands to empty, the mouth may be irrigated with a stringent mouth wash just before inserting the impression material, or the palate may be wiped with gauze. On the other hand (xerostomia) or dry mouth where the case of insufficient saliva may cause loss of retention. In addition mucosa will not tolerate dentures so that such a problem should be discussed with the patient and it may be necessary to limit denture use to short periods and to restrict the diet to moist foods that are relatively soft or even liquid. Lastly, the viscosity of the saliva is very important a thick saliva is good for retention of the denture, but may cause gagging, on the other hand can be of bad effect on the efficiency of denture seal. 21