First antagonistic muscle pair
The first antagonistic muscle pair in the angle of the mouth comprises the orbicular muscle of the mouth and the buccinator muscle as well as their accessory muscles. They form the muscular basis of the lips and cheeks. The fibers of these two muscles cross at the modiolus.
The orbicular muscle of the mouth (musculus orbicularis oris) is the whole muscular mass lying around the orifice of the mouth, which can be regarded as the closing muscle of the mouth. Only a small part of the fibers contained in this muscle follow a wholly orbicular (ring-shaped) course; instead, the orbicular muscle is made up of fibers from adjacent muscles that radiate in from the upper and lower lips or the cheeks.
As a whole, the fibers following a circular path bring about closure of the orifice of the mouth. If the peripheral fibers are mainly activated, the orifice is narrowed in a circular fashion and the lips are pursed. Activation of the central fibers presses the lips together until the redness of the lips disappears (ie, slides inward). When contracted, the orbicular muscle lies firmly against the incisors and narrows the anterior part of the oral vestibule.
Because the orbicular muscle itself does not have an origin in the skull, it is held by various muscles. For instance, the muscle fibers of the masseter radiate in from the side over the modiolus. From above, the two incisive muscles run into the upper lip. These two muscles have their origin in the skull and therefore give the orbicular muscle support.
The superior incisive muscle (musculus incisivi superioris) has its origin at the alveolar mounds of the incisors and ideally extends to the angle of the mouth. The posterior incisive muscle (musculus incisivi inferioris) has its origin at the alveolar mounds of the canine and the first premolars and extends to the upper lip and the angle of the mouth. When contracted, these muscles give the mouth a round shape, eg, when whistling or forming the sounds "oh" and "you"
The buccinator muscle (musculus buccinator) forms the anterior region of the cheek and can be seen as an antagonist of the orbicular muscle. Its origin is on the outer surface of the alveolar process of the maxilla
. In the mandible, its path of insertion corresponds to the oblique line as far as the crista buccinatoria (crus laterale of the trigo-num retromolare) and up to the pterygomandibular raphe (plica pterygomandibularis). This raphe forms the boundary between the buccinator and the muscles of the throat.
The fibers of the buccinator initially run forward in a horizontal direction and converge on the angle of the mouth, where they cross and extend into the lips: the lower fibers run into the upper lip, the upper fibers into the lower lip, and the middle fibers directly to the angle of the mouth. The buccinator is perforated by the duct of the parotid gland (Stensen duct), whose opening in the oral vestibule is located at the maxillary second molar. The buccinator is the only muscle of facial expression to be covered by a fascia, which gives way to the tendinous plate of the modiolus. The fascia increases the strength and elasticity of the muscle and the cheek.
When activated, the buccinator brings about shortening of the cheek in a sagittal direction, which narrows the vestibule while widening the orifice of the mouth. The activity of this muscular foundation of the cheek works in a twofold way as an antagonist for the tongue: During chewing, the food is pressed back between the masticatory surfaces by the buccinator. On the other hand, the tissue tension while at rest maintains a balance against the pressure of the tongue, as a result of which shaping of the dentition takes place. During sucking (drinking), the buccinator forms the cylinder wall to which the tongue acts as a "suction valve" The buccinator is particularly important when playing wind instruments, which is where it derives its name (Latin, bucinator = trumpeter).
The risorius muscle (musculus risorius; Latin, risus = laughter; therefore, also called "laughing muscle") is a small muscle that runs parallel to the buccinator; it is usually poorly developed and only present on one side or entirely absent. Its fibers overlie the buccinator or are interwoven with the fibers of the buccinator. The risorius muscle originates in the fascia of the masseter muscle. It produces the dimple in the cheek by contracting and drawing the skin of the cheek toward the angle of the mouth and pulling the angle of the mouth backward. It has no significance in prosthetic work.
The external surface of denture bodies is shaped to grip the muscles, which involves fashioning lip shields for the orbicular muscle in the anterior area, starting from the modiolus. In the area of the posterior teeth, buccinator supports can be created; the muscular tracts are redrawn on the ligaments of the cheek. The areas in need of reduction should be heeded.