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(Contemporary Surgical Techniques) - Coggle Diagram

- - - - 1. The Hierarchy of Stability and Predictability
        
        The Hierarchy of Stability and Predictability Stability after surgical repositioning of the jaws depends on the
        1.direction of movement,
        2.the type of fixation, and
        3.the surgical technique, largely in that order of importance.
        The most stable orthognathic procedure is superior repositioning of the maxilla, closely followed by mandibular advancement in patients whose anterior facial height is maintained or increased.
        These procedures, the key ones in correcting severe Class II problems, can be considered to produce a highly stable result even without rigid fixation, and this remains true when they are combined in the treatment of patients with mandibular deficiency and a long face—but only if rigid fixation is used.
        In the treatment of patients with Class III problems, the maxilla remains forward just where it was put in about 80% of the patients, and there is almost no tendency for major relapse (4 mm or more).
        With rigid fixation, the combination of maxillary advancement and mandibular setback is acceptably stable.
        In contrast, isolated mandibular setback often is unstable.
        So is downward movement of the maxilla that creates downward backward rotation of the mandible.
        For this reason, almost all Class III patients now have maxillary advancement, either alone or (more frequently) combined with mandibular setback.
        Surgical widening of the maxilla has the least stable result among the orthognathic surgical procedures.
        Widening the maxilla stretches the palatal mucosa, and its elastic rebound is the major cause of the relapse tendency.
        Strategies to control relapse include overcorrection initially and careful retention afterward, with either a heavy orthodontic archwire or a palatal bar during the completion of orthodontic treatment, and then a palate-covering retainer for at least the first postsurgical year.
        SARPE is preferred over a three segment maxillary osteotomy if only expansion is required, but SARPE is not advantageous when vertical and/or anteroposterior change is needed, because then it would be the first stage of an unnecessary two-stage procedure.
      - 2. Influences on Stability
        Three principles that influence postsurgical stability help to put this in perspective:
        
        Neuromuscular adaptation is essential for stability. Fortunately, there is good neuromuscular adaptation to most orthognathic procedures
        
        A. When the maxilla is moved up, the postural position of the mandible alters in concert with the new maxillary position, and occlusal forces tend to increase rather than decrease. This controls any tendency for the maxilla to immediately relapse downward and contributes to the excellent stability of this surgical movement.
        B. Repositioning of the tongue to maintain airway dimensions (i.e., a change in tongue posture) occurs as an adaptation to changes produced by mandibular osteotomy, so surgical reduction of the tongue is not needed when the mandible is set back
        C. When the mandible is moved forward, a similar adaptation in tongue posture can be advantageous in patients with sleep disordered breathing, and a lower border osteotomy to bring the chin forward also produces forward movement of the tongue because the tongue is attached to the genial tubercles
        In contrast, neuromuscular adaptation does not occur when the pterygomandibular sling is stretched during mandibular osteotomy, as when the mandible is rotated to close an open bite as it is advanced or set back, so movement of the mandible that stretches the elevator muscles should be avoided.
        
        Stability is greatest when soft tissues are relaxed during the surgical procedure and least when they are stretched. Moving the maxilla up relaxes tissues. Moving the mandible forward stretches tissues, but rotating it up at the gonial angle and down at the chin decreases the amount of stretch.
        
        It is not surprising that the most stable mandibular advancements rotate the mandible in this way, whereas the least stable advancements are those that rotate it in the opposite direction, lengthening the ramus and rotating the chin up.
        The least stable orthognathic surgical procedure, widening the maxilla, stretches the heavy, relatively inelastic palatal mucosa.
        
        Neuromuscular adaptation affects the length of the masticatory muscles but not their orientation, and adaptation to a new orientation cannot be expected.
        
        This concept is best illustrated by the effect of changing the inclination of the mandibular ramus when the mandible is set back or advanced.
        Successful mandibular advancement requires keeping the ramus in an upright position rather than letting it incline forward as the mandibular body is brought forward.
        The same is true, in reverse, when the mandible is set back: a major cause of instability appears to be the tendency at operation to push the ramus posteriorly when the chin is moved back, thus changing its orientation.
        The orientation is restored when jaw function resumes after operation, and that moves the jaw forward again. The stability of two-jaw Class III correction is better than mandibular setback alone, apparently because it is easier to avoid pushing the mandible back when a two-jaw operation is done
    - - 1. Chin Augmentation or Reduction
        There are two approaches to repositioning the chin relative to the rest of the mandible: A. a lower border osteotomy to slide it to its new location or
        B. (Augmentation only) placement of an alloplastic implant.
        
        The lower border osteotomy to advance the chin has the advantages of well-documented predictability and stability, and (because it advances the genial tubercles) it tightens the suprahyoid musculature and produces desirable changes in chin–neck contour.
        It is particularly advantageous in patients with a combination of vertical excess and anteroposterior deficiency, which results in strain to bring the lips together
        Sliding the chin upward and forward in patients with this facial pattern is a “functional genioplasty,” in that it allows normal lip function as well as correction of the facial disproportion
        Functional genioplasty provides
        1.not only improved facial proportions but also
        2.normalization of lip function
        3.decreasing lip separation at rest and allowing the lips to be brought into contact without muscle strain
        Bone remodeling thickens the alveolar process below the teeth, and the notch above the repositioned chin fills in with new bone that extends up to the alveolar crest, creating new bone in an area where it is needed for future gingival stability
        In later periodontal therapy to control gingival stripping, that area often is bone grafted
        Functional genioplasty, and augmentation with sliding of the chin forward more generally, are more successful when done before age 15 so this is just the opposite of an orthognathic procedure that should wait until growth is essentially completed.
        The more the lower incisors erupt after the genioplasty, the better the formation of new bone above the displaced chin
        How early can it be done? The practical limit is eruption of the mandibular canines, which would be in the way of the osteotomy until they have moved into the oral cavity.
        Bone remodeling occurs above and behind the repositioned chin, but the bony chin does not remodel and is remarkably stable over time
        
        Augmentation genioplasty with alloplastic materials is favored by plastic surgeons, who prefer to avoid osteotomy and often are working with late adolescents or adults—although lower border osteotomy still remodels well in older patients.
        With greater chin advancement in older patients prominence of the lateral border of the chin that creates a notch along the lower border can become a problem.
        To compensate for this, there are two possibilities:
        1.either splitting the chin so the posterior margins can be moved medially to eliminate the notch
        2.or bone grafts or alloplastic augmentation materials over the lower border to fill in the posterior notch.
        Notch in grower:
        When a lower border osteotomy is used to move the chin forward, remodeling along the lower border smooths the bony contour there. New bone formation above the prominence of the chin fills in the notch that is present immediately after the operation, and this extends all the way to the alveolar crest in patients who are still growing vertically
        A chin implant has two advantages:
        1.the possibility of removal if the patient is unhappy with the result, and
        2.less risk of decreased sensation in the lower lip (from trauma to the nerve that emerges from the mental foramen to innervate the lip, which must be avoided in any genioplasty)
        It also has a major disadvantage, particularly with silicone implants: erosion of the implant into the surface of the bone or migration into the neck.
        Newer implant materials placed into a soft tissue pocket rather than directly against the bone provide much better stability and have almost totally replaced silicone.
        Removal of one of these implants, however, is difficult, and undesirable soft tissue changes may result if this is necessary.
        There are two aspects of deformity of the chin in patients with excessive mandibular growth: prominence of the chin relative to the dentition, and excessive height of the chin.
        In patients with a prominent chin, if a lower border osteotomy is used to simply slide the bony chin backward, and especially if the chin surface is cut away, the soft tissue chin tends to look like an underinflated ball because of the loss of skeletal volume. In patients with excess chin height, however, vertical reduction of the chin by removing a wedge of bone above the chin prominence and then repositioning the chin upward (and often slightly forward) can greatly improve the facial appearance and result in a measurable improvement in the patients’ quality of life.
        
        This surgical procedure is more effective if it is done before age 15
        
        Vertical growth and eruption of the lower incisors increase the amount of new bone formation at the symphysis.
        
        A lower border osteotomy should not be done until eruption of mandibular canines has cleared the way for the osteotomy.
        
        Genioplasty at the end of active orthodontics often is the optimum time
      - 2. Rhinoplasty
        
        The smile is framed by the chin below and the nose above. It may be necessary to change both to achieve optimal changes in facial appearance. Mandibular surgery repositions the chin relative to the rest of the face, and as we have seen, repositioning the chin relative to the jaw also may be needed. Maxillary surgery via Le Fort I osteotomy rarely has a positive effect on the appearance of the nose and may compromise it.
        Moving the maxilla up and/or forward can have two major deleterious effects on the nose:
        1.rotation of the nasal tip upward, resulting in deepening of the supratip depression, and
        2.widening of the alar base.
        Rhinoplasty, simultaneous with orthognathic surgery or staged to follow it can prevent these problems, so it is indicated for this purpose as well as for correction of a preexisting nasal deformity
        Although Le Fort II and III procedures do move the nose along with the upper parts of the maxilla, these more extensive and riskier operations are indicated only in the most severe deformities
        Rhinoplasty usually is focused on the contour of the nasal dorsum, the shape of the nasal tip, and the width of the alar base. All these aspects can be significantly improved with modern surgical techniques. Because the soft tissue contours around the nose will be affected by repositioning of the jaws, rhinoplasty follows the orthognathic procedure.
        It can be done immediately afterward, as part of the same surgical experience, with a switch from nasal to oral intubation after the jaw operation is completed.
        This is technically more difficult and requires excellent interaction between the orthognathic and rhinoplasty surgeons but greatly increases the chance that the rhinoplasty actually will be accomplished.
      - 3. Facial Soft Tissue Contouring With Implants
        
        Implants on the surface of the face can greatly improve soft tissue contours and are particularly advantageous for correction of two problems:
        
        The paranasal deficiency that often accompanies maxillary deficiency and
        2.The soft tissue deficiencies that accompany facial syndromes such as hemifacial microsomia.
        
        Onlay grafts in the paranasal area can be done successfully with use of the patient’s own bone, freeze-dried cadaver bone, or alloplastic materials.
        The more extensive implants needed in patients with congenital anomalies usually are made from alloplastic materials that can be shaped in advance.
      - 4. Lip Procedures
        
        Instead of changing soft tissue contours indirectly with skeletal surgery, lip procedures directly augment or reduce the lips.
        Lip augmentation rarely accompanies orthognathic procedures; this usually is done to counteract the loss of lip fullness that accompanies aging.
        Although injections of collagen or other materials into the lips can be successful, the results tend to be temporary.
        A more permanent increase in lip projection can be obtained with AlloDerm (human dermis in sheet form), a synthetic material such as Gore-Tex, or the patient’s own soft tissue harvested during a simultaneous face lift procedure.
        These are placed by creating a tunnel beneath the mucosa and threading the material into this space.
        This approach is preferred when lip augmentation is needed for orthognathic patients.
        Lip reduction rarely is performed now but can greatly improve outcomes for the rare patients with extremely thick and prominent lips.
        It is accomplished via intraoral incisions parallel to the vermilion border and excision of soft tissue, avoiding the removal of muscle but including submucosal glands.
      - 5. Submental Procedures
        
        Correction of an unesthetic throat form often is needed as an adjunct to orthognathic procedures in older patients
        Advancing the mandible improves throat form, and a lower border osteotomy to advance the chin tightens sagging throat tissues even more, but the orthognathic procedures alone are not sufficient to correct a “double chin” or “turkey gobbler” deformity
        This often requires a combination of removal of excessive submental fat and tightening the platysma muscle sling
        Both can be done readily at the time of the orthognathic surgery.
        Localized fat deposits superficial to the platysma can be removed with liposuction.
        Fat below the platysma requires an approach through the muscle that allows direct removal of the fat, then closure of the platysma muscle layer.
        Loose musculature in the area can be tightened as this is done
    - - Distraction osteogenesis is based on manipulation of a healing bone
        stretching an osteotomized area before calcification has occurred to generate the formation of additional bone and investing soft tissue
        Advantages
        (1) larger distances of movement are possible than with conventional orthognathic surgery and
        (2) deficient jaws can be increased in size at an earlier age.
        The Great disadvantage is that precise movements are not possible
        With distraction, the mandible or maxilla can be moved forward, but there is no way to position the jaw or teeth in exactly a preplanned place, as can be done routinely with orthognathic procedures.
        This means that patients with craniofacial syndromes are the prime candidates for distraction of the jaws.
        They are likely to need intervention at early ages and large distances of movement, and precision in establishing the posttreatment jaw relationship is not so critical for them.
      - Moderately severe hemifacial microsomia, in which a rudimentary ramus is present on the affected side, is a major indication for distraction because it is the only way to generate new bone to replace the missing part
        Distraction is not needed in the milder forms of this syndrome in which mandibular asymmetry exists but the mandible is reasonably complete (for these patients, growth modification is possible),
        and it cannot be used as the initial stage of treatment in patients so severely affected that the entire distal portion of the mandible is absent. For them, a bone graft is necessary, and distraction later can be one way to lengthen the graft.
      - The timing of treatment for patients with moderately severe hemifacial problems remains controversial, but social acceptability becomes a factor in the decision.
        To improve the child’s facial appearance, intervention to advance the mandible on the affected side often is considered at ages 6 to 8, and at that time both of its advantages make distraction a frequent choice.
        Early distraction, however, is unlikely to be followed by normal growth of the distracted area, and later orthognathic surgery or a second round of distraction probably will be required. Patients with facial syndromes that include severe maxillary deficiency (e.g., Crouzon, Apert also are candidates for early distraction.
        In these patients, appropriate bone cuts in the posterior and superior areas of the maxilla can allow advancement of the entire midface, similarly to what can be achieved with Le Fort III surgery but without the need for extensive bone grafts.
        For patients with problems of this type, the precision with which the teeth can be placed in proper occlusion simply becomes a secondary consideration.
        The fact that later orthodontic and surgical treatment will be required reinforces this attitude toward the initial treatment.
        Distractor fitted on stereolithographic models made from a computed tomography scan
        Creating new mandibular bone with distraction, as a general rule, is more effective than placing bone grafts, but distraction cannot be used to replace grafts in all circumstances.
      - For less severe maxillary or mandibular deficiency, however, distraction offers no advantage over a sagittal split or Le Fort I osteotomy, and after a period of initial enthusiasm about treatment of these patients, it is almost never used now.
        The orthognathic procedures allow the teeth and jaws to be precisely positioned, and an excellent clinical result can be anticipated. For these patients, distraction is a more difficult way to accomplish a surgical result that requires more extensive postsurgical orthodontics.
        One of the things that cannot be done with orthognathic surgery is widening the mandibular symphysis, because there is not enough soft tissue to cover a bone graft in that area.
        Distraction makes this possible and provides additional space in the incisor area. Does that make it an acceptable method for nonextraction treatment of lower incisor crowding? Usually, no.
        When crowded incisors are aligned with orthodontic expansion, this is accomplished at the expense of incisor protrusion and doubtful stability, especially if mandibular canines are expanded without also retracting them.
        The important clinical questions therefore are whether symphysis distraction provides a more stable and less protrusive result than nonextraction orthodontics, and whether either approach to expanding the mandibular dental arch gives a better result than premolar extraction to provide space for alignment.
      - With distraction at the symphysis
        not only osteogenesis (formation of new bone) but also histogenesis (formation of new soft tissue) occurs
        The formation of new periosteum over the distracted area is what makes widening of the symphysis possible.
        To relieve lip and cheek pressure against expanded mandibular canines, however, soft tissue changes would have to extend to the muscles of facial expression at the corners of the mouth.
        To date, there is no evidence that expansion with distraction is more stable than conventional expansion, and given the distance from the osteotomy site to the soft tissues at the corner of the mouth, it seems unlikely that this would be the case. As with distraction for mandibular advancement, symphysis distraction has been abandoned except for patients with what amounts to a midline cleft, with both missing incisors and the bone to support them
        Mandibular symphysis distraction to provide greater width to the anterior mandible.
        Distraction begins after a 5- to 7-day latency period, with the screw activated 2 turns (0.5 mm) twice a day
    - - Recovery From Inadvertent
        Tooth Movement
        
        Although it is not orthognathic surgery, it is part of orthodontic management of patients to include coordinated periodontal surgery to help with correction of malposed tooth roots, which can happen when teeth undergo unplanned tooth movement after archwires or fixed retainers break or are distorted.
        Induction of new bone formation now is possible with local bone-stimulating hormone therapy and bone grafts so that when displaced roots are repositioned into the alveolar process with torque force, root fenestration can be corrected as well
        This requires close coordination between the periodontist and orthodontist.
      - Repositioning Alveolar Segments
        
        Segments of the dentoalveolar process can be repositioned surgically in all three planes of space , but there are important limitations with this procedure.
        1.The principal one is the distance of movement that is possible: in most instances, only a few millimeters.
        2.A significant but less important limitation is the size of the segment:
        a three-tooth or larger segment is preferred,
        a two-tooth segment is acceptable but less predictable, and
        a one-tooth segment is a problem waiting to occur.
        The reason for both limitations is the same. After an osteotomy beneath the bone segment and teeth, the blood supply is the surprisingly good collateral circulation via the facial and lingual mucosa. This must be preserved to maintain the vitality of the teeth and the integrity of the bone. The further a segment is moved and the smaller it is, the greater the chance of interrupting not only the usual blood supply but also the collateral supply.
        An osteotomy below the root apices cuts the nerves to the pulp of the teeth in that segment, and of course there is no collateral innervation.
        The short-term result is something that dentists rarely observe: a vital but denervated pulp that does not respond to electrical stimulation.
        At that point, pulp vitality can be demonstrated by the maintenance of either normal pulp temperature (temperature probe) or blood flow (Doppler flow meter), and re-innervation of the pulp often occurs after a few months.
        Even though the major vessels to the tooth pulp are cut, less than 2% of the involved teeth require endodontic treatment. Even if the apex of a tooth is inadvertently cut off, pulp vitality is likely to be maintained by blood flow through auxiliary foramina.
        The key to surgery to reposition dentoalveolar segments is maintaining an adequate blood supply to the bone and teeth through intact labial or lingual mucosa. In the mandibular posterior area, temporarily lifting the inferior alveolar neurovascular bundle out into the cheek allows cuts to be made safely beneath the teeth. Although the nerve supply to the teeth is interrupted, sensation usually returns and endodontic treatment almost never is required
    - - The maxilla can be moved up and forward with excellent stability,
        moved down with difficulty because of instability,
        and moved back only with great difficulty because of all the structures behind it that are in the way.
        Fortunately, protruding anterior teeth can be moved back via segmental osteotomy, so there is no reason to move the posterior maxilla back. Segmental osteotomy also allows the maxilla to be widened or narrowed, but widening it tends to be unstable because of the pull of stretched palatal tissues.
        constriction that is possible is less than the possible amount of expansion.
        Anterior movement of the maxilla is similarly limited to 6 to 8 mm in most circumstances; the possibility of relapse or speech alteration from nasopharyngeal incompetence increases with larger movements.
        Posterior movement of the entire maxilla, although possible, is difficult and usually unnecessary.
        Instead, posterior movement of protruding incisors up to the width of a premolar is accomplished by removal of a premolar tooth on each side, followed by segmentation of the maxilla
        When the posterior maxilla is repositioned vertically, both the postural (rest) and occlusal positions of the mandible change.
        Orthopedic palatal expansion of the type used in adolescents is not feasible in adults because of the increasing resistance from interdigitated midpalatal and lateral maxillary sutures, although recent studies have shown that a small increase in maxillary width still can be achieved when palatal anchorage is used
      - Le Fort I osteotomy with downfracture of the maxilla dominates contemporary maxillary surgery just as the sagittal split dominates mandibular surgery
        It allows the maxilla to be moved up and/or forward with excellent stability
        Moving the entire maxilla back is quite difficult because of the structures behind it, but this is not necessary when the upper teeth are protrusive
        A segmental osteotomy, closing the space where a premolar was extracted, allows the anterior teeth to be retracted and posterior teeth to be moved superiorly so that anterior open bite is closed as the mandible rotates upward and forward
        Segmental osteotomies also allow the posterior maxilla to be widened or (less frequently) narrowed
        Expansion is done with parasagittal osteotomies in the lateral floor of the nose or medial floor of the sinus that are connected by a transverse cut anteriorly
        In a two-piece osteotomy, a midline extension runs forward between the roots of the central incisors; this may or may not be included in a three-piece osteotomy
        If constriction is desired, bone is removed at the parasagittal osteotomy sites.
        In expansion, either bone harvested in the downfracture or bank bone is used to fill the void created by lateral movement of the posterior segments
        
        Two pieces maxillary Osteotomy
        The location of lateral para-midline and anterior midline interdental osteotomies to widen the maxilla in two pieces and resection of cartilage of the nasal septum so that the maxilla can be moved up are shown in this view of the maxilla in a downfractured position during Le Fort I osteotomy.
        A major advantage of Le Fort I osteotomy over surgically assisted transverse expansion is that the maxilla can be repositioned in all three planes of space rather than just transversely.
        
        Three pieces maxillary Osteotomy
        The location of lateral para-midline and anterior interdental osteotomies for a three-piece maxilla.
        This allows widening posteriorly and differential vertical movement of the anterior and posterior segments.
      - Surgically assisted rapid palatal expansion (SARPE)
        using bone cuts to reduce the resistance followed by expansion of the jackscrew to separate the halves of the maxilla, is another possible treatment approach for adult patients with a narrow maxilla
        The original idea of surgically assisted expansion was that cuts in the lateral buttress of the maxilla would decrease resistance to the point that the midpalatal suture could be forced open (i.e., microfractured) in older patients.
        
        Although this usually works in patients in their late teens or early 20s, the chance of inadvertent fractures in other areas is a concern, especially for patients in their 30s or older.
        For SARPE now, surgeons usually make all the cuts needed for a Le Fort I osteotomy, omitting only the final step of downfracture.
        This allows widening of the maxilla against only soft tissue resistance, manipulating the osteotomy sites with what amounts to distraction osteogenesis.
        If only expansion is desired, this provides a somewhat less invasive approach than segmental osteotomy.
        Unfortunately, however, there is no guarantee of symmetric expansion—one of the main disadvantages of this procedure.
        The implication of SARPE is that the problem affects only the transverse plane of space, and this is when it is most useful.
        One of its purported advantages over segmental osteotomy was better stability, and some surgeons advocated a preliminary phase of SARPE before Le Fort I osteotomy to move the maxilla anteroposteriorly or vertically.
        Current data, however, show that relapse of the dental expansion accompanies SARPE and that its long-term stability is similar to that with segmental osteotomy.
        It is difficult, therefore, to justify the additional cost and morbidity of surgically assisted expansion as a first stage of surgical treatment in a patient who would require another operation later to reposition the maxilla in the anteroposterior or vertical planes of space.
        The primary indication for preliminary SARPE is such severe maxillary constriction that segmental expansion of the maxilla in the Le Fort I procedure might compromise the blood supply to the segments.
        Almost all relapse was dental rather than skeletal whereas skeletal changes were stable and unaffected by time after operation
    - - The mandible can be moved forward or backward, rotated, and moved down anteriorly to increase the mandibular plane and anterior face height
        But rotating it up with a ramus osteotomy to decrease the mandibular plane angle and decrease anterior face height will create instability unless the maxilla is moved up posteriorly at the same time so that this rotation does not lengthen the ramus and stretch the elevator muscles.
        The mandible can be narrowed anteriorly
        But widened only with distraction osteogenesis
        The only transverse movement easily achieved in the mandible is constriction, although limited expansion now is possible with distraction osteogenesis.
      - Anterior movements of the mandible greater than approximately 10 mm create considerable tension in the investing soft tissues and tend to result in increasing instability with greater advancement.
        The major limitation of posterior movement of the mandible is its effect on the appearance of the throat. When the mandible is moved back, the tongue moves down as the airway is maintained, and a “turkey gobbler” prominence appears below the chin.
      - In contemporary treatment, a lower border osteotomy of the mandible to reposition the chin (Genioplasty) relative to the mandibular body is a major adjunct to ramus procedures, especially when the mandible is advanced.
        It is used in about 30% of the patients who receive a ramus osteotomy and in about the same number of patients with maxillary surgery.
        The lower border procedure allows the chin to be moved transversely, forward or back, and up or down.
        The chin can be sectioned anterior to the mental foramen and repositioned in all three planes of space.
        The lingual surface remains attached to muscles in the floor of the mouth, which provide the blood supply.
        
        Moving the chin anteriorly, upward, or laterally usually produces highly favorable esthetic results.
        Moving it back or down may produce a “boxy” appearance.
        Sliding the chin upward and forward in patients with this facial pattern is a functional genioplasty in that it allows normal lip function as well as correction of the facial disproportion Functional genioplasty, and augmentation with sliding of the chin forward more generally, are more successful when done before age 15
        How early can it be done?
        The practical limit is eruption of the mandibular canines
        genioplasty at the end of active orthodontics often is the optimum time.
        This provides not only
        1.improved facial proportions but also
        2.normalization of lip function,
        3.decreasing lip separation at rest
        4.allowing the lips to be brought into contact without muscle strain.
        Notch in grower:
        When a lower border osteotomy is used to move the chin forward, remodeling along the lower border smooths the bony contour there. New bone formation above the prominence of the chin fills in the notch that is present immediately after the operation, and this extends all the way to the alveolar crest in patients who are still growing vertically
        Notch in older patients:
        With greater chin advancement in older patients, prominence of the lateral border of the chin that creates a notch along the lower border can become a problem.
        To compensate for this, there are two possibilities:
        1.splitting the chin so the posterior margins can be moved medially to eliminate the notch
        2.bone grafts or alloplastic augmentation materials over the lower border to fill in the posterior notch.
        Other mandibular procedures are used primarily for major advancements or surgery involving the condyles.
        An extraoral approach often is required, and a bone graft is likely to be needed.
        Rarely, a midline osteotomy of the mandible with removal of an incisor is used to narrow it anteriorly.