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Micrognathia
MICROGNATHIA 1 By ~OHN MARQUIS CONVERSE,M.D. From the Institute of Reconstructive Plastic Surgery, New York University Medical Center, and the Department of Plastic Surgery, Manhattan Eye, Ear and Throat Hospital, New York, N.Y.
THE term micrognathia, from its Greek derivation, signifies a small jaw; retrognathia (backward jaw) is also employed to designate mandibular retrusion. Micrognathia is the result of mandibular atresia. The term retrognathism indicates the type of dental occlusal relationships usually found in the micrognathic patient ; the teeth of the mandible are in lingual occlusion with the teeth of the maxilla. When the jaw is very small in micrognathia it is usually edentulous. Because of the characteristic appearance of the patient's face, the Germans use the term "Vogelgesicht" (bird-face). In the United States we often familiarly refer to this type of facies as the Andy Gump facies, because of its identification with a well-known cartoon personality. On the basis of ~etiology, micrognathia is classified as congenital, developmental, .or acquired. The most frequent cause of congenital micrognathia is the maldevelopment of the mandible as one aspect of the maldevelopment of the first (mandibular) and the second (hyoid) arches. A relatively frequent maldevelopment of this type is observed in the microtia syndrome. In these patients, in association with auricular malformation, an underdevelopment of the entire affected side of the skull is also observed ; the mastoid process is poorly developed ; the tympanic bone is frequently absent ; as a result the condyle of the mandible on the affected side is situated in a more posterior position. Micrognathia of the Pierre Robin type appears to be the result of intra-uterine compression ; it is the only type of micrognathia which is self-correcting during the postnatal period. Maldevelopment o f the condylar region may lead to the infant being born with temporomandibular ankylosis. Agenesis of the temporomandibular joints, a rare condition, has been .observed in generalised mandibular atresia. The developmental type of micrognathia is probably more frequent than is usually assumed. Injury by the application of forceps at birth may result in compression of the temporomandibular region with ankylosis. A fall in infancy may cause sufficient damage to the condyles to arrest growth. Many other causes than trauma may also result in interference with condylar growth, such as suppurative disease originating from the mastoid process, extending into the zygomatic cells and into the temporomandibular joint, a complication less frequent since the advent of antibiotics. Intempestive application of X-ray therapy is :another cause of arrest of condylar growth. Micrognathia is also produced by bone resection for turnout of the mandible or loss of bone from a gunshot wound, when the fragments are allowed to collapse and the anterior portion of the mandibular arch is displaced backward in relation with the maxillary arch. Mandibular micrognathia is characterised by an inadequate development in ]ength as well as in width of both the ramus and the body of the mandible. When the pathology is bilateral, generalised micrognathia is observed ; the 1 Read at a Meeting of the British Association of Plastic Surgeons, July 1962.
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jaw is uniformly underdeveloped, the chin eminence is absent, and the lower lip is retracted downward, unable to occlude with the upper lip. In unilateral micrognathia the growth disturbance affects one-half of the mandible. The entire
FIG. I Micrognathia trcated by intra-oral approach for elongation of the body of the mandible. A, Extreme micrognathia with te_mporomandibular ankylosis resulting from forceps injury a~ birth. B, Improved contour obtained by elongation osteotomy and contour restoring bone grafting through the intra-oral approach. C, Profile view of the micrognathic patient. D, Improved contour following surgery.
mandible is affected, however, for the affected side of the mandible is shorter, the chin deviated toward the shorter side, and the unaffected side of the mandible is flat and usually appears to be the deformed portion of the mandible to an uninitiated observer. When the jaw is very smaU the patient suffers a serious disfigurement ; in addition to the deformity, there is a certain amount of ridicule attached to the appearance of the patient because of the absence of normal projection of the lower part of the face (Fig. I). A serious functional disability also accompanies most cases of micrognathia when the condition is accompanied by temporomandibular
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ankylosis. The relief of the temporomandibular ankylosis does not, however, solve all of the patient's functional problems. Mastication cannot take place effectively if the mandibular teeth are in a markedly lingual position to the maxillary teeth, nor can the problem of mastication be effectively solved in edentulous patients by a denture. The re-establishment of the centric relations of the teeth of the upper and lower jaws is essential for the proper functioning of a denture, and requires the surgical advancement of the alveolar process of the mandible into suitable centric relationships with the alveolar process of the maxilla if effective use of the denture is to be made possible. Restoration of contour obtained by a skin graft inlay and a downward extension of the denture is not an entirely satisfactory method of treatment. It is a compromise which is justifiable in patients of the older age group. Restoration of contour by bone grafts or other types of transplants is another compromise procedure as the occlusal problem remains. In unilateral micrognathia contour-restoring grafts restore symmetry to the face in repose ; asymmetry reappears, however, when the mouth is opened, the chin swinging over to the short side of the mandible. The maxilla may also show abnormalities in shape, the most frequent being a protrusion of the anterior dento-alveolar segment of the maxilla, which may be explained by the inadequacy of the labial musculature, the lips being unable to occlude because of the abnormal backward position of the lower lip. It can also be assumed that the tongue, too large for the mandibular arch, tends to protrude into the anterior part of the palate. Orthodontic treatment and, occasionally, surgical recession of the anterior portion of the protruding superior dentoalveolar arch are required in these patients. TREATMENT
The first phase of treatment usually consists in the relief of the temporomandibular ankylosis, if present, by resection of sufficient bone to permit free movement of the mandible. At the completion of the temporo-mandibular operation a strip of autogenous fascia lata is packed into the cavity. The fascia is removed through a small incision over the lower portion of the iliotibial band, and a stripper with a guillotine attachment is employed to remove the fascia through this single incision. Bilateral ankylosis operation is avoided ; an interval of six weeks to two months is usually allowed between operations. By thus staging the ankylosis operations, it is not necessary to interpose cartilage or other material between the bone fragments in order to avoid shortening of the ramus and an open-bite deformity. In a few patients the lengthening of the micrognathic mandible was done prior to the relief of the temporomandibular ankylosis. It was felt that with fixation of the posterior fragment the ramus would be controlled, and that an elongation procedure on the body of the mandible could be done more satisfactorily. This advantage, however, is overweighed by the inconvenience of the ankylosed jaw, difficulty with anaesthesia, and the care of the patient during the post-operative period.
Diagnosis and Planning.--Pre-operative measures include clinical examination, the making of impressions and plaster casts of the dentition for the study of the malocclusion, and sectioning of the study casts for the purpose of
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planning the operation. Roentgenograms and cephalograms are essential tools in the pre-operative planning. The cephalograms are of considerable assistance in determining the final position of the fragments after elongation osteotomy, and also the dimensions of the contour-restoring bone grafts which may be required. The study casts of the dentition in patients who have a dentulous micrognathic mandible and the sectioning of the casts permit determining in advance the best possible occlusal relationships obtained by the surgical procedure. Tracings
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Fig. 2 . - - T h e " d r a w e r " procedure (Limberg and Ginestet). A, In a preliminary stage, a bone graft is placed in subperiosteal contact with the lower border of the micrognathic jaw. B, In a second stage the mandible is sectioned and the anterior fragment pulled forward. Bone continuity is maintained through the bone graft. Fig. 3 . - - A to D, Osteotomy anterior to the mental foramen. E to H, Osteotomy posterior to the mental foramen with preservation of the inferior alveolar neurovascular bundle.
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made over the cephalograms of both the soft tissue and the bony contour furnish an excellent opportunity for pre-operative studies in both dentulous and edentulous patients. By means of cut-outs an outline of the contour that can be obtained by the elongation osteotomy may be determined. At the completion of the pre-operative studies the site of osteotomy is decided upon. The choice of a number of sites of osteotomy of the body or the ramus of the mandible and of a variety of surgical techniques must be made. Elongation Osteotomy of the Body of the Mandible.mVarious techniques have been employed to elongate the body of the mandible, while still maintaining contact between the fragments. Step osteotomy of the body of the mandible was employed by von Eiselsberg (I9O6) to lengthen the bone and by Blair (19o7) to correct open-bite. Limberg (i928) placed a bone graft in a preliminary stage along the lower border of the mandible. In a second stage elongation osteotomy was done using the previously implanted bone graft as a means of maintaining bony continuity. Ginestet has more recently employed a similar method using a long bone graft (Fig. 2). Oblique osteotomy was employed by Blair (19o7) in the edentulous mandible
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and more recently by Kazanjian (I939). Step osteotomy is the most frequently employed operation (Fig. 3) and is successful through an intra-oral approach. The cut through the bone is made in the shape of a letter " L " which is resting on its back. The horizontal branch of the step osteotomy extends below the inferior alveolar neurovascular bundle and does not interfere with the vascularisation and innervation of the teeth; it may be lengthened when the mandible requires considerable elongation. The reverse step osteotomy is usually employed in mandibles which require the closure of an open-bite deformity as well as elongation (Fig. 3, a and B). Step osteotomy, or simple linear osteotomy, must be combined with bone grafting to fill the defect when the mandible is very small and requires considerable lengthening. Sites of Osteotomy.~The position of the line of osteotomy through the body of the mandible depends upon the availability of space in the dental arch ; usually gaps exist in the dental arch from loss of teeth through caries and extraction. FIG. 4 Should the micrognathic mandible be fully dentulous, a tooth T h e retromolar step osteotomy (see text). should be extracted in a preliminary step to make room for the line of osteotomy; usually a premolar tooth is extracted for this purpose. In the retromolar osteotomy preliminary extraction of a tooth is not necessary (Fig. 4). When the vertical branch of the osteotomy extends through the portion of the mandible situated anterior to the mental foramen, only the anterior-inferior alveolar neurovascular bundle is sectioned (see Fig. 3, A to D). The loss of function of this portion of the nerve is relatively unimportant as it supplies the innervation to the anterior teeth. Severance of the main trunk of the inferior alveolar nerve which supplies the sensory innervation of the lower lip is more serious. Some surgeons have stated that the inferior alveolar nerve can be severed with impunity as it always regenerates. This fact has not been confirmed by clinical observation. Considerable overlap in the sensory innervation occurs in the insensitive lower lip from adjacent sensory nerves, but the lower lip rarely recovers full sensation through this process. It is for this reason that preservation of the continuity of the inferior alveolar bundle has been recommended. New and Erich (I94I) first emphasised the need for preservation of the continuity of the inferior alveolar nerve in osteotomy of the body of the mandible. Dingman (1944) developed a two-stage operation for the correction of prognathism ; he performed a first operation removing a block of bone from the portion of the mandible situated above the inferior alveolar nerve. Having thus located the nerve, in a second stage he resected the bone from the inferior portion of the body of the mandible below the nerve, thus preserving the continuity of the nerve. Converse and Shapiro (1952) described a one-stage technique of step osteotomy with preservation of the neurovascular bundle done through the intra-oral approach. The essential landmark for the location of the inferior alveolar bundle is the mental foramen. I f the outer cortex of the mandible is removed posteriorly to the mental foramen the nerve which is superficial in this area is readily exposed. The outer table of the inferior alveolar canal is removed as far back as necessary to insure that the nerve is visible during the osteotomy. Elongation of the inferior alveolar bundle is no problem, and lengthening of 2 to 3 cm. has been done without
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apparent excessive tension of the nerve. The neurovascular bundle may also be lengthened at the expense of the mental nerve and vessels. In a few cases it has been found expedient to sever the mental nerve as far from the mental foramen as possible in order to obtain better exposure of the mandible; the ends of the nerve are sutured with one fine silk suture at the end of the operation and regeneration has taken place. The retromolar step osteotomy has the merit of avoiding extraction of a tooth, but it is more laborious to perform (Fig. 4). The inferior alveolar nerve is situated deeper in the bone because it extends obliquely toward the medial table of the bone to exit at the internal alveolar foramen; a thick layer of bone must, therefore, be removed before the nerve is exposed. The site of osteotomy is usually approached through a skin incision as the intra-oral approach is difficult. One serious inconvenience of the retromolar osteotomy is the difficulty in fixation of the edentulous posterior ramus fragment ; in employing this technique it is advantageous to do the step osteotomy prior to the relief of the temporomandibular ankylosis in order to benefit from the fixation of the ankylosed posterior fragments. FIG. 5 The Extra-oral versus the Intra-oral Elongation of micrognathic mandible by Approach.--An external submandibular inbone grafting. A, Illustrates bone graft cision placed in a natural skin fold leaves an placed in defect after osteotomy and inconspicuous scar. Precautions should be forward displacement of the anterior segment. B, Prior to osteotomy a flap taken to avoid severance of the marginal of mucoperiosteum and buccal mucosa mandibular branch of the facial nerve, and is raised from over the mandible, the snlcus, and the cheek wall. C, After the landmarks described by Dingman and placement of the bone graft it is covered Grabb 0962) should be observed. by the flap. T h e sulcus is later restored The intra-oral approach has the merit by a skin graft inlay. of giving excellent exposure without the inconvenience of the external scar. One problem that may occur, however, is the possibility of deficiency of soft tissue covering when considerable lengthening of the mandible is required. This inconvenience also occurs when the mandible is approached through an external incision. A large horseshoe-shaped flap extending laterally on to the cheek wall will prevent intra-oral exposure of the site of the osteotomy. When larger gaps are present it is possible to reverse the design of the flap, placing the base of the pedicle on the cheek wall. All of the mucous membrane situated in the vestibule is raised from the bone, and at the completion of the elongation procedure the flap is sutured to the mucous membrane on the medial aspect of the alveolar process (Fig. 5). The flap obliterates the buccal sulcus and requires, after consolidation of the bone, that a skin graft inlay be done to restore the depth of the sulcus (Fig. 6). Fixation of Fragments.--In order to obtain the most satisfactory dental occlusal relationships it is necessary to make a careful preliminary study of the position which will be assumed by the mobilised fragments. Anterior displacement of the
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posterior fragments and a post-operative recurrence of the retrognathism are avoided by fixation of the posterior fragments. If teeth are present this is no problem as intermaxillary fixation with the maxillary teeth can be obtained. When the posterior fragment is edentulous, a careful pre-operative calculation made
Fie. 6 Micrognathia resulting from radiation of the left side of the mandible in childhood. A, Appearance of patient with micrognathia with marked shortening of the left half of the mandibular body. B, Post-operative appearance following bilateral osteotomy and bone grafting of the body of the mandible through the intra-oral approach. C, Pre-operative profile view. D, Post-operative view showing elongation of the body of the mandible.
from the sectioned dental cast will provide a useful guide ; the fragments must be wired by direct interosseous fixation by means of stainless-steel wire ligature. Interosseous wiring by stainless steel is remarkably well tolerated, even in the intra-oral approach technique. As a general principle, these wires should be avoided unless they are essential. Accurately made arch and band appliances provide good fixation in patients with teeth. An excellent method is the circumferential wire which may be removed after consolidation of the bony fragments. A problem that arises following elongation osteotomy of the body of the mandible is the backward tilt of the symphysis of the mandible produced by the pull of the suprahyoid musculature. Three measures may be taken to avoid this
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complication. The first is the subperiosteal disinsertion of these muscles, which temporarily releases the traction exerted by them on the bones. The second is the employment of traction on a circumferential wire placed around the symphysis ; the wires twisted together at the lower border of the symphysis are brought out through the skin under the fat pad of the chin. A forward and upward traction is then exerted by an external traction apparatus with cranial fixation assured by a plaster headcap. The third method, which is often adequate and which may permit dispensing with the external traction apparatus, is a circumferential wire around the symphysis which is looped over the nasal spine.
Elongation Osteotomy through the Ramus.--Operations through the ramus have been done since Blair (1915) first did this procedure, which was popularised by Kostecka (1931); these operations were done for the correction of prognathism. They consisted in a horizontal section of the ramus above the entrance of the inferior alveolar nerve into the internal alveolar foramen. This operation had two inconveniences : (I) The loss of contact or inadequate contact of the bony fragments because of the pull exerted by the external pterygoid muscle displacing the upper fragment medially, and (2) loss of the vertical dimension of the ramus through the pull exerted by the masseter and internal pterygoid muscle causing an overlap between the fragments. In order to prevent medial displacement of the upper fragment, Kazanjian (1954) did an oblique section with an osteotome through the external approach thus increasing the surface contact between the fragments. More recently the surface of contact between the fragments has been greatly increased by step osteotomy of the ramus by the technique of Trauner and Obwegeser (I957), Schuchardt (I958), and Dal Pont (1959). This operation is done through the intra-oral approach. Because of the increased surface of bony contact between the fragments obtained by this technique, it can be considered a satisfactory technique for advancing the body of the mandible in cases of moderate micrognathia. Vertical Section of the Rarnus.--A major departure in the approach to ramus surgery was made by Caldwell and Letterman (I954), who described an operation for prognathism in which they split the ramus vertically into two fragments through an external approach, producing a wide overlap of one fragment over the other and thus a wide surface of contact (Fig. 7)- Whereas the previously described operations section the ramus either horizontally across or split the ramus in the sagittal plane, this type of vertical osteotomy sections the ramus in the frontal plane. The advantage of the technique is that the fragments are not influenced by the vertical pull of the masseter and internal pterygoid muscles, which tends to cause an overlap of the fragments in all operations cutting through the ramus horizontally, and the danger of an open-bite resulting from shortening of the ramus length. Robinson (1957) and Hinds (1958) have recently employed an oblique osteotomy of the ramus for the correction of prognathism producing an overlap of the posterior fragment over and lateral to the anterior fragment. Oblique vertical section of the ramus was described by Limberg (1925) in the treatment of micrognathia with open-bite (Fig. 8), and he later (1928) proposed the addition of a costal bone graft in the treatment of micrognathia, a method employed by Thoma (1961). Robinson (1957) employed an iliac bone graft, and Caldwell and Amaral (196o) also combined an iliac bone graft with vertical section of the ramus.
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Wassmund (1927) described a vertical section with a forward angulation above the lingula extending through the base of the coronoid process. Schuchardt (1958) has utilised this type of osteotomy and inserted a bone graft for the treatment of micrognathia (Fig. 9). I have performed the vertical section of the ramus with a nasal saw or a straight blade mounted on the Stryker oscillating saw. The gap between the fragments, after advancement of the mandible, is filled with an iliac bone graft or split-rib grafts pc (in children ; after Longacre, 1955) wedged ,¢ ~j 1., between the fragments (Fig. IO). This technique
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Fig. 7.--Vertical osteotomy for the correction of prognathism (after Caldwell and Letterman, 1954). A, Outline of prognathic mandible. B, T h e outer cortex of the ramus is perforated by numerous drill holes. Vertical section is done extending from the sigmoid notch to a point anterior to the mandibular angle. C, T h e outer cortex of the anterior fragment is removed with an osteotome. T h e base of the coronoid process is cut through. D, T h e posterior fragment overlaps the anterior fragment and is maintained by direct wire fixation after intermaxillary fixation of the teeth has been established. Fig. 8.--Limberg's vertical osteotomy of the ramus for the correction of micrognathia with open-bite.
is too new to evaluate in terms of results ; it appears to have excellent prospects for the future. Elongation by Horizontal Section o f the Ramus and Bone G r a f t . I T h e ramus is approached through an external incision placed in a natural skin fold in the submandibular region. In addition, a second incision may be placed in the pre-auricular area to give added exposure. The ramus is exposed by wide subperiosteal elevation of the masseter and internal pterygoid muscles. If the separation of the fragments is to be successfully achieved, it is essential that the muscles be entirely separated from the bone. A horizontal cut is then made above the internal alveolar foramen and the lower fragment is separated from the upper fragment. A block of iliac bone is then wedged into the interval between the bone fragments. It is essential to maintain the downward position of the lower fragment by means of a bite block placed in the molar region. I did an operation of this type in collaboration with Rushton in 1941 ; successful follow-up of this patient was reported by Rushton (1942, 1944) and by Gillies and Millard (1957).
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Treatment of Unilateral Micrognathia.mAs previously stated, it is often necessary in unilateral micrognathia to perform a bilateral osteotomy in order to advance the unaffected side as well as the grossly shortened affected side (see Fig. 6). Even in cases where the unaffected side appears to be only slightly deformed, the elongation of the shortened side imposes a movement of rotation of the condyle on the unaffected side, which may disturb the function of the joint. In order to avoid this inconvenience a vertical osteotomy of the unaffected ramus permits bending the mandibular arch without disturbing the position of the condyle.
FIG. 9 Schuchardt's curved osteotomy of the ramus for micrognathia extending behind and above the inferior alveolar canal. Interposition of an iliac bone graft restores the continuity of the sectioned ramus,
FIG. IO Vertical osteotomy of the ramus for micrognathia with interposition of split rib bone grafts.
Mierogenia and Mierognathia.--Minor degrees of micrognathia are frequently observed and are usually referred to as microgenia (small chin). These patients have a relatively small mandible, an underdeveloped chin, and characteristic dental occlusal relationships ; the lower incisor teeth are situated lingually to the upper incisor teeth which thus assume a marked position of "over-bite." This type of dental occlusion further increases the deformity by diminishing the height of the lower third of the face. In patients with moderate micrognathia the following procedures can be employed : (I) A bone graft or other implant material over the symphysis done routinely through the intra-oral approach (Converse, 195o) (Figs. i i and 12); (2) horizontal osteotomy of the body of the mandible (Obwegeser, 1957) (Fig. 13) ; and (3) the Babcock (1937) operation has recently been employed by Trauner and Obwegeser (1957) consisting in the implantation of cartilage in the temporomandibular joint posterior to the condyle. This procedure forces the mandible forward and changes the occlusal relationships of the teeth from a marked over-bite to a more satisfactory type of occlusion. In moderate types of micrognathia it may be possible to advance the body of the mandible by the step osteotomy of the ramus through the intra-oral approach.
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When protrusion of the superior-anterior dento-alveolar segment is present, correction by orthodontic and/or surgical means completes the rehabilitation of the micrognathic or microgenic patient. An excellent technique is the advancement of the lower portion of the body of the mandible following a horizontal osteotomy done along a horizontal plane below the inferior alveolar
~ac,~q from ce/o~alojrum FIG.
ii
Cephalometric planning for contour restoration in microgenia. A, Tracing of soft tissue and bone contour made on transparent acetate. B, The angle between two planes (nasion to sella turcica ; nasion to menton) should be 72 degrees. These measurements are of assistance but not indispensable as demonstrated in C to D. C, Modified profile line established. D , The distance between the original profile line and the new profile establishes the thickness and shape of the contour-restoring bone graft. E, Tracing of cephalogram taken after implantation of the bone graft.
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canal (Fig. I3). The mandible is exposed by the " degloving " technique through the intra-oral approach (Fig. i4). The fragment is advanced to provide an adequate chin projection. An added improvement is the placing of bone grafts in the gap left posteriorly by the shifting forward of the sectioned bone (Fig. I3). Fixation is obtained by a circumferential wire on each side, which is removed after consolidation. The problem of establishing a chin prominence in the microgenic patient is a more complicated one in the patient with tight covering soft tissues than in the patient in whom the soft tissues are lax. Fortunately in most of the congenital and developmental types of micrognathia the soft tissues appear to be adequate. In patients in whom the tissues are deficient or whose tissues have been scarred, i.e., by a burn, the pressure exerted by the tense soft tissue tends to produce a diminution in size of the onlay bone graft placed over the mandible. In these cases three solutions can be offered. The first is to employ the sliding horizontal osteotomy technique advancing the lower border of the bone; this type of bone transplant has a greater chance of maintaining its bulk than a free graft. The
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second method is to place the bone graft over the symphysis area prior to the horizontal elongation osteotomy. Once the bone graft is well established its chances of resisting the overlying soft tissue pressure after elongation osteotomy
FIG. I2
Contour restoration by bone grafting through the intra-oral approach in microgenia. A, Pre-operative appearance of microgenic patient. B, Post-operative contour obtained by bone grafting.
FIG. 13
Horizontal osteotomy of the mandibular body with advancement of the symphysis for micrognathia with minimal malocclusion and as an additional contour-restoring procedure following elongation step osteotomy of the mandible.
are greater than the fresh bone autograft. A third technique which has been advocated is the use of synthetic implant materials (Brown et all., I96o) which appear to be well tolerated in the region of the symphysis ; as with all foreign implants the tolerance of the host to these materials awaits the test of time. CONCLUSIONS
Many techniques are available for the correction of micrognathia. The choice between these techniques appears to depend upon the anatomy of the malformation. In some patients elongation of the body of the mandible is the method of choice ;
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in others vertical section of the ramus with interposed bone grafts is preferable; it is possible that in some cases both procedures may be employed, the patient undergoing, in two separate stages, a section of the ramus with a bone graft to fill the: interval between the sectioned ramus fragments, followed by a section of the body and a bone graft over the mandibular symphysis. A major problem which remains controversial is that of deciding at what age such corrective
A
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FIG. 14 Horizontal osteotomy of the mandible. A, Intra-oral exposure obtained by the " degloving" technique. B, Horizontal osteotomy done by means of the Stryker oscillating saw. C, Fragment being moved forward into position of adequate projection.
procedures should be done in the developmental type of micrognathia. There is a growing trend toward operation in early childhood. Further study and long-term follow-up of these patients will undoubtedly provide the answer. SUMMARY Micrognathia is of three types, congenital, developmental, and acquired, and in its severe form is not only a disfigurement but also a functional handicap to the patient. Surgical treatment is necessary to restore facial form and the centric relationships of the dento-alveolar processes in order to re-establish the masticatory function. Corrective and reconstructive surgical procedures include selective sectioning of the body of the mandible or the ramus for the purpose of elongating the bone, and bone grafting for restoration of adequate size and contour of the mandible. Most of these procedures can be done through the intra-oral approach.
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REFERENCES BABCOCK,W. W. (1937). Ann. Surg., xo5, 11o5. BLAIR, V. P. (19o7). Surg. Gynec. Obstet., 4, 67. -(1915). lnt. ff. Orthod., i, 395. BROWN, J. B., OI~LWlLER,D. A., and FRYER, M.P. (196o). Ann. Surg., 152, 534. CALl)WELL, J. B., and AMARAL,W. J. (196o). ft. oral Surg., I8, 3. CALl)WELL, J. B., and LETTERMAN,G. S. (1954)- ft. oral Surg., 12, 185. CONVERSE, J. M. (195o). Plast. reconstr. Surg., 6, 295. CONVERSE,J. M., and SHAPIRO, H. H. (1952). Plast. reconstr. Surg., IO, 473. DAL PONT, G. (1959). Minerva chit., 14, 1138. DINGMAN, R. O. (1944). Amer. ft. Orthodont., 30, 683. DINGMAN, R. O., and GRABB, W. C. (1962). Plast. reconstr. Surg., 29, 266. EISELSB~RG, F. YON (1906). Wien. klin. lVschr., 50, 75o5. GILLIES, H. D., and MILLAm), D. R. (1957). " Principles and Art of Plastic Surgery," p. 312. Boston : Little Brown & Co. HINDS, E. C. (1958)..7. oral Surg., 16, 2o 9. KAZANJIAN,V. H. (1939). Amer. ft. Surg., 43, 249. - - " (1954)- Amer. ft. Surg., 87, 691. KOSTECKA, F. (1931). Zahndrzd. Rsch., 4o, 67o. LIMBERG, A. A. (1925). Dent. Cosmos, 67, 1191. (1928). `7. Amer. dent. Ass., I5, 851. NEw, G. B., and ERICH, J. B. (1941). Amer. ft. Surg., 53, 2. OBWEGESER,H., and TRAUNER,R. (1957). Oral Surg., IO, 677. ROBINSON, M. (1957). Oral Surg., 1o, 1125. RUSHTON, M. (1942). Dent. Rec., 62, 272. -(1944). Brit. dent. `7., 76, 58. SCrlUCHAm)T, K. (1958). Arch. klin. Chit., 289, 651. THOMA, K. H. (1961). Oral Surg., I4, 23. TRAUNER~R., and OBW~GESER,H. (1957). Oral Surg., IO, 677. WASSMUNI), M. (1927). Fracturen und Luxationen des Gesichtsschadels unter Berucksichtigung der Komplikationen des Hirnschadels. In Klinik und Therapie. Praktisches Lehrbuch., 20, 384. Berlin : H. Mensser. Submitted for publication, November 1962.
THE term micrognathia, from its Greek derivation, signifies a small jaw; retrognathia (backward jaw) is also employed to designate mandibular retrusion. Micrognathia is the result of mandibular atresia. The term retrognathism indicates the type of dental occlusal relationships usually found in the micrognathic patient ; the teeth of the mandible are in lingual occlusion with the teeth of the maxilla. When the jaw is very small in micrognathia it is usually edentulous. Because of the characteristic appearance of the patient's face, the Germans use the term "Vogelgesicht" (bird-face). In the United States we often familiarly refer to this type of facies as the Andy Gump facies, because of its identification with a well-known cartoon personality. On the basis of ~etiology, micrognathia is classified as congenital, developmental, .or acquired. The most frequent cause of congenital micrognathia is the maldevelopment of the mandible as one aspect of the maldevelopment of the first (mandibular) and the second (hyoid) arches. A relatively frequent maldevelopment of this type is observed in the microtia syndrome. In these patients, in association with auricular malformation, an underdevelopment of the entire affected side of the skull is also observed ; the mastoid process is poorly developed ; the tympanic bone is frequently absent ; as a result the condyle of the mandible on the affected side is situated in a more posterior position. Micrognathia of the Pierre Robin type appears to be the result of intra-uterine compression ; it is the only type of micrognathia which is self-correcting during the postnatal period. Maldevelopment o f the condylar region may lead to the infant being born with temporomandibular ankylosis. Agenesis of the temporomandibular joints, a rare condition, has been .observed in generalised mandibular atresia. The developmental type of micrognathia is probably more frequent than is usually assumed. Injury by the application of forceps at birth may result in compression of the temporomandibular region with ankylosis. A fall in infancy may cause sufficient damage to the condyles to arrest growth. Many other causes than trauma may also result in interference with condylar growth, such as suppurative disease originating from the mastoid process, extending into the zygomatic cells and into the temporomandibular joint, a complication less frequent since the advent of antibiotics. Intempestive application of X-ray therapy is :another cause of arrest of condylar growth. Micrognathia is also produced by bone resection for turnout of the mandible or loss of bone from a gunshot wound, when the fragments are allowed to collapse and the anterior portion of the mandibular arch is displaced backward in relation with the maxillary arch. Mandibular micrognathia is characterised by an inadequate development in ]ength as well as in width of both the ramus and the body of the mandible. When the pathology is bilateral, generalised micrognathia is observed ; the 1 Read at a Meeting of the British Association of Plastic Surgeons, July 1962.
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jaw is uniformly underdeveloped, the chin eminence is absent, and the lower lip is retracted downward, unable to occlude with the upper lip. In unilateral micrognathia the growth disturbance affects one-half of the mandible. The entire
FIG. I Micrognathia trcated by intra-oral approach for elongation of the body of the mandible. A, Extreme micrognathia with te_mporomandibular ankylosis resulting from forceps injury a~ birth. B, Improved contour obtained by elongation osteotomy and contour restoring bone grafting through the intra-oral approach. C, Profile view of the micrognathic patient. D, Improved contour following surgery.
mandible is affected, however, for the affected side of the mandible is shorter, the chin deviated toward the shorter side, and the unaffected side of the mandible is flat and usually appears to be the deformed portion of the mandible to an uninitiated observer. When the jaw is very smaU the patient suffers a serious disfigurement ; in addition to the deformity, there is a certain amount of ridicule attached to the appearance of the patient because of the absence of normal projection of the lower part of the face (Fig. I). A serious functional disability also accompanies most cases of micrognathia when the condition is accompanied by temporomandibular
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ankylosis. The relief of the temporomandibular ankylosis does not, however, solve all of the patient's functional problems. Mastication cannot take place effectively if the mandibular teeth are in a markedly lingual position to the maxillary teeth, nor can the problem of mastication be effectively solved in edentulous patients by a denture. The re-establishment of the centric relations of the teeth of the upper and lower jaws is essential for the proper functioning of a denture, and requires the surgical advancement of the alveolar process of the mandible into suitable centric relationships with the alveolar process of the maxilla if effective use of the denture is to be made possible. Restoration of contour obtained by a skin graft inlay and a downward extension of the denture is not an entirely satisfactory method of treatment. It is a compromise which is justifiable in patients of the older age group. Restoration of contour by bone grafts or other types of transplants is another compromise procedure as the occlusal problem remains. In unilateral micrognathia contour-restoring grafts restore symmetry to the face in repose ; asymmetry reappears, however, when the mouth is opened, the chin swinging over to the short side of the mandible. The maxilla may also show abnormalities in shape, the most frequent being a protrusion of the anterior dento-alveolar segment of the maxilla, which may be explained by the inadequacy of the labial musculature, the lips being unable to occlude because of the abnormal backward position of the lower lip. It can also be assumed that the tongue, too large for the mandibular arch, tends to protrude into the anterior part of the palate. Orthodontic treatment and, occasionally, surgical recession of the anterior portion of the protruding superior dentoalveolar arch are required in these patients. TREATMENT
The first phase of treatment usually consists in the relief of the temporomandibular ankylosis, if present, by resection of sufficient bone to permit free movement of the mandible. At the completion of the temporo-mandibular operation a strip of autogenous fascia lata is packed into the cavity. The fascia is removed through a small incision over the lower portion of the iliotibial band, and a stripper with a guillotine attachment is employed to remove the fascia through this single incision. Bilateral ankylosis operation is avoided ; an interval of six weeks to two months is usually allowed between operations. By thus staging the ankylosis operations, it is not necessary to interpose cartilage or other material between the bone fragments in order to avoid shortening of the ramus and an open-bite deformity. In a few patients the lengthening of the micrognathic mandible was done prior to the relief of the temporomandibular ankylosis. It was felt that with fixation of the posterior fragment the ramus would be controlled, and that an elongation procedure on the body of the mandible could be done more satisfactorily. This advantage, however, is overweighed by the inconvenience of the ankylosed jaw, difficulty with anaesthesia, and the care of the patient during the post-operative period.
Diagnosis and Planning.--Pre-operative measures include clinical examination, the making of impressions and plaster casts of the dentition for the study of the malocclusion, and sectioning of the study casts for the purpose of
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planning the operation. Roentgenograms and cephalograms are essential tools in the pre-operative planning. The cephalograms are of considerable assistance in determining the final position of the fragments after elongation osteotomy, and also the dimensions of the contour-restoring bone grafts which may be required. The study casts of the dentition in patients who have a dentulous micrognathic mandible and the sectioning of the casts permit determining in advance the best possible occlusal relationships obtained by the surgical procedure. Tracings
i,t
,
'
B
FIG. 2
Fig. 2 . - - T h e " d r a w e r " procedure (Limberg and Ginestet). A, In a preliminary stage, a bone graft is placed in subperiosteal contact with the lower border of the micrognathic jaw. B, In a second stage the mandible is sectioned and the anterior fragment pulled forward. Bone continuity is maintained through the bone graft. Fig. 3 . - - A to D, Osteotomy anterior to the mental foramen. E to H, Osteotomy posterior to the mental foramen with preservation of the inferior alveolar neurovascular bundle.
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made over the cephalograms of both the soft tissue and the bony contour furnish an excellent opportunity for pre-operative studies in both dentulous and edentulous patients. By means of cut-outs an outline of the contour that can be obtained by the elongation osteotomy may be determined. At the completion of the pre-operative studies the site of osteotomy is decided upon. The choice of a number of sites of osteotomy of the body or the ramus of the mandible and of a variety of surgical techniques must be made. Elongation Osteotomy of the Body of the Mandible.mVarious techniques have been employed to elongate the body of the mandible, while still maintaining contact between the fragments. Step osteotomy of the body of the mandible was employed by von Eiselsberg (I9O6) to lengthen the bone and by Blair (19o7) to correct open-bite. Limberg (i928) placed a bone graft in a preliminary stage along the lower border of the mandible. In a second stage elongation osteotomy was done using the previously implanted bone graft as a means of maintaining bony continuity. Ginestet has more recently employed a similar method using a long bone graft (Fig. 2). Oblique osteotomy was employed by Blair (19o7) in the edentulous mandible
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and more recently by Kazanjian (I939). Step osteotomy is the most frequently employed operation (Fig. 3) and is successful through an intra-oral approach. The cut through the bone is made in the shape of a letter " L " which is resting on its back. The horizontal branch of the step osteotomy extends below the inferior alveolar neurovascular bundle and does not interfere with the vascularisation and innervation of the teeth; it may be lengthened when the mandible requires considerable elongation. The reverse step osteotomy is usually employed in mandibles which require the closure of an open-bite deformity as well as elongation (Fig. 3, a and B). Step osteotomy, or simple linear osteotomy, must be combined with bone grafting to fill the defect when the mandible is very small and requires considerable lengthening. Sites of Osteotomy.~The position of the line of osteotomy through the body of the mandible depends upon the availability of space in the dental arch ; usually gaps exist in the dental arch from loss of teeth through caries and extraction. FIG. 4 Should the micrognathic mandible be fully dentulous, a tooth T h e retromolar step osteotomy (see text). should be extracted in a preliminary step to make room for the line of osteotomy; usually a premolar tooth is extracted for this purpose. In the retromolar osteotomy preliminary extraction of a tooth is not necessary (Fig. 4). When the vertical branch of the osteotomy extends through the portion of the mandible situated anterior to the mental foramen, only the anterior-inferior alveolar neurovascular bundle is sectioned (see Fig. 3, A to D). The loss of function of this portion of the nerve is relatively unimportant as it supplies the innervation to the anterior teeth. Severance of the main trunk of the inferior alveolar nerve which supplies the sensory innervation of the lower lip is more serious. Some surgeons have stated that the inferior alveolar nerve can be severed with impunity as it always regenerates. This fact has not been confirmed by clinical observation. Considerable overlap in the sensory innervation occurs in the insensitive lower lip from adjacent sensory nerves, but the lower lip rarely recovers full sensation through this process. It is for this reason that preservation of the continuity of the inferior alveolar bundle has been recommended. New and Erich (I94I) first emphasised the need for preservation of the continuity of the inferior alveolar nerve in osteotomy of the body of the mandible. Dingman (1944) developed a two-stage operation for the correction of prognathism ; he performed a first operation removing a block of bone from the portion of the mandible situated above the inferior alveolar nerve. Having thus located the nerve, in a second stage he resected the bone from the inferior portion of the body of the mandible below the nerve, thus preserving the continuity of the nerve. Converse and Shapiro (1952) described a one-stage technique of step osteotomy with preservation of the neurovascular bundle done through the intra-oral approach. The essential landmark for the location of the inferior alveolar bundle is the mental foramen. I f the outer cortex of the mandible is removed posteriorly to the mental foramen the nerve which is superficial in this area is readily exposed. The outer table of the inferior alveolar canal is removed as far back as necessary to insure that the nerve is visible during the osteotomy. Elongation of the inferior alveolar bundle is no problem, and lengthening of 2 to 3 cm. has been done without
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apparent excessive tension of the nerve. The neurovascular bundle may also be lengthened at the expense of the mental nerve and vessels. In a few cases it has been found expedient to sever the mental nerve as far from the mental foramen as possible in order to obtain better exposure of the mandible; the ends of the nerve are sutured with one fine silk suture at the end of the operation and regeneration has taken place. The retromolar step osteotomy has the merit of avoiding extraction of a tooth, but it is more laborious to perform (Fig. 4). The inferior alveolar nerve is situated deeper in the bone because it extends obliquely toward the medial table of the bone to exit at the internal alveolar foramen; a thick layer of bone must, therefore, be removed before the nerve is exposed. The site of osteotomy is usually approached through a skin incision as the intra-oral approach is difficult. One serious inconvenience of the retromolar osteotomy is the difficulty in fixation of the edentulous posterior ramus fragment ; in employing this technique it is advantageous to do the step osteotomy prior to the relief of the temporomandibular ankylosis in order to benefit from the fixation of the ankylosed posterior fragments. FIG. 5 The Extra-oral versus the Intra-oral Elongation of micrognathic mandible by Approach.--An external submandibular inbone grafting. A, Illustrates bone graft cision placed in a natural skin fold leaves an placed in defect after osteotomy and inconspicuous scar. Precautions should be forward displacement of the anterior segment. B, Prior to osteotomy a flap taken to avoid severance of the marginal of mucoperiosteum and buccal mucosa mandibular branch of the facial nerve, and is raised from over the mandible, the snlcus, and the cheek wall. C, After the landmarks described by Dingman and placement of the bone graft it is covered Grabb 0962) should be observed. by the flap. T h e sulcus is later restored The intra-oral approach has the merit by a skin graft inlay. of giving excellent exposure without the inconvenience of the external scar. One problem that may occur, however, is the possibility of deficiency of soft tissue covering when considerable lengthening of the mandible is required. This inconvenience also occurs when the mandible is approached through an external incision. A large horseshoe-shaped flap extending laterally on to the cheek wall will prevent intra-oral exposure of the site of the osteotomy. When larger gaps are present it is possible to reverse the design of the flap, placing the base of the pedicle on the cheek wall. All of the mucous membrane situated in the vestibule is raised from the bone, and at the completion of the elongation procedure the flap is sutured to the mucous membrane on the medial aspect of the alveolar process (Fig. 5). The flap obliterates the buccal sulcus and requires, after consolidation of the bone, that a skin graft inlay be done to restore the depth of the sulcus (Fig. 6). Fixation of Fragments.--In order to obtain the most satisfactory dental occlusal relationships it is necessary to make a careful preliminary study of the position which will be assumed by the mobilised fragments. Anterior displacement of the
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posterior fragments and a post-operative recurrence of the retrognathism are avoided by fixation of the posterior fragments. If teeth are present this is no problem as intermaxillary fixation with the maxillary teeth can be obtained. When the posterior fragment is edentulous, a careful pre-operative calculation made
Fie. 6 Micrognathia resulting from radiation of the left side of the mandible in childhood. A, Appearance of patient with micrognathia with marked shortening of the left half of the mandibular body. B, Post-operative appearance following bilateral osteotomy and bone grafting of the body of the mandible through the intra-oral approach. C, Pre-operative profile view. D, Post-operative view showing elongation of the body of the mandible.
from the sectioned dental cast will provide a useful guide ; the fragments must be wired by direct interosseous fixation by means of stainless-steel wire ligature. Interosseous wiring by stainless steel is remarkably well tolerated, even in the intra-oral approach technique. As a general principle, these wires should be avoided unless they are essential. Accurately made arch and band appliances provide good fixation in patients with teeth. An excellent method is the circumferential wire which may be removed after consolidation of the bony fragments. A problem that arises following elongation osteotomy of the body of the mandible is the backward tilt of the symphysis of the mandible produced by the pull of the suprahyoid musculature. Three measures may be taken to avoid this
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complication. The first is the subperiosteal disinsertion of these muscles, which temporarily releases the traction exerted by them on the bones. The second is the employment of traction on a circumferential wire placed around the symphysis ; the wires twisted together at the lower border of the symphysis are brought out through the skin under the fat pad of the chin. A forward and upward traction is then exerted by an external traction apparatus with cranial fixation assured by a plaster headcap. The third method, which is often adequate and which may permit dispensing with the external traction apparatus, is a circumferential wire around the symphysis which is looped over the nasal spine.
Elongation Osteotomy through the Ramus.--Operations through the ramus have been done since Blair (1915) first did this procedure, which was popularised by Kostecka (1931); these operations were done for the correction of prognathism. They consisted in a horizontal section of the ramus above the entrance of the inferior alveolar nerve into the internal alveolar foramen. This operation had two inconveniences : (I) The loss of contact or inadequate contact of the bony fragments because of the pull exerted by the external pterygoid muscle displacing the upper fragment medially, and (2) loss of the vertical dimension of the ramus through the pull exerted by the masseter and internal pterygoid muscle causing an overlap between the fragments. In order to prevent medial displacement of the upper fragment, Kazanjian (1954) did an oblique section with an osteotome through the external approach thus increasing the surface contact between the fragments. More recently the surface of contact between the fragments has been greatly increased by step osteotomy of the ramus by the technique of Trauner and Obwegeser (I957), Schuchardt (I958), and Dal Pont (1959). This operation is done through the intra-oral approach. Because of the increased surface of bony contact between the fragments obtained by this technique, it can be considered a satisfactory technique for advancing the body of the mandible in cases of moderate micrognathia. Vertical Section of the Rarnus.--A major departure in the approach to ramus surgery was made by Caldwell and Letterman (I954), who described an operation for prognathism in which they split the ramus vertically into two fragments through an external approach, producing a wide overlap of one fragment over the other and thus a wide surface of contact (Fig. 7)- Whereas the previously described operations section the ramus either horizontally across or split the ramus in the sagittal plane, this type of vertical osteotomy sections the ramus in the frontal plane. The advantage of the technique is that the fragments are not influenced by the vertical pull of the masseter and internal pterygoid muscles, which tends to cause an overlap of the fragments in all operations cutting through the ramus horizontally, and the danger of an open-bite resulting from shortening of the ramus length. Robinson (1957) and Hinds (1958) have recently employed an oblique osteotomy of the ramus for the correction of prognathism producing an overlap of the posterior fragment over and lateral to the anterior fragment. Oblique vertical section of the ramus was described by Limberg (1925) in the treatment of micrognathia with open-bite (Fig. 8), and he later (1928) proposed the addition of a costal bone graft in the treatment of micrognathia, a method employed by Thoma (1961). Robinson (1957) employed an iliac bone graft, and Caldwell and Amaral (196o) also combined an iliac bone graft with vertical section of the ramus.
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Wassmund (1927) described a vertical section with a forward angulation above the lingula extending through the base of the coronoid process. Schuchardt (1958) has utilised this type of osteotomy and inserted a bone graft for the treatment of micrognathia (Fig. 9). I have performed the vertical section of the ramus with a nasal saw or a straight blade mounted on the Stryker oscillating saw. The gap between the fragments, after advancement of the mandible, is filled with an iliac bone graft or split-rib grafts pc (in children ; after Longacre, 1955) wedged ,¢ ~j 1., between the fragments (Fig. IO). This technique
~3
FIG. 7
FiG. 8
Fig. 7.--Vertical osteotomy for the correction of prognathism (after Caldwell and Letterman, 1954). A, Outline of prognathic mandible. B, T h e outer cortex of the ramus is perforated by numerous drill holes. Vertical section is done extending from the sigmoid notch to a point anterior to the mandibular angle. C, T h e outer cortex of the anterior fragment is removed with an osteotome. T h e base of the coronoid process is cut through. D, T h e posterior fragment overlaps the anterior fragment and is maintained by direct wire fixation after intermaxillary fixation of the teeth has been established. Fig. 8.--Limberg's vertical osteotomy of the ramus for the correction of micrognathia with open-bite.
is too new to evaluate in terms of results ; it appears to have excellent prospects for the future. Elongation by Horizontal Section o f the Ramus and Bone G r a f t . I T h e ramus is approached through an external incision placed in a natural skin fold in the submandibular region. In addition, a second incision may be placed in the pre-auricular area to give added exposure. The ramus is exposed by wide subperiosteal elevation of the masseter and internal pterygoid muscles. If the separation of the fragments is to be successfully achieved, it is essential that the muscles be entirely separated from the bone. A horizontal cut is then made above the internal alveolar foramen and the lower fragment is separated from the upper fragment. A block of iliac bone is then wedged into the interval between the bone fragments. It is essential to maintain the downward position of the lower fragment by means of a bite block placed in the molar region. I did an operation of this type in collaboration with Rushton in 1941 ; successful follow-up of this patient was reported by Rushton (1942, 1944) and by Gillies and Millard (1957).
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Treatment of Unilateral Micrognathia.mAs previously stated, it is often necessary in unilateral micrognathia to perform a bilateral osteotomy in order to advance the unaffected side as well as the grossly shortened affected side (see Fig. 6). Even in cases where the unaffected side appears to be only slightly deformed, the elongation of the shortened side imposes a movement of rotation of the condyle on the unaffected side, which may disturb the function of the joint. In order to avoid this inconvenience a vertical osteotomy of the unaffected ramus permits bending the mandibular arch without disturbing the position of the condyle.
FIG. 9 Schuchardt's curved osteotomy of the ramus for micrognathia extending behind and above the inferior alveolar canal. Interposition of an iliac bone graft restores the continuity of the sectioned ramus,
FIG. IO Vertical osteotomy of the ramus for micrognathia with interposition of split rib bone grafts.
Mierogenia and Mierognathia.--Minor degrees of micrognathia are frequently observed and are usually referred to as microgenia (small chin). These patients have a relatively small mandible, an underdeveloped chin, and characteristic dental occlusal relationships ; the lower incisor teeth are situated lingually to the upper incisor teeth which thus assume a marked position of "over-bite." This type of dental occlusion further increases the deformity by diminishing the height of the lower third of the face. In patients with moderate micrognathia the following procedures can be employed : (I) A bone graft or other implant material over the symphysis done routinely through the intra-oral approach (Converse, 195o) (Figs. i i and 12); (2) horizontal osteotomy of the body of the mandible (Obwegeser, 1957) (Fig. 13) ; and (3) the Babcock (1937) operation has recently been employed by Trauner and Obwegeser (1957) consisting in the implantation of cartilage in the temporomandibular joint posterior to the condyle. This procedure forces the mandible forward and changes the occlusal relationships of the teeth from a marked over-bite to a more satisfactory type of occlusion. In moderate types of micrognathia it may be possible to advance the body of the mandible by the step osteotomy of the ramus through the intra-oral approach.
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When protrusion of the superior-anterior dento-alveolar segment is present, correction by orthodontic and/or surgical means completes the rehabilitation of the micrognathic or microgenic patient. An excellent technique is the advancement of the lower portion of the body of the mandible following a horizontal osteotomy done along a horizontal plane below the inferior alveolar
~ac,~q from ce/o~alojrum FIG.
ii
Cephalometric planning for contour restoration in microgenia. A, Tracing of soft tissue and bone contour made on transparent acetate. B, The angle between two planes (nasion to sella turcica ; nasion to menton) should be 72 degrees. These measurements are of assistance but not indispensable as demonstrated in C to D. C, Modified profile line established. D , The distance between the original profile line and the new profile establishes the thickness and shape of the contour-restoring bone graft. E, Tracing of cephalogram taken after implantation of the bone graft.
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canal (Fig. I3). The mandible is exposed by the " degloving " technique through the intra-oral approach (Fig. i4). The fragment is advanced to provide an adequate chin projection. An added improvement is the placing of bone grafts in the gap left posteriorly by the shifting forward of the sectioned bone (Fig. I3). Fixation is obtained by a circumferential wire on each side, which is removed after consolidation. The problem of establishing a chin prominence in the microgenic patient is a more complicated one in the patient with tight covering soft tissues than in the patient in whom the soft tissues are lax. Fortunately in most of the congenital and developmental types of micrognathia the soft tissues appear to be adequate. In patients in whom the tissues are deficient or whose tissues have been scarred, i.e., by a burn, the pressure exerted by the tense soft tissue tends to produce a diminution in size of the onlay bone graft placed over the mandible. In these cases three solutions can be offered. The first is to employ the sliding horizontal osteotomy technique advancing the lower border of the bone; this type of bone transplant has a greater chance of maintaining its bulk than a free graft. The
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second method is to place the bone graft over the symphysis area prior to the horizontal elongation osteotomy. Once the bone graft is well established its chances of resisting the overlying soft tissue pressure after elongation osteotomy
FIG. I2
Contour restoration by bone grafting through the intra-oral approach in microgenia. A, Pre-operative appearance of microgenic patient. B, Post-operative contour obtained by bone grafting.
FIG. 13
Horizontal osteotomy of the mandibular body with advancement of the symphysis for micrognathia with minimal malocclusion and as an additional contour-restoring procedure following elongation step osteotomy of the mandible.
are greater than the fresh bone autograft. A third technique which has been advocated is the use of synthetic implant materials (Brown et all., I96o) which appear to be well tolerated in the region of the symphysis ; as with all foreign implants the tolerance of the host to these materials awaits the test of time. CONCLUSIONS
Many techniques are available for the correction of micrognathia. The choice between these techniques appears to depend upon the anatomy of the malformation. In some patients elongation of the body of the mandible is the method of choice ;
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in others vertical section of the ramus with interposed bone grafts is preferable; it is possible that in some cases both procedures may be employed, the patient undergoing, in two separate stages, a section of the ramus with a bone graft to fill the: interval between the sectioned ramus fragments, followed by a section of the body and a bone graft over the mandibular symphysis. A major problem which remains controversial is that of deciding at what age such corrective
A
B
C
FIG. 14 Horizontal osteotomy of the mandible. A, Intra-oral exposure obtained by the " degloving" technique. B, Horizontal osteotomy done by means of the Stryker oscillating saw. C, Fragment being moved forward into position of adequate projection.
procedures should be done in the developmental type of micrognathia. There is a growing trend toward operation in early childhood. Further study and long-term follow-up of these patients will undoubtedly provide the answer. SUMMARY Micrognathia is of three types, congenital, developmental, and acquired, and in its severe form is not only a disfigurement but also a functional handicap to the patient. Surgical treatment is necessary to restore facial form and the centric relationships of the dento-alveolar processes in order to re-establish the masticatory function. Corrective and reconstructive surgical procedures include selective sectioning of the body of the mandible or the ramus for the purpose of elongating the bone, and bone grafting for restoration of adequate size and contour of the mandible. Most of these procedures can be done through the intra-oral approach.
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