CONGENITAL ANOMALIES AND PEDIATRIC PLASTIC SURGERY
CHAPTER 24 CRANIOFACIAL MICROSOMIA AND PRINCIPLES OF CRANIOFACIAL DISTRACTION
JOSEPH G. Mc.CARTHY
Craniofacial microsomia, a variable hypoplasia of the skeleton and soft tissue, is the second most common congenital deformity of the head and neck region, with an incidence as high as 1 in 3,500 live births.
The deformity has been known by a variety of terms. In Europe, the term dysostosis otomandibularis has been used. Gorlin and Pindborg preferred hemifacial microsomia, but this term implies that the syndrome is unilateral and that the deformity is confined to the face. I prefer the term unilateral craniofacial microsomia or, when there is bilateral involvement, bilateral craniofacial microsomia.
Bilateral craniofacial microsomia can be confused with Treacher Collins syndrome (Chapter 28), but the latter shows a well-defined pattern of inheritance and the pathology is relatively symmetrical. Treacher Collins syndrome has other distinguishing features (absence of the medial lower eyelashes, lower eyelid hypoplasia, and antegonial notching of the mandible), findings that are absent in craniofacial microsomia. Likewise, craniofacial microsomia should be distinguished from micrognathia of the development or posttraumatic type where the underdevelopment is restricted to the mandible and there is no evidence of facial paralysis, ear anomalies, or hypoplasia of the soft tissue.
The hypoplasia is variable and can be manifested in any structure(s) derived from the first and second branchial arches (Table 24.1), accounting for the wide spectrum of the deformity.
The genetics of the syndrome are poorly understood. In a series of 102 affected patients, only 4 had a sibling or parent with evidence of craniofacial microsomia; indeed, only a few pedigrees of the syndrome have been reported. Despite the possibility of an occasional autosomal dominant transmission, only a 2% to 3% recurrence rate was found in a study of first-degree relatives.
Several theories have been proposed in an attempt to understand the etiopathogenesis. Stark and Saunders invoked the concept of mesodermal deficiency, the current theory for cleft lip/palate formation. Tessier, in a classification system of orbitofacial clefts, invoked a clefting mechanism. He described three clefts involving the orbitozygomatic complex in patients with craniofacial microsomia (see Chapter 26).
The most commonly accepted is one of vascular insult, with hemorrhage formation in the developing first and second branchial arches. The stapedial artery is a temporary embryonic collateral of the hyoid artery. Defects of this temporary vessel may result in hemorrhage, accounting for injury to the developing first and second branchial arches.
Laboratory phenocopies of craniofacial microsomia have been created following the administration of triazine to the developing mouse and thalidomide to the monkey. Histologic studies demonstrated hematoma formation before formation of the stapedial artery. The spectrum of the pathology varied depending on the volume of hemorrhage, ranging from involvement of only the external ear and auditory ossicles to a larger defect involving the zygomatic complex and the entire mandible on the affected side of the mouse model. Moreover, the laboratory findings were supported by the clinical documentation in Germany of approximately 1,000 severe cases and an additional 2,000 less severe cases of craniofacial microsomia following the widespread use of thalidomide as a tranquilizer in pregnant women.
The incidence of the syndrome is not accurately known in the United States but has been reported to be as high as 1 in 3,500 live births. If all infants with preauricular skin tags and so-called isolated microtia are included, the incidence of maldevelopment of the first and second branchial arches is much higher.
Similarly, the sex ratio is not accurately known; in a series of 102 patients, 63 were males and 39 were females. Another series reported an almost equal sex ratio (59 males and 62 females). Studies of “isolated” microtia patients, on the other hand, all report a clear male preponderance.
The incidence of bilateral involvement is said to be 10% to 15%. The true incidence is probably higher when one considers the presence of preauricular skin tags and subtle radiographic abnormalities of the mandible on the contralateral, “unaffected” side.
There is a wide variety of pathologic expression of craniofacial microsomia in the following anatomic regions: jaws, other craniofacial skeletal components, muscles of mastication, ears, soft tissue, and nervous system (Figure 24.1).
The most obvious deformity is the mandible, especially the ascending ramus, which is reduced in the vertical dimension. The size of the condyle usually reflects the degree of hypoplasia of the ramus. Involvement of the temporomandibular joint ranges from mild hypoplasia to a pseudoarticulation at the cranial base to complete absence of the condyle. In addition to being short, the ramus is usually displaced toward the midline.
The chin is deviated toward the affected side and there is a corresponding cant of the mandibular occlusal plane, which is paralleled in the corresponding planes of the maxillary sinus floors and pyriform apertures. Similarly, the maxillary and mandibular dentoalveolar complexes are reduced in the vertical dimension on the affected side.
FIGURE 24.1. Patient with left-sided craniofacial microsomia demonstrating the characteristic occlusal cant upward on the affected side with associated cheek hypoplasia and ear anomaly. The oral commissure is also elevated on the affected side. The chin point is deviated to the affected side.
Pruzansky proposed a classification of the mandibular deficiency, which was later modified by Mulliken and Kaban (Figure 24.2):
I. Mild hypoplasia of the ramus with minimally affected mandibular body.
II. The condyle and ramus are small; the head of the condyle is flattened; the glenoid fossa is absent; the condyle is hinged on a flat, often convex, infratemporal surface; the coronoid may be absent.
III. The ramus is reduced to a thin lamina of bone or is completely absent. There is no evidence of a temporomandibular joint.
The above classification was subsequently modified by subdividing type II based on the pathology of the temporomandibular joint region. In type IIA, although the ramus and condyle are abnormal in size and shape, the glenoid fossa–condyle relationship is maintained. Temporomandibular joint function is almost normal. In contrast, in type IIB, the condyle is hypoplastic and malformed and displaced toward the midline relative to the contralateral side. Patients open with restricted hinge-like functioning of the mandible on the ipsilateral side.
Other Skeletal Components
The maxilla is reduced in the vertical dimension and, depending on the degree of hypoplasia of the mandible, there is a corresponding cant of the occlusal surface of the maxillary dentition.
FIGURE 24.2. Pruzansky’s proposed (1969) classification of the mandibular deformity in craniofacial microsomia as modified by Mulliken and Kaban (1987). A. Type I: The condyle and ramus are reduced in size but the overall morphology is maintained. B. Type IIA: The ramus and condyle demonstrate abnormal morphology but the glenoid fossa has maintained a position in the temporal bone similar to that of the contralateral side. C. Type IIB: The ramus/condyle is hypoplastic, malformed, and displaced toward the midline. D. Type III: The ramus is essentially absent without any evidence of temporomandibular joint.
The zygomatic complex can be reduced in all dimensions; the zygomatic arch can be decreased in length or absent. These findings, combined with soft-tissue deficiency, result in a reduction in the distance between the oral commissure and ear vestige on the affected side.
The temporal bone can also be involved, although the petrous portion is spared. The mastoid process can be hypoplastic and there can be partial or complete lack of pneumatization of the mastoid air cells. The styloid process can be shortened or absent. The orbit is often reduced in all dimensions, and occasional patients have microphthalmos. The frontal bone can be flattened, giving the illusion of a plagiocephaly although the ipsilateral coronal suture is patent.
Malformations of the cervical vertebrae are not uncommon and include the presence of hemivertebrae, fused vertebrae, and even a basilar impression syndrome. Goldenhaar described a variant of craniofacial microsomia characterized by epibulbar dermoids/lipodermoids, associated vertebral abnormalities (usually cervical), and occasional rib anomalies.
Muscles of Mastication
The syndrome is not restricted to the skeleton; the associated muscles of mastication are hypoplastic. The deficiency, however, is not always proportional to the skeletal deficiency. A three-dimensional computed tomography (CT) scan study compared the volume of the mandibular deformity with that of the adjacent muscles of mastication and noted that there was not always a 1:1 relationship in the degree of pathologic involvement.
Muscle function is impaired, especially evident in lateral pterygoid muscle function on the affected side. The lateral pterygoid muscle is responsible for movement of the mandible and chin point to the contralateral side. Consequently, in patients with unilateral craniofacial microsomia, who attempt a protrusive chin movement, the chin deviates to the affected side during opening and during forceful protrusion. The hypoplastic lateral pterygoid muscle on the affected side is overpowered by its unaffected counterpart. Moreover, mouth opening is also adversely affected by the hypoplastic ramus and malpositioned temporomandibular joint.
Involvement of the auricle occurs in most patients (Chapter 27). Meurmann proposed a classification of the external ear deformities: Grade I, distinctly smaller malformed auricle but all components are present; Grade II, only a vertical remnant of cartilage and skin with aural atresia (complete atresia of the external auditory canal); Grade III, almost complete absence of the auricle except for a small remnant, usually a soft-tissue lobule. Nagata’s classification (Chapter 27) is more useful for the surgeon. The great majority of patients with microtia also have aural atresia. A small percentage do have a canal, but in many of these patients the canal is stenotic, atypical, or in abnormal position.
On the affected side, preauricular skin tags are common, and the skin and subcutaneous tissue of the cheek show varying degrees of hypoplasia. Hypoplasia or aplasia of the parotid gland has been documented. The soft-tissue deficiency occurs in all three dimensions.
Lateral facial clefts (macrostomia) are common associated findings and also contribute to the overall cheek hypoplasia.
Overt clefts of the soft palate are said to occur in 25% of patients, and the soft palate may deviate to the affected side on voluntary function.
Cerebral abnormalities, although rare, can occur and include hypoplasia of the cerebrum and corpus callosum, as well as hydrocephalus of the communicating and obstructive types. The brain stem can be involved secondarily because of anomalies of the cervical vertebrae, resulting in disturbances such as impression of the brain stem.
The most common cranial nerve anomaly is an incomplete facial palsy of varying degrees, attributed to the following (alone or in combination): absence of the intracranial portion of the facial nerve and nucleus in the brain stem, aberrant pathway of the nerve in the temporal bone, or agenesis of the facial muscles. Absence of facial nerve function in the distribution of the marginal mandibular branch is seen in approximately 25% of patients, with weakness of other components, such as the buccal and zygomatic branches, occurring in a smaller percentage.
Several classifications have been described based on the clinical findings of the patient with unilateral craniofacial microsomia. Harvold, Vargervik, and Chierici proposed the following classification:
I (A). The classic type characterized by unilateral facial underdevelopment without microphthalmos or ocular dermoids but with or without abnormalities of the vertebrae, heart, or kidneys.
I (B). Similar to type I (A) except for the presence of microphthalmos.
I (C). Bilateral asymmetric type in which one side is more severely involved.
I (D). Complex type that does not fit the above but does not display limb deficiency, frontonasal phenotype, or ocular dermoids.
II. Limb deficiency type (unilateral or bilateral) with or without ocular abnormalities.
III. Frontonasal type. Relative unilateral underdevelopment of the face in the presence of hypertelorism with or without ocular dermoids and vertebral, cardiac, or renal abnormalities.
IV. (A) Unilateral or (B) bilateral. Goldenhar type with facial underdevelopment in association with other dermoids, with or without upper lid coloboma.
Munro and Lauritzen proposed a clinical classification system (Figure 24.3) that was designed as an aid in planning surgical correction:
Type IA: The craniofacial skeleton is only mildly hypoplastic and the occlusal plane is horizontal.
Type IB: The skeleton is similar to IA, but the occlusal plane is canted.
Type II: The condyle and part of the affected ramus are absent.
Type III: In addition to the findings in type II, the zygomatic arch and glenoid fossa are absent.
Type IV: This is an uncommon type with hypoplasia of the zygoma and medial and posterior displacement of the lateral orbital wall.
Type V: The most extreme type has inferior displacement of the orbit with a decrease in orbital volume.
Vento and colleagues proposed the nosologic OMENS classification system in an effort to standardize reporting between treatment centers. The acronym OMENS designates each of the five major areas of involvement in craniofacial microsomia: O = orbital, M = mandibular, E = ear, N = facial nerve, and S = soft tissue. The orbital gradations were based on size and position; the mandible was scored as noted above; the ear anomaly was categorized essentially according to the Meurmann classification; the facial nerve was according to which branches were involved, and the soft tissue was according to the degree of subcutaneous and muscular deficiency.
FIGURE 24.3. Classification of unilateral craniofacial microsomia proposed by Munro and Lauritzen in 1985. The circle in Figure IA designates the usual site of skeletal involvement. The midsagittal, midincisor, occlusal, and orbital planes are designated. See text for details of each type. (From McCarthy JG, Grayson BH, Coccaro PJ, et al. Craniofacial microsomia. In: McCarthy JG, ed. Plastic Surgery. Philadelphia, PA: WB Saunders; 1990, with permission.)
A complete clinical evaluation is mandatory, because other organ systems, such as the kidneys and heart, can be involved. The role of the pediatric otolaryngologist is critical in assessing the airway and auditory function. The patient with airway problems will require endoscopy and sleep studies. Medical photographs are obtained, including frontal, lateral, oblique, submental vertex, and occlusal views. Cephalograms (posteroanterior, lateral, and basilar) and a panoramic roentgenogram (Panorex) are likewise obtained. The optimal way to define the various skeletal deformities is with a three-dimensional CT scan (Figure 24.4), which can be reformatted to give a dentascan and document the location of tooth follicles in the younger patient in whom cephalograms and Panorex cannot be obtained. i-CAT scans have recently been added as a tool in planning skeletal surgery.
No standardized treatment program exists for the child with craniofacial microsomia. The pathology, as emphasized before, is variable, and other factors, such as growth and development and prior therapy, are considered before recommending an individualized treatment program.
Younger Than Two Years of Age
Excision of the preauricular skin tags and cartilage remnants is often satisfying to the parents, because it removes some of the stigmata of the syndrome. Likewise, macrostomia can be corrected by a commissuroplasty on the affected side or on both sides in bilateral cases. In the occasional patient with involvement of the fronto-orbital region, characterized by severe retrusion of the supraorbital bar and frontal bone, a fronto-orbital advancement may be necessary.
Mandibular distraction is indicated in the newborn or infant with sleep apnea (with or without a tracheostomy). Distraction can correct not only the sleep apnea but also the associated alimentary or feeding problems (e.g., swallowing and gastroesophageal reflux). The principles of distraction and the applications in the mandible and other parts of the craniofacial skeleton are addressed in a separate section later in this chapter.
Two to Six Years of Age
In the child with mild deformity, such as Pruzansky type I mandible and a horizontal occlusal plane (Munro and Lauritzen type IA), no surgical treatment is recommended at this early age.
In the child with severe reduction in the vertical height of the mandibular ramus (Pruzansky types I and II), airway problems, and obvious aesthetic deformity, the technique of distraction osteogenesis (Figure 24.5) is considered. Sufficient clinical experience with mandibular distraction has accumulated to demonstrate that this technique improves the airway, lengthens the affected ramus, and also augments the associated soft tissue and muscles of mastication. The gradual nature of the distraction process lowers relapse rates. Studies also demonstrate that the distracted ramus/condyle remodel to assume a more anatomic size, shape, and position. Studies have demonstrated that the distracted mandible does not grow as much as the contralateral side, necessitating secondary distraction or definitive jaw surgery when craniofacial growth is completed (approximately age 17 years), or both.
In the patient with a Pruzansky type III deformity without evidence of a ramus, condyle, and glenoid fossa (or zygomatic arch), a preliminary costochondral rib or iliac bone graft reconstruction is performed at approximately age 4 years. In this technique, the glenoid fossa, zygomatic arch, and ascending ramus are reconstructed in a single surgical procedure (Figure 24.6). If there is a persistent mandibular deficiency, especially manifested by airway obstruction, distraction, as a secondary procedure, is considered.
In the child with bilateral craniofacial microsomia (Pruzansky types I and II mandibular deformity) with associated sleep apnea (with or without tracheostomy), bilateral mandibular distraction can be performed after sleep studies have established the diagnosis and the latter has been confirmed by endoscopy. In these children, the treatment can result in removal of the tracheostomy. If no mandibular rami exist, bilateral costochondral graft reconstruction is required.
Six to Sixteen Years of Age
This is the period of orthodontic treatment, including possible functional appliance therapy to promote eruption and growth of the dentoalveolus on the affected side. Distraction can be considered in the patient with chronic low-grade sleep apnea and in the patient with severe dysmorphism who has never received treatment. Ear reconstruction is undertaken during this period (Chapter 27). Serial autogenous fat injections or insertion of a microvascular free flap to augment the facial soft tissue and improve facial contour results in considerable aesthetic improvement.
Older Than Sixteen Years of Age
Surgery is often indicated in the period of skeletal maturity because of residual deficiency resulting from inadequate growth and development on the affected side, severe malocclusion, or failure of the patient to seek treatment previously.
At this point in time, when craniofacial growth and development are complete, the following procedures are considered: (a) limited autogenous bone grafting of deficient portions of the craniofacial skeleton; (b) bilateral mandibular advancement in patients with mild to moderate mandibular micrognathia; (c) combined Le Fort I osteotomy, bilateral mandibular osteotomy, and genioplasty (Figure 24.7); and (d) serial autogenous fat injection or insertion of a microvascular free flap to augment the soft tissue of the face on the affected side.
FIGURE 24.4. Three-dimensional CT scans of three unilateral microsomia cases demonstrating increasing severity from left to right. The affected side of each case is on the top panel, with the corresponding normal contralateral side on the lower panel. The three cases correspond to the modified Pruzansky classification of mandibular deformity: class I (left), class II (center), class III (right). (From Mathes SJ. Plastic Surgery. Philadelphia, PA: WB Saunders; 2005, with permission.)
FIGURE 24.5. Mandibular distraction. A semi-buried device, placed with an oblique vector, is depicted. The bony generate is seen in the distraction gap. (From Nelligan P. Plastic Surgery. Philadelphia, PA; WB Saunders; 2012, with permission.)
FIGURE 24.6. The technique of reconstruction of an absent ramus, zygomatic arch, and the temporomandibular joint with costochondral rib grafts. Note the cartilage graft simulating the disc and the cartilaginous portion of the rib graft simulating the condyle. Rigid skeletal fixation (screws and plates) is also utilized. (Modified from Munro JR, Lauritzen CG. Classification and treatment of hemifacial microsomia. In: Caronni EP, ed. Craniofacial Surgery. Boston, MA: Little, Brown; 1985:391–400, with permission.)
PRINCIPLES OF CRANIOFACIAL DISTRACTION
Distraction osteogenesis is an established therapeutic tool, especially in the craniofacial skeleton where it has the enormous advantage of eliminating bone grafts and alloplastic materials, almost completely eliminating infections after osteotomies, and decreasing the rate and extent of osteotomy relapse.
The technique is unique in that it applies gradual and incremental traction force/tension to surgically separated bony segments to produce additional bone. In essence, it releases inherent biologic forces to generate tissues, that is, bone and the associated neuromuscular/soft-tissue complex. The technique could actually be called distraction histogenesis in that distraction of the skeleton also causes enlargement of the overlying or surrounding soft tissue. Distraction osteogenesis represents one of the first examples of surgically induced tissue engineering.
Skeletal molding has been practiced for centuries. In certain African tribes, serial applications of metal necklaces at a young age result in elongation of the neck. Mayan cultures performed cranial molding with the application of helmets to the skulls of infants.
Early in the 20th century, Codivilla reported a technique involving an osteotomy of the femur and application of external traction to lengthen the lower extremity. A similar report was provided by Abbot in 1927. The biologic principles were insufficiently studied; the devices were poorly designed; infection, fibrous union, nerve palsy, and joint contractures resulted; and the concept was abandoned.
Ilizarov1,2 conducted laboratory studies and popularized the concept of distraction osteogenesis in the long (endochondral) bones of the extremities for limb lengthening and for the closure of bony defects. McCarthy and colleagues at New York University3-5 applied the technique to the bones (membranous) of the craniofacial skeleton in a series of canine mandible studies and introduced clinical craniofacial distraction in 1989.
While distraction in the extremities has fallen out of favor, distraction in the craniofacial skeleton (for deficiencies of the mandible, maxilla, midface, zygomas, and cranium) has assumed a much larger role.
The biologic concept of targeted bone growth/deposition is best demonstrated by cranial sutures. As the rapidly enlarging brain in the growing neonate separates the individual cranial bones, the sutures react by depositing new bone. In this manner the cranial vault increases in surface area to provide a skeleton of adequate volume for protection of the brain. Maxillary arch expansion by activation of a device placed across the palatine suture, as routinely practiced by orthodontists, is another example of distraction osteogenesis.
The concept is simple.
1. The bone is separated into segments either by a full- thickness osteotomy or by a low-energy corticotomy (sparing the endosteum or marrow space). The location of the bony separation is termed thedistraction zone.
FIGURE 24.7. The combined Le Fort I and bilateral sagittal split osteotomy and genioplasty, in a patient with right-sided hemifacial microsomia. A. (left) Lines of osteotomy. The osteotomy and site of vertical impaction are illustrated on the left maxilla. The solid circlesdesignate the midpoints of the chin, maxilla, and orbital region (midsagittal plane). The arrow shows the direction of the jaw movements. B. (right) Following movement of the maxillary, mandibular, and chin segments and the establishment of rigid skeletal fixation with plates and screws. Note the interpositional bone graft in the right maxilla. The solid circles line up along the midsagittal plane. (Modified from Obwegeser HL. Correction of the skeletal anomalies of otomandibular dysostosis. J Maxillofac Surg. 1974;2:73, with permission.)
2. Time is allowed (5 to 7 days) for reparative callus formation in the distraction zone (the latency period).
3. Gradual distraction forces are applied to separate the edges and elongate the intersegmentary callus under tension (the activation period).
a. Rigidity of the distraction device is critical to maintain the intersegmentary gap tissues in a direction or vector parallel to the orientation of the device (the vector of distraction).
b. A rhythm of 0.25 mm four times a day is preferable (0.5 mm twice a day is generally acceptable in a clinical setting).
4. At the end of activation, the external fixation must be maintained in position to allow consolidation of the newly formed bone (distraction generate). The consolidation period usually lasts approximately 8 weeks.
The three types of distraction osteogenesis are as follows (Figure 24.8):
1. Unifocal—a single osteotomy with distraction forces applied by a device attached by screws on either side of the osteotomy;
2. Bifocal—a single osteotomy with one set of pins adjacent to the osteotomy and the other set on the distal side of a skeletal defect, with a single distraction device spanning the transport segment;
3. Trifocal—two osteotomies used to fill a skeletal defect in a bidirectional manner with a spanning distraction device.
The transport segment is delivered into the skeletal defect by forces applied by the distraction device. The leading edge of the segment has a fibrocartilage cap. Bone grafting is usually required after the transport segment has been finally “docked,” the fibrocartilage is resected, and the defect is replaced by a bone graft. However, after temporomandibular ankylosis release with a gap arthroplasty, the cartilaginous leading edge of the transport segment simulates a neocondyle and no grafting is performed.
The most usual type of distraction is transosteotomy (or transcorticotomy) distraction. However, in very young patients, successful distraction can be performed across patent or open sutures (trans-sutural distraction). Palatal or maxillary expansion with an orthodontic appliance is an example of the latter, and trans-sutural midface distraction has been successfully achieved in an immature canine model.
FIGURE 24.8. The three types of distraction osteogenesis: unifocal, bifocal, and trifocal. The solid gray zone represents the newly generated bone at the osteotomy/corticotomy site. The arrows designate the direction of the distraction (strain) forces. The transport segments are white. (Adapted from Aro H. Biomechanics of distraction. In: McCarthy JG, ed. Distraction of the Craniofacial Skeleton. New York, NY: Springer; 1999.)
Distraction ostoegenesis represents a unique form of fracture healing. In contrast to fracture healing that occurs via a cartilaginous intermediate, distraction of both the membranous bone of the craniofacial skeleton and the endochondral bone of the extremities occurs without a cartilaginous intermediate.
The histologic changes occurring in the distraction zone have been elucidated in animal experiments. A circumferential corticotomy was performed in the region of the angle of the mandible at a position posterior to the molars in a canine model.4 After the application of a distraction device, and a latency period of 7 days, the device was activated at the rate and rhythm of 0.5 mm twice a day for a total of 20 mm (activation period). The distracted mandibles were harvested at several time points during the activation and consolidation periods and subjected to histologic and microradiographic examination.
During the latency period (after osteotomy and before activation of the device), bone repair is similar to that observed after fracture healing—hematoma formation and the migration of inflammatory cells into the osseous gap or distraction zone.
Microscopic examination after activation of the distraction device demonstrates the presence of tapered cells, similar to fibroblasts, and new blood vessels, which form a fibrovascular matrix aligned longitudinally in the direction of the distraction vector. Osteoid synthesis and mineralization are not apparent until almost 14 days after the initiation of activation.
At approximately 3 weeks after activation, calcification of the linear-oriented collagen bundles is noted, followed by the appearance of osteoblasts along the collagen bundles and formation of bony spicules that extend from the edges of the osteotomy toward the central portion of the distraction zone. With progressive calcification of the generate, there is bony closure of the distraction defect. Continued remodeling of the newly formed bone, as evidenced by the appearance of osteoclasts, results in lamellar bone with marrow elements of adequate volume.
In summary, the following four temporal zones are observed in the distraction gap (Figure 24.9):
1. Fibrous central zone (mesenchymal proliferation)— longitudinally oriented fibrous bundles;
2. Transition zone—osteoid formation along the collagen bundles in the distraction gap;
3. Remodeling zone—the appearance of osteoclasts with remodeling of the newly formed bone; and
4. Mature bone zone.
The development of a laboratory rat model of mandibular distraction has permitted the study of a relatively large number of animals with the potential for detailed biomolecular analysis of the distraction zone.6 At the end of the latency and in the early activation periods, there is a metabolically active, heterogeneous cell population (endothelial cells, fibroblasts, and polymorphonuclear leukocytes) in the distraction zone, all associated with the presence of type I collagen bundles. The latter become organized and oriented as a fibrovascular bridge in a plane parallel to the distraction vector. The arrival of large osteoblasts at the edges of the osteotomized bone is associated with osteoid deposition along the collagen bundles; this is followed by mineralization of the generate in the distraction gap.
FIGURE 24.9. Schematic of the temporal stages of bone generation in unifocal distraction. See text for details. (From Karp NS, McCarthy JG, Schreiber JS, et al. Membranous bone lengthening: a serial histologic study. Ann Plast Surg. 1992;29:2, with permission.)
A marked increase in transforming growth factor β1 (TGF-β1) is demonstrated as early as 3 days into the latency period. Expression of this cytokine peaks during the late stages of the activation period.7 It returns to near-normal levels toward the end of the consolidation period. These findings imply a regulatory mechanism for TGF-β1 in inducing collagen deposition and noncollagen extracellular matrix proteins involved in the mineralization and remodeling of bones. TGF-β1 is also important in the activation of VEGF (vascular endothelial growth factor) and basic FGF (fibroblast growth factor). TGF-β1 also plays a regulatory role in osteoblast migration, differentiation, and bone remodeling. Neovascularization is critical to the success of distraction.8
Although osteocalcin (a noncollagenous matrix protein) expression is decreased during the latency period, an increased expression is observed early in the activation period, and it is increased to normal levels by the end of the consolidation period. Osteocalcin plays an important role in mineralization and bone remodeling. The key quality of bone, that is, its rigidity or hardness, is attributable to the mineralization of the linear-oriented extracellular matrices.
A more complete understanding of the biomolecular regulation of distraction osteogenesis offers the possibility of future clinical manipulation of the distraction zone, for example, increasing the rate of activation and decreasing the length of the consolidation period. If the latter goals can be achieved, the length of the overall distraction treatment period would be significantly reduced.
In distraction osteogenesis, the tensile forces delivered to the developing callus at the osteotomy site cause elongation of the callus. The mechanical environment in the distraction zone is determined by the following factors: the rigidity of the distraction device, the applied distraction forces, the inherent physiologic loading (muscle action), and the properties of all of the local soft tissues.9
Tensile strain is defined as the amount of elongation as a fraction of the original bone length.9 At an activation rate of 1.0 mm/d and an osteotomy defect of 1.0 mm, the strain is 100% during the first day of activation. By activation day 10, when there is a 10-mm gap, the tensile strain has decreased to 10%. Because bone can tolerate only 1% to 2% tensile strain (“ultimate tensile strain”), bone tissue cannot survive for long a load exceeding more than 1% to 2% tensile strain. Consequently, bone formation is not observed in the distraction zone until approximately 4 weeks of activation, that is, the period when the tensile strength is at or below the ultimate tensile level.
The process by which mechanical forces are converted to cellular signals is termed mechanical transduction.10 A studied pathway is the integrin-mediated signal transduction cascade. In the rat mandibular model of distraction, the demonstration of immunolocalization of focal adhesion kinase and other molecular mediators supports the hypothesis that bone formation in mandibular distraction is regulated by mechanical forces, signaling integrin-mediated single-transduction pathways at the molecular level.
The mandible was the obvious first choice for craniofacial distraction.5 It is an accessible, somewhat tubular bone in which changes can be easily documented by measurement of radiographic and occlusal changes. In addition, a clinical need existed for a therapeutic paradigm shift, especially in pediatric patients with deficiency of the mandibular ramus and life-threatening respiratory problems.
In contrast to classic mandibular osteotomies, distraction permits surgery at a younger age without the need for bone grafts, blood transfusions, prolonged operations, and extended hospital stays. There is also an associated expansion or lengthening of the overlying soft tissues and muscles (distraction histiogenesis). The relapse rate is lower, as the bone is lengthened gradually at the rate of 1 mm/d, in contrast to an acute intraoperative forceful skeletal advancement against deficient and restrictive soft tissues.
A variety of mandibular distraction devices are available and the surgeon must choose between an external (extraoral) and a buried (intraoral) device (Figure 24.10). In general, extraoral devices are associated with more successful and consistent outcomes. They are especially indicated when the skeletal site for the osteotomy and pin insertions is diminutive in area and volume. A distinct disadvantage is that it leaves an external scar, which can be obvious and hypertrophic in some patients.
Although intraoral or semi-buried devices are associated with better scar formation, it is usually also necessary to place a transcutaneous (submandibular) incision for their insertion. Consequently, there is always a resulting, albeit fine line, scar. The actual progress of the activation cannot be observed externally, and in the infant in whom serial radiographs are not possible, there can be an undetected mechanical problem. While molding of the generate is not possible, the semi-buried device (with the activation knob accessible transcutaneously) is ideal for patients requiring a vertical vector. Intraoral devices are also more difficult to remove.
FIGURE 24.10. Mandibular distraction. A. External technique: an intraoral incision along the oblique line of the mandibular remnant. B. Sites of the pinholes and proposed osteotomy (dotted line). C. Pins in place. D. Completing the osteotomy (cortices only). E. Distraction device in place. F. Activation of the device with formation of the bony generate (shaded). (From Nelligan P. Plastic Surgery. Philadelphia, PA: WB Saunders; 2012, with permission.)
The mandible is approached by individual or combined transcutaneous (submandibular) or intraoral incisions.
The first decision is the choice of a vector.11 The vertical vector is defined as one at 90° to the maxillary occlusal plane and is indicated when there is a vertical deficiency of the ramus (Figure 24.11). In patients with severe micrognathia associated with deficiency of the mandibular body, the horizontal vector (parallel to the maxillary occlusal plane) is selected. The oblique vector is selected when there is a deficiency in both the vertical ramus and the horizontal body of the mandible.
Clinical studies12,13 have demonstrated that in unilateral distraction, a vertical vector results in lengthening of the ramus with autorotation of the mandible and anterior projection of the chin and tongue base. In bilateral mandibular distraction, vertical vectors again result in autorotation of the mandible with more projection of the chin (e.g., Treacher Collins syndrome, bilateral craniofacial microsomia). In contrast, in bilateral mandibular distraction, horizontal vectors increase the anteroposterior dimensions of the body of the mandible but result in less chin projection (e.g., Pierre Robin sequence).
FIGURE 24.11. The vectors of mandibular distraction. A. Vertical vector. B. Horizontal vector. C. Oblique vector. Note the vectors referenced to the maxillary occlusal plane (red). (From Nelligan P. Plastic Surgery. Philadelphia, PA: WB Saunders; 2012, with permission.)
The indications for mandibular distraction are both functional and aesthetic. Distraction has revolutionized the treatment of the infant or young patient with sleep apnea and the associated alimentary problems of eating and swallowing. The technique can be employed in the neonate and has obviated the need for tracheostomy. Moreover, it has permitted decannulation of tracheotomies in infants and young children. Sleep apnea is also an unrecognized problem in older patients, accounting for learning disabilities and behavioral problems. Mandibular distraction can have a positive impact on the quality of life of such children.
Mandibular distraction is also indicated for patients with respiratory functional problems and facial dysmorphism in such conditions as craniofacial microsomia, developmental micrognathia, and Treacher Collins syndrome. Postablative mandibular defects and temporomandibular joint ankylosis are other conditions that can be treated by transport distraction. Alveolar ridge distraction is indicated to increase alveolar ridge bone volume for the insertion of dental implants or for orthodontic tooth movements.
Even more than most surgical procedures, the surgeon is intimately involved during the postoperative period. After the completion of the latency period, the surgeon and orthodontist oversee device manipulation (activation). In addition to lengthening of the device and mandible, it may also be necessary to “mold the generate” with orthodontic rubber bands or manipulation of multiplanar distraction devices to correct or ameliorate malocclusions.
In unilateral mandibular distraction, as in the patient with unilateral craniofacial microsomia, the treatment end points are the movement of the chin to the contralateral side with lowering of the ipsilateral oral commissure, the inferior border of the mandible, and the occlusal plane to a level below that of the contralateral side. Such “overcorrection” is especially indicated in the growing child. Inbilateral mandibular distraction, the treatment end points include the achievement of a slight anterior crossbite, especially in the growing child.
Maxillary Le Fort I distraction is indicated for the correction of maxillary retrusion usually associated with cleft lip and palate patients and the maxillary deformity in craniofacial microsomia. The latter can be treated by combined maxillomandibular distraction.
The advantages of maxillary distraction are that it can be performed at a younger age, especially in the child with respiratory obstruction or a severe malocclusion (anterior crossbite) and midface retrusion that impact on psychosocial functioning. During the period of mixed dentition, the surgeon must be aware of unerupted maxillary teeth that lie along the path of the Le Fort I corticotomy. Because activation is at the rate of only 1 mm/d, and because the associated soft tissue is also being distracted, the relapse problems associated with a classic Le Fort I advancement in the patient with severe palatal scarring are reduced. A corollary is that significantly greater maxillary advancement can be achieved (in excess of 15 mm). It should be emphasized, however, that a second maxillary advancement will most likely be required when the patient achieves craniofacial maturity at the age of 17 or 18 years.
Several types of maxillary distraction devices are available. An external head frame (RED) provides relative stability and the ability to change the vector during activation (Figure 24.5), but is somewhat cumbersome; a variety of buried or intraoral devices are also available.
A maxillary degloving incision is made and the osteotomy or corticotomy is carefully performed along the traditional Le Fort I lines, with care being taken to avoid injury to unerupted maxillary teeth. A vector is usually chosen in a forward and downward direction.
The treatment’s end point is a class II malocclusion or overjet in the growing child (“overcorrection”). The consolidation period is approximately 2 months.
The clinical technique of midface or subcranial Le Fort III distraction is based on laboratory studies.14 It has several advantages in that it avoids the need for bone grafts and the application of plates and screws. The length of the surgical procedure and the volume of blood transfusion are reduced, as is the length of hospitalization. Infection almost never occurs. Moreover, the aesthetic results are superior to those of the traditional Le Fort III advancement with bone grafts because of more zygomatic projection and a lower relapse rate. A greater degree of midface distraction (up to 20 mm) can be achieved.15,16 Serial CT studies have demonstrated bone deposition along the entire Le Fort III osteotomy line as well as expansion of the nasopharyngeal airway space with relief of obstructive sleep apnea.
Midface distraction is especially indicated in the syndromic craniofacial synostosis patient with exorbitism, malocclusion, sleep apnea, midface retrusion, and severe dysmorphism. Patients with orbitofacial clefts are also candidates. When performed in a growing child, it must be emphasized to the family that a second midface procedure will be required when the child completes craniofacial skeletal growth in late adolescence.
There are two types of available distraction devices: head frames and buried devices that can be directly applied to the craniofacial skeleton through a coronal incision (Figure 24.12).
The ideal vector of distraction is in an anterior direction along a plane parallel to the maxillary occlusal surface. One should guard against a vector that closes the anterior open bite and increases the vertical dimension of the face and orbit. Treatment end points in the growing child include overcorrection with an overjet or class II occlusion and maximal orbitozygomatic advancement.
FRONTOFACIAL (MONOBLOC) DISTRACTION
Frontofacial or monobloc distraction is similar to subcranial midface distraction except that the superior part of the orbits and frontal bones is distracted along with the midface fragment. Collaboration with a neurosurgeon is required for the craniotomy and intracranial exposure. Severe exorbitism and the need for expansion of orbital volume are ideal indications, as well as patients who require expansion of the cranial vault (anterior) for symptoms of increased intracranial pressure.
Because the process is gradual, monobloc distraction does not create the intracranial dead space that the standard osteotomy/advancement does. This almost eliminates the risk of infection and cerebrospinal fluid leakage, common complications when monobloc osteotomies were performed without distraction. Patients with functional ventriculoperitoneal shunts, however, remain at risk for these complications.
Research studies17 and clinical reports demonstrate that cranial bone (i.e., cranial vault) distraction is clinically feasible. It is a form of bifocal or trifocal distraction in that an osteotomized segment of cranial bone is moved into a cranial defect with bony generation in the donor defect.
FIGURE 24.12. Midface distraction devices. A and B. External (RED) device, C. Buried device with an activation arm that penetrates the scalp. (From Mathes SJ. Plastic Surgery. Philadelphia, PA: Elsevier; 2005, with permission.)
The possibilities of craniofacial distraction are only beginning to be realized. It has been demonstrated that all components of the craniofacial skeleton—the mandible, maxilla, zygoma, orbits, and cranial bone—can be successfully distracted. As the devices are miniaturized and automated, it is possible that multiple bones could be individually distracted concurrently without the need for external devices. In the infant or young patient, it may be possible to perform transutural distraction without the need for osteotomies. As the molecular biology of the distraction zone is more fully understood, the rate of activation may be increased beyond 1.0 mm/d and the consolidation period reduced far below the current requirement of 8 weeks, thereby significantly reducing the overall length of treatment.
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