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The Versatility of Distraction Osteogenesis in Craniofacial Surgery
Mario J. Imola, MD, DDS, FRCSC;
David D. Hamlar, MD, DDS;
Gentry Thatcher, MD;
Khalid Chowdhury, MD, FRCSC
Arch Facial Plast Surg. 2002;4:8-19.
ABSTRACT
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Objectives To review our preliminary results using distraction osteogenesis for
the correction of craniofacial deformities and to determine its role in treating
anatomic deformities and functional deficits relative to conventional craniofacial
surgery.
Design and Setting Retrospective clinical review; tertiary care center.
Methods Twenty-four consecutive patients were treated with distraction osteogenesis
during a 34-month period. Outcomes were compared with preexisting anatomic
deformities and functional deficits using records of clinical assessments,
photodocumentation, diagnostic imaging, and treatment planning aids.
Main Outcome Measures Distraction achieved vs planned distraction based on clinical and radiographic
assessment, clinical status of functional deficits before and after treatment,
and objective rating of aesthetic improvement.
Conclusions Preliminary results demonstrated good-to-excellent outcome in correcting
facial skeletal deformity in 80% of patients. Functional outcomes included
resolution or significant improvement of upper airway obstruction in 13 of
14 patients and correction of corneal exposure for all 5 patients with preexisting
exorbitism. Correction of malocclusion was less reliable. Problems related
to the distraction devices, including failure of the advancement mechanism
and fixation, were the most prevalent complications. Distraction osteogenesis
represents an exciting new development in craniofacial surgery with several
potential benefits, including less invasive surgery, the ability for earlier
intervention, and the potential for correction of more severe deformities
with improved posttreatment stability. The exact role of distraction osteogenesis
relative to conventional techniques requires ongoing assessment.
INTRODUCTION
ALTHOUGH orthopedic surgeons have used distraction osteogenesis (DO)
for several decades to repair long bone defects, the procedure is only now
gaining acceptance for correction of craniofacial deformities. Bone distraction
was first introduced by Codvilla1 nearly 100
years ago and subsequently popularized during the 1940s by Ilizarov,2-3 who developed a single-stage procedure
to lengthen long bones without the use of grafting material. The feasibility
of applying Ilizarov's principles to different craniofacial deformities was
not considered until several decades after his pioneering work in the peripheral
skeleton. Strictly speaking, the first reports of craniofacial DO were in
the early 1960s, at which time rapid expansion of the palate was carried out
in growing subjects; however, this involved distraction of a naturally occurring
growth interface.4-5 Therefore,
it was not until 1973, when Snyder et al6 first
described the Ilizarov technique to lengthen a canine mandible, that DO was
first applied to surgical osteotomies of the facial skeleton. Interest in
craniofacial distraction was slow to grow initially, with sporadic experimental
reports appearing throughout the ensuing 2 decades.7-8
However, in the early 1990s, experimental investigation intensified following
reports that examined lengthening canine mandibles and the use of DO to successfully
close canine segmental lower jaw defects.9-13
Thereafter, several studies14-29
demonstrated the ability to apply osteodistraction at several different sites,
including the mandible, lower maxilla, midface, and cranial vault, within
a variety of animal models. The first clinical results of craniofacial DO
were reported in 1992 by McCarthy et al30 in
a small series of patients with congenital mandible deformities. Since then,
several larger series with longer follow-up periods have appeared.30-35
More recently, the technique has been successfully used for midface and upper
craniofacial skeletal defects.36-42
The underlying principle of DO, as described by Ilizarov, is "the mechanical
induction of new bone between bony surfaces that are gradually distracted."2-3 The process of DO begins with careful
preoperative assessment and planning, which are critical to success. During
the initial surgery, osteotomies are performed and the distraction device
is inserted. A waiting period (latency phase) is allowed to elapse so that
osseous healing is initiated at the bony gap, periosteal integrity is restored,
and callus formation begins.43 The bone segments
at either end of the gap are then progressively distracted over several days
(distraction phase) during which osteogenesis is induced, producing a so-called
regenerate of immature bone laid down between the cut bone ends. With time,
the bone remodels into a more mature state (consolidation phase), and the
surrounding soft tissues accommodate to their new positions and lengths.43
Distraction can be divided into either elongation DO or transport DO,
depending on whether a foreshortened bone is being increased in length (elongation)
or a segmental defect is being repaired within a bone that is otherwise of
normal length (transport). In addition, DO has been categorized into monofocal,
bifocal, and trifocal types, depending on the number of foci at which osteogenesis
occurs (Figure 1). Monofocal elongation
DO currently represents most of the clinical applications in the craniofacial
skeleton. The histologic and physiologic principles that underlie DO have
been well documented in long bones and more recently in the craniofacial skeleton.2-3,10, 43-48
During the distraction phase, bone formation occurs in response to the tension-stress
forces exerted on the regenerate, and healing proceeds primarily by a reparative
membranous ossification process (Figure 2).10, 43-45
The middle of the regenerate consists of a fibrous central zone where osteoid
is deposited with collagen fibers oriented parallel to the direction of distraction.
Ossification occurs as a primary mineralization front advances from either
end of the fibrous central zone, resulting in a bridge of immature bone across
the distraction gap. Bone remodeling begins during the consolidation phase
and continues throughout a 1- to 2-year period, eventually transforming the
regenerate into a mature osseous structure similar in size and shape to the
adjacent preexisting bone. Although the volume and architecture of the new
bone are comparable to the adjacent bones, animal studies have shown that
mineral content and radiodensity are less. The tensile strength of the regenerated
segment is approximately 75% of the native bone.13
In addition to bony changes, there are effects on the adjacent soft tissues
that occur in response to osseous distraction.46
Muscle and soft tissue mass increases via a process referred to as distraction histogenesis. Clinically, this offers a distinct advantage
since several craniofacial anomalies have soft tissue hypoplasia in addition
to deficient bony structures. Neurovascular elements contained within distracted
bony segments are also stimulated to elongate. Experimental studies in dogs
have demonstrated a recanalized mandibular canal that contains both neural
and vascular elements.47 The functional level
of the regenerated neurovascular structures, however, was less than normal.
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Figure 1. Three types of distraction osteogenesis
have been described: monofocal, bifocal, and trifocal. A, Monofocal distraction
is used to lengthen abnormally shortened bones and involves separation of
2 bone segments across a single osteotomy. B, Bifocal distraction is used
to repair a segmental defect and requires creation of a transport disk, which
is then distracted across the defect until it docks with the opposing bony
segment. C, Trifocal distraction is similar to bifocal distraction attempts
to halve the distraction time by transporting 2 disks from opposite ends of
a defect to dock in the middle. Arrows indicate distraction vectors; large
arrow heads, distraction regenerate; and small arrow heads, docking site.
Reprinted from Costantino PD, Shybut G, Friedman CD, et al.11(p543)
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Figure 2. Schematic representation of the
stages of bone formation during distraction osteogenesis: (1) zone of fibrous
tissue, (2) zone of bone mineralization, (3) zone of bone remodeling, and
(4) zone of mature bone. Adapted from Karp et al.10(p7)
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Several factors influence the physiologic process of DO, and these can
be separated into 2 basic groups: bone healing factors and distraction factors
as outlined here:
Factors that affect bone healing can be local or systemic in nature.
Viability of osteocytes and osteoblasts is essential to provide an adequate
source of osteogenic activity at the distraction site. Hence, careful surgical
technique should be used to minimize thermal or mechanical injury to the periosteum
and endosteum, which are the main sources of osteoblast precursors. Similarly,
an adequate blood supply to the distraction site is critical to osteogenesis.
Arterial insufficiency may lead to ischemic fibrogenesis within the regenerate,
yielding a loose, irregular collagen network instead of the desirable dense,
regular collagen pattern. Venous outflow obstruction has been associated with
cystic degeneration of the regenerate. The clinician, therefore, needs to
ensure that the soft tissues that surround the site of the proposed distraction
are well vascularized. Early studies in long bones concluded that both an
intact periosteum and endosteum were critical to successful osteogenesis,
and therefore many advocated that a corticotomy-only bone cut be performed
through a minimal periosteal opening.2-3
More recently, however, investigators have demonstrated that the periosteum
alone can provide sufficient osteogenic capacity to form a viable regenerate
in the well-vascularized membranous bone of the craniofacial skeleton.9, 11, 18, 21-22,26-27,43
Therefore, although some clinicians continue to advocate corticotomy, most
reports31, 35, 43 of
craniofacial DO describe the use of a complete osteotomy, taking care to preserve
as much of the surrounding periosteum as possible. Prior radiation therapy
to the distraction site has been shown to not adversely influence the results
of distraction in the canine model, and when using DO to repair segmental
defects, the status of the surrounding soft tissues will likely be the key
factor that influences outcome.48
Latency, rate, and rhythm of distraction are variables that influence
the quality of the regenerate. Of these factors, the effect of latency is
the most controversial.37, 39, 43
Most craniofacial surgeons have empirically applied the conclusions from long
bone studies and recommend waiting periods of 4 to 7 days following osteotomy
and before initiating the distraction process. In younger children, the high
rate of bone metabolism favors a shorter waiting period. Some clinicians,
however, use a zero latency period and begin distracting right at the time
of appliance insertion. They claim no adverse effects on outcome while substantially
shortening the treatment period.37, 39
Waiting too long before distraction (beyond 10 to 14 days) substantially increases
the risk of premature bone union. In contrast to latency, the rate and frequency
(rhythm) of distraction are believed to be important factors.43
If widening of the osteotomy site occurs too rapidly (>2 mm per day), then
a fibrous nonunion will result, whereas if the rate is too slow (<0.5 mm
per day), premature bony union prevents lengthening to the desired dimension.
These findings in long bones have been empirically applied to the craniofacial
skeleton, and most studies have described a rate of 1.0 mm per day. According
to Ilizarov's work in long bones, the ideal rhythm of DO is a continuous steady-state
separation of the bone fragments.2-3,44-45
However, this is impractical from a clinical standpoint, and therefore, most
reports30-33,35, 38
recommend distraction frequencies of 1 or 2 times daily. The length of the
consolidation phase has been recommended as ranging from 6 to 12 weeks in
long bones, depending on the length of the distraction segment. In the craniofacial
skeleton, most authors advocate 4 to 8 weeks, with the general rule that the
consolidation period should be at least twice the duration of the distraction
phase.30-33,35, 38
Distraction in load-bearing bones, such as the mandible, is an indication
for a longer consolidation time. Finally, appliance rigidity during distraction
and consolidation is a critical element to ensure that bending or shearing
forces do not result in microfractures of the immature columns of new bone
within the regenerate, which lead to focal hemorrhage and cartilage interposition.43
We have been using DO since early 1997 for several indications within
the craniofacial skeleton, including the correction of form and function.
This study was undertaken to review our experience with DO and evaluate its
role and future place in the management of the difficult challenges that craniofacial
patients typically present.
PATIENTS AND METHODS
A medical record review was conducted to identify all patients who had
undergone DO within the craniofacial skeleton since we began using this technique.
A total of 24 patients who were treated during the 34-month period from March
1, 1997 to December 31, 1999, were identified and formed the case series for
this study. Each patient underwent a comprehensive preoperative assessment,
including clinical examination, photodocumentation, and appropriate diagnostic
imaging. Cephalometric and Panorex x-ray films were used in 19 patients, and
in 15 individuals computed tomographic (CT) scans with 3-dimensional reconstruction
were obtained. Dental study casts were fabricated from impressions in 18 patients,
and in 6 cases acrylic replicas of pertinent facial skeletal segments were
constructed from preoperative CT scans. The preoperative data were analyzed
to define the nature of the deformity and the objectives and goals of the
reconstructive procedure being contemplated. The value of DO was weighed relative
to other surgical options before selecting DO as the reconstructive procedure
of choice. Skeletal tracings were used in most cases to plan the necessary
osteotomies and desired vector of distraction.
The distraction procedures included 6 midface and 29 mandibular osteotomies
with a total of 40 distraction devices inserted. Several designs are available
from the various suppliers, and the devices used in this series are shown
in Figure 3. Distraction devices
can be broadly grouped into internal or external types based on the location
of the main body of the distraction mechanism. Different designs are suited
to different applications, and careful consideration is required to ensure
that the appropriate device is selected for a given clinical situation.
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Figure 3. Distraction appliances can be
divided into internal and external types. Internal appliances have the main
body of the distraction mechanism submerged below the skin or mucosa and are
fixated to bone using conventional plate and screw metal hardware. A connecting
rod passes from the distraction mechanism and exits through the skin cover
or mucosa to allow activation. Internal appliances used in this study include
the Stryber-Leibinger (Kalamazoo, Mich) intraoral device (A), KLS Martin (Jacksonville,
Fla) modular (B), and Stryber-Leibinger modular internal distractor (C). External
devices are positioned outside the body and are secured to bony segments using
transcutaneous pin fixation. External appliances used in this study include
the Leibinger multiguide (D) and the KLS rigid external distractor (E).
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Latency time ranged from 4 to 7 days after placement of the devices,
and in general a shorter waiting period was used in the younger patients.
Perioperative antibiotics were used in all cases. During the distraction phase,
all the devices were advanced at the rate of 1 mm per day. Turnings were performed
twice daily (0.5 mm each time) if possible and once daily (1.0 mm) if the
former was not convenient. Once the distraction was completed, the devices
were left in place from 4 to 17 weeks with a mean of 8.3 weeks. The consolidation
time was determined by considering the site of distraction, the magnitude
of distraction, and the presence of any adverse bone healing factors. In addition,
weekly radiographs were obtained to semiquantitatively assess mineralization
of the regenerate. Following consolidation, all the devices were removed in
a second operative procedure and patients were monitored periodically in clinic.
For each patient, the results were reviewed with consideration of the
following variables: planned vs actual distraction (magnitude and direction)
achieved, improvement in facial form measured using an objective semiquantitative
scale, and functional improvements. The magnitude of distraction was ascertained
from radiographic monitoring and clinical measurement using reliable reference
surface landmarks such as the dentition wherever applicable. Functional evaluation
included assessment of sinonasal symptoms (obstruction, sinusitis, sleep disturbances),
ophthalmologic symptoms (corneal exposure, conjunctivitis, lacrimal drainage,
visual disturbances), and masticatory function (occlusion, mandibular movement).
The nature of any complications and their outcome were also noted. Data were
then analyzed to formulate relevant observations and conclusions regarding
the clinical utility of craniofacial DO based on our experience.
RESULTS
Congenital deformities comprised most of the cases (22), whereas 2 patients
had acquired defects. Table 1
gives the various underlying diagnoses present in the overall group. The youngest
group consisted of 8 patients who presented with glossoptosis-micrognathia
and upper airway obstruction as the main indication for distraction. These
patients had a mean age of 32 months (range, 6 days to 5 years) and consisted
of 6 individuals with isolated glossoptosis-micrognathia (3 with Pierre Robin
sequence), 1 patient with Nager syndrome, and 1 patient with maxillomandibular
syngnathia. Craniofacial dysostosis patients (3 with Crouzon syndrome, 1 with
Apert syndrome, and 1 with Pfeiffer syndrome) who required midfacial advancement
had a mean age of 5 years, whereas 6 patients with oroauriculovertebral spectrum
(hemifacial microsomia) underwent distraction at a mean age of 7.6 years.
Follow-up in the overall group ranged from 6 to 36 months, with a mean of
14 months following the end of the consolidation phase.
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Table 1. Number, Age, and Diagnoses of Patients Included in the Study
Group
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The cases were separated into midface and mandible procedures (Table 2), and the actual length of distraction
achieved was compared with the preoperative desired length. In 18 patients,
the devices were turned the planned number of revolutions, and in 14 of these
the desired length of distraction was fully achieved, whereas in the other
4 the distracted length was within 2 mm of the desired length based on radiologic
follow-up. In 6 other patients, the devices could not be advanced the desired
number of revolutions because of mechanical problems with the distraction
devices (4), compliance problems (1), and soft tissue breakdown with device
exposure at the osteotomy site (1).
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Table 2. Distraction Procedures Performed and Lengths of Distraction
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Cosmetic results were rated objectively by the treating surgeon using
an arbitrary scale of excellent, good, fair, or unsatisfactory. The results
were tallied for 20 patients for whom aesthetic considerations were applicable,
and the results were good to excellent in 80% of the group (Table 3). Midfacial advancements achieved the best cosmetic outcome
(Figure 4). Most hemifacial microsomia
patients were rated as good; however, within this group it was noted that
lower face asymmetry was difficult to fully correct. Cosmetic results were
less predictable in 8 patients, with symmetric mandibular deficiency ranging
from fair to excellent (Figure 5
and Figure 6).
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Table 3. Cosmetic Results in 20 Patients
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Figure 4. A 5-year-old boy with Crouzon
syndrome who had undergone prior bifrontal-orbital advancement and cranial
vault remodeling during infancy. A, Preoperative lateral cephalogram demonstrating
severe horizontal and vertical midfacial deficiency. Functional deficits included
exorbitism with chronic conjunctival exposure, oronasal airway obstruction
with obstructive sleep apnea, and recurrent rhinosinusitis. B, Corresponding
cephalometric tracing illustrating proposed vector of distraction. We planned
for 20 mm of anterior displacement of orbitale (Or) and 12 mm of inferior
midface repositioning. Addition of the 2 component vectors yielded the resultant
distraction vector with a magnitude of 23 mm and direction as illustrated
(green triangle). S indicates sella; N, nasion; Po, porion; Ba, basion; SN,
sella-nasion plane; MxP, maxillary plane; ANS, anterior nasal spine; and Me,
menton. C, Preoperative articulated dental models showing class III molar
relationship and severe negative overjet. D, A Le Fort III osteotomy was performed
and bilateral Stryber-Leibinger modular internal distractors were secured
along the planned line of distraction. E, Photograph of the patient during
the distraction phase with activation rods exiting the temporal region. F,
Lateral cephalogram following distraction and consolidation phases. Comparison
to preoperative cephalogram reveals marked improvement in midfacial proportions.
G and H, Preoperative frontal and lateral photographs. I and J,
Postoperative frontal and lateral views 16 months following distraction demonstrating
excellent cosmetic outcome with correction of exorbitism and a normalized
facial profile. K, Before and L, after base views showing improved midfacial
projection. M, Pretreatment and N, posttreatment intraoral photographs illustrating
occlusal correction into a class I relationship and interval exfoliation of
the maxillary anterior deciduous incisors.
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Figure 5. A 5-year-old girl with primary
mandibular-maxillary syngnathia and associated glossoptosis-micrognathia requiring
a tracheostomy since infancy. A, Preoperative acrylic model used for treatment
planning demonstrates a markedly micrognathic and retrognathic mandible. B,
Intraoperative photograph showing bilateral posterior body osteotomies and
insertion of internal Stryber-Leibinger (Kalamazoo, Mich) intraoral devices.
Panorex x-ray films: C, before and D, after 21 mm of horizontal advancement.
Profile views: E, before and F, 12 months after treatment showing good improvement
in lower face profile.
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Figure 6. A 31-year-old woman with Hallerman-Streiff
syndrome had residual micrognathia and severe malocclusion following a previous
attempt at correction with bilateral sagittal split osteotomies and conventional
mandibular advancement. A, Lateral 3-dimensional computed tomographic scan
demonstrating mandibular hypoplasia and marked displacement of the condyles
anteriorly out of the hypoplastic glenoid fossae. Treatment consisted of posterior
body osteotomies and distraction using internal distraction appliances. Planned
bilateral advancement was 30 mm. The distraction vector was horizontal in
orientation. Midway during the distraction phase, one of the devices was noted
to have failed with lack of advancement when activated. B, Despite replacement
of the device, only 24 mm of distraction was achieved because of premature
consolidation at the osteotomy sites. Profile views: C, before and D, after
show a fair cosmetic outcome 18 months following surgery.
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Functional deformities fell into 3 basic groups: airway obstruction,
exorbitism (corneal exposure), and malocclusion. Airway obstruction was identified
in 14 patients as follows: 8 individuals with obstruction as the main indication
for distraction (6 with glossoptosis-micrognathia, 1 with Nager syndrome,
1 with maxillomandibular syngnathia) and 6 patients in whom obstruction was
an associated finding (5 with midfacial deficiency, 1 with Hallermann-Streiff
syndrome). The group treated primarily for airway obstruction included 2 infants
with glossoptosis-micrognathia who required urgent endotracheal intubation
after birth for whom consultations were requested for tracheostomy to manage
the airway. Both of these patients underwent external bilateral mandibular
body distraction and eventually were successfully extubated, thereby avoiding
tracheostomy. The remaining 6 patients had preexisting tracheotomies: 4 of
these patients underwent decannulation, 1 was pending tube removal following
unrelated surgery, and 1 has failed attempts at decannulation. All 5 patients
who underwent Le Fort III advancement demonstrated some degree of preexisting
oropharyngeal or nasopharyngeal obstruction and disturbed sleep patterns.
Formal sleep studies were available for 3 of these patients before and after
advancement, demonstrating resolution of moderate sleep apnea based on the
respiratory index. One patient with Hallerman-Streiff syndrome noted subjective
clinical improvement of associated airway obstruction. Exorbitism and varying
degrees of problems with corneal exposure were present in all 5 patients with
craniofacial dysostosis. Midface advancement was able to sufficiently improve
corneal protection and decrease the conjunctival inflammation in these patients.
Malocclusion and related masticatory disturbances were the most prevalent
functional problem in our group of patients, presenting in 16 of 24 patients
overall. Unfortunately, they were also the most refractory to correction.
In the 6 patients with maxillary deficiency, anteroposterior malrelationships
ranged from moderate to severe class III skeletal disharmony. These patients
underwent midfacial advancement and 3 were corrected to class 1, whereas the
remaining 3, although substantially improved, were left with some degree of
class III relationship (Figure 4).
Hemifacial microsomia patients (6) presented with mild to moderate degrees
of ipsilateral class II molar relationships, midline shift, and occlusal cant.
Although the anteroposterior relationships underwent favorable changes, 5
of these patients developed posterior open bites on the distracted side, which
did not fully close with postoperative guided eruption of the posterior maxillary
teeth. Furthermore, shifts in the mandibular midline to the contralateral
side were noted in 4 patients. Distraction of the shortened hemimandible to
the desired length was complicated by deviation of the mandibular midline
to the opposite side and development of crossbites in 4 of the 6 patients.
Of the various preexisting occlusal deformities, open bite and transverse
crossbites were the most refractory to correction. In 6 patients with anterior
open bites, 1 experienced closure and 1 was improved, whereas the remaining
4 were no better at the end of treatment. Posterior crossbite, present in
8 patients before DO, was corrected in only 3.
A variety of complications manifested during the distraction phase of
the process. Problems with the distraction device occurred in 4 patients.
In 2 cases, the turning mechanism failed and revision surgery with device
replacement was undertaken (Figure 6). In the other 2 patients, the fixation became unstable during the distraction
phase and surgical reinsertion was also performed. Notably, in 3 of these
4 patients, the desired direction or magnitude of distraction was not attained
at the completion of the distraction phase. In 2 other patients, patient noncompliance
was clearly identified, also resulting in significantly less advancement than
planned for. Late complications were primarily related to adverse soft tissue
effects, including linear scarring at the transcutaneous pin sites with external
devices (4), temporal wasting (2), transient trismus (2), and infection (2).
The patient with the oncologic segmental mandibular defect developed wound
breakdown, infection, and hardware exposure, leading to abandonment of the
distraction procedure. Notably, there were no cases of fibrous nonunion across
the distraction sites, and at the time of internal device removal, healthy
bone regenerate was verified (Figure 7).
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Figure 7. Intraoperative view at the time
of removal of internal distraction devices following the consolidation phase
in a young patient who underwent Le Fort III distraction. Note the quality
of the maturing bone regenerate in the distraction gap (instrument tip).
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COMMENT
Our results demonstrate that DO is a useful technique and can be a powerful
tool for the correction of various structural deformities and functional deficits
throughout the craniofacial skeleton. The early outcomes described herein
support ongoing evaluation to assess the long-term efficacy of craniofacial
DO and its role relative to conventional craniofacial techniques. The extent
of bony advancement that we achieved has been retained throughout the follow-up
period in all patients, which would suggest an increased resistance to relapse,
a well-documented problem with conventional craniofacial advancements. Furthermore,
in patients with severe skeletal deficiencies or soft tissue scarring, the
advancements planned and achieved using DO were well beyond what could have
been hoped for using conventional surgery. In most of our cases, the desired
advancements were achieved within 2 mm based on clinical or radiographic follow-up.
Factors identified as contributing to a shortfall in the distraction length
included device failure and patient noncompliance.
Careful selection and planning are critical to a successful outcome.49-53
Initially, all of the necessary preoperative data are acquired and analyzed
to precisely define the existing deformities. The various subunits of the
craniofacial skeleton that need mobilization and repositioning can then be
identified to determine the nature of the desired osteotomies and the vector
(direction and magnitude) of distraction (Figure 8). Preoperative considerations for mandibular distraction
include the preexisting shape of the lower jaw (ramus height, body length,
gonial angle), mandibular size (mandibular length), position of the lower
jaw relative to the maxilla and cranial base, presence of asymmetry, and mandibular
plane angles. Midface considerations during planning include the degree of
exorbitism, the presence of anteroposterior deficiency within the midfacial
bony structures, midface position relative to the cranial base and the mandible,
vertical midface deficiency or elongation, maxillary plane, and occlusal plane
angles. Whenever the desired movements will affect occlusion, a complete occlusal
analysis needs to be undertaken, including anteroposterior occlusal relationships,
transverse relationships, the presence of open bite deformities, occlusal
plane angles, and incisor inclinations. The planning phase requires cooperation
between the surgeon and orthodontist if the skeletal movements will affect
the dentition. Once the osteotomy design and vector of displacement have been
determined, selection of the appropriate mode of distraction and the specific
appliances to be used requires careful evaluation.54-58
Internal appliances are more suited for uncomplicated movements where displacement
in only one direction is required. Once internal appliances are in situ, modification
of the distraction vector is not possible. External devices have the advantage
of allowing multidirectional distraction in several different planes of movement.
They provide a greater degree of freedom in selecting the desired vector of
distraction (both angular and linear movements are possible) and the ability
to change the vector of incremental advancements during the distraction phase
as necessary. This is advantageous in controlling occlusal relationships.
Furthermore, external appliances are typically retained via transcutaneous
threaded pins, which facilitates use in very young patients whose facial skeleton
size may preclude using the plate and screw fixation necessary to secure internal
devices. Lastly, external devices are easily removed following consolidation
in contrast to internal devices, which often require a major surgical procedure
to expose and retrieve. Disadvantages with external devices are unavoidable
scars as a result of the transcutaneous fixation pins. These produce at times
a severe linear scar that grows as the pins are splayed during the distraction
process. Also, external devices can be bulky, and having the appliances on
for 8 to 12 weeks can be a drawback for some patients.
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Figure 8. Careful preoperative planning
to determine the vector of distraction is a critical element for success.
A, Distraction of the lower jaw can be horizontal, vertical, or oblique, depending
on the nature of the preexisting deformity. Assessment should consider the
height of the posterior ramus (Ar-Go), the length of the mandibular body (Go-B),
and the overall length of the mandible (Ar-B). A horizontal vector (H) is
used to primarily correct deficient length of the mandibular body. A vertical
vector (V) is selected to increase an abnormally short posterior ramus. Oblique
vectors (O) are useful to increase both ramal height and body length. Note
that cases with an abnormal gonial angle (Ar-Go-Me) are best managed with
an external device capable of angular distraction. Go indicates gonion; B,
"B" point; Ar, articulare; SN, sella-nasion plane; MP, mandibular plane; and
Me, menton. B, Planning for midface distraction should consider the linear
and angular measurements that describe the size and relationship of the orbitomaxillary
complex to the cranial base. Key measurements are illustrated and the accompanying
numeric values are approximate norms for adults. Each patient should be assessed
relative to age-specific norms, keeping in mind, however, that inherent growth
deficiencies will persist despite distraction osteogenesis. It is therefore
useful to overcorrect close to adult values when treating patients who have
completed most of their midfacial growth. S indicates sella; N, nasion; Po,
porion; Or, orbitale; Ba, basion; MxP, maxillary plane; ANS, anterior nasal
spine; Me, menton; SE, sphenoethmoid intersect; PNS, posterior nasal spine;
and A, "A" point.
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In our series, the best aesthetic results were obtained in the midfacial
advancement cases and to a lesser degree in patients with hemifacial microsomia.
Cases with bilateral micrognathia did not achieve as favorable a cosmetic
outcome. This may have been contributed to by the younger age group of the
micrognathia patients. Because of the lack of a proven track record using
distraction techniques in the very immature facial skeleton, we tended to
be conservative in setting our distraction length objectives with these patients.
Furthermore, in these patients, the primary objective of distraction was to
improve or eliminate upper airway obstruction.
Functional results were encouraging, particularly in resolving or significantly
improving airway obstruction. Distraction osteogenesis holds great potential
for addressing the structural problems that contribute to acute respiratory
obstruction in patients with glossoptosis-micrognathia or other disorders
with small jaws. Our results indicate that early intervention in infants and
young children with DO can prevent the need for tracheotomy or allow early
decannulation in children with existing tracheotomy. Because most glossoptosis-micrognathia
patients improve spontaneously over time, the cases should be carefully selected.
We have reserved mandibular distraction for those with severe micrognathia
and obstruction in which case conservative surgical techniques have a lower
success rate. These patients include those who would otherwise go on to tracheotomy
or those with an existing tracheotomy tube who are not demonstrating "catch-up"
growth. Less severe cases can still be adequately managed with more conservative
techniques, such as lip-tongue adhesion. Impressive improvements in the nasopharyngeal
airway were also seen in the patients undergoing midfacial advancement. Furthermore,
these patients experienced resolution of conjunctival exposure associated
with exorbitism. Occlusal correction proved less amenable to correction using
DO. Anteroposterior discrepancies had the best improvement; this is not surprising
considering that the planned vectors of distraction used to correct the skeletal
base disharmony coincided with the necessary movements needed to improve the
dental bases in most patients. Transverse deficiencies, crossbites, and open
bites, on the other hand, were not of primary concern when planning the distraction
strategy. Notably, most of the patients were left with some occlusal discrepancies
that are within the realm of orthodontic correction, although a few will eventually
require finishing orthognathic procedures.
The timing of intervention has always been a contested issue in craniofacial
surgery. During the past several years, improved expertise and biotechnology
have allowed better results during manipulation of immature bone using conventional
techniques. Nonetheless, considerable drawbacks exist when attempting to reposition
and internally fixate bone segments in young patients. Uncontrolled and excessive
fragmentation during manipulation of fragile bone segments, inadequate bone
stock in which to secure metal plates and screws, and injury to developing
tooth buds are all well-recognized risks in this setting. Distraction osteogenesis
represents a major advancement in addressing this problem owing to the lack
of intraoperative repositioning and the minimal fixation requirements.
The less invasive nature of DO permits advancements in earlier age groups
than conventional surgery whenever such intervention would be appropriate.
In our group, the distraction procedures were carried out well ahead of the
age groups considered typical when using conventional surgical repositioning.
The results yielded substantial improvements in form and function at an earlier
developmental period. Another argument in favor of DO that was borne out in
our study is the ability to achieve very large degrees of advancement. When
attempting such movements with conventional craniofacial repositioning, the
restrictive soft tissue envelope often physically precludes extreme advancements.
It seems that the gradual movement in DO is able to overcome the soft tissue
pull through a stepwise adaptive process. The long-term outcome of earlier
intervention with DO, particularly as it relates to stability, remains to
be seen because most studies still do not have long enough follow-up. Early
reports39, 58 demonstrate excellent
stability of distraction procedures relative to conventional craniofacial
repositioning, particularly in cases with large advancements or restrictive
soft tissue envelopes. However, despite the potential for improved results,
the distraction procedure will not reverse or correct the underlying syndrome-specific
abnormalities, and growth will eventually lag at the affected areas as the
patient ages after treatment.34 Thus, intervention
at an earlier age with DO may or may not preclude additional surgery. In severe
deformities, however, the patient will have several years of considerably
improved form and function, and if revision procedures are necessary at the
end of skeletal maturity, they will be technically easier to perform.
CONCLUSIONS
In summary, DO has great potential for several congenital and acquired
osseous defects that can be encountered within the craniofacial skeleton.
Numerous advantages have been cited among the growing number of advocates
for craniofacial DO: (1) The surgery is less invasive and associated with
a shorter hospital stay, less tissue dissection and bone manipulation, and
decreased blood loss. (2) There is no need for bone grafting to maintain repositioned
segments. (3) There is no risk of growth restrictions secondary to plate fixation.
(4) In severe skeletal deficiencies, there is the potential for substantially
larger osseous movements and greater stability postoperatively, especially
in cases with a scarred soft tissue bed. (5) There is the potential for improved
soft tissue augmentation associated with distraction histogenesis. (6) Finally,
surgical intervention is possible in younger age groups. Experience with the
technique is still limited, and more investigation is necessary to refine
technique and improve the design and reliability of distraction devices. Investigations
are under way to determine the feasibility of combining endoscopic techniques
to perform the initial osteotomies with insertion of external distraction
appliances. As experience with distraction increases, its exact role in correcting
craniofacial deformities will eventually become apparent. Currently, it seems
that DO is most useful for more severe anomalies in which there is a need
for earlier intervention, the risk of relapse with conventional techniques
is high, and conventional osteotomies are not feasible.
AUTHOR INFORMATION
Accepted for publication May 7, 2001.
Corresponding author and reprints: Mario J. Imola, MD, DDS, FRCSC,
Craniofacial–Skull Base Center, 1601 E 19th Ave, Denver, CO 80218 (e-mail: MJImola{at}qwest.net).
From the Craniofacial–Skull Base Center, Denver, Colo (Drs Imola
and Chowdhury); the Department of Otolaryngology–Head and Neck Surgery,
University of Colorado, Denver (Drs Imola and Chowdhury); and the Department
of Otolaryngology–Head and Neck Surgery, Division of Facial Plastic
and Reconstructive Surgery, University of Minnesota, Minneapolis (Drs Imola,
Hamlar, and Thatcher).
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