Biomechanical and Clinical Implications of Distraction Osteogenesis In

Biomechanical and clinical implications of distraction osteogenesis in

craniofacial surgery

Ulrich Meyer, Johannes Kleinheinz, Ulrich Joos

Department of Cranio-Maxillofacial Surgery (Head: Prof. Dr. Dr. Dr. h.c. U. Joos MD, DMD), Universityof M .unster, Germany

SUMMARY. Introduction: Craniofacial distraction osteogenesis is an established surgical procedure to correctbony malformations. Force transduction through the osteotomized bone fragments elicits defined biologicalresponses in the gap tissue, which determines the clinical success of the distraction treatment. Objective: Thepurpose of this investigation was to evaluate clinically a new distraction protocol based on an analysis of thebiological and biomechanical parameters executing direct effects on bone regeneration during distraction. Studydesign: A multistep distraction protocol was used in 39 patients and the clinical outcome was monitoredpostoperatively. Results: All the distraction cases were successful with a single exception. Segmentaldisplacements were stable clinically and radiologically. Conclusion: In order to improve the clinical success ofdistraction osteogenesis, individual treatment protocols are recommended. r 2003 European Association forCranio-Maxillofacial Surgery.

Keywords: Distraction osteogenesis; Biomechanics; Mechanical strain

INTRODUCTION

Distraction osteogenesis is a widely employed surgi-cal approach in the treatment of patients with bonymalformations. The clinical success of craniofacialand alveolar distraction procedures relies on multiplefactors, particularly bone anatomy and biomecha-nics, the distraction forces and the surgical techniquesapplied. This article reviews some basic principles ofdistraction biology and focuses on its clinicalapplication. The implications of this treatmentstrategy, as described, rely on guidelines developedin our department.

BONE BIOMECHANICS

The biomechanical impact of distraction osteogenesison regenerating bone tissue is a highly complex anddynamic process (Burger and Veldhuijzen, 1993).Physical and biological parameters affecting thesuccess of distraction osteogenesis include the macro-and microscopical bone anatomy, the direction andamount of the applied distraction forces, and theregenerative capacity of the tissues involved. Forcetransduction via adjacent structures (joints, liga-ments, muscles, and soft tissue) influences theregeneration of the tissue between the bone fragmentsby modulating the stress produced within the callus(Fig. 1).The reaction of bone to distraction can be analysed

at least at two levels. Firstly, we can assess thematerial properties of bone independently of itsanatomical structure and geometry by performing

standardized mechanical tests on uniform specimens(Turner et al., 1995). Secondly, by examining themechanical behaviour of bone as an anatomicalentity, the contributions made by its structuralproperties can be assessed. Mechanically, theseproperties determine how bone responds to forcesin a clinical setting.Load is a commonly used and poorly defined term.

It is not defined in terms of a physical determinablecondition. It is mostly used when a complexmechanically related state is meant. In contrast toload, various other terms are precisely defined. Theinternal resistance to an applied force is known asstress. Stress, defined as equal in magnitude butopposite in direction to the applied force, isdistributed over the cross-sectional area of the boneand callus tissue. It is expressed in units of force perunit area: stress=d=force/area. In distraction osteo-genesis, mechanical forces are transduced through theseparated bony fragments, the regenerating tissue,and the adjacent tissues. Due to the different materialproperties of bone, callus, and other tissues, theforce/stress relationship is more complex in distrac-tion osteogenesis than it is in intact bone.Most stress patterns are combinations of three

stress types, namely tension, compression, and shear(Einhorn, 1996). Bending, for example, produces acombination of tensile forces on the convex side of amaterial and compression on the concave side.Torsion or twisting produces shear stress along theentire length of a material, while tensile stress tends toelongate it and compressive stress tends to shorten it.Distraction osteogenesis can be regarded as anuniaxial elongation leading to the development of

ARTICLE IN PRESSJournal of Cranio-Maxillofacial Surgery (2004) 32, 140–149

r 2003 European Association for Cranio-Maxillofacial Surgery.

doi:10.1016/S1010-5182(03)00131-8, available online at http://www.sciencedirect.com

140

tensile stress. The measurement of deformationnormalized to the original length of the specimen iscalled strain:

strain ¼ E

¼ change in length=original length

¼ ðdeformed length-orig: lengthÞ=ðorig: lengthÞ:

Strain is expressed as a percentage change.At low levels of stress, there is a linear relationship

between the stress applied and the resultant deforma-tion (in bone) in terms of strain (Curey, 1962). Thisproportionality is called the modulus of elasticity orYoung’s modulus. It is a measured from the slope ofthe linear portion of the curve and is calculated bydividing the stress by the strain at any point alongthis linear portion of the curve. If the stress/strainarea were to be generated by testing a whole bone asopposed to a uniform specimen, this measurement ofthe linear slope would give the stiffness or rigidity ofthe bony or callus tissue. With respect to distractionosteogenesis, it is important to recognize thatYoung’s modulus is nearly stable in the separatedbony fragments, but varies in the regenerating tissuedepending on its maturation stage. Thus, themodulus of elasticity of the elongated gap tissuealters during the distraction treatment.Energy put into deformation of an elastic material

just prior to reaching the yield point can be recoveredby removing the stress (Chamay, 1970). The energyrecovered is known as resilience and is a measure ofthe ability of the material to store energy. Althoughthis energy is not always recoverable in a useful form,it will not be lost as long as the material does notundergo permanent deformation. The ability to storeenergy highlights the need to stabilize an elongatedspecimen at the end of the distraction treatment inorder to prevent an immediate relapse (Mc Tavishet al., 2000).The relation between force application and resul-

tant bone deformation is crucial for the outcome ofcraniofacial distraction. A variety of powerful ap-proaches has generated a comprehensive picture ofhow the gap tissue responses to deformation. Load-dependent deformation of bones can be evaluated byvarious approaches. Indirect measurements of defor-

mation are measured indirectly by means of intraoraldevices (Ahmad et al., 1982; Mosley et al., 1997) orstrain gauges (Ahrendts and Sigolotto, 1989, 1990).Additionally, holographic interferometry has beenperformed to assess the impact of mechanical forceson bone deformation (Ferre et al., 1985). One majordisadvantage of all experimental studies using eitherstrain gauges or holographic interferometry is theinability to determine strains at defined positionswithin the specimen. Biomechanical research by thesemethods is limited to surface deformations andneither stresses nor dislocations within the gap tissuecan be measured directly.Computer-aided simulation based on finite element

analysis (FEA) has addressed the adequacy ofmathematical models to relate mechanical factorssuch as load transfer to the biomechanical behaviourof tissue specimens. Given a high correlation betweenthe calculated FEA and the measured experimentaldata, various biomechanical parameters can bedetermined within the distracted tissue (Meyer et al.,2000; Vollmer et al., 2000). The accuracy of FEA indescribing the biomechanical behaviour of non-livingbony specimens has been demonstrated by variousauthors (Hart et al., 1992; Voo et al., 1996; Koriothand Verslius, 1997).In contrast to explanted tissue, living bone has a

very different structure. This is due to the highdynamics of tissue remodelling. Intact bone as well asseparated bony fragments bridged by callus tissueundergo deformation in a changing mechanicalenvironment and react biologically when a force isapplied (Aronson, 1994). If the bone or the fragmentsare fixed so that they cannot move, or if equal andopposing forces are applied to them, deformation willoccur, resulting in the generation of internal resis-tance to the applied force (Brown and Ferguson,1978). Whereas a number of in vitro studies haveinvestigated the relationship between the appliedforces and the resulting bone deformation, much lessis known about the in vivo behaviour of intact orosteotomized bones. Because of the technical pro-blems involved, it is difficult to measure directly theforce required to distract the various skull bones andthere is only one recent report on this subject in theliterature (Robinson et al., 2001). On account of theirmore simplified geometry, the distraction forces havemainly been measured in long bones. However, thishas yielded a better insight into the mechanicalproperties of callus tissue.

PRINCIPLES OF LOAD-RELATEDBONE REGENERATION

The purpose of bone is to provide structural strengthcorresponding to its mechanical use (Brown andFerguson, 1978). This means that bones provideenough strength to keep voluntary physical loadsfrom causing pain or damage as a result of fractures.However, recent research has suggested that bonesalso display an extraordinary adaptive behaviour

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Fig. 1 – Different biomechanical parameters involved in thedistraction osteogenesis.

Biomechanical and clinical implications of distraction osteogenesis in craniofacial surgery 141

towards a changing mechanical environment, whichis often regarded as phenotype plasticity (Aegerterand Kirkpatrick, 1973; Frost, 2000a, 2000b; Losos,2001; Frost et al., 2002). Specific strain-dependentsignals are thought to control this adaptive mode ofbony tissue modelling (Meyer et al., 1997b). Theadaptive mechanisms include basic multicellular units(BMUs) of bone remodelling. Effector cells withinBMUs have been shown to function in an inter-dependent manner. While hormones may bring aboutas much as 10% of the postnatal changes in bonestrength and mass, 40% are determined by mechan-ical effects. This has been shown by the loss ofextremity bone mass in patients with paraplegia(more than 40%).Modelling occurs by separate formation and

resorption drifts to reshape, thicken, and strengthena bone or trabecula by moving its surfaces around intissue space (Frost, 1964a, 1964b, 1983, 1986;Takahashi, 1995). Remodelling also involves bothresorption and formation of bone. BMUs turn boneover in small packets through a process in which anactivating event causes some bone resorption andbone formation is following (Frost, 1964a, 1964b;Martin et al., 1998; Jee, 1999). This BMU-basedremodelling operates in two modes: ‘‘conservationmode’’ and ‘‘disuse mode’’ (Frost and Sch .onau, 2000).A specific strain threshold range controls which ofthese two modes is active at any given time (Martinet al., 1998).

MECHANOTRANSDUCTIONOF OSTEOBLASTS

As distraction healing is a highly dynamic cellularprocess, tensile strains are the leading stimuli forbone regeneration (Frost, 1992). It is generallysuggested that distraction forces leading to cellulardeformation are signalled to the cellular genomethrough mechanotransduction. Mechanotransduc-tion, or the conversion of a biophysical force into acellular response, is an essential mechanism in bonebiology (Meyer et al., 1997a). It allows bone cells torespond to a changing mechanical environment.Mechanotransduction can be categorized in anidealized manner into (1) mechanocoupling, whichmeans the transduction of mechanical force appliedto the tissue into a local mechanical signal perceivedby a bone cell; (2) biochemical coupling, the transduc-tion of a local mechanical signal into biochemicalsignal cascades altering gene expression or proteinactivation; (3) transmission of signals from the sensorcells to effector cells, which actually form or removebone; and ultimately (4) the effector cell response.When loads are applied to bone, the tissue begins todeform causing local strains (typically reported inunits of microstrain; 10,000 microstrain=1% changein length). It is well known that osteoblasts andosteocytes act as the sensors of local bone strains andthat they are appropriately located in the bone forthis function.

Various investigators have revealed that reactiveloads give rise to relatively high strains at frequenciesthat extend from 1 to 10Hz. It was found that peakstrain magnitudes measured in a wide variety ofspecies are remarkably similar, ranging from 2000 to3500 microstrain. Lanyon et al. (1975) showed that,within a single period of loading, the remodellingprocess seemed to be saturated after only a few (o50)loading cycles. Further strain repetitions then pro-duced no extra effect.There have been many suggestions for biophysical

transduction mechanisms on a cellular level (Andoet al., 1988; Binderman et al., 1988; Jones et al., 1991;Takei et al., 1998). Most studies have suggested thatmechanical strain transduction might be directlyrelated to mechanical deformation of ultrastructuralorganelles or proteins, thereby converting the me-chanical information into a biochemical signal.Although hydrostatic compression has been pro-posed as being analogous to physiological strain andre-creates physiological strain effects, no significantdistortion or compression of the fluid-filled cell canoccur until very high pressures are attained (Bogi andBurger, 1989). Cell elongation seems to be the drivingforce for signal transduction. Studies of cell elonga-tion in vitro have demonstrated that physiologicalloading of osteoblast-like cells induces the differen-tiation of osteoblasts, whereas hyperphysiologicalload tends to dedifferentiate cells towards a fibro-blastic phenotype (Meyer et al., 2001c; Fig. 2). It isassumed that the strain sensor is linked to thecytoskeleton, although other hypotheses have beensuggested. If the strain sensor is located in thecytoskeleton, then deformations of this structurewould tend not to be homogeneous because differentcompartments would have different mechanicalproperties and the weakest link would deform themost. In this model, elongation of bone cells appearsto influence subsequent transcriptional events.

LOAD-RELATED DISTRACTION HEALING

In clinical terms, gradual distraction of bonesmechanically elongates the gap tissue. Because the

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Fig. 2 – Load-related cellular responses as assessed in cell cultures.High strains (>3000mstrain) lead to a fibroblastic, whereas lowerstrains (300–3000mstrain) result in an osteoblastic differentiation.

142 Journal of Cranio-Maxillofacial Surgery

osteoblast is the principal cell for bone growth andregeneration, straining of osteoblasts seems to be themajor determinant influencing the subsequent tissueresponses in distraction osteogenesis. Several investi-gations have been performed to evaluate basicmechanisms of bone formation following distractionof the mandible (Karp et al., 1992; Stewart et al.,1998). Treatment protocols differed substantiallyamong the various authors, thus making it nearlyimpossible to directly compare the effects of tissuestrains. The chosen schedules of daily bone lengthen-ing in these studies were based mainly on clinicalexperience. In rabbits, mandibular osteotomies weresubjected to defined daily strains using an implantedmechanical distractor. This animal model allowed theapplication of defined strain magnitudes (Frost,1996). These studies demonstrated that bone regen-eration is primarily strain related (Meyer et al.,1999c). It appeared that differentiation and matrixproduction in the bone cell, are influenced by theapplied peak strain magnitudes. Different histologicalpatterns corresponding to various stages of bonehealing were distinguished. Woven and, in someareas, lamellar ossification predominated at physio-logic strain magnitudes (B2000 microstrain). Athigher strain magnitudes (B20,000 microstrain)trabecular bone formation is detected, and fibroustissue formation is observed at ‘‘hyperphysiological’’strain magnitudes (200,000 and 300,000 microstrain,respectively). Interestingly, the application of multi-ple increments of distraction (10 cycles per day vs. 1cycle per day) does not change the histologicalpattern of bone regeneration, but leads to an overallgreater extent of bone deposition (Meyer et al.,1999a; Fig. 3). In specimens distracted at higherstrain magnitudes (B20,000 microstrain), areas con-taining chondroid tissue are often observed in theosteotomy gap. Micromovements within the osteo-tomy gap may also lead to the development ofcartilage (Klotch et al., 1995; Meyer et al., 2001b).Growth factors are likely to play an important role

in distraction osteogenesis. Mechanical strain hasbeen shown to increase the expression of transform-

ing growth factor beta in mandibular lengthening(Farhadieh et al., 1999). It was demonstrated that, inresponse to cyclical strain, human osteoblasts re-leased transforming growth factor beta into theconditioned culture medium (Neidlinger-Wilke et al.,1995). Furthermore, insulin-like growth factor I andbasic fibroblast growth factor have both beenidentified in the distracted region of osteotomizedmandibles suggesting that these proteins promoteosteoblast proliferation and formation from mes-enchymal precursor cells (Farhadieh et al., 1999). Ithas been shown that the oestrogen receptor isinvolved in the adaptive response of osteoblasts tomechanical strain (Damien et al., 1998, 2000). Stressin mechanically loaded bone may activate themitogen-mediated protein kinase (MAPK) pathway,finally leading to phosphorylation and activation oftranscription factors such as c-Jun N-terminal kinase(Matsuda et al., 1998). Recent evidence suggests thatmechanical loading of bones induces immediate earlygene expression such as Egr-1 and nuclear transloca-tion of NF-kappa B (Dolce et al., 1996: Granet et al.,2001). Other modulators of mechanotransduction inbone are parathyroid hormone, prostanoids, andextracellular calcium (Mikuni-Takagaki, 1999).Bone cells are assumed to act in vivo by secreting

cytokines in a strain-related manner. Differentcytokines are involved in paracrine and autocrinesignalling cascades (Cheal et al., 1991; Meyer et al.,2001a; Bouletreau et al., 2002; Fig. 4).A reduced vascular supply with diminished oxygen

tension might be an additional factor controlling theclinical outcome of distraction osteogenesis. Somestudies indicated that vascular formation was notprominent in the regenerated tissue and that angio-genesis was not found until later stages of tissueregeneration (Sawaki et al., 1996). In contrast, Roweet al. (1999) described an intense vascular responseassociated with mandibular distraction in the earlystages of osteogenesis.Numerous studies have recently demonstrated

that both the collagen microarchitecture and themineralization process of bone are influenced by

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Fig. 3 – The histological appearance of the elongated mandibular tissue critically depends on the applied strain magnitude and frequency.


mechanical stress. Tensile strains were found to beassociated with an increased tissue anisotropy.Collagens have been identified as important structur-al proteins, which are sensitive to the application ofmechanical strains. Fratzl et al. (1998) have shownthat low strains lead to a straightening of collagenfibres, whereas higher strains induce a moleculargliding within the fibrils, resulting ultimately in thedisruption of the fibrillar organization.Ultrastructural data from various studies support

the hypothesis of a structural alteration of collagenassembly under tension stress (Farrar et al., 1978;Mosley et al., 1997; Fratzl et al., 1998; Meyer et al.,2001d). The spatial orientation of collagen fibresappears to be associated with different modes ofmineralization, as judged by electron microscopy,diffraction analysis, and element determination byanalysis of X-rays. In rabbit mandibles distracted atlow strains (2000 microstrain), electron diffractionanalysis demonstrated mature crystal formation, asindicated by the appearance of multiple reflectionpatterns. In contrast, even moderately elevated strainapplications (20,000 microstrain) result in immaturecrystal formation, containing predominantly devel-oping apatite crystals.The stimulating effect of mechanical distraction on

osteoblast proliferation and extracellular matrixsynthesis is well characterized (Karp et al., 1992;Stewart et al., 1998). However, little is known aboutthe involvement of apoptosis in bone regenerationand distraction osteogenesis. Using light microscopyto detect apoptotic bodies in injured rat tibia, Landryco-workers (1997) have suggested that osteoblasts areremoved from the injury site via apoptosis. Recentinvestigations into distraction osteogenesis haverevealed that incremental traction of osteotomizedmandibles results in an enhanced rate of apoptosis(Fig. 5). As a result of hyperphysiological strainapplication, some osteoblastic cells in the newlyformed tissue at osteotomy sites undergo apoptosis.In contrast, mandibles exposed to low magnitudes ofstrain display only minimal, if any, evidence of

programmed cell death (Meyer et al., 1999b). Recentfindings have implied that in human osteoblasts thesmall G-protein H-Ras induces apoptosis and in-hibits integrin-mediated adhesion (Tanaka et al.,2002).

MULTISTEP DISTRACTION PROTOCOL

The fact that large gaps between bone fragmentsimpair bone healing (Watson-Jones, 1943) has beenwell documented. However, with the Ilizarov proce-dure, healing is possible even in very large gaps if theycommence as small gaps and increase in size veryslowly (Ilizarov, 1989). This contradicts the formerview that micromotion between fracture or osteot-omy fragments impair bone healing (Watson-Jones,1943). While excessive motion can obviously preventhealing, the extent to which small strains actuallyhelp to guide the remodelling in bone healing is, atpresent, unclear (Carter et al., 1988; Kenwright andGoodship, 1989; Hanafusa et al., 1995; Buckwalterand Grodzinshy, 1999; Meyer et al., 1999a, 1999c). Itshould be noted that fracture and distraction repairsproceed by the same cellular healing process,although the strain rate as related to ossification is

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Fig. 4 – Feedback regulation of tissue development. Osteoblasts synthesize the extracellular matrix that mineralizes according to the strainhistory. The newly developing tissue influences subsequent osteoblast reactions depending on the applied load and the stability of theregenerate.

Mineralized tissue Soft tissue/Apoptosis

Strain magnitude

<0,3% >3%

Distraction healing

0,3-3%

Callus tissue

Fig. 5 – Strain-related distraction healing. Physiological strain leadsto callus formation, maximum strains (>20,000mstrain) to non-union.


different. Ossification does not occur in fracturerepair when the mobility of the distracted fragment isexcessive. Non-union of bone is more likely to occurwhen the critical gap widens and fragment movementincreases. In distraction osteogenesis, with slowlyincreasing gap size, ossification occurs as long asstrain rates do not exceed a certain threshold.Theoretical considerations and experimental datasuggest a general upper limit of distraction strain inwhich the formation of a bony bridge is hindered(Fig. 5). To facilitate bony consolidation, a multistepapproach of strain application seems to be advisable.

CLINICAL CONSIDERATIONS

The clinical applicability of distraction osteogenesis isdependent upon device-related and tissue-relatedfactors. Device-related factors affect the mechanicalintegrity of the distractor and the stability of bonefixation (Mc Tavish et al., 2000). The number, length,and diameter of fixation pins, the rigidity of thedistractor fixation, and the material properties of thedevice affect the clinical result of the distractionprocedure (Brunner et al., 1994). Additionally, theorientation of the distraction device and the resultingdistraction vector relative to the anatomical axis ofthe distracted bone segments (as well as – in the caseof jaws – the occlusal plane and the joint position) areimportant considerations (Cope et al., 1999). Thesignificance of device orientation has been establishedin clinical settings and refinements have been made tooptimize the treatment outcome. Tissue-related para-meters affecting the quality of the distraction tissuegenerated include the geometric shape, the cross-sectional area, the density of the distracted bonesegments, the length of the distraction gap, and thetension of the soft tissue envelope (Hollis et al., 1992).In cranio-maxillofacial and alveolar distraction

osteogenesis it is important to consider dental aspectsin the planning of distraction osteogenesis. Theseaspects include predistraction orthodontics, osteo-tomy design and location, selection of the distractiondevice, orientation of the distraction vector, use ofdistraction splints, postdistraction orthodontics, andfunctional loading of the generated bone (Skerryet al., 1989; Smolka et al., 2001). As elongation of themandible leads to force transmission to the tempo-romandibular joints, structural alterations in theanatomy of the joints as well as the overlying softtissue might also be expected. Distraction proceduresshould take these joint effects into account (Kruse-L .osler et al., 2001).The correction of complex malformations should

be based not only on plain radiographs but also oncomputed tomographical (CT) data. Computer-assisted planning tools allow the virtual determina-tion of the osteotomy lines as well as the simulationof distraction progress (Meyer et al., 2002). Astereolithography model may be used as an addi-tional tool. The planning of the distraction procedurein maxillo-mandibular movements should be based

on the established principles of orthognathic surgery.Model operations with gliding splints and endpoint markers in the splint should be used to monitorthe distraction process in order to gain optimalocclusion.

CLINICAL OUTCOME

Distraction osteogenesis was performed in 39 cranio-facial cases by a multistep approach in our depart-ment (Table 1). In all of these, a multiple stepdistraction protocol was executed by a standardizedapplication of 10 cycles of 20,000 mstrain per day. Forthe application of 20,000 mstrain the initial osteotomygap was measured. The initial distraction length (2%of the gap length) was calculated according to thedistractor parameters. Subsequent distractions werethen performed in 2% increments of distractor axisrotation, i.e. in a strain dependant manner, indepen-dently of the applied forces. All but one multistepdistractions were clinically successful. In one case,elongation of the maxillary segment failed due to anincomplete osteotomy. No signs of instability of thesegments were observed in any patient. No prematureossification of the callus was present. Segmentdisplacements were stable during the follow-upperiods (6 month–4 years) in all cases as revealedby radiographic and clinical investigations (lateraland a.p. radiographs, dental casts, clinical measure-ments). Two representative cases illustrate the treat-ment approach.Case #1: Distraction was planned in an 18-year-old

patient with severe deficiency of the midface, Angle’sclass III skeletal relationship and open bite anteriorly(Fig. 6). Preoperative planning included a predistrac-tion orthodontic treatment, plain radiography andcomputed tomography. Based on the computedtomography, a stereolithographic model was con-structed. The distraction vector was defined by avirtual computer operation as well as by a plaster andstereolithography model operation. A multiple dis-traction protocol was applied to advance the midfacefollowing a Le-Fort III osteotomy (Fig. 7). A glidingsplint was used to control and monitor the midfacemovement. Elastics were applied for 3 weeks follow-ing the distraction procedure to stabilize the maxillo-

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Table 1 – Number of patients and extent of bone elongation,achieved by the various types of craniofacial treatment

Treatment Number ofpatients

Bone elongation(mm)

Mandibular advancement 8 10–21Mandibular widening 1 11Maxillary advancement 6 5–12Maxillary widening 15 4–8Midfacial advancement 9 5–9Total (mean) 39 8.7


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Fig. 7 – Same patient as in Fig. 6 before (a) and directly at the end (b) of the distraction. (c,d) Occlusion after removal of the gliding splint.

Fig. 6 – Planning and execution of midface advancement. (a) 18-year-old patient with severe deficiency of the midface prior to treatment. (b)Lateral radiograph demonstrates class III relation with an open bite. (c) Computed tomography and (d) stereolithographic model.


mandibular relationship. Two weeks later orthodon-tic treatment was resumed to refine dental occlusion.Case #2: Mandibular widening is demonstrated

in a 24-year-old patient with crowding of the frontteeth due to a transverse deficiency of the mandible(Fig. 8). A median osteotomy (initial gap size: 2mm)was performed under general anaesthesia and adistractor (Modus, Switzerland) was placed transver-sely across the osteotomy line on the labial aspect ofthe symphysis. After a latent period of 4 days,distraction was performed by application of 20,000microstrains 10 times a day. The daily increment wascalculated individually based on the initial gap of2mm. Distraction was performed for 14 days untilthe preoperatively planned positions were reached.The distraction was monitored continuously byclinical and instrumental examinations. Sonographicimages in combination with radiographs revealednormal mineral formation during the distractionperiod. At all times the gap tissue displayed ahomogenous structure. When bony consolidationwas complete 3 weeks later, mature bone formationwas observed in the gap tissue with a spontaneousmovement of teeth into the newly formed bone. Thiswas completed using postsurgical orthodontictreatment for 9 months. The patient was re-examined1 year after distractor removal and the mandibularwidening was found to be radiographically stable.The patient was preoperatively informed thatthe aesthetic impairment of the broadened chincan be corrected surgically. However, the patientrefused to undergo such a genioplasty becauseshe considered the final facial appearance asacceptable.

CONCLUSION

Taking biomechanical aspects of bone regenerationinto consideration allows a better understanding ofhow distraction osteogenesis operates in cranio-maxillofacial surgery.

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Ulrich Meyer MD, DMD, PhD

Department of Cranio-Maxillofacial SurgeryUniversity of M .unsterWaldeyerstr 30M .unster D-48149 Germany

Tel: +49 251 83 47 201Fax: +49 251 83 47 203E-mail: [email protected]

Paper received 22 September 2002

Accepted 23 September 2003

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Biomechanical and Clinical Implications of Distraction Osteogenesis In