Research Paper

Orthognathic Surgery

Int. J. Oral Maxillofac. Surg. 2012; 41: 1361–1368http://dx.doi.org/10.1016/j.ijom.2012.04.012, available online at http://www.sciencedirect.com

Comparative study of bonerepair in mandibular bodyosteotomy between metallic andabsorbable 2.0 mm internalfixation systems. Histologicaland histometric analysis indogs: a pilot studyC. E. Sverzut, R. B. Kato, A. L. Rosa, A. E. Trivellato, A. T. Sverzut, K. M. da Silveira,P. T. de Oliveira: Comparative study of bone repair in mandibular body osteotomybetween metallic and absorbable 2.0 mm internal fixation systems. Histological andhistometric analysis in dogs: a pilot study. Int. J. Oral Maxillofac. Surg. 2012; 41:1361–1368. # 2012 International Association of Oral and Maxillofacial Surgeons.Published by Elsevier Ltd. All rights reserved.

Abstract. The objective of this study was to compare the bone repair along amandibular body osteotomy stabilized with 2.0 mm absorbable and metallicsystems. 12 male, adult mongrel dogs were divided into two groups (metallic andabsorbable) and subjected to unilateral osteotomy between the mandibular third andfourth premolars, which was stabilized by applying two 4-hole plates. At 2 and 18weeks, three dogs from each group were killed and the osteotomy sites wereremoved and divided equally into three parts: the upper part was labelled the tensionthird (TT), the lower part the compression third (CT), and the part between the TTand CT the intermediary third (IT). Regardless of the treatment system, unionbetween the fragments was observed at 18 weeks and the CT showed moreadvanced stages of bone repair than the TT. Histometric analysis did not reveal anysignificant differences among the 3 parts or systems in the distance between bonefragments at 2 weeks. Although at 18 weeks the proportions of newly formed bonedid not differ among TT, IT and CT, significantly enhanced bone formation wasobserved in all sections for the metallic group. The patterns of repair were distinctbetween treatments.

0901-5027/01101361 + 08 $36.00/0 # 2012 International Association of Oral and Maxillofacial Surge

C. E. Sverzut1, R. B. Kato1,A. L. Rosa1, A. E. Trivellato1,A. T. Sverzut1, K. M. da Silveira1,P. T. de Oliveira2

1Department of Oral and MaxillofacialSurgery and Periodontology, School ofDentistry of Ribeirao Preto, University of SaoPaulo, Brazil; 2Departament of Morphology,Stomatology and Physiology, School ofDentistry of Ribeirao Preto, University of SaoPaulo, Brazil

Key words: internal fixation; osteotomy; bonerepair; absorbable system.

Accepted for publication 16 April 2012Available online 26 May 2012

ons. Published by Elsevier Ltd. All rights reserved.

1362 Sverzut et al.

Fig. 1. Plates positioned and fastened in the metallic group. Two of the bicortically engagedscrews passing through the lingual cortical surface can be observed.

Internal fixation (IF) systems have beenwidely used on the facial skeleton for thetreatment of fractures and osteotomies.Since the early 1980s, IF systems manu-factured with titanium (Ti) or its alloyshave been developed to achieve adequatemechanical behaviour, biocompatibilityand optimization of the bone repair pro-cess at the fracture or osteotomy site.1

Although the treatment of mandibularfractures and osteotomies with metallicIF systems is a well-established method,several problems have been associatedwith the use of these permanent devices,including infection, breakage, intraoralexposure, osteoporosis or bone atrophydue to the stress shielding effect, changesin anatomical contour, palpability, sensi-tivity to cold, screw migration, corrosion,interference in imaging examinations andradiation therapy, allergic reaction, inter-ference with the facial growth of paedia-tric patients and deposition of Ti particlesin scar tissue and lymph nodes.1–7 Toaddress these concerns, efforts have beenfocused on the development of a new classof materials: absorbable polymers.1,6,8

IF systems manufactured with absorb-able polymers were first used experimen-tally in monkeys and dogs,9–11 with lateruse in humans for the treatment of zygo-matic and orbital floor fractures.12,13

Absorbable polymers with different mole-cular weights, crystallinities, mechanicalproperties and absorption times have beenapplied. Among the most commonlyapplied polymers are polylactic (PLA)and polyglycolic (PGA) acids, which havebeen manufactured separately or in com-bination with each other, with their iso-mers or with other substances such ashydroxyapatite (HA).1,2,14–17

The absorbable IF system must main-tain the stability of bone fragments duringthe bone repair process, gradually trans-ferring the stress function to the bonewhile the IF system is resorbing.1,5,6 Someauthors propose that absorbable IF sys-tems have sufficient mechanical propertiesto allow effective bone repair at the frac-ture or osteotomy site,1,3 others disagree.15

Those in the latter group question the useof these systems in the mandible, wherethe forces resulting from muscular activityare greater than those sustained by themiddle or upper third of the face.

The dynamics of bone repair in dogswere clearly affected by changes in themechanical stimuli that occurred along theosteotomy site when a 2.0 mm metallicsystem was applied.18 This effect wasevident on histological analysis, althoughno statistically significant difference wasdetected in a histometric analysis.

Although some clinical,7,19,20 ani-mal1,2,16,21–23 and in vitro studies3,24 haveevaluated the mechanical behaviour of theabsorbable IF systems, few studies haveevaluated the influence of this behaviouron the bone repair process at the fracture orosteotomy site.9–11,25 A better understand-ing of this relationship can help optimizeprocedures, techniques, instrumentationand system manufacturing.

The objective of this study was to con-duct a histological and histometric analy-sis of bone repair along a mandibular bodyosteotomy stabilized with a 2.0 mmabsorbable IF system and to comparethe outcomes of these analyses with thoseobtained with the metallic IF system,which is considered to be the gold stan-dard in the literature.

Materials and methods

The study protocol was reviewed andapproved by the Institutional Animal Careand Use Committee of the University ofSao Paulo (Process 07.1.188.53.9) and allanimal care was conducted at the Experi-mental Animal Care Facility II under thecare of the same veterinarian. Twelvehealthy, male, adult mongrel dogs agedbetween 3 and 8 years old and weighingbetween 15 and 20 kg were used as sub-jects. The dogs were randomly divided intotwo groups after a simple draw according towhich IF system was applied (metallic andabsorbable groups). The previously men-tioned veterinarian administered generalanaesthesia, intravenous zolazepam/tileta-mine (0.12 ml/kg), which was maintainedwith inhalation of isoflurane and oxygenvia orotracheal intubation. After animalswere assigned to groups, an iatrogenicosteotomy was performed between thethird and fourth mandibular premolarsthrough an intraoral approach using a sur-gical micro-reciprocating saw. A surgical

blade with a thickness of 0.2 mm was posi-tioned perpendicular to the mandibularbody, and a continuous defect was created.The fragments were reduced manually andstabilized by applying 155 mm self-lockreduction forceps (Synthes, Oberdorf,Switzerland).

Metallic group

Two 4-hole Ti plates fixed with 2.0 mm Tialloy self-tapping screws in the 2.0 mmnon-locking system (Synthes, Oberdorf,Switzerland) were used according to therecommendation of the Association forOsteosynthesis–AO for a transverse frac-ture line without dislocation.26 Two1.5 mm drill bits with stop lengths of6.0 mm and 20.0 mm were used to drillholes near the alveolar process and themandibular base, respectively. The platepositioned along the alveolar process wasfastened monocortically with 6.0 mm self-tapping screws first, and then the otherplate was fastened bicortically along themandibular base. To ensure bicorticalengagement, the length of the self-tappingscrews was selected according to the buc-cal–lingual dimension of the mandibularbase, which varied from 8.0 to 12.0 mm(Fig. 1). After copious irrigation withsterile saline solution, the surgical woundwas continuously sutured with a 4-0absorbable thread (Poliglactina 910).

Absorbable group

Two 4-hole plates manufactured with acombination of L-polylactic acid (LPLA),D,L-polylactic acid (DLPLA) and trimethy-lene carbonate (TMC) were fixed with2.0 mm non-locking screws composedof the same material (Inion, Tampere,Finland). With the exception of thehandling of the plates and screw insertion,which were performed according to the

Bone repair in mandibular body osteotomy 1363

Fig. 2. Positioned and fixed plates in the absorbable group.

manufacturer’s recommendations, and themanufacture of the dental splint, the surgi-cal steps conducted in this group wereidentical to those in the metallic group.

To achieve adequate adaptation, theplates were immersed in a sterile salinesolution, warmed to 55 8C in a water bath(Thermo, Inion, Tampere, Finland) for5 min, and manipulated to the contoursof bone surface of the buccal cortical bonevia digital manipulation. Two 1.75 mmdrill bits with stops of 6.0 mm and22.0 mm were applied to generate holesnear the alveolar process and the mandib-ular base, respectively. Afterward, a tapwas manually inserted to the full drilldepth to provide threading for the screwsbefore insertion. The plate positionedalong the alveolar process was fastenedmonocortically with 7.0 mm screws first,and then the other plate was fastened alongthe mandibular base. To ensure bicorticalengagement, the length of the screws wasselected according to the buccal–lingualdimension of the mandibular base, whichvaried from 9.0 to 13.0 mm (Fig. 2).

Similar to the metallic group, aftercopious irrigation with saline solution,

Fig. 3. Dental splint created with 0.8 mm orthoinvolving the lingual surface of the first molar anfourth premolars and the cuspid.

the surgical wound was continuouslysutured with a 4-0 absorbable thread (Poli-glactina 910). Afterward, a dental splintwas created using a 0.8 mm orthodonticwire and light-cured composite resin invol-ving the lingual surface of the first molarand the buccal surfaces of the second, thirdand fourth premolars, as well as the canine(Fig. 3). The final configuration of thedental splint was obtained to avoid dentalcontacts during mandibular movementsand, consequently, some form of occlusioninterferences. Regardless of the group, nofunctional restriction was applied and allanimals were placed on a regular chow diet(dry granulated fodder) immediately fol-lowing the surgical procedure.

At 2 and 18 weeks post-treatment, threedogs from each group were killed per timeperiod. The mandibular body was resectedclose to the plates’ extremities and imme-diately placed in a 10% neutral bufferedformalin solution. After fixation, the regioncontaining the osteotomy site was carefullyresected with a 0.10 mm thick double-facedflexible diamond disc (Sorensen, Cotia, SaoPaulo, Brazil) and decalcified in 10% tri-chloroacetic acid. Decalcified specimens

dontic wire and light cured composite-resin,d the buccal surfaces of the second, third and

from the osteotomy site and the adjacentbone were divided into three equal parts.The upper part that included the crest of thealveolar process was labelled the tensionthird (TT), the lower part that included themandibular base was labelled the compres-sion third (CT), and the part between the TTand CT was labelled the intermediary third(IT). For each specimen, the upper site ofeach part was oriented horizontally andembedded in paraffin. The 5.0 mm thicksections were obtained and stained withhaematoxylin and eosin (HE) and Mal-lory’s trichrome (TM) for light microscopicanalysis.

Histological analysis

Histological analysis of the entire osteot-omy area was performed using conven-tional and polarized light microscopy(DMLB2, Leica, Wetzlar, Germany).

Histometric analysis

Three randomlyselectedsections fromeachpart were used. The images were acquiredwith a digital camera (DFC300FX, Leica,Wetzlar, Germany) coupled to an opticalmicroscope (DMLB2, Leica, Wetzlar, Ger-many) and the IM50 Image Manager soft-ware (Leica, Wetzlar, Germany). Theregion of interest was restricted to theosteotomy site at the cortical level.

At 2 weeks post-treatment, the osteot-omy osseous margins were identified basedon the typical interface between the parentlamellar bone and the non-mineralized con-nective tissue. The distance between thebone fragments was measured linearly atthe most external surface of the lingual andbuccal cortical bone.

At 18 weeks, after clear identification ofthe interface between old and newly formedbone under polarized light at the osteotomysite, the total area delimited by the old bonesurface in each cortical bone was deter-mined using QWin Plus software (Leica,Wetzlar, Germany). The area of newlyformed bone, including woven bone, par-allel-fibred bone, and lamellar bone, wasdetermined using the same software. Botharea values (total and newly formed bone)were measured and the percentage of newlyformed bone was determined. The endo-steal and periosteal reactions were notincluded in this measurement.

To compare the percentages of newlyformed bone at the buccal and lingualcorticals, the references used to identifythe cortical for the TT and IT, whetherbuccal or lingual, were the proximity ofthe tooth roots, which are closer to thebuccal cortical than the lingual cortical. In

1364 Sverzut et al.

contrast, for the CT, the proximity of themandibular canal was used, which is clo-ser to the buccal cortical. After testing thenormality of the data distribution with theShapiro–Wilk test, statistical analysis wasperformed using Student’s t-test and two-way analysis of variance (ANOVA) with asignificance level of 0.05.

Results

Histological analysis

2-Week period

Regardless of the experimental group, theamount of newly formed immature bone at

Fig. 4. Conventional (A–E and G–I) and polarizeand absorbable TT(C) and CT (D). 18 week resultcortical lamellar bone adjacent to the osteotomy sresult of the periosteal and endosteal reactions. (Eweeks under conventional (E) and polarized (F)

weeks (G). The absorbable group exhibited newlObjectives: 2.5� for A–D, G–I; 10� for E and

2 weeks was not sufficient to promoteunion of the bone edges created by theosteotomy (Fig. 4A–D).

The histological sections for metallicTT showed well-defined buccal and lin-gual cortical lamellar bone, with themedullary bone area predominantly char-acterized by loose connective tissue, dif-ferent densities of collagen fibres, focalareas of inflammatory infiltrate and hae-morrhage. The osteotomy site was par-tially filled with non-mineralizedconnective tissue. Bone remodelling wasobserved for the cortical bone (both buccaland lingual) as revealed by the presence ofHowship’s lacunae containing osteoclasts

d (F) light microscopy of histological sections ofs for metallic TT (E and F) and CT (G), and absorbite showed areas of bone resorption (arrowheads)

and F) The interface between parent lamellar bolight. (G–I) The metallic group showed more comy formed bone with typical features of spongy bon

F.

at the periosteal and endosteal surfaces,which was associated with a discrete endo-steal reaction characterized by the pre-sence of interconnected trabeculae ofimmature, woven bone (Fig. 4A).

For metallic CT, a remarkable endostealreaction was observed adjacent to theosteotomy site, consisting of intercon-nected trabeculae of immature bone in acontinuum with the internal cortical sur-face that was previously resorbed byosteoclasts. The osteotomy site and themedullary bone area showed dense con-nective tissue with focal areas of inflam-matory infiltrate and haemorrhage(Fig. 4B).

2 week results for metallic TT (A) and CT(B),able TT (H) and CT (I). (A–D) At 2 weeks, theand apposition of immature bone (arrows) as ane and parallel-fibred bone (arrowheads) at 18pact cortical bone at the osteotomy site at 18e (H and I) at 18 weeks. Mallory’s trichrome.

Bone repair in mandibular body osteotomy 1365

Metallic IT shared histological featureswith metallic TT and CT, as describedabove.

For absorbable TT, the osteotomy siteand the medullary bone area were primar-ily characterized by loose connective tis-sue with moderate to intense inflammatoryinfiltrate and small haemorrhagic foci. Theendosteal and periosteal reaction exhibitedareas of bone resorption and newly formedimmature bone with interconnected trabe-culae in a continuum with the parentlamellar bone (Fig. 4C).

Adjacent to the osteotomy site forabsorbable CT, the inner surface of buccaland lingual cortical bone exhibited early,immature bone formation and resorptionlacunae. The osteotomy site and themedullary bone area showed dense con-nective tissue with focal areas of inflam-matory infiltrate and haemorrhage. Somesections also revealed a periosteal reactioncharacterized by newly formed immaturebone (Fig. 4D).

Absorbable IT shared histological fea-tures with absorbable TT and CT, asdescribed above.

18-Week period

Regardless of the IF system applied, thegap created by osteotomy was completelyfilled with newly formed bone (Fig. 4E–I).For both the metallic and absorbablegroups, the TT, IT and CT shared histo-logical features, with the exception of thepresence of the mandibular canal in theCT.

For the metallic TT, IT and CT, thebone edges created by the osteotomy wereconnected by compact parallel-fibred boneexhibiting areas of both immature andlamellar bone and Haversian canals of

Fig. 5. The mean � SD of the distance (mm) betwthirds (TT, IT and CT). No statistically signific

varying diameter (Fig. 4E–G). The inter-face between the parent lamellar bone andthe newly formed bone was clearly iden-tified under polarized light (Fig. 4E and F).

For the absorbable TT, IT and CT, theosteotomy site was completely filled withbone tissue exhibiting typical features ofspongy bone. Interconnected parallel-fibred bone trabeculae defined bone mar-row areas filled with highly vascularizedconnective tissue (Fig. 4H and I). Someearly Haversian systems could also benoticed. The interface between the parentlamellar bone and the newly formed bonewas visualized under polarized light.

Histometric analysis

2-Week period

The distances between the bone edgescreated by the osteotomy for the metallicand absorbable groups and the thirds (TT,IT and CT) are presented in Fig. 5. Stu-dent’s t-test did not reveal any significantdifferences (p = 0.67) among the cortices(buccal and lingual) or thirds (TT, IT andCT). Two-way ANOVA did not reveal anysignificant differences among the vari-ables analysed, which were the IF systemapplied (p = 0.99), the thirds (p = 0.07)and the interaction of the IF system andthirds (p = 0.66).

18-Week period

The percentages of newly formed bone inthe metallic and absorbable groups andthirds (TT, IT and CT) are presented inFig. 6. Two-way ANOVA did not demon-strate a statistically significant differenceamong the thirds (p = 0.14); but there werestatistically significant differencesbetween the IF groups (p = 0.01) and

een the bone fragments at 2 weeks post-treatmenant differences were detected.

when the data from thirds and two groupswere crosschecked (p = 0.03).

Discussion

Studies of the absorbable IF system haverevealed a series of advantages overmetallic systems, including noncorrosivematerial, easier plate bending, eliminationof an additional surgery to remove thesystem, low incidence of infection, lackof interference with imaging examinationsand radiotherapy, gradual transference ofthe functional stress to the bone (prevent-ing the stress shielding phenomenon) andthe absence of cold sensitivity.1–3,6,19–

21,25,27–30 Some disadvantages have alsobeen associated with these systems, suchas the high cost, the need for a warmingbath for plate moulding and the use of athread formation device prior to screwinsertion, which increases the intraopera-tive surgical time.6,20,27–29,31 Some of theproperties of the absorbable IF systemshave also been discussed, such as resis-tance, rigidity, stability, quality of bonerepair, absorption time, foreign body reac-tion and treatment predictabil-ity.6,7,19,20,27,28,30 Currently, theabsorbable IF system should not be con-sidered a substitute for the metallic sys-tem, but an additional alternative in selectcases. Some contraindications for the useof absorbable IF systems are cited in theliterature, such as alcoholism, smoking,illicit drug use and high risk for failureto follow postoperative recommenda-tions.6,19,20,27,32

Although clinical and experimental invivo evaluation has demonstrated goodresults with absorbable IF systems,1,2,22–

24,27,32 some authors warn that the stabilitybetween fragments is still lower than that

t for the metallic and absorbable groups and the

1366 Sverzut et al.

Fig. 6. The mean � SD of the percentage of newly formed bone at 18 weeks post-treatment for the metallic and absorbable groups and the thirds(TT, IT and CT). Asterisks (*) indicate statistically significant differences at p = 0.01.

offered by the metallic systems.6,7,19–

22,24,32,33 In the present study, at the 2-week period, the distances between bonefragments did not differ based on the IFsystem applied. Nonetheless, moderatemobility between the bone fragmentswas clinically observed for the absorbablegroup only. During the weekly assess-ment, this mobility gradually decreaseduntil it was absent at 4 weeks post-treat-ment. Therefore, some type of additionalstabilization, such as a dental splint,should be recommended with the absorb-able IF system during the first weeks toprevent malocclusion and interfragmen-tary instability.6,19,20,27,32

At 2 weeks, qualitative differencesbetween the IF systems were detected interms of histological features. The find-ings of increased areas of previous osteo-clastic activity and more abundantinflammatory infiltrate in the medullaryregion, with a more discrete endostealreaction associated with the corticalbones, for the absorbable group are con-sistent with those described in both earlystudies9–11 and more recent reports.1,23,25

Despite these differences, the amount ofnewly formed bone in both groups wasinsufficient to connect the bone edgescreated by the osteotomy; thus, the bonerepair process was not completed at thispoint.

At 18 weeks, the newly formed bonefilled the osteotomy site, connecting thebone edges created by the osteotomy forboth groups. Differences were observed interms of bone quality and quantity.The metallic group exhibited compactbone and the absorbable group showedspongy bone (23.3% and 36.3% of

non-mineralized spaces, respectively),with marrow spaces filled with a highlyvascularized connective tissue, which isconsistent with the definition of spongybone.34–36 Although the presence ofwoven bone or lamellar bone has beenshown in the bone repair area at 18 and24 weeks in different experimental mod-els,1,23,24 no description of the quality ofthe newly formed bone along the osteot-omy site has been provided to date.

Previous studies evaluating the patternof bone repair process after using a metal-lic IF system showed the occurrence of thegap healing type and, less frequently, thepresence of bone callus.18,37 Bone callusformation has also been reported after theuse of absorbable IF systems, and in somecases, the bone callus volume increased oncompletion of the repair process.21–23 Dif-ferences between buccal and lingual peri-osteal reactions are expected. Accordingto Reitzik and Schoorl38 and Rasubalaet al.,39 the enhanced periosteal reactionat the lingual cortical could be due tointerfragmentary movements, which weregreatest at the lingual site. Conversely, atthe buccal site, movements would berestricted by the fixation device to rotationabout a transverse axis, resulting in a lessperiosteal callus formation. Freitag andLandau40 stated that the amount and dis-tribution of callus varied according to theknown mechanical effects of plates andscrews on the bone fragments, while Rasu-bala et al.39 also considered surgicaltrauma (i.e. stripping of the periosteumand the surrounding soft tissue) to be animportant factor in this phenomenon. Inthe present study, bone callus formationwas observed in both groups, but the

periosteal reaction was more marked inthe absorbable group.

Although early studies evaluating theuse of an absorbable system in the treat-ment of facial fractures reported completebone repair process with mature, lamellarbone formation, the presence of multinu-cleated giant cells has also beendescribed.9–11 More recent studies of IFsystems manufactured with LPLA orDLPLA combined with PGA haverevealed a minimal inflammatory processand no foreign body reaction during thefirst 24 postoperative months. This findingis probably a result of the absorbablepolymer applied, which is degraded moreslowly than the systems manufacturedwith only PGA or PLA.7,14,28,29,32,36,41

The abundance of giant cells and macro-phages may represent reduced biocompat-ibility of the absorbable polymer, and anacute inflammatory process and foreignbody reaction may occur.6,9–11,28 In thepresent study, no multinucleated giantcells were observed at either 2 or 18weeks, a finding that could be explainedby three hypotheses: the absorbable poly-mer applied undergoes slow degradation,which prevents the excessive accumula-tion of residue; the time points evaluateddid not span the degradation phase thatinvolves the presence of these cells; and/orthe analysed areas were relatively distantfrom the polymer surface where these cellswere present. Some authors have sug-gested that when LPLA is combined withDLPLA, the giant cells and macrophagesappear only 1 year after the surgical pro-cedure.2,6,21,22

In conclusion, although the IF systemsused here were able to facilitate the bone

Bone repair in mandibular body osteotomy 1367

repair process at the osteotomy site after18 weeks post-treatment, differences inbone quality and quantity were detected.More compact bone was observed in themetallic group, and spongy bone wasobserved at the osteotomy site in theabsorbable group. Although the low num-ber of animals indicates the need to inter-pret the statistical analysis with caution,these findings led the authors to questionwhether they represent different pro-cesses, or two phases of the same phenom-enon. The authors propose that twodifferent patterns of bone repair occurred,depending on the distinct biomechanicalenvironments provided by the two IF sys-tems. Further studies are needed to eval-uate whether the present results can bereproduced consistently and are clinicallyrelevant.

Funding

This research study was supported by TheState of Sao Paulo Research Foundation(FAPESP Grant No. 07/00892-8).

Competing interests

None declared.

Ethical approval

Institutional Animal Care and Use Com-mittee of the University of Sao Paulo(Process 07.1.188.53.9).

Acknowledgements. The authors wouldlike to thank Mr. Sebastiao Carlos Biancoand Ms. Adriana Luisa GoncalvesAlmeida for technical assistance.

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Address:Cassio Edvard SverzutDepartment of Oral and Maxillofacial Sur-gery and PeriodontologySchool of Dentistry of Ribeirao PretoUniversity of Sao PauloAvenida do Cafesem numero – Bairro Monte Alegre – CampusUniversitario da USP – 14040.904 RibeiraoPretoSao PauloBrazilTel: +55 16 3602 3980;Fax: +55 16 3602 4781E-mail: cesve@forp.usp.br