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J Oral Maxillofac Surg66:1625-1629, 2008

Predictability of Prebent AdvancementPlates for Use in Maxillomandibular

Advancement SurgeryKok Weng Lye, BDS, MDS,* Peter D. Waite, MPH, DDS, MD,†

Deli Wang, PhD,‡ and Somsak Sittitavornwong, DDS, MS§

Purpose: The purpose of this study was to determine the predictability when using prebent advance-ment (PBA) plates in the maxillary osteotomies of maxillomandibular advancement (MMA) surgery.

Materials and Methods: A retrospective review of the MMA surgeries at University of Alabama atBirmingham was carried out. Thirty-six obstructive sleep apnea patients who underwent MMA with PBAwere included in this study. All cases had fixation of the maxillary osteotomies with KLS Martin PBAtitanium plates and screws (KLS Martin, Jacksonville, FL). Lateral cephalometric radiographs were takenwithin 1 month before surgery (T1), and between 2 to 4 weeks postsurgically (T2). All cephalometricradiographs were analyzed using Planmeca Dimaxis Imaging Software (Helsinki, Finland). Vertical andhorizontal measurements were made to several skeletal landmarks including Sella (S), A point (APT), andupper incisor (UPI). The amount of advancement was measured through pre- and postoperative lateralcephalographs and compared with the magnitude of the advancement plates used.

Results: All data of 36 patients were included and used for statistics. The data showed that the meandifference was 0.74 mm (advancement is more than the size of the PBA plates) and the mean percentagedifference was 11.35%. Fifty percent of the cases had the magnitude of advancement within 1 mm of thePBA size. The predictive analysis found that the PBA size was a significant predictor for the advancementachieved (P � .0001) but not the absolute difference between the advancement achieved and the platesize (P � .0726) and the relative advancement difference (P � .0612). The absolute difference and therelative difference take into account the size of the PBA and are better parameters to judge the predictabilityof the advancement achieved. The rationale for this unpredictability is largely due to the anatomy of themaxilla and pyriform rims and the adaptation of the PBA to the maxilla.

Conclusion: There are distinct advantages to the use of PBA plates. The PBA plates have been provento significantly affect the advancement. However, when very precise movements are required, the sizeof the PBA used does not predictably correlate to and frequently underestimates the amount ofadvancement achieved. Therefore accurate model surgeries are still required for most cases.© 2008 American Association of Oral and Maxillofacial Surgeons

J Oral Maxillofac Surg 66:1625-1629, 2008

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ne of the recent modifications in the field of cranio-acial plating is the use of prebent advancement (PBA)lates. These plates are manufactured with 2 oppos-

ng right angles to produce a step in the middlenterval portion of the plates, as seen in Figure 1.here are a range of different designs to suit differentegions. They are also produced with a differentength between the right angles to provide a range of

eceived from the University of Alabama at Birmingham, Birmingham, AL.

*Fellow, Department of Oral and Maxillofacial Surgery.

†Professor and Chairman, Department of Oral and Maxillofacial

urgery.

‡Assistant Professor, Medical Statistics Section, Division of He-

atology/Oncology, Department of Medicine.

§Assistant Professor, Department of Oral and Maxillofacial Surgery. d

1625

dvancement, as seen in Figure 2. They are mainlyanufactured for advancement of bony segments dur-

ng osteotomies. The PBA titanium plates are placedt the pyriform rim and anterior wall on each side ofhe maxilla. The concept behind their creation is toeduce if not eliminate the need for bending andontouring of the titanium plates during orthognathicurgery. A study showed that bending of reconstruction

Address correspondence and reprint requests to Dr Lye: Depart-

ent of Oral and Maxillofacial Surgery, National Dental Centre, 5

econd Hospital Avenue, Singapore 168938; e-mail: kokwenglye@

ahoo.com

2008 American Association of Oral and Maxillofacial Surgeons

278-2391/08/6608-0011$34.00/0

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1626 PBA IN MMA SURGERY

lates creates stresses and microcracks in the materialhat weakens the plates.1 Another study showed that theBA plates provide the best resistance to displacement,ermanent deformation, and load for breaking whenompared with other systems.2 This may not be crucialn routine orthognathic surgery but could be a potentialssue in a large and unstable advancement. The use ofhese plates has several advantages and potential disad-antages.The advantages include: 1) reduction in bending

ecreases the compromise to the integral strength ofhe plates; 2) reduction in operating time with lessontouring; 3) ability to estimate the amount of move-ent gained; 4) the prebent 90° angles and thicker

esign enables them to resist straightening out; and 5)he larger and more rigid plates are more robust. Theisadvantage is larger and more rigid plates may beore palpable.In maxillomandibular advancement (MMA) surgery,

he main purpose is to have a sizable and stabledvancement of both the maxilla and the mandible viasteotomies. The movement is usually a pure ad-ancement with minimal vertical and rotational move-ents. Comparatively, traditional orthognathic sur-

ery for dentofacial deformities usually consists ofome vertical movement, change in occlusal angle,otation to correct midline discrepancies, and evenultiple pieces osteotomies. These require very exact

D contouring of the plates that cannot be prefabricatedithout complex planning. The large advancement inMA also equate to lesser stability due to decreasedony contact and soft tissue stretching. This means thathe plates need to be stronger in order to resist the largertresses. Therefore the added strength of the prebentlates from both the increased bulk and the lack ofeakening are greatly appreciated.It is important to know if the magnitude of advance-

FIGURE 1. PBA 2.0 miniplate.

ye et al. PBA in MMA Surgery. J Oral Maxillofac Surg 2008.

ent created by use of the PBA plates is predictable. L

ven though we have not converted to using PBAlates solely to judge the movement, we are explor-

ng the feasibility of this alternative. Good predictabil-ty of the advancement would mean a lesser need foretailed model surgeries to position the maxilla dur-

ng MMA surgery. Discrepancies in other dimensions—ertical, cant, yaw, and midline—can be correctedased on clinical visualization of the bone cuts, man-ibular plane of occlusion, external pins, and the foxlane. If predictable, PBA could be used in orthog-athic cases for advancement of the maxilla without

ntermediate splints. However, there have not beenny studies in the literature regarding the use of PBAlates in the maxillofacial region.

aterials and Methods

A retrospective review of the MMA surgeries per-ormed by the Department of Oral and Maxillofacialurgery at the University of Alabama, Birminghamrom 2003 to 2006 was carried out. Thirty-six obstruc-ive sleep apnea patients who underwent MMA withBA plates were involved in an Institutional Reviewoard–approved study investigating the use of KLSartin (Jacksonville, FL) PBA titanium plates and

crews. The inclusion criteria were patients with) MMA for obstructive sleep apnea; 2) minimal ver-ical changes during the maxillary osteotomy; espe-ially down grafts; and 3) fixation with PBA titaniumlates and screws. Lateral cephalometric radiographsere taken within 1 month before surgery (T1), andto 4 weeks postoperatively (T2). All cephalometric

adiographs were analyzed using the Dimaxis Imagingoftware (Dimaxis Pro 4.1X, Planmeca, Helsinki, Fin-and) in the digital format. Vertical and horizontal

easurements were made to several skeletal land-arks including Sella (S), A point (APT), and upper

FIGURE 2. Different designs and magnitudes of PBA plates.

ye et al. PBA in MMA Surgery. J Oral Maxillofac Surg 2008.

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ncisor (UPI). The horizontal reference line throughella and rotated 6° down from the Sella-Nasion (SN)lane was referred to as SN � 6.3,4 The vertical ref-rence was established perpendicular to SN � 6, withts origin at Sella.4,5 The maxillary landmarks (APT andPI) were identified and measured from the vertical

eference. Each point (APT and UPI) was measuredwice and assigned the magnitude of advancement. Ifhere was a discrepancy, the average of the 2 pointsas used.

SURGERY

Prior to the operation, model surgery was con-ucted on dental casts and the planned movement ofhe maxilla was between 4 and 10 mm. During theMA surgery, the mandible was advanced according

o the intermediate prefabricated custom splint andxated with interpositional screws. The maxilla wasositioned to the mandible by occlusion with maxil-

omandibular fixation wires or a final splint, if neces-ary. The appropriate PBA was used for fixation of theaxilla.

STATISTICAL METHOD

Descriptive statistics were used to describe pa-ients’ demographic characteristics, including age,thnic origin, and gender, and advancement afterMA. One sample t test was used to test the absolute

nd relative changes of advancement after MMA withBA plates. General linear regression models weresed to detect correlations between dependent vari-bles, such as the absolute difference of advancementfter MMA with PBA, the relative changes of advance-ent after MMA with PBA plates, with the PBA plate

ize as the predictor after adjusting for the demo-raphic variables. We used .05 as the cut-off criteria toetermine the significance of tests.

esults

A total of 36 patients had the complete data re-uired for this study. Table 1 shows the patientsharacteristics, sizes of the PBA plates used, and thedvancement gained.

The magnitude of advancement was comparedith the size of the PBA plates used. The absolute

ifference between the advancement and the size ofhe PBA plate (advancement calculated by theephalographs�the size of the PBA plate) and theelative difference (the absolute difference/size ofhe PBA plate) also were calculated and analyzedccordingly.

The data showed that the mean absolute differenceas 0.74 mm and the mean relative difference was

1.35%. In 50% of the cases, the magnitude of ad-

ancement was within 1 mm of the PBA plate size. c

he case with the biggest discrepancy involved PBAlates of 6 mm but resulted in an advancement of 11.1m; an absolute difference of 5.1 mm and a relative

ifference of 85%.The univariate analysis showed that the advance-ent (P � .0001), the absolute difference (P �

0001), and the relative difference (P � .007) werell statistically significant.

The predictive analysis found that the PBA plateize is a significant predictor for the advancementchieved (P � .0001) but not the absolute differenceP � .0726) and the relative difference (P � .0612).he absolute difference and relative difference take

nto account the size of the PBA plates and are betterarameters to judge the predictability of the advance-ent achieved.Therefore, the PBA plates have been proven to

ignificantly affect the advancement. However, whenrecise movements are required, the size of the PBAlate used does not predictably correlate to and fre-uently underestimates the amount of advancementchieved.

iscussion

The main reason for the unpredictability the ad-ancement achieved might be the variability of the 3D

Table 1. CHARACTERISTICS OF MMA CASES USINGDIFFERENT SIZES OF PBA PLATES

n (%) Mean

ge (yr) 47.3 (range, 21-67)enderMale 27 (75.0)Female 9 (25.0)

thnicityCaucasian 34 (94.4)African American 2 (5.6)

ize of PBA plate(mm)

4 7 (19.4)6 9 (25.0)8 9 (25.0)10 11 (30.6)Overall (mm) 7.39agnitude of

advancement(mm)

4 4.136 7.788 8.8510 10.41Overall (mm) 8.14 (range: 2.8-13.65)

bbreviations: MMA, maxillomandibular advancement;BA, prebent advancement.

ye et al. PBA in MMA Surgery. J Oral Maxillofac Surg 2008.

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1628 PBA IN MMA SURGERY

he PBA plates. The following is the rationalization ofow the contours of the maxilla can be significantactors for the advancement.

1. Axially, the angle of the anterior maxillary wallat the pyriform rim to the mid sagittal plane isquite variable from patient to patient; therefore

IGURE 3. The pyriform region (blue line) is 45° to 55° to the midagittal plane (red line) and the PBA plates placed perpendicular tohe pyriform region.

ye et al. PBA in MMA Surgery. J Oral Maxillofac Surg 2008.

IGURE 4. Figures are predictive tracings based on actual anat-my based on patients’ CT scans. Top, Flat pyriform region. Bot-

om, Steep pyriform angles resulting in different advancement.

ye et al. PBA in MMA Surgery. J Oral Maxillofac Surg 2008. L

the true anterior movement of the maxilla var-ies. This is illustrated in Figures 3 and 4. As asurgical comment, we noticed the decreasedbone contact at the maxillary walls of the sinuswith larger advancements, which behooves thesurgeon to consider more stable fixation andpossibly bone grafting as well.

2. In the vertical axis, there is also variation amongstpatients. This could result in an oblique vector ofprojection for the PBA plates and thereforechange the advancement. This is illustrated inFigure 5.

3. The region of the maxilla where the plating isperformed is often a concave surface. Therefore,if the PBA plates are not well contoured to fitpassively to the surface of the maxilla, the ad-

IGURE 5. Figures are predictive tracings based on actual anat-my based on patients’ CT scans. Left, Flat anterior maxillaryurface. Right, Concave anterior maxillary surface resulting in dif-erent advancement.

ye et al. PBA in MMA Surgery. J Oral Maxillofac Surg 2008.

IGURE 6. Figures are predictive tracings based on actual anat-my based on patients’ CT scans. Left, Properly contoured plate.ight, Poorly contoured plate resulting in a different magnitude ofdvancement.

ye et al. PBA in MMA Surgery. J Oral Maxillofac Surg 2008.

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vancement can also be affected. This is illus-trated in Figure 6.

In view of this, we cannot solely use the size of theBA plates to predict the magnitude of advancement.owever, if we use additional reference points andids like external pin fixations and accurate calipers toeasure the movements and prevent large discrepan-

ies, the intermediate prefabricated custom splintsay be redundant for cases with minimal vertical,

cclusal cant, and midline issues. This would equateo the use of PBA plates to establish the maxillarydvancement intraoperatively followed by using thecclusion or final splint to guide the mandible.Therefore, if these PBA plates are to be used in

rthognathic surgery for very specific prescriptivedvancement of the maxilla, one cannot assume theurgical movement to be equal to the plate. However,he use of PBA plates for its inherent advantageshould be encouraged. Further studies should also belanned to compare the accuracy of the new tech-ique of using PBA plates without splints versus the

onventional technique and also to compare the sta-

ility and relapse rates of the surgery using the PBAlates versus the traditional craniofacial plates.

cknowledgment

We would like to thank the staff of the Department of Oral andaxillofacial Surgery at the Kirklin Clinic, UAB for their help with

he medical records.

eferences. Martola M, Lindqvist C, Hanninen H, et al: Fracture of titanium

plates used for mandibular reconstruction following ablativetumor surgery. J Biomed Mater Res B Appl Biomater 80:345,2007

. Araujo MM, Waite PD, Lemons JE: Strength analysis of Le Fort Iosteotomy fixation: Titanium versus resorbable plates. J OralMaxillofac Surg 59:1034, 2001

. Eppley BL, Sarver D, Pietrzak B: Biomechanical testing of resorb-able screws used for mandibular sagittal split osteotomies. J OralMaxillofac Surg 57:1431, 1999

. Sittitavornwong S, Waite PD, Dann JJ, et al: The stability ofmaxillary osteotomies fixated with biodegradable mesh in or-thognathic surgery. J Oral Maxillofac Surg 64:1631, 2006

. Bailey LJ, Phillips C, Proffit WR, et al: Stability following superior

repositioning of the maxilla by Le Fort I osteotomy: Five-yearfollow-up. Int J Adult Orthodont Orthognath Surg 9:163, 1994