Measurement of Torque During MandibularDistraction

Ailanta, Georgia, USA

Fernando D. Burstein, MD, FAAP, FACS,* Saylan Lukas, MS,t Dina Forsthoffer, PA*

In a prospective study, 26 patients aged 9 days to 12years old underwent mandibular distraction. Therewere 18 bilateral and 8 unilateral distractionsperformed. Five patients had previous distraction.Torque measurements were performed during thedistraction process. A modest linear increase intorque was noted during the distraction process.Older patients required more torque to achieve thesame distraction length as younger patients. Theresults of this study suggest that distraction forcesare relatively modest, which may allow for greaterfreedom of distractor design.

Key Words: Mandibular clistraction, torque, force

Resorbable one-stage distractors are findingincreasin~ utilization among craniofacialsurgeons. -4 Manclibular distractors in par-ticular are being used to lengthen the

mandible in a variety of conditions. These includePierre Robin sequence, Treacher Collins syndrome,Nagers syndrome, and craniofacial microsomia. Thecurrent generations of distractors were designedusing strength parameters derived from metallicdistractors. Currently; the Food and Drug Adminis-tration has approved resorbable mandibular distrac-tors for use in children younger than 2 years. Moreinformation is necessary to help guide the design andmanufacture of these devices. Very little is knownregarding the forces that are applied to the distractorsduring clinical use. This study was undertaken toclarify 1 parameter of the force vectors that are activeduring mandibular distraction. Torque, as an indirect

From the "Center for Craniofacial Disorders, Children's Health-care of Atlanta, Emory University, Atlanta, Georgia; and tBiometMicrofixation, Jacksonville, Florida.

Address correspondence and reprint requests to FernandoBurstein, MD, FAAP, FACS, Suite 500,975 Johnson Ferry Rd,Atlanta, GA 30342; E-mail: FBurstein@aol.com

644

measure of force, can be easily measured during theprocess of distraction using an electronic torquegauge (Imada, Northbrook, IL). We realized that wedid not know if the devices were subjected to moreforce in early, mid, or late distraction phases. Thenature of the force necessary during single vectordistraction over time has not been well described inmanclibular distraction. In addition, we felt thatmonitoring the amount of force during distractioncould yield potentially valuable clinical information.Knowing if the torque is rising rapidly on one side ofthe mandible but not the other might imply that one

. side had reached full clistraction before the other.Alternatively, a very low unilateral reading couldmean that the device has become detached.

MATERIALS AND METHODS

Twenty-six patients undergoing manclibular dis-traction, using resorbable distraction devices,had

torque measurements taken during clistraction. AnImada DSD-4digital torque wrench (Imada)was usedfor all measurements. There were 14 males and 12females, ages 9 days to 12 years, with a mean of 2.6years. All underwent mandibular clistractionfrom 20to 30 rom (mean, 25 rom). Three models of resorbableclistractorswere used, mandibular infant, manclibularpecliatric, and L shape (see proceeding article in thisissue). The choice of which device to use was basedlargely on the shape and size of the mandible. Theapproximate linear force required to distract themancliblecan be calculated by correlating the materialand mechanical characteristics of the clistractiondevice and the recorded clinical torsion data. Labora-tory testing was conducted to establish the correlationbetween torque applied to a clistraction drive screwand the corresponding linear force of the platestransmitted to the proximal and distal bone segments(Fig 1). This test established that 1 in-oz of torquecorrelates to 1 Ib of linear force. Measurements weretaken at the time of device insertion, approximatelyhalf way during the distraction and when distraction

Ii

IIp!~

If

Boxplot of Distraction Force [Ibf] vs Distraction LengthI144 *

TORQUE DURING MANDIBULAR DISTRACTION I Burstein et al

Fig 1 Drawing of laboratory setup used to correlate torqueapplied to resorbable clistractordrive screw and linear forcethat the plates exert on the bone segments. The torquewrench records torsional data at the same time that load cellrecords linear force.

was nearly complete. For purposes of tabulation, wedivide these periods into initial one third, middle, andlast third of distraction. Distraction procedures wereperformed as previously described (see precedingarticle). A 48-hour latency period was followed by2-mm/ d distraction until full length had beenachieved. Five weeks was allowed for bone consolida-tion before removing the distraction drive screw.

RESULTS

All patients successfully completed their course ofdistraction. There were no complications in this

group. Figures 2 and 3 give a graphic representation ofour findings. There were modest increases in torquemeasurements with increasing mandibular length and

12* *!1O

~Ii! 8c.111:1j;a 4

**

$....,.,....... :...:,::

'" ",' :- ~'.,

~

.:,;-. : -.'~' .....

. ',,", .

1 2 3Data pt (1 = intra op, 2 = half way through distraction, 3 = near completetlon)

Fig 2 Graphic representation of data. Distraction forceversus distraction length. Note the consistentforcethrough-out active distraction process. (*); outliers in data.

patient age. Torque measurements ranged from 1.0 to13.6 in-oz (mean, 5.0 in-oz). There was no spike intorque as distraction neared completion. In the bilateraldistraction cases, torque measurements increasedsymmetrically. All torque measurements were con-verted to pounds of linear force (lbf) using the dataderived from our laboratory testing (Fig 1). These testsdemonstrated that every in-oz of force torque appliedto the distraction drive screw translated into 1 lb oflinear force transmitted through the plates attachedto the surrounding bone. This made the data morerelevant to common clinical units of measure. Theresults in Figures 2 and 3 were statistically significant(P < 0.005). Although redistraction cases had slightlyhigher torque measurements when compared withprimary distraction cases, the difference did not reachstatistical Significance.

Boxplot of Torque Value (in-oz) vs Age (d)w~ I

16

m ~

* h14

N

.~ 12

g 10

~~

~ 6J ~'. B4: n:~~ U ~

' ~ * ~.' :: 8H ' ~, ·10 12 15 30 60 150 210 510 1095 1825 2555 3285 3650 4015 ~380

Age,d

Fig 3 Graphic representation of force versus patient age.Note the increased force necessary for distraction withincreasing age of patient. (*); outliers in data.

645

~

THE JOURNAL OF CRANIOFACIAL SURGERY / VOLUME 19, NUMBER 3 May 2008

DISCUSSION

The principles of distraction elucidated by Ilizarof"predict that through gradual distraction, soft

tissues including skin, muscle, and nerves will bestretched at a rate that does not exceed their elasticitymodulus. The main opposing force to the distractionvector is the resistance of the soft tissues to elongation.It follows that being able to quantitate this opposingforce by measuring torque during distraction wouldbe important in designing distraction devices. Whentesting the mechanical attributes of a new distractordesign, the construct's ability to withstand the linearforce needed to lengthen bony segments and canti-lever bending forces (needed to withstand masticatoryforces) are tested in the laboratory.Y In the presentstudy, we studied the more clinically relevant mea-sure, the torque necessary to turn the distractor drivescrew in resorbable devices in patients undergoingmandibular distraction. In contrast to the mechanicaltesting measured in the laboratory, our in vivomeasurements include the forces applied from thesurrounding soft tissues. This type of in vivo clinicalstudy is quite rare. Robinson et al reported on a limitedstudy of 8patients undergoing mandibular distractionusing titanium devices." Interestingly, his torquemeasurements were consistent with our findings.Ours is the first study to measure torque duringmandibular distraction using resorbable devices.Torque measurements can be converted to pounds offorce using laboratory-derived measurements (Fig 1).Our measurements show that the force on thedistractors is relatively constant throughout thedistraction process (Figs 2 and 3). There was nospike in the amount of force required to turn thedistractors as the maximal length was obtained. Thesefindings imply that the elasticity coefficient of theregenerate, nerves, muscles, and skin is not exceededduring 2-mml d distraction. As such, the total forcethat these devices must withstand is relatively con-stant and does not increase significantly with increas-ing distraction length. This observation held true evenin reoperative caseswhere soft tissue resistance wouldbe expected to be higher with much less elasticity.Themaximal force we measured, 13.6 lbf was in a patientundergoing his third course of distraction in 6 years.Even in this extreme case, the forces applied to thedistractor were far below the design parameters.Older patients required slightly more force thanyounger patients for an equivalent amount of man-dibular lengthening. This can be explained by thegreater weight of the bone and soft tissues that arebeing lengthened (Fig 2). It would seem that makingthe distraction devices significantly smaller and

646

thinner would be possible with little chance of devicefailure. If this were possible, the devices could be moreeasily applied with greater freedom of design. Currentdesign parameters have been derived from laboratorybench testing. Previous benchmark laboratory testingperformed on resorbable mandibular distractorsresulted in setting a design acceptance criterion ofwithstanding 30 lbf Current resorbable devices are alldesigned to withstand 60 lbf of force, which gives thedesign a safety factor of 2 at the time of implantation(due to the resorption process, the safety factor dropsto 1.4 at 8 weeks). In contrast, our study, which had awide range of clinical conditions and age ranges,demonstrated that none of the distractors weresubjected to greater than 13.6 lbf of force in mandib-ular distraction ofup to 30 mm. Our data indicate thatcurrent design guidelines provide a safety factor of4.4times the force exerted during initial active phase ofmandibular distraction (factoryof safety drops to 3 at 8weeks). Use of these devices in patients older than 2years is considered "off label." This is due to concernsregarding masticatory forces during the resorptionperiod of the resorbable distractors. In the authors'experience with resorbable mandibular distractionduring the last five years, in patients ranging in agefrom 5 days to 14years, structural devicefailurehas notbeen observed when the devices were properlyapplied.l-3 We speculate that studies such as ours willshow that in vivo the forcesexerted on these devicesarerelatively modest and that current strength guidelinesmay be excessive. Smaller thinner devices wouldfacilitateapplication and increase the rate of resorption,since less material needs to be broken down. Wefoundthat the torque measurements were clinicallyuseful inmonitoring the course of distraction, sincewe were ableto measure initial torque readings at the time ofplacement. A dramatic increase or decrease in forcemeasurements would indicate device failure or dis-lodgement from the underlying bone. This study waslimited in scope,measuring only the forceapplied in thedirection of the distraction vector. We did not measurethe forces generated during mastication, vocalization,or other activities. Despite these limitations, we wereable to start to understand the true magnitude of theforceexcretedon these devicesin the clinicalsetting.Wehope that these data will help to advance the design ofthe next generation of resorbable distractors.

CONCLUSIONS

Measurements of torque during mandibulardistraction can be converted to pounds of

linear force. The force exerted during distraction of2 mml d remains relatively constant throughout

3. Burstein FD, Williams JK.Mandibular distraction osteogenesis inPierre Robin sequence: application of a new single-stageresorbable device. Plast Reconstr Surg 2005;115:61-67

4. llizarof GA. A new principle of osteosynthesis with the use ofcrossing pins and rings. In: Collection of Scientific Works of theKurgan Regional Scientific Medical Society. USSR: Kurgan,1954:145-160

5. Haug RH, Nuveen EJ, Barber IE, et al. An in vitro evaluation ofdistractors used for osteogenesis. Oral Surg Oral Med OralPathol Oral Radiol Endodon 1998;86:648--659

6. Robinson RC, Oneal PI, Robinson GH. Mandibular distractionforce: laboratory data and clinical correlation. J Oral MaxillofacSurg 2001;59:539-544

TORQUE DURING MANDIBULAR DISTRACTION I Burstein et al

r-

distraction. The linear force of distraction is far belowthe current design guidelines. Measurement of thetorque during distraction can provide valuable clin-ical information regarding the state of the devices.

REFERENCES

1. Burstein FD, Wiliams JK, Hudgins R. Single stage distractionusing resorbable devices. Craniofac Surg 2002;13:776-782

2. Burstein FD, Wiliams JK. Resorbable bone distraction: currentstatus and future directions. Clin Plast Surg 2004;31:407--414

647