Effects of latency and rate on bone formation in a porcine mandibular distraction model

J Oral

Max~llofoc

58 507-Z

Surg

13 2000

Effects of Latency and Rate on Bone Formation in a Porcine Mandibular Distraction Model Maria David

J. Troulis,

DDS, MSc, *Julie

H. Pen-Ott, DDS, MD,*

and Leonard

GIowacki, B. Kaban,

PhD, f DMD, MDJ

Purpose: Long treatment protocols currently limit the application of distraction osteogenesis (DO). The purpose of this study was to develop a porcine model for DO and to define the effects of latency and distraction rate on bone formation. Materials

and Methods: Distracters were placed through submandibular incisions. For analysis of latency, mandibular osteotomies were distracted after 0 (n = 3) or 4 (n = 2) days at a rate of 1 mm/d (7 days) with 14 days fixation. For analysis of rate, osteotomies were distracted at 1 mm (n = 4), 2 mm (n = 4), or 4 mm (n = 4) per day to produce a 12-mm gap with 24 days fixation. DO wounds were assessed in vivo with bimanual palpation to detect mobility across the gap. Harvested specimens were evaluated by gross appearance and bimanual palpation. Standardized radiographic techniques were used to estimate bone density.

During DO, animals showed normal activity. There were no infections, and no distracters were removed prematurely. Clinical and radiographic evaluation of the groups that underwent distraction after 0 or 4-day latency showed equivalent healing. In the comparison of distraction rates, stability was greatest in the group distracted at 1 mm/d.

Results:

A porcine model for mandibular DO has been developed in which mandibular lengthening was successfully performed without latency and at a rate of 1 mm/d. The relationships among latency, gap size, rate, and duration of fixation are poorly understood and can be defined with relevant animal models. 0 2000 American Association of Oral and Maxillofacial Surgeons

Conclusions:

Recriwd

from

the

and Skeletal

Biolom

tal, Harvard

School

‘AO/ASIF

Department Research of Drntal

Research

of Oral

and Maxillofacial

Center,

Massachusetts

Medicine.

Boston,

Fellow

in Pediatric

Since the original report of distraction osteogenesis (DO) by Codivilla* in 1905, there have been large numbers of published studies on skeletal expansion by this technique. These include the pioneering orthopedic work of Ilizarov2-’ in the late 1950s and the clinical and experimental investigation of long bone and craniomaxillofacial DO.%la Although DO offers a promising alternative to conventional osteotomies and bone grafting procedures, its application for the broad range of craniomaxillofacial skeletal deformities has been somewhat limited. The cumbersome nature of distraction devices, the necessity for cutaneous incisions, the resulting pin track scars, the lack of 3dimensional control of vectors of movement, and the lengthy treatment period have all been identified as problems. Distraction protocols currently used in the crania maxillofacial region are derived from clinical and experimental studies in the long bones2” and from studies in the canine mandible.b Most protocols include a 4 to 6day latency period, a distraction rate of 1 mm/d, and a stabilization period twice the duration

Surgeq

Gener4

Hospi-

MA.

Oral

and

Mzxillofacial

Surgery. tAssociate

Professor

and

Director

Associate

Professor.

of Skeletal

Biology

Research

Center. *Formerly,

Department

of Oral and Maxillofa-

cial Surgery. $W.C

Gur&ick

and Maxillofxial Presented Orleans.

Professor

in part

at the 8Vh Annual

LA, Srptcmbrr

the AO/ASIF

Foundation,

facial,

PA.

Pwli,

Address Department

and Chairman.

Departmtxt

of Oral

Surgery. Meeting

of AAOMS,

New

1998. and was supported

by a grant

from

Davos,

correspondence

Switzerland.

and

of Oral Sr 1Ma?rillofacial

setts Grnrral

Hospital,

SD 2000 American

Boston,

Association

0278.2391/00,‘58050008$3 dot IO 1053/10

2000

and Synthes

reprint

requests

Surgery.

Warren

to

MaxillaDr

Kaban:

120 1. Massachu-

MA 02114.

of Oral and Mox~llofac~oi

Suryeons

OO/‘O 5527

507

508

PORCINE MANDIBULAR

DO: LATENCY

AND RATE

of distraction. DO in the craniomaxillofacial region might be applicable to a broader range of deformities if the treatment period were shorter. In this study, a porcine model was developed, and the effects of latency and rate of distraction on bone formation in the mandible were investigated.

Materials

and

Methods

MINIPIG MODEL Nineteen Yucatan female minipigs (Charles River Laboratories, Wilmington, MA) in the mixed dentition stage (age 6 months), weighing between 25 and 30 kg, were used in these studies. After sedation (ketamine 20 mg/kg, intramuscularly), the anlmal was transferred to the operating room for preoperative radiographs and photographs. A pulse oximeter was placed and general anesthesia was induced and maintained with isoflurane (1% to 2%). A peripheral intravenous catheter was inserted for antibiotics (Ancef, SmithKline Beecham Pharmaceuticals, Philadelphia, PA; 1 g, intravenously). The pig was prepared and draped in a sterile fashion, and dissection and exposure of the right mandibular body and ramus were performed through a submandibular incision. A lateral corticotomy extending from the junction of the ramus and body (anterior) to the angle of the mandible (posterior) was created with a side-cutting bur. A custom-modified, singlevector, semiburied mandibular distraction device (Synthes Maxillofacial, Paoli, PA) was positioned across the corticotomy and fixed with four 2.4rnn-1 diameter, 12-mm length bicortical screws. Radiopaque markers (1.5mm screws) were placed halfway between the distractor footplates and the inferior border of the mandible, and the osteotomy was completed with osteotomes (Fig 1). A thin rod attached to a universal joint was used to activate the distractor. The exit wound was just below the pig’s ear and posterior to the mandibular ramus. The wound was copiously irrigated with sterile saline and closed in layers using 3-O chromic catgut sutures. Analgesics (buprenorphine, 0.3 mg intramuscularly and fentanyl, 25 ug/h patch) were given Immediately postoperatively. Postoperative radiographs and photographs were taken. An Elizabethan collar was placed, and the animal was returned to its cage and received a pureed diet ad lib&urn. EXPERIMENTALGROUPS Effect of Latency In the first part of this study, the effects of the latency period were determined. In 3 pigs, distraction was initiated immediately, that is, Oday latency, and in 2 pigs distraction was initiated after a 4&y latency period. In both groups, lengthening was achieved by

FIGURE 1. A, Custom-modified, single-vector, semi-buried mondibular distractor. A rod attached to a universal joint is used to activate the distroctor. 6, Introoperative photograph illustrating the dissection, exposure, osteotomy, and position of the semi-buried distroctor on the right mandible. The distractor has been activated to 4 mm. Note the bone markers (arrows].

distraction at a rate of 1 mm/d (0.5 mm every 12 hours) for 7 days followed by a 14-day period of neutral fixation. In the control group (n = 2) the submandibular dissection, exposure, and osteotomy were performed, and the distraction device was fixed into position but not activated. Effect of Rate In the second distraction was distraction for period. Fixation days (24 days).

of Distraction part of the study, the effect of rate of determined. Six animals underwent a 12-mm gap after a Oday latency periods were twice the gap size in The rate of distraction was varied at

Calculated Gap Size (mm) (mm/d X no. of days) Control 0 mm/d X 7 days = 0 n=2 lmm/dX7days=7 n=5 lmm/dX 12days= 12 n=4 2mm/d X Gdays = 12 n=4 4mm/dX3days= 12 n=4

Radiographic Gap Size (mm) (change between bone markers)

Clinical Gap Size (mm) (change between bone markers)

0

0

7.2 21 0.9

7.1 k 0.8

11.6 k 1.0

11.6 f 0.4

11.9 ? 0.5

11.9 + 0.5

12.5 2 0.5

12.0 2 0.3

509

FIGURE 2. Photograph showing the occlusion and mandibular deviation after unilateral distraction (Olotency, 1 mm/d x 12 doys). Note the asymmetric anterior crossbite and mandibular dental midline deviation (arrows indicate maxillary and mandibular midlines).

1 mm (n = 2) 2 mm (n = 2) or 4 mm (n = 2) per day. The experiment was then repeated using a 4day latency with distraction rates of 1 mm (n = 2) 2 mm (n = 2) and 4 mm (n = 2) per day, 12mm gap size, and 24 days fixation. ASSESSMENT Standardized lateral cephalometric radiographs and photographs of all animals were taken preoperatively,

FIGURE at the end 1 mm/d orientatior otomy, an

3. Lateral cephalogram of distraction (C-latency, x 12 days), showing 1 of the pig’s.head, oste d distractor.

immediately postoperatively, mid-distraction, immediately premortem, and postmortem. The cephalograms were used to monitor the progress of distraction as evidenced by changes in the distance between the bone markers. The photographs were used to document occlusal changes. At the same intervals, animals underwent clinical examination to evaluate mandibular stability. After sacrifice, at the end of the fixation period, the mandibles were harvested and the specimens were examined, photographed, and radiographed. Ex vivo clinical appearance of the gap was rated on a semiquantitative 4-point scale: 0 = Osteotomy margins clearly visible; 1 = Greater than 50% of osteotomy length visible; 2 = Less than 50% of osteotomy length visible; 3 = Osteotomy not visible, that is, bone tilling entire gap. Stability across the gap was rated on a 4point scale: 0 = Unstable (nonunion); 1 = Mobility in 2 planes (vertical and horizontal); 2 = Mobility in 1 plane; 3 = No mobility (clinical union). All radiographs were obtained in a standard fashion using an aluminum, step-wedge penetrometer (E.M. Parker, Wilmington, MA). Specimens were fixed in 2% paraformaldehyde and 0.1 mol/L cacodylate buffer, pH 7.4. Segments were excised across the osteotomies at a consistent landmark region just below the inferior alveolar canal. They were trimmed and prepared for nondecalcified histologic evaluation by dehydration and embedment in methylmethacrylate. Thick sections were ground to

510

PORCINE

approximately blue.

50 urn and stained with 0.5% toluidine

Results PORCINE

MODEL

OF

MANDIBULAR

DISTRACTION

OSTEOGENESIS

All animals survived the surgical procedure. They fed well on a pureed diet and exhibited a normal pattern of activity. There were no postoperative infections or wound dehiscences. All distraction devices remained stable, and none were removed prematurely. The Elizabethan collar blocked the pig’s peripheral vision, allowing for distractor activation (twice daily) with no need for sedation. Animal behavior was not affected by distractor manipulation. Serial lateral cephalograms showed the progress of

MANDIBUIAR

DO:

LATENCY

AND

RATE

distraction. The distance between the bone markers was measured at each interval and was correlated with the calculated amount of distraction (number of turns to date X 0.5 mm per turn) (Table 1). This analysis validated that the distractor was functioning as designed. EFFECT

OF

LATENCY

PERIOD

At the end of the distraction period, afl animals had developed an occlusal crossbite with deviation of the jaw toward the nondistracted side (Fig 2). At the end of neutral duration, the jaw was clinically united across the distraction gap. Radiographically, the desired elongation was achieved and maintained during the neutral fixation period (Fig 3). Ex vivo inspection, bimanual palpation of stability,

FIGURE 4. A, Ex vivo radiographic assessment at the end of fixation period (Odoy latency with 1 mm/d distraction rote) showing unrform opocification across the gap. 5, Ex vivo rodiogrophic assessment ot the end of fixation pertod (4doy latency with 1 mm/d distraction rate) showing uniform opacification across the gap except for o small area inferiorly. C, Ex vivo radiographic ossessment at the end of frxotron period (O-day latency with 2 mm/d distraction rate] showing minimol opocification. D, Ex viva radiographic ossessment ot the end of frxotron period (O-day latency with 4 mm/d distraction rate) showing incomplete and irregular opocificotion.

511

TROULIS ET AL and radiographic examination (Fig 4A, B) showed equivalent clinical healing, stability, and radiographic density in both the 0- and 4-day latency groups (Table 2). EFFECTOF RATEOF DISTRACTION

Control

All animals (1 mm/d, 2 mm/d, and 4 mm/d rates) developed the expected occlusal crossbite and jaw deviation toward the nondistracted side. Evaluation of the harvested specimens included inspection, bimanual palpation of stability, and radiographic examination (Fig 4). On inspection, the mandibles that were distracted at 1 mm/d showed the most uniform bone formation along the length of the osteotomy. Average scores were 3.0, 2.0, and 1.5 at O-day latency and 3.0, 2.0, and 2.0 at 4day latency for 1 mm/d, 2 mm/d, and 4 mm/d rates, respectively (Table 3). The most sensitive detectable difference between the groups was in the rating of stability (Fig 5). Average scores were 3.0, 1.5, and 1.5 at O-day latency and 3.0, 2.0, and 1.5 at Cday latency for 1 mm/d, 2 mm/d, and 4 mm/d rates, respectively. The histologic results correlated best with the stability index by bimanual palpation. A random section from an osteotomy gap that was distracted at 1 mm/d after no latency showed exuberant woven and lamellar bone across the 7mm gap (Fig CA). A comparable section from an osteotomy gap that was dis tracted at 4 mm/d after no latency showed a vacant 7-mm gap, with no evidence of fibrous or osseous tissue (Fig 6~).

Discussion Distraction protocols currently used for the craniomaxillofacial skeleton have been developed in clinical and experimental long bone models2” and in the canine mandible modelG9 The goal of this study was to validate a minipig model for DO of the mandible. This species is relatively inexpensive and easy to handle,i5 and its chewing pattern, temporomandibular joint function, and its mandibular size and shape

Subject Number

Protocol

Control O-day Latency 1 mm/d X 14 days 4day Latency 1 mm/d X 14 days

7 days, fixation 7 days, fixation

Subject Number

Protocol

Appearance’

Latency (12~mm gap, 24day fixation) 1 mm/d

Appearance*

StabiIityt

2-5 129-1

3 3

3 3

1063 111-2 120-l 130-6 456 441

3 3 2 2 1 2

3 3 1 2 1 2

636 644 60-2 137-l 120-3 1326

3 3 2 2 2 2

3 3 2 2 2 1

Oday

2 mm/d 4 mm/d

4day Latency (12~mm gap, lmm/d

24day

fixation)

2 mm/d 4 mm/d

l Ex vivo clinical appearance of gap (@3). tEx viva stability by bimanual palpation (O-3).

are similar to humans.16~17Furthermore, the pig’s bone turnover rate (sigma) is equal to that of the human, an important factor when studying the biology of the mandibular DO wound.1s*19 The effects of latency and distraction rate were evaluated to determine the shortest possible treatment duration. A latency period of 7 to 21 days has been reported in long bone DO, and 4 to 10 days has been the standard for craniomaxillofacial D0.2-5*2022 The guidelines are based on Ilizarov’s original experiments in the long bone. He concluded that for optimal bone formation in canine long bones, a latency period of at least 5 days must be observed.2-4 The optimal latency period for craniomaxillofacial DO is not known. The anatomy of the craniomaxillo facial region, especially its rich blood supply, is significantly different from that of long bones. Therefore, it may be possible that a shorter or even O-day

Stabilityt

2-5 129-l 443 465 112-1 9-l

3 3 3 3 3 3

3 3 3 3 3 3

30-4

3

3

O-day

lday LStMCy

‘Ex vivo gross appearance of gap (O-3). tEx vivo stability by bimanual palpation

(O-3).

FIGURE 5. Effects of rate and latency on clinical stability gaps. Values represent the mean of scores for 2 specimens exvivo. I@, 1 mm/d; 0, 2 mm/d; q ,4 m/d.)

of 12-mm evaluated

PORCINE

MANDIBULAR

DO;

LATENCY

AND

RATE

FIGURE 6. A, Photomicrograph of a histologic section across a 7-mm gap lorrows) created after no latency and distraction at 1 mm/d (Tbluidine blue, originol maonificotion x6.1 I. B. Photomicrograph of o histologk section across 0 7-mm gap (arrows) cre oted after no l&n&y ond distroction at 4 mm/d IToluidine blue, originol magnkaiion x6.1)

latency would be feasible in the mandible. Tavakoli et aI23 performed mandibular DO in sheep (rate of 1 mm/d for 20 days and fixation period of 20 days) and found that latency periods of 0,4, and 7 days resulted in equivalent bone formation on histologic evaluation. The biomechanical properties and bone density of the regenerated bone were equal in all groups. Similarly, O- and 4-day latency periods (with 1 mm/d rate and Cxation twice the number of days of distraction) produced equivalent clinical and radiographic bone

healing in the current study. Aronson and colleagues24*25 also found that bone regeneration and consolidation were best after a O-day latency in metaphyseal and diaphyseal long bone lengthening. In the minipig model reported here, mandibles that were distracted with rates of 2 mm/d and 4 mm/d (gap size of 12 mm and fixation 24 days) were less stable on bimanual examination than those mandibles distracted at 1 mm/d. This was even true when latency was increased from 0 to 4 days, allowing for metabolic

TROULIS

513

ET AL

upregulation and initial callus formation. These results are consistent with those of Aronson and Shen,L-Lwho found that rat tibiae distracted at 2 mm/d showed delayed or nonunion. The 3 outcome measures used to assess the distraction wound in this study were inspection, bimanual palpation, and radiographic bone density. Stability across the gap is most important for the clinician because it reflects clinical bone union. Specimens with clinical stability showed bone formation across the gap (Fig 6A) when examined histologically. Unstable specimens, when examined histologically, showed poor ossification (Figure 6B). Despite differences in clinical appearance and stability with different distraction rates, standard radiographs were not sensitive enough to detect these differences. These findings correlate with previous reports that bony union on plain radiographs is not a reliable indicator of clinical stability.‘GzM Other techniques such as ultrasound’9 and computed tomography” may be more useful for quantitative assessment of bone density.

Conclusions The use of DO is limited by the lengthy treatment period, which is determined by 4 parameters: latency, gap size and rate, and duration of fixation. In these experiments, a reproducible and relevant porcine model for mandibular DO was established. When mandibles were distracted after 0- or 4day latency periods, there was equivalent stability across the resulting gaps. Preliminary assessment of the effect of rates of distraction suggest that 1 mm/d may give the most consistent osteogenesis. The relationships among latency, gap size, rate, and duration of fixation are poorly understood and can be defined with appropriate animal models. A better understanding of the distraction wound may allow a shortened treatment time, while enhancing the quality and quantity of bone formation. Acknowledgment The authors thank Drs Ralph Holmes University of California, San Diego, CA, histologic specimens.

and Mark Shusterman, for assistance with the

References 1. Codivilla A: On the means of lengthening, in the lower limbs, the muscles and tissues which are shortened through deformity. AmJ Onhop Surg 2:353, 1905 2. llizarov G: The tension-stress effect on the genesis and growth of tissues. Part 1. Clin Orthop 238249, 1989 3. llizarov G: The tension-stress effect on the genesis and growth of tissues. Part Il. Clin Orthop 239:263. 1989 4. Bizarov G: The principles of the Ilizarov method. Bull Hosp Joint Dis Orthop Inst 48: 1, 1988 5. McCarthy J. Stalfenberg DA, Wood RL, et al: Lengthening the human mandible by gradual distraction. Plast Reconstr Surg 89:l. 1992

6. Snyder C, Levine GA, Swanson HM, et al: Mandibular lengthen ing by gmdual distraction: Preliminary report. Plast Reconstr Surg 51:506. 1973 7. IMicheli S, Miotti B: Lengthening of mandibular body by gradual surgical orthodontic distraction. J Oral Surg 35:187. 1977 8. Costantino PD. Shybut G. Friedman CD, et aI: Segmental mandibular regenention by distraction osteogenesis: An experimental study. Arch Otolaryngol Head Neck Surg 116:535, 1990 9. Klotch DW. Ganey TM, Slater-Haase A, et al: Assessment of bone formation during osteogenesis: A canine model. Otohuyngo1 Head Neck Surg 112:291, 1995 IO. Perrott DH. Berger R, Vargen+k K. et al: Use of a skeletal distraction device to widen the mandible: A case report. J Oral Maxillofac Surg 51:435. 1993 11. Chin M. Toth B: Distraction osteogenesis in maxlIlofacial surgery using internal devices: Review of five cases, J Oral ~Maxillofac Surg 54:45. 1996 12. Chin IM. Toth B: LeFon 111 advancement with gradual distraction using internal devices. Plast Reconstr Surg 100:819, 1997 13. Monastcrio FO. Molina F. Andrade L, et al: Simultaneous mandibular and maxillary distraction in hemifacial microsoma in adults: Avoiding occlusal disasters. Plast Reconstr Surg 100:852. 1997 14. Padwd BL. Keams GJ, Todd R. et al: Simultaneous maxillary and mandibular distraction osteogenesis: A technique and case report. Int J Oral Maxillofac Surg 28:2, 1999 15. Svendsen P: Anesthesia and basic experimental surgery of minipigs: A practical exercise. Pharmacol Toxic01 80:23, 1997 16. Kuboki T. Shinoda IM. Orsini MC, et al: Viscoelastic properties of the pig temporomandibular joint articular soft tissues of the condyle and disc. J Dent Res 76: 1760, 1997 17. Ciochon RL. Nisbett RA. Corruccini RS: Dietary consistency and craniofacial development related to mastlcatory function in minlpigs. J Craniofac Genet Dev Biol 17:96, 1997 18. de Vemejoul MC, Pointillart A, Golenzer CC, et al: Effects of iron overload on bone remodeling in pigs. Am J Path01 116:377, 1984 19. Gerard DA, Gotcher JE. Cooper EC, et al: Histomorphometrlc study of endosseous implants in minipigs: A pilot study. J Dent Res 70:460. 1991 20. Rachmiel A, Levy M. Laufer D: Lengthening of the mandible by distrdction osteogenesis: Report of cases. J Oral Maxlllofac Surg 53:838. 1995 2 1. Klein C, Howaldt HP: Lengthening of the hypoplastic mandible by gradual distraction in childhood: A preliminary report. J Craniomaxillofac Surg 23:68, 1995 22. White SH, Kenwright J: The importance of delay in distraction osteotomies. Orthop Clin North Am 22:569, 1991 23. Tavakoli K. Walsh WR, Smart R, et aI: The role of latency ln mandibular osteodistraction. J Cmniomaxillofac Surg 26:209, 1998 24. Aronson J. Shen X: Experimental healing of dlstractlon osteogenesis comparing metaphyseal to diaphyseal sites. Clin Orthop 301:25, 1993 25. Aronson J, Good B, Stewart C. et al: Preliminary studies of mineralization during distraction osteogenesis. Clin Otthop 250:43. 1990 26. Panjabi MM, Walter SD, Karuda M, et al: Correlations of radiographic analysis of healing fractures with strength: A statistical analysis of experimental osteotomies. J Orthop Res 3:212. 1985 27. Blane CE. Hetzenberg. DiPietro MA: Radiographic imaging for llizarov limb lengthening in children. Pediatr Radio1 21:117, 1991 28. Hughes TH, Maffulli. Green V. et al: Imaging in bone lengthening. Clin Orthop 308:50, 1994 29. Gluer CC, Wu CY. Jet-gas SA, et al: Three quantitative ultrasound parameters reflect bone structure. Calcif Tissue lnt 55:46, 1994 30. Harp JH. Aronson J. Hollis M: Noninvasive determination of bone stillness for distraction osteogenesis by quantitative computed tomography scans. Clin Orthop 301:42,1994