Rat mandibular distraction osteogenesis: part III. Gradual distraction versus acute lengthening

Distraction osteogenesis is a well-established method of endogenous tissue engineering. This technique has significantly augmented our armamentarium of reconstructive craniofacial procedures. Although the histologic and ultrastructural changes associated with distraction osteogenesis have been extensively described, the molecular mechanisms governing successful membranous distraction remain unknown. Using an established rat model, the molecular differences between successful (i.e., osseous union with gradual distraction) and ineffective (i.e., fibrous union with acute lengthening) membranous bone lengthening was analyzed. Herein, the first insight into the molecular mechanisms of successful membranous bone distraction is provided. In addition, these data provide the foundation for future targeted therapeutic manipulations designed to improve osseous regeneration. Vertical mandibular osteotomies were created in 52 adult male Sprague-Dawley rats, and the animals were fitted with customized distraction devices. Twenty-six animals underwent immediate acute lengthening (3 mm; a length previously shown to result in fibrous union) and 26 animals were gradually distracted (after a 3-day latency period, animals were distracted 0.25 mm twice daily for 6 days; total = 3 mm). Four mandibular regenerates were harvested from each group for RNA analysis on 5, 7, 9, 23, and 37 days postoperatively (n = 40). Two mandibular regenerates were also harvested from each group and prepared for immunohistochemistry on postoperative days 5, 7, and 37 (n = 12). In addition to the 52 experimental animals, 4 control rats underwent sham operations (skin incision only) and mandibular RNA was immediately collected. Control and experimental specimens were analyzed for collagen I, osteocalcin, tissue inhibitor of metalloproteinase-1, and vascular endothelial growth factor mRNA and protein expression. In this study, marked elevation of critical extracellular matrix molecules (osteocalcin and collagen I) during the consolidation phase of gradual distraction compared with acute lengthening is demonstrated. In addition, the expression of an inhibitor of extracellular matrix turnover, tissue inhibitor of metalloproteinase-1, remained strikingly elevated in gradually distracted animals. Finally, this study demonstrated that neither gradual distraction nor acute lengthening appreciably alters vascular endothelial growth factor expression. These results suggest that gradual distraction osteogenesis promotes successful osseous bone repair by regulating the expression of bone-specific extracellular matrix molecules. In contrast, decreased production or increased turnover of bone scaffolding proteins (i.e., collagen) or regulators of mineralization (i.e., osteocalcin) may lead to fibrous union during acute lengthening.     

Removal of mandibular tooth follicles before distraction osteogenesis

Distraction osteogenesis is an innovative technique that has transformed the treatment of craniofacial malformations in young children. Bone generation obviates the need for graft material, which is in short supply in young patients, thus making possible surgical procedures on the craniofacial skeleton in young children. Sufficient mandibular volume is required for the osteotomy and placement of the device screws and/or pins. To have adequate bone stock and to facilitate distraction, the authors preoperatively examined all patients radiographically and selected those with tooth follicles that precluded successful osteotomy and pin placement for planned mandibular distraction. This report is of the first 13 children, aged 9 months to 6 years, who underwent predistraction enucleation. The osteotomy and device placement were performed successfully at least 4 months after enucleation. The described procedure has minimal morbidity and has resulted in successful subsequent distraction. The advantages, disadvantages, and cost-benefit issues are discussed.     

Expression of bone morphogenetic proteins during mandibular distraction osteogenesis

Distraction osteogenesis is a form of in vivo tissue engineering in which the gradual separation of cut bone edges results in the generation of new bone. In this study, the temporal and spatial expression of bone morphogenetic proteins (BMPs) 2, 4, and 7 was examined in a rabbit model of mandibular distraction osteogenesis. Fourteen skeletally mature male rabbits were studied. After osteotomy, a distractor was applied to one side of the mandible. After 1 week of latency, distraction was initiated at 0.25 mm every 12 hours for 3 weeks (distraction period), followed by a 3-week consolidation period. Two animals were killed each week after surgery. The generate bone was analyzed for the expression of BMP-2, -4, and -7 by using standard bone histological and immunohistochemical techniques. BMP-2 and -4 were highly expressed in osteoblastic cells during the distraction period and in chondrocytes during the consolidation period. BMP-7 demonstrated relatively minor expression in osteoblastic cells during the distraction period. All BMPs were strongly expressed in vascularized connective tissue during the distraction period. These data indicate that BMPs participate in the translation of mechanical stimuli into a biological response during mandibular distraction osteogenesis.     

Modeling distraction osteogenesis: analysis of the distraction rate

Distraction osteogenesis is a useful technique aimed at inducing bone formation in widespread clinical applications. One of the most important factors that conditions the success of bone regeneration is the distraction rate. Since the mechanical environment around the osteotomy site is one of the main factors that affects both quantity and quality of the regenerated bone, we have focused on analyzing how the distraction rate influences on the mechanical conditions and tissue regeneration. Therefore, the aim of the present work is to explore the potential of a mathematical algorithm to simulate clinically observed distraction rate related phenomena that occur during distraction osteogenesis. Improvements have been performed on a previous model (Gómez-Benito et al. in J Theor Biol 235:105–119, 2005) in order to take into account the load history. The results obtained concur with experimental findings: a slow distraction rate results in premature bony union, whereas a fast rate results in a fibrous union. Tension forces in the interfragmentary gap tissue have also been estimated and successfully compared with experimental measurements.     

Focal adhesion kinase expression during mandibular distraction osteogenesis: evidence for mechanotransduction

Distraction osteogenesis is an established treatment strategy in the reconstruction of the craniofacial skeleton. The underlying mechanisms that drive bone formation during this process are largely unknown, but a regulatory role for mechanical force is believed to be critical. The integrin-mediated signal transduction cascade is a primary pathway by which signal transduction of mechanical stimuli (i.e., mechanotransduction) occurs. Focal adhesion kinase (FAK) is a significant regulator in this pathway. The authors hypothesize that mechanical forces created during distraction osteogenesis are responsible for the osteogenic response that takes place, and that these changes arise through integrin-dependent mechanotransduction. Using a rat model of distraction osteogenesis, the authors examined the expression of FAK in critical size defects (n = 15), subcritical size defects (n = 15), and mandibles undergoing distraction osteogenesis (n = 15). Their findings demonstrated FAK immunolocalization in mandibles undergoing distraction osteogenesis, but not in the critical size defects or in subcritical size defects, despite varying degrees of bone formation in the latter two groups. Furthermore, bone sialoprotein mRNA in situ hybridization patterns were found to mirror FAK immunolocalization patterns in mandibles undergoing distraction osteogenesis, demonstrating an association of FAK expression with the osteogenic process specific to distraction osteogenesis. These findings suggest that the bone formation in distraction osteogenesis is regulated by mechanical force by means of integrin-dependent mechanotransduction pathways.     

Long-term skeletal stability after maxillary advancement with distraction osteogenesis using a rigid external distraction device in cleft maxillary deformities

Rigid external distraction is a highly effective technique for correction of maxillary hypoplasia in patients with orofacial clefts. The clinical results after correction of sagittal maxillary deformities in both the adult and pediatric age groups have been stable. The purpose of this retrospective longitudinal cephalometric study was to review the long-term stability of the repositioned maxilla in cleft patients who underwent maxillary advancement with rigid external distraction. Between April 1, 1995, and April 1, 1999, 17 consecutive patients with cleft maxillary hypoplasia underwent maxillary advancement using rigid external distraction. There were 13 male patients and four female patients, with ages ranging from 5.2 to 23.6 years (mean, 12.6 years). After a modified complete high Le Fort I osteotomy and a latency period of 3 to 5 days, patients underwent maxillary advancement with rigid external distraction until proper facial convexity and dental overjet and overbite were obtained. After active distraction, a 3- to 4-week period of rigid retention was undertaken; this was followed by removable elastic retention for 6 to 8 weeks using, during sleep time, an orthodontic protraction face mask. Cephalometric radiographs were obtained preoperatively, after distraction, at 1 year after distraction, and 2 or more years after distraction. The mean follow-up was 3.3 years (minimum, 2.1 years; maximum, 5.3 years). The following measurements were obtained in each cephalogram: three linear horizontal and two linear vertical maxillary measurements, two angular craniomaxillary measurements, and one craniomandibular measurement. Differences between the preoperative and postoperative cephalometric values were analyzed by paired t tests (p < 0.05). The cephalometric analysis demonstrated postoperatively significant advancement of the maxilla. In addition, the mandibular plane angle opened 1.2 degrees after surgery. After the 1- to 3-year follow-up period, the maxilla was stable in the sagittal plane. Minimal anteroposterior growth was observed in the maxilla compared with that exhibited in the anterior cranial base. However, there was significant vertical maxillary growth over the 3-year observation period. The mandibular plane angle tended to decrease during the follow-up period. The cephalometric data from this study support the clinical impression of maxillary stability after maxillary advancement with rigid external distraction in cleft patients. This effective and stable technique is now considered for all pediatric patients with severe cleft maxillary hypoplasia and for adolescent and adult patients with moderate to severe deformities.     

Effects of mandibular distraction osteogenesis on the inferior alveolar nerve: an experimental study in monkeys

A series of experimental studies were performed in monkeys to study the effect of mandibular distraction osteogenesis on the inferior alveolar nerve at different times before and after distraction. A mandible osteotomy was performed and distraction was carried out unilaterally in 10 young rhesus monkeys and bilaterally in six. The intact nerves on the contralateral side of the 10 monkeys were used for the control. Care was taken to avoid destroying the integrity of the inferior alveolar nerve during the surgical procedure. After a 5-day latency period, the distraction device was activated at a rate of 0.5 mm twice each day for 15 days. Sensory nerve action potential testing was applied before and 1 day after the operation, at completion of distraction, and at 2, 4, 6, 9, and 12 weeks of fixation. Necropsy was performed at the completion of distraction and 2, 4, 6, and 12 weeks of fixation. The mental nerves were taken, sectioned, and stained with lead citrate and uranyl acetate, and examined with a transmission electron microscope. The inferior alveolar nerves in the distraction gap were obtained, and paraffin slides were made and stained with hematoxylin and eosin, Luxol fast blue, and Bodian methods. The authors found that immediately after the mandible osteotomy, most nerves showed signs of slight acute injury; the latency was increased by 5.553 percent, and the amplitude was decreased to 1808 microV. This might be caused by the surgical procedure or by compressions produced by swelling tissues around the nerves. When distraction was completed, the latency was prolonged for an average of 22.18 percent, and the amplitude average had attenuated to 28.54 percent (804 microV) of the preoperative value on the distracted side. Most nerve fibers exhibited signs of degeneration, such as myelin disruption, swelling of cell organs greatly increased in axoplasm, axon tearing, and myelin fragments engulfed by macrophages. These were nerve reactions to the tensions produced by mandible lengthening. As time elapsed, the nerve's action potential recovered gradually because of its repairing ability, the latency shortened, amplitude increased, Schwann cells proliferated and formed new myelin sheaths, and the tearing axons reconnected. After 12 weeks of consolidation, there was still a latency of 12.384 percent prolongation because of the prolonged conduction distance, and the average amplitude was restored to 2786 microV, the approximate preoperative value. The nerve seemed to be repaired completely; its myelin thickness, axon diameter, and ultrastructure were all similar to those of the control. It was concluded that mandibular distraction osteogenesis can produce some degree of harmful effects on the function and structure of inferior alveolar nerves, but it is reversible and relatively slight. Along with the regeneration of the nerve's myelin and axon, the nerve function can gradually rehabilitate to a normal level.     

Vascular leak syndrome of Sprague-Dawley rats in a mandibular distraction osteogenesis study

Vascular leak syndrome (VLS) is a common and often fatal sequela of multiple bone traumas, and of infectious, toxic, and allergic insults in human patients. Although an animal model for VLS has not been fully established, rats have shown sensitivity to the syndrome that approximates that of the human population. We describe cases of VLS in three-month-old adult and one-month-old Sprague-Dawley rats in an osteogenesis study aimed at optimizing correction of bone hypoplasias and other craniofacial deformities in children, using a mandibular distraction device. In the study reported here, VLS was diagnosed in 40% of the rats that were necropsied after dying or being euthanized early, subsequent to mandibular osteotomy, a procedure that involves minimal bone trauma. The gross and histologic findings, as well as the clinical course of VLS in the rats of the osteogenesis study, were similar to those of documented human cases. Hence, the rat may be a useful animal model to h elp characterize the physiologic and molecular events that accompany this syndrome.     

Remodeling of the temporomandibular joint following mandibular distraction osteogenesis in the transverse dimension

Transverse mandibular distraction osteogenesis involves moving the osteotomized segments of the mandible in either a varus or valgus direction. This maneuver allows for widening of the bigonial distance or for a lateral shift of an asymmetric mandibular midline. During this process, a significant amount of torque is placed on the mandibular condyles, because they act as the pivot point for the mandibular translation. Although standard linear distraction osteogenesis induces transient, reversible changes in the temporomandibular joint, it is not known what effect the varus and valgus stresses of transverse distraction have on the temporomandibular joint. We therefore designed a study to document the temporomandibular joint changes following various degrees of transverse distraction.Bilateral transverse mandibular distraction was performed on 10 adult, female mongrel dogs using an external, multiplanar mandibular distraction device. The distraction protocol was as follows: (1) complete osteotomy at the angle of the mandible, (2) 5-day latency period, (3) distraction rate of 1 mm/day, (4) rhythm of one turn per day, (5) linear activation 16 to 30 mm bilaterally, and (6) 8-week consolidation period. A variety of varus and valgus distraction vectors were applied to the mandible only after 10 mm of initial linear distraction had been achieved. Posteroanterior and lateral cephalograms were performed throughout the entire process. Pre-distraction and post-consolidation computed tomographic scans were also performed. Changes in mandibular conformation, axis of rotation, temporomandibular joint structure, and glenoid fossa changes were directly assessed by evaluating the postmortem craniofacial skeleton. The findings were compared with those of normal, age-matched mongrel dog skulls.Significant remodeling changes were observed in the temporomandibular joints of all animals involved in the study. The mandibular condyles demonstrated varying degrees of flattening and erosion at all contact points with the craniofacial skeleton. In some cases, the condyle became part of the distraction regenerate process and was hypertrophied in all dimensions. The condyles were frequently displaced out of the glenoid fossa, particularly on the side in the direction of varus distraction. When the latter occurred, a new fossa was created on the undersurface of the zygomatic arch. Varying degrees of mandibular rotation in the sagittal plane were also observed, which led to abnormal torquing of the condyles in the coronal plane, depending on whether the axis of rotation occurred primarily around the condyle or around the distraction regenerate zone.In conclusion, transverse mandibular distraction is an effective means of producing a varus or valgus shift in the gonion relative to the midsagittal plane. However, unlike linear or angular mandibular distraction, transverse distraction has a multitude of nontransient effects on the temporomandibular joint. Therefore it must be emphasized that in clinical practice, transverse distraction should be used cautiously. One must also be aware that such a maneuver in distraction can have negative effects on the temporomandibular joint.     

Central cable system--fully automatic, continuous distraction osteogenesis for the lengthening treatment of large bone defects]

Distraction osteogenesis involving bone transport enables the reconstruction of large bone defects. The main bone fragments are usually stablilised externally, an intermediate bone segment is separated and moved through the defect at a rate of about 1 mm/day. New high-quality bone is built up in the constantly enlarging osteotomy gap. A major problem associated with the method is the fact that the fixation pins are also moved over the same distance, and cut through the soft tissue, often resulting in painful pin tract infections and ugly scars. An automatic motorized bone transport system employing a single central cable now eliminates this problem. The system can be combined with any external fixateur, since the relevant implanted parts for bone transport are independent of the external stabilizer. The surgical procedure, which is easy on the patient, consists of bone segment separation, central cable fixation, and stabilisation of the main fragments, and requires the use of numerous special tools. The distraction itself results in significantly less soft tissue irritation and pain. Pin tract infections are rare, so that changeover to internal fixation after completion of bone transport carries little risk of infection. This article details the technical features of the stabilizing system and the transport and the control systems, and describes the clinical application in a patient.     

A new device to quantify regenerate torsional stiffness in distraction osteogenesis.

We present a newly developed torsional stiffness measurement device with the potential to quantitatively assess the in vivo torsional stiffness of bone regenerate during distraction osteogenesis. We describe the form and function of this device and its application in a model of regenerate consolidation. The device was able to produce data to assess stiffness of the regenerate with an accuracy between +/- 3 and +/- 9% for material stiffness ranging between 0.1 and 2.4 Nm/o and with a precision of +/- 3.6%. This method provides advantages over similar methods of bone fracture healing assessment with guaranteed maintenance of bone axis, minimized risk of bone misalignment during the bone healing process and a close relation to the functional loading pattern in torsion of bones such as tibia and femora.