Computer planning for distraction osteogenesis

Distraction osteogenesis of the mandible has found an application in the treatment of patients with a variety of different mandibular deformities. Compared with the relatively simple unidirectional distraction of long bones as described by Ilizarov, the three-dimensional distraction of the mandible is extremely complex. Whereas experience with orthognathic surgery clearly demonstrates that careful presurgical planning is necessary to achieve predictable outcomes, there are few reported methods for the planning of mandibular distraction. The authors have developed a method for planning distraction osteogenesis of the mandible that involves the use of three-dimensional modeling and animation to simulate distraction osteogenesis in virtual reality. The first step in the authors' treatment planning process is to obtain a three-dimensional computerized scan of the facial skeleton. From this scan, a three-dimensional wire-mesh model is built using animation software. With the same software, a virtual distractor is built and installed on the wire-mesh model. The osteotomies and the distraction process are then simulated. Finally, a recipe for sequencing the linear and angular changes of the distractor is calculated. The authors have used this planning process in seven patients (age range, 4 to 10 years): four with unilateral mandibular deformities and three with bilateral. The planning process has yielded predictable and reproducible results.     

Distraction osteogenesis of costochondral bone grafts in the mandible

Costochondral grafting for reconstruction of the Pruzansky type III mandible has given variable results. Lengthening of the rib graft by means of distraction had been advocated when subsequent growth of the grafted mandible is inadequate. This retrospective study reviews a series of patients with mandibular costochondral grafts who underwent subsequent distraction osteogenesis of the graft. A retrospective review identified two patient groups: group 1 consisted of individuals (n = 9) who underwent costochondral rib grafting of the mandible followed by distraction osteogenesis several months later at a rate of 1 mm/day. Group 2 consisted of patients with Pruzansky type II mandibles who had distraction osteogenesis without prior rib grafting (n = 9). The biomechanical parameters, orthodontic treatment regimens, and complications were examined versus patient age and quality of the rib graft. Distraction osteogenesis was successfully performed in six of the rib graft patients (group 1) and in all of the group 2 individuals. On the basis of the Haminishi scale, the computed tomographic scan appearance of the regenerate was classified as "standard or external" in six of the group 1 patients and as either "agenetic" or "pillar" (fibrous union) in the remaining three patients. In group 1, the average device was expanded 23 mm (range, 20 to 30 mm). Group 2 mandibular distraction results were all classified as either standard or external, and there was an average device expansion of 22.4 mm (range, 16 to 30 mm). The length of consolidation averaged 12.6 weeks in group 1, compared with 8.5 weeks in the traditional mandibular distraction patients (group 2). The mean shift of the dental midline to the contralateral side was 2.5 mm in group 1 versus 4.0 mm in group 2. Complex multiplanar and transport distractions were successfully performed on grafts of adequate bony volume. All four patients in group 1 with tracheostomies were successfully decannulated after consolidation. Rib graft distraction complications included pin tract infections in two patients, hardware failure with premature pin pullout in one patient, and evidence of fibrous nonunions in three young patients with single, diminutive rib grafts. In group 2, there were no distraction failures. Distraction osteogenesis can be successfully performed on costochondral rib grafts of the mandible; however, the complication rate is higher than in non-rib-graft patients. Performing the technique on older, more cooperative individuals seems to reduce this risk. In addition, placement of a double rib graft or an iliac bone graft of sufficient volume to create a neomandible with greater bone stock is an absolute requirement to decrease the risk of fibrous nonunion and provide a bone base of sufficient size for retention of the distraction device and manipulation of the regenerate.     

Three-dimensional histomorphometric analysis of distraction osteogenesis using an implanted device for mandibular lengthening in sheep

The aim of this study was to lengthen the sheep mandible with a fully buried device and to quantitatively analyze the tissue regenerate in the distraction gap by means of two-dimensional and three-dimensional histomorphometry. A custom-made device for continuous distraction was used in five adult sheep and fixed with three bicortical screws on either side of an osteotomy, anterior to the premolar region of the mandible. A cable-connected power and control unit was implanted in the neck region. After a 5-day latency period, distraction was activated every 2 hours and advanced at a rate of 1.01 mm per day. The distraction period was planned for 14 days, but because of stability problems and cable breakage, the actual distraction period ranged from 2 to 17 days, resulting in gap distances from 1.7 to 17.1 mm (mean, 0.95 mm/day). Osteogenesis was followed by radiographic imaging, and after a 6-week consolidation period, the harvested mandibles were serially sectioned for histologic and two-dimensional histomorphometric analysis, with three-dimensional reconstruction. Histologic examination of the specimens demonstrated predominantly membranous bone formation with remodeling bridging the distraction gap mainly in the periosteal region of the lingual side. In addition, cartilaginous areas and chondral bone formation were observed where the bridging appeared incomplete. Because of device fixation on the buccal side of the mandible, the preservation of the lingual periosteum seemed to play the major role for sufficient bone repair in the distraction gap. Cartilage within the distraction gap suggests fixation instability in this animal model.     

Effect of distraction rate on biomechanical, mineralization, and histologic properties of an ovine mandible model

Craniofacial microsomia is a common congenital malformation. Ilizarov's method of distraction osteogenesis applied to the mandible has yielded promising results both experimentally and clinically. Because the technique is used predominantly in a pediatric population, length of treatment and compliance may be problematic. To date, the limits of distraction rate in the craniofacial skeleton have not been defined. This study was designed to investigate the effects of distraction rate, in a large animal model, on the mineralization, biomechanical, and histologic properties of lengthened mandibles. Clinically faster distraction rates would decrease the overall treatment time. Twenty-four animals were divided into four groups, with varying rates of distraction (1, 2, 3, and 4 mm/day). A uniaxial distractor at the angle of the mandible was used. The mandibles were lengthened to 24 mm and fixed for a period of 5 weeks, when the animals were killed. The specimens were analyzed with respect to mineralization using dual energy x-ray absorptiometry, biomechanical strength, through a modified three-point bending test, and histologic properties with hematoxylin and eosin stains. Biomechanical, mineralization, and histologic analyses of the samples indicated that group 1 (1 mm/day) samples were significantly superior (p<0.05) to those of group 4 (4 mm/day). Although bone formation was achieved in all groups, group 1 (1 mm/day) demonstrated the strongest biomechanical and histologic properties. Bone mineral density obtained using dual energy x-ray absorptiometry may be clinically useful as a reliable, noninvasive, and relatively cheap predictor for removal time of the fixator.     

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.     

Callus stimulation in distraction osteogenesis

Distraction osteogenesis has been described as in vivo tissue engineering. The ability to stimulate this process for the repair of bony defects or lengthening of congenitally shortened facial structures is likely to significantly impact the field of craniofacial surgery. The purpose of this study was to determine whether mechanical stimulation of the distracted rabbit mandible would accelerate the maturation of the bony callus when applied during the early consolidation period. Twenty adult New Zealand White rabbits underwent unilateral mandibular osteotomy. A uni-directional internal distractor device (Synthes, Paoli, Pa.) was positioned along a plane perpendicular to the line of osteotomy. After a 7-day latency period, distraction was commenced at a rate of 1.0 mm/day for 12 days in all animals. In a control group of 10 rabbits, a consolidation period of 8 weeks was observed before they were killed. In the experimental group of 10 rabbits, daily alternate compression and distraction of 1 mm (sequential compression and distraction) was performed for 3 weeks followed by a 5-week period of rigid fixation. Each animal received a dose of a fluorescent label at three different time points during the study: at the end of the distraction period, 3 weeks after the completion of the distraction phase, and 3 days before it was killed. All animals were killed 8 weeks after the completion of the distraction phase. Undecalcified histologic analysis and 3-point bending tests to failure were performed on the extracted mandibles. The results of the experimental and control groups were compared.Four animals in the control group and three animals in the experimental group were excluded from the study because of screw loosening resulting in distractor dislodgment or because of infection. On histologic analysis, cortical thickness at the center of the callus was found to be significantly greater in the experimental group compared with the control group when normalized to the contralateral hemimandible (83 percent versus 49 percent, respectively; p < 0.007). The ratio of cortical to cancellous bone in the distracted callus was uniformly found to be greater in the experimental specimens. The mineral apposition rate was calculated by using fluorescence microscopy and found to be significantly greater in the experimental group both during the period of sequential compression and distraction (3.2 microm/day versus 2.1 microm/day, p = 0.02) and after the period of sequential compression and distraction (1.4 microm/day versus 1.1 microm/day, p = 0.006). Mechanical testing revealed no significant differences in bending strength or stiffness between experimental or control groups (p = 0.54 and 0.47, respectively). This study has demonstrated that daily alternating compression and distraction of 1 mm amplitude during the early consolidation period has a stimulatory impact on callus formation with respect to osteoblastic activity, remodeling, and maturation of bone. Optimal timing and amplitude of sequential movement, long-term biomechanical differences, and molecular pathways have yet to be elucidated.     

Volumetric change of the medial pterygoid following distraction osteogenesis of the mandible: an example of the associated soft-tissue changes

Mandibular distraction osteogenesis lengthens not only the affected skeleton but also the associated muscles of mastication. The purpose of this study was to determine medial pterygoid volume before and after distraction by using computed tomography. Using computed tomographic scans, the volume of the medial pterygoid muscle was determined before and after mandibular distraction in six pediatric patients. In four unilateral distraction patients (average age, 65 months), the average increase of the medial pterygoid muscle on the distracted side of the mandible was 29 percent, and on the contralateral nondistracted side, 10 percent. The average increase in medial pterygoid muscle volume in two bilateral distraction patients (each aged 8 months) was 75 percent. Results of this study demonstrate that distraction osteogenesis of the human mandible not only lengthens deficient bone, but it also increases the volume of the attached musculature.     

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.     

Long-term outcome study of bilateral mandibular distraction: a comparison of Treacher Collins and Nager syndromes to other types of micrognathia

A long-term follow-up study of patients who underwent bilateral mandibular distraction is presented, and the results of patients with Treacher Collins syndrome and Nager syndrome are compared with results for other forms of congenital micrognathia. It was hypothesized that the factors responsible for the predetermined, syndrome-specific shape of the mandible in patients with Treacher Collins and Nager syndromes would alter the long-term results of linear (uniplanar) distraction of the mandible. Thus, over time, the mandibles would remodel to preoperative form while maintaining the increase in volume. To investigate this hypothesis, all patients treated with bilateral mandibular distraction who had at least 1.5 years of follow-up, including satisfactory cephalometric examinations, were retrospectively reviewed. Two groups were identified. Group 1 (n = 6) were Treacher Collins and Nager syndrome patients (ages, 2 to 13 years; mean, 5.2 years) and group 2 (n = 6) included other forms of bilateral, congenital micrognathia (ages, 1.5 to 19 years; mean, 8.4 years). Serial cephalometric measurements were recorded before distraction, after distraction, and at least 18 months after distraction. Mandibular mean linear distraction distance (as recorded on the device) averaged 24.5 mm in group 1 and 26.2 mm in group 2. In group 1, the antegonial angle (angle from the mandibular plane to the top of the antegonial notch) decreased after distraction by 3.8 degrees, and the antegonial notch height was reduced by 1.6 mm. The posttreatment morphologic change was modified significantly over time, with a 3.7-degree increase of the antegonial angle and a 1.2-mm deepening of the antegonial notch. In group 2, the immediate reduction in height of the antegonial notching was subtler; however, long-term recurrence of the antegonial notching was also observed. At the end of distraction, the mean group 1 gonial angle became 8 degrees more obtuse. In contrast, patients in group 2 developed a more acute angle (mean, 8 degrees). The mandibles of the Treacher Collins syndrome patients (group 1) maintained their more obtuse postdistraction gonial angle during the period of follow-up, whereas over time this change was reversed in group 2 patients. In conclusion, experience with bilateral mandibular distraction has demonstrated that long-term determination of mandibular form is more complex than either the amount of distraction or the direction of the distraction vector. The underlying genotype and the musculoskeletal milieu must be taken into account when planning distraction, as these factors tend to remodel the mandible into its preoperative shape over time, despite the fact that the increased mandibular volume and projection are maintained.     

The effects of magnitude and frequency of distraction forces on tissue regeneration in distraction osteogenesis of the mandible.

Callus distraction has become an accepted treatment procedure to lengthen hypoplastic mandibles in humans. For this purpose, extraoral and intraoral devices have been applied successfully. The effects of the distraction vector, distractor stability, and rate and frequency of callus distraction on the regenerating tissues have been examined in various studies. In an experimental animal trial on pigs (n = 12), a newly developed microhydraulic osteodistractor was tested. The animals were evenly assigned to two groups to perform a continuous and noncontinuous osteodistraction of the mandible. Initially, the forces necessary to distract the pig mandibles were recorded from a noncontinuous distraction procedure; the results were then used to perform continuous bone distraction. Besides testing the new distractor, the study proved that in continuous osteodistraction, intramembranous bone regeneration occurred, whereas in noncontinuous osteodistraction the bone regeneration process followed a chondroid ossification. In continuous osteodistraction, the bone regeneration proceeded at a higher speed with the lower distraction forces compared with noncontinuous distraction, thereby reducing the consolidation period. Clinical and microscopical results are presented.     

Palatal distraction in a canine cleft palate model

The purpose of this study was to determine whether the canine hard palate can be lengthened by distraction osteogenesis in a cleft palate model using a mostly submucosal distractor. Five mongrel dogs were used. After raising mucoperiosteal flaps, a midline strip of bone was removed from the hard palate of each dog to simulate the bony defect seen in a cleft palate. A transverse osteotomy was then made to separate the posterior segment of the hard palate from the anterior segment. Posterior osteotomies were also made laterally parallel to the teeth so that the 2 posterior segments (one on either side of the bony cleft) were mobile. An intraoral distractor that was mostly submucosal was attached to the anterior hard palate and both segments of the mobilized posterior hard palate. Radiopaque bone markers were placed, and x-rays were obtained. After a 10-day latency period, the distractor was expanded 0.675 mm per day until it had been lengthened 10.125 mm. Distractors were left in place for an additional 8 weeks. After distractor removal, animals were observed for an additional 8 weeks before euthanization. Follow-up x-rays and histologic examinations were performed. New bone formation was found at the site of distraction in all dogs at the time of death. This new bone was seen on the follow-up x-rays and on histologic examination of the hard palates using both hematoxylin and eosin staining and Masson's trichrome stain. Distraction osteogenesis using a mostly submucosal device is an effective technique for lengthening the hard palate in a canine cleft palate model. The technique may eventually provide an alternative treatment for velopharyngeal incompetence in humans that is more precise and involves less morbidity than existing treatments.     

The effect of rate of distraction osteogenesis on structure and function of anterior digastric muscle fibers

The authors hypothesized that distraction at a rate of 3 mm/day, compared with mandibular distraction at a rate of 1 mm/day, would produce a maladaptive response in adjacent muscles of mastication. The authors further hypothesized that the maladaptive response would manifest at the single fiber level by means of increased sarcomeric heterogeneity, decreased maximum force output, and increased susceptibility to stretch-induced injury. In an ovine model, distraction osteogenesis of the right hemimandible was performed at either 1 mm/day for 21 days (n = 2) or 3 mm/day for 7 days (n = 2) to achieve a total distraction distance of 21 mm. The left hemimandibles served as controls. After a consolidation period of 2 days, the anterior digastric muscles were harvested; in six randomly selected single fibers from each muscle, maximum calcium-activated force (Po) was measured at optimal sarcomere length. The amount of damage to the sarcomeres in each fiber was assessed microscopically. To test susceptibility to contraction-induced injury, each fiber was given an activated stretch of 20 percent. Compared with control fibers and fibers distracted at 1 mm/day, maximum tetanic force (Po) was significantly lower in fibers distracted at 3 mm/day. Compared with control fibers, specific Po (Po/cross-sectional area) was lower in fibers distracted at 3 mm/day. The number of sarcomeres appearing damaged in fibers distracted at 3 mm/day was significantly higher than in control fibers or in fibers distracted at 1 mm/day. A greater deficit in Po was observed after a single activated stretch in fibers distracted at 3 mm/day than in control fibers or in fibers distracted at 1 mm/day. The authors conclude that distraction of the anterior digastric muscle in sheep at 3 mm/day produces a maladaptive response in the muscle fibers but a rate of 1 mm/day is tolerated by the muscle fibers. These data are consistent with the hypothesis that distraction of skeletal muscle at high rates results in increased heterogeneity of sarcomere lengths and that this increase in heterogeneity is the most likely potential mechanism resulting in whole muscle force deficits and in increased susceptibility to stretch-induced injury in distracted muscles.     

Dual midfacial distraction osteogenesis: Le Fort III minus I and Le Fort I for syndromic craniosynostosis

Midfacial hypoplasia has been corrected by Le Fort III or monobloc forward advancement in one stage in syndromic craniosynostosis, but recently developed distraction osteogenesis has been in use. Whereas the amount of forward mobilization in Le Fort III conventional osteotomy is determined by the preplanned fabricated interdental splint, that in Le Fort III distraction is determined by the positions of the inferior orbital rim, malar complex, and nose. Therefore, the forward mobilization of the upper part of the midface may sometimes be insufficient when one focuses on the final occlusion, and the occlusion might not be satisfied when the forward mobilization is sufficient. Correction of the midfacial hypoplasia should be considered differently in the upper and lower portions of the midface. The upper portion contains the inferior orbit and nose, and the lower portion contains the occlusal structure of the maxillary dentoalveolar portion with the mandible. Separating the midface into two portions and conducting the distraction osteogenesis in both portions separately in different amounts and vectors of distraction is described in this article. Although distraction of the upper portion of the midface can be conducted in one direction with an internal device, distraction of the lower portion of the midface is preferred for conduction by a controllable device because of the need to obtain the preferred occlusion. To obtain better functional and aesthetic results in midfacial distraction in adults and adolescents with syndromic craniosynostosis, dual Le Fort III minus I and Le Fort I midfacial distraction osteogenesis was performed in four cases (in two patients with Crouzon syndrome and in two patients with Apert syndrome). Two females and two males are described (age range, 13 to 26 years). An internal device was used for the upper portion of the midface and an external device was used for the lower portion. The amount of distraction ranged from 14 to 21 mm in the upper portion of the midface and from 11 to 18 mm in the lower portion. No particular complications were noticed over a follow-up period of 10 to 38 months (average follow-up, 19.8 months).     

Anterosuperior protraction of maxillae using the extraoral device,RAMPA; finite element method

Abstract

This paper presents a boosting effect against gravity by analyzing the displacement and stress distribution of craniofacial structures due to the protraction of the extraoral device, the Right Angle Maxillary Protraction Appliance(RAMPA) system, including semi-rapid maxillary expansion (sRME) using the finite element method. In addition, a patient case was illustrated and compared with the results calculated from a simulation. The results from the finite element method were obtained for 0.5?mm activation using the screw of the intraoral device, gHu-1. This study reveals that RAMPA rotates the patient’s maxilla and mandible in the forward direction and forces them to move forward and upward.     

Immediate versus delayed midface distraction in a primate model using a new intraoral internal device

The theoretic advantage of distraction osteogenesis of the craniofacial skeleton, especially in cases of severe midface retrusion and in the presence of maxillary scarring, is prevention of relapse following significant advancements. The purpose of this study is to demonstrate the utility of a new low-profile, intraoral, internal device for midface distraction at the conventional or high Le Fort I level. In addition, the present study compares the efficacy of immediate versus delayed distraction on subsequent maxillary relapse. Four adult rhesus Macaca mulatta monkeys were divided into two groups. Group 1 underwent immediate midface distraction; group 2 underwent delayed distraction. All four monkeys underwent a modified Le Fort I osteotomy through an upper buccal sulcus incision and bilateral application of the intraoral midface distraction devices. No other osteotomies or incisions were necessary. Immediate distraction, performed in group 1, entailed intraoperative activation of the devices and distraction of 10 mm followed by a 5-day lag period before postoperative activation and distraction of an additional 10 mm at the rate of 1 mm/day. Delayed distraction, performed in group 2, entailed a 5-day postoperative lag period before device activation and distraction of 20 mm at the rate of 1 mm/day. Both groups thus underwent 20 mm of midface distraction. All devices were removed 6 weeks after completion of distraction. All monkeys tolerated the devices and daily distraction uneventfully. On the basis of serial cephalograms and dental models obtained throughout the experimental period, there was no evidence of relapse in either the immediate or delayed groups 6 months after distraction. In addition, on the basis of histologic, ultrastructural, and dry skull analysis, no significant differences were observed in the quality of regenerate bone obtained when comparing the immediate and delayed distraction groups. Significant midface advancement is thus feasible using this new internal, intraoral distraction device, which presents several advantages over other internal devices that require coronal incisions and additional osteotomies to achieve midface advancement. In addition, immediate distraction may abbreviate the distraction period without adverse sequelae.     

Jaw muscles and the skull in mammals: the biomechanics of mastication.

Among non-mammalian vertebrates, rigid skulls with tight sutural junctions are associated with high levels of cranial loading. The rigid skulls of mammals presumably act to resist the stresses of mastication. The pig, Sus scrofa, is a generalized ungulate with a diet rich in resistant foods. This report synthesizes previous work using strain gages bonded to the bones and sutures of the braincase, zygomatic arch, jaw joint, and mandible with new studies on the maxilla. Strains were recorded during unrestrained mastication and/or in anesthetized pigs during muscle stimulation. Bone strains were 100-1000 micro epsilon, except in the braincase, but sutural strains were higher, regardless of region. Strain regimes were specific to different regions, indicating that theoretical treatment of the skull as a unitary structure is probably incorrect. Muscle contraction, especially the masseter, caused strain patterns by four mechanisms: (1) direct loading of muscle attachment areas; (2) a compressive reaction force at the jaw joint; (3) bite force loading on the snout and mandible; and (4) movement causing new points of contact between mandible and cranium. Some expected patterns of loading were not seen. Most notably, strains did not differ for right and left chewing, perhaps because pigs have bilateral occlusion and masseter activity.     

Compressive strains and displacement in a partially dentate lower jaw rehabilitated with two different treatment modalities

doi: 10.1111/j.1741‐2358.2011.00572.x Compressive strains and displacement in a partially dentate lower jaw rehabilitated with two different treatment modalities Background: Understanding of the biomechanical consequences of the stresses generated to the supporting bone during occlusal loading is significant for improving the design and clinical planning process in partial edentulism therapy. Objective: The aim of this study was to analyse the distribution of strain and displacement on the partially dentate lower jaw rehabilitated with an partial denture (RPD) and to compare it to the strain and displacement distribution on a partially dentate lower jaw rehabilitated with a cantilever fixed partial denture (FPD). Material and methods: The experimental models were a partially dentate mandible with full‐arch PFM crowns and RPD and a partially dentate mandible rehabilitated with a full‐arch cantilever FPD. Strains and displacement were measured using the Digital Image Correlation Method. Results: Displacement values of the first experimental model ranged from 0.31 to 0.54 mm with strains from 1.35 to 2.34%. Analysis of the second experimental model results showed displacement values from 0 to 0.34 mm, while strains were in the range of 0–1.40%. Conclusion: Higher displacements and strains of bone tissue were observed below the RPD, especially in the region of the distal abutment and distal portion of the free‐end saddle. Strains within bone and the bone‐denture contact area were mostly influenced by the teeth and denture vertical displacement.     

Distraction osteogenesis of the porcine mandible: histomorphometric evaluation of bone

Distraction osteogenesis is a technique for skeletal lengthening that exploits the body's innate capacity for bone formation in response to tension forces on the repair callus. The authors developed a distraction osteogenesis model with a semiburied device in the Yucatan minipig mandible because of similarities between human and porcine mandibular anatomy, temporomandibular function, chewing patterns, and bone turnover rates. The purpose of this study was to measure histomorphometric bone fill after different latency periods, rates of distraction, and duration of neutral fixation in the minipig mandible. In addition, the relationship between histomorphometric bone fill and clinical stability was investigated. Mandibular osteotomies in 20 female Yucatan minipigs weighing 25 to 30 kg were distracted with modified semiburied distraction devices. Variables included 0-day or 4-day latency; 1-mm, 2-mm, or 4-mm daily distraction rates; gap size of 7 or 12 mm; and evaluation after neutral fixation for various lengths of time. Specimens were fixed in 2% paraformaldehyde, pH 7.4, before being embedded in methylmethacrylate. Sections were prepared from the region just below the inferior alveolar canal. The area of new bone formation within the gap was measured and expressed as a percentage of the total area of the distraction gap. Bone fill ranged from 0 to 100 percent. A pilot study with 7-mm advancements showed similar bone fill with 0-day or 4-day latency, but with poor reproducibility. Mandibles that were distracted to 12 mm at 1 mm per day exhibited nearly complete bone fill, either with 0-day latency (average, 93 percent) or 4-day latency (average, 100 percent). Mandibles that had been distracted for 3 days at 4 mm per day showed moderate osteogenesis and clinical stability with increasing time of neutral fixation. Bone fill was significantly correlated with clinical stability (Spearman r = 0.801, p = 0.001). Histological examination showed exuberant periosteal osteogenesis in distracted mandibles, even in those that showed poor bone fill and clinical stability. Thus, the periosteum appears to be a major source of new bone formation. These results show that osteogenesis was nearly complete with 1 mm per day and 0-day or 4-day latency. These results are consistent with the authors' previously reported clinical and radiographic observations that a latency period is not necessary for successful healing of the mandibular distraction osteogenesis wound.     

Molding of the regenerate in mandibular distraction: clinical experience

Initial clinical experience with distraction osteogenesis has demonstrated the risk of developing postdistraction malocclusion that requires secondary orthodontic correction. In addition, optimal mandibular form is not always achieved. Both animal studies and preliminary clinical investigations have suggested that the regenerate can be successfully "molded" during active mandibular distraction. The authors have applied this concept clinically to obtain a more desirable occlusal relationship in a group of mandibular distraction patients. Eleven patients are described in whom angulation of the distraction device or intermaxillary/interdental elastics were employed to mold the regenerate. Two representative case studies are provided to illustrate the principles. When using elastic traction to close an anterior open bite, care must be taken that extrusion of individual teeth is minimized by distributing the force over the entire dental arch, especially the basilar portions of the jaws. The authors demonstrate that molding of the regenerate can be successfully accomplished not only during device activation but also early in the consolidation period. The outer limit of the time window in which molding is effective remains to be defined.     

Distraction osteogenesis: a new surgical technique for use with the multiplanar mandibular distractor

If distraction osteogenesis is to reach its full potential and achieve the level of accuracy that is possible with orthognathic surgery, its outcomes need to be as predictable. To this end, the authors developed a planning process for distraction osteogenesis similar to that used in orthognathic surgery. However, the success of the planning process depends on the authors' ability to execute the plan at the time of surgery. As a result, the authors needed to develop a surgical technique that would enable them to precisely install the distractor as indicated in the presurgical plan. The surgical technique presented in this article was developed for this purpose. The authors used this technique in seven patients (four boys and three girls; age range, 4 to 10 years). Four patients presented with unilateral deformities, and three patients presented with bilateral deformities. The follow-up period in this group of patients ranged from 12 to 33 months. The purpose of the technique is to replicate the position of the distractor on the mandible as determined by the presurgical plan. To this purpose, a custom drill guide and a surgical template have been developed. Both of these are used following the principles of triangulation to establish the pin position and orientation of the distractor. In the authors' hands, the use of this surgical technique has resulted in outcomes close to those predicted by the planning process.