The Effect Of Low-intensity Pulsed Ultrasound On Bone Formation During Mandible Distraction Osteogenesis And Bone Healing Around Dental Implants

The treatment of maxillofacial bone defect due to tumor and trauma is notonly to reconstruct the facial contour but also to resume mastication function.Distraction osteogenesis (DO) has had an enormous impact and played a majorrole in the reconstruction of congenital and acquired deformities of thecraniofacial skeleton. Dental implants can resume mastication functionefficiently. However, the main disadvantage of DO is the long treatment timewhich requires the distracted bone segment be fixed by the distraction device fora certain period of consolidation to facilitate the new bone mature andremodeled. This imposes a psychological effect on the patient and familybecause several months are needed before the woven bone becomes compactbone to allow for distractor removal. Ilizarov suggested that the consolidationtime should be no less than two or three times as long as(6～8weeks usually)the distraction time. Long-time retention can result in inconvenience andexpensive inpatient cost and there were also some complications such as pseudarthrosis and relapse were reported. After dental implants inserted intobone, it costs at least 2～3 months to achieve osseointegration before acceptingdenture loading, which means dental implant is almost completely covered by acompact, mature, newly formed bone. An early good biological fixation mayallow the shortening of time before loading the implant, favoring the clinicalprocedure of early or immediate implant loading. Now, how to shorten theconsolidation time and enhance the mechanical property of the new distractedbone is the focus research field of DO. Also how to intensify osseointegration ofdental implants is the front study of dentistry.Low-intensity pulsed ultrasound (LIPUS) is a form of mechanical energywhich is transmitted through and into biological tissues as an acoustic pressurewave and has been widely used in medicine as a diagnostic and therapeutic tool.Application of LIPUS (30～50mW/cm2) was considered to have little thermaleffect and to produce stable cavitation and streaming. There are many papers anddocuments published to establish LIPUS can accelerate the healing of fresh bonefracture and nonunion. LIPUS is an ideal physical stimulus to enhance boneformation. If ultrasound stimulation could accelerate the rate of the distractedcallus formation and remodification, the treatment period could be shortened,complications would decrease. Similarly, If ultrasound stimulation couldimprove the osseointegration of dental implants, patients would eat what theywant to as soon as possible. In this study, we observed the effect of LIPUS onthe enhancement of bone formation during mandible distraction osteogenesis indogs and improvement of osseointegration of dental implant in rabbits.Experiment one was the animal model creation of enhancement of boneformation of mandible distraction osteogenesis by LIPUS. Bilateral surgical cutswere made in the mandible of seven dogs between the first and second premolar region. The anterior mandibles were lengthened by 20mm at the rate of 1mm/d,twice a day. During the distraction period one lateral distraction gap wasirradiated by LIPUS 10 minutes, twice a day and the other side was shamirradiated as control. After distraction was completed, the dogs were sacrificedon 0,1,2,4,6,8,12 week. The mandible samples were harvested.Experiment two was 99mTc-methylene diphosphonate (MDP) bone imaging.Before the animals sacrifice, the animals were injected bolus 740 MBq 99mTc-MDP intravenously. Four hours later, delayed static bone scanning was obtainedby SPECT. For semiquantitative analysis, we set manually the region of interest(ROI) on the LIPUS stimulated distraction area and set a symmetric ROI on thecontralateral area . The result showed the uptake of 99mTc-MDP in theexperimental side was significant (P<0.05) higher than that of the control side inthe early period of consolidation (before the fourth week). But later the situationwas reversed, the uptake of 99mTc-MDP in the control side was significanthigher than that of the experimental side (P<0.05). There were no significantradioactivity differences between two sides at the 12th week. It was indicated theLIPUS stimulated area had more blood supply and metabolic activity. LIPUSradiation had positive effect on the healing and osteogenesis course of the newbone.Experiment three was X-ray plain, three-dimension (3D) CT scan,bonemineral density (BMD) and biomechanical property measurement. The wholemandible samples were harvested and regular plain X-ray photographs weretaken. The front part of the mandibles were scanned with LightSpeed VCT 64-slice Scanner and 3D-images was obtained. Input 3D-CT scan data of themandible to the computer and used graphic information processing software tocalculate the volume and the surface area of the distracted bone. The BMD of the distracted bone was measured by dual-energy X-ray absorptiometry. Half of thebone samples were stored in saline at–20°C. Before testing, the bone wasthawed and dried for 48 hours at room temperature. The bone block was sectioninto 3mm thick slices by diamond blade. The biomechanical test was performedin a computed servohydraulic materials testing system. The slices were placed onthe on the smooth surface of a steel disk and axially compressed by the smoothsurface of a steel rod attached to load cell at a constant speed of 1 mm/min. Bothdisplacement and load were recorded for later analysis.X-ray plain showed the new bone in the experimental side was matureearlier than that of the control side. The volume and the surface areameasurement of distracted area showed there was not significant differencebetween the LIPUS stimulated side and the control side ( paired t test, P > 0.05) .BMD measurement showed at the same time the BMD of LIPUS stimulated sidewas significantly higher than that of the control side (paired t test, P < 0.05). Theresult showed LIPUS could enhance the biomechanical property of new bone.It was indicated the LIPUS radiation accelerate bone mature and increasethe BMD and mechanical property during mandible distraction osteogenesis buthad no effect on the volume of the new bone.Experiment four was histological examination. One half of the bone sampleswere fixed in 4% paraformaldehyde for 48 hours and decalcified in 20% EDTAfor four weeks. The specimens were dehydrated in ethanol and embedded inparaffin, sectioned longitudinally at 4μm thick, stained for HE and Masson'sstain. The trabecula of the experimental side was more and thicker than that ofthe control side at the early period of consolidation. But in the late period, therewas not significant difference. We observed endochondral bone formation in theexperimental side at the 2w and 4w of consolidation. LIPUS may stimulate synthesis of extracellular matrix proteins altering chondrocyte maturation andendochondral bone formation. The modulation by ultrasound may occur byaccelerating endochondral ossification through action on chondrocytes, yetdistraction osteogenesis is mainly intramembranous.Experiment five was the effect of LIPUS on the osseointegration of titaniumdental implants. 10 New Zealand rabbits were used in this study. Epiphyses ofboth femur and tibiae were inserted one screw titanium implants (? 2mm)separately in bilateral knee joints, total forty. One lateral knee joint includingimplants was irradiated by LIPUS(40mW/cm2)10 minutes, twice a day for 20days. The other side was dammed as control. The dogs were sacrificed on 0, 2, 4,6, 8 week after LIPUS irradiation. The bone samples including implants wereharvested. X-ray of dental implants apical image was taken. Half of the sampleswere stored in saline at–20°C. For pull-out test of dent implant, the bone wasthawed and dried. The bone block was fixed stable, the external part of dentimplant was held fast by steel pliers linked to load cell and distracted at aconstant displacement rate of 0.5mm/min. Both displacement and load wererecorded for later analysis. The result showed both the max load and extractiondisplacement were greater significantly in the experimental group than in thecontrol group (paired t test, P<0.05). The result indicated LIPUS can improve theretention of dental implant.Experiment six was histological examination of the mental-bone surface ofdental implants. Half of the samples were dehydrated in ethanol and embeddedin methylmethacrylate, sectioned longitudinally at 100-200μm thick forGoldner's stain. The result showed fibrillar calcified layer and trabecular bonearound the dental implant observable at the implant surface earlier in theexperimental group than in the control group. Later, trabecular bone was gradually substituted by lamellar bone and mature earlier in the experimentalgroup. The result indicated the LIPUS can accelerate osseointegration of dentalimplants.In summary, based on our study showed LIPUS could enhance blood supplyand metabolic activity at the early period of consolidation and had positive effecton the healing and osteogenesis course of the new bone. LIPUS radiationaccelerated bone formation and mature, increased the BMD and biomechanicalproperty but had no effect on the volume of the new bone. LIPUS may stimulatesynthesis of extracellular matrix proteins altering chondrocyte maturation andendochondral bone formation. LIPUS also can accelerate osseointegration andenhance the retention of dental implants.