Effects of near-field ultrasound stimulation on new bone formation and osseointegration of dental titanium implants in vitro and in vivo

Revolutionizing Dental Implant Research: A Systematic Review on Three-Dimensional In Vitro Models

Dental implants have been clinically used for almost five decades with high success rates. In vitro research models used in implant dentistry are limited to two-dimensional experiments, which are reproducible and well adapted to evaluate a single parameter but do not reproduce the complexity of clinical settings. On the contrary, the in vivo research models using animals offer similar histological and anatomical features to humans, and tissue healing can be close to a clinical situation, but those models are usually accompanied with ethical concerns, and their outcomes could not be extrapolated to humans because of interspecies variabilities. This makes the development of novel in vitro models that recapitulate physiological events occurring during dental implant placement of particular interest for current research in dentistry. Also, such models could be challenged by setting a pathological environment (peri-implantitis) to better understand the disease and eventually serve as a platform to evaluate novel treatment modalities. The aim of this systematic literature review was to cover all the in vitro three-dimensional (3D) complex models available for research in implant dentistry. To accomplish this, a comprehensive search of the literature present on Scopus and PubMed databases was done using specific keywords, as well as inclusion/exclusion criteria. Out of 1334 articles found, we have finally included 27 articles in this review with publication dates between 2001 and 2022. In those articles, the 3D models were designed to study tissue-implant interface behavior in bone or gingival tissue. The articles focused on simulating implant integration, evaluating the effect of different conditions on implant integration, or developing an infection model for the implant integration process. The methods used involved implant material and cells organized in a specific 3D structure. The 3D models developed were able to simulate the process of dental implant osseo- and soft tissue integration and lead to results comparable with conventional in vitro and in vivo models. A relatively limited number of articles were obtained, which indicates that this is an emerging field, highly dependent on progresses made in biotechnologies and tissue engineering, and that further investigation is needed to enhance these 3D in vitro models.

The Effect of Low-Intensity Pulsed Ultrasound on Bone Regeneration and the Expression of Osterix and Cyclooxygenase-2 during Critical-Size Bone Defect Repair

Low-intensity pulsed ultrasound (LIPUS) is a form of ultrasound that utilizes low-intensity pulsed waves. Its effect on bones that heal by intramembranous ossification has not been sufficiently investigated. In this study, we examined LIPUS and the autologous bone, to determine their effect on the healing of the critical-size bone defect (CSBD) of the rat calvaria. The bone samples underwent histological, histomorphometric and immunohistochemical analyses. Both LIPUS and autologous bone promoted osteogenesis, leading to almost complete closure of the bone defect. On day 30, the bone volume was the highest in the autologous bone group (20.35%), followed by the LIPUS group (19.12%), and the lowest value was in the control group (5.11%). The autologous bone group exhibited the highest intensities of COX-2 (167.7 ± 1.1) and Osx (177.1 ± 0.9) expression on day 30. In the LIPUS group, the highest intensity of COX-2 expression was found on day 7 (169.7 ±1.6) and day 15 (92.7 ± 2.2), while the highest Osx expression was on day 7 (131.9 ± 0.9). In conclusion, this study suggests that LIPUS could represent a viable alternative to autologous bone grafts in repairing bone defects that are ossified by intramembranous ossification.

Active osseointegration in an ex vivo porcine bone model

Achieving osseointegration is a fundamental requirement for many orthopaedic, oral, and craniofacial implants. Osseointegration typically takes three to 6 months, during which time implants are at risk of loosening. The aim of this study was to investigate whether osseointegration could be actively enhanced by delivering controllable electromechanical stimuli to the periprosthetic bone. First, the osteoconductivity of the implant surface was confirmed using an in vitro culture with murine preosteoblasts. The effects of active treatment on osseointegration were then investigated in a 21-day ex vivo model with freshly harvested cancellous bone cylinders (n = 24; Ø10 mm × 5 mm) from distal porcine femora, with comparisons to specimens treated by a distant ultrasound source and static controls. Cell viability, proliferation and distribution was evident throughout culture. Superior ongrowth of tissue onto the titanium discs during culture was observed in the actively stimulated specimens, with evidence of ten-times increased mineralisation after 7 and 14 days of culture (p < 0.05) and 2.5 times increased expression of osteopontin (p < 0.005), an adhesive protein, at 21 days. Moreover, histological analyses revealed increased bone remodelling at the implant-bone interface in the actively stimulated specimens compared to the passive controls. Active osseointegration is an exciting new approach for accelerating bone growth into and around implants.

Influence of Ultrasound Stimulation on the Viability, Proliferation and Protein Expression of Osteoblasts and Periodontal Ligament Fibroblasts

Among the adjunctive procedures to accelerate orthodontic tooth movement (OTM), ultrasound (US) is a nonsurgical form of mechanical stimulus that has been explored as an alternative to the currently available treatments. This study aimed to clarify the role of US in OTM by exploring different stimulation parameters and their effects on the biological responses of cells involved in OTM. Human fetal osteoblasts and periodontal ligament fibroblasts cell lines were stimulated with US at 1.0 and 1.5 MHz central frequencies and power densities of 30 and 60 mW/cm2 in continuous mode for 5 and 10 min. Cellular proliferation, metabolic activity and protein expression were analyzed. The US parameters that significantly improved the metabolic activity were 1.0 MHz at 30 mW/cm2 for 5 min and 1.0 MHz at 60 mW/cm2 for 5 and 10 min for osteoblasts; and 1.0 MHz at 30 mW/cm2 for 5 min and 1.5 MHz at 60 mW/cm2 for 5 and 10 min for fibroblasts. By stimulating with these parameters, the expression of alkaline phosphatase was maintained, while osteoprotegerin synthesis was induced after three days of US stimulation. The US stimulation improved the biological activity of both osteoblasts and periodontal ligament fibroblasts, inducing their osteogenic differentiation.

Evaluate the effects of low-intensity pulsed ultrasound on dental implant osseointegration under type II diabetes

Objective: The objective of this study is to assess the impact of low-intensity pulsed ultrasound (LIPUS) therapy on the peri-implant osteogenesis in a Type II diabetes mellitus (T2DM) rat model. Methods: A total of twenty male Sprague-Dawley (SD) rats were randomly allocated into four groups: Control group, T2DM group, Control-LIPUS group, and T2DM-LIPUS group. Implants were placed at the rats' bilateral maxillary first molar sites. The LIPUS treatment was carried out on the rats in Control-LIPUS group and T2DM-LIPUS group, immediately after the placement of the implants, over three consecutive weeks. Three weeks after implantation, the rats' maxillae were extracted for micro-CT, removal torque value (RTV), and histologic analysis. Results: Micro-CT analysis showed that T2DM rats experienced more bone loss around implant cervical margins compared with the non-T2DM rats, while the LIPUS treated T2DM rats showed similar bone heights to the non-T2DM rats. Bone-implant contact ratio (BIC) were lower in T2DM rats but significantly improved in the LIPUS treated T2DM rats. Bone formation parameters including bone volume fraction (BV/TV), trabecular thickness (Tb.Th), bone mineral density (BMD) and RTV were all positively influenced by LIPUS treatment. Histological staining further confirmed LIPUS's positive effects on peri-implant new bone formation in T2DM rats. Conclusion: As an effective and safe treatment in promoting osteogenesis, LIPUS has a great potential for T2DM patients to attain improved peri-implant osteogenesis. To confirm its clinical efficacy and to explore the underlying mechanism, further prospective cohort studies or randomized controlled trials are needed in the future.

Musculoskeletal Biomaterials: Stimulated and Synergized with Low Intensity Pulsed Ultrasound

Clinical biophysical stimulating strategies, which have significant effects on improving the function of organs or treating diseases by causing the salutary response of body, have shown many advantages, such as non-invasiveness, few side effects, and controllable treatment process. As a critical technique for stimulation, the low intensity pulsed ultrasound (LIPUS) has been explored in regulating osteogenesis, which has presented great promise in bone repair by delivering a combined effect with biomaterials. This review summarizes the musculoskeletal biomaterials that can be synergized with LIPUS for enhanced biomedical application, including bone regeneration, spinal fusion, osteonecrosis/osteolysis, cartilage repair, and nerve regeneration. Different types of biomaterials are categorized for summary and evaluation. In each subtype, the verified biological mechanisms are listed in a table or graphs to prove how LIPUS was effective in improving musculoskeletal tissue regeneration. Meanwhile, the acoustic excitation parameters of LIPUS that were promising to be effective for further musculoskeletal tissue engineering are discussed, as well as their limitations and some perspectives for future research. Overall, coupled with biomimetic scaffolds and platforms, LIPUS may be a powerful therapeutic approach to accelerate musculoskeletal tissue repair and even in other regenerative medicine applications.

Effectiveness and Mechanisms of Low-Intensity Pulsed Ultrasound on Osseointegration of Dental Implants and Biological Functions of Bone Marrow Mesenchymal Stem Cells

Dental implant restoration is the preferred choice for patients with dentition defects or edentulous patients, and obtaining stable osseointegration is the determining factor for successful implant healing. The risk of implant failure during the healing stage is still an urgent problem in clinical practice due to differences in bone quality at different implant sites and the impact of some systemic diseases on bone tissue metabolism. Low-intensity pulsed ultrasound (LIPUS) is a noninvasive physical intervention method widely recognized in the treatment of bone fracture and joint damage repair. Moreover, many studies indicated that LIPUS could effectively promote the osseointegration of dental implants and improve the osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs). This review is aimed at investigating the research progress on the use of LIPUS in dental implant medicine from three aspects: (1) discuss the promoting effects of LIPUS on osseointegration and peri-implant bone regeneration, (2) summarize the effects and associated mechanisms of LIPUS on the biological functions of BMSCs, and (3) introduce the application and prospects of LIPUS in the clinical work of dental implantation. Although many challenges need to be overcome in the future, LIPUS is bound to be an efficient and convenient therapeutic method to improve the dental implantation success rate and expand clinical implant indications.

Stimulation of dental implant osseointegration by low-Intensity pulsed ultrasound: An in vivo preliminary study in a porcine model

PURPOSES

Several studies have evaluated the interest of Low Intensity Pulsed Ultrasound (LIPUS) in the osseointegration of dental implants in murine or rabbit models. However, the thinness and narrowness bones make it difficult to study the effect of LIPUS. The purpose of this study is to assess the ability of LIPUS to stimulate bone formation in contact with a titanium dental implant in a porcine model.

METHODS

Eight adults mini-pigs were used. An implant is placed on each tibial crest in the metaphysis. The right side was treated with LIPUS at 1 MHz and 300 mW/cm² of acoustic intensity during 15 minutes per day on 5 consecutive days and during 42 days. The left side was not treated. The Bone Volume/Total Volume ratio (BV/TV), the Intersection Surface (IS) of the volume of interest by the binarized bone and the Trabecular bone Thickness (TbTh) around the implant were analyzed.

RESULTS

At 42 days, BV/TV ratio is significantly higher on the treated side (42,1+/-8,76% versus 32,31+/-10,11%, p < 0,02); as well as TbTh with 0,13+/-0,01 mm versus 0,10+/-0,01 mm (p < 0,01). IS is also significantly higher on the treated side (40,7 +/- 12,68 mm² versus 33,68+/-9,44 mm² at 200 μm from the implant surface; p < 0,01).

CONCLUSION

The present study showed that LIPUS can significantly increase bone formation and accelerate the healing process at the bone-implant interface in a porcine model. Its low toxicity, low immunogenicity and non-invasion make it a complementary treatment of choice for improving the bone formation around titanium implants.

The effect of low intensity pulsed ultrasound on mandibular condylar growth in young adult rats

There is a need for more effective methods to enhance mandibular growth in young adults with mandibular deficiency. Previous studies suggest that low intensity pulsed ultrasound (LIPUS) can enhance mandibular growth in growing individuals. This study aimed to evaluate the potential growth changes of the mandible following 4-week LIPUS application in young adult rats. Nineteen ≈120-day-old female rats were allocated to experimental (n = 10) and control (n = 9) groups. The animals in the experimental group were treated with LIPUS to their temporomandibular joints (TMJs) bilaterally, 20 min each day for 28 consecutive days. Animals were then euthanized; gross morphological evaluation was performed on 2D photographs and 3D virtual models of hemi-mandibles, and microstructural assessment was done for the mandibular condyle (MC). Evaluation of mineralization and microarchitecture properties of subchondral cancellous bone was performed by micro-computed tomography (μCT) scanning. Qualitative and histomorphometric analysis was done on condylar cartilage and subchondral bone following Alcian Blue/PAS and Goldner's Trichrome staining. Vital flourochrome (calcein green) labeling was also utilized to determine the amount of endochondral bone growth. Gross morphological evaluations showed a slight statistically non-significant increase especially in the main condylar growth direction in the LIPUS group. Moreover, 3D evaluation depicted an enhanced periosteal bone apposition at the site of LIPUS application. Microstructural analysis revealed that LIPUS stimulates both chondrogenesis and osteogenesis and enhances endochondral bone formation in young adult rat MC. Furthermore, the effect of LIPUS on osteogenic cells of subchondral cancellous bone was notable. To conclude, LIPUS can enhance young adult rats' MC residual growth potential.

Low Intensity Pulsed Ultrasound for Bone Tissue Engineering

As explained by Wolff's law and the mechanostat hypothesis, mechanical stimulation can be used to promote bone formation. Low intensity pulsed ultrasound (LIPUS) is a source of mechanical stimulation that can activate the integrin/phosphatidylinositol 3-OH kinase/Akt pathway and upregulate osteogenic proteins through the production of cyclooxygenase-2 (COX-2) and prostaglandin E2 (PGE2). This paper analyzes the results of in vitro and in vivo studies that have evaluated the effects of LIPUS on cell behavior within three-dimensional (3D) titanium, ceramic, and hydrogel scaffolds. We focus specifically on cell morphology and attachment, cell proliferation and viability, osteogenic differentiation, mineralization, bone volume, and osseointegration. As shown by upregulated levels of alkaline phosphatase and osteocalcin, increased mineral deposition, improved cell ingrowth, greater scaffold pore occupancy by bone tissue, and superior vascularization, LIPUS generally has a positive effect and promotes bone formation within engineered scaffolds. Additionally, LIPUS can have synergistic effects by producing the piezoelectric effect and enhancing the benefits of 3D hydrogel encapsulation, growth factor delivery, and scaffold modification. Additional research should be conducted to optimize the ultrasound parameters and evaluate the effects of LIPUS with other types of scaffold materials and cell types.

Ultrasonic Propagation in a Dental Implant

Ultrasound techniques can be used to characterize and stimulate dental implant osseointegration. However, the interaction between an ultrasonic wave and the implant-bone interface (IBI) remains unclear. This study-combining experimental and numerical approaches-investigates the propagation of an ultrasonic wave in a dental implant by assessing the amplitude of the displacements along the implant axis. An ultrasonic transducer was excited in a transient regime at 10 MHz. Laser interferometric techniques were employed to measure the amplitude of the displacements, which varied 3.2-8.9 nm along the implant axis. The results demonstrated the propagation of a guided wave mode along the implant axis. The velocity of the first arriving signal was equal to 2110 m.s^-1, with frequency components lower than 1 MHz, in agreement with numerical results. Investigating guided wave propagation in dental implants should contribute to improved methods for the characterization and stimulation of the IBI.

Simplified Theoretical Model for Temperature Evaluation in Tissue–Implant–Bone Systems during Ultrasound Diathermy

Deep heating procedures are helpful in treating joint contractures that frequently occur with fractures and joint diseases involving surgical implants and artificial joint prostheses. This study uses a one-dimensional composite medium model consisting of parallel slabs as a simplified approach to shed light on the influences of implants during ultrasound diathermy. Analytical solutions for the one-dimensional transient heat generation and conduction problem were derived using the orthogonal expansion technique and a Green’s function approach. The analytical solutions provided deep insight into the temperature profile by therapeutic ultrasound heating in the composite system. The effects of the implant material type, tissue thickness, and ultrasound operation frequency on temperature distribution were studied for clinical application. In addition, sensitivity analyses were carried out to investigate the influences of material properties on the temperature distribution during ultrasound diathermy. Based on the derived analytical solutions, the numerical simulations indicate that materials with high density, high specific heat, and low thermal conductivity may be optimal implant materials. Among available implant materials, a tantalum implant, which can achieve a lower temperature rise within the tissue (hydrogel) and bone layers during ultrasound diathermy, is a better choice thanks to its thermodynamics.

Reflection of an ultrasonic wave on the bone-implant interface: A numerical study of the effect of the multiscale roughness

Quantitative ultrasound is used to characterize and stimulate osseointegration processes at the bone-implant interface (BII). However, the interaction between an ultrasonic wave and the implant remains poorly understood. This study aims at investigating the sensitivity of the ultrasonic response to the microscopic and macroscopic properties of the BII and to osseointegration processes. The reflection coefficient R of the BII was modeled for different frequencies using a two-dimensional finite element model. The implant surface roughness was modeled by a sinusoidal function with varying amplitude h and spatial frequency L. A soft tissue layer of thickness W was considered between bone tissue and the implant in order to model non-mineralized fibrous tissue. For microscopic roughness, R is shown to increase from around 0.55 until 0.9 when kW increases from 0 to 1 and to be constant for kW > 1, where k is the wavenumber in the implant. These results allow us to show that R depends on the properties of bone tissue located at a distance comprised between 1 and 25 μm from the implant surface. For macroscopic roughness, R is highly dependent on h and this dependence may be explained by phase cancellation and multiple scattering effects for high roughness parameters.

Bone Texture Fractal Dimension Analysis of Ultrasound-Treated Bone around Implant Site: A Double-Blind Clinical Trial

Objectives

To evaluate the efficacy of bone texture fractal dimension (FD) analysis method in predicting implant stability from intraoral periapical radiographs using two implant protocols.

Materials and Methods

A double-blind clinical trial was conducted on 22 subjects who needed dental implants. The participants were randomized into two groups, the control group with standard implant protocol treatment and the intervention group with added low-intensity power ultrasound treatment (LIPUS) besides the standard implant protocol. The FD values of bone density were carried out on the mesial and distal sides of the implant on digital intraoral radiographs using the box-counting method. Both resonance frequency (RF) and fractal dimension (FD) were assessed in three time intervals: after surgery and before and after loading.

Results

FD on both the mesial and distal sides serve as very good-to-excellent tests with high validity (ROC area exceeding 0.8) in predicting high implant stability (ISQ ≥ 70). The mesial side measurements were consistently better than the distal side among the intervention groups. The optimum cutoff value for the FD-mesial side that predicts a highly stable implant (ISQ ≥ 70) is ≥1.505. At this optimum cutoff value, the mesial side FD is associated with a perfect sensitivity (100%) and fairly high specificity (86.5%).

Conclusion

The FD analysis could be recommended as an adjunctive quantitative method in prediction of the implant stability with very high sensitivity and specificity. This trial is registered with ISRCTN72648040.

Ultrasound: A Revenant Therapeutic Modality in Dentistry

An ultrasound (US) gives a visible image of the organs that are present inside the body. In medicine it serves for diagnosing and also its therapeutic benefits are well established for bone healing, osteointegration and soft tissue healing. In dentistry it is widely used for diagnostic purposes. When it was discovered it was introduced for therapeutic purposes, but due to lack of clinical studies its use as therapy was remittent in dentistry. The aim of the present paper was to establish the efficiency of therapeutic US in maxillofacial region for alleviating the pain and to see the other applications. Our search included the English terms like ultrasonography, applications, dentistry in Google search engine, PubMed and Medline from 1980 to 2015. We found very few articles showing the effects of therapeutic Ultrasound (US) in treatment of pain and healing in dentistry. We concluded that clinical benefits of in vivo studies were very little and demands further rigorous research to strive for the therapeutic success of US.

Low-intensity pulsed ultrasound stimulates matrix metabolism of human annulus fibrosus cells mediated by transforming growth factor β1 and extracellular signal-regulated kinase pathway

PURPOSE

There are limited strategies to restore the damaged annulus fibrosus (AF) of the intervertebral disc. Low-intensity pulsed ultrasound (LIPUS) has positive effects on the proliferation of several types of cells and the repair of damage tissue in vivo. However, scientific evidence of therapeutic effects of LIPUS on AF cells remains limited. The purpose of this study is to evaluate the feasibility of applying LIPUS to the repair of the AF.

MATERIALS AND METHODS

We used an in vitro model of human AF cells subjected to LIPUS stimulation to examine its effects on cell proliferation and matrix metabolism. Cell viability, synthesis of collagen and glycosaminoglycan (GAG), expression of matrix metalloproteinases (MMPs) and transforming growth factor β1 and pathways involving mitogen-activated protein kinases (MAPKs) were investigated.

RESULTS

LIPUS significantly enhanced proliferation of AF cells after 5 days of treatment. LIPUS with an intensity of 0.5 W/cm(2) increased the collagen and GAG synthesis and decreased the expressions of MMP-1 and -3 of human AF cells. Real-time polymerase chain reactions and western blotting analysis revealed that LIPUS could increase transforming growth factor β1 (TGF-β1) and activate extracellular signal-regulated kinase (ERK) pathway. In addition, TGF-β receptor kinase inhibitor could suppress the ultrasound-induced alterations in cell viability and matrix metabolism.

CONCLUSIONS

The findings suggested that LIPUS could be useful as a physical stimulation of cell metabolism for the repair of the AF.

Biomechanical determinants of the stability of dental implants: influence of the bone-implant interface properties

Dental implants are now widely used for the replacement of missing teeth in fully or partially edentulous patients and for cranial reconstructions. However, risks of failure, which may have dramatic consequences, are still experienced and remain difficult to anticipate. The stability of biomaterials inserted in bone tissue depends on multiscale phenomena of biomechanical (bone-implant interlocking) and of biological (mechanotransduction) natures. The objective of this review is to provide an overview of the biomechanical behavior of the bone-dental implant interface as a function of its environment by considering in silico, ex vivo and in vivo studies including animal models as well as clinical studies. The biomechanical determinants of osseointegration phenomena are related to bone remodeling in the vicinity of the implants (adaptation of the bone structure to accommodate the presence of a biomaterial). Aspects related to the description of the interface and to its space-time multiscale nature will first be reviewed. Then, the various approaches used in the literature to measure implant stability and the bone-implant interface properties in vitro and in vivo will be described. Quantitative ultrasound methods are promising because they are cheap, non invasive and because of their lower spatial resolution around the implant compared to other biomechanical approaches.

The rabbit as experimental model for research in implant dentistry and related tissue regeneration

The use of rabbits for experimental research has a long historical tradition. The aim of this review consists in outlining the use of the rabbit for research in implant dentistry and related tissue regeneration. Rabbits appear as a first-hand choice for fundamental implant design studies because of their size, easy handling, short life span, and economical aspects in purchasing and sustaining. In the following, the various anatomical sites in the rabbit will be summarized to provide an overview of current possibilities and limitations of this model for bone research in oral implantology.

Enhanced osteoblastic cell response on zirconia by bio-inspired surface modification

Excellent esthetic properties and limited plaque adhesion make zirconia ceramics an ideal material for implants in the fields of dentistry and orthopedics. Unfortunately, the physicochemical stability of zirconia makes it difficult to improve biocompatibility through surface modification. The dopamine-derived residue, 3,4-dihydroxy-L-phenylalanine (L-DOPA), has been identified as an important molecule secreted by marine mussels for the formation of adhesive pads. This study coated zirconia with L-DOPA to improve the biocompatibility of ZrO2. As confirmed by contact angle and X-ray photoelectron spectroscopy (XPS), the formation of L-DOPA film can be controlled by varying the process temperature. Results from scanning electron microscopy (SEM) and atomic force microscopy (AFM) show that the topography of the zirconia substrate was preserved after being coated with a film of L-DOPA. Specifically, the thickness of the coating and initial cell spreading ability were both enhanced by preparing samples at higher temperatures. L-DOPA coated zirconia demonstrated better cyto-compatibility than uncoated specimens, as indicated by cell responses such as cell spreading and proliferation. These preliminary results suggest that L-DOPA film could be used to improve the cyto-compatibility of zirconia and further has the potential to immobilize other biofunctional molecules in biomedical applications.

Current status of low intensity pulsed ultrasound for dental purposes

Over the past few years, tissue engineering applied to the dental field has achieved relevant results. Tissue engineering can be described by actions taken to improve biological functions. Several methods have been described to enhance cellular performance and low intensity pulsed ultrasound (LIPUS) has shown to play an important role in cell metabolism. The present article provides an overview about the current status of LIPUS as a tissue engineering tool to be used to enhance tooth and periodontal regeneration.

Low intensity ultrasound stimulates osteoblast migration at different frequencies

This study investigated the effects of different frequencies of low intensity ultrasound on osteoblast migration using an in vitro scratch-wound healing assay. Mouse calvarial-derived MC3T3-E1 osteoblasts in culture were exposed to continuous 45 kHz ultrasound (25 mW/cm(2)) or pulsed 1 MHz ultrasound (250 mW/cm(2)) for 30 min followed by 2 days' culture. Ultrasound treatment with either kHz or MHz output similarly and significantly increased cell numbers after 2 days in culture compared with untreated control cultures. In the scratch-wound healing assay the presence of the cell proliferation inhibitor mitomycin C (MMC) did not influence scratch-wound closure in control cultures indicating that cell migration was responsible for the in vitro wound healing. Application of ultrasound significantly stimulated wound closure. MMC did not affect kHz-stimulated in vitro wound healing; however, MMC reduced in part the scratch-wound closure rate in MHz-treated cultures suggesting that enhanced cell proliferation as well as migration was involved in the healing promoted by MHz ultrasound. In conclusion, both continuous kHz and pulsed MHz ultrasound promoted osteoblastic migration; however, subtle differences were apparent in the manner the different ultrasound regimens enhanced in vitro scratch-wound healing.