Analysis of implant stability changes in immediate loading using a laser displacement sensor in vivo and comparison of its sensitivity with that of resonance frequency analysis.

OBJECTIVE: The aim of this study was to analyze the stability changes in immediately loaded implants by using an in vivo quantitative measurement of micromotion under functional dynamic loading and to verify the sensitivity of Resonance Frequency Analysis (RFA) as compared to that of actual micromotion. MATERIALS AND METHODS: The micromotions of immediately loaded implants placed in the tibia of 11 rabbits were monitored using a laser displacement sensor. Functional dynamic loading forces were applied 5 days a week for 6 weeks. The implant stability quotient (ISQ) was monitored using RFA. RESULTS: The micromotion of the almost-loaded implants increased to peak values the day after loading was started and subsequently reached a plateau gradually. The ISQ changes in the loaded implants closely correlated with the alterations of the actual micromotion (r = -0.98, p < .01). Although the ISQ value itself correlated with the measured micromotion at the time of initial fixation (r = 0.73, p < .05), it did not correlate with the micromotion of the implant that acquired integration. No close correlation was observed between the ISQ and the histomorphometrical data. CONCLUSION: The immediately loaded implants showed the lowest stability immediately after the start of loading, which gradually increased thereafter. RFA is considered a useful method for examining stability changes and initial stability; however, it cannot determine the absolute magnitude of the stability after integration.

Theoretical efficacy of preventive measures for pathologic fracture after surgical removal of mandibular lesions based on a three-dimensional finite element analysis.

PURPOSE: Pathologic fracture of the mandible after removal of a lesion historically has been a clinical problem. The present study aimed to evaluate mandibular strength after removal of a lesion and to illustrate the theoretical efficacy of preventive measures against pathologic fracture based on a 3-dimensional finite element (FE) analysis. MATERIALS AND METHODS: A computed tomographic (CT)-based FE model of the mandible of a patient with a dentigerous cyst including a third molar was constructed. Using this model, the decrease of mandibular strength after virtual removal of the lesion was analyzed. The effect of the decrease of occlusal force and reinforcement by a miniplate was analyzed using a simple FE model of the mandible. Based on these analyses, removal of the cyst with the third molar was performed with a decrease of occlusal force and reinforcement by a miniplate. The validity of these procedures was analyzed using a CT-based FE model constructed after surgery. RESULTS: The von Mises stress in a CT-based FE model after virtual removal of the cyst with the third molar was markedly greater than that in the original FE model. In the analysis using a simple FE model, the stress around the fenestrated area was decreased after premolar loading compared with that after molar loading. In addition, miniplate placement around the fenestrated area markedly decreased the stress. Based on these results, the cast crowns of the first and second molars were removed and the fenestrated area of the mandible was reinforced with a 1.5-mm locking miniplate in the actual surgery. The von Mises stress in the fenestrated area was decreased and primarily borne by the miniplate in the analysis of a CT-based FE model constructed after surgery. CONCLUSION: The present study illustrated the theoretical efficacy of plate application for the decrease of stress on the mandible after surgical removal of a cyst including a third molar based on a simulation by FE analysis.

Effect of clenching on biomechanical response of human mandible and temporomandibular joint to traumatic force analyzed by finite element method.

PURPOSE: The purpose of the present study was to analyze the effect of clenching on the biomechanical response of human mandible and temporomandibular joint (TMJ) to traumatic force by the finite element (FE) method. MATERIAL AND METHODS: FE models of the mandible and the TMJ in resting and clenching positions were prepared. Distribution and magnitude of von Mises stress were analyzed by applying force as a point load in the symphyseal, canine, body and angle regions of the mandible. In addition, strain energy density (SED) at the articular disc and in posterior connective tissue of TMJ was analyzed. RESULTS: In the resting position, von Mises stress was mainly concentrated at the condylar neck and in the retromolar region of the mandible. In the clenching position, the stress at the condylar neck decreased in all loadings. The stress in the retromolar region similary decreased in the symphyseal, canine and body loading, respectively; however, higher stress was observed in the retromolar region on the loading side in the angle loading. High SED was generated at the articular disc and in posterior connective tissues of TMJ in the resting position. The SED in these tissues decreased in all loadings in the clenching position. CONCLUSIONS: Clenching generally reduces stress at the condylar neck and in the retromolar region of the mandible, and strain energy at the articular disc and in posterior connective tissue of TMJ by traumatic forces on the mandible; however, clenching induces greater stress in the retromolar region on the loading side by traumatic force to the angle region.

Biomechanical analysis of the strength of the mandible after marginal resection.

PURPOSE: This study investigated the biomechanical behavior of the mandible after marginal resection by tensile test in a human cadaveric mandible and finite element (FE) analysis. MATERIALS AND METHODS: Human cadaveric mandibular models after marginal resection were prepared with residual heights of 5, 10, and 15 mm. The strength in each of these mandibular models was examined by tensile testing. In addition, FE models of the mandible after marginal resection were prepared with residual heights of 5, 7.5, 10, 12.5, and 15 mm. Distribution and magnitude of von Mises stress were analyzed by applying bite forces of 151 N as a point load on the incisal region and 355.2 and 478.1 N on the premolar and molar regions on the nonresected and resected sides, respectively. At the molar region of the resected side, bite forces of 368.5 N and 286.9 N (80% and 60%, respectively, of 478.1 N) were also applied. RESULTS: On tensile testing, all cadaveric mandibular models were broken at the posterior resection corner. The tensile force was significantly larger in the model with a residual height of 15 mm compared with that of those with a 5- or 10-mm residual height. On FE analysis, von Mises stress was concentrated at the resection corner. The region of maximal von Mises stress concentration in FE models was consistent with that showing destruction on tensile testing. The relationship between the residual height and von Mises stress in the resection area was linear in models of the incisal, premolar, and molar loading on the nonresected side and quadratic in models of the premolar and molar loading on the resected side. The maximal von Mises stress in the resection area was highest during molar loading on the resected side under the present loading condition and exceeded the threshold for the development of pathologic fracture in the model with a residual height of around 10 mm or less. However, the maximal von Mises stress decreased in parallel with the reduction of bite force in the molar region of the resected side. CONCLUSIONS: The residual height and bite force are critical factors for the prevention of pathologic fracture of the mandible after marginal resection. Currently, a residual height of more than 10 mm and reduction of bite force are recommended to reduce the risk of fracture.

Direct 3-D morphological measurements of silicone rubber impression using micro-focus X-ray CT.

Three-dimensional computer models of dental arches play a significant role in prosthetic dentistry. The microfocus X-ray CT scanner has the advantage of capturing precise 3D shapes of deep fossa, and we propose a new method of measuring the three-dimensional morphology of a dental impression directly, which will eliminate the conversion process to dental casts. Measurement precision and accuracy were evaluated using a standard gage comprised of steel balls which simulate the dental arch. Measurement accuracy, standard deviation of distance distribution of superimposed models, was determined as +/-0.050 mm in comparison with a CAD model. Impressions and casts of an actual dental arch were scanned by microfocus X-ray CT and three-dimensional models were compared. The impression model had finer morphology, especially around the cervical margins of teeth. Within the limitations of the current study, direct three-dimensional impression modeling was successfully demonstrated using microfocus X-ray CT.

Biomechanical analysis of miniplate osteosynthesis for fractures of the atrophic mandible.

PURPOSE: The purpose of this study was to investigate the biomechanical behavior of miniplate osteosynthesis for fracture of the edentulous mandible with various degrees of atrophy by finite element (FE) analysis. MATERIALS AND METHODS: Three-dimensional FE models simulating various atrophic or nonatrophic edentulous mandibles were constructed. The models were divided into 3 groups based on the height: 20 mm, 15 mm, and 10 mm. A model 30-mm high was defined as a nonatrophic mandible. Fracture in the premolar region was simulated. Single or double miniplate osteosynthesis was assumed to fix the fracture. In each case, models of fractures with and without bone contact between bone fragments were prepared. A bite force of 62.8 N was applied in the FE models as a point load on the anterior point. RESULTS: There were no noticeable differences in compressive stress level in the bone around screws among the single miniplate models or double miniplate models with bone contact. Single miniplate models without bone contact showed markedly greater compressive stress than that of models with bone contact. The use of double miniplates showed a great influence on von Mises stress reduction in the miniplates. Without bone contact, greater interfragmentary displacements occurred; however, interfragmentary displacements were within the limit of not causing a malunion of the fractured bone in all models. CONCLUSION: Double miniplate fixation may be a reliable method for treating fracture of the atrophic mandible from a biomechanical viewpoint.

The behavior of vascular smooth muscle cells and platelets onto epigallocatechin gallate-releasing poly(l-lactide-co-epsilon-caprolactone) as stent-coating materials.

Localized drug delivery from drug-eluting stents has been accepted as one of the most promising treatment methods for preventing restenosis after stenting. However, thrombosis, inflammation, and restenosis are still major problems for the utility of cardiovascular prostheses such as vascular grafts and stents. Epigallocatechin-3-O-gallate (EGCG), a major polyphenolic constituent of green tea, has been shown to have anti-thrombotic, anti-inflammatory and anti-proliferative activities. It was hypothesized that controlled release of EGCG from biodegradable poly(lactide-co-epsilon-caprolactone, PLCL) stent coatings would suppress migration and invasion of vascular smooth muscle cells (VSMCs) as well as platelet-mediated thrombosis. EGCG-releasing PLCL (E-PLCL) was prepared by blending PLCL with 5% EGCG. The surface morphology, roughness and melting temperature of PLCL were not changed despite EGCG addition. EGCG did, however, EGCG appreciably increase the hydrophilicity of PLCL. EGCG was found to be uniformly dispersed throughout E-PLCL without direct chemical interactions with PLCL. E-PLCL displayed diffusion controlled release of EGCG release for periods up to 34 days. E-PLCL significantly suppressed the migration and invasion of VSMCs as well as the adhesion and activation of platelets. E-PLCL coatings were able to smooth the surface of bare stents with neither cracks nor webbings after balloon expansion. The structural integrity of coatings was sufficient to resist delamination or destruction during 90% dilatation. These results suggest that EGCG-releasing polymers can be effectively applied for fabricating an EGCG-eluting vascular stent to prevent in-stent restenosis and thrombosis.

3D morphological measurements of dental casts with occlusal relationship using microfocus X-ray CT.

In the diagnosis of dental occlusion, it is necessary to quantitatively measure interocclusal contacts and transfer them to a computer model. In this aspect, three-dimensional computer models of upper and lower dental casts play a significant role. In this study, we proposed a new method to measure occlusal interaction by using a microfocus X-ray CT technique. Measurement accuracy was determined as +/-0.03 mm in comparison with a coordinate measuring machine. A superimposition procedure for two sets of three-dimensional dental cast models was also established. Using the same dental cast, the standard deviation between the two sets of models was +/-0.015 mm - which was defined as measurement precision. Between an optical laser scanner and the microfocus X-ray CT system, the standard deviation measured between the two models was +/-0.05 mm. Data were acquired when upper and lower dental casts mounted on the bite impression were scanned, and then occlusal interaction, contacts, and distance distribution between the casts were visualized by a colored map on the cast models. Within the limitations of the current study, it was successfully demonstrated that microfocus Xray CT was well poised for quantitative measurement of occlusal interaction.

Effects on gingival cells of hydroxyapatite immobilized on poly(ethylene-co-vinyl alcohol).

Hydroxyapatite was immobilized on poly(ethylene-co-vinyl alcohol) (EVA) by alternate soaking in aqueous CaCl(2) and Na(2)HPO(4) solutions, followed by carboxyl groups introduction through ozone exposure in order to investigate the nature of the gingival cells, to control their proliferation and properties and to develop a highly organized hybrid implant possessing periodontium. Human gingival cells were cultured on the ozone-exposed EVA, collagen-immobilized EVA, hydroxyapatite-immobilized EVA, and a conventional tissue culture dish. Cell proliferation was highest on the tissue culture dish and lowest on the hydroxyapatite-immobilized EVA. The results of RT-PCR of gingival cells on hydroxyapatite-immobilized EVA shows that mRNAs expressed in bone and periodontal ligament were determined. Furthermore, alkaline phosphatase activity and ELISA assay revealed that gingival cells acquired the osteoblastic properties when cultured on hydroxyapatite-immobilized EVA, suggesting that the periodontium might be regenerated around implants using gingival cells.

The effects of bone density and crestal cortical bone thickness on micromotion and peri-implant bone strain distribution in an immediately loaded implant: a nonlinear finite element analysis.

PURPOSE: This study investigated the effects of bone density and crestal cortical bone thickness at the implant-placement site on micromotion (relative displacement between the implant and bone) and the peri-implant bone strain distribution under immediate-loading conditions. METHODS: A three-dimensional finite element model of the posterior mandible with an implant was constructed. Various bone parameters were simulated, including low or high cancellous bone density, low or high crestal cortical bone density, and crestal cortical bone thicknesses ranging from 0.5 to 2.5 mm. Delayed- and immediate-loading conditions were simulated. A buccolingual oblique load of 200 N was applied to the top of the abutment. RESULTS: The maximum extent of micromotion was approximately 100 µm in the low-density cancellous bone models, whereas it was under 30 µm in the high-density cancellous bone models. Crestal cortical bone thickness significantly affected the maximum micromotion in the low-density cancellous bone models. The minimum principal strain in the peri-implant cortical bone was affected by the density of the crestal cortical bone and cancellous bone to the same degree for both delayed and immediate loading. In the low-density cancellous bone models under immediate loading, the minimum principal strain in the peri-implant cortical bone decreased with an increase in crestal cortical bone thickness. CONCLUSIONS: Cancellous bone density may be a critical factor for avoiding excessive micromotion in immediately loaded implants. Crestal cortical bone thickness significantly affected the maximum extent of micromotion and peri-implant bone strain in simulations of low-density cancellous bone under immediate loading.

Assessment of correlation between computerized tomography values of the bone and cutting torque values at implant placement: a clinical study.

PURPOSE: The relationship between computerized tomography (CT) values of bone surrounding endosseous implants and the cutting torque values required for self-tapping during implant placement was examined for the purpose of predicting the initial stability (bone quality) during implant placement by presurgical CT scan examinations and determining whether it can be quantified. MATERIALS AND METHODS: The study sample consisted of 13 subjects with 56 implants. Sites for implant placement were determined based on CT data using implant planning software. The average CT values of the bone surrounding the simulated implants were calculated by the software. Using a stereolithographic drill guide, implants were placed at the locations indicated by the protocol. The cutting torque values required for self-tapping were measured during implant placement. The resulting CT values and cutting torque values were analyzed statistically for correlation. RESULTS: The correlation was considered significant at a level of .01 or less, and the correlation coefficient was 0.77. DISCUSSION: There was a strong correlation between CT values and cutting torque values in the clinical cases evaluated. These results indicate that it may be possible to predict and quantify initial implant stability and bone quality from presurgical CT diagnosis and implant simulation. CONCLUSION: Presurgical CT examination may be an effective technique for predicting initial stability of the implant and bone quality.

Development of a polyvinyl alcohol hydrogel for temperature measurement by MRI.

This paper describes the development of a Polyvinyl alcohol hydrogel (PVA-H) model for calibration and measurement of temperature using image contrast on MRI using DMSO/H_2O as a solvent to alter the freezing and melting points of PVA-H. Tissues exposed to temperature changes above 41 degrees C or below 0 degrees C exhibit increasingly extensive and irreversible damage, depending on the exposure duration. MR images can provide a map of temperature if there is sufficient tissue signal. To evaluate treatment principles using temperature changes (hyperthermia, cryotherapy), a model simulating tissue may be useful to provide a reproducible test environment. PVA-H is water soluble and can be seen on MRI. It can be used to construct complicated shapes such as vascular structure, soft tissues, and so on. Therefore, PVA-H can be useful to measure temperatures and assume the distribution of temperature under treatment. In this paper, we applied PVA-H as a temperature detector and calibrated temperature from image contrast. The results exhibit good capability as a temperature detector not only of high temperature (around 41 degrees C), but also of low temperature (as low as -23 degrees C).

Surface modification of poly(ethylene-co-vinyl alcohol): hydroxyapatite immobilization and control of periodontal ligament cells differentiation.

To reveal and control the differentiation and proliferation of the periodontal ligament (PDL) cells and to develop a highly organized hybrid implant possessing periodontium, hydroxyapatite (HAP) was immobilized on the poly(ethylene-co-vinyl alcohol) (EVA) by alternate soaking method following with carboxyl groups' introduction through ozone exposure. Human PDL cells were cultured on the ozone-exposed EVA, collagen-immobilized EVA, HAP-immobilized EVA, HAP plate, tricalcium phosphate plate, and conventional tissue culture dish. Cell proliferation was highest on the collagen-immobilized EVA and lowest on the HAP-immobilized EVA. Alkaline phosphatase activity and osteocalcin secretion were highest on the HAP-immobilized EVA. These results suggest that PDL cells were differentiated toward bone-like cells on the HAP-immobilized EVA.

Ex vivo mechanical evaluation of carbonate apatite-collagen-grafted porous poly-L-lactic acid membrane in rabbit calvarial bone.

Appropriate mechanical recovery is an important parameter in successful restoration of skeletal defects. Carbonate apatite and type I atelocollagen mixture (CAp) was grafted on a porous poly-L-lactic acid (PLLA) membrane to produce a CAp bilayered PLLA membrane (CAp+PLLA). After implantation, regional mechanical change in the membrane was investigated in rabbit calvarial bone defects. Dynamic viscoelasticity and elastic modulus of the implanted specimen were measured after 2, 4, 8, 12, 16, 26, and 52 weeks. The modulus of the peripheral part was higher than that of the central part of the implanted area, whereas the central part demonstrated a gradual increase. This phenomenon indicates that regeneration initially occurs from the periphery of the calvarial bone. After 26 weeks, stiffness of the central part became similar to that of the periphery in the CAp+PLLA-implanted area. According to this result, measuring viscoelasticity of an implanted biodegradable material would be a useful method to determine degrees of regeneration and replacement of an implanted region.

Control of pore size in L-lactide/epsilon-caprolactone copolymer foams for tissue regeneration by the freeze-drying method.

In the regeneration and repair of missing tissues, synthetic polymer scaffolds need many pores to involve cells and to supply cells with nutrients. The control of the pore size of biodegradable L-lactide/epsilon-caprolactone copolymer foams was studied by changing the polymer concentration and the cooling temperature in the freeze-drying method. The mixtures of polymer and 1, 4-dioxane solution were poured into an 18-8 stainless steel Petri dish and frozen. The pore size of a polymer foam tends to increase from the bottom towards the top of a Petri dish. The pore size decreased to one-half with increasing polymer concentration (1 to 10 wt%). The mean pore size in foams of 8% polymer concentration decreased from 100 microm to 20 microm as cooling temperature was lowered. This suggests that the higher cooling rate due to lower cooling temperature can produce smaller ice-crystals and result in smaller pores.

Poly-vinyl alcohol hydrogel vascular models for in vitro aneurysm simulations: the key to low friction surfaces.

PURPOSE: To develop a vessel model with poly-vinyl alcohol hydrogel (PVA-H) with low surface friction for in vitro simulations of vascular pathologies. MATERIALS AND METHODS: Simulations of variable vascular pathologies were based on human vessel casts reproduced in wax. Constructions of vascular models were then obtained by lost wax techniques. As materials, liquid silicon or PVA in organic solvents were used. After solidification, the wax was drained by heating. Comparison of PVA-H with silicone models included evaluation of surface characteristics (friction coefficient), of visibility (transparence and compatibility with medical imaging techniques), and of stiffness (pulsatility of vascular lumen). Both model types were integrated in a circulation circuit allowing for flow simulations and testing with endovascular medical devices. RESULTS AND CONCLUSION: Both model types were transparent and compatible with current medical imaging methods. PVA-H models exhibited much lower friction characteristics. PVA-H stiffness was closer to soft tissue when compared to silicone, allowing for pulsations during flow simulations.

Control of proliferation and differentiation of osteoblasts on apatite-coated poly(vinyl alcohol) hydrogel as an artificial articular cartilage material.

One of the key challenges in employing biomaterials is determining how to fix them into the surrounding tissue. To enhance the interaction with surrounding bone, amorphous hydroxyapatite (HA) was coated onto the surface of the bio-inert poly(vinyl alcohol) hydrogel (PVA-H), as an artificial cartilage material, by a pulsed laser deposition technique. Next we examined the binding effects of the HA thin film (300 nm thick) to the underlying bone using osteoblast proliferation and differentiation. A mouse osteoblast cell line, MC3T3E1, was cultured on the HA-coated and noncoated PVA-H with a water content of 33% or 53% for 3 weeks. Cell proliferation, alkaline phosphatase (ALP) activity, and levels of osteocalcin were evaluated for biocompatibility and differentiation. HA coating enhanced the cell proliferation, the ALP activity, and the levels of osteocalcin on both low and high water-content PVA-Hs. The cell growth rates on the PVA-H were lower than on tissue culture dishes even after the HA coating was added; however, osteoblastic differentiation was highly promoted by the HA coating on low water content PVA-H. These results suggested that the HA coating on the PVA-H enhanced the affinity between the bone and the PVA-H as an artificial cartilage material in surface replacement arthroplasty.

In vivo structural analysis of articular cartilage using diffusion tensor magnetic resonance imaging.

PURPOSE: The articular cartilage is a small tissue with a matrix structure of three layers between which the orientation of collagen fiber differs. A diffusion-weighted twice-refocused spin-echo echo-planar imaging (SE-EPI) sequence was optimized for the articular cartilage, and the structure of the three layers of human articular cartilage was imaged in vivo from diffusion tensor images. MATERIALS AND METHODS: The subjects imaged were five specimens of swine femur head after removal of the flesh around the knee joint, five specimens of swine articular cartilage with flesh present and the knee cartilage of five adult male volunteers. Based on diffusion-weighted images in six directions, the mean diffusivity (MD) and the fractional anisotropy (FA) values were calculated. RESULTS: Diffusion tensor images of the articular cartilage were obtained by sequence optimization. The MD and FA value of the specimens (each of five examples) under different conditions were estimated. Although the articular cartilage is a small tissue, the matrix structure of each layer in the articular cartilage was obtained by SE-EPI sequence with GRAPPA. The MD and FA values of swine articular cartilage are different between the synovial fluid and saline. In human articular cartilage, the load of the body weight on the knee had an effect on the FA value of the surface layer of the articular cartilage. CONCLUSION: This method can be used to create images of the articular cartilage structure, not only in vitro but also in vivo. Therefore, it is suggested that this method should support the elucidation of the in vivo structure and function of the knee joint and might be applied to clinical practice.

Analysis of the mandibular movement by simultaneous multisection continuous ultrafast MRI.

PURPOSE: Using simultaneous multiple cross-sectional imaging, we imaged four cross sections, including the mandibular midline and the right and left temporomandibular joints, to observe one movement of mouth opening and closing and analyze the movement of the mandible and temporomandibular joints. MATERIALS AND METHODS: Four cross sections, including the midsagittal section of the mandible, the sagittal sections of the right and left temporomandibular joints and the horizontal section containing the heads of right and left temporomandibular joints, were imaged simultaneously. The imaging was conducted in 10 male and female volunteers. RESULTS: In all volunteers, the relationship of the mandibular movement on the median line with the right and left temporomandibular joints was observed. Images of the volunteers with trismus indicated the condition in which the right and left temporomandibular joints did not move in keeping with each other but moved independently from each other. CONCLUSION: Complex movement of the temporomandibular joint was first evaluated by simultaneous multiple cross-sectional MRI for the movement of mandible and temporomandibular joints.

Histological evaluation and surface componential analysis of modified micro-arc oxidation-treated titanium implants.

The objective of this study is to investigate soft tissue and bone tissue reaction to titanium implants treated by a modified micro arc oxidation (MAO) technique, and analyze the surface components and implant-bone contact ratio by animal experiments to evaluate the osseointegration condition of implants with this modified MAO surface. MAO titanium plates were installed subcutaneously in rabbits. Tissue reaction was evaluated by HE sections. MAO titanium implants designed for endosseous examination were installed in Beagles' femurs. Bone tissue surrounding implants was analyzed histologically. Surfaces of retrieved implants were observed and examined by SEM and EDX. All procedures were performed under the control of untreated pure titanium implants. Thin homogeneous fibrous envelope could be found without apparent inflammation cells infiltration around the subcutaneously imbedded MAO titanium plates, which was almost same as control group. Fast osteoid deposition comprising high content of calcium, phosphor, carbon, and nitrogen elements was found on the retrieved MAO implant surfaces, while comparatively less amount of carbon and nitrogen elements were found on the retrieved implants of control group. Matured bone tissue comprising bone trabeculae and Haversian canals appeared in 8 weeks, while it took 12 weeks needed to form matured bone tissue in control group. In conclusion, MAO titanium materials shows good biocompatibility and calcium phosphate inducement capability in vivo and could accelerate bone tissue growth and shorten the osseointegration time.