Finite element analysis of four thread-form configurations in a stepped screw implant

The purpose of this study was to determine the optimal thread form configuration for an experimental stepped screw implant. Two-dimensional finite element analysis was applied to model the experimental stepped screw implant in a standard cross-section of the posterior human mandible digitized from a CT-generated patient data set. Four different thread form configurations: v-thread (V), thin-thread (T), and two square-thread forms of 0.24 mm (S1) and 0.36 mm (S2) thread width were compared under oblique load in normal cortical bone condition. The support-type constraint position changed from middle to the base of the bone segment. In middle support-type constraint position only the thin-thread (T) model demonstrated significantly different stress distribution from the other three models, however, in base support-type constraint position T and S1 models demonstrated significantly different stress distribution from the other two models. The results implies that v-thread (V) or large square-thread (S2) are optimal thread form for the experimental stepped screw implant. While, minimal support constraints allow clearer differentiation of the stress picture between the different stepped screw types at the trabecular bone-implant interface.     

Comparison of implant body designs and threaded designs of dental implants: a 3-dimensional finite element analysis

PURPOSE: Stress analysis was performed for various implant designs using 3-dimensional finite element analysis approaches. MATERIALS AND METHODS: Six implant designs were included: 3 parallel-sided implants (no thread, triangular thread, and squared thread), 2 stepped configurations (non-thread and triangular thread), and a tapered body of implant with squared thread. All threads had spiral characteristics. The mandibular model was constructed from computed tomographic (CT) images of a human mandible, and the material properties were anisotropic (different in different directions). A 100-N oblique force was applied at a 45-degree angle to the long axis of the implants at the buccal cusp as the loading condition. RESULTS: Compared with cylindric implants, threaded implants (either triangular or squared) demonstrated increased peak stress at the crestal bone. The bone stress of stepped implants was decreased in the cortical region but was increased in the trabecular region. However, both threaded and stepped designs showed decreased interfacial stresses of bone near the valleys of the threaded and stepped areas. The tapered design decreased stresses by up to 32% in the cortical region and 17% in the trabecular region. CONCLUSIONS: Although threaded implants could not decrease the peak stress at the crestal bone, both threaded and stepped designs show an ability to dissipate the interfacial stresses of bone. The use of tapered implants could reduce peak stress in both cortical and trabecular bone.     

Implant stability and histomorphometry: a correlation study in human cadavers using stepped cylinder implants

The aim of the present study was to determine the correlation between the primary stability of dental implants placed in edentulous maxillae and mandibles, the bone mineral density and different histomorphometric parameters. After assessing the bone mineral density of the implant sites by computed tomography, 48 stepped cylinder screw implants were installed in four unfixed human maxillae and mandibles of recently deceased people who had bequeathed their bodies to the Anatomic Institute I of the University of Erlangen-Nuremberg for medical-scientific research. Peak insertion torque, Periotest values and resonance frequency analysis were assessed. Subsequently, histologic specimens were prepared, and bone-to-implant contact, the trabecular bone pattern factor (TBPf), the density of trabecular bone (BV/TV) and the height of the cortical passage of the implants were determined. The correlation between the different parameters was calculated statistically. The mean resonance frequency analysis values (maxilla 6130.4+/-363.2 Hz, mandible 6424.5+/-236.2 Hz) did not correlate with the Periotest measurements (maxilla 13.1+/-7.2, mandible -7.9+/-2.1) and peak insertion torque values (maxilla 23.8+/-2.2 N cm, mandible 45.0+/-7.9 N cm) (P=0.280 and 0.193, respectively). Again, no correlations could be found between the resonance frequency analysis, the bone mineral density (maxilla 259.2+/-124.8 mg/cm(3), mandible 349.8+/-113.3 mg/cm3), BV/TV (maxilla 19.7+/-8.8%, mandible 34.3+/-6.0%) and the TBPf (maxilla 2.39+/-1.46 mm-1, mandible -0.84+/-3.27 mm-1) (P=0.140 and 0.602, respectively). However, the resonance frequency analysis values did correlate with bone-to-implant contact of the oral aspect of the specimens (maxilla 12.6+/-6.0%, mandible 35.1+/-5.1%) and with the height of the crestal cortical bone penetrated by the implants in the oral aspect of the implant sites (maxilla 2.1+/-0.7 mm, mandible 5.1+/-3.7 mm) (P=0.024 and 0.002, respectively). The Periotest values showed a correlation with the height of the crestal cortical bone penetrated by the implants in the buccal aspect of the implant sites (maxilla 2.5+/-1.2 mm, mandible 5.4+/-1.2 mm) (P=0.015). The resonance frequency analysis revealed more correlations to the histomorphometric parameters than the Periotest measurements. However, it seems that the noninvasive determination of implant stability has to be improved in order to give a more comprehensive prediction of the bone characteristics of the implant site.     

Initial stability and bone strain evaluation of the immediately loaded dental implant: an in vitro model study

Objectives: The aim of this study was to evaluate the effects of cortical bone thickness and trabecular bone elastic modulus on the strain in the bone surrounding an immediately loaded implant. We also examined the correlations between bone structure and the following indices of primary implant stability: insertion torque value (ITV), Periotest value (PTV), and implant stability quotient (ISQ). Material and methods: The ITV, PTV, and ISQ were measured in 24 artificial jaw bone models representing cortical bone with four thicknesses (0, 1, 2, and 3 mm) and trabecular bone with four elastic moduli (137, 47.5, 23, and 12.4 MPa). Two loading conditions were applied (force of 130 N applied vertically and at 45° laterally), and the strains in the crestal region were measured by rosette strain gauges with a data acquisition system. Results: When the cortical bone thickness and the elastic modulus of trabecular bone decreased, the bone strains increased by 10.3–52.1% and 39–73.1%, respectively, for vertical loading and by 35–62% and 42.4–56.2% for lateral loading. The cortical bone thickness has a stronger correlation (R²=0.95–0.71) with ITV, PTV, and ISQ than the elastic modulus of trabecular bone (R²=0.89–0.59). Conclusions: The initial stability at the time of implant placement is influenced by both the cortical bone thickness and the elastic modulus of trabecular bone; however, these parameters are not totally linearly correlated with ITV, PTV, and ISQ. The placement of an immediately loaded implant in cases with thin cortical bone and/or weak trabecular bone can induce extreme bone strains and may increase the risk of implant failure. To cite this article:
Huang H‐L, Chang Y‐Y, Lin D‐J, Li Y‐F, Chen K‐T, Hsu J‐T. Initial stability and bone strain evaluation of the immediately loaded dental implant: an in vitro model study.
Clin. Oral Impl. Res. 22 , 2011; 691–698
doi: 10.1111/j.1600‐0501.2010.01983.x     

The Effects of Cortical Bone Thickness and Trabecular Bone Strength on Noninvasive Measures of the Implant Primary Stability Using Synthetic Bone Models

Purpose: This study investigated how the primary stability of a dental implant as measured by the insertion torque value (ITV), Periotest value (PTV), and implant stability quotient (ISQ) is affected by varying thicknesses of cortical bone and strengths of trabecular bone using synthetic bone models. Materials and Methods: Four synthetic cortical shells (with thicknesses of 0, 1, 2, and 3 mm) were attached to four cellular rigid polyurethane foams (with elastic moduli of 137, 47.5, 23, and 12.4 MPa) and one open‐cell rigid polyurethane foam which mimic the osteoporotic bone (with an elastic modulus 6.5 MPa), to represent the jawbones with various cortical bone thicknesses and strengths of trabecular bone. A total of 60 bone specimens accompanied with implants was examined by a torque meter, Osstell resonance frequency analyzer, and Periotest electronic device. All data were statistically analyzed by two‐way analysis of variance. In addition, second‐order nonlinear regression was utilized to assess the correlations of the primary implant stability with the four cortex thicknesses and five strengths of trabecular bone. Results: ITV, ISQ, and PTV differed significantly (p < .05) and were strongly correlated with the thickness of cortical bone (R² > 0.9) and the elastic modulus of trabecular bone (R² = 0.74–0.99). Conclusions: The initial stability at the time of implant placement is influenced by both the cortical bone thickness and the strength of trabecular bone; however, these factors are mostly nonlinearly correlated with ITV, PTV, and ISQ. Using ITV and PTV seems more suitable for identifying the primary implant stability in osteoporotic bone with a thin cortex.     

Bicortically stabilized implant load transfer

PURPOSE: Questions exist as to the potential advantages of bicortical stabilization of implants in the mandible through engagement of the lingual cortical plate. The purpose of this investigation was to determine photoelastically the effect of lingual cortical plate engagement on implant load transfer. MATERIALS AND METHODS: Composite photoelastic models of an edentulous posterior segment of a mandible were fabricated using plastics of different modulus to simulate cortical and trabecular bone. One model included a 3.75 x 15-mm threaded implant that engaged the simulated lingual cortical plate, while in the other model the implant was centrally located within the simulated trabecular bone. A metal superstructure was cast using an abutment cylinder. Simulated vertical occlusal loads were applied to the superstructure over the implant and at fixed buccal cantilever lengths. Stresses that developed within the model were monitored photoelastically and recorded photographically. Peri-implant defects were then formed in the models and the loading and recording procedures were repeated. RESULTS: Lingual cortical plate engagement generated the highest stresses at the lingual cortical plate and the buccal crestal cortical layer at the implant neck. Stress intensity within the buccal plate at the implant neck was lower than that in the centrally placed implant. In the presence of a peri-implant defect, for all load conditions, more load was borne by the trabecular bone. Increasing cantilever lengths caused asymmetric load transfer with higher maximum stresses. DISCUSSION: For both implant placements, a large portion of the applied load was taken by the crestal cortical bone simulant. Engagement of the lingual cortical plate reduced maximum stress in the crestal cortical bone by approximately 25%. With peri-implant defects, the simulated trabecular bone provided the main support of the applied load. Longer buccal cantilever lengths increased maximum stresses for all placement and crestal bone conditions. CONCLUSIONS: The results of this investigation do not indicate a clear load transfer advantage to apical engagement of the lingual cortical plate in this model.     

Topography of the outer mandibular symphyseal region with reference to the autogenous bone graft

The mandibular symphysis is preferred as a donor site for the relatively small grafts needed for the autogeneous bone graft procedure. This study was undertaken to determine the morphology and composition of the cortical and trabecular bone in the mandibular symphyseal region using 35 mandible specimens from Koreans. The topographical patterns through the thickness of the cortical plate and the width of the trabecular bone were observed. In this study, the labial cortical plate of the mandible became thicker from the superior to the inferior aspects (P < 0.05). However, the trabecular bone width exhibited a different distribution pattern compared to the thickness of the labial cortical plate. This observation concerning the cortical and trabecular bones assists in determination of the depth of osteotomy. The results provide useful information on the mandibular symphysis graft prior to dental implant placement. These results will enable the volume of the cortical plate in the mandibular symphyseal region and its proper size, depth, and location to be predicted when removing a graft block.     

上下颌游离缺失Brnemark种植义齿早期临床效果的比较研究

目的:比较上下颌游离缺失Branemark种植义齿修复早期临床效果。方法 :5例上颌 (17颗 )和7例下颌(22颗 )游离缺失Branemark种植体植入金合金烤瓷桥修复行使功能18个月后 ,比较上下颌游离缺失Branemark种植体支持固定桥义齿的稳固情况、种植体周牙龈指数和种植体周骨吸收量。结果 :上下颌游离缺失Branemark种植体支持固定桥义齿种植体周骨吸收量无显著性差异 (P>0.05) ;两者种植体周牙龈均处于正常状态 ;上下颌游离缺失种植义齿均未见种植体松动或脱落 ,基台和全冠的稳固性均良好。结论 :上下颌游离缺失Branemark种植体支持种植固定桥义齿修复效果良好     

Stress analysis of implant-supported partial prostheses in anisotropic mandibular bone: in-line versus offset placements of implants

Three-dimensional, anisotropic finite element models were executed to investigate the biomechanical effects of in-line and offset placements of implants on implant-supported partial prostheses. Three implant placements of finite element models were created: in-line, buccal offset and lingual offset placements. The mesh models of a cadaver mandibular segment and a three-united crown containing the 2nd premolar, the 1st molar and the 2nd molar were constructed by computer tomography images. The material properties of mandible were applied as transversely isotropic and linearly elastic. Two loading modes (100N), vertical and oblique, were evaluated in all models. Insignificant difference was observed in implant stresses between the in-line and offset placements under the vertical loading mode. Under the oblique loading, however, the offset placement decreased the implant stress by a maximum of 17%. The maximum stress at cortical bone and trabecular bone around each implant did not show conspicuous difference between the in-line and offset placements. This study demonstrated the mechanisms of how stresses were distributed between the in-line and offset placements. Even though the offset placements showed the benefit of decreasing implant stresses, justified from the bone stress the offset placements provided no advantage for the stress decreasing over the in-line placement.     

Tension More种植体骨界面的光弹应力分析

目的 研究不同锥度设计的Tension More（TM）种植体对种植体骨界面应力分布的影响。方法 医用纯钛制作5组种植体,分别为圆柱状螺纹种植体、上1/3 TM种植体（锥度长度为3 mm）、中1/2 TM种植体（锥度长度为5 mm）、下1/3 TM种植体（锥度长度为7 mm）、全长变化TM种植体（锥度长度为10 mm）。每组种植体各自包埋于由松质骨及1 mm皮质骨构成的复合光弹模型中,共建立5个复合光弹模型。每一模型先后分别予以垂直及斜向（45°）静态加载力。利用光弹应力分析法比较5组种植体骨界面的生物力学特征。结果 垂直加载下,上1/3 TM种植体、中1/2 TM种植体、下1/3 TM种植体比圆柱状螺纹种植体在皮质骨区及松质骨区的局部应力集中小;斜向加载下,4组TM种植体皮质骨区局部应力集中均低于圆柱状螺纹种植体。无论在垂直、斜向加载下,上1/3 TM种植体皮质骨区局部应力集中均最小。结论 合理锥度设计的TM种植体周围皮质骨、松质骨应力分布均匀合理,在不同载荷条件下,上1/3 TM种植体骨界面生物力学表现最优。     

A model for the evaluation of mandibular bone response to implant materials

SUMMARY An in vivo animal (goat) mandibular bone model has been evaluated for the cortical and trabecular bone response to biomaterials with different elastic modulus and/or different surfaces. The effect of the elastic modulus on the bone formation was also studied. The difficulties encountered in orientation of the implants probably could have been surmounted if pre-operative radiographs has been available. It was also established that there is a large variance in the amount of trabecular and cortical bone in the mandible of the goat'one model'.     

Influence of cortical bone quality on stress distribution in bone around dental implant

Using finite element method (FEM), this study sought to investigate how the thickness and Young's modulus of cortical bone influenced stress distribution in bone surrounding a dental implant. The finite element implant-bone model consisted of a titanium abutment, a titanium fixture, a gold alloy retaining screw, cancellous bone, and cortical bone. The results showed that von Mises equivalent stress was at its maximum in the cortical bone surrounding dental implant. Upon investigation, it was found that maximum von Mises equivalent stress in bone decreased as cortical bone thickness increased. On the other hand, maximum von Mises equivalent stress in bone increased as Young's modulus of cortical bone increased. In conclusion, it was confirmed that von Mises equivalent stress was sensitive to the thickness and Young's modulus of cortical bone.     

Influence of implant length and bicortical anchorage on implant stress distribution

Background: Short implants present superior failure rates for everybody. Purpose: The aim of this theoretic study was to assess to what extent implant length and bicortical anchorage affect the way stress is transferred to implant components, the implant proper, and the surrounding bone. Materials and Methods: Stress analysis was performed using finite element analysis. A three‐dimensional linear elastic model was generated. All implants modeled were of the same diameter (3.75 mm) but varied in length, at 6, 7, 8, 9, 10, 11, and 12 mm (Branemark System®, Nobel Biocare AB, Gothenburg, Sweden). Each implant was modeled with a titanium abutment screw and abutment, a gold cylinder and prosthetic screw, and a ceramic crown. The implants were seated in a supporting bone structure consisting of cortical and cancellous bone. An occlusal load of 100 N was applied at a 30° angle to the buccolingual plane. Results: With the selected model and bone properties, the coronal cortical anchorage was dominating, and the bone stress concentrated to that area. Conclusions: The maximum bone stress was virtually constant, independent of implant length and bicortical anchorage. The maximum implant stress, however, increased somewhat with implant length and bicortical anchorage.     

Bone Structure Changes in Iliac Crest Grafts Combined with Implants

Background: Remodeling of onlay grafts combined with implants to the mandible results in predictable changes in the graft's radiographic density. We studied the relationship between changes in radiographic density and trabecular structure during the first year after onlay grafting with simultaneous implant placement to the mandible. Purpose: The aim of this study was to evaluate changes in bone structure after onlay grafting. Materials and Methods: Standardized extraoral radiographs were taken regularly of 16 mandibular sides. Bone structure was measured using the Carl Zeiss Vision KS 400 3.0 imaging system. The parameters studied were trabecular area and perimeter, cavity area and perimeter, end points, branching points, skeleton length, branch angle and direction, and texture. Results: No differences were found between measurements ventrally versus dorsally of the implant, nor close to versus away from the implant. Early cortical changes suggest partial resorption and formation of a more complex structure. In the fourth quarter after surgery, progressive resorption is seen in the graft's upper cortex. In the graft's upper spongiosa, most parameters indicate bone formation during the first postoperative year. Loading‐induced structure changes could not yet be found. Conclusion: The technique can be used to study changes in the architecture of bone grafts. Changes found in the graft's architecture are in accordance with changes in bone density.     

Morphologic Characteristics of Bony Edentulous Jaws

PURPOSE: The objective of this study was to examine bone tissue characteristics of edentulous arches and residual ridges in different regions of the human jaws. MATERIALS AND METHODS: For the study, 24 maxillary and 99 mandibular completely edentulous dry specimens were examined macroscopically and by linear and caliper measurements. Width and length of the edentulous arches were registered from the molar regions to the crest of the incisor zone. Residual ridges were quantified at the incisor, premolar, and molar regions. RESULTS: Arches and ridges varied in size and shape. A trabecular bone track was present at the edentulous surface. In the maxillary arch, trabecular tissue was external to the cortical palatal vault, and in the mandible, the trabecular bony stretch was inside the cortical plates along the entire edentulous surface of the jaw. Of the ridges, 38% were thin knife-edged. In the maxilla, bone resorption was centripetal, and the crest of the edentulous arch was external to the osseous base of the cranium. In the mandible, resorption was centrifugal, forming an edentulous crest lingual to the mental protuberance anteriorly and to the mandibular base posteriorly. This discrepancy produced a reverse horizontal overlap of the residual crests, where the edentulous maxilla was at the same level or internal to the facing edentulous mandible. CONCLUSIONS: This study, performed on 123 human edentulous dry bone specimens, indicates that the edentulous arch and the residual ridge take many forms. At the occlusal surface of the edentulous jaw a trabecular track is the remaining scar after tooth extraction. To accommodate an entire metal osseointegrated implant within the residual bone volume, crest reduction and bone grafts may be required as preprosthetic surgical measures. After tooth loss, maxillary resorption was centripetal and apical, whereas mandibular resorption was centrifugal and also apical, resulting in a reversed horizontal relationship in fully edentulous subjects.     

用Br(a)nemark种植系统对全口牙列缺失患者进行固定修复的临床探讨

目的:观察Branemark种植系统对全口牙列缺失患者进行全口种植固定修复后的临床效果。方法:全口牙列缺失患者 ,用Branemark种植系统对1例患者的上下颌骨内各种植6个Branemark种植体 ,然后进行了全口固定修复。在临床及技术室的操作过程中 ,对降低成本 ,改进临床操作和工艺制作进行了探讨和4年的临床随访。结果:通过4年的随访 ,获得了满意的临床效果。患者自述异物感明显减小 ,咀嚼功能明显提高。X线全景片显示 ,所有12个种植体骨整合效果良好 ,未见明显吸收现象。结论:对全口牙列缺失患者应用Branemark种植系统进行全口固定修复临床效果良好。     

不同种植体外形设计对上颌窦提升术后种植体周围应力分布的影响

目的：比较不同种植体外形设计对上颌窦提升术后种植体周围生物力学的影响。方法：在D3型上颌骨简化模型上利用三维有限元法分析3种不同外形设计的种植体在植骨与不植骨条件下的应力分布情况。假设所有材料都是线弹性、连续材料,向种植体施加150 N的倾斜力,测量种植体周围骨组织的最大等效力（equivalent von-Mises,EQV）。采用Ansys Workbench 14.5软件包对数据进行测量分析,采用SPSS 17.0软件包对数据进行统计学分析。结果：各组应力集中区域均位于种植体颈部皮质骨区域。不同种植体外形设计对种植体周围皮质骨最大EQV值无显著影响,但锥形种植体较其他种植体的周围松质骨最大EQV值显著上升,上颌窦提升术后植骨可以降低各组种植体周围最大EQV值。结论：锥形种植体用于上颌后牙区种植修复时,可能引起种植体周围松质骨应力变大,增加种植体周围骨吸收的风险。上颌窦提升后植骨,可降低种植体周围压力负载。     

Association of cortical shape of the mandible on panoramic radiographs with mandibular trabecular bone structure in Japanese adults: a cone-beam CT-image analysis

Objectives

The purpose of this study was to assess the association between the cortical shape of the mandible, as detected on panoramic radiographs, and trabecular bone structure, as assessed by cone-beam computed tomography (CBCT), in Japanese adults.

Methods

Panoramic radiographs and CBCT images of the mandibles of 50 subjects (18 men, 32 women), aged 45–86 years, were evaluated. An experienced oral and maxillofacial radiologist categorized the cortical shape of the mandible as detected on panoramic radiographs as normal, mildly to moderately eroded, and severely eroded cortices, respectively. All mandibles were scanned using CBCT. Four bone structure parameters of the basal portion of the mandible were calculated in three dimensions using an image-analysis system: total bone volume (mm³); cortical bone volume fraction (%); trabecular bone volume fraction (%); fractal dimension. One-way analysis of covariance with Bonferroni correction was employed to evaluate differences in the four bone parameters among the three cortical shape groups. Pearson’s correlation coefficient was calculated to examine correlations between age and cortical and trabecular bone volume fractions.

Results

Progression of cortical bone erosion was significantly associated with increased trabecular bone volume fraction (P < 0.001) and increased fractal dimension (P = 0.01). Cortical bone volume fraction decreased significantly with age (P = 0.04). However, trabecular bone volume fraction tended to increase with age (P = 0.06).

Conclusions

The change in the trabecular bone structure of the mandible may differ from that of the general skeleton in Japanese adults.     

Three-dimensional finite element analyses of four designs of a high-strength silicon nitride implant

The effects of implant shape and size on the stress distribution around high-strength silicon nitride implants under vertical and oblique forces were determined using a three-dimensional finite element analysis. Finite element models were designed using as a basis the serial sections of the mandible. Using Auto-CAD software, the model simulated the placement of implants in the molar region of the left mandible. Results of the analyses demonstrated that mainly the implant root shape and the directions of bite forces influence the stress distributions in the supporting bone around each implant. Implant size is a lesser factor. The serrated implants presented a larger surface area to the bone than either the cylindrical or tapered implants, which resulted in lower compressive stress around the serrated implants. With increasing implant diameter and length, compressive stress decreased. The mean compressive stress distribution on the serrated implants was more flat (platykurtic) than on either the cylindrical or tapered implants. Results of studies on two load directions (vertical and oblique) showed that, in either case, the compressive stress in the cortical bone around the neck of the implant was higher than in the cancellous bone along the length of the implant. The most extreme principal compressive stress was found with oblique force. This study provides the first information on the relationship between shape of the silicon nitride implant and stress on the supporting bone.     

Indentation modulus of the alveolar process in dogs

One mechanism of bone adaptation is alteration in tissue level material properties. We hypothesized that alteration in the indentation modulus of the alveolar process is an adaptive response to the localized mechanical environment. Forty-eight specimens representing anterior and posterior regions of the maxilla and mandible were obtained from 6 mature male beagle dogs. The indentation properties of the alveolar bone proper and more distant osteonal cortical bone were estimated. The bone types were further divided into 3 regions (coronal, middle, and apical), with 27 indents being made in each region of tooth-supporting bone. There was a significant difference (p < 0.001) in the indentation moduli of the jaws (maxilla/mandible), location (anterior/posterior), and bone type (alveolar bone proper vs. cortical bone). However, statistical interactions exist which preclude the simple interpretation of results. The distribution of relative stiffness provides a better understanding of bone adaptations in the alveolar process.