Design of functionally graded dental implant in the presence of cancellous bone

In a previous work by the author [Hedia HS, Mahmoud NA. Biomed Mater Eng 2004;14(2):133--143], a functionally graded material (FGM) dental implant was designed without cancellous bone in the model. In this investigation, the effect of the presence of cancellous bone as a thin layer around the dental implant was investigated. It is well known that the main inorganic component of natural bone is hydroxyapatite (HAP) and that the main organic component is collagen (Col). HAP implants are not bioabsorbable, and because induction of bone into and around the artificially made HAP is not always satisfactory, loosening or breakage of HAP implants might occur after implantation in the clinical application. The development of a new material that is bioabsorbable and that has osteo-conductive activity is needed. Therefore, the aim of the current investigation was to design an implant, in the presence of cancellous bone as a thin layer around it, from FGM. In this study, a novel biomaterial, Col/HAP, as a FGM, was developed using the finite element and optimization techniques that are available in the ANSYS package. These materials have a self-organized character similar to that of natural bone. The investigations have shown that the maximum stress in the cortical bone and cancellous bone for the Col/HAP functionally graded implant has been reduced by about 40% and 19%, respectively, compared with currently used titanium dental implants.     

Optimum gradient material for a functionally graded dental implant using metaheuristic algorithms

Despite dental implantation being a great success, one of the key issues facing it is a mismatch of mechanical properties between engineered and native biomaterials, which makes osseointegration and bone remodeling problematical. Functionally graded material (FGM) has been proposed as a potential upgrade to some conventional implant materials such as titanium for selection in prosthetic dentistry. The idea of an FGM dental implant is that the property would vary in a certain pattern to match the biomechanical characteristics required at different regions in the hosting bone. However, matching the properties does not necessarily guarantee the best osseointegration and bone remodeling. Little existing research has been reported on developing an optimal design of an FGM dental implant for promoting long-term success. Based upon remodeling results, metaheuristic algorithms such as the genetic algorithms (GAs) and simulated annealing (SA) have been adopted to develop a multi-objective optimal design for FGM implantation design. The results are compared with those in literature.     

Effect of cancellous bone on the functionally graded dental implant concept

In a previous work by the author [H.S. Hedia and M. Nemat-Alla, Design optimization of functionally graded dental implant, submitted to be published in the J. Bio-Medical Materials and Engineering], a functionally graded material dental implant was designed without cansellous bone in the model. In this investigation the effect of presence cancellous bone as a thin layer around the dental implant was investigated.It is well known that the main inorganic component of natural bone is hydroxyapatite (HAP) and that the main organic component is collagen (Col). Hydroxyapatite HAP implants are not bioabsorbable, and because induction of bone into and around the artificially made HAP is not always satisfactory, loosening or breakage of HAP implants may occur after implantation in the clinical application. The development of a new material which is bioabsorbable and which has osteoconductive activity is needed. Therefore, the aim of the current investigation is to design an implant, in the presence of cancellous bone as a thin layer around it, from functionally graded material. In this study, a novel biomaterial, collagen/hydroxyapatite (Col/HAP) as a functionally graded material (FGM), was developed using the finite element and optimization techniques which are available in the ANSYS package. These materials have a self-organized character similar to that of natural bone. The investigations have shown that the maximum stress in the cortical bone and cancellous bone for the Col/HAP functionally graded implant has been reduced by about 40% and 19% respectively compared to currently used titanium dental implants.     

Correlation between hydroxyapatite osseointegration and Young's Modulus

From the standpoint of hard tissue response to implant materials, calcium phosphate is probably the most compatible of materials known. During the last few years, much attention has been paid to hydroxyapatite and beta-tricalcium phosphate as potential biomaterials for a bone substitute. Good implantation of biomaterials in the skeleton is evidenced by an ability to reach full integration of the non-living implant with living bone. The aim of this study is to correlate hydroxyapatite osseointegration with Young's Modulus. Cylinders (5-6 mm in diameter) of these ceramics were packed into holes made in the femur diaphysis of a mature sheep. At 2, 4, 8, 12, 16, 20, 28, 36 and 48 weeks after the operation, samples of the bone/implant interface were embedded in polymethylmethacrylate. We used the PIXE method (Particles Induced X-rays Emission) to measure the distribution of mineral elements (Ca, P, Sr, Zn, Mn and Fe) at the bone/implant interface. At 4, 8, 12, 16, 20, 28, 36 and 48 weeks after implantation we studied Young's Modulus on a biopsy of the ceramic. Young's Modulus increased with time after implantation and is linked with biomaterials integration into cortical bone.     

Improved design of cementless hip stems using two-dimensional functionally graded materials

Increasingly, it is acknowledged that bone resorption around cementless hip implants may cause future problems. The solution is frequently sought in reducing implant stiffness. However, this confronts the designer with a true design conflict: how to reduce the stiffness without excessively loading the proximal bone/prosthesis interface? The aim of this work is to improve the design of cementless hip stem material, using two-dimensional (2D) functionally graded material (FGM) concept in order to solve the above problems. Two models were used in this analysis, using three materials with different elastic moduli, E(1), E(2), and E(3). In model I, the elastic moduli E(1) and E(2) gradually change along the upper stem surface, while E(3) is maintained constant along all the lower surface of the stem. However, in model II, the elastic moduli E(1) and E(2) gradually change along the lower stem surface, while E(3) is maintained constant all along the upper stem surface. It is found that the recommended model is model I, which has three distinct materials of hydroxyapatite, Bioglass, and collagen. The recommended design of 2D FGM is expected to reduce the stress shielding by 91% and 12%, respectively, compared with titanium stem and model II of FGM. It is found that this new design reduces the maximum interface shear stress at the lateral and medial sides of the femur by about 50%, compared with titanium stem. Furthermore, the maximum interface shear stress is reduced by about 17% and 11% at the lateral and medial sides of the femur, respectively, compared with that of model II of FGM.     

A bioactive titanium foam scaffold for bone repair

While titanium has been clinically successful as an orthopedic or dental implant material, performance problems still persist related to implant-bone interfacial strength and mechanical modulus mismatch between titanium and tissue. We describe here the preparation of a titanium foam as a better mechanical match to tissue with surfaces attractive to bone cells through deposition of an organically-modified apatite layer (organoapatite). In a rotating bioreactor, these organoapatite-coated foams are successfully colonized by preosteoblastic cells. Finite element analyses suggest that ingrown tissue in these systems may improve both implant performance and tissue formation through load-sharing and stress distribution. The novel metal-ceramic-polymer hybrid materials described here hold great promise for bone tissue engineering.     

Biocompatibility of electrophoretical deposition of nanostructured hydroxyapatite coating on roughen titanium surface: in vitro evaluation using mesenchymal stem cells

A nano hydroxyapatite (HAp) layer was coated on a roughen titanium surface by means of electrophoretic deposition with an acetic anhydride solvent system. The objectives of this current study are to investigate whether nano-HAp can improve mechanical strength at a lower sintering temperature and biocompatibility. Densification temperature was lowered from usual 1000 to 800 degrees C. The coating interfacial bonding strength, phase purity, microstructure, and biocompatibility were investigated. Degradation of HA phase was not detected in XRD. A porous TiO2 layer acts as a gradient coating layer with an intermediate thermal expansion coefficient between hydroxyapatite and titanium that reduces the thermal stress. From SEM image, the coating does not contain any crack. Mesenchymal stem cell (MSC) is the progenitor cell for various tissues in mature animals, which can improve integration of bone tissue into implant. In this in vitro study, rabbit MSCs culture indicated that the HAp/Ti nanocomposite biomaterial had good biocompatibility and bioactivity. Around materials and on its surface cell grew well with good morphology. Proliferation of the MSCs on the nano-HAp coating was higher than its micron counterpart in XTT assay. These properties show potential for the orthopaedic and dental applications.     

钛基植人物生物陶瓷涂层的研究进展

医用钛合金材料属于生物惰性材料,广泛应用于硬组织的替换与修复领域.采用表面改性技术在钛基材料表面制成生物陶瓷涂层可改善钛基材料的生物活性和生物相容性.羟基磷灰石涂层已在临床上获得应用,但使用效果仍然受其较低的结合强度和结晶度所制约.为了获得综合性能更好的植入材料,制备了多种新型生物陶瓷涂层,其具有良好的生物活性、较好的...     

A 3-D FEM analysis of single and multiple screw-root dental implant fixed in a mandible

Replacement of single tooth using a threaded titanium screw root coated with hydroxyapatite (HA) for faster bone apposition to implant site is common. Uncoated pure titanium is also used for osseointegration. Usually bone fixation of implant takes four to six months in either case. Quite often, a good number of teeth in a mandible or maxilla are replaced. Usually it is often said that bones appear to be well designed from the point of view of structural engineering. The "maximum-minimum law" claimed by Roux is a rational concept that states that bone provides maximum strength with a minimum of construction material. According to this proposition, stress distribution in bone will be almost uniform under a set of loading conditions. This was found to be true in the case of normal human mandibular bone as examined by some Japanese scientists. The present authors are interested in examining the stress distribution during multiple single tooth replacements using 3D-FEM technique to ascertain how the stress pattern changes with such implantation of three screws in a row in the human mandible, as we are in the process of clinical trial of hydroxyapatite coated and uncoated titanium implant. This will be of considerable interest to the dental surgeons, who prompted us to address this problem. Our results depicted the mean values of various stress in cortical and cancellous bone while applying the maximum masticatory load of 50 N in each teeth. The generated stress level is within the safe range of stress for bones. However, such screws cannot be applied to osteoporotic or other diseased mandible where bone strength may be quite low.     

Permucosal implantation pilot study with HA-coated dental implant in dogs.

The histological response of transmucosal one-stage titanium dental implants coated with hydroxyapatite is described. The gingival adhesion to the implant was examined with regard to coated, partially coated or non-coated surfaces in the cervical region. From each coating type, 9 implants were inserted into dogs. Six months after the insertion, 19 implants could be evaluated, but 8 implants were lost. From these 19 implants, 6 implants showed severe pockets with inflammation up to the bony tissue. The 13 successful implants showed direct bone bonding with the hydroxyapatite coating and adhesion of submucosal connective tissue to the implant surface, with inflammation. The marginal gingiva showed slight inflammation. A totally coated implant will probably introduce inflammation by debris formation against the rough implant surface more easily. The hydroxyapatite coating often disappeared in the soft tissue or in the oral cavity. Bone which directly adapted to the coating seemed to prevent it from resorption.     

Biomechanical behavior of hydroxyapatite as bone substitute material in a loaded implant model. On the surface strain measurement and the maximum compression strength determination of material crash

Many investigators have advocated that hydroxyapatite ceramics may be extremely prospective bone substitute material mainly through evidence with its given biocompatibility towards bone and demonstrated continuity between living bone and hydroxyapatite. Its mechanical brittleness and strength, however, have been the most serious considerations. In this paper, a study has been performed using animal experiments, massive hydroxyapatite ceramics are implanted into rectangular bone defect created by operation at the load-bearing area in close contact with tibia plateau. The changes of surface strain at the hydroxyapatite implantation of retrieved tibiae are measured at any week after implantation when the compressive stress were applied in the direction of long axis in order to clarify the mechanical behaviors of hydroxyapatite living bone complex. The compression strength of hydroxyapatite implant crash was also determined at each week after implantation. Results revealed that the hydroxyapatite living bone complex has been proven to have sufficient flexibility such that it shows no hysteresis in stress versus strain relationship up to 200 kg of applied load. The strain pattern on the surface of hydroxyapatite implant develops quite similar to that of natural cortical bone. The material crash of hydroxyapatite implant tolerates up to 500 kg of load at 52 weeks after implantation. These characteristics suggest that it achieves normal skeletal function in the points of biomechanical properties in vivo.     

羟基磷灰石基人工骨的研究进展

目的 研究羟基磷灰石基人工骨作为最有发展前途的生物硬组织替代材料之一，在生物医用材料和医学研究领域的应用。方法制备具有良好生物相容性、力学性能、骨诱导性与治疗功能的羟基磷灰石基人工骨，用于人体骨组织的缺失的修复，及相关疾病的治疗。结果通过制备性能优良的羟基磷灰石粉体，结合纳米技术、组织工程技术和生物技术将不同材料通过合适的方法组合成羟摹磷灰石人工骨，从而获得良好的临床应用性能。结论从临床角度，综合了国内外近十年相关文献的基础上，评述了羟基磷灰石人工骨的研究进展。     

Interface between hydroxyapatite and mandibular human bone tissue.

Samples from intraosseous dental implants, removed from patients for mechanical failures, were examined to analyse the interaction between hydroxyapatite as plasma sprayed coating on titanium supports and human bone. The implantation time varied up to 8 years. No failures had arisen from problems at the interface between the hydroxyapatite coating and bone. The number of samples examined and the implantation times give good statistical conclusions. Histological and microchemical studies showed the good performance and compatibility of this sprayed hydroxyapatite. We present evidence from the best samples which show close bonding with the surrounding bone tissue. New bone is seen all around the coated implant. The composition of the calcium phosphate deposited on the hydroxyapatite and cellular approach were determined, and demonstrate the efficiency of the interaction between this plasma sprayed hydroxyapatite and the bone.     

羟基磷灰石基人工骨的研究进展

目的研究羟基磷灰石基人工骨作为最有发展前途的生物硬组织替代材料之一,在生物医用材料和医学研究领域的应用.方法制备具有良好生物相容性、力学性能、骨诱导性与治疗功能的羟基磷灰石基人工骨,用于人体骨组织的缺失的修复,及相关疾病的治疗.结果通过制备性能优良的羟基磷灰石粉体,结合纳米技术、组织工程技术和生物技术将不同材料通过合适的方法组合成羟基磷灰石人工骨,从而获得良好的临床应用性能.结论从临床角度,综合了国内外近十年相关文献的基础上,评述了羟基磷灰石人工骨的研究进展.     

Evaluation of porous collagen membrane in guided tissue regeneration

Porous collagen membrane was prepared with collagen protein, which was extracted from bovine tendon by enzyme digestion, by freeze-drying method. The animal, clinical experiments of the membrane used in artificial dental implant system were studied. In the animal experiments, pure titanium spiral implants, which were prepared according to Adell etc. method, were implanted in the mandibular dental alveus of adult hybrid dog and covered with collagen membrane. Then the animals were killed after 4, 18 weeks individually. In the clinical research, the implants (phi 3.3 mm) were used in 33 patients of different age groups. The implant was put on the buccal lateral deficiency of implantation cavity wall, and covered with collagen membrane on the buccal lateral, then observed after 3, 6 months individually. The results of animal experiments proved the collagen membrane could guide osseous tissue regeneration around the bone integral implant which was implanted in the fresh tooth extraction fossa, be helpful to repair the fissural bone deficiency produced when implanted the implant, increase the bone content around the implant significantly, and improve the structure of new bone to a certain extent. The results of clinical research proved that collagen membrane was used in the patients with density deficiency, irregularity of alveolar ridge, or artificial dental of shorter tooth extraction, could significantly improved the bone density of artificial implant's shoulder.     

Keratin-hydroxyapatite composites: biocompatibility, osseointegration, and physical properties in an ovine model

Reconstituted keratin has shown promise as an orthopaedic biomaterial. This in vivo study investigates the biological response of composite materials prepared from reconstituted keratin containing a high content of hydroxyapatite (HA) (40 wt % HA), implanted for up to 18 weeks in the long bones of sheep. Keratin-HA composites were compared with a commercially available polylactic acid (PLA) HA composite (BIO RCI HA?, Smith and Nephew). Porous keratin-HA materials displayed excellent biocompatibility and osseointegration, with full integration into bone by 12 weeks. Dense keratin-HA materials also showed excellent biocompatibility, with a more limited osseointegration, involving the penetration of new bone into the periphery of the implant after eight weeks. In contrast, the PLA-HA implant did not integrate with surrounding tissue. Microindentation showed that porous keratin-HA implants were initially soft, but became stiffer as new bone penetrated the implant from four weeks onwards. In contrast, although the initial rigidity of dense keratin-HA composites was maintained for at least two weeks, the implant material weakened after four weeks. The PLA-HA implant maintained its physical properties throughout the course of the trial. This study demonstrates the increased osseointegration/osteoconduction capacity of keratin-HA composites and provides further evidence supporting the suitability of keratin-based materials, such as bone graft substitutes and soft tissue fixation devices.     

A novel nano-porous alumina biomaterial with potential for loading with bioactive materials

Nano-porous alumina, with the potential for being loaded with bioactive materials, has been proposed as a novel material for coating implants. In this study, the shear strength of the interface between such nano-porous anodic aluminium oxide (AAO) coatings and titanium substrates, their biocompatibility, and their potential for pore loading have been investigated. An interface shear strength in excess of 29 MPa was obtained which is comparable with that of conventional plasma sprayed hydroxyapatite implant coatings. The viability and differentiation of MG63 osteoblastic cells co-cultured on the coating was found to be broadly comparable to that of similar cells co-cultured on conventional bioinert implant materials such as titanium and fully dense alumina. Extensive pore loading with silica nano-particles of different sizes and in different combinations was demonstrated throughout the thickness of AAO layers 1 microm and 60 microm thick. This work has demonstrated, that with suitable choice of pore filling materials, this novel coating might simultaneously combat infection, encourage bone regeneration, and secure fixation of the implant to bone.     

Integration of metallic endoprotheses in dog femur studied by near-infrared Fourier-transform Raman microscopy.

The integration of hydroxyapatite-coated implants in dog femur was studied by near-infrared Fourier-transform Raman microscopy. Raman spectra were taken in lateral scans in step widths of 10-40 microm from the implant surface up to a distance of 320 microm into the bone tissue. The spectra were subjected to a component analysis for the quantitative determination of the protein and the inorganic components. This quantitative analysis is shown to be more reliable than conventional band fitting procedures and allows, for the first time, the quantitative distinction between the hydroxyapatite form of mature bone tissue and synthetic hydroxyapatite introduced by the implant coating. It is demonstrated that full mineralization of the ongrowing bone is not achieved after 6 months. In contrast, after a residence time of 18 months in the body, the Raman spectra reveal a complete calcification of the new bone tissue as indicated by content of biological hydroxyapatite that is the same as in mature bone tissue throughout the whole implant/bone interface. On the other hand, the content of synthetic hydroxyapatite is strongly reduced in the sample prepared after eighteen months implantation whereas for the shorter implantation time. substantial contributions of synthetic hydroxyapatite are found even at positions beyond the thickness of the implant coating. These results indicate that the coating material is actively involved in the mineralization of ongrowing bone. Possible mechanisms for the underlying transport processes in the implant/bone interface are discussed.     

An evaluation of variables influencing implant fixation by direct bone apposition

A systematic mechanical and histologic evaluation of design variables affecting bone apposition to various biocompatible materials was undertaken. The variables investigated included material elastic modulus, material surface texture, as well as material surface composition. The implant materials included polymethylmethacrylate (PMMA), low-temperature isotropic (LTI) pyrolytic carbon, commercially pure (C.P.) titanium, and aluminum oxide (Al2O3). Implant surface texture was varied by either polishing or grit-blasting the various materials. Implant surface composition was varied by applying a coating of ultra-low temperature isotropic (ULTI) pyrolytic carbon to the various implants. A total of 12 types of implants were evaluated in vivo by placement transcortically in the femora of adult mongrel dogs for a period of 32 weeks. Following sacrifice, mechanical push-out testing was performed to determine interface shear strength and interface shear stiffness. The results obtained from mechanical testing indicate that for implants fixed by direct bone apposition, interface stiffness and interface shear strength are not significantly affected by either implant elastic modulus or implant surface composition. Varying surface texture, however, significantly affected the interface response to the implants. For each elastic modulus group the roughened surfaced implants exhibited greater strengths than the corresponding smooth surfaced implants. Undecalcified histologic evaluation of the implants demonstrated that the roughened implants exhibited direct bone apposition, whereas the smooth implants exhibited various degrees of fibrous tissue encasement. Thus, for implants utilizing direct bone apposition fixation, it appears that of the parameters investigated, implant surface texture is the most significant.     

Development of a new system for evaluating the biocompatibility of implant materials using an osteogenic cell line (MC3T3-E1)

A new culture system was developed to clarify the biocompatibility of implant materials with bone tissue using the MC3T3-E1 osteogenic cell line. The cells were inoculated onto specimens such as aluminium oxide, titanium, dental casting silver-palladium alloy (PD), and a plastic coverslip. To study the effects of these materials on cell growth, differentiation, and calcification, DNA and protein content, alkaline phosphatase activity, and calcium content, respectively, were determined. The results from biochemical analysis suggest titanium and aluminum oxide to have adequate biocompatibility, while PD has an irritant effect on cell metabolism. It is clear that an objective view of the differentiation and calcification processes of osteogenic cells can be understood through such analysis. From the results of this study, our culture system appears suitable for evaluating the biocompatibility of implant materials with bone tissue.