Stress analysis in bone tissue around single implants with different diameters and veneering materials: A 3-D finite element study

The aim of this study was to evaluate the stress distribution on bone tissue with a single prosthesis supported by implants of large and conventional diameter and presenting different veneering materials using the 3-D finite element method. Sixteen models were fabricated to reproduce a bone block with implants, using two diameters (3.75 × 10 mm and 5.00 × 10 mm), four different veneering materials (composite resin, acrylic resin, porcelain, and NiCr crown), and two loads (axial (200 N) and oblique (100 N)). For data analysis, the maximum principal stress and von Mises criterion were used. For the axial load, the cortical bone in all models did not exhibit significant differences, and the trabecular bone presented higher tensile stress with reduced implant diameter. For the oblique load, the cortical bone presented a significant increase in tensile stress on the same side as the loading for smaller implant diameters. The trabecular bone showed a similar but more discreet trend. There was no difference in bone tissue with different veneering materials. The veneering material did not influence the stress distribution in the supporting tissues of single implant-supported prostheses. The large-diameter implants improved the transference of occlusal loads to bone tissue and decreased stress mainly under oblique loads. Oblique loading was more detrimental to distribution stresses than axial loading.     

A numerical investigation of porous titanium as orthopedic implant material

Porous titanium is being developed as an alternative orthopedic implant material to alleviate the inherent problems of bulk metallic implants by reducing the stiffness to be comparable to bone stiffness and allowing complete bone ingrowth. However, a porous microstructure is susceptible to local permanent plastic strain and residual stress under cyclic loading which reduces damage tolerance and therefore limits their application as orthopedic implants. The mechanical properties of porous titanium are governed by the microstructural configurations such as pore morphology, porosity, and bone ingrowth. To understand the influence of these features on performance, the macroscopic and microscopic responses of porous Ti are studied using three-dimensional finite element models. The models are generated based on simulated microstructures of experimental materials at porosities of 15%, 32% and 50%. The results show the effect of porosity and bone ingrowth on Young’s modulus, yield stress, and microscopic stress and strain distribution. Importantly, simulations predict that the bone ingrowth reduces the stress and strain localization under cyclic loading so significantly that it counteracts the concentration condition caused by the increased porosity of the structure.     

The effect of bead diameter on the accuracy of two current techniques used to quantify bone ingrowth in porous-coated implants

The effect of the bead diameter on the accuracy of two techniques used in bone ingrowth quantification, microradiography and backscattered electron imaging-scanning electron microscopy (BEI-SEM), was assessed using porous-coated implants. Two groups of seven titanium porous implants (group A: bead size 250–350 m and group B: 500–700 m) were implanted for 12 weeks in a canine model. After euthanasia, the same histological slides were prepared for microradiography and BEI-SEM. The percentage of bone, bone ingrowth, bone ongrowth, porosity and bone index were determined by a point counting method using images from both techniques. ANOVA and Tukey's test were used to compare the results from the different bead sizes and techniques. The results showed significant higher bone ingrowth in microradiography groups, and significant lower porosity in only the fine-bead microradiography group (group A size). Microradiography also obtained significantly higher bone ongrowth, but only for the coarse bead size group (group B). From these results it was concluded that microradiography decreases the porosity of the porous coating compared with BEI-SEM. This effect seems to be dependent on the bead diameter. The smaller the diameter, the greater the effect. Furthermore, microradiography increases bone ingrowth which seems to be affected independently of the bead diameter, becoming the most sensitive parameter to increase.     

The effects of loading conditions and specimen environment on the nanomechanical response of canine cortical bone

Bone is a viscoelastic connective tissue composed primarily of mineral and type I collagen, which interacts with water, affecting its mechanical properties. Therefore, both the level of hydration and the loading rate are expected to influence the measured nanomechanical response of bone. In this study, we investigated the influence of three distinct hydration conditions, peak loads and loading/unloading rates on the elastic modulus and hardness of canine femoral cortical bone via nanoindentation. Sections from three canine femurs from multiple regions of the diaphysis were tested for a total of 670 indentations. All three hydration conditions (dry, moist and fully hydrated tissue) were tested at three different loading profiles (a triangular loading profile with peak loads of 600, 800 and 1000 μN at loading/unloading rate of 60, 80 and 100 μN/s, respectively; each test was 20 s in duration). Significant differences were found for both the elastic modulus and hardness between the dry, moist and fully hydrated conditions (p ≤ 0.02). For dry bone, elastic modulus and hardness values were not found to be significantly different between the different loading profiles (p > 0.05). However, in both the moist and fully hydrated conditions, the elastic modulus and hardness were significantly different under all loading profiles (with the exception of the moist condition at the 600- and 800-μN peak load). Given these findings, it is critical to perform nanoindentation of bone under fully hydrated conditions to ensure physiologically relevant results. Furthermore, this work found that a 20-s triangular loading/unloading profile was sufficient to capture the viscoelastic behavior of bone in the 600- to 1000-μN peak load range. Lastly, specific peak load values and loading rates need to be selected based on the structural region for which the mechanical properties are to be measured.     

Structural differences in the cortical bone of Turkey tibia

Different regions namely anterior, posterior, medial and lateral, of the cortical bone in six sections in the diaphysis from proximal to the distal end of a Turkey tibia were evaluated for structural differences. The predominant feature, Haversian canals showed high area fraction (can be viewed as high porosity) in the posterior region as compared to the anterior region and this difference is attributed to the high compressive and tensile stress states in the respective regions. With increasing size, the Haversian canals deviated from circular shapes in cross section. The distance from the lacunae to the center of the Haversian canal showed an increase with the size of the canals. However, the canal to lacunae-canal distance ratio is independent of the anterior and posterior regions. It was found that majority of the lacunae are placed at a distance of about 2.5 to 3 times the Haversian canal radius. At the ultrastructural level, periodic banding of the collagen fibers showed a bimodal banding in the anterior region while only one finer banding distribution in the posterior region and could probably the result of the different stress states in the two regions. The Haversian canals in the three dimensional reconstruction showed branching, twisted canals and Haversian space and resemble Cohen and Harris model. Differences in stress in different regions cause structural changes in bone.     

Finite element analysis of bone loss around failing implants

Dental implants induce diverse forces on their surrounding bone. However, when excessive unphysiological forces are applied, resorption of the neighbouring bone may occur. The aim of this study was to assess possible causes of bone loss around failing dental implants using finite element analysis. A further aim was to assess the implications of progressive bone loss on the strains induced by dental implants. Between 2003 and 2009 a total of 3700 implant operations were performed in a private clinic. Ten patients with 16 fixtures developed severe marginal bone defects. Finite element analysis was used to assess the effective strains produced at the bone-implant interface under unidirectional axial loading. These simulations were carried out on 4 specific implant types – Camlog Plus, Astra Osseo Speed, Straumann BL and Straumann S/SP. All implant types exhibited degraded performance under circular and horizontal bone loss conditions. This is evidenced by increased distribution of pathological strain intensities (>3000 με), in accordance with the mechanostat hypothesis, in the surrounding bone. Among the implants, the Camlog design seemed to have performed poorly, especially at the chamfer in the implant collar (>25000 με). Implants are designed to perform under nearly ideal conditions from insertion till osseointegration. However, when the surrounding bone undergoes remodelling, implant geometries can have varied performance, which in some cases can exacerbate bone loss. The results of this study indicate the importance of evaluating implant geometries under clinically observed conditions of progressive bone loss.     

Filling of bone defects with porous hydroxyapatite reinforced with polylactide or polyglycolide fibres

Hydroxyapatite has intrinsically poor mechanical resistance for loading bone replacements and its clinical applications are mainly limited to use as a filling device. In the present study, hydroxyapatite blocks were reinforced with resorbable polyglycolide (PGA) or polylactide (PDLLA) fibres so that most of the implant pores remained open assuming intimate contact with the host bone. The reinforced blocks, 2×3×4 mm in size (Interpore 200), were implanted into the proximal and diaphyseal tibiae of rabbits in order to study the tissue and bone ingrowth into the implants. The samples were studied by histological, histomorphometrical, microradiological, and oxytetracycline fluorescence analyses. The results suggested that PGA or PDLLA fibre reinforcement does not hinder bony ingrowth into the hydroxyapatite implant. Maximal bone ingrowth was observed at 6 weeks but thereafter the amount of ingrowth remained constant up to the end of the 24-week follow-up period. Modest foreign body type reactions around the fibres were histologically seen and there was no difference between the two types fibres in relation to the bone ingrowth. With implants used in this study the bone ingrowth as measured with histomorphometry was 12.9±1.4% in the cancellous implantation and 17.1±1.5% in the cortical implantation. It seems that fibre reinforcement does not hinder bone ingrowth into the coralline hydroxyapatite implants and supports their further development as bone graft substitute in high loading conditions.     

Strain energy density distribution of vertebral body of two motion segment model under combined compression and sagittal bending moment – an in vitro porcine spine biomechanical study

Abstract

The purpose of the current study is to find the strain energy density (SED) distribution of a vertebral body during different compression loadings, combined with sagittal bending moments. The combined flexion and extension, which are generated by applying an eccentric pointed loading on the motion segment, is to mimic different postures of trunk and loading on the spine. Two strain gage rosettes were applied at an anterior site and a posterior site of a vertebral body. The total SED, deviatoric SED and dilatation SED were obtained from the measurements of the two rosettes. Three major phenomena are observed in the current study; first, the anterior site on the vertebra is at higher risk compared to the posterior site on the vertebra when the motion segment is in compression combined with extreme flexion and extension. Second, the SED is minimal when the loading is applied along the trajectories of the spinal canal and joint facets. Third, the major contribution to SED is from the deviatoric SED. The distribution of SED within the vertebral body during different loading conditions can serve as the baseline for treatment to protect the vertebral body from the risk of compression fracture.     

An animal study on the bone behaviour of Ca–P-coated implants: influence of implant location

Four different implant materials were installed into the mandibular corner of goats to investigate the trabecular bone response in a mainly unloaded model. The implants were installed using a standardized technique and were left in situ for 12 weeks. One goat had to be sacrificed after surgery because of a broken rib; the other animals healed uneventfully. After sacrifice of the animals, the bone response to the uncoated and the three different Ca–P implants was evaluated histologically and histomorphometrically. Four sections of each implant were evaluated; two were located in the cortical and two in the trabecular bone. Of the 44 retrieved implants, 20 implants appeared to be installed partially in the mandibular canal, as clearly visible on the X-rays. These implants were not used in the histomorphometrical measurements. Histological evaluation showed that the trabecular and cortical bone reactions were similar; there was no significant difference in the percentage of bone contact nor in the amount of bone in contact with the implants. In conclusion this study showed that the mandibular corner is an unsatisfactory model for the installation of implants because of anatomical restrictions. Also, the experiment remained inconclusive about the influence of loading conditions on bone behaviour. Nevertheless, the histological results confirmed the bioactive properties of Ca–P coatings.     

Induced tissue integration of bone implants by coating with bone selective RGD-peptides in vitro and in vivo studies

The optimal function of medical implant materials used in tissue substitution is often limited due to its healing properties. This effect is linked to reduced interactions of the implants with the surrounding tissue. Implant surfaces biologically functionalized with arginine-glycine-aspartic acid (RGD) peptides, a class of cellular adhesion factors, are described in this paper. The RGD-peptides are either bound via bovine serum albumin linking on culture plastic dishes as a model surface or via acrylic acid coupling on PMMA surface as a potential implant material. Resulting functionalized surfaces aquire the capability to bind cultured osteoblasts in high levels and show high proliferation rates in vitro. These results are observed for osteoblast cultures as well as from different species with different preparation procedures. A critical minimum distance between the bioactive portion of the RGD-peptides and the implant surface of 3.0–3.5 nm is crucial for the induction of an optimum cell binding process. In vivo animal studies in the rabbit show that newly formed bone tissue generated a direct contact with the RGD-peptide coated implants. In contrast uncoated implants are separated from newly formed bone tissue by a fibrous tissue layer thereby preventing the formation of a direct implant–bone bonding.     

Enhanced osteointegration of orthopaedic implant gradient coating composed of bioactive glass and nanohydroxyapatite

We conducted histologic and histomorphometric studies to evaluate the osteointegration of gradient coatings composed of bioactive glass and nanohydroxyapatite (BG–nHA) on titanium-alloy orthopaedic implants and surrounding bone tissue in vivo. Titanium-alloy implants with a gradient coating (gradient coating group), uncoated implants (uncoated group), and implants with a conventional hydroxyapatite (HA) coating (HA coating group) were randomly implanted in bilateral femoral condyles of 36 male New Zealand rabbits. The bone–implant contact at 12 and 24 weeks and the new bone volume in the notch created for observing bone ingrowth at 4, 12, and 24 weeks were found greater in the gradient coating group than those in both the uncoated group and the HA coating group (p < 0.05). Fluorescence micrographs showed active osteogenesis in the gradient coating group at 4 weeks after implantation. These findings indicated that BG–nHA gradient coatings could enhance the osteointegration of orthopaedic implant.     

Microfracturing characteristics in brittle material containing structural defects under biaxial loading

The purpose of this study is to specify the influence of structural defects and confining pressure on microfracturing characteristics in rocklike brittle materials. The stress field and microfracturing events of rock samples containing structural defects under biaxial loading are simulated by the finite element software, Rock Failure Progress Analysis (RFPA^2D). The simulated rock samples are compressed by an increasing vertical displacement of 0.001 mm/step and confined with different constant horizontal lateral pressures 0, 5, 10 and 15 MPa, respectively. The characteristics of microfracturing events with stress field evolution in rock failure process are visually represented by damaged elements. The results show that rock failure with great structural effects is greatly dependent on the arrangement of structural defects. As the outer loading increases, the stresses in rock mass are built up gradually. When the stress of an element reaches a certain critical value, a microfracturing event occurs. Two neighboring damaged elements owning one same edge are considered in the same damaged element group. Based on the newly-developed statistic function of RFPA^2D on damaged element group, the scales (element number in a group) and counts of damaged groups (micro cracks) are recorded. The distribution of microfracturing events for all rock samples under different lateral pressures represents self-similar fractal features. However, rescaled range analysis indicates that microfracturing events do not exhibit the similar scale-invariant property strictly under higher lateral pressures.     

Bioactive glass granules and polytetrafluoroethylene membrane in the repair of bone defects adjacent to titanium and bioactive glass implants

An experimental animal model was used to investigate the effect of bioactive glass (BG) granules and nonresorbable polytetrafluoroethylene (PTFE) membrane on the repair of cortical bone defects adjacent to titanium and BG implants. Thirty-two Astra® (diameter 3.5 mm) dental implants were inserted bicortically and 42 conical BG implants (diameter 2.5–3.0 mm) monocortically, into fitted holes of rabbit tibia. Before implantation, a standardized bone defect was created by drilling an extra hole (diameter 3.0 mm) adjacent to each implant site. Twenty-eight defects were filled with BG granules (diameter 630–800 m) (BG group) and 28 defects were left empty but covered with PTFE membrane (PTFE group). No material was used in 18 control defects (control group). Morphometrical evaluation with a digital image analysis system was used to measure bone repair as percentages of the defect area on scanning electron microscopy (SEM) and light microscopy pictures. Bone–implant contact was measured as percentages of the thickness of the cortical bone. At 6 and 12 wk, bone repair in defects in connection with titanium implants was 23.2% and 36.6% in the BG group, 23.2% and 32.4% in the PTFE group, and 47.2% and 46.2% in control defects. Corresponding figures for BG implants were 33.2% and 40.1% in the BG group, 16.6% and 33.5% in the PTFE group, and 25.7% and 54.9% in control defects, BG granules and new bone together filled 82.7% and 68.5% of the defect area adjacent to titanium implants, and 75.9% and 74.4% of the defect adjacent to BG implants at 6 and 12 wk, respectively. Better bone–implant contact was achieved at the defect side with BG than titanium implants (77.0% versus 45.0% at 12 wk). The results indicate that BG granules are useful in treatment of bone defects adjacent to dental implants. BG coating of the implant seems to improve osseointegration in the defect area.     

Bone tissue response to titanium implant surfaces modified with carboxylate and sulfonate groups

The present study assessed in vivo new bone formation around titanium alloy implants chemically grafted with macromolecules bearing ionic sulfonate and/or carboxylate groups. Unmodified and grafted Ti–6Al–4V exhibiting either 100% carboxylate, or 100% sulfonate, or both carboxylate and sulfonate groups in the percent of 50/50 and 80/20 were bilaterally implanted into rabbit femoral condyle. Neither toxicity nor inflammation were observed for all implants tested. After 4 weeks, peri-implant new bone formation varied as a function of the chemical composition of the titanium surfaces. The percent bone-implant contact (BIC) was the lowest (13.4 ± 6.3%) for the implants modified with grafted carboxylate only. The value of BIC on the implants with 20% sulfonate (24.6 ± 5.2%) was significantly (P < 0.05) lower than that observed on 100% sulfonate (38.2 ± 13.2%) surfaces. After both 4 and 12 weeks post-implantation, the BIC value for implants with more than 50% sulfonate was similar to that obtained with the unmodified Ti–6Al–4V. The grafted titanium alloy exhibiting either 100% sulfonate or carboxylate and sulfonate (50% each) groups promoted bone formation. Such materials are of clinical interest because, they do not promote bacteria adhesion but, they support new bone formation, a condition which can lead to osseointegration of bone implants while preventing peri-implant infections.     

Nanoscale characterization of bone–implant interface and biomechanical modulation of bone ingrowth

Bone–implant interface is characterized by an array of cells and macromolecules. This study investigated the nanomechancial properties of bone–implant interface using atomic force microscopy in vitro, and the mechanical modulation of implant bone ingrowth in vivo using bone histomorphometry. Upon harvest of screw-type titanium implants placed in vivo in the rabbit maxilla and proximal femur for 4 weeks, nanoindentation was performed in the bone–implant interface at 60-μm intervals radially from the implant surface. The average Young's Moduli (E) of the maxillary bone–implant interface was 1.13 ± 0.27 MPa, lacking significant differences at all intervals. In contrast, an increasing gradient of E was observed radially from the femur bone–implant interface: 0.87 ± 0.25 MPa to 2.24 ± 0.69 MPa, representing significant differences among several 60-μm intervals. In a separate experiment, bone healing was allowed for 6 weeks for proximal femur implants. The right femoral implant received axial cyclic loading at 200 mN and 1 Hz for 10 min/d over 12 days, whereas the left femoral implant served as control. Cyclic loading induced significantly higher bone volume, osteoblast numbers per endocortical bone surface, mineral apposition rate, and bone formation rate than controls. These data demonstrate nanoscale and microscale characterizations of bone–implant interface, and mechanical modulation of bone ingrowth surrounding titanium implants.     

Preparation of highly concentrated aqueous hydroxyapatite suspensions for slip casting

This paper reports the preparation of highly concentrated aqueous hydroxyapatite (HA) suspensions for slip casting of dense bone implants. The dispersing behaviour of HA powders in aqueous media was monitored by viscosity and zeta potential analyses as a function of pH of the slurry. The rheological properties of concentrated aqueous hydroxyapatite suspensions have been characterized with varying pH, NH₄PAA concentration and solids loading. The intrinsic pH of the suspension was found suitable for slip casting. The optimum dispersant concentration is 0.75 wt.% for 75 wt.% solid loading. A stable suspension with 75 wt.% solid was suitable for slip casting with viscosity of 0.36 Pa s at 100 s⁻¹. Finally, crack-free and dense microstructures have been obtained successfully with a grain size of 2–5 μm.     

The effect of drilling parameters on bone

For orthopaedic biomaterial implantation testing, specimens are often implanted into cortical bone defects. The implantation site is assumed to be one of the factors that influence the bone response to biomaterials. The aim of this study was to investigate the bone-healing process in drilled cortical defects at different sites with respect to time. Sheep metatarsus was implemented, since it is a long straight bone with four flat faces. Thus, the different drilling sites were obtained by changing the longitudinal level (proximal, middle and distal) and bone aspect (anterior, lateral and posterior). Metatarsi were obtained at 1, 2, 3 and 4 months post-operatively and with non-decalcified sections the newly formed bone area was measured using a microscope connected to an image analyser. The rate of bone formation was higher in the anterior aspect (P<0.05). The new bone did not form concentrically from the hole edge towards the centre, and the principal direction of bone growth was different between the anterior and the posterior aspects (P<0.05). However, there was no difference with respect to the longitudinal axis. These results indicate that the implantation site must be considered when analysing the bone response to biomaterials implanted in cortical defects.     

No evidence to indicate topographic dependency on bone formation around cp titanium implants under masticatory loading

In vitro studies have proved the topographic dependency upon osteogenesis on titanium plate by investigating the cell-adhesion, -shape, -proliferation, -differentiation, ALP activity and osteocalcin production of osteogenic stem cells, MG36, MC3T3-E1 and wild strains of bone formative cells from animal and human. However, this in vivo study on bone growth around cp titanium dental implants under masticatory loading did not demonstrate significant difference among the different surface roughness in the range of Ra 0.4–1.9 μm, Rz 2.8–11.2 μm, Rmax 3.6–28.1 μm and Sm 2.9–41.0 μm, which was estimated by measuring the bone contacts, bone occupancies and bone bonding strengths at the implant/bone marrow interface. It is revealed that the topographic dependency on the osteogenetic activity is apt to be covered with wide variation in bone healing potential under the clinical condition with functional biting load.     

Bone remodeling adjacent to total hip replacements: A naturally occurring material design problem

Summary The reaction of bone to orthopedic implants is an example of a self-adjusting material which changes from a normal state to an altered state, based on the mechanical features of the implant and the loads applied to it. The changes in bone around cemented and uncemented femoral total hip components are well documented, and many numerical characterizations of the material reaction to stress have attempted to mimic the natural remodeling process. In this study we review the development of a simple material remodeling rule which yields a stable structure which is optimal and which allows a unique solution. We then use this algorithm to assess the effect of prosthesis stiffness and the presence of a compliant layer on bone remodeling around these implants. An axisymmetric model for axial loading is used to model changes in bone density through the thickness of the cancellous bone around the implants. With cortical remodeling left out of the simulation, the simulations showed density distributions that agreed in general with the results in the literature, and showed a marked difference in response if a compliant layer was added to the prosthesis.     

Tissue response to a braided poly-L-lactide implant in an experimental reconstruction of anterior cruciate ligament

During follow-up periods of 6, 12, 24 and 48 weeks, the tissue response to a braided poly-L-lactide (PLLA) implant, 3.2 mm in diameter, was investigated in the reconstruction of experimental anterior cruciate ligament (ACL) ruptures in 32 sheep. In 16 sheep the cut ACL was removed and reconstructed with the fascia lata augmented with a PLLA implant. In 16 sheep the ACL was cut from its midportion, sutured, and thereafter augmented with a PLLA implant. The tissue reactions were typical of a scant non-specific-foreign-body reaction. The number of inflammatory and giant cells was greatest at six weeks, decreasing thereafter. Degradation of the PLLA was incomplete at 48 weeks. No signs of synovitis or changes in the cartilaginous surfaces were observed. The reconstructions in both groups were anchored to the bone by fibroconnective tissue, and remodelling of the bone was seen along the drill channels. After 48 weeks the maturation of the fibroconnective tissue and the orientation of the collagen fibres were higher (p<0.01) in the fascia-lata-PLLA group than in the primarysuture-PLLA group. Histologically, the braided PLLA implants proved to be suitable for ACL repair in sheep. The augmentation of the fascia lata with the PLLA implant seemed to be preferable to that of the primary suture of the ACL.