Mechanical regulation of localized and appositional bone formation around bone-interfacing implants

Bone Ingrowth Around an Uncemented Femoral Implant Using Mechanoregulatory Algorithm: A Multiscale Finite Element Analysis

The primary fixation and long-term stability of a cementless femoral implant depend on bone ingrowth within the porous coating. Although attempts were made to quantify the peri-implant bone ingrowth using the finite element (FE) analysis and mechanoregulatory principles, the tissue differentiation patterns on a porous-coated hip stem have scarcely been investigated. The objective of this study is to predict the spatial distribution of evolutionary bone ingrowth around an uncemented hip stem, using a three-dimensional (3D) multiscale mechanobiology-based numerical framework. Multiple load cases representing a variety of daily living activities, including walking, stair climbing, sitting down, and standing up from a chair, were used as applied loading conditions. The study accounted for the local variations in host bone material properties and implant-bone relative displacements of the macroscale implanted FE model, in order to predict bone ingrowth in microscale representative volume elements (RVEs) of 12 interfacial regions. In majority RVEs, 20-70% bone tissue (immature and mature) was predicted after 2 months, contributing toward a progressive increase in average Young's modulus (1200-3000 MPa) of the interbead tissue layer. Higher bone ingrowth (mostly greater than 60%) was predicted in the anterolateral regions of the implant, as compared to the posteromedial side (20-50%). New bone tissue was formed deeper inside the interbead spacing, adhering to the implant surface. The study helps to gain an insight into the degree of osseointegration of a porous-coated femoral implant.

Osteogenesis Modulation: Induction of Mandibular Bone Growth in Adults by Electrical Field for Aesthetic Purposes

BACKGROUND

A new technique in plastic surgery termed Osteogenesis Modulation is described. This technique uses a surgically implanted, battery-operated medical device to deliver customized electrical pulses to produce mandibular bone growth. This device was designed to be a temporary, nonpermanent implant. The purpose of this study was to review both the safety and efficacy of Osteogenesis Modulation.

METHODS

This study comprises two phases. Phase I involved experimental technology development and animal experiments. Phase II included technology development for clinical use and a clinical trial. In Phase II, four patients with a diagnosis of mandibular hypoplasia and microgenia underwent surgical implantation of the novel medical device over the chin bone. Once a satisfactory change of contour of mandibular bone was achieved, the devices were removed. In all patients, the devices were left in place for 12 months, then surgically removed under local anesthesia. Preoperative and long-term postoperative cephalometric controls were done.

RESULTS

In all patients, symmetrical mandibular bone growth was observed with good-to-excellent aesthetic results. The overall follow-up period was 39 months. Cephalometric controls taken 3 to 6 months after the device removal showed an average increase in mandible length of 5.26mm (range, 2.83-7.60mm) CONCLUSIONS: Preliminary clinical results suggest that Osteogenesis Modulation is a safe, minimally invasive, and effective alternative treatment for the correction of mandibular hypoplasia in selected cases.

LEVEL OF EVIDENCE IV

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Collagen Fibres Orientation in the Bone Matrix around Dental Implants: Does the Implant's Thread Design Play a Role?

The aim of this study was to analyse the influence of different thread shapes of titanium dental implant on the bone collagen fibre orientation (BCFO) around loaded implants. Twenty titanium dental implants, divided for thread shapes in six groups (A–F) were analysed in the present study. All implants were immediately loaded and left in function for 6 months before retrieval. The parameters evaluated under scanning electron microscope were the thread width, thread depth, top radius of curvature, flank angle, and the inter-thread straight section. Two undecalcified histological sections were prepared from each implant. Birefringence analysis using circularly polarized light microscopy was used to quantitively measure BCFO. For groups A–F, respectively, transverse BCFO was 32.7%, 24.1%, 22.3%, 18.2%, 32.4%, and 21.2%, longitudinal BCFO was 28.2%, 14.5%, 44.9%, 33.1%, 37.7%, and 40.2%. The percentage differences between transverse and longitudinal orientation were 4.50% (A), 9.60% (B), −22.60% (C), −14.90% (D), −5.30% (E), and −19.00% (F). Following loading, the amount of transverse and longitudinal BCFO were significantly influenced by the thread shape. The greater flank angles and narrower inter-thread sections of the “V” shaped and “concave” shaped implant threads of groups A and B, respectively, promoted the predominance of transverse BCFO, compared to groups C-F (p < 0.05). A narrow inter-thread straight section promotes transverse BCFO, as do “V” shaped and “concave” shaped threads, which can thus be considered desirable design for implant threads.

The limit of tolerable micromotion for implant osseointegration: a systematic review

Much research effort is being invested into the development of porous biomaterials that enhance implant osseointegration. Large micromotions at the bone-implant interface impair this osseointegration process, resulting in fibrous capsule formation and implant loosening. This systematic review compiled all the in vivo evidence available to establish if there is a universal limit of tolerable micromotion for implant osseointegration. The protocol was registered with the International Prospective Register for Systematic Reviews (ID: CRD42020196686). Pubmed, Scopus and Web of Knowledge databases were searched for studies containing terms relating to micromotion and osseointegration. The mean value of micromotion for implants that osseointegrated was 32% of the mean value for those that did not (112 ± 176 µm versus 349 ± 231 µm, p < 0.001). However, there was a large overlap in the data ranges with no universal limit apparent. Rather, many factors were found to combine to affect the overall outcome including loading time, the type of implant and the material being used. The tables provided in this review summarise these factors and will aid investigators in identifying the most relevant micromotion values for their biomaterial and implant development research.

The influence of macro-textural designs over implant surface on bone on-growth: A computational mechanobiology based study

The longerterm secondary stability of an uncemented implant depends primarily on the quality and extent of bone in-growth or on-growth at the bone-implant interface. Investigations are warranted to predict the influences of implant macro-textures on bone on-growth pattern. Mechanoregulatory tissue differentiation algorithms can predict such patterns effectively. There is, however, a dearth of volumetric in silico study to assess the influence of macro-textures on bone growth. The present study investigated the influence of macro-textural grooves/ribs on changes in tissue formation at the bone-implant interface by carrying out a 3D finite element (FE) analysis. Three distinct macro-textures, loosely based on commercially viable hip stem models, were comparatively assessed for varying levels of interfacial micromotion. The study predicted elevated fibrogenesis and chondrogenesis, followed by a suppressed osteogenesis for higher levels of micromotion (60 μm and 100 μm), resulting in weak bone-implant interface strength. However, small judicious modifications in implant surface texture may enhance bone growth to a considerable extent. The numerical scheme can further be used as a template for more rigorous parametric and multi-scale studies.

The impact of implant abutment surface treatment with TiO2 on peri-implant levels of angiogenesis and bone-related markers: a randomized clinical trial

The goal of this randomized, blinded, split-mouth controlled clinical trial was to assess the influence of abutment surface treatment on tissue healing. Fifteen patients received two implants distributed randomly to two groups: test (TiO₂ abutment surface), control (standard abutment surface). Levels of epidermal growth factor (EGF), bone morphogenetic protein 9 (BMP-9), endothelin 1 (ET-1), fibroblast growth factor (FGF), placental growth factor (PlGF), and vascular endothelial growth factor (VEGF) were quantified in the peri-implant fluid after 3, 14, 30, and 60 days. Inter-group comparisons indicated higher levels of EGF, BMP-9, ET-1, FGF, and PlGF in the test group after 30days (P<0.05). PlGF levels were also higher in the test group after 60 days. In the test group, intra-group analysis revealed different levels of ET-1 and FGF between days 3 and 30, and days 3 and 60 (P<0.05); furthermore, VEGF levels were significantly higher on day 60 than on day 3 (P <0.05). In the control group, intra-group analysis demonstrated significantly different levels of ET-1, FGF, and PlGF between days 3 and 60 and of PlGF between days 14 and 60 (P<0.05). In conclusion, abutment surfaces treated with TiO₂ influenced the levels of angiogenesis and bone-related markers.

Nanomechanical Assessment of Bone Surrounding Implants Loaded for 3 Years in a Canine Experimental Model

PURPOSE

This work evaluated the nanomechanical properties of bone surrounding submerged and immediately loaded implants after 3 years in vivo. It was hypothesized that the nanomechanical properties of bone would markedly increase in immediately and functionally loaded implants compared with submerged implants.

MATERIALS AND METHODS

The second, third, and fourth right premolars and the first molar of 10 adult Doberman dogs were extracted. After 6 months, 4 implants were placed in 1 side of the mandible. The mesial implant received a cover screw and remained unloaded. The remaining 3 implants received fixed dental prostheses within 48 hours after surgery that remained in occlusal function for 3 years. After sacrifice, the bone was prepared for histologic and nanoindentation analysis. Nanoindentation was carried out under wet conditions on bone areas within the plateaus. Indentations (n = 30 per histologic section) were performed with a maximum load of 300 μN (loading rate, 60 μN per second) followed by a holding and unloading time of 10 and 2 seconds, respectively. Elastic modulus (E) and hardness (H) were computed in giga-pascals. The amount of bone-to-implant contact (BIC) also was evaluated.

RESULTS

The E and H values for cortical bone regions were higher than those for trabecular bone regardless of load condition, but this difference was not statistically significant (P > .05). The E and H values were higher for loaded implants than for submerged implants (P < .05) for cortical and trabecular bone. For the same load condition, the E and H values for cortical and trabecular bone were not statistically different (P > .05). The loaded and submerged implants presented BIC values (mean ± standard deviation) of 57.4 ± 12.1% and 62 ± 7.5%, respectively (P > .05).

CONCLUSION

The E and H values of bone surrounding dental implants, measured by nanoindentation, were higher for immediately loaded than for submerged implants.

Wound healing in immediately loaded implants

The orthopedic field has accumulated ample evidence that bone formation is related to functional loading and in general to physical activity. However, despite evidence that immediately loaded implants can be predictably successful, many clinicians still use the classical (delayed loading) treatment protocol. This paper examines the effects of loading on dental implants and discusses the advantages of immediate loading. The role of loading on augmented alveolar ridges is also addressed and provides evidence that early bone resorption may be controlled when bone is functionally loaded. Similar data are emerging for advanced augmentation techniques in order to control crestal bone loss.

The feasibility of a custom-made endoprosthesis in mandibular reconstruction: Implant design and finite element analysis

This work studies the feasibility of custom-made endoprosthesis in the reconstruction of major mandibular defects. The natural anatomical and occlusal relations are used to accurately reconstruct a mandibular defect. The customized implant allows the accurate restoration of the facial profile and aesthetics. The biomechanical behaviour of mandibular endoprosthesis was validated with Finite Element Analysis for three masticatory tasks, namely incisal, right molar and left group clenching. The implanted mandible shows displacement fields and stress distributions very similar to the intact mandible. The strain fields observed along the bone-implant interface may promote bone maintenance and ingrowth. The preliminary results show that this implant may be a reliable alternative to other prosthetic mandibular reconstruction approaches.

Micromotion-induced strain fields influence early stages of repair at bone-implant interfaces

Implant loading can create micromotion at the bone-implant interface. The interfacial strain associated with implant micromotion could contribute to regulating the tissue healing response. Excessive micromotion can lead to fibrous encapsulation and implant loosening. Our objective was to characterize the influence of interfacial strain on bone regeneration around implants in mouse tibiae. A micromotion system was used to create strain under conditions of (1) no initial contact between implant and bone and (2) direct bone-implant contact. Pin- and screw-shaped implants were subjected to displacements of 150 or 300 μm for 60 cycles per day for 7 days. Pin-shaped implants placed in five animals were subjected to three sessions of 150 μm displacement per day, with 60 cycles per session. Control implants in both types of interfaces were stabilized throughout the healing period. Experimental strain analyses, microtomography, image-based displacement mapping, and finite element simulations were used to characterize interfacial strain fields. Calcified tissue sections were prepared and Goldner trichrome stained to evaluate the tissue reactions in higher and lower strain regions. In stable implants bone formation occurred consistently around the implants. In implants subjected to micromotion bone regeneration was disrupted in areas of high strain concentrations (e.g. >30%), whereas lower strain values were permissive of bone formation. Increasing implant displacement or number of cycles per day also changed the strain distribution and disturbed bone healing. These results indicate that not only implant micromotion but also the associated interfacial strain field contributes to regulating the interfacial mechanobiology at healing bone-implant interfaces.

Evaluation of Optimal Taper of Immediately Loaded Wide-Diameter Implants: A Finite Element Analysis

This study aimed to evaluate the effects of different tapering angles of an immediately loaded wide-diameter implant on the stress/strain distribution in bone and implant after implant insertion in healed or fresh molar extraction sockets. A total of 10 finite element (FE) implant-bone models, including 8.1-mm diameter implant, superstructure, and mandibular molar segment, were created to investigate the biomechanical behavior of different implant taper angles in immediate and delayed placement conditions. The degrees of implant taper ranged from 2° to 14°, and the contact conditions between the immediately loaded implants and bone were set with frictional coefficients (μ) of 0.3 in the healed models and 0.1 in the extracted models. Vertical and lateral loading forces of 189.5 N were applied in all models. Regardless of the degree of implant tapering, immediate loading of wide-diameter implants placed in molar extraction sockets generated higher stress/strain levels than implants placed in healed sockets. In all models, the von Mises stresses and strains at the implant surfaces, cortical bone, and cancellous bone increased with the increasing taper angle of the implant body, except for the buccal cancellous bone in the healed models. The maximum von Mises strains were highly concentrated on the buccal cortical struts in the extracted models and around the implant neck in the healed models. The maximum von Mises stresses on the implant threads were more concentrated in the non-tapered coronal part of the 11° and 14° tapered implants, particularly in the healed models, while the stresses were more evenly dissipated along the implant threads in other models. Under immediate loading conditions, the present study indicates that minimally tapered implants generate the most favorable stress and strain distribution patterns in extracted and healed molar sites.

A paradigm for the development and evaluation of novel implant topologies for bone fixation: in vivo evaluation

While contemporary prosthetic devices restore some function to individuals who have lost a limb, there are efforts to develop bio-integrated prostheses to improve functionality. A critical step in advancing this technology will be to securely attach the device to remnant bone. To investigate mechanisms for establishing robust implant fixation in bone while undergoing loading, we previously used a topology optimization scheme to develop optimized orthopedic implants and then fabricated selected designs from titanium (Ti)-alloy with selective laser sintering (SLS) technology. In the present study, we examined how implant architecture and mechanical stimulation influence osseointegration within an in vivo environment. To do this, we evaluated three implant designs (two optimized and one non-optimized) using a unique in vivo model that applied cyclic, tension/compression loads to the implants. Eighteen (six per implant design) adult male canines had implants surgically placed in their proximal, tibial metaphyses. Experimental duration was 12 weeks; daily loading (peak load of ±22 N for 1000 cycles) was applied to one of each animal's bilateral implants for the latter six weeks. Following harvest, osseointegration was assessed by non-destructive mechanical testing, micro-computed tomography (microCT) and back-scatter scanning electron microscopy (SEM). Data revealed that implant loading enhanced osseointegration by significantly increasing construct stiffness, peri-implant trabecular morphology, and percentages of interface connectivity and bone ingrowth. While this experiment did not demonstrate a clear advantage associated with the optimized implant designs, osseointegration was found to be significantly influenced by aspects of implant architecture.

Interface micromotions increase with less-conforming cementless glenoid components

BACKGROUND

The optimal degree of conformity between the glenoid and humeral components in total shoulder arthroplasty for best performance and durability is still a matter of debate. The main aim of this study is to evaluate the influence of joint conformity on the bone-implant interface micromotions in a cementless glenoid implant.

MATERIALS AND METHODS

Polyethylene inlays with different degrees of conformity (radial mismatch of 0, 2, 4, and 6 mm) were mounted on a cementless metal back and then implanted in a bone substitute. These glenoid components were loaded by a prosthetic humeral head during a force-controlled experiment. Normal-to-interface micromotions and bone substitute deformations were measured at different points of the interface. Rim displacement and humeral head translation were also measured. A finite element (FE) model of the experiments was implemented to estimate the normal- and tangent-to-interface micromotions in the entire bone-implant interface.

RESULTS

All measured variables increased with less-conforming PE inlays. Normal-to-interface micromotions were significantly larger (P < .05) when the radial mismatch was 6 mm compared with the fully conforming inlay. The FE model was in agreement and complemented the experimental results. FE model-predicted interface micromotions were already significantly larger when the radial mismatch was equal to 4 mm.

DISCUSSION

In a force-controlled experiment with a cementless glenoid component, a non-conforming PE inlay allows larger interface micromotions than a conforming inlay, reaching a magnitude that may hamper local bone ingrowth in this type of component. This is mainly because of the larger humeral head translation that boosts the effects of the so-called rocking-horse phenomenon.

The effect of primary stability on load transfer and bone remodelling within the uncemented resurfaced femur

One of the major causes of aseptic loosening in an uncemented implant is the lack of any attachment between the implant and the bone. The implant’s stability depends on a combination of primary stability (mechanical stability) and secondary stability (biological stability). The primary stability may affect the implant–bone interface condition and thus influence the load transfer and mechanical stimuli for bone remodelling in the resurfaced femur. This paper reports the results of a study into the affect of primary stability on load transfer and bone adaptation for an uncemented resurfaced femur. Three-dimensional finite element models were used to simulate the intact and resurfaced femurs and the bone remodelling. As a first step towards assessing the immediate post-operative condition, a debonded interfacial contact condition with varying levels of the friction coefficient (0.4, 0.5, and 0.6) was simulated at the implant–bone interface. Then, using a threshold value of micromotion of 50 µm, the implant–bone interfacial condition was varied along the implant–bone boundary to mechanically represent non-osseointegrated or osseointegrated regions of the interface. The considered applied loading conditions included normal walking and stair climbing. Resurfacing leads to strain shielding in the femoral head (20–75 per cent strain reductions). In immediate post-operative conditions, there was no occurrence of elevated strains in the cancellous bone around the proximal femoral neck–component junction resulting in a lower risk of neck fracture. Predominantly, the micromotions were observed to remain below 50 µm at the implant–bone interface, which represents 97–99 per cent of the interfacial surface area. The predicted micromotions at the implant–bone interface strongly suggest the likelihood of bone ingrowth onto the coated surface of the implant, thereby enhancing implant fixation. For the osseointegrated implant–bone interface, the effect of strain shielding was observed in a considerably greater bone volume in the femoral head as compared to the initial debonded interfacial condition. A 50–80 per cent periprosthetic bone density reduction was predicted as compared to the value of the intact femur, indicating bone resorption within the superior resurfaced head. Although primary fixation of the resurfacing component may be achieved, the presence of high strain shielding and peri-prosthetic bone resorption are a major concern.

Heterotypic cell-cell interaction on micropatterned surfaces

PURPOSE

The aim of this paper was to study the influence of chemical and topographical signals on cell behavior and to obtain a heterotypic cell-cell interaction on microstructured domains.

METHODS

The polysaccharide hyaluronic acid (Hyal) was photoimmobilized on glass surfaces in order to obtain a pattern with squares and rectangles of different dimensions and chemistry. The microstructured surfaces were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The behavior of human coronary artery endothelial cells (HCAEC) and human tumoral dermal fibroblasts (C54) was investigated on these micropatterned surfaces by adhesion studies. Moreover heterotypic interaction among C54 and HCAEC adherent on patterned surfaces was evaluated by time-lapse video microscopy

RESULTS

Surface analysis revealed the presence of a pattern consisting of alternating glass and Hyal microstructures whose dimensions decreased from the center to the edge of the sample. Neither HCAEC nor C54 adhered to the immobilized Hyal but both adapted their shape to the different sizes of the glass squares and rectangles. The number of adherent cells depended on the dimensions of both the glass domains and the nuclei of the cells. Co-cultured C54 on HCAEC patterned surfaces showed a heterotypic cell-cell interaction in the same chemical and topographic domain.

CONCLUSIONS

A heterotypic cell-cell interaction occurred in the same chemical and topographic micro-domains but in narrow areas only. Moreover, the number of cells adhering to the glass domains and cell morphology depended on the dimensions of both adhesive areas and cell nuclei.

Nanoscale surface modifications of medically relevant metals: state-of-the art and perspectives

Evidence that nanoscale surface properties stimulate and guide various molecular and biological processes at the implant/tissue interface is fostering a new trend in designing implantable metals. Cutting-edge expertise and techniques drawn from widely separated fields, such as nanotechnology, materials engineering and biology, have been advantageously exploited to nanoengineer surfaces in ways that control and direct these processes in predictable manners. In this review, we present and discuss the state-of-the-art of nanotechnology-based approaches currently adopted to modify the surface of metals used for orthopedic and dental applications, and also briefly consider their use in the cardiovascular field. The effects of nanoengineered surfaces on various in vitro molecular and cellular events are firstly discussed. This review also provides an overview of in vivo and clinical studies with nanostructured metallic implants, and addresses the potential influence of nanotopography on biomechanical events at interfaces. Ultimately, the objective of this work is to give the readership a comprehensive picture of the current advances, future developments and challenges in the application of the infinitesimally small to biomedical surface science. We believe that an integrated understanding of the in vitro and particularly of the in vivo behavior is mandatory for the proper exploitation of nanostructured implantable metals and, indeed, of all biomaterials.

Finite element sub-modeling analyses of damage to enamel at the incisor enamel/adhesive interface upon de-bonding for different orthodontic bracket bases

This study investigates the micro-mechanical behavior associated with enamel damage at an enamel/adhesive interface for different bracket bases subjected to various detachment forces using 3-D finite element (FE) sub-modeling analysis. Two FE macro-models using triangular and square bracket bases subjected to shear, tensile and torsional de-bonding forces were established using μCT images. Six enamel/adhesive interface sub-models with micro- resin tag morphology and enamel rod arrangement were constructed at the corresponding stress concentrations in macro-model results. The boundary conditions for the sub-models were determined from the macro-model results and applied in sub-modeling analysis. The enamel and resin cement stress concentrations for triangular and square bases were observed at the adhesive bottom towards the occlusal surface under shear force and at the mesial and distal side planes under tensile force. The corresponding areas under torsional force were at the three corners of the adhesive for the triangular base and at the adhesive bottom toward/off the occlusal surface for the square base. In the sub-model analysis, the concentration regions were at the resin tag base and in the region around the etched holes in the enamel. These were perfectly consistent with morphological observations in a parallel in vitro bracket detachment experiment. The critical de-bonding forces damaging the enamel for the square base were lower than those of the triangular base for all detached forces. This study establishes that FE sub-modeling can be used to simulate the stress pattern at the micro-scale enamel/adhesive interface, suggesting that a square base bracket might be better than a triangular bracket. A de-bonding shear force can detach a bracket more easily than any other force with a lower risk of enamel loss.

3D micro-crack propagation simulation at enamel/adhesive interface using FE submodeling and element death techniques

This study investigates micro-crack propagation at the enamel/adhesive interface using finite element (FE) submodeling and element death techniques. A three-dimensional (3D) FE macro-model of the enamel/adhesive/ceramic subjected to shear bond testing was generated and analyzed. A 3D micro-model with interfacial bonding structure was constructed at the upper enamel/adhesive interface where the stress concentration was found from the macro-model results. The morphology of this interfacial bonding structure (i.e., resin tag) was assigned based on resin tag geometry and enamel rod arrangement from a scanning electron microscopy micrograph. The boundary conditions for the micro-model were determined from the macro-model results. A custom iterative code combined with the element death technique was used to calculate the micro-crack propagation. Parallel experiments were performed to validate this FE simulation. The stress concentration within the adhesive occurred mainly at the upper corner near the enamel/adhesive interface and the resin tag base. A simulated fracture path was found at the resin tag base along the enamel/adhesive interface. A morphological observation of the fracture patterns obtained from in vitro testing corresponded with the simulation results. This study shows that the FE submodeling and element death techniques could be used to simulate the 3D micro-stress pattern and the crack propagation noted at the enamel/adhesive interface.

Effect of bone to implant contact percentage on bone remodelling surrounding a dental implant

Dental implants are an effective, safe and predictable solution for patients suffering from tooth loss, but implant placement changes the normal mechanical environment of the jawbone leading to bone density redistribution and 'remodelling', in order to adapt to the new environment. Many bone remodelling theories assume the presence of 100% contact between bone and implant, which is inconsistent with clinical reality. About 50-80% bone-implant contact is commonly seen with clinically successful implants. The influence of different percentages of bone-implant contact on bone remodelling has not been investigated adequately. This study aims to evaluate this influence using a newly proposed remodelling algorithm through a 2D finite element model. Four different degrees of bone-implant contact (25, 50, 75 and 100%) are considered and their influences on the density distribution of the jawbone are evaluated. The predicted results indicate that no matter what the initial percentage of bone-implant contact (25-100%), the final outcome is about 58-60% contact when an equilibrium state is reached by bone remodelling. The results are consistent with clinical observations and findings.

A mathematical approach to bone tissue engineering

Tissue engineering is becoming consolidated in the biomedical field as one of the most promising strategies in tissue repair and regenerative medicine. Within this discipline, bone tissue engineering involves the use of cell-loaded porous biomaterials, i.e. bioscaffolds, to promote bone tissue regeneration in bone defects or diseases such as osteoporosis, although it has not yet been incorporated into daily clinical practice. The overall success of a particular bone tissue engineering application depends strongly on scaffold design parameters, which do away with long and expensive clinical protocols. Computer simulation is a useful tool that may reduce animal experiments and help to identify optimal patient-specific designs after concise model validation. In this paper, we present a novel mathematical approach to bone regeneration within scaffolds, based on a multiscale framework. Results are presented over an actual scaffold microstructure, showing the potential of computer simulation, and how it can aid in the task of making bone tissue engineering a reality in clinical practice.

Numerical investigation of macro- and micro-mechanics of a ceramic veneer bonded with various cement thicknesses using the typical and submodeling finite element approaches

OBJECTIVES

This study investigates the influence of cement thickness on the macro- and micro-mechanical responses in a ceramic veneer adjacent to an incisal overlapped incisor.

METHODS

Seven finite element (FE) ceramic veneer macro-models with different cement thicknesses (10-180mum) were generated. A 10N load was applied with an angulation of 60 degrees to the longitudinal tooth axis. Seven FE micro-models corresponding to the macro-models were constructed at an enamel-adhesive interface where the stress concentration was found. Based on an interfacial scanning electron microscope (SEM) micrograph, morphology of resin tags in the micro-models was generated. The micro-model boundary conditions were determined from the macro-model results. The principal stress on each node in the macro- and micro-models was calculated to investigate interfacial mechanics. A tensile test was performed to obtain an ultimate cement tensile strength to determine the material failure parameters.

RESULTS

The highest stress concentration within the cement was found at the resin tag base of the enamel-adhesive interface in lingual side. Maximum stress values from 10.6 to 14.7MPa for the micro-models were higher (44-48%) than that from 7.2 to 10.0MPa for the macro-models when the cement layers increased. Based on the ultimate tensile strength (11.8MPa), bonding failure could found when the micro-models with the cement layers presented more than about 50mum. This seems to correspond with data from previous studies.

CONCLUSIONS

Higher stresses develop in the adhesive as the cement thickness increases. Cement thicknesses less than 50mum might reduce the adhesive bonding failure.

Histodynamics of bone tissue formation around immediately loaded cylindrical implants in the rabbit

OBJECTIVES

The local mechanical environment influences early peri-implant tissue formation. It is still unclear whether immediate loading limits or promotes peri-implant osteogenesis and which mechanical parameters are important herein. The present study evaluated the influence of well-controlled mechanical stimuli on the tissue response around immediately loaded cylindrical turned titanium implants at two different observation periods.

MATERIAL AND METHODS

A repeated sampling bone chamber, consisting of dual-structure perforated hollow cylinders with a cylindrical implant, was installed in the tibia of 14 rabbits and used to conduct three displacement-controlled immediate loading experiments: (i) 30 microm - 400 cycles/day - 1 Hz frequency - 2 x/week - 6 weeks; (ii) 30 microm - 400 cycles/day - 1 Hz - 2 x/week - 6 weeks, followed by another 6 weeks with a 50 microm - 800 cycles/day - 1 Hz - 2 x/week loading protocol; and (iii) 0 microm implant displacement for 12 weeks. A linear mixed model and logistic mixed model with alpha=5% were conducted on the data set.

RESULTS

The tissue area fraction was significantly the highest after 12 weeks of loading. The bone area fraction was significantly different between all three loading conditions, with the highest values for the 12-week loading experiment. Twelve-week stimulation resulted in a significantly higher mineralized bone fraction than 6 weeks. Loading did have a significantly positive effect on the mineralized bone fraction. The incidence of osteoid-to-implant and bone-to-implant contact increased significantly when loading the implant for 12 weeks.

CONCLUSION

Immediate loading had a positive effect on the tissue differentiation and bone formation around cylindrical turned titanium implants. Controlled implant micro-motion up to 50 microm had a positive effect on the bone formation at its interface.

Effect of mechanical stimuli on skeletal regeneration around implants

Due to the aging population and the increasing need for total joint replacements, osseointegration is of a great interest for various clinical disciplines. Our objective was to investigate the molecular and cellular foundation that underlies this process. Here, we used an in vivo mouse model to study the cellular and molecular response in three distinct areas of unloaded implants: the periosteum, the gap between implant and cortical bone, and the marrow space. Our analyses began with the early phases of healing, and continued until the implants were completely osseointegrated. We investigated aspects of osseointegration ranging from vascularization, cell proliferation, differentiation, and bone remodeling. In doing so, we gained an understanding of the healing mechanisms of different skeletal tissues during unloaded implant osseointegration. To continue our analysis, we used a micromotion device to apply a defined physical stimulus to the implants, and in doing so, we dramatically enhanced bone formation in the peri-implant tissue. By comparing strain measurements with cellular and molecular analyses, we developed an understanding of the correlation between strain magnitudes and fate decisions of cells shaping the skeletal regenerate.

Influence of controlled immediate loading and implant design on peri-implant bone formation

AIM

Tissue formation at the implant interface is known to be sensitive to mechanical stimuli. The aim of the study was to compare the bone formation around immediately loaded versus unloaded implants in two different implant macro-designs.

MATERIAL AND METHODS

A repeated sampling bone chamber with a central implant was installed in the tibia of 10 rabbits. Highly controlled loading experiments were designed for a cylindrical (CL) and screw-shaped (SL) implant, while the unloaded screw-shaped (SU) implant served as a control. An F-statistic model with alpha=5% determined statistical significance.

RESULTS

A significantly higher bone area fraction was observed for SL compared with SU (p<0.0001). The mineralized bone fraction was the highest for SL and significantly different from SU (p<0.0001). The chance that osteoid- and bone-to-implant contact occurred was the highest for SL and significantly different from SU (p<0.0001), but not from CL. When bone-to-implant contact was observed, a loading (SL versus SU: p=0.0049) as well as an implant geometry effect (SL versus CL: p=0.01) was found, in favour of the SL condition.

CONCLUSIONS

Well-controlled immediate implant loading accelerates tissue mineralization at the interface. Adequate bone stimulation via mechanical coupling may account for the larger bone response around the screw-type implant compared with the cylindrical implant.

Preferred collagen fiber orientation human peri-implant bone after a short- and long-term loading period: a case report

Immediate loading of dental implants offers treatment cost advantages to patients and avoids the functional and psychological problems caused by the wearing of provisional dentures. There is evidence that the amount of transverse collagen fiber orientation in bone is influenced by mechanical stresses and strains. Two osseointegrated dental implants in humans were used in the present study. Two implants inserted in the maxilla were analyzed: 1 short-term implant (implant A) immediately loaded and retrieved after 4 months of loading and 1 long-term implant (implant B) immediately loaded and retrieved after 12 years. We hypothesized that the bone functional strain caused by immediate loading correlated well with the collagen fiber organization occurring after both short- and long-term functional healing. Circularly polarized light (CPL) was used to assess the area fraction extension related to the transverse collagen fiber orientation in the bone matrix. After evaluating a total of 68 digitized images taken at x50 magnification, birefringence measurements were performed all around the implant surfaces by using 2 central sections from each implant. The results showed that the bone-to-implant contact (BIC) percentage for implant A was 67.9% +/- 9.5% (mean +/- SD), whereas the BIC percentage for implant B was 74.6% +/- 11.2% (mean +/- SD). The area fraction extension was 2.7% +/- 1.4 % (mean +/- SD) for implant A, whereas the area fraction extension was 4.7% +/- 1.2% (mean +/- SD) for implant B. The CPL measurements of the birefringence for transverse collagen fibers of implant A vs implant B indicated that the bone fraction area difference was not high. In the bone near both dental implants, no differences were found in the amount of transverse collagen fibers. Immediate loading seemed to determine and maintain the collagen fiber's orientation over a long period.

Mineralization at the interface of implants

Osseointegration of implants is crucial for the long-term success of oral implants. Mineralization of the bone's extracellular matrix as the ultimate step of a mature bone formation is closely related to implant osseointegration. Osteogenesis at oral implants is a complex process, driven by cellular and acellular phenomena. The biological process of the maintenance and emergence of minerals in the vicinity of oral implants is influenced to a great extent by biophysical parameters. Implant-related structural and functional factors, as well as patient-specific factors, govern the features of osteogenesis. To understand the influence of these factors in peri-implant bone mineralization, it is important to consider the basic biological processes. Biological and crystallographic investigations have to be applied to evaluate mineralization at implant surfaces at the different hierarchical levels of analysis. This review gives insight into the complex theme of mineral formation around implants. Special focus is given to new developments in implant design and loading protocols aimed at accelerating osseointegration of dental implants.

Peri-implant osteogenesis in health and osteoporosis

Long-term clinical success of endosseous dental implants is critically related to a wide bone-to-implant direct contact. This condition is called osseointegration and is achieved ensuring a mechanical primary stability to the implant immediately after implantation. Both primary stability and osseointegration are favoured by micro-rough implant surfaces which are obtained by different techniques from titanium implants or coating the titanium with different materials. Host bone drilled cavity is comparable to a common bone wound. In the early bone response to the implant, the first tissue which comes into contact with the implant surface is the blood clot, with particular attention to platelets and fibrin. Peri-implant tissue healing starts with an inflammatory response as the implant is inserted in the bone cavity, but an early afibrillar calcified layer comparable to the lamina limitans or incremental lines in bone is just observable at the implant surface both in vitro than in vivo conditions. Just within the first day from implantation, mesenchymal cells, pre-osteoblasts and osteoblasts adhere to the implant surface covered by the afibrillar calcified layer to produce collagen fibrils of osteoid tissue. Within few days from implantation a woven bone and then a reparative trabecular bone with bone trabeculae delimiting large marrow spaces rich in blood vessels and mesenchymal cells are present at the gap between the implant and the host bone. The peri-implant osteogenesis can proceed from the host bone to the implant surface (distant osteogenesis) and from the implant surface to the host bone (contact osteogenesis) in the so called de novo bone formation. This early bone response to the implant gradually develops into a biological fixation of the device and consists in an early deposition of a newly formed reparative bone just in direct contact with the implant surface. Nowadays, senile and post-menopausal osteoporosis are extremely diffuse in the population and have important consequences on the clinical success of endosseous dental implants. In particular the systemic methabolic and site morphological conditions are not favorable to primary stability, biological fixation and final osseointegration. An early good biological fixation may allow the shortening of time before loading the implant, favouring the clinical procedure of early or immediate implant loading. Trabecular bone in implant biological fixation is gradually substituted by a mature lamellar bone which characterizes the implant ossoeintegration. As a final consideration, the mature lamellar bone observed in osseointegrated implants is not always the same as a biological turnover occurs in the peri-implant bone up to 1mm from the implant surface, with both osteogenesis and bone reabsorption processes.

Cementless implant fixation--toward improved reliability

Cementless implants offer the advantage of fixation by direct bone-to-implant osseointegration, thereby avoiding the use of a synthetic intermediary material (such as acrylic bone cement) of limited mechanical strength. Successful osseointegration, however, depends on several conditions being satisfied during the peri-implant bone healing period, including the need for limited early loading resulting in minimal relative movement at the implant-bone interface. Sintered porous- and plasma spray-coated implants represent the most common cementless orthopedic implants in current clinical use, although novel cast structures also are being investigated. All stand to benefit from surface modifications currently being explored to enhance osteoconductive or osteoinductive characteristics of the implants. The faster osseointegration that such modified surface designs potentially might offer would result in more reliable and convenient (from the patient perspective) cementless implants. Encouraging results of early animal-based studies exploring such modifications have been reported.

Calcium phosphate sol–gel-derived thin films on porous-surfaced implants for enhanced osteoconductivity. Part II: Short-term in vivo studies

Osseointegration rates of porous-surfaced Ti6Al4V implants with control (unmodified sintered coatings) were compared to porous-surfaced implants modified through the addition of either an Inorganic or Organic Route-formed-Ca-P film. Implants were placed in distal femoral rabbit condyle sites and, following a 9-day healing period, implant fixation strength was evaluated using a pull-out test. Three groups of ten rabbits each were evaluated. Inorganic Route Ca-P-coated implants were compared with control implants in Group I. Organic Route Ca-P-coated implants with control implants in Group II, and Inorganic- with Organic Route-Ca-P-coated implants in Group III. Maximum pull-out force and interface stiffness were compared while selected extracted implants were examined by SEM to characterise failure surfaces. Both types of Ca-P coatings significantly enhanced the early rate of bone ingrowth and fixation as evidenced by higher pull-out force and interface stiffness compared with controls. However, there was no significant difference between Ca-P-coated implants prepared using the two different methods. The enhanced osteoconductivity observed with the Organic Route-formed films despite the absence of any obvious new surface topographic features introduced with the films suggests that the increased rate of bone ingrowth was due primarily to altered surface chemistry rather than changes in topography, at least for these sintered porous-surfaced implants.

Interbody fusion cage design using integrated global layout and local microstructure topology optimization

STUDY DESIGN

An approach combining global layout and local microstructure topology optimization was used to create a new interbody fusion cage design that concurrently enhanced stability, biofactor delivery, and mechanical tissue stimulation for improved arthrodesis.

OBJECTIVE

To develop a new interbody fusion cage design by topology optimization with porous internal architecture. To compare the performance of this new design to conventional threaded cage designs regarding early stability and long-term stress shielding effects on ingrown bone.

SUMMARY OF BACKGROUND DATA

Conventional interbody cage designs mainly fall into categories of cylindrical or rectangular shell shapes. The designs contribute to rigid stability and maintain disc height for successful arthrodesis but may also suffer mechanically mediated failures of dislocation or subsidence, as well as the possibility of bone resorption. The new optimization approach created a cage having designed microstructure that achieved desired mechanical performance while providing interconnected channels for biofactor delivery.

METHODS

The topology optimization algorithm determines the material layout under desirable volume fraction (50%) and displacement constraints favorable to bone formation. A local microstructural topology optimization method was used to generate periodic microstructures for porous isotropic materials. Final topology was generated by the integration of the two-scaled structures according to segmented regions and the corresponding material density. Image-base finite element analysis was used to compare the mechanical performance of the topology-optimized cage and conventional threaded cage.

RESULTS

The final design can be fabricated by a variety of Solid Free-Form systems directly from the image output. The new design exhibited a narrower, more uniform displacement range than the threaded cage design and lower stress at the cage-vertebra interface, suggesting a reduced risk of subsidence. Strain energy density analysis also indicated that a higher portion of total strain energy density was transferred into the new bone region inside the new designed cage, indicating a reduced risk of stress shielding.

CONCLUSION

The new design approach using integrated topology optimization demonstrated comparable or better stability by limited displacement and reduced localized deformation related to the risk of subsidence. Less shielding of newly formed bone was predicted inside the new designed cage. Using the present approach, it is also possible to tailor cage design for specific materials, either titanium or polymer, that can attain the desired balance between stability, reduced stress shielding, and porosity for biofactor delivery.

Ultrastructural characterization of the implant/bone interface of immediately loaded dental implants

Primary stability and an optimized load transfer are assumed to account for an undisturbed osseointegration process of implants. Immediate loaded newly designed titanium dental implants inserted in the mandible of minipigs were used for the characterization of the interfacial area between the implant surface and the surrounding bone tissue during the early healing phase. Histological and electron microscopical studies were performed from implant containing bone specimens. Two different load regimens were applied to investigate the load related tissue reaction. Histological and electron microscopical analysis revealed a direct bone apposition on the implant surfaces, as well as the attachment of cells and matrix proteins in the early loading phase. A striking finding of the ultrastructural immunocytochemical investigations was the synthesis and deposition of bone related proteins (osteonectin, fibronectin, fibronectin receptor) by osteoblasts from day one of bone/biomaterial interaction. Calcium-phosphate needle-like crystallites were newly synthesized in a time-related manner directly at the titanium surface. No difference in the ultrastructural appearance of the interface was found between the two loading groups. Our experimental data suggest that loading of specially designed implants can be performed immediately after insertion without disturbing the biological osseointegration process.

Cyclic strain enhances matrix mineralization by adult human mesenchymal stem cells via the extracellular signal-regulated kinase (ERK1/2) signaling pathway

Physical stimuli play critical roles in the development, regeneration, and pathology of many mesenchymal tissues, most notably bone. While mature bone cells, such as osteoblasts and osteocytes, are clearly involved in these processes, the role of their progenitors in mechanically mediated tissue responses is unknown. In this study, we investigated the effect of cyclic substrate deformation on the proliferation and osteogenic differentiation of human mesenchymal stem cells (hMSCs). Application of equibiaxial cyclic strain (3%, 0.25Hz) to hMSCs cultured in osteogenic media inhibited proliferation and stimulated a 2.3-fold increase in matrix mineralization over unstrained cells. The strain stimulus activated the extracellular signal-regulated kinase (ERK1/2) and p38 mitogen-activated protein kinase pathways, but had no effect on c-Jun N-terminal kinase phosphorylation or activity. Strain-induced mineralization was largely mediated by ERK1/2 signaling, as inhibition of ERK1/2 attenuated calcium deposition by 55%. Inhibition of the p38 pathway resulted in a more mature osteogenic phenotype, suggesting an inhibitory role for p38 signaling in the modulation of strain-induced osteogenic differentiation. These results demonstrate that mechanical signals regulate hMSC function, suggesting a critical role for physical stimulation of this specific cell population in mesenchymal tissue formation.