Effect of hydroxyapatite impregnation on skeletal bonding of porous coated implants

Forged to heal: The role of metallic cellular solids in bone tissue engineering

Metallic cellular solids, made of biocompatible alloys like titanium, stainless steel, or cobalt-chromium, have gained attention for their mechanical strength, reliability, and biocompatibility. These three-dimensional structures provide support and aid tissue regeneration in orthopedic implants, cardiovascular stents, and other tissue engineering cellular solids. The design and material chemistry of metallic cellular solids play crucial roles in their performance: factors such as porosity, pore size, and surface roughness influence nutrient transport, cell attachment, and mechanical stability, while their microstructure imparts strength, durability and flexibility. Various techniques, including additive manufacturing and conventional fabrication methods, are utilized for producing metallic biomedical cellular solids, each offering distinct advantages and drawbacks that must be considered for optimal design and manufacturing. The combination of mechanical properties and biocompatibility makes metallic cellular solids superior to their ceramic and polymeric counterparts in most load bearing applications, in particular under cyclic fatigue conditions, and more in general in application that require long term reliability. Although challenges remain, such as reducing the production times and the associated costs or increasing the array of available materials, metallic cellular solids showed excellent long-term reliability, with high survival rates even in long term follow-ups.

Bone Tissue Engineering (BTE) of the Craniofacial Skeleton, Part I: Evolution and Optimization of 3D-Printed Scaffolds for Repair of Defects

Bone tissue regeneration is a complex process that proceeds along the well-established wound healing pathway of hemostasis, inflammation, proliferation, and remodeling. Recently, tissue engineering efforts have focused on the application of biological and technological principles for the development of soft and hard tissue substitutes. Aim is directed towards boosting pathways of the healing process to restore form and function of tissue deficits. Continued development of synthetic scaffolds, cell therapies, and signaling biomolecules seeks to minimize the need for autografting. Despite being the current gold standard treatment, it is limited by donor sites' size and shape, as well as donor site morbidity. Since the advent of computer-aided design/computer-aided manufacturing (CAD/CAM) and additive manufacturing (AM) techniques (3D printing), bioengineering has expanded markedly while continuing to present innovative approaches to oral and craniofacial skeletal reconstruction. Prime examples include customizable, high-strength, load bearing, bioactive ceramic scaffolds. Porous macro- and micro-architecture along with the surface topography of 3D printed scaffolds favors osteoconduction and vascular in-growth, as well as the incorporation of stem and/or other osteoprogenitor cells and growth factors. This includes platelet concentrates (PCs), bone morphogenetic proteins (BMPs), and some pharmacological agents, such as dipyridamole (DIPY), an adenosine A 2A receptor indirect agonist that enhances osteogenic and osteoinductive capacity, thus improving bone formation. This two-part review commences by presenting current biological and engineering principles of bone regeneration utilized to produce 3D-printed ceramic scaffolds with the goal to create a viable alternative to autografts for craniofacial skeleton reconstruction. Part II comprehensively examines recent preclinical data to elucidate the potential clinical translation of such 3D-printed ceramic scaffolds.

Three-Dimensional Printing Bioceramic Scaffolds Using Direct-Ink-Writing for Craniomaxillofacial Bone Regeneration

Defects characterized as large osseous voids in bone, in certain circumstances, are difficult to treat, requiring extensive treatments which lead to an increased financial burden, pain, and prolonged hospital stays. Grafts exist to aid in bone tissue regeneration (BTR), among which ceramic-based grafts have become increasingly popular due to their biocompatibility and resorbability. BTR using bioceramic materials such as β-tricalcium phosphate has seen tremendous progress and has been extensively used in the fabrication of biomimetic scaffolds through the three-dimensional printing (3DP) workflow. 3DP has hence revolutionized BTR by offering unparalleled potential for the creation of complex, patient, and anatomic location-specific structures. More importantly, it has enabled the production of biomimetic scaffolds with porous structures that mimic the natural extracellular matrix while allowing for cell growth-a critical factor in determining the overall success of the BTR modality. While the concept of 3DP bioceramic bone tissue scaffolds for human applications is nascent, numerous studies have highlighted its potential in restoring both form and function of critically sized defects in a wide variety of translational models. In this review, we summarize these recent advancements and present a review of the engineering principles and methodologies that are vital for using 3DP technology for craniomaxillofacial reconstructive applications. Moreover, we highlight future advances in the field of dynamic 3D printed constructs via shape-memory effect, and comment on pharmacological manipulation and bioactive molecules required to treat a wider range of boney defects.

The presence of 3D printing in orthopedics: A clinical and material review

The field of additive manufacturing, 3D printing (3DP), has experienced an exponential growth over the past four decades, in part due to increased accessibility. Developments including computer-aided design and manufacturing, incorporation of more versatile materials, and improved printing techniques/equipment have stimulated growth of 3DP technologies within various industries, but most specifically the medical field. Alternatives to metals including ceramics and polymers have been garnering popularity due to their resorbable properties and physiologic similarity to extracellular matrix. 3DP has the capacity to utilize an assortment of materials and printing techniques for a multitude of indications, each with their own associated benefits. Within the field of medicine, advances in medical imaging have facilitated the integration of 3DP. In particular, the field of orthopedics has been one of the earliest medical specialties to implement 3DP. Current indications include education for patients, providers, and trainees, in addition to surgical planning. Moreover, further possibilities within orthopedic surgery continue to be explored, including the development of patient-specific implants. This review aims to highlight the use of current 3DP technology and materials by the orthopedic community, and includes comments on current trends and future direction(s) within the field.

Corrosion Behavior in Magnesium-Based Alloys for Biomedical Applications

Magnesium alloys exhibit superior biocompatibility and biodegradability, which makes them an excellent candidate for artificial implants. However, these materials also suffer from lower corrosion resistance, which limits their clinical applicability. The corrosion mechanism of Mg alloys is complicated since the spontaneous occurrence is determined by means of loss of aspects, e.g., the basic feature of materials and various corrosive environments. As such, this study provides a review of the general degradation/precipitation process multifactorial corrosion behavior and proposes a reasonable method for modeling and preventing corrosion in metals. In addition, the composition design, the structural treatment, and the surface processing technique are involved as potential methods to control the degradation rate and improve the biological properties of Mg alloys. This systematic representation of corrosive mechanisms and the comprehensive discussion of various technologies for applications could lead to improved designs for Mg-based biomedical devices in the future.

Promises of Functionally Graded Material in Bone Regeneration: Current Trends, Properties, and Challenges

Functionally graded materials (FGMs) are emerging materials systems, with structures and compositions gradually changing in a particular direction. Consequently, the properties of the materials gradually change in the desired direction to achieve particular nonhomogeneous service demands without abrupting the compositional and behavioral interface at the macroscale. FGMs have been found to have high potential as orthopedic implants; because the functional gradient can be adapted in such a manner that the core of FGM should be compatible with the density and strength of bone, interlayers can maintain the structural integrity and outermost layers would provide bioactivity and corrosion resistance, thus overall tailoring the stress shielding effect. This review article discusses the typical FGM systems existing in nature and the human body, focusing on bone tissue. Further, the reason behind the application of these FGMs systems in orthopedic implants is explored in detail, considering the physical and biological necessities. The substantial focus of the present critical review is devoted to two primary topics related to the usage of FGMs for orthopedic implants: (1) the synthesizing techniques currently available to produce FGMs for load-bearing orthopedic applications and (2) the properties, such as mechanical, structural, and biological behavior of the FGMs. This review article gives an insight into the potential of FGMs for orthopedic applications.

Structural and elemental characterization of glass and ceramic particles for bone surgery

OBJECTIVE

Clinically used bioceramics have been characterized previously with different kinds of methods and comparison of results have proven to be difficult due to varieties of the material properties of interest. Therefore, in this study we compared clinically commonly used bioceramics of hydroxyapatite and carbonate apatite, two bioactive glasses 45S5 and S53P4, and alumina with respect of properties which according to the present knowledge are significant for bone biology.

METHODS

Physicochemical properties of the materials were characterized by various methods. Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) was used to analyze the material vibrational features. X-ray Power Diffraction (XRD) was used to characterize the material crystal structure and scanning electron microscopy-energy-dispersive x-ray analysis (SEM-EDXA) was used to evaluate the morphology and size of the materials and to calculate their oxide content. The dissolution behavior of the materials, ion release and pH changes in Tris buffer in a continuous flow-through reaction for 24-hours were determined. The change of the surface of the bioactive glasses by interfacial reaction during the Tris immersion was examined and the thickness of the surface reaction layer of the materials was studied.

RESULTS

SEM examination showed that the particle morphology of BG 45S5, BG S53P4 and alumina particle's surface was smooth. The surface of HAP was porous, but also CAP showed some surface porosity. An increase in the pH of the immersion solution was observed especially for BG 45S5 and BG S53P4. HAP, CAP and alumina caused only a minor increase in pH. BGs 45S5 and S53P4 showed a rapid initial release of sodium and calcium ions, followed by the release of silicon species. Minor release of sodium ions was registered for HAP, CAP and alumina. Calcium ion release was low but constant over the experimental time while only a minor initial dissolution was measured for HAP.

SIGNIFICANCE

The in vitro study showed differences in the materials' properties, which are considered to be important for biological suitability and in clinical applications, such as materials tomography, ion release and pH changes.

Dental Implants

The goal of modern dentistry is to return patients to oral health in a predictable fashion. The partial and complete edentulous patient may be unable to recover normal function, esthetics, comfort, or speech with a traditional removable prosthesis. The patient’s function when wearing a denture may be reduced to one sixth of the level formerly experienced with natural dentition; however, an implant prosthesis may return the function to near-normal limits. The esthetics of the edentulous patient is affected as a result of muscle and bone atrophy. In order to replace a missing tooth, the development of materials science and technology improved the materials for implant application. Nowadays, titanium has become the most popular implant material due to its advantages. The first submerged implant placed by Strock was still functioning 40 years later. Recently, zirconia implants and innovative surface designs are being researched and practiced. In this chapter, these materials will be comparatively discussed through contemporary literature and research.

Modifications of Dental Implant Surfaces at the Micro- and Nano-Level for Enhanced Osseointegration

This review paper describes several recent modification methods for biocompatible titanium dental implant surfaces. The micro-roughened surfaces reviewed in the literature are sandblasted, large-grit, acid-etched, and anodically oxidized. These globally-used surfaces have been clinically investigated, showing survival rates higher than 95%. In the past, dental clinicians believed that eukaryotic cells for osteogenesis did not recognize the changes of the nanostructures of dental implant surfaces. However, research findings have recently shown that osteogenic cells respond to chemical and morphological changes at a nanoscale on the surfaces, including titanium dioxide nanotube arrangements, functional peptide coatings, fluoride treatments, calcium–phosphorus applications, and ultraviolet photofunctionalization. Some of the nano-level modifications have not yet been clinically evaluated. However, these modified dental implant surfaces at the nanoscale have shown excellent in vitro and in vivo results, and thus promising potential future clinical use.

Biological Activation of Inert Ceramics: Recent Advances Using Tailored Self-Assembled Monolayers on Implant Ceramic Surfaces

High-strength ceramics as materials for medical implants have a long, research-intensive history. Yet, especially on applications where the ceramic components are in direct contact with the surrounding tissue, an unresolved issue is its inherent property of biological inertness. To combat this, several strategies have been investigated over the last couple of years. One promising approach investigates the technique of Self-Assembled Monolayers (SAM) and subsequent chemical functionalization to create a biologically active tissue-facing surface layer. Implementation of this would have a beneficial impact on several fields in modern implant medicine such as hip and knee arthroplasty, dental applications and related fields. This review aims to give a summarizing overview of the latest advances in this recently emerging field, along with thorough introductions of the underlying mechanism of SAMs and surface cell attachment mechanics on the cell side.

Effect of placement angle on the stability of loaded titanium microscrews in beagle jaws

OBJECTIVE

To evaluate the effect of insertion angle on stability of loaded titanium microscrews in beagle jaws.

MATERIALS AND METHODS

Forty-eight microscrews were inserted at four different angles (30°, 50°, 70°, and 90°) into the intraradicular zones of the mandibular first molars and third premolars of 12 beagles and immediately loaded with a force of 2 N for 8 weeks. Microcomputed tomography (micro-CT) and biomechanical pull-out tests were used to assess osseointegration of the interface.

RESULTS

All micro-CT parameters and maximum pull-out force (FMAX) of the microscrews were affected by insertion angles of microscrews. Higher micro-CT parameters and FMAX were seen for implants inserted at angles between 50° and 70° (P < .05). Excessive oblique and vertical insertion angles resulted in reduced stability (P < .05).

CONCLUSION

An insertion angle of 50° to 70° is more favorable than excessive oblique or vertical angles to achieve stability of microscrews.

Immobilized antibiotics to prevent orthopaedic implant infections

Many surgical procedures require the placement of an inert or tissue-derived implant deep within the body cavity. While the majority of these implants do not become colonized by bacteria, a small percentage develops a biofilm layer that harbors invasive microorganisms. In orthopaedic surgery, unresolved periprosthetic infections can lead to implant loosening, arthrodeses, amputations and sometimes death. The focus of this review is to describe development of an implant in which an antibiotic tethered to the metal surface is used to prevent bacterial colonization and biofilm formation. Building on well-established chemical syntheses, studies show that antibiotics can be linked to titanium through a self-assembled monolayer of siloxy amines. The stable metal-antibiotic construct resists bacterial colonization and biofilm formation while remaining amenable to osteoblastic cell adhesion and maturation. In an animal model, the antibiotic modified implant resists challenges by bacteria that are commonly present in periprosthetic infections. While the long-term efficacy and stability is still to be established, ongoing studies support the view that this novel type of bioactive surface has a real potential to mitigate or prevent the devastating consequences of orthopaedic infection.

Surface modification of magnesium alloys developed for bioabsorbable orthopedic implants: a general review

As a bioabsorbable metal with mechanical properties close to bone, pure magnesium or its alloys have great potential to be developed as medical implants for clinical applications. However, great efforts should be made to avoid its fast degradation in vivo for orthopedic applications when used for fracture fixation. Therefore, how to decease degradation rate of pure magnesium or its alloys is one of the focuses in Research and Development (R&D) of medical implants. It has been recognized that surface modification is an effective method to prevent its initial degradation in vivo to maintain its desired mechanical strength. This article reviews the recent progress in surface modifications for prevention of fast degradation of magnesium or its alloys using in vitro testing model, a fast yet relevant model before moving towards time-consuming and expensive in vivo testing. Pros and cons of various surface modifications are also discussed for the goal to design available products to be applied in clinical trials.

Calcium orthophosphates and human beings: a historical perspective from the 1770s until 1940

The historical development of a scientific knowledge on calcium orthophosphates from the 1770s until 1940 is described. Many forgotten and poorly known historical facts and approaches have been extracted from old publications and then they have been analyzed, systematized and reconsidered from the modern point of view. The chosen time scale starts with the earliest available studies of 1770s (to the best of my findings, calcium orthophosphates had been unknown before), passes through the entire 19th century and finishes in 1940, because since then the amount of publications on calcium orthophosphates rapidly increases and the subject becomes too broad. Furthermore, since publications of the second half of the 20th century are easily accessible, a substantial amount of them have already been reviewed by other researchers. The reported historical findings clearly demonstrate that the substantial amount of the scientific facts and experimental approaches have been known for very many decades and, in fact, the considerable quantity of relatively recent investigations on calcium orthophosphates is just either a further development of the earlier studies or a rediscovery of the already forgotten knowledge.

Bioactive Coating on Stainless Steel 316 L through Sol-Gel Method

In this work hydroxyapatite coating through Sol-Gel method on stainless steel 316 L was investigated. Biocompatible additives such as P2O5, Na2CO3, KH2PO4 and HA commercial powder were used for preparation of hydroxyapatite Sol-Gel. Corrosion behavior of the ground and passivated stainless steel was evaluated. Characterization of the coated samples was performed by using SEM and XRD. The results showed that by increasing the sintering temperature, a denser structure of hydroxyapatite was obtained, and volume fraction and size of porosities decreased. Thick hydroxyapatite coatings with a thickness range of 50-70 µm free from cracks were obtained.

Effect of surgical fit on integration of cancellous bone and implant cortical bone shear strength for a porous titanium

Porous scaffold dowels of Ti(6)Al(4)V were prepared and implanted into cancellous and cortical bone sites in adult sheep. Cancellous implants were examined under gap, line-to-line, and press-fit conditions, whereas line-to-line implantation was used in cortical sites. Cortical shear strength increased significantly with time and reached 26.1 ± 8.6 MPa at 12 weeks, accompanied by a concomitant increase in bone integration and remodeling. In cancellous sites, bone integration was well established at 4 and 12 weeks under conditions of press-fit and line-to-line match between implant and surgical defect. New bone growth was also found in the gap conditions, although to a lesser extent. These findings suggest that the porous Ti(6)Al(4)V could prove an effective scaffold material for uncemented fixation in cortical and cancellous sites.

Effect of placement angle on the stability of loaded titanium microscrews: a microcomputed tomographic and biomechanical analysis

INTRODUCTION

The aim of this study was to evaluate the impact of the placement angle on the stability of loaded microscrews.

METHODS

Forty-eight microscrews were placed at 4 angles ( 30°, 50°, 70°, and 90°) into the tibiae of 12 beagles, loaded with a force of 2 N immediately, and maintained for 8 weeks. Microcomputed tomography and pullout tests were used for morphometric and biomechanical analyses, respectively.

RESULTS

All microcomputed tomography parameters and the peak loads at extraction of the microscrews were influenced by the placement angles of the microscrews. The higher microcomputed tomography parameters and the peak load at extraction were measured at angles from 50° to 70°. Oblique and vertical placement angles resulted in reduced stability of the loaded microscrews (P <0.05).

CONCLUSIONS

To achieve the best stability of microscrews, a placement angle of 50° to 70° is advisable.

In-Vivo Degradation of a Poly(Propylene-Fumarate) Biodegradable, Particulate Composite Bone Cement

We have developed a biodegradable particulate composite bone cement consisting of a poly(propylene glycol-fumarate)-(methylmethacrylate) matrix mixed with calcium carbonate and tricalcium phosphate particulates. Previous ex-vivo studies suggest that this system provides sufficient strength for a number of potential clinical applications including structural reinforcement of osseous defects, supplementation of internal fixation of age-related fractures, and delivery of antibiotics to treat osteomyelitis. Ex-vivo degradation assays have also shown that the cement approximates physiologic conditions of bone remodeling as it degrades. In order to evaluate the in-vivo responses to this material, we implanted cement specimens subcutaneously in rats for up to 84 days. Compressive strength of the subcutaneous implants increased linearly through day 21 to 4.91 MPa, then decreased linearly by day 84 to less than 1 MPa. We conclude that this PPFMMA system is biocompatible and biodegradable, and has the potential for use as an orthopedic bone cement. Future studies will be directed toward characterizing the intraosseous histological response and at coordinating the rate of cement degradation with bony ingrowth.

A cross-linking model for estimating Young's modulus of artificial bone tissue grown on carbon nanotube scaffold

Carbon nanotubes (CNTs) provide a suitable environment for growth and proliferation of bone cells. The elastic properties exhibited by CNTs can enhance mechanical characteristics of bone mineral phase, hydroxyapatite (HAp), precipitated on such a scaffold. In this article, a simplified model for estimating the axial Young's modulus of a representative volume element (RVE) of CNT-HAp composite is presented. The model is based on the idea of HAp formation on functionalized sites on CNTs as cross-links between HAp matrix and CNT. Modeling results show that the reinforcement role contributed by CNT in the RVE causes a significant increase in the Young's modulus of the composite material which is a direct consequence of transferring stresses from the HAp matrix to the CNT through the cross-links. Similar conclusions may be suggested regarding the improvement of overall mechanical properties of the material. The prediction made by the model lies reasonably well within the limits proposed by conventional Rule-of-Mixtures, and sliding below Voigt's model. The Young's modulus predicted by the model lies adjacent to the Hashin-Shtrikman upper bound as a function of the RVE length (or equivalently CNT aspect ratio). The model simulation indicates that an increase in the CNT aspect ratio and/or number of cross-links in the RVE, results in the prediction to move closer to the estimation made by Voigt as the assumption of perfect bonding between composite phases is approached.

Increased risk of revision of acetabular cups coated with hydroxyapatite

BACKGROUND

Hydroxyapatite (HA) is the main inorganic component of bone, and HA coating is widely used on acetabular cups in hip arthroplasty. It has been suggested that this surface finish improves cup survival.

METHODS

All patients registered in the Swedish Hip Arthroplasty Register between 1992 and 2007 with an uncemented acetabular implant that was available either with or without HA coating were identified. 8,043 total hip arthroplasties (THAs) with the most common cup types (Harris-Galante, Romanus, and Trilogy) were investigated. A Cox regression model including type of coating, age, sex, primary diagnosis, cup type, and type of stem fixation was used to calculate adjusted risk ratios (RRs) for the risk of revision.

RESULTS

HA coating was a risk factor for cup revision due to aseptic loosening (adjusted RR 1.7; 95% CI: 1.3-2). Age at primary arthroplasty of < 50 years, a diagnosis of pediatric hip disease, the use of a cemented stem, and the Romanus and Harris-Galante cup types were also associated with statistically significantly increased risk of cup revision due to aseptic loosening.

INTERPRETATION

Our findings question the routine use of HA-coated cups in primary total hip arthroplasty. With some designs, this practice may even increase the risk of loosening-resulting in revision surgery.

Computational mechanobiology to study the effect of surface geometry on peri-implant tissue differentiation

The geometry of an implant surface to best promote osseointegration has been the subject of several experimental studies, with porous beads and woven mesh surfaces being among the options available. Furthermore, it is unlikely that one surface geometry is optimal for all loading conditions. In this paper, a computational method is used to simulate tissue differentiation and osseointegration on a smooth surface, a surface covered with sintered beads (this simulated the experiment (Simmons, C., and Pilliar, R., 2000, Biomechanical Study of Early Tissue Formation Around Bone-Interface Implants: The Effects of Implant Surface Geometry," Bone Engineering, J. E. Davies, ed., Emsquared, Chap. A, pp. 369-379) and established that the method gives realistic results) and a surface covered by porous tantalum. The computational method assumes differentiation of mesenchymal stem cells in response to fluid flow and shear strain and models cell migration and proliferation as continuum processes. The results of the simulation show a higher rate of bone ingrowth into the surfaces with porous coatings as compared with the smooth surface. It is also shown that a thicker interface does not increase the chance of fixation failure.

Structural models of bioactive glasses from molecular dynamics simulations

The bioactive mechanism, by which living tissues attach to and integrate with an artificial implant through stable chemical bonds, is at the core of many current medical applications of biomaterials, as well as of novel promising applications in tissue engineering. Having been employed in these applications for almost 40 years, soda-lime phosphosilicate glasses such as 45S5 represent today the paradigm of bioactive materials. Despite their strategical importance in the field, the relationship between the structure and the activity of a glass composition in a biological environment has not been studied in detail. This fundamental gap negatively affects further progress, for instance, to improve the chemical durability and tailor the biodegradability of these materials for specific applications. This paper reviews recent advances in computer modelling of bioactive glasses based on molecular dynamics simulations, which are starting to unveil key structural features of these materials, thus contributing to improve our fundamental understanding of how bioactive materials work.

Growth of new bone guided by implants in a murine calvarial model

New methods to increase vertical bone growth are needed to permit dental implant placement in patients with low alveolar ridge height after extended periods of tooth loss. While ectopic rodent models are typically used to evaluate new osteogenic implant surface coatings, a more relevant intramembraneous rodent model was needed to address the particular clinical need to grow a new layer of bone above an existing layer of bone. In this study we report on a novel murine calvaria model in which successful vertical bone growth around miniaturized dental implants was achieved when using non-glycosylated bone morphogenetic protein-2 (ng/rhBMP-2). Twenty CD-1 mice received two Ti implants each consisting of a Ti ring implant stabilized by a Ti screw into the occipital calvarial bone. Four groups were evaluated: control Ti, Ti+20 mug ng/rhBMP-2, hydroxyapatite (HA)-coated Ti, and HA+20 mug ng/rhBMP-2. The mice were sacrificed 21 days following implant placement. MicroCT analysis showed no new bone formation around the untreated Ti or the HA-coated implants, but demonstrated new bone growth in every dimension around and above the Ti+ng/rhBMP-2 and the HA+ng/rhBMP-2 treated implants. Histopathologic analysis showed that a thin fibrous capsule covered the untreated Ti implants. Limited bone-to-implant contact (BIC) was observed for the HA-coated implants, while in contrast both ng/rhBMP-2 treated groups exhibited extensive new supracalvarial woven bone that covered the implant and merged with the calvarial plate. Histomorphometrically, supracalvarial bone heights and bone widths and BIC were not statistically different from one another for the two ng/rhBMP-2 treated groups. However, the total supracalvarial bone surface area was significantly greater (p<0.05) for the Ti+ng/rhBMP-2 implants (7.2 mm(2)) than the HA+ng/rhBMP-2 (4.0 mm(2)) treated implants. The bone density within 1 mm around the implant was also significantly greater (p<0.05) for the Ti+ng/rhBMP-2 implants (9.9%) than the HA+ng/rhBMP-2 (4.0%) implants, indicating that HA coatings may not be required for sustained release when non-glycosylated BMP-2 is used. This new murine model is capable of discriminating between various bone augmentation strategies and may represent a clinically more relevant model for alveolar bone augmentation than the commonly used ectopic muscle pouch or long bone models.

Human osteoblast response to silicon-substituted hydroxyapatite

Human osteoblasts were cultured on hydroxyapatite (HA), 0.8 wt % silicon substituted hydroxyapatite (Si-HA) and 1.5 wt % Si-HA discs. The influence of these substrates on cell behaviour in vitro was assessed by measuring total protein in the cell lysate and the production of several phenotypic markers: collagen type I (COL I), alkaline phosphatase (ALP), osteocalcin (OC), and the formation of bone mineral. After 7 days, beta-glycerophosphate and physiological levels of hydrocortisone were added to the culture medium to stimulate cell differentiation and mineral production. There was a significantly higher production of ALP on 1.5 wt % Si-HA at day 7 following which, the addition of hydrocortisone promoted the differentiation of cells on the other two substrates. Hydrocortisone addition also decreased the production of OC. During the period, when hydrocortisone was present, no significant difference in behavior was seen between cells on Si-HA and HA; however, following removal of hydrocortisone, cells responded to 0.8 wt % Si-HA with a significant increase in protein production. Using fluorescence microscopy, nodular structures labeled with tetracycline were observed on the surface of all substrates after 21 days. These structures were deposited on areas of high cell density but were not related to the presence or level of silicon in the substrate. These results indicate that human osteoblasts are affected by the presence of silicon in the HA substrate and that the timing of these effects may be dependent upon the level of silicon substitution.

Alloplastic Reconstruction of Large Cranio-Orbital Defects

Norian CRS, Bone Cement (Synthes CMF), and Mimix (Lorenz) have been used to reconstruct large cranio-orbital defects in 85 patients. Resorbable mesh (Macropore), used in combination in selected patients, obviates dura pulsations that have been postulated to cause fragmentation of alloplastic material. Norian is composed of monocalcium phosphate, monohydrate, alpha-tricalcium phosphate, and calcium carbonate. Admixture with NaPO4 creates dahllite, which has a higher carbonate content (4%--6%) than hydroxyapatite (0%). CRS is soluble at low pH, facilitating its resorption and replacement by bone. In contradistinction, Mimix is converted to aqueous solution at 37 degrees C, supports fibrovascular ingrowth and bony interdigitation at the implant-material surface. Forty-five adults (mean age=42 years) and 40 children (mean age=8 years) were evaluated with respect to etiology of defect, size, location, gram usage of alloplast material, type of alloplast, postoperative clinical course, and complications. A minimum of 3-year follow-up is available; 22 adult patients additionally underwent resorbable mesh reconstruction. There were 7 (8%) complications, including infection, extrusion, a sterile loculated fluid collection and fragmentation. No difference in complication rate was noted between biomaterials. Two additional patients exhibited resorption (Norian), necessitating reaugmentation. Alloplastic replacement of cranio-orbital defects has recently advanced dramatically. Bivalved cranial bone grafting with its attached morbidity and sequelae can be avoided. Resorbable mesh allows for the placement of alloplast material in larger defects while avoiding dura pulsation causing alloplast fragmentation. In avoiding titanium type reconstruction, it obviates any interference with radiologic diagnosis and radiotherapeutic modalities. Long-term results are needed to assess bone growth within alloplast and to study bone growth in alloplastic reconstructed pediatric patients.

Extensor mechanism reconstruction after proximal tibial replacement

We developed an in vivo model of the attachment of a patellar tendon to a metal implant to simulate the reconstruction of an extensor mechanism after replacement of the proximal tibia. In 24 ewes, the patellar tendon was attached to a hydroxyapatite (HA)-coated titanium prosthesis. In 12, the interface was augmented with autograft containing cancellous bone and marrow. In the remaining ewes, the interface was not grafted.

Kinematic gait analysis showed nearly normal function of the joint by 12 weeks. Force-plate assessment showed a significant increase in functional weight-bearing in the grafted animals (p = 0.043). The tendon-implant interface showed that without graft, encapsulation of fibrous tissue occurred. With autograft, a developing tendon-bone-HA-implant interface was observed at six weeks and by 12 weeks a layered tendon-fibrocartilage-bone interface was seen which was similar to a direct-type enthesis.

With stable mechanical fixation, an appropriate bioactive surface and biological augmentation the development of a functional tendon-implant interface can be achieved.

Preparation and characterization of novel tricalcium silicate bioceramics

Novel tricalcium silicate (Ca3SiO5) ceramics were successfully fabricated. The mechanical properties of the ceramics were dependent remarkably on sintering temperature. The fracture toughness, Young's modules, and bending strength of Ca3SiO5 ceramics sintered at 1500 degrees C were 1.93 MPa . m1/2, 36.7 GPa, and 93.4 MPa, respectively. These findings suggest that the Ca3SiO5 ceramics possess good mechanical properties, and might be a potential bone implant material.

Enhanced fixation of implants by bone ingrowth to titanium fiber mesh: Effect of incorporation of hydroxyapatite powder

Tight fixation between bone and implant materials is of great importance for a successful outcome of procedures such as total knee arthroplasty (TKA) and total hip arthroplasty (THA). Titanium fiber mesh is an attractive structure for the establishment of tight fixation between bone and implant by bone ingrowth into the spaces among the fibers. Enhancement of bone ingrowth is desired not only for tight fixation but also for a fast recovery. Our hypothesis is that just the presence of hydroxyapatite (HA) particles ensures improved bone ingrowth, and that long-term stability can be obtained by mechanical anchoring of bone in the spaces among titanium fibers. In this study, we examine our hypothesis by in vivo experiment using dog femur. HA particles were incorporated in titanium fiber mesh coated on titanium alloy rod by dipping in a slurry of HA with hydroxy-propyl-cellulose in an ethanol solution. Specimens were implanted for 3, 5, and 8 weeks, and were then compared with the results from specimens without the use of HA. Bonding strength was evaluated by push-out test, and histomorphometric measurements were made with analysis software to calculate the average value of bone ingrowth. A significantly higher bonding strength was observed for the specimens with HA-incorporated implant at 3 and 5 weeks, and larger bone ingrowth deep inside the titanium fiber mesh was measured at 3 weeks. Our proposed method has the additional advantage of not requiring a high temperature that may result in changes in characters of HA powder such as phase transition, grain growth, and decomposition. Moreover, this technique of HA powder incorporation without high-temperature treatment allows the use of several types of metallic fiber mesh, as well as the application to fiber mesh made of organic polymers. We conclude that this simple modification of titanium fiber mesh with HA powder can improve the fixation of implant to bone in the initial stage after operation.

A histomorphological study of tendon reconstruction to a hydroxyapatite-coated implant: regeneration of a neo-enthesis in vivo

The attachment of tendons and ligaments to massive endoprostheses remains a clinical challenge due to the difficulty in achieving a soft tissue implant interface with a mechanical strength sufficient to transmit the forces necessary for locomotion. We have used an in vivo animal model to study patellar tendon attachment to an implant surface. The interface generated when the patellar tendon was attached to a hydroxyapatite (HA) coated implant was examined using light microscopy and a quantitative histomorphological analysis was performed. In the Autograft Group, the interface was augmented with autogenous cancellous bone and marrow graft, and at six weeks an indirect-like insertion was observed. At twelve weeks, the interface was observed to be a layered neo-enthesis, whose morphology was similar to a normal direct tendon insertion. In the HA Group, the tendon-implant interface was not augmented, and the implant was enveloped by a dense collagenous fibrous tissue. This study shows that a tendon-implant neo-enthesis can develop in situ by employing a suitable implant surface in association with biological augmentation.

Bone bonding to hydroxyapatite and titanium surfaces on femoral stems retrieved from human subjects at autopsy

The success of clinical results obtained with many hydroxyapatite (HA)-coated prosthetic designs has deflected attention from the need to extend the life of the HA coating on the device. In the current study the percentages of HA and titanium surfaces to which bone was bonded, on HA-coated and non-coated titanium femoral stems retrieved from human subjects, were evaluated. Plasma-sprayed hydroxyapatite (PSHA)-coated devices demonstrated wide variability in the percentage of the PSHA coating remaining on the stems. The coating was missing from a substantial portion of a stem after only about 6 months of implantation. The percentage of revealed metal to which bone was bonded was significantly less than the percentage of the HA coating demonstrating such bonding. The revealed metal to which bone was bonded was comparable to the same value for a separate group of non-PSHA-coated titanium stems. If HA-coatings degrade over time precipitous decline in performance may occur even after several functional years. Many ultrastructural features of the bone bonded to the HA coatings on these implants from human subjects were comparable to those found on HA-coated devices implanted in a canine model.

In vivo evaluation of a biomimetic apatite coating grown on titanium surfaces

Osteoconductive mineral coatings represent an established technology for enhancing the integration of orthopedic implants with living bone. However, current coatings have limitations related to fabrication methods, attachment strength to metal substrates, and in vivo performance. Low temperature biomimetic growth is a coating technique wherein the device to be coated is immersed in a meta-stable saturated solution of the coating constituents and growth of the coating is then allowed to proceed on the surface of the device. This study focused on the in vivo evaluation of a biomimetic apatite coating fabricated under these conditions. The experiment was designed to specifically test the amount of bone ingrowth into the coated channels versus the uncoated channels of an established bone chamber system, with emphasis placed on the amount of bone present on the coupon surface. Three types of measurements were taken on each channel: linear ingrowth %, area ingrowth %, and continuous bone apposition %. The experiments demonstrated that under controlled conditions, the apatite coating appears to resorb in 8 weeks and did stimulate early osseointegration with the metal surface with a reduction in fibrous tissue encapsulation. This coating may, therefore, be useful in facilitating early bone ingrowth into porous surfaces without the potential for coating debris, macrophage infiltration, fibrous tissue encapsulation, and eventual coating failure as may occur with current plasma-sprayed hydroxapatite coating techniques.

A low-temperature biomimetic calcium phosphate surface enhances early implant fixation in a rat model

The present study demonstrates increased early mechanical fixation of titanium implants coated with a new biomimetic apatite surface in a rat model. Male Sprague-Dawley rats received unilateral femoral medullary implants for periods of 1-4 weeks. The strength of fixation of the implant to the host bone increased more rapidly in the group receiving apatite-treated implants compared with the control group as evidenced by the apatite group's 21-fold greater fixation strength at 1 week (p = 0.009), 4-fold greater fixation strength at 2 weeks (p = 0.041), and 2-fold greater fixation strength at 4 weeks (p = 0.093) compared with the control. Fixation strength was correlated with bone-implant contact as determined from micro computed tomography assessment of the specimens (r2 = 0.338, p = 0.011 in the control group and r2 = 0.543, p < 0.001 in the apatite group). Furthermore, for a given amount of bone-implant contact, the fixation strength was higher in the apatite group than in the control group (p = 0.011), suggesting that the bone formed a stronger bond to the apatite coating than to the titanium. This difference in bonding strength accounted for the difference in mechanical behavior.

The effect of different titanium and hydroxyapatite-coated dental implant surfaces on phenotypic expression of human bone-derived cells

Roughened titanium (Ti) surfaces have been widely used for dental implants. In recent years, there has been the tendency to replace Ti plasma-sprayed surfaces by sandblasted and acid-etched surfaces in order to enhance osseous apposition. Another approach has been the utilization of hydroxyapatite (HA)-coated implants. This study examines the effect of two roughened Ti dental implant surfaces on the osteoblastic phenotype of human bone-derived cells (HBDC) and compares this behavior to that for cells on an HA-coated surface. Test materials were an acid-etched and sandblasted Ti surface (Ti-DPS), a porous Ti plasma-sprayed coating (Ti-TPS), and a plasma-sprayed porous HA coating (HA). Smooth Ti machined surfaces served as control (Ti-ma). HBDC were grown on the substrata for 3, 7, 14, and 21 days, counted and probed for various bone-related mRNAs and proteins (type I collagen, osteocalcin, osteopontin, osteonectin, alkaline phosphatase, and bone sialoprotein). All dental implant surfaces significantly affected cellular growth and the temporal expression of an array of bone-related genes and proteins. HA-coated Ti had the most effect on osteoblastic differentiation inducing a greater expression of an array of osteogenic markers than recorded for cells grown on Ti-DPS and Ti-TPS, thus suggesting that the HA-coated surface may possess a higher potency to enhance osteogenesis. Furthermore, Ti-DPS surfaces induced greater osteoblast proliferation and differentiation than Ti-TPS.

Electrophoretic deposition of porous hydroxyapatite scaffold

Bioactive porous hydroxyapatite (HA) scaffold was fabricated using electrophoretic deposition (EPD) technique in the present work. Bulk HA scaffold was achieved by repeated deposition. The green scaffold was sintered at 1200 degrees C to 82% of the theoretical density. Scanning electron microscopy examination and mercury porosimetry measurement have shown that the porosity remains interconnected and a range of pore size from several microns to hundreds of microns was obtained. X-ray diffraction analysis was performed and confirmed that there is no HA decomposition during the sintering process. Mechanical characterization has also shown that the EPD scaffold possesses excellent properties. Cell culturing experiment was carried out and the result shows that the scaffold bioactivity is not only dependent on the interconnectivity of the pores, but also the pore size.

A new electrochemically graded hydroxyapatite coating for osteosynthetic implants promotes implant osteointegration in a rat model

Hydroxyapatite (HAP) is widely used as an osteoconductive coating for orthopedic implants. So far standard coating methods like plasma spraying produce a relatively thick coating layer (>30 microm). In addition, the chemical structure of the HAP may be altered because of the heating throughout the coating process. This may have negative effects on the coating stability, implant fixation, and induction of bone formation. The relatively thick layer may detach from the implant with the risk of wear debris. In the present study the potential of a newly developed HAP coating of implants on osteointegration was investigated in a rat model. The coating method, based on an electrochemical process, is applied in a graded manner and results in a biodegradable HAP coating with a thickness of approximately 2 mum. Coated versus uncoated titanium Kirschner wires (1.4-mm diameter) were inserted into the medullary cavity of the right femora of 5-month old female Sprague Dawley rats (n=36) in a retrograde fashion. Throughout an experimental period of 2 months the osteointegration was traced radiologically. After this time the animals were sacrificed and the implant integration was tested biomechanically with the use of a push-out test. To analyze the bone-implant interface, histological sections (80 mum) were investigated with an image analyzing system. The biomechanical testing revealed a significantly higher implant fixation in the group treated with the HAP-coated implant (shear strength: 27.8 +/- 6.7 MPa) compared to control (shear strength: 8.08 +/- 3.4 MPa). The histological analyses demonstrated a better ingrowth of the implants in the HAP group with significantly more direct bone-implant contacts compared to the control group. The results demonstrate that the HAP coating promotes implant osteointegration in a rat model.

IGF-I and TGF-beta 1 incorporated in a poly(D,L-lactide) implant coating stimulates osteoblast differentiation and collagen-1 production but reduces osteoblast proliferation in cell culture

Previous in vivo studies revealed a stimulating effect of locally applied IGF-I and TGF-beta1 released from poly(D,L-lactide)-coated titanium implants on rat and porcine fracture healing. The purpose of the present study was to evaluate the effect of IGF-I (5% w/w) and TGF-beta1 (1% w/w) and the carrier PDLLA on osteoblasts in cell culture to improve the understanding of these growth factors. The well-characterized human osteoblast cell line hFOB 1.19 was used in the study. The implants and cells were cocultured in a noncontact manner. The cells were incubated for 10 days in total, and the implants (n = 6 each group and time point) were added for 1 h, 12 h, 24 h, 2 d, 4 d, or 10 d. To analyze a possible effect of the growth factors or the coating, cell proliferation, metabolism, and differentiation were investigated. As an indicator for differentiation the production of collagen I was chosen. All experimental groups showed comparable cell vitality. No change in the pH of the medium was detectable between the analyzed groups. When the effect of the titanium implant and the PDLLA coating were compared with the control culture, no differences in proliferation, metabolic activity, and collagen I production were detectable. The osteoblasts treated with IGF-I and TGF-beta1 released from PDLLA revealed a significantly enhanced collagen I production with a decrease in proliferation and metabolic activity compared to the other groups. No significant differences in collagen I production were seen due to the incubation time points. None of the experimental groups evoked an immunological response on mouse macrophages. In conclusion, the PDLLA-carrier showed no negative effect on osteoblasts, whereas the incorporated growth factors stimulated osteoblast differentiation.

The effect of demineralized bone matrix gel on bone ingrowth and fixation of porous implants

The presence of demineralized bone matrix (DBM) gel did not enhance or accelerate attachment strength or bone ingrowth and resulted in a significant decrease in implant interface attachment strength at 3 weeks. Hydroxyapatite (HA) coating resulted in significant increases in interface shear strength and bone ingrowth compared with non-HA-coated porous implants at all time periods. The HA-coated implants achieved greater attachment strength and bone ingrowth at earlier time periods and maintained greater attachment strength at long-term periods. The results of this study indicate that in the presence of a good bone-implant interference fit, there is no beneficial effect in applying DBM gel to a porous-coated or HA-coated porous implant surface. The small amount that can be applied and the degree of osteoinductivity of DBM seem to preclude it from having a significant biologic effect.

Structural evolution of sol-gel-derived hydroxyapatite

Structural evolution upon transformation of sol to gel, and gel to final ceramic during the synthesis of hydroxyapatite is investigated using Fourier transform infrared (FTIR) analysis, X-ray diffraction (XRD), thermal behavior (DTA and TGA), and electron microscopy examination (SEM/TEM). The sol was first thermally aged at 45 C for various time periods up to 120 min. The colloidal sol, which may have an oligomeric structure, was relatively stable against coagulation. Upon drying, the sol particles consolidated into dry gel through van der Waals attraction, and showed X-ray amorphous phosphate structure. The solid gels showed a particulate microstructure, composed of primary particles of about 8-10 nm in diameter. The amorphous gel transformed into crystalline apatite at temperatures > 300 C. The calcined gels showed a nano-scale microstructure, with grains of 20-50 nm in diameter. Through an appropriate heat treatment between 300 and 400d degrees C. the apatite prepared using current process exhibits a nano-scale, low-crystallinity, carbonated apatitic structure, which closely resembles that of human bone apatite.