Calcium phosphate coatings for fixation of bone implants: Evaluated mechanically and histologically by stereological methods

Calcium phosphate coated nanoparticles for drug delivery: where are we now?

INTRODUCTION

For three decades since the term 'biomaterial' was defined in the late 1960s, the interest of the biomaterials research community in calcium phosphates (CaPs) constantly increased. After this interest reached its peak in the mid-1990s, however, it has begun its steady decline, which lasts to this day, the reasons being manifold, many of which are explicated in this review piece. As of this turning point onwards, one solution for CaP to regain its relevance has involved its use in composite structures where properties of complementary components are intended to mitigate each other's weaknesses. A major type of such hybrid particulate structures has included CaP as a surface coating, the goal being to augment bioactivity, promote an intimate interaction with living tissues, facilitate cellular uptake and/or impart smart, pH-sensitive properties to the particles, among other intended effects.

AREAS COVERED

In this review article, historical remarks, recent examples, challenges and opportunities pertaining to CaP-coated nanoparticles for drug delivery are elaborated. Discussion is supplemented with a bibliographic analysis and framed within a chronological timeline.

EXPERT OPINION

Phenomenal properties and functions are bound to be elicited by composite structures containing CaP coatings and it is imperative that the exploration of these hybrids continues in decades that follow.

Computational models of bone fracture healing and applications: a review

Fracture healing is a very complex physiological process involving multiple events at different temporal and spatial scales, such as cell migration and tissue differentiation, in which mechanical stimuli and biochemical factors assume key roles. With the continuous improvement of computer technology in recent years, computer models have provided excellent solutions for studying the complex process of bone healing. These models not only provide profound insights into the mechanisms of fracture healing, but also have important implications for clinical treatment strategies. In this review, we first provide an overview of research in the field of computational models of fracture healing based on CiteSpace software, followed by a summary of recent advances, and a discussion of the limitations of these models and future directions for improvement. Finally, we provide a systematic summary of the application of computational models of fracture healing in three areas: bone tissue engineering, fixator optimization and clinical treatment strategies. The application of computational models of bone healing in clinical treatment is immature, but an inevitable trend, and as these models become more refined, their role in guiding clinical treatment will become more prominent.

Near-Surface Studies of the Changes to the Structure and Mechanical Properties of Human Enamel under the Action of Fluoride Varnish Containing CPP-ACP Compound

Changes to the features of the enamel surface submitted to induced demineralisation and subsequent remineralisation were studied. The in vitro examination was conducted on polished slices of human molar teeth, divided in four groups: the untreated control (n = 20), challenged by a demineralisation with orthophosphoric acid (H₃PO₄) (n = 20), and challenged by a demineralisation following remineralisation with fluoride (F) varnish containing casein phosphopeptides (CPP) and amorphous calcium phosphate (ACP) compounds (n = 20). The specimens’ enamel surfaces were subjected to analysis of structure, molecular arrangement, mechanical features, chemical composition, and crystalline organization of apatite crystals. Specimens treated with acid showed a significant decrease in crystallinity, calcium, and phosphorus levels as well as mechanical parameters, with an increase in enamel surface roughness and degree of carbonates when compared to the control group. Treatment with fluoride CPP–ACP varnish provided great improvements in enamel arrangement, as the destroyed hydroxyapatite structure was largely rebuilt and the resulting enamel surface was characterised by greater regularity, higher molecular and structural organisation, and a smoother surface compared to the demineralised one. In conclusion, this in vitro study showed that fluoride CPP–ACP varnish, by improving enamel hardness and initiating the deposition of a new crystal layer, can be an effective remineralising agent for the treatment of damaged enamel.

The efficacy of poly-d,l-lactic acid- and hyaluronic acid-coated bone substitutes on implant fixation in sheep

Background/Objective

The present study investigated the efficacy of poly-d,l-lactic acid (PDLLA) and hyaluronic acid (HyA) on implant fixation when coated onto hydroxyapatite/beta-tri-calcium phosphate (HA/βTCP) granules.

Methods

The effect was assessed in a clinically relevant in vivo gap model in sheep. Thus, four titanium implants combined with either allograft (control), pure HA/βTCP, HyA infiltrated HA/βTCP, or PDLLA reinforced HA/βTCP granules were bilaterally inserted into the trabecular bone of the distal femurs in eight sheep. The insertion created a 2-mm peri-implant gap. After 12 weeks, histomorphometry and push-out test was used for quantification of newly formed bone in the gap, bone-implant contact, and implant fixation.

Results

The histomorphometric analysis revealed the presence of newly formed bone in all groups, though substitute groups showed fragments of nonabsorbed substitute material. A significant larger bone volume was found in the allograft group versus the HA/βTCP-PDLLA group (Zone 1), and in Zone 2 a statistically significantly larger bone volume was found in the allograft compared with the HA/βTCP group. The mechanical properties and the bone-implant contact revealed no statistically significant differences between the groups.

Conclusion

This study demonstrates that HA/βTCP granules coated with PDLLA and HyA have similar bone ingrowth and implant fixation as those with allograft, and with mechanical properties resembling those of allograft in advance, they may be considered as alternative substitute materials for bone formation in sheep.

Bioinspired electrohydrodynamic ceramic patterning of curved metallic substrates

Template-assisted electrohydrodynamic atomisation (TAEA) has been used for the first time to pattern curved metallic surfaces. Parallel lines of ceramic titania (TiO2) were produced on titanium substrates, convex and concave with diameters of ~25 mm, at the ambient temperature. Optimal results were obtained with 4 wt% TiO2 in ethanol suspension deposited over 300 s during stable cone-jetting at 20 µl/min, 10kV and collection distance 80 mm. A high degree of control over pattern line width, interline spacing and thickness were achieved. Nanoindentation load-displacement curves were continuous for the full loading and unloading cycle, indicating good adhesion between pattern and substrate. At a loading rate of 1 μN/s and a hold time of 1 s, pattern hardness decreased as load increased up to 7 μN and remained at 0·1 GPa up to higher loads. Elastic modulus behaved similarly, and both were not sensitive to loading rate. The effect of heat treatment to further consolidate the patterned deposits was also investigated. Hardness of the patterns was not markedly affected by heating. This work shows that TAEA is highly controllable and compatible on a range of substrate geometries. Extending TAEA capabilities from flat to curved surfaces, enabling the bioactive patterning of different surface geometries, takes this technology closer to orthopaedic engineering applications.

Comparison of Physical-chemical and Mechanical Properties of Chlorapatite and Hydroxyapatite Plasma Sprayed Coatings

Chlorapatite can be considered a potential biomaterial for orthopaedic applications. Its use as plasma-sprayed coating could be of interest considering its thermal properties and particularly its ability to melt without decomposition unlike hydroxyapatite. Chlorapatite (ClA) was synthesized by a high-temperature ion exchange reaction starting from commercial stoichiometric hydroxyapatites (HA). The ClA powder showed similar characteristics as the original industrial HA powder, and was obtained in the monoclinic form. The HA and ClA powders were plasma-sprayed using a low-energy plasma spraying system with identical processing parameters. The coatings were characterized by physical-chemical methods, i.e. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy, including distribution mapping of the main phases detected such as amorphous calcium phosphate (ACP), oxyapatite (OA), and HA or ClA. The unexpected formation of oxyapatite in ClA coatings was assigned to a side reaction with contaminating oxygenated species (O2, H2O). ClA coatings exhibited characteristics different from HA, showing a lower content of oxyapatite and amorphous phase. Although their adhesion strength was found to be lower than that of HA coatings, their application could be an interesting alternative, offering, in particular, a larger range of spraying conditions without formation of massive impurities.

Titanium nanofeaturing for enhanced bioactivity of implanted orthopedic and dental devices

Titanium (Ti) is used as a load-bearing material in the production of orthopedic devices. The clinical efficacy of these implants could be greatly enhanced by the addition of nanofeatures that would improve the bioactivity of the implants, in order to promote in situ osteo-induction and -conduction of the patient's stem and osteoprogenitor cells, and to enhance osseointegration between the implant and the surrounding bone. Nanofeaturing of Ti is also currently being applied as a tool for the biofunctionalization of commercially available dental implants. In this review, we discuss the different nanofabrication strategies that are available to generate nanofeatures in Ti and the cellular response to the resulting nanofeatures. In vitro research, in vivo studies and clinical trials are considered, and we conclude with a perspective about the future potential for use of nanotopographical features in a therapeutic setting.

The effects of a novel-reinforced bone substitute and Colloss®E on bone defect healing in sheep

Hydroxyappatite-β-tricalciumphosphate (HA/β-TCP) was reinforced with poly(D,L)-lactic acid (PDLLA) to overcome its weak mechanical properties. Two substitutes with porosities of 77% and 81% HA/β-TCP reinforced with 12 wt % PDLLA were tested in compression. The effects of allograft, substitute (HA/β-TCP-PDLLA), Colloss®E, and combination of substitute with Colloss®E on bone formation in vivo were evaluated. Cylindrical critical size defects were created at distal femoral condyles bilaterally in sheep. Titanium implant with concentric gap filling with one of the four materials was inserted. After 9 weeks, the sheep were sacrificed. Implants with surrounding bone were harvested and sectioned into two parts: one for microcomputed tomography scanning and push-out test, and one for histomorphometry. The 77% HA/β-TCP reinforced with PDLLA had similar mechanical properties to human cancellous bone and was significantly stronger than the HA/β-TCP without PDLLA. Microarchitecture of gap mass was significantly changed after implantation for all groups. Allograft had stronger shear mechanical properties than the other three groups, whereas there were no significant differences between the other three groups. Significant new bone formation could be seen in vivo in all four groups and there were no significant differences between them. The PDLLA-reinforced substitute seems to be good alternative substitute material for bone healing in sheep. Further investigations should be performed to validate this novel substitute material.

The effect of adding an equine bone matrix protein lyophilisate on fixation and osseointegration of HA-coated Ti implants

Joint replacements should be firmly anchored in vital bone to avoid early implant subsidence and late aseptic loosening. We investigated whether the fixation of orthopedic implants could be improved by adding an osteoinductive extract of lyophilized equine bone matrix proteins (Colloss E, Ossacur AG, Germany), between the implant and the surrounding bone. Eighteen uncemented HA-coated implants were inserted pairwise in the proximal tibia of nine dogs. All implants were surrounded by a 2 mm concentric defect. In each dog, the intervention implant was added 20 mg protein lyophilisate. The contralateral control implant was inserted untreated. After four weeks, the treated HA-coated implants had better mechanical fixation than the untreated control implants. The treated implants were better osseointegrated, there was more newly formed bone around these implants, and fibrous tissue was eliminated. The mechanical implant fixation had a strong positive correlation to new bone formation on and around the implant, and a strong negative correlation to fibrous tissue encapsulation. The results suggest that bone protein extracts such as the Colloss E device may augment early implant fixation of even HA-coated Ti implants and thereby reduce the risk of long-term failure. This may be particularly useful in revision arthroplasty with bone loss.

Microarchitectural adaptations in aging and osteoarthrotic subchondral bone issues

The human skeleton optimizes its microarchitecture by elaborate adaptations to mechanical loading during development and growth. The mechanisms for adaptation involve a multistep process of cellular mechanotransduction stimulating bone modelling, and remodeling resulting in either bone formation or resorption. This process causes appropriate microarchitectural changes tending to adjust and improve the bone structure to its prevailing mechanical environment. Normal individual reaches peak bone mass at age between 25 and 30 years, and thereafter bone mass declines with age in both genders. The bone loss is accompanied by microarchitectural deterioration resulting in reduced mechanical strength likely leading to fragility fractures. With aging, inevitable bone loss occurs, which is frequently the cause of osteoporosis; and inevitable bone and joint degeneration happens, which often results in osteoarthrosis. These diseases are among the major health care problems in terms of socio-economic costs. The overall goals of the current series of studies were to investigate the age-related and osteoarthrosis (OA) related changes in the 3-D microarchitectural properties, mechanical properties, collagen and mineral quality of subchondral cancellous and cortical bone tissues. The studies included mainly two parts. For human subjects: aging- (I–IV) and early OArelated (V–VI) changes in cancellous bone properties were assessed. For OA guinea pig models (VII–IX), three topics were studied: firstly, the spontaneous, age-related development of guinea pig OA; secondly, the potential effects of hyaluronan on OA subchondral bone tissues; and thirdly, the effects on OA progression of an increase in subchondral bone density by inhibition of bone remodeling with a bisphosphonate. These investigations aimed to obtain more insight into the age-related and OA-related subchondral bone adaptations.

Histological and histomorphometric investigations on bone integration of rapidly resorbable calcium phosphate ceramics

Resorbable ceramics can promote the bony integration of implants. Their rate of degradation should ideally be synchronized with bone regeneration. We report here the results of a histological study of implants with two resorbable calcium phosphate ceramic coatings: Ca(2)KNa(PO(4))(2)-(GB14) and Ca(10)[K/Na](PO(4))(7)-(602020). The results attained with these ceramic-coated implants show the benefits of these materials with regard to bioactive bone-healing stimulation, compared with uncoated implants. The GB14 ceramic coating exhibited greater bone regeneration and differentiation on its surface than the conventional hydroxyapatite coating and helped bone tissue achieve more extensive contact free of connective tissue. Not until the coating disintegrated did the histological features of GB14- and 602020-coated implants converge-both implant types were integrated into bone. Rapid disintegration of the coating material, as with 602020, supports osteoblast proliferation but has negative effects on bone mineralization. Both resorbable ceramics tested, GB14 and 602020, demonstrated bioactivity; even metal surfaces coated with these materials were populated by mature bone tissue without connective tissue after disintegration of their ceramic coating. The less rapidly degrading material, GB14, achieved better results. Degradable calcium phosphate coatings have the potential to stimulate bone regeneration. From the histological viewpoint, the resorbable ceramics examined here can be recommended as coating materials for clinical use.

Early bone apposition in vivo on plasma-sprayed and electrochemically deposited hydroxyapatite coatings on titanium alloy

Three different implants, bare Ti-6Al-4V alloy, Ti-6Al-4V alloy coated with plasma-sprayed hydroxyapatite (PSHA), and Ti-6Al-4V alloy coated with electrochemically deposited hydroxyapatite (EDHA), were implanted into canine trabecular bone for 6 h, 7, and 14 days, respectively. Environmental scanning electron microscopy study showed that PSHA coatings had higher bone apposition ratios than those exhibited by bare Ti-6Al-4V and EDHA coatings after 7 days; however, at 14 days after implantation, EDHA and PSHA coatings exhibited similar bone apposition ratios, much higher than that for bare Ti-6Al-4V. The ultrastructure of the bone/implant interface observed by transmission electron microscope showed that the earliest mineralization (6 h-7 days) was in the form of nano-ribbon cluster mineral deposits with a Ca/P atomic ratio lower than that of hydroxyapatite. Later-stage mineralization (7-14 days) resulted in bone-like tissue with the characteristic templating of self-assembled collagen fibrils by HA platelets. Though adhesion of EDHA coatings to Ti-6Al-4V substrate proved problematical and clearly needs to be addressed through appropriate manipulation of electrodepositon parameters, the finely textured microstructure of EDHA coatings appears to provide significant advantage for the integration of mineralized bone tissue into the coatings.

A novel method for the simultaneous, titrant-free control of pH and calcium phosphate mass yield

Calcium phosphate (CaP) bioceramics have long been of interest for the unique properties that they exhibit as bone substitute materials. By harnessing the unique bone-bonding capacity of CaP's, biomaterials scientists have made great strides over the past 2 decades to produce novel materials to assist in the treatments of defects caused by trauma, disease, or both. In recent years, however, it has become apparent that the traditional set of techniques used to produce calcium phosphates does not satisfy all of the requirements necessary to meet the challenges of emerging applications. In particular, recent interest in (i) the synthesis of coprecipitated CaP/bioorganic composites and (ii) the investigation of the mechanisms of biomineralization has highlighted the need for new methods to control pH and CaP mass yield.

Anatomically adapted, HA coated SBG stem – Ten years of successful implantation / Anatomisch adaptierte, HA beschichtete SBG Prothese – 10 Jahre erfolgreiche Anwendung

For stable implantation of anatomically conforming femur prostheses, modifications that accommodate curving and torsion are necessary. In accordance to this concept, the anatomically shaped SBG stem has been developed. The first consecutive implantations were evaluated. 194 primary arthroplasties with uncemented anatomical, HA coated SBG stems were performed. Mean patient age was 61 years. 143 patients with 151 (78%) SBG stems were followed-up clinically and radiologically after an average of 10 (9-11) years. So far, only one implant had to be revised. Survivorship with revision of the femoral component is 99.5% at ten years. The mean postoperative Harris Hip Score was 92. Patients over 75 years had a score of 89; younger patients under 40 years scored 94. Sclerotic lines were seen in zone I or VII in 8%. Minor femoral remodeling (Type 1) occurred proximally in 75%. Minor thickening of the femur at the level of the tip of the prosthesis could be observed in 27% of the cases. There is no correlation between compacta thickening and clinical symptoms (p=0.15). The anatomical shape, the oval diameter and the longitudinal grooves secure stable primary fixation. Together with the HA coating, which enhances osseointegration, the SBG stem has a high success rate in the medium and long term.