Formation of Self-Organising Interface

FORMATION OF SELF-ORGANISING INTERFACE
BETWEEN THE BONE AND IMPLANTS
WITH NANOSTRUCTURED BIOACTIVE COATINGS: PHYSICS, CHEMISTRY AND MECHANICS

A.I. Mamaev, V.A. Mamaeva

Institute of Strength Physics and Materials Science, Siberian Branch of the Russian Academy of Science, Tomsk, Russian Federation

D.A. Malanin

Volgograd Scientific Centre of the Russian Academy of Sciences and Volgograd State Medical University, Volgograd, Russian Federation

V.I. Kalita, A.G. Gnedovets, D.I. Komlev

A.A. Baikov’s Institute of Metallurgy and Materials Science of the Russian Academy of Sciences, Moscow, Russian Federation

A. Yerokhin and A. Matthews

Department of Engineering Materials, University of Sheffield, Sheffield, U.K.

Abstract. The paper discusses effects of novel nanostructured composite coatings on mechanical properties of bone-implant systems. The composite coatings comprise thick Ti underlayer featured by 3-dimentional capillary-porous morphology and thin top ceramic layer with enhanced biological activity. The Ti layer is produced by plasma spraying under low incident angles and the bioceramic layer is produced by plasma electrolytic oxidation. The coated Ti samples were implanted into animals and resided there up to 2 years. Tensile tests were performed to evaluate shear strength at the bone-implant interface. It was found that the composite coatings allowed faster and better integration of Ti implants into bone tissue. The coatings provided 24% increase in the interfacial shear strength. It was suggested that the composition of the bioactive layer is principally responsible for the shear strength growth rate.

Introduction

To facilitate osseointegration of intrabone prosthetic implants they should possess a specific porous surface structure, with porosity ranging between 50 and 500 mm and stiffness being intermediate between that of the bone and the implant material. These characteristics of the porous structure promote osteoconductivity at the bone-implant interface and ensure its high adhesion strength. However, none of the known conventional porous structures satisfy these criteria in full. A preferable bioactive compound for prosthetic implants is hydroxyapatite (Ca10(PO4)6(OH)2, HA). These considerations are put into the base of development of principally new 3-dimensional capillary-porous (3DCP) composite coating deposited by plasma spraying of Ti wire or powder [1] followed by surface conversion by plasma electrolytic oxidation (PEO) [2,3]. The 3D structure of Ti coating comprises alternating hills and valleys, beginning at the coating-substrate interface. The region of the highest porosity dedicated for bone ingrowth is located in the valleys and smaller capillary pores – in the bulk of the hills and on their surfaces. The original 3DCP structure allowed obtaining high shear strength, about 250 MPa, which is up to 6 times of that for conventional porous structures [1]. Surface conversion of Ti results in formation of 5 to 50 mm thick bioactive ceramic (BC) layer with titania matrix. The layer is featured by fine porosity, ranging between 0.1 and 10 mm and comprising 10 to 40% of its volume. Such structure of the top BC layer promotes formation of new bone material and retain its bioactivity during desorption since titania matrix is thermodynamically stable in the bone. Generally, the composite 3DCP + BC-PEO coatings are capable to adapt easily to bone systems of live bodies and can therefore be categorised as healing promoters.

The main purpose of this work was to study effects of the nanostructured composite coatings on mechanical properties of the bone-implant system during medium-term in-vivo residence.

Materials and methods

The studies were carried out in vivo using Ti wire implants. The wire samples of 2 mm in diameter were coated with 3DCP Ti layer using arc plasma spray method [1]. The mean average thickness of the 3DCP coating was 650 mm. BC top layers containing hydroxyapatite (HA), either alone or in the mixture with calcium phosphates (CP), were deposited in electrolyte solutions by pulsed current PEO methods [2, 3].

Integration of implants with bone were studied using 12 mature underbred dogs (24 knee joints), 3 to 4 years old, weighted 9 to 11 kg. The test protocol in parts of experimental design, animal selection, keeping and bringing out of the experiment was agreed with local ethical committee. The animals were randomly split into 3 groups to implant samples with (i) 3DCP Ti, (ii) 3DCP Ti + BC (HA) and (iii) 3DCP Ti + BC (HA + CP) coatings respectively. At the first stage, on the side of joint surface, some channels were formed in distal epiphyses of femoral bones of the animals. The coated wire implants with length of 7 mm were subchondrically tight-fit inserted in these channels. Biopsy of the samples was carried out after 24, 48, 72 and 96 weeks of implantation. The samples were taken off in such way that it is fully surrounded by >1 cm thick bone tissue. To prevent subsequent changes in bone tissue the samples were kept in physiological NaCl solution at 4oC.

Fig. 1. Typical appearance of the bone-implant sample for shear tests.

At the second stage, within 24 to 48 hours after the biopsy, mechanical testing of the samples was carried out to evaluate shear strength of the bone-implant interface. The samples were coaxially placed inside Al cylinders of 20 mm in diameter, with surrounding space filled in by epoxy resin (Fig. 1). Each time, 2 to 5 samples of each type of coating were tested. The tests were carried out using ‘Instron’ model 1115 tensile tester at 0.5 mm min-1 strain rate.

Finally, implant surfaces after the shear tests were analysed using a binocular microscope under x30 magnification. Polished cross-sections of the bone-implant system and implants taken out of the bone were additionally studied by optical microscopy under x1000 magnification.

Results

Shear strength of bone-implant system with different types of coatings is collated in Table 1. Visual observations of implants tested after 24 weeks (0.5 yrs) of implantation showed that fracture always occurred in the bone and shear strength represent that of the new bone formed at the interface. For 3DCP Ti and 3dCP Ti + BC (HA) coatings the fracture in some cases proceeded via tops of the coating hills whereas a presence of transparent fibrous structures of different density was observed in valleys.

Table 1. Mean average shear strength (MPa) in bone-implant system.

Coating type / Implantation time (yrs) / Mean average strength
0.5 / 1.0 / 1.5 / 2.0
3DCP Ti / 1.3 / 1.6 / 7.6 / 3.1 / 3.4
3DCP Ti + BC (HA) / 1.2 / 9.8 / 9.4 / 4.5 / 6.2
3DCP Ti + BC (HA + CP) / 2.0 / 5.1 / 5.5 / 6.2 / 4.7

In 48 weeks (1 year) after implantation, all groups fractured via highest tops of the hills. Porous bone constituents with lamellar morphology remained between the hills. Similar behaviour was observed for 3DCP Ti + BC (HA) samples after 72 weeks (1.5 yrs) , whereas for other groups the fracture occurred via the surrounding bone tissue. In the latest tests (96 weeks or 2 yrs), the fracture always occurred through the bone, on some distance from the interface.

At 24 weeks (0.5 yrs) after implantation, the shear strength of implants with 3DCP Ti and 3DCP Ti +BC (HA) coatings was similar (1.3 and 1.2 MPa respectively) and 3DCP Ti +BC (HA + CP) coating showed a higher shear strength of 2 MPa (Fig. 2). During the following 24 weeks, the value of shear strength of the samples with 3DCP Ti +BC (HA) coating increased significantly and became 9.8 MPa, exceeding that of other groups. At 72 weeks (1.5 yrs), the maximum strength was also shown by 3DCP Ti +BC (HA) coatings; the strength of 3DCP Ti coating increased from 1.6 to 7.6 MPa and 3DCP Ti +BC (HA + CP) increased slightly, reaching 5.5 MPa.

Fig. 2. Effect of implantation time on shear strength in bone-implant systems with different types of composite coatings.

Fig. 3. Typical surface morphology of implants pulled out from the bone during shear tests. Left to right: 3DCP Ti; 3DCP Ti +BC (HA) and 3DCP Ti +BC (HA + CP). x30.

At the latest stage (96 weeks or 2 yrs) the shear strength for 3DCP Ti and 3DCP Ti + BC (HA) coatings decreased to 3.1 and 4.5 MPa respectively but increased for 3DCP Ti + BC (HA + CP) coating to 6.2 MPa.

Visual observations at x16 to x32 magnifications did not reveal any additional surface films that could be formed on the implant surface during its residence in the animal body (Fig. 3). However, some white matter was observed on the surfaces of 3DCP Ti +BC (HA) and 3DCP Ti +BC (HA + CP) samples, which could be relatively easily removed by scribing with a needle. Additionally, relatively thick non-transparent globular features (probably grown in bone tissue) were present on the surface of 3DCP Ti +BC (HA + CP) coating, covering up to 85% of the surface area. The above surface morphologies were characteristic for each type of coating regardless the implantation time.

The presence of bone tissue in the coating pores was also confirmed by the cross-sectional analysis of implants both remaining in the bone and pulled out of it. The latter provided better interfacial observations since the coating hills were free of the bone tissue and relatively hard epoxy resin engaged tightly with the surface. In this case a 10 to 25 mm thick BC film was clearly observed. Thus it can be deduced that BC coatings do not decompose fully in the body during at least 2 years, while forming surface products that are dissimilar from those formed on 3DCP Ti coating.

Discussion

According to the literature data, implant integration with bone tissue is mainly determined by its surface profile or porosity. These requirements have been fully obeyed in all experimental groups. This is supported by the fact that bare Ti implants could be removed from the bone with much lower force event at the sampling stage [1].

Important role in ensuring high interfacial adhesion strength in the bone-implant system is also played by the bioactivity of the implant surface that enhances osseoinduction and osseointegration. The samples with BC coatings (i.e. of 3DCP Ti +BC (HA) and 3DCP Ti +BC (HA + CP) groups) studied in this work exhibited better mechanical integration with the bone and therefore better shear strength, indicating good osseoinductive and osseoconductive characteristics. Histological and immuno-histochemical justification of this was provided elsewhere [1].

Current results are in agreement with data of interfacial shear strength of Ti- and HA-coated implants [4] in unloaded (0.63 MPa) and loaded (0.73 MPa) state. However, in our case, the shear strength is almost an order of magnitude higher, probably due to the highly convoluted surface morphology of 3DCP coating. Comparing data for 3DCP Ti +BC (HA) and 3DCP Ti +BC (HA + CP) coatings, it should be noted that at 24 weeks the former showed relatively low shear strength, making only a half of that for the latter. However at 48 and 72 weeks, the situation reversed. This is probably associated with differential coating desorption rate and corresponding formation of the new bone. It is interesting that at these particular times the fracture of samples with 3DCP Ti +BC (HA) coatings occurred via highest tops of the hills, whereas for 3DCP Ti +BC (HA + CP) coatings it went on through the surrounding bone. The same argument can be put forward to explain increased shear strength of 3DCP Ti +BC (HA + CP) coatings at 96 weeks, while further verification is required for 3DCP Ti +BC (HA) coating data as there are some concerns regarding sample keeping in this batch.

Due to the limited amount of samples available for the tests, it made a sense to evaluate time-averaged shear strength for each coating group (Table 1). In regard to this characteristic, the coatings can be ranked in the following ascending order:

3DCP Ti < 3DCP Ti +BC (HA+ CP) < 3DCP Ti +BC (HA)

(3.4 MPa) (4.7 MPa) (6.2 MPa)

Thus the account for bioactive properties of the coatings allows explaining implant behaviour during the period of its residence, in particular differential rate of increase in shear strength for 3DCP Ti +BC (HA) and 3DCP Ti +BC (HA + CP) coatings.

Conclusions

The results of in vivo studies have shown that composite 3DCP Ti + BC coatings allowed faster and better integration of Ti implants into bone tissue. In average, the composite coatings provide 24% higher shear strength of the bone-implant interface. A composition of the bioactive top layer appears to be the main factor determining the rate of increase the shear strength.

Acknowledgements

The work was supported by the Russian State contract #02.523.11.3007. International collaboration was supported by the Royal Society and is acknowledged with thanks.

References

1.  Kalita VI, Gnedovets AG, Mamaev AI, Mamaeva VA, Pisarev VB, Malanin DA, Mamonov VI, Snigur GL and Krainov YeA: Fizika I Khimia Obrab. Materialov, 2005 3 39-47.

2.  Mamaev AI, Mamaeva VA and Vybornova SN: ‘Method of surface modification of medical devices’. Patent No RU 2206642, 2000.

3.  Yerokhin A and Matthews A: ‘Method for forming a bioactive coating’. UK Patent application, 2007

4.  Mouzin O, Soballe K, Bechtold JE: J. Biomed. Mater. Res. (Appl Biomater) 2001 1 (58) 61-68.

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