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Fabrication, characterization and fracture study of a machinable hydroxyapatite ceramic
69
Fabrication, characterization and fracture study of a machinable hydroxyapatite ceramic M.Y. Shareef and P.F. Messer Department of Engineering Materials, University of Sheffield, Sheffield Sl 3JD, UK
R. v a n N o o r t Department of Restorative Dentistry, School of Clinical Dentistry, University of Sheffield, Sheffield S10 2SZ, UK
In this study the preparation of a machinable hydroxyapatite from mixtures of a fine, submicrometer powder and either a coarse powder composed of porous aggregates up to 50pm or a medium powder composed of dense particles of 3pm median size is described. These were characterized using X-ray diffraction, transmission and scanning electron microscopy and infra-red spectroscopy. Test-pieces were formed by powder pressing and slip casting mixtures of various combinations of the fine, medium and coarse powders. The fired test-pieces were subjected to measurements of firing shrinkage, porosity, bulk density, tensile strength and fracture toughness. The microstructure and composition were examined using scanning electron microscopy and X-ray diffraction. For both processing methods, a uniform interconnected microporous structure was produced of a high-purity hydroxyapatite. The maximum tensile strength and fracture toughness that could be attained while retaining machinability were 37 MPa and 0.8 MPa m 1/2 respectively.
Keywords: Hydroxyapatite, tensile properties Received 4 February 1992; revised 9 March 1992; accepted 18 June 1992
H y d r o x y a p a t i t e [Calo(PO4}6{OH)2 ] is a highly biocom-
patible ceramic, which is used in a variety of oral and maxillofacial applications such as bone recontouring, alveolar ridge augmentation and infilling of extraction sockets ~-3. It is available in dense blocks, macroporous blocks and granules. Each of these forms of HAP has its drawbacks: dense HAP is difficult to machine without causing large-scale fracture, granules tend to migrate and the macroporous material leaves a ragged surface finish. These problems were addressed by seeking a means of producing a material with a partially bonded microstructure 4.s, which can readily be shaped either in the dental surgery or operating theatre with conventional dental rotary instruments. The partially bonded microstructure in the present case is composed of polycrystalline aggregates bonded together over small areas and has a porosity which is interconnected. When such a material is mechanically loaded, the highest stresses are developed in the bonded regions. A crack will propagate across such a region but will rapidly run into a pore space. This allows bond breakage to occur without cracks propagating through the bulk of the material. The objective of this study was to produce a machinCorrespondence to Dr R. van Noort. ,~ 1993 Butterworth-Heinernann Ltd 0142-9612/93/010069-07
able, high-purity HAP in such a way that complex shapes could be formed. Flexibility in the production of complex shapes was provided by developing the slip casting route in addition to that of powder pressing. Purity was considered important, since decomposition products resulting from the firing cycle may produce a different hard tissue response than would be expected from HAP. Also, pure HAP is considered to be insoluble in the biological environment and this may be upset by the presence of other calcium phosphates.
MATERIALS AND METHODS Characterization of starting powders The two starting powders of HAP used in this study, designated as fine and coarse, were obtained from Plasma Biotal Ltd, Tideswell, UK. The coarse powder had been produced by the supplier by calcining the fine powder. Previous work indicated these powders to be of exceptionally high purity compared with other powders available 6. The chemical characteristics of the powders were determined using X-ray diffraction (XRD} and infrared spectroscopy {IRS}, while the physical characteristics Biomaterials 1993, Vo]. 14 No. 1
70
Machinable hydroxyapatite ceramic: M.Y. Shareef eta/.
and coarse powders. The powder mixtures were granulated and subsequently sieved to produce pressbodies 5. These had granule size ranges of < 710 pro, < 500pm, < 250 pm and < 100 pm. Any granules greater than the sieve being used were crushed and regranulated. Testpieces were produced from each of the pressbodies by double-ended uniaxial compaction in a steel die at 47 MPa. These were fired in air at 1250°C for 3 h in an electrically heated muffle kiln. The various stages in the production of the granules and the test-pieces are shown in the flow diagram of Figure 1. The test-pieces consisted of annular rings with an external diameter of 27 ram, internal diameter of 19 m m and height 5 mm, as shown in Figure 2.
were determined using scanning and transmission electron microscopy (SEM and TEM). XRD measurements were carried out using CuKa radiation (a = 0.154050 nm] in a Philips X-ray diffractometer (PW 1050/25 goniometer and PW 1730 generator) with 50 kV tube voltage and 30 mA current. A scan rate of 2°/min was used over a range of 10-70 ° for all samples. The positions and intensity of the peaks were read from the X-ray trace and the calculated d-values compared with those listed in the ASTM powder diffraction files. The infra-red absorption spectra of the two HAP powders were recorded from 400 to 4000 cm -1 at ambient temperature using a Perkin-Elmer 683 infra-red spectrophotometer. Samples were pelletized with KBr [0.5% dilution] at a pressure of 765 MPa, which was carried out in an evacuated die for 10 min so as to minimize water sorption. Samples of the HAP powders were examined using a Philips P-500 scanning electron microscope. The fine powder was also examined under a Philips 301 transmission electron microscope.
Preparation of slip cast and sintered test-pieces A suspension was prepared containing 18 vol% of the submicrometer powder in deionized water, To disperse the agglomerates, Calgon (sodium hexameta phosphate) was added as a deflocculant. A few porcelain balls were placed in the plastic pot containing the suspension and the pot was rotated on rollers for 15 h to effect the dispersion. Using a Rheomat 30 rotating cylinder viscometer, the suspension was found to exhibit essentially Newtonian hehaviour. The required amounts of coarse powder were added to suspensions of the fine powder to produce mixtures of fine and coarse powders of 80/40, 70/30 and 80/20 respectively. A 2 wt% addition of Carhowax (polyethylene glycol) relative to the solids content was added as a binder. A further addition of Calgon was made to the suspensions, to retain just a semblance of pseudoplastic hehaviour which helped to prevent sedimentation of the larger particles. Mixing was carried out for a further 3 h. Suspensions were poured into a one-piece plaster of
Preparation of pressed and sintered test-pieces Previous work 5'8 has shown that compacts of the fine powder have a dense microstructure close to the theoretical density, while the coarse powder produces a very weak highly porous microstructure. To maximize the strength and fracture toughness of the machinable HAP, test-pieces were prepared from mixtures of the fine and coarse powder in the ratios by weight of 60/40, 50/50 and 40/60. Pressing of these simple mixtures would result in a non-uniform packing. Therefore, uniform filling of the die cavity of the mould on pressing required the production of suitable granules of the mixtures of fine
Jc°arse I
I F'ne I
powder
Water (deionlzed)
powder
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Dispersion of fine powder Mixed for 15 h
1
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40,50,60w/orelativl to fine
Preparation of powder mixture Mixed for 3 h vI
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4 w/o relative to solids content
1
Powder cake formation Slip dried at 100°C for 24 h
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Classification Passed through a 100,250,500 and 710 IJm sieve Forming Double-ended uniaxial compaction in a steel die at 47MPa
Figure
l_ ]~
I
Binder [ (Carbowax) ]
Agglomerated powder granulation Crushed, moistened and rubbed
P
Oversized particles crushed and regranulated
1
1
Deflocculant (Calgon)
Firing Fired in air at t250°C for 3h
Flow diagram showing the production stages for the granules and the test-pieces of hydroxyapatite using powder pressing.
Biomaterials 1993, Vol. 14 No. 1
Machinable hydroxyapatite ceramic: M.Y. Shareef et al.
71
Characterization of the fired test-pieces
,'" ~ - ~ . ~ ' t t ~ , t t ~ h . . ~ . ~ .
The fired test-pieces were characterized by measuring the firing shrinkage, apparent and true porosities and bulk density. The shrinkages of the test-pieces were determined from their green and fired dimensions, which were measured using a travelling microscope. The densities and porosities were determined using the standard technique involving boiling in wateF. The microstructure and composition were examined using SEM and XRD respectively. For SEM examination, the samples were impregnated with epoxy resin, polished and the resin removed by firing at 400°C.
r..~
5 cm !_
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Measurement of tensile strength and fracture toughness Figure 2
Ring-shaped test-piecesof hydroxyapatite prepared by powder pressing.
The modified Stanford ring bursting test s was used to determine the tensile strength of the pressed and sintered HAP. In this test, the test-piece is pressurized internally to generate a tensile hoop stress in the ring. The internal pressure is increased until the test-piece fails. The average tensile strengths were determined using 15 testpieces for each pressbody. Similar tests were not performed for the slip cast material because of problems in producing a consistent surface finish. Fracture toughness values for both pressed and slip cast test-pieces were determined using the technique described above, except that each test-piece was prenotched radially on its outer surface to a depth of approximately 2 mm. A 200pm thick diamond-impregnated circular blade was used to form the notches. The exact depths of the cuts were measured using a travelling microscope. Seven test-pieces were used to determine the average fracture toughness value for each type of test-piece. The tensile strength and fracture toughness values were calculated using formulae published in the literature 9, lo
Paris mould to a depth of about 80 mm. The mould had a cylindrical cavity of 34 m m in diameter and was 160 mm long. Casting was continued for 15 min and then the surplus slip drained from the mould. Cast tubes were thoroughly dried and then fired in air in an electrically heated muffle kiln for either 3 or 6 h at 1250°C. Up to eight rings, 6 m m in height, were cut from a fired tube with a diamond-impregnated saw. The plane ends of the rings were carefully ground flat on SiC papers and radially notched using a 2 0 0 p m diamond-impregnated saw to produce the test-pieces for the measurement of fracture toughness. Following tests on the fired slip cast test-pieces, which showed them to be too porous, the coarse porous aggregates were replaced by a medium-sized powder composed of dense particles with a median size of 3/Jm. This powder was produced by ball milling the coarse powder. Medium powder was added to a dispersion of the fine powder to produce a mixture of 60 fine/40 medium. This also had Carbowax added as a binder and was mixed for 3 h. The various stages are shown in Figure 3.
Coarse
[M_illedf o r
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I Medium ] -I powder
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The machinability of the test-pieces was assessed using a stee~ fissure bur in a slow speed dental handpiece by
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Figure 3
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Drying and firing [ Dried at room temperature for 2q h and fired in air at 1250°C for 3 or 6h
Flow diagram showing the production stages for the test-pieces of hydroxyapatite using slip casting,
Biomaterials 1993, Vol. 14 No. 1
72
Machinable hydroxyapatite ceramic: M. Yo Shareef et al.
attempting to remove material by applying only light finger pressure.
RESULTS AND D I S C U S S I O N The XRD trace for the as-received fine powder is presented in Figure 4. Only peaks attributable to HAP could be detected for both powders, which have a well-defined crystal structure as indicated by the sharpness of the peaks. The peaks correspond exactly with those of pure HAP by the ASTM standard card and are identical to those reported by Xingdong et al. 11. The infra-red absorption spectrum for the fine powder is shown in Figure 5 and corresponds well to those presented by Rootare and Craig lz for pure hydroxyapatite. The XRD and IRS for the coarse powder were identical. The XRD traces for the fired test-pieces formed by both pressing and slip casting were identical to those of the starting powders, indicating that any breakdown of the hydroxyapatite on firing was below the level that could be detected when using this technique. It was noted though that whereas the starting powders were white, the fired test-pieces did take on a slightly bluish tint. This can readily be avoided by firing the material in an atmosphere of steam, when the samples remain white 4. These data confirm the high purity of the HAP used in this study, which is an important consideration in subsequent studies of the biological response to the machinable HAP 13. The presence of impurities can cause breakdown of the HAP, particularly the formation of tricalcium phosphates such as/~-whitlockite, which can be more or less biodegradable than hydroxyapatite 14. The SEM micrograph of the fine powder (Figure 6)
I
70
I
50
30
shows that this powder consists of loosely clustered, poorly defined agglomerates with individual agglomerates up to 20/lm in diameter. The TEM micrograph (Figure 7) shows that these agglomerates are made up of rodshaped particles less than 0.5/lm in length. The coarse powder (Figure 8} consists of porous aggregates. These aggregates, which are up to 50 tim in diameter, are composed of single crystals between 1 to 3/Jm in size, partially bonded together to yield a microporous structure. The granules produced for the powder pressing process are agglomerates of a rounded form, as shown in Figure 9 for the pressbody passed through the 500 pm sieve. The average values for the linear firing shrinkage, apparent and true porosities and bulk density for the pressed and slip cast sintered test-pieces are presented in Tables 1 and 2 respectively. It is evident from these data that the firing shrinkage and bulk density increased, and the apparent and true porosities decreased, as the content of the fine powder was increased. A reduction in the maximum granule size for the pressbodies resulted in an increase in the firing shrinkage and bulk density and a concomitant reduction in the porosity. It is considered
Figure 6 Scanning electron micrograph of the fine hydroxyapatite powder.
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Figure4 X-ray diffraction trace for the as-received fine hydroxyapatite powder. Max= IO0.OOT
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1500
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1000
500
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Figure5 Infra-red absorption spectrum for the as-received fine hydroxyapatite powder. Biomaterials
1 9 9 3 , Vol. 14 N o . 1
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Figure? Transmission electron micrograph of the fine hydroxyapatite powder.
Machinable hydroxyapatite ceramic: M.Y. Shareef et al.
73
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that the lower porosity results from the progressive break-up of the coarser aggregated particles during the granulation process. The finerthe granules produced, the more break-up of aggregate particles occurred. The apparent and true porosity values are very closely matched suggesting that there is virtually no closed porosity in the samples, all the porosity being fully interconnected. The microporous structureof the polished surface for a pressed and sintered sample made from the 6 0 % fine/40% coarse composition for a m a x i m u m granule size of 100/Jm is shown in Figure 10. The interconnected pores have diameters which are of the order of
•
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Granules of hydroxyapatite produced for the pressbody passed through the 500/Jm sieve.
Figure9
1/~m. The appearance of the slip cast materials is
virtually identical. The tensile strength and fracture toughness data as a function of composition and m a x i m u m granule size for the pressbodies are presented in Table 3. The tensile strengths were taken as the m a x i m u m hoop stresses at the inner walls of the test-pieces at failure. Both the average tensile strength and average fracture toughness increased as the proportion of fine powder was increased and as the size of the granules was reduced. Increasing the proportion of fine powder in the granules or increasing the firing time from 3 to 6 h at 1250°C resulted in a
Densification data for hydroxyapatite test-pieces formed by powder pressing, fired for 3 h at 1250°C, as a function of composition and maximum granule size.
Table I
Maximum granule size (pm) Parameter
Composition (% fine)
710
500
250
100
% Firing shrinkage
40 50 60 40 50 60 40 50 60
7.9 10.8 13.2 28.7 26.8 24.2 29.2 27.3 24.8
8.2 11.2 13.8 28.2 25.7 21.3 28.7 26.2 21.7
9.5 11.9 14.0 22.2 18.0 14.9 22.7 18.3 15.2
10.5 12.2 14.3 20.2 15.8 12.0 20.6 16.1 12.2
% Apparent porosity
% True porosity
Bulk density (g/cm 3)
40 50 60
2.23 2.29 2.37
2.25 2.33 2.47
2.44 2.58 2.67
2.50 2.65 2.77
Table 2
Data for hydroxyapatite test-pieces prepared by the slip casting technique for various compositions of fine and coarse powders, using two firing schedules at a sintering temperature of 1250°C.
Composition
60 fine/40 coarse
70 fine/30 coarse
80 fine/20 coarse
Sintering time (h)
3
6
3
6
3
6
% Firing shrinkage
13.3
15.8
17.2
19.9
22.2
29.2
% Apparent porosity
34.9
27.8
29.8
24.9
19.5
16.0
% True porosity
35.0
28.0
30.0
25.0
20.0
16.4
Bulk density (g/cm 3)
2.05
2.28
2.22
2.37
2.53
2.64
B i o m a t e r i a l s 1993, Vol. 14 No, 1
74
Machinable hydroxyapatite ceramic: M. Y. Shareef et al.
Table 3 Tensile strength and fracture toughness data for hydroxyapatite test-pieces prepared by powder pressing as a function of composition and maximum granule size, fired for 3 h at 1250°C.
Maximum granule size (pro) Property
Composition (% fine)
710
500
250
100
Tensile strength (MPa)
40 50 60
20.0 23.6 24.2
23.5 25.7 29.3
29.2 31.9 33.3
31.8 32.8 37.1
Fracture toughness (MPa m ~/~)
40 50 60
0.48 0.54 0.55
0.51 0.55 0.62
0.61 0.66 0.68
0.67 0.68 0.73
Table 4 Fracture toughness data for hydroxyapatite test-pieces prepared by sllp casting as a function of composition and firing times of 3 and 6 h at a sintering temperature of 1250°C. Composition
60 fine/40 coarse
70 fine/30 coarse
80 fine/20 coarse
60 fine/40 medium
Sintering time (h)
3
6
3
6
3
6
3
6
Fracture toughness (MPa m 1/~)
0.48
0.56
0.51
0.59
0.61
0.65
0.66
0.79
denser and stronger material. This material was found not to be machinable, with large pieces fragmenting away due to catastrophic crack propagation throughout the bulk of the material. Thus the values for the tensile strength of 37 MPa and the fracture toughness of 0.73 MPa m ~/z represent the m a x i m u m that was achievable using powder pressing whilst retainingmachinability. The test-pieces made by slip casting mixtures of fine and coarse powders also increased in density as the content of fines increased {see Table 2}. The data for the fracture toughness of different ratios of fine and coarse powder are presented in Table 4 for two firingschedules, showing again an increase in fracture toughness with increased amounts of the fine powder and longer sintering times. However, the samples made with the highest fine powder content, which had the highest density were not as tough as those obtained from powder pressing, the m a x i m u m value for the fracture toughness being 0.65 M P a m ~/~. Whereas the fracture toughness m a y be expected to increase by increasing the density, increasing the proportion of fine powder to achieve this was not possible as a fine powder content in excess
of 80% caused cracking on drying due to excessive shrinkage. Since it had been noted that the density of the powder-pressed test-pieces increased with the breakup of the coarse aggregates, a mixture was produced with the coarse material premilled to produce a medium-sized powder of approximately 3/~m. A mixture of 60 fine/40 medium was prepared in the same w a y as the other mixtures. Test-pieces were fired for 3 and 6 h at 1250°C, resulting in a true porosity of 17.2% and 11.8% respectively. The latter yielded material which was machinable and had a fracture toughness of 0.79 MPa m ~/2 as shown in Table 4. This value was comparable to the m a x i m u m value of the fracture toughness achieved for the pressbodies and close to the limiting value for machinability.
CONCLUSIONS It was found that changes in powder formulation and processing routes have a significant influence on the characteristic properties of hydroxyapatite. Using carefully formulated starting powders, it is possible to produce a uniformly microporous form of hydroxyapatite which is readily machinable in the dental surgery or operating theatre. It has been shown that two processing routes can be used providing a high degree of flexibility in manufacturing dental implant shapes. However, given the low tensile strength and fracture toughness, the use of these materials is restricted to low tensile stressbearing applications such as restoration of bone contour or possibly tooth root replicas.
REFERENCES
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Figure lO Scanning electron micrograph of a polished section of 60% fine/40% coarse composition for a maximum hydroxyapatite granule size of 100 #m fired at 1250°C for 3 h. Biomaterials 1993, Vol. 14 No. 1
2
Frame, ].W. and Brady, C.L.,The versatilityof hydroxyapatite blocks in maxillofacial surgery, Br. ]. Oral Maxillo£ac. Surg. 1987, 25, 452-464 Schwartz,H.C. and Relle,R.].,Extraoralplacement of a subperiostialfissure expander for reconstructionwith H A of the severelyatrophic mandibular alveolarridge, J. Oral Maxillo[ac. Surg. 1990, 48, 157-161
Machinable hydroxyapatite ceramic: M. Y. Shareef et aL 3
4
5
6
7 8 9
10
11
12
13
14
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Dennissen, H.W., Kalk, W., Veldhinis, A.A.M. et al., Eleven year study of HA implants, J. Prosthet. Dent. 1989, 61, 706-711 Shareef, M.Y., Fabrication, characterisation and fracture behaviour of machinable hydroxyapatite ceramic, PhD thesis, University of Sheffield, 1991 Shareef, M.Y., Messer, P.F. and van Noort, R., The fracture behaviour of machinable hydroxyapatite, Am. Ceram. Soc. Ceramic Transactions, Vol. 17, Fractography of Classes and Ceramics II (Ed. V.D. Frechette and ].R. Varner] 1991, pp. 79-99 Shareef, M., Messer, P.F. and van Noort, R., Fabrication of machinable hydroxyapatite, Br. Ceram. Proc. 1990, 45, 59-70 ASTM Designation: C373-72 (Reapproved 1977) Messer, P.F., Die designs for laboratory use, Trans. J. Br. Ceram. Soc. 1982, 81, 66-67 Banda, J.S. and Messer, P.F., Fracture-initiating flaws in whitewares, Am. Cerarn. Soc., Advances in Ceramics, Vol. 22, Fractography of Classes and Ceramics (Ed. V.D. Frechette and ].R. Varner) 1988, pp 363-375 Rowcliffe, D.J., ]ones, R.L. and Cran, J.D., A notch ring facture toughness test for ceramics, Fracture Mechanics of Ceramics, Vol. 3 (Ed. R.C. Bradt, D.P.H. Hasselman and F.F. Lang) Plenum, New York, USA, 1978, 473-482 Xingdong, Z., ]iyong, C., ]truing, Z. et aL, Porous hydroxyapatite granules: their synthesis, application and characterisation, Clin. Mater. 1989, 4, 319-320 Rootare, H.M. and Craig, R.G., Characterisation of hydroxyapatite powders and compacts at room temperature and after sintering at 1200°C, J. Oral Rehab. 1978, 5, 293-307 Heughebaert, J.C. and Bonel, G., Composition, structures and properties of calcium phosphates of biological interest, in Biological and Biomechanical Performance of Biomaterials (Eds P. Christel, A. Meunier and A.].C. Lee}, Elsevier Science Publishers B.V., Amsterdam, The Netherlands, 1986, pp 9-14 Klein, C.P.A.T., de Groot, K., Driessen, A.A. and van der Lubbe, H.B.M., A comparative study of different ~whitlockite ceramics in rabbit cortical bone with regard to their biodegradation behaviour, Biomaterials 1986, 7, 144-146
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Biomaterials 1993, Vol. 14 No. 1
Fabrication, characterization and fracture study of a machinable hydroxyapatite ceramic M.Y. Shareef and P.F. Messer Department of Engineering Materials, University of Sheffield, Sheffield Sl 3JD, UK
R. v a n N o o r t Department of Restorative Dentistry, School of Clinical Dentistry, University of Sheffield, Sheffield S10 2SZ, UK
In this study the preparation of a machinable hydroxyapatite from mixtures of a fine, submicrometer powder and either a coarse powder composed of porous aggregates up to 50pm or a medium powder composed of dense particles of 3pm median size is described. These were characterized using X-ray diffraction, transmission and scanning electron microscopy and infra-red spectroscopy. Test-pieces were formed by powder pressing and slip casting mixtures of various combinations of the fine, medium and coarse powders. The fired test-pieces were subjected to measurements of firing shrinkage, porosity, bulk density, tensile strength and fracture toughness. The microstructure and composition were examined using scanning electron microscopy and X-ray diffraction. For both processing methods, a uniform interconnected microporous structure was produced of a high-purity hydroxyapatite. The maximum tensile strength and fracture toughness that could be attained while retaining machinability were 37 MPa and 0.8 MPa m 1/2 respectively.
Keywords: Hydroxyapatite, tensile properties Received 4 February 1992; revised 9 March 1992; accepted 18 June 1992
H y d r o x y a p a t i t e [Calo(PO4}6{OH)2 ] is a highly biocom-
patible ceramic, which is used in a variety of oral and maxillofacial applications such as bone recontouring, alveolar ridge augmentation and infilling of extraction sockets ~-3. It is available in dense blocks, macroporous blocks and granules. Each of these forms of HAP has its drawbacks: dense HAP is difficult to machine without causing large-scale fracture, granules tend to migrate and the macroporous material leaves a ragged surface finish. These problems were addressed by seeking a means of producing a material with a partially bonded microstructure 4.s, which can readily be shaped either in the dental surgery or operating theatre with conventional dental rotary instruments. The partially bonded microstructure in the present case is composed of polycrystalline aggregates bonded together over small areas and has a porosity which is interconnected. When such a material is mechanically loaded, the highest stresses are developed in the bonded regions. A crack will propagate across such a region but will rapidly run into a pore space. This allows bond breakage to occur without cracks propagating through the bulk of the material. The objective of this study was to produce a machinCorrespondence to Dr R. van Noort. ,~ 1993 Butterworth-Heinernann Ltd 0142-9612/93/010069-07
able, high-purity HAP in such a way that complex shapes could be formed. Flexibility in the production of complex shapes was provided by developing the slip casting route in addition to that of powder pressing. Purity was considered important, since decomposition products resulting from the firing cycle may produce a different hard tissue response than would be expected from HAP. Also, pure HAP is considered to be insoluble in the biological environment and this may be upset by the presence of other calcium phosphates.
MATERIALS AND METHODS Characterization of starting powders The two starting powders of HAP used in this study, designated as fine and coarse, were obtained from Plasma Biotal Ltd, Tideswell, UK. The coarse powder had been produced by the supplier by calcining the fine powder. Previous work indicated these powders to be of exceptionally high purity compared with other powders available 6. The chemical characteristics of the powders were determined using X-ray diffraction (XRD} and infrared spectroscopy {IRS}, while the physical characteristics Biomaterials 1993, Vo]. 14 No. 1
70
Machinable hydroxyapatite ceramic: M.Y. Shareef eta/.
and coarse powders. The powder mixtures were granulated and subsequently sieved to produce pressbodies 5. These had granule size ranges of < 710 pro, < 500pm, < 250 pm and < 100 pm. Any granules greater than the sieve being used were crushed and regranulated. Testpieces were produced from each of the pressbodies by double-ended uniaxial compaction in a steel die at 47 MPa. These were fired in air at 1250°C for 3 h in an electrically heated muffle kiln. The various stages in the production of the granules and the test-pieces are shown in the flow diagram of Figure 1. The test-pieces consisted of annular rings with an external diameter of 27 ram, internal diameter of 19 m m and height 5 mm, as shown in Figure 2.
were determined using scanning and transmission electron microscopy (SEM and TEM). XRD measurements were carried out using CuKa radiation (a = 0.154050 nm] in a Philips X-ray diffractometer (PW 1050/25 goniometer and PW 1730 generator) with 50 kV tube voltage and 30 mA current. A scan rate of 2°/min was used over a range of 10-70 ° for all samples. The positions and intensity of the peaks were read from the X-ray trace and the calculated d-values compared with those listed in the ASTM powder diffraction files. The infra-red absorption spectra of the two HAP powders were recorded from 400 to 4000 cm -1 at ambient temperature using a Perkin-Elmer 683 infra-red spectrophotometer. Samples were pelletized with KBr [0.5% dilution] at a pressure of 765 MPa, which was carried out in an evacuated die for 10 min so as to minimize water sorption. Samples of the HAP powders were examined using a Philips P-500 scanning electron microscope. The fine powder was also examined under a Philips 301 transmission electron microscope.
Preparation of slip cast and sintered test-pieces A suspension was prepared containing 18 vol% of the submicrometer powder in deionized water, To disperse the agglomerates, Calgon (sodium hexameta phosphate) was added as a deflocculant. A few porcelain balls were placed in the plastic pot containing the suspension and the pot was rotated on rollers for 15 h to effect the dispersion. Using a Rheomat 30 rotating cylinder viscometer, the suspension was found to exhibit essentially Newtonian hehaviour. The required amounts of coarse powder were added to suspensions of the fine powder to produce mixtures of fine and coarse powders of 80/40, 70/30 and 80/20 respectively. A 2 wt% addition of Carhowax (polyethylene glycol) relative to the solids content was added as a binder. A further addition of Calgon was made to the suspensions, to retain just a semblance of pseudoplastic hehaviour which helped to prevent sedimentation of the larger particles. Mixing was carried out for a further 3 h. Suspensions were poured into a one-piece plaster of
Preparation of pressed and sintered test-pieces Previous work 5'8 has shown that compacts of the fine powder have a dense microstructure close to the theoretical density, while the coarse powder produces a very weak highly porous microstructure. To maximize the strength and fracture toughness of the machinable HAP, test-pieces were prepared from mixtures of the fine and coarse powder in the ratios by weight of 60/40, 50/50 and 40/60. Pressing of these simple mixtures would result in a non-uniform packing. Therefore, uniform filling of the die cavity of the mould on pressing required the production of suitable granules of the mixtures of fine
Jc°arse I
I F'ne I
powder
Water (deionlzed)
powder
I
Dispersion of fine powder Mixed for 15 h
1
l
40,50,60w/orelativl to fine
Preparation of powder mixture Mixed for 3 h vI
I I
I
~[ I-
'
4 w/o relative to solids content
1
Powder cake formation Slip dried at 100°C for 24 h
I
1
Classification Passed through a 100,250,500 and 710 IJm sieve Forming Double-ended uniaxial compaction in a steel die at 47MPa
Figure
l_ ]~
I
Binder [ (Carbowax) ]
Agglomerated powder granulation Crushed, moistened and rubbed
P
Oversized particles crushed and regranulated
1
1
Deflocculant (Calgon)
Firing Fired in air at t250°C for 3h
Flow diagram showing the production stages for the granules and the test-pieces of hydroxyapatite using powder pressing.
Biomaterials 1993, Vol. 14 No. 1
Machinable hydroxyapatite ceramic: M.Y. Shareef et al.
71
Characterization of the fired test-pieces
,'" ~ - ~ . ~ ' t t ~ , t t ~ h . . ~ . ~ .
The fired test-pieces were characterized by measuring the firing shrinkage, apparent and true porosities and bulk density. The shrinkages of the test-pieces were determined from their green and fired dimensions, which were measured using a travelling microscope. The densities and porosities were determined using the standard technique involving boiling in wateF. The microstructure and composition were examined using SEM and XRD respectively. For SEM examination, the samples were impregnated with epoxy resin, polished and the resin removed by firing at 400°C.
r..~
5 cm !_
I
I
I
I
Measurement of tensile strength and fracture toughness Figure 2
Ring-shaped test-piecesof hydroxyapatite prepared by powder pressing.
The modified Stanford ring bursting test s was used to determine the tensile strength of the pressed and sintered HAP. In this test, the test-piece is pressurized internally to generate a tensile hoop stress in the ring. The internal pressure is increased until the test-piece fails. The average tensile strengths were determined using 15 testpieces for each pressbody. Similar tests were not performed for the slip cast material because of problems in producing a consistent surface finish. Fracture toughness values for both pressed and slip cast test-pieces were determined using the technique described above, except that each test-piece was prenotched radially on its outer surface to a depth of approximately 2 mm. A 200pm thick diamond-impregnated circular blade was used to form the notches. The exact depths of the cuts were measured using a travelling microscope. Seven test-pieces were used to determine the average fracture toughness value for each type of test-piece. The tensile strength and fracture toughness values were calculated using formulae published in the literature 9, lo
Paris mould to a depth of about 80 mm. The mould had a cylindrical cavity of 34 m m in diameter and was 160 mm long. Casting was continued for 15 min and then the surplus slip drained from the mould. Cast tubes were thoroughly dried and then fired in air in an electrically heated muffle kiln for either 3 or 6 h at 1250°C. Up to eight rings, 6 m m in height, were cut from a fired tube with a diamond-impregnated saw. The plane ends of the rings were carefully ground flat on SiC papers and radially notched using a 2 0 0 p m diamond-impregnated saw to produce the test-pieces for the measurement of fracture toughness. Following tests on the fired slip cast test-pieces, which showed them to be too porous, the coarse porous aggregates were replaced by a medium-sized powder composed of dense particles with a median size of 3/Jm. This powder was produced by ball milling the coarse powder. Medium powder was added to a dispersion of the fine powder to produce a mixture of 60 fine/40 medium. This also had Carbowax added as a binder and was mixed for 3 h. The various stages are shown in Figure 3.
Coarse
[M_illedf o r
[ powder |
I
Stage
L
I
30 h
I Medium ] -I powder
[
'
The machinability of the test-pieces was assessed using a stee~ fissure bur in a slow speed dental handpiece by
I
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Deflocculant (Ca gon)
¢
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Machinability
Deflocculated suspension Wet ball milled for 15h
~
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4
Binder I I(Carbowax)
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Wet ball milled for 3 h 1
2 wlo relative to solids content
Slip casting Drain casting of cylindrical tube using a piaster of Paris mould
I
Figure 3
1
Drying and firing [ Dried at room temperature for 2q h and fired in air at 1250°C for 3 or 6h
Flow diagram showing the production stages for the test-pieces of hydroxyapatite using slip casting,
Biomaterials 1993, Vol. 14 No. 1
72
Machinable hydroxyapatite ceramic: M. Yo Shareef et al.
attempting to remove material by applying only light finger pressure.
RESULTS AND D I S C U S S I O N The XRD trace for the as-received fine powder is presented in Figure 4. Only peaks attributable to HAP could be detected for both powders, which have a well-defined crystal structure as indicated by the sharpness of the peaks. The peaks correspond exactly with those of pure HAP by the ASTM standard card and are identical to those reported by Xingdong et al. 11. The infra-red absorption spectrum for the fine powder is shown in Figure 5 and corresponds well to those presented by Rootare and Craig lz for pure hydroxyapatite. The XRD and IRS for the coarse powder were identical. The XRD traces for the fired test-pieces formed by both pressing and slip casting were identical to those of the starting powders, indicating that any breakdown of the hydroxyapatite on firing was below the level that could be detected when using this technique. It was noted though that whereas the starting powders were white, the fired test-pieces did take on a slightly bluish tint. This can readily be avoided by firing the material in an atmosphere of steam, when the samples remain white 4. These data confirm the high purity of the HAP used in this study, which is an important consideration in subsequent studies of the biological response to the machinable HAP 13. The presence of impurities can cause breakdown of the HAP, particularly the formation of tricalcium phosphates such as/~-whitlockite, which can be more or less biodegradable than hydroxyapatite 14. The SEM micrograph of the fine powder (Figure 6)
I
70
I
50
30
shows that this powder consists of loosely clustered, poorly defined agglomerates with individual agglomerates up to 20/lm in diameter. The TEM micrograph (Figure 7) shows that these agglomerates are made up of rodshaped particles less than 0.5/lm in length. The coarse powder (Figure 8} consists of porous aggregates. These aggregates, which are up to 50 tim in diameter, are composed of single crystals between 1 to 3/Jm in size, partially bonded together to yield a microporous structure. The granules produced for the powder pressing process are agglomerates of a rounded form, as shown in Figure 9 for the pressbody passed through the 500 pm sieve. The average values for the linear firing shrinkage, apparent and true porosities and bulk density for the pressed and slip cast sintered test-pieces are presented in Tables 1 and 2 respectively. It is evident from these data that the firing shrinkage and bulk density increased, and the apparent and true porosities decreased, as the content of the fine powder was increased. A reduction in the maximum granule size for the pressbodies resulted in an increase in the firing shrinkage and bulk density and a concomitant reduction in the porosity. It is considered
Figure 6 Scanning electron micrograph of the fine hydroxyapatite powder.
10
28 degrees
Figure4 X-ray diffraction trace for the as-received fine hydroxyapatite powder. Max= IO0.OOT
#
/ 'v 4000 3500300025002000
1500
'
1000
500
Min = 0.00T
Wave number (cm - I )
Figure5 Infra-red absorption spectrum for the as-received fine hydroxyapatite powder. Biomaterials
1 9 9 3 , Vol. 14 N o . 1
0 . 5 IJm ~.,
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Figure? Transmission electron micrograph of the fine hydroxyapatite powder.
Machinable hydroxyapatite ceramic: M.Y. Shareef et al.
73
.......
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.,
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Figure6
of the coarse
that the lower porosity results from the progressive break-up of the coarser aggregated particles during the granulation process. The finerthe granules produced, the more break-up of aggregate particles occurred. The apparent and true porosity values are very closely matched suggesting that there is virtually no closed porosity in the samples, all the porosity being fully interconnected. The microporous structureof the polished surface for a pressed and sintered sample made from the 6 0 % fine/40% coarse composition for a m a x i m u m granule size of 100/Jm is shown in Figure 10. The interconnected pores have diameters which are of the order of
•
.,~
Granules of hydroxyapatite produced for the pressbody passed through the 500/Jm sieve.
Figure9
1/~m. The appearance of the slip cast materials is
virtually identical. The tensile strength and fracture toughness data as a function of composition and m a x i m u m granule size for the pressbodies are presented in Table 3. The tensile strengths were taken as the m a x i m u m hoop stresses at the inner walls of the test-pieces at failure. Both the average tensile strength and average fracture toughness increased as the proportion of fine powder was increased and as the size of the granules was reduced. Increasing the proportion of fine powder in the granules or increasing the firing time from 3 to 6 h at 1250°C resulted in a
Densification data for hydroxyapatite test-pieces formed by powder pressing, fired for 3 h at 1250°C, as a function of composition and maximum granule size.
Table I
Maximum granule size (pm) Parameter
Composition (% fine)
710
500
250
100
% Firing shrinkage
40 50 60 40 50 60 40 50 60
7.9 10.8 13.2 28.7 26.8 24.2 29.2 27.3 24.8
8.2 11.2 13.8 28.2 25.7 21.3 28.7 26.2 21.7
9.5 11.9 14.0 22.2 18.0 14.9 22.7 18.3 15.2
10.5 12.2 14.3 20.2 15.8 12.0 20.6 16.1 12.2
% Apparent porosity
% True porosity
Bulk density (g/cm 3)
40 50 60
2.23 2.29 2.37
2.25 2.33 2.47
2.44 2.58 2.67
2.50 2.65 2.77
Table 2
Data for hydroxyapatite test-pieces prepared by the slip casting technique for various compositions of fine and coarse powders, using two firing schedules at a sintering temperature of 1250°C.
Composition
60 fine/40 coarse
70 fine/30 coarse
80 fine/20 coarse
Sintering time (h)
3
6
3
6
3
6
% Firing shrinkage
13.3
15.8
17.2
19.9
22.2
29.2
% Apparent porosity
34.9
27.8
29.8
24.9
19.5
16.0
% True porosity
35.0
28.0
30.0
25.0
20.0
16.4
Bulk density (g/cm 3)
2.05
2.28
2.22
2.37
2.53
2.64
B i o m a t e r i a l s 1993, Vol. 14 No, 1
74
Machinable hydroxyapatite ceramic: M. Y. Shareef et al.
Table 3 Tensile strength and fracture toughness data for hydroxyapatite test-pieces prepared by powder pressing as a function of composition and maximum granule size, fired for 3 h at 1250°C.
Maximum granule size (pro) Property
Composition (% fine)
710
500
250
100
Tensile strength (MPa)
40 50 60
20.0 23.6 24.2
23.5 25.7 29.3
29.2 31.9 33.3
31.8 32.8 37.1
Fracture toughness (MPa m ~/~)
40 50 60
0.48 0.54 0.55
0.51 0.55 0.62
0.61 0.66 0.68
0.67 0.68 0.73
Table 4 Fracture toughness data for hydroxyapatite test-pieces prepared by sllp casting as a function of composition and firing times of 3 and 6 h at a sintering temperature of 1250°C. Composition
60 fine/40 coarse
70 fine/30 coarse
80 fine/20 coarse
60 fine/40 medium
Sintering time (h)
3
6
3
6
3
6
3
6
Fracture toughness (MPa m 1/~)
0.48
0.56
0.51
0.59
0.61
0.65
0.66
0.79
denser and stronger material. This material was found not to be machinable, with large pieces fragmenting away due to catastrophic crack propagation throughout the bulk of the material. Thus the values for the tensile strength of 37 MPa and the fracture toughness of 0.73 MPa m ~/z represent the m a x i m u m that was achievable using powder pressing whilst retainingmachinability. The test-pieces made by slip casting mixtures of fine and coarse powders also increased in density as the content of fines increased {see Table 2}. The data for the fracture toughness of different ratios of fine and coarse powder are presented in Table 4 for two firingschedules, showing again an increase in fracture toughness with increased amounts of the fine powder and longer sintering times. However, the samples made with the highest fine powder content, which had the highest density were not as tough as those obtained from powder pressing, the m a x i m u m value for the fracture toughness being 0.65 M P a m ~/~. Whereas the fracture toughness m a y be expected to increase by increasing the density, increasing the proportion of fine powder to achieve this was not possible as a fine powder content in excess
of 80% caused cracking on drying due to excessive shrinkage. Since it had been noted that the density of the powder-pressed test-pieces increased with the breakup of the coarse aggregates, a mixture was produced with the coarse material premilled to produce a medium-sized powder of approximately 3/~m. A mixture of 60 fine/40 medium was prepared in the same w a y as the other mixtures. Test-pieces were fired for 3 and 6 h at 1250°C, resulting in a true porosity of 17.2% and 11.8% respectively. The latter yielded material which was machinable and had a fracture toughness of 0.79 MPa m ~/2 as shown in Table 4. This value was comparable to the m a x i m u m value of the fracture toughness achieved for the pressbodies and close to the limiting value for machinability.
CONCLUSIONS It was found that changes in powder formulation and processing routes have a significant influence on the characteristic properties of hydroxyapatite. Using carefully formulated starting powders, it is possible to produce a uniformly microporous form of hydroxyapatite which is readily machinable in the dental surgery or operating theatre. It has been shown that two processing routes can be used providing a high degree of flexibility in manufacturing dental implant shapes. However, given the low tensile strength and fracture toughness, the use of these materials is restricted to low tensile stressbearing applications such as restoration of bone contour or possibly tooth root replicas.
REFERENCES
1 t~r.~'.
.-t
.
.~--
~
=
~=
~
~
~_
L._~r
~q~
-
Figure lO Scanning electron micrograph of a polished section of 60% fine/40% coarse composition for a maximum hydroxyapatite granule size of 100 #m fired at 1250°C for 3 h. Biomaterials 1993, Vol. 14 No. 1
2
Frame, ].W. and Brady, C.L.,The versatilityof hydroxyapatite blocks in maxillofacial surgery, Br. ]. Oral Maxillo£ac. Surg. 1987, 25, 452-464 Schwartz,H.C. and Relle,R.].,Extraoralplacement of a subperiostialfissure expander for reconstructionwith H A of the severelyatrophic mandibular alveolarridge, J. Oral Maxillo[ac. Surg. 1990, 48, 157-161
Machinable hydroxyapatite ceramic: M. Y. Shareef et aL 3
4
5
6
7 8 9
10
11
12
13
14
75
Dennissen, H.W., Kalk, W., Veldhinis, A.A.M. et al., Eleven year study of HA implants, J. Prosthet. Dent. 1989, 61, 706-711 Shareef, M.Y., Fabrication, characterisation and fracture behaviour of machinable hydroxyapatite ceramic, PhD thesis, University of Sheffield, 1991 Shareef, M.Y., Messer, P.F. and van Noort, R., The fracture behaviour of machinable hydroxyapatite, Am. Ceram. Soc. Ceramic Transactions, Vol. 17, Fractography of Classes and Ceramics II (Ed. V.D. Frechette and ].R. Varner] 1991, pp. 79-99 Shareef, M., Messer, P.F. and van Noort, R., Fabrication of machinable hydroxyapatite, Br. Ceram. Proc. 1990, 45, 59-70 ASTM Designation: C373-72 (Reapproved 1977) Messer, P.F., Die designs for laboratory use, Trans. J. Br. Ceram. Soc. 1982, 81, 66-67 Banda, J.S. and Messer, P.F., Fracture-initiating flaws in whitewares, Am. Cerarn. Soc., Advances in Ceramics, Vol. 22, Fractography of Classes and Ceramics (Ed. V.D. Frechette and ].R. Varner) 1988, pp 363-375 Rowcliffe, D.J., ]ones, R.L. and Cran, J.D., A notch ring facture toughness test for ceramics, Fracture Mechanics of Ceramics, Vol. 3 (Ed. R.C. Bradt, D.P.H. Hasselman and F.F. Lang) Plenum, New York, USA, 1978, 473-482 Xingdong, Z., ]iyong, C., ]truing, Z. et aL, Porous hydroxyapatite granules: their synthesis, application and characterisation, Clin. Mater. 1989, 4, 319-320 Rootare, H.M. and Craig, R.G., Characterisation of hydroxyapatite powders and compacts at room temperature and after sintering at 1200°C, J. Oral Rehab. 1978, 5, 293-307 Heughebaert, J.C. and Bonel, G., Composition, structures and properties of calcium phosphates of biological interest, in Biological and Biomechanical Performance of Biomaterials (Eds P. Christel, A. Meunier and A.].C. Lee}, Elsevier Science Publishers B.V., Amsterdam, The Netherlands, 1986, pp 9-14 Klein, C.P.A.T., de Groot, K., Driessen, A.A. and van der Lubbe, H.B.M., A comparative study of different ~whitlockite ceramics in rabbit cortical bone with regard to their biodegradation behaviour, Biomaterials 1986, 7, 144-146
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