Biodegradation and bioresorption of calcium phosphate ceramics

Clinical Muterids 14 (1993) 65-88

Review paper Biod.egradation and Bioresorption Phosphate Ceramics

of Calcium.

Racquel Z. LeGeros New York University College of Dentistry, 345 East 24th Street, New York, New York 10010, USA (Received 1 July 1991; accepted 1 December 1991)

Abstract: The use of several calcium phosphate (Ca-P) materials for bone repair, augmentation, substitution and as coatings on metal implants has gained clinical acceptance in many dental and medical applications. These Ca-P materials may be of synthetic or natural origin, available in different physical forms (dense or macroporous, particles or blocks) and are used in bulk as coatings for metallic and non-metallic substrates or as components in composites, cements and bioactive glasses. Biodegradation or bioresorption of calcium phosphate materials implies cell-mediated degradation in vitro or in vivo. Cellular activity during biodegradation or bioresorption occurs in acid media; thus the factors affecting the solubility or the extent of dissolution (which in turn depends on the physicochemical properties) of the Ca-P materials are important. Enrichment of the microenvironment due to the release of calcium and phosphate ions from the dissolving Ca-P materials affects the proliferation and activities of the cells. The increase in the concentrations of the calcium and phosphate ions promotes the formation of carbonate apatite which are similar to the bone apatite. The purpose of this invited paper is to discuss the processes of biodegradation or bioresorption of Ca-P materials in terms of the physico-chemical properties of these materials and the phenomena involved including the formation of carbonate apatite on the surfaces and in the vicinity of these materials. This phenomenon appears to be related to the bioactivity of the material and the ability of such materials to directly attach to bone and to form a uniquely strong material-bone interface.

Figs 1 and 2), and can be prepared in different physical forms, e.g. dense, microporous and/or macroporous or particles or blocks (Fig. 3). These materials can be used in bulk or #as coatings on metafic25, 37,39,46,93,107,114 or polymeric substrates;12@ 17*or admixed with other inorganic: materials, e.g. gypsum, alumina,2’ I3916* g1ass,59j97lto2,195 or poly~~~~~2~16~114~119~191 materials to form composites. Experimental materials include calcium phosphate cements, CPC,20,791127,16’ apatite with CO3 or F 5,94-96,105,112

INTRODUCTION The use of some Ca-P materials for bone substitution, augmentation and repair has gained clinical acceptance in many areas of orthopedics and dentistry. Dental applications include fillers for periodontal bony defects, alveolar ridge augmentation, immediate tooth root replacement, coatings for dental metal implants and maxillofacial reconstruction 4,9,26,36,41,42,50,61,79,89,91,112-114,137,141, 151-154,162,171,1~6,187,194

Medical applications include ear implants, spine fusion, repair of bony defects, and coatings for orthopedic metal implants. 1,2,32,37,59,64,73,78,80,81,87,114,160,165,173,185-187,190

Biodegradation is the process caused by the action of living systems (e.g. microorganisms, cells) when a material breaks down into its simpler components; reduces the complexity of a chemical compound or wears away by erosion (Webster’s dictionary, paraphrased). Resorption of bone

ca_p

materials differ in composition from each other, reflecting the differences in their origin (e.g. natural or synthetic) and methods of preparation (Table 1; 65

Clinical Mak+als 0267-6605/93/$6.00 0 1993 Elsevier Science Publishers Ltd, England

66 Table 1. Ca-P materials

for bone repair, substitution

and a~~rne~t~tio~

Materials (1) Natural biomaterials (a) Allogenic bones (b) Coralline HA (Interpore”) (c) Bovine bone (Kielbone *, Pyrost*, Snrgibone*;

~axi~~ofaci~~~ cranial bone defects; dental and orthopedic Dental and orthopedic surgery Dental and orthopedic surgery TBC**, BonAP)

(2) Bioactive glass ceramics (Bioglass*, Ceravital*, AW-GC**) (3) Calcium phosphate materials HA (Calcitite*, Osteograp, Bioapatite*); b-TCP (SynthograF); BCP (Triosit*); non-ceramic apatite (Osteogen*) Experimental

surgery

Ca-P

Ear implant; maxillofacial metal implants ental, orthopedics, implants Fillers for composites;

* Commercial; ** Experimental Dental applications: periodontal defects; alveolar ridge augmentation; tooth-rook Orthopedic applications: bone defects; spine fusion; coating for metal prosthesis

in vitro or in vivo has been described as a cellmediated degradation of the inorganic and organic phases of bone.81 171211643g01191 For this paper,

reconstruction;

dentai; coafkgs ii01

ear surgery, maxillofacial:

coatings

coatings

for metal

on polymers

repiacements.

$X.Xdegradation

in witrs or in viva.

C

(b)

Fig. 1. X-ray diffraction (XRD) patterns of calcium phosphate ceramics: (A) tricalcium phosphate, b-Ca3(P04)2, b-TCP; (B) biphasic calcium phosphate, BCP, consisting of a mixture of HA and b-TCP, 40/60, b-TCP/HA; (C) calcium hydroxyapatite, HA, Caro(PO,k(OH)z.

Fig. 2. XRD patterns comparirmg (A) bone mineral coralline HA and (C) HA ceramic.

with (

Biodegradation and bioresorption of calcium phosphate ceramics

Fig. 3. Scanning electron microscopy (SEM) of (A) dense and W w-c IUScalcium phosphate materials. The macroporosity terial (B) was intentionally introduced; the micrc jporosity in (A) and (B) is due to sintering conditions.

after contact with the biological environment (e.g. cell culture, serum - in vitro; or after implantation). ‘The physical changes may include disintegration or breaking away into smaller particles, loss of mechanical strength, loss of density, changes in th’e micro- and macroporosities or changes in implant size and/or weight.28’381421491981 99,145 The chemical changes include reduction in pH in the implant environment causing partial dissolution of the material which will be exhibited in terms of physical changes mentioned above on micro and macro levels, reduction in size of the particles and of individual Ca-P crystals of the implant material., elevation in the concentrations of calcium (Ca) and phosphate (P) ions in the microenvironment, leading to the formation/deposition of other Ca-P phases intimately associated with organic matrices on the surfaces of the Ca-P materials or incorporation (of these crystals into the new bone formed at the implant/host bone interface 11,24~,28,30,45,58,62,76,88,92,114,118,135, 179 Biodegradation/bioresorption has been assumed to be related to the bioactivity of the @a-P materials.

36,45,70,71,73-75,86,98-101,158,159

67

of ma-

As wilth some special glasses, Ca-P ceramics may display characteristics of biocompatibility and have a unique property (bioactivity) which allows them to form a chemical bond with the bone tissue resulting in a very strong bone/ material interface.24>45169-711 73-75 This property is believed to be related to the ability of the bioactive materials e.g. glasses or apatite/glass composites to form a Ca-P compound on the implant surfaces upon exposure to biological or simulated biological fluids 28,59,74,83,102,118,135,185 Biodegradation/bioresorption of the material is assumed also to be related to solubility of the Ca-P materials,24’45’ 87,99~101,132,161,185 which in turn is, related to the bioactivity, and ultimately to the hone formation of the bone/material interface. Factors affecting biodegradation/bioresorption are both materially and environmentally related concerning the properties of the Ca-P materials themselves and those of the biological environment. In order to assess the factors relating to the properties of the materials, they have to be appropriately characterized in terms of their composition and

68

Racquel Z. LeGeros

physico-chemical properties (e.g. crystallinity). Likewise, the biological factors need to be fully defined, taking into consideration the influence of such variables as animal species and age, implantation sites (osseous or non-osseous; if osseous, cancellous or cortical; types of bone), metabolic activity, diseased states (e.g. presence of infection), and the types of cells involved. The apparent disagreement on what types of Ca-P ceramics are ‘biomay be due degradable ,40,41,45,50,87,98-100,150,151,158 to inappropriate or insuthcient characterization csf the Ca-P materials,l12 limitations of methods for assessing biodegradation/bioresorption,31V451145 an failure to consider differences in implant sites (e.g. cortical vs. cancellous bone; osseous versus non-osseous sites). The purpose of this invited paper is to discuss the biodegradation or bioresorption of calcium phosphate (Ca-P) materials in terms of their physicochemical properties and the processes involved on the cellular and crystal levels. This paper will discuss the following topics: bone and bone mineral, calcium phosphate (Ca-P) materials, biodegradation or bioresorption of Ca-P materials, physico-chemical and cellular events involved, formation of carbonate apatite crystals on surfaces of Ca-P materials and bone-material interface.

CaE

Fig. 4. IR spectraof (A) bone miner after remaval of the organic phase and ( ) bone mineral associated with the organic phase, indicated by the absorption bands due to C-H and N-H vibrations. The bone mineral is a CC+qwdit~ (A).

Bone and bone mineral

Bone is an intimately integrated composite of inorganic and organic phases (Fig. 4B) with the inorganic/organic ratio of approximately 75/Z by weight and 65/35 by volume. The organic phase consists principally of collagen (Type I) and small amounts of non-collageneous proteins, acidic glycoproteins, phospho-proteins, serum proteins, lipids and small proteoglycans.“’ 1ga63The inorganic or mineral phase has been idealized as a calcium hydroxyapatite, CaIo(P04)6(0H)2, (HA), and still often erroneously described as a hydroxyapatite. Actually, the bone mineral is a carbonate-substituted apatite (Fig. 4B), associated with minor (e.g. sodium, magnesium, potassium, chloride, HP042-, fluoride, etc.) and trace (e.g. strontium, zinc, copper, iron, etc.) constituents, with the Ca/P molar ratio below or above l-67 (the stoichiometric value for pure HA), depending upon the specie, age 113,121The bone apatite and type of bone. 63,1Os,ttO, microcrystals have rod-like or plate-like Omorphology, with average dimensions of 250 A x 30A. The bone apatite crystallinity (reflecting crystal size

among

D patterns showing the difleerences in crystallimty the biological apatites in the mineral phases of (A) human enamel, (B) dentin and (6) bone.

Biodegradation and bioresorption of calcium phosphate ceramics

responsi’ole for bone resorption in vitro and in vivo are osteoclasts.7’ 17,21a 9o The biodegradation (dissolution) o:f the organic and inorganic phases caused by osteoclast activity is characterized by the pres#ence of resorption pits as observed by scanning electr’on microscopy (SEM), a commonly used method for the assav of bone resornAutogeneous bone and allogenic or banked bones are commonly used as bone grafts.51 However, the second surgery and longer healing period required for the use of autogeneous bone, the risk and grealter expense associated with these types of bone graft, spurred the search for other materials for bone substitution, augmentation and repair. The experimental and commercial Ca-P materials or materials which form Ca-P (e.g. bioactive glasses with and without apatite) are similar to the bone mineral in terms of having calcium (Ca) and phosphate (P) ions as their principal components. However, differences in the Ca/P molar ratio, composition and in several properties are evident (Fig. 2; Table 2). Calcium phosphate materials Preparation

and composition

‘Triple calcium phosphate’ and ‘tribasic calcium phospha.te’ reagents were reported to promote new bone formation when used in bony defects.” 164 Unfortunately, these reagents were not characterized. Based on the variability in composition of presently available commercial calcium phosphate reagents, it can be safely assumed that the Ca-P reagents used in these early studies were just as varied in composition as the Ca-P reagents available at the present time, as shown by X-ray diffraction analyses.“” ‘I4 For example, commercial Ca-P reagents labeled as ‘calcium phosphate, tribasic’, “hydroxyapatite’, ‘CarO(PO&(OH)2’ or ‘spheroidal hydroxyapatite’; and commercial Ca-P material described as ‘resorbable HA’ show that Table 2. Tensile strength Material Cortical bone Ti-6Al-4V alloy Polyethylene Bioglass* Hydroxyapatite, Dense Porous

of bone and bone graft materials Tensile strength (MPa)

HA

69-110 900 3 42 799196 42

69

(b)

Cd)

1 I 25

I 27

I I 29

I

I

I

31

I 33 2’

28

DIFFRACTION ANGLE

Fig. 6. XRD patterns of calcium phosphate reagents labeled as ‘Tribasic, calcium phosphate’; some are given an ‘approximate formula, Ca10(OH)2(P0&’ from different manufacturers.‘r3 (A) and (D) are from the same manufacturer, but different lot numbers. Only apatite calcium phosphate pattern is observed with (A); apatite mixed with other calcium phosphate phase, e.g. monetite (M) is observed in (IB)) (C) and (D).

labels and commercial description can be misleading (Figs 6 and 7). Slurries of three types of ‘biodegradable’ calcium phosphates (‘tricalcium phosphate’ (TCP); monetite, CaHP04; and ‘hydrated ca1ciu.m phosphate’, CaH4(P04) 2.H20) were compared in their abilities to repair surgically created bone defects in the proximal end of the rat tibiam6t ‘TCP’ was reported to degrade more slowly than CaHP04, but both materials promoted bone healing as rapidly as blood clotting. On the other hand, the hydrated Ca-P was slow to resorb and defects containing this material showed the least bone rep;air. In this study, it was not clear whether the ‘tricalcium phosphate’ used was really a tricalcium phosphate compound, i.e. Ca3(P04)2y with a whitlockite structure; or a Ca-P reagent labeled as ‘tricalcium phosphate’. Levin et aZ.137 reporteld the successful application of a ‘biodegradable’ material described as ‘tricalcium phosphate’ or “TCP’ ceramic. Nery et al. used ‘tricalcium phosphate’ ceramic implants in surgically produced infrabony defects, and reported new bone formation but no significant biodegradation or bioresorption of the ‘TCP’ cera-

Racquel Z. LeGeros

70 (4

Diffraction

Ln,qle

Fig. 8.

, 25

t

I

27

I

I

29

I

!

31

I

I

33

I,

350 26

DIFFRACTION ANGLE

Fig. 7. XRD pattern of reagents labeled as ‘spheroidal hydroxyapatite’, same manufacturer, different lot numbers. These materials are identified as (A) pure beta-tricalcium phosphate, b-TCP; and (B) a mixture of b-TCP and HA.

Obviously, the ‘TCP ceramic materials referred to by Levin et al.‘37 and Nery et LZ~.“~‘15’ ~ showed different types of biodegradation because they were not similar, although both ceramics were prepared by sintering Ca-P reagents labeled as ‘tribasic’ or ‘tricalcium phosphate’. The ‘TCP’ used by Nery et al., prepared by Hubbard,‘” was later analyzed by this author as a biphasic Ca-P ceramic, consisting of a mixture of b-TCP and HA in the approximate b-TCP/HA weight ratio of 20/ 80.’l2 (Subsequent publications using this material referred to it as ‘biphasic calcium phosphate’ or BCP.” Thus, this apparent lack of agreement on the ‘biodegradability’ of TC ceramics was due to the variability of the b-TCP/HA ratio in sintered apatites prepared by different methods,1111161 as shown in Figs 8 and 9. It has been demonstrated that b-TCP or whitlockite, Ca3(P04)2, is more soluble and biodegrades to a greater extent than HA, 36,44,87,99-101,109,112,113,158,159It has Calo(Po&(oH). also been demonstrated that the b-TCP/HA ratio in the ceramic affects its dissolution and in-vivo performance: the higher the b-TCP/HA ratio, the

patterns of calcium phosphate reagent labeied as ‘tribasi ’ fnxn the same rna~~fa~~t~re~~ but one made in France an one in Germany, giving similar XRD patterns; (A) after sintering at 900°C, the product is b-TCP; (B) from product made in Germany and HA with small amount of b-TCP (about 5% by weight); CC) loom product made En France.

e

extent of ~~ssoI~ti0~

an

biodegradation2*’ ‘l*,152 tial dissolution of b-T

IIlk. 15'a15'

!i’

Fig. 9. XRD patterns (A) before sintering ofcaicium reagent labeted as “calcium phosphat sintering at 800°C product gives an (C) after sintering at 900 u and 1 lQO”C, pattern of b-TCP + HA

~~~~~~~~~

Biodegradation and bioresorption of calcium phosphate ceramics

71

McCarty22 and Evans et a1.52 consist of such mixtures. The composition and Ca/P of the starting apatitic material depends upon the pH, temperature (of preparation or sintering), solution composition and mode of preparation. t13 In turn, the composition and Ca/P ratio determines the amou.nt of HA in the sintered product: if the Ca/P is lower than I.67 (the stoichiometric value for pure A), or if prepared at low pH, b-TCP appears with HA after sintering above 900 “C, in varying b-TCP/HA ratios; if the Ca/P of the starting apatitic Ca-P is higher than 1~67,or prepared at very high pH or in the presence of carbonate (C03), the sintered product will contain CaO with HA 15,11*>‘131124-1261 I*’ The temperature of sintering also affects the crystallinity and the b-TCP/ HA ratio in the sintered product. For example, sintered apatitic Ca-P showe the ‘HA’ phase after sintering at 800 “C; a .TCP/HA after [A ceramics do sintering above 900 “C (Fig. not differ significantly i the b-TCP/HA ratio but differ in crystallinity, r ecting crystal size and/or strain. Commercial HA ceramics obtained by synthetic methods differ in their crystallinity. HA ceramics produced in France (e.g. Bioapatite* by PRED) and Japan (e.g. by itsubishi) generally have a lower crystallinity n the HA from the US (e.g. Calcitite * from Calcitek; Osteograf Fig. 10. XFID of biphasic calcium phosphate materials (A) before, from Sterio-Oss), probably reflecting the differ(B) after exposure to acid buffer for 1 h and (C) after exposure to acid buffer for 3 h. Note the preferential dissolution of the ences in their sintering temperatures. ’l4 Differences b-TCP (T) phase. in sintering temperatures will cause differences in extent of dissolution and therefolre differences in bioactivity reported by some147,153 but not by available Ca-P materials may be classified as follows: others.85 The differences in biodegradation/biodis(1) calcium hydroxyapatite, Ca10(P04)6(OH)2 (HA); solution of Ca-P materials of similar composition (2) tricalcium phosphate, Ca3(P04)2 (b-TCP); (3) but of different crystallinity may be associated biphasic calcium phosphate (BCP), consisting of with crystal sizes and in the amount of lattice mixed b-TCP/HA (e.g. 20/80, 40/60) phases; (4) defects present. Smaller crystals will dissolve unsintered calcium phosphates; (5) coralline HA more readily than larger crystals of the same (derived from coral); and (6) bone-derived materials composition, due to the larger surface area exposed (Table 1). Commercial and experimental HA, to the biological environment. ikewise, crystals b-TCP ;and BCP materials are usually prepared with a greater number of lattice defects will diseither by sintering apatitic Ca-P obtained by solve more readily than those with less lattice precipitation B5,40,72,86,113,114,118,141,143,161,167 or by defects. Daculsi et al. demonstramd that crystals hydrolysis of acidic Ca-P compounds. These acidic compounds may include dicalcium phosphate of ceramics prepared at 900 “C had. a greater number of lattice defects than those prepared at dihydrate (DCPD) of brushite (CaHP04.2H20); 1200 oc.29>33 dicalcium phosphate anhydrous, DCP or The origin of the Ca-P materials also influences monetite (CaHP04), octacalcium phosphate, (CasH2(P04)6.5H20 (OCP).ii31122,r25,1X133,175,190 their composition. For example, coralline HA is obtained by the hydrothermal conversion of the Incomplete hydrolysis results in the formation of coral, Porites (whose mineral phase consists of calmixed Ca-P phases, e.g. apatitic Ca-P mixed with cium carbonate, CaCOJ, in aragonite form). The DCPD, DCP or OCP (Fig. 6). For example, a comconversion is accomplished in the presence of mercial material described as ‘resorbable HA’ and ammonium phosphate,r7” ‘87 according to the the Ca-P materials reported by Cheung and

72

14

25

I

/

27

/

I

i7

I1

23

3i

DIFFRACTION

33

350 ze

ANGLE

vr

4000

‘:equency 3000

2000

1400

1000

ml

Fig. 11. (A) XRD pattern and (B) 1 spectrum of coralline HA. in (A), it is shown that the HA is mixed with small amounts of Mg-substituted b-TCP; in (B) the presence of carbonate (A) substituting for OH and for PO4 groups in the apatite is

reaction

etals

“coated’ or “im

However, X-ray difractisn and infrared analyses show that coralline HA is not pure in coralline HA co and the apatite is Mg-substituted tri (Fig. 11) as previously reported.112~‘30~ 13L134 Bone-derived material include bone matrix, DMB641 I46 166 bank processed bovine bone of the organic phases. preparations (e.g. Kiel yrost”) appear to contain some organic pha e other commercial (Bio-Oss*) and experimen preparations do not cant sintered ‘O41”4,‘2O, 183 The trace elements The relative Calcium implants

phosphate

Coatings

of metal

material

implants

coatings

wit

#

-4

WJ

2. (A) MA ceramic 16 s starting material; i &SEEsprayed coating showing of poor crystallinit ) ionsputtered coating showing q3hous calcium phosphate with ~yrop~osp~~te (P-O groups.

+ 6 (NH4.)2

from bone will contain minor a associated with bone apatites.““>

-7.

‘803

400 cc’

FREWENCV

10 Ca03

2m ‘--i?dX cm -:

abu

73

Biodegradation and bioresorption of calcium phosphate ceramics

1 ACP

(b)

25

27

29

31

DIFFRACTIW

: IDiffraction

angle, ’

Fig. 13. (A) XRD patterns of HA ceramic used for plasmaspraying; (B) outer and (C) inner layer of the coatings show difference in composition with (C) which has more of the ACP phase. The presence of a- and b-TCP phases is evident in both layers.

ing reduces the ACP component while autoclaving or hydrothermal treatment reduces the ACP and TCP components. Since different Ca-P phases have different solubilities 39,43,68,113,139,182 it. is expected that the stability r of the coating in vivo, and therefore its biodegradation/bioresorption will be affected by the composition of the coating. Physico-chemical

properties of Ca-P materials

Density The density of the Ca-P material (HA or b-TCP) depends, upon the pressing conditions and temperature of sintering. The greater the pressure and the higher the temperature, the greater the density. Porosity Microporosity is a consequence of sintering temperature and duration of sintering. The higher the temperature and the longer the period of sintering, the lower the degree of microporosity. On the other hand, macroporosity is deliberately introduced by the addition of volatile agents, such as hydrogen

33

35

37

u2e

AHGIE

Fig. 14. Plasma-sprayed coatings on two commercial dental implants described as ‘HA-coated’. “The coatings (B) and (C) consist of a mixture of different Ca-P phases: HA of lower crystallinity than the starting HA ceramic; a- and b-TCP; tetracalcium phosphate (TTCP); and amorphous calcium phosphate (ACF’).

peroxide,167 napthalene” or organic materials,143 heating below 100 “C to release the volatile agents and sintering at high temperatures (usually above 1100 “6). The higher the agent/Ca-P material ratio, the greater the macroporosity. With regards to the Ca-P materials of natural origin (e.g. coralline HA bone-derived Ca-P), the micro- and macro-porosity of the original material (coral) is conservedgla t70,187as shown m Dissolution properties Some of the important factors affecting the extent of dissolution of calcium phoslphate materials (Figs 16(A) and 16(B)) relate to ,their properties, which include (a) physical form; (b) composition; (c) crystal structure; and (d) crystallinity (reflecting crystal size and perfection and/or strain). Other factors relate to the solution properties which include (a) pH23t 53 and (b) solution compo;sition.831 t13 Ca-P materials of identical composition but different physical forms (e.g. dense versus porous; powder versus particulate) will iffer in their extent of dissolution (Fig. 17); the material with greater density or lower degree of microporosit:y/macroporosity will dissolve to a lesser extent than those with lower density or higher degree of microporosity/macroporosity. This has been demonstrated in vitro in

acquek Z. LeGeros

Fig. 15. Conservation

Comparative

of macroporosity

from experimental

material;

(A.) BonAP;

(

Extent of Dissolution (O.lYK&AC Buffer. pEEi.37C)

Fig. 16A. Comparative extent of dissolution Periograf (P) and b-TCP ceramic, Synthograf

of human bone

mineral (FIB), synthetic bone minera j; HA cera_mic, Cakitite (Cl (S) expressed as ppm Ca/ml of acid buffer. m was made in 0. I M KAc buffer, pw 5, 37 “C, 1 h’s 114 Fig. 16B. Comparative extent of dissolution of different Ca-P materials in 0.1 M tic 5, 4Qmin. (A) Amo phosphate, ACP, containing Mg and COs; (B) a-TCP; (C) b-TCP; (ID) experime consisting of 15/&5 b-TCP/HA; (E) HA ceramic, Bioapatite; (F) coralline HA, Interpore; (G) HA ceramic All materials were pow acid exposure. The difference between the two commercial HA ceramics (E) and (G) may reflect the difference in sintering temperature.

Biodegradation and bioresorption

B B’ C C’ D D’ u POWDER ia PARTICULATE

Fig. 17. Comparative extent (A, B and C) and particulates Ca-P, unsintered; (B,B’) HA sintereid at 1100 “6; (D,D’)

of dissolution between powder (A’, B’ and C’) of: (A,A’) apatitic sintered at 900°C; (C,C’) HA coralline HA, Interpore 200.

buffers.“7>‘09> 112,‘74

Besides the crystal size, the crystal perfection also affects the extent and mode of dissolution as shown in studies on the dissolution proper-tiles of biological apatites27)291lo6 and ceramic apatites sintered at different temperatures.29133 Ca-P materials of different composition differ in their crystallographic structure and therefore in internal bonding strength which is reflected in their stability and solubility or dissolution properties.431 &en Ca_P materials 68,105,110,115~139,144~181~182,193 acid

of identical composition but different crystallographic structure (e.g. a-TCP versus b-TCP) vary considerably in their dissolution properties in vitro (Fig. 16) and are also expected to vary in extent of biodegr#adation. The order of relative solubility of some Ca-P compounds are as follows: ACP >> DCP > TTCP > a-TCP > b-TCP >, HA (ACP =: amorphous calcium phosphate; DCP = monetite, CaHP04; TTCP = tetracalcium phosphate, Ca4P,09; a- and b-TCP, Ca3(P04)2; HA = CaIo(P04)6(OH)2). Substitutions in the TCP or HA structure will affect their extent of dissolution; e.g. Al-for-Ca substitution causes an increase, and Mg-for-Ca substitution, a decrease in the extent of dissolution of b-TCP. Similarly, F substitution causes a decrease while CO,, Mg or Sr causes an increase in the extent of dissolution of synthetic and biological apatites.‘0~~110,1~3,115-117,123,134,1~, 149Matlerials which have been appropriately characterized (i.e. by X-ray diffraction) as b-TCP were shown in vitro to dissolve to a greater extent than

qf calcium phosphate ceramics

75

similarly characterized HA; and the extent of dissolution of biphasic calcium phosphate (BCP) ceramics depended upon their b-TCP/HA ratio: the higher the ratio, the greater the extent of dissolu&ports (3n in-vitro dis_ tion 38,s7,9s_rOr,109,112,174 solution of b-TCP ranged from 3 to 12 times faster than HA ceramic.39a87a991l”‘Jlr12 This large range of difference is probably due to the difference in conditions of the dissolution experiments (e.g. particle size; solid/solution ratio; buffer pH; types of buffers used; etc.). Crystallinity (reflecting crystal size and perfection) affects the extent of dissolution of Ca-P materials. Crystallinity is affected by conditions of preparation such as sintering temperature. Besides affecting crystal size, the sintering tern erature affects the crystal perfection. HAS prepared at 900 “C were shown to have greater lattice defects than those prepared at 1200 oC.33This is reflected in the higher dissolution rate of HA prepared at a lower temperature.147 Powdered coralline HA (Enterpore*) was shown to have a greater extent of dissolution than ceramic HA (Calcitite*). This is not due only to the difference in their composition (coralline HA contains CO3 and Mg which are known to promote greater dissolution of apatite’lO.“‘Z1‘15 but also to the difference in their crystallinity (Fig. 2). These differences in crystallinity and composition are due to the differences in their ori,gin and method of preparation.t3” 13%136 Solution conditions (e.g. pH, composition) also influence the dissolution of Ca-P materials,23’83,‘I3 In addition, HA incorporated in a resorbable polymer (polylactic acid, PLA) was shown to dissolve (biodegrade) to a greater extent than without the polymer,‘9’ probably because of added acidity from the lactic acid produced by the breakdown of PLA. The greater acidity (lower pH) will promote greater dissolution of HA.23 Biodegradation/bioresorption

The extent of biodegradation/bioresorption of Ca-P materials reflect their solubility or dissolution properties. The factors influencing the dissolution properties of solubility were similar to affecting biodegradation/bioresorption, those namely: physical forms (degrees of microcomposition density); porosity/macroporosity; (TCP ver3u.s HA; AP/glass composites versus HA ceramic) and crystallinity (e.g. coralline HA versus HA ceramic). This was demonstrated in vitro, in cell culture 7’22,34,52,62,65%67,82,130,136,155-157,186and

acquel Z. LeGeros

76

Fig. 18. TEM showing G-P

crystals engulfed by phagossme.

(S) is a rnag~~fi~at~~~ of (A).

in viva 10,28,38,49,98~101,118,147,153,158;159,172,174, ‘85,186

With regard to coated imp1 expected that the composition of affect the coating’s (and therefo long-term stability, since biodegradation/bioresorption is related to the dissolution properties of the Ca-P materials. It is reasonable to assume that Ca-P coatings containing relatively higher concentrations of more soluble (more biodegradable) Ca phases (e.g. ACP, TTCP, a- and b-TCP) will be le stable than those which consist principally of wellcrystallized HA.

conditions i;a Vi00 of ~egr~~ati~~ (b

are cause

is due to the fact cesses (cellular or ac acidic ~o~~~~i~~s.

Biodegradation and bioresorption of calcium phosphate ceramics

Fig. 19. TEM showing dissolution

along the grains (arrow) of the crystals (A); dissolution of a single Ca-P crystal :showing dissolution from the core and from the surfaces of the crystals.‘29

osteoclasts involved with the resorption of living bone and Ca-P materials;‘7’ 51264)192 and multinucleated cells and macrophages involved with phagocytosis and with1 resorption of bioactive (bioglass, Ca-P ceramaterials. composites) mics, AP/glass 10,34,35,

@i-67,69-71,74,75,82,83,90,98-103,136,137,

145; 146,155~157,163,166,

77

172,177,184~186,188,192

Extracefllular

biodegradation is also associated with acidic enzymes and conditions.“86 SEM demonstrated similarities in resorption pits in bone and in Ca-P materials.7)90’ 145 Some examples of biodegradation/bioresorption of Ca-P materials in vitro and in vivo are shown in Figs 18 and 19. Assessment of biodegradation/ ioresorption has

acqueFZ.

78

LeGeros

Table 3. ~o~~b~~ity product

and CaI? molar ratio

constants,

---

---.-._--__.-_.---_-

Galciuan phosphate.9 (sjmthetic)

Ca’lrlk

KiP

-

Brushite, dicalcium phosphate CaHPQ4.H20 Monetite, dicalcium phosphate CaHP04 Octacalcium phosphate, OC Ca8HZ(P04)6”5H20 Tricalcium phosphate, b-TCP Q3

&hydrate 2.39 X io--‘i

:$

hydroxya~a~te,

tb *.5_, ‘z

2.83

x

]o-“:

{.j()

.i,;q __.

,, /\

;o

HA

W0W4MOW2

Calcium fluorapatite, FA Ca,o(PW& Tetracalcium phosphate Ca4P209

---_--__.-

-i!:

,I); .L3 x :0-b” 2

____

ization of the Ca‘s7,150,I” leading ~Q~c~~s~~~$.1~2~ Fig. 19 (contd.)

Fig. 20. Difference in the microporosity of the surface of (2) the BCP ceramic and (3) implant core; (1) is before, and (2) and (3) are after implantation. The b-TCP/HA ratios in the experimental BCP ceramics were: (A) W/IS; (B) 35/65; (C) 15/?~5.*~

i .05 x: 1p1

W4)2

Calcium

been reported in terms of changes in macroporosity, density or loss of material or degra of the Ca-P implant observed SCOPY>98-101a 159)1851186 changes in meters and average crystal size with time from the surface and the core of the Ca-P implant material (Fig. 20) as observed by scanning (SEM) and electron (TEM) microscopy.28l 118)lJ5 The great difference in the resolutions of the analytical methods used for such assessments give rise to differences in interpretation.28131’145For example, reported observations of no significant resorption of HA ceramics were made based on observations from light microscopy98s10’while reports of evidence of resorption were based on SEM and TEM measurements 28,66,67,118,145,157 Another reason for Back of agreement is the lack of or inappropriate character-

3,(j

anhydrous

to the “TCP’ showing no signifi reported by Nery Rk’6zP0, I51

__~~_

0

Biodegradation

and bioresorption

of calcium phosphate

79

ceramics

Fig. 21. (A) TEM of large crystals of Ca-P ceramics (shown in (C)) associated with microcrystals (arrow) after im:plantation. (B) The epitaxial g,rowth of the microcrystals on the ceramic Ca-P crystal. (C) Electron diffraction of (a) microcrystal; (b) bone apatite crystals; (c) Ca-P crystal.

ACP, DCPD, OCP and CO,-AP. The formation of Ca-P phases and their transformation to other Ca-P phrases may be promoted or inhibited, depending upcm solution conditions (e.g. pN) and composition. The presence of Mg *+ in solution will stabilize ACP, DCPD and OCP and suppress their transformation to apatite; 113,117,128,129,133,189 the presence of critical concentrations of pyrophosphate, P2074-, aluminum (A13+), zinc (Zn+*), other elements and some proteins will suppress, while other ions (e.g. C03*-, jF_) a n d some protein molecules will pro-

e.g.

mote the formation of apatite.14; is, 19a55163)113)125 Ca-P phases may also form on the Ca-P material surfaces by seeded growth31’48 instead of, or in addition to, forming by dissolution/precipitation processes. Formation of apatite microcrystals on surfaces of Ca-P macrocrystals of the @a-P imlplant have been observed

in

yitro83> 130,131, ‘35, ‘36, ‘57,

;md

3’>5s> 76>ss,92,iis>135>136>154,179 The intimate

in

vi70

11,289

association

of these microcrystals with an organic phase and their identification as C03-apatite, similar to bone apatite (Figs 21-23) was first reported by the author

Racquel Z. LeGeros

80

Fig. 2%. TGA profiles of IIA ceramic (A) before implantation in scraped from implants non-osseous sites, and materials after (IS) 40, (C) 180, and (D) 365 days.sO The weight and 200°C is r heating between room temperature associated with adsorbed H20; between 200 and 400”C, ?; between 400-900 “6, CO, in the apatite.

Biodegradationlbioresorptio

‘17l135Cell ~r~~~feratio mitogenesis, increased 1800

1400 800 400 Frequency (cm’“1)

the increase

in intra

Fig. 22. IR spectra of BCP (A) before and (B) after implantation from the core, (C) from the interface between the ceramic and host bone, i.e. the new bone region and (D) in the area of the host bone furthest from the interface. The spectra in (A) shows principally the absorption bands of the PO4 (P-O) and OH (O-H) groups of the HA or TCP; (B) absorption bands due to the CO3 (C-O) groups of the COa-apatite and due to the NH2 (N-H) groups of the organic matrix intimately associated with the COa-apatite crystals.

and her colleagues. 303~ 11sIt is proposed that the acid conditions at the implant site cause partial dissolution of the crystals of the Ca-P biomaterials, causing an increase in the levels of calcium and phosphate ions in the biological fluids in the immediate environment and, subsequently, precipitation of apatite microcrystals incorporating ions (Ca2’t Mg2+, Na+, HP04-, POd3-, etc.) and organic molecules co32-, present in the fluid (Fig. 23). The formation of C03apatite can also occur by an indirect process through non-apatitic Ca-P phases, which are more stable under acid conditions (e.g. DCPD, OCP) later hydrolyze to COswhich can and 113,123,128,133,181 apatite.

/ Precipitation

--_.-__-.“A

!

Fig. 24. Schematic ~e~~ese~t~tio~ of the partial dissolution of Ca-P crystals of the implants and the formation of the CO3apatite microcrystals

Biodegradation and bioresorption of calcium phosphate ceramics

81

Fig. 25. Some of the events in biodegradation/biodissolution associated with bone formation: (A) cell adhesion; (B) and (C) phagocytosis of Ca-P crystals; (D) dissolution of Ca-P crystals and precipitation of C03-apatite microcrystals; (E) production of collagen; (F) mineralization of collagen; (G) bone formation.

b_TCP

c-erami,-ee.6> _I

7, “,65~67,150~157,163

The

increase

in the concentration of calcium and phosphate ions resulting from the dissolution of Ca-P materials also affects bone-cell activity.54’ w 192One of the principal effects reported was increased inhibition of bone resorption due to reduced osteoclast formation

and decreased activity of mature osteoclasts.‘92 Other elements which may be released by the dissolution of Ca-P materials containing these ions (e.g. A13+ or F - ions) may affect osteoblast and/or osteoclast activities,94-96) 169,r*s and ultimately, bone formation.

82

Racquel Z. LeGeros

Biodegradation/bioresorption: Effect on bone bonding A sequence of events at the surface of the bioactive materials promotes bone bonding or bonding osteo-

~~S~~~~~~ iI1 iB interface.24’ ‘% I59 (a> biodegra y extra~e~~~~ar an ;bCtiW processes in an acidic en

genesis, material includes

Biodegradation

and bioresorption

differentiation leading to the production of adhesive proteins and collagen fibrils containing extracellular matrix; (c) formation of C03-AP microcrystals (intimately associated with organic matrices) on surfaces of degrading Ca-P crystals of the implant; (d) simultaneous mineralization of the collagen librils and incolrporation of the new apatite crystals associated with organic matrix in the remodelling of the new bone 6,7,24,34,35,&I, 68-71,73-75,135,145,158,159 The ‘direct’ or ‘biological’ or ‘chemical’ bond responsible for the uniquely strong bone/material interface is characterized by the interdigitating of the collagen fibrils wjith the surface of the Ca-P material.7l 111 35 The morphological organization of the initial collagen bonding with the implant surface was shown in vitro to be influenced by the surface properties and composition of the Ca-P materials.35 The surface chemistry (which depends upon composition and crystallinity) and physical properties (e.g. density, porosity) of the bioactive materials determine the rate and extent of biodegradation/bioresorption, a primary factor in bone bonding. Some of these events are illustrated in Fig. 25.

ACKNOWLEDGEMENTS The author gratefully acknowledges the collaboration of colleiagues (Drs G. Daculsi, M. Gineste, G. Gregoire, M. Heughebaert, J. Kazimiroff, B. and M.-L. Kerebel, J. P. LeGeros, G. Bonel, K. Lynch, E. B. Nery and I. Orly) in some of the work cited in this paper. The valuable technical assistance of M. Retino, J.-L. Wong and R. Zheng; of G. Merlin0 and D. Corn (for photography) and V. Fronjian (word processing) are also gratefully acknowledged. This work was supported in part by Research Grants from NIHjNIDR Nos. DE-04123, DE-072;!3, and 2 SO7 RR 07062.

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