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Clinical applications of calcium phosphate biomaterials: A review
Ceramics hlternational 19 (1993) 363-366
Review Paper
Clinical Applications of Calcium Phosphate Biomaterials: A Review
phosphates at room temperature, among them being several that are thermodynamically stable only at high temperatures. A few calcium phosphate salts have been sintered into a bioceramic, sintering being a heat treatment involving the transition of a very fine powder (particle size about 1 Ibm or less) into a solid or porous form. Table 1 presents two c o m m o n synthetic routes, and Table 2 shows the classification in terms of porosity. This classification is of interest for both mechanical properties and behaviour in terms of biodegradation. The solubilities of various calcium phosphates differ greatly: not only are the chemical crystallographical structures themselves important, but variables such as pH, the specific nature of the buffer, temperature, porosities (or, better, specific surfaces) are also significant. Table 3 shows the differences clearly for three variables: crystallographic structure, pH, and specific nature of the buffer. 4
Klaas de G r o o t Biomaterials Research Group, Department of Biomaterials, School of Medicine, Leiden University, Building 55, Rijnsburgcrweg 10, 2333 AA Lcidcn, The Netherlands (Received 28 May 1992; accepted 12 September 1992) Abstract: It has been known for more than twenty years that ceramics made of calcium phosphate salts can be used successfully for replacing and augmenting bone tissue, t'2 This can be understood from the structure of bone itself: it is a composite structure, consisting of a continuous phase (made of collagenous proteins and other biological polymers, and physiological fluid), in which small calcium phosphate crystals are dispersed. Thus bioceramics made of calcium phosphate are not foreign and hence are biologically compatible with living bone. In this paper, several aspects of bioceramics of calcium phosphates are discussed, namely, their physical-chemical structure, mechanical properties, and biodegradation, their use as coatings on mechanically strong substrates, and their clinical
COMPATIBILITY OF C A L C I U M PHOSPHATE SURFACES
uses.
For two materials, HA and fl-tricalcium phosphate, biodegradation after implantation has been studied by Klein e t al., 5 and by van Blitterswijk e t al. 6 Table
PHASE D I A G R A M S OF THE SYSTEM CaO-PaOs-H20
Table 1. S y n t h e t i c
Phase diagrams clearly show that the combination of CaO, P205, and H20 allows the existence of many calcium phosphate salts. Not only is water essential in defining thermodynamically stable salts, but the temperature also has a large effect. As illustration, several phase diagrams are shown in Figs 1, 2, 3 and 4. 3 They are explained in the captions. If one takes into account that some solidstate transitions may occur slowly (the time ranging from days to many years), then it is obvious that, by selected heat treatments, under chosen vapour pressures, it is possible to obtain numerous calcium
routes for calcium materials
phosphate
(1) Starting powder of the required stoichiometry (Cal0(PO4)e(OH)2 for apatite; Ca3(PO~) 2 for tricalcium phosphate), either bought commercially or synthesized in the laboratory, results, after sintering at 1200-1300' C for several hours, in a solid shape. Micropores and macropores can be introduced as described in Ref. 1. (2) Calcium carbonate shapes (for example: coral) can be transformed into calcium phosphate shapes after heating in phosphate-rich aqueous solutions under pressure at 300-400' C. (Example: Interpore.)
363 Ceramics h~ternational 0272-8842/93/$6.00 ((" 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain
364
KlaasdeGroot
17OO
C3P -I- C4P ~
Li( uid o.CaP+ Liq
m
~O
1600
m__
1500
Ap
C
a
O C4P
CaO
+
~
Ap
"_3
1400
C3P
P205
C a O -I.- C4P
c:P
13OO
P205 (*1.) Fig. 1. Phasediagram of CaO/P205 mixtures.
T (*C) 1600 1500 1400 I I
4
-r F: 2 - C 4 o
Ap + CaO
P +
o.z 1 m Liq
, =c:P
oO0 -
J
I
5.0
I 5.5
I
I 6-0 I041T (K)
I.
I 6.5
65 / 60 55\ 5O C4P C3P CaO (wt *1.)
Fig. 4. Phase diagram of CaO/P20 s mixtures (water pressure 500 mm Hg).
1300 I
\
N
1200 70
7"0
4 shows that biodegradation increases with porosity and that H A degrades hardly at all. Van Blitterswijk studied H A in more detail and found that the surface dissolves at a rate o f about 20-30/lm per year. Thus only for small particles (less than several hundred /am) is this effect visible. In all cases, a direct bond between bone and ceramic has been found by many authors.
Fig. 2. Phase diagram of CaO/P20 s mixtures showing the influence of water pressure.
Table 2. Porosities Micropores: diameter < 5 pm
Liquid 0"C31~+ Liq
_
Macropores: diameter > 100 l~m Dense: total volume of micro- and macropores: < 5%
17OO
16OC
C4P-
15OO
Table 3. Solubility of several calcium phosphates as
eL'CaP+ ~'C3P
-CaO+ C4P
14OO -
1300 -
120070
+Ap ="
.
/
~ / ,~1"
C4P +Ap + i 136C? ~- Liq + n CaO + Ap U O,C 3 P + .~ C2P /5 /i 60 I A/p 55\ I I 50 (n
c4P
%P
CaO (wt*l.)
Fig. 3. Phase diagram of CaO/P205 mixtures.
function of pH and specific buffer in relative units Buffer
HA"
TetraCPb
TCPc
pH
Lactate Citrate Michaelis Gomoris Lactate Citrate Michaelis Gomoris Aq. dest
27 81 14 9 27 38 12 8 2, 3
163 337 94 140 100 86 100 108 4, 7
142 245 30 37 93 91 15 14 2, 7
6, 2 6, 2 6, 2 6, 2 7, 2 7, 2 7,2 7, 2 --
aHydroxylapatite. bTetracalcium phosphate. CTricalcium phosphate.
Clinical applications of cak'ium phosphate hiomaterials
Table 4. Biodegradation of HA and TCP (in rabbits) Material
Biodegradation (loss/3 months) O%
HA, 30vo1% macropores HA, 30 vol% macropores + 40 vol% micropores TCP, 30vo1% macropores TCP, 30vo1% macropores + 40 vol% micropores
0% 15+_6% 30 + 4%
CALCIUM PHOSPHATE COATINGS As mentioned in the literature, 7 the fatigue behaviour of bulk calcium phosphate ceramics is poor. Clinical use (see the next section) is therefore limited. An increasingly popular way to circumvent this problem is to coat a strong substrate with a thin layer of calcium phosphate. A combination is then obtained that not only has good biocompatibility but also possesses resistance towards fatigue failure. The method used to put a coating onto a substrate is plasma-spraying, the principle of which is depicted in Fig. 5. Literature on calcium-phosphate-coated implants clearly shows that, compared with uncoated implants, their clinical performance is better. For example, clinical studies on hydroxylapatitecoated titanium hip implants resulted in higher Harris-scores than uncoated hip implants. 8 Although the general consensus now is that hydroxylapatite coatings degrade after implantation, the quantitative results of i n - v i v o degradationrate studies are not always reproducible. Degradation is not only a property of the coating itself but also depends on the specific biological environment Plasma
spraying ~t Powder"
Anode m
lil
-\-
--
)
Plasma
Fig, 5. Schematic drawing of plasma-spray process. Between anode and cathode, a continuous electric discharge heats a gas (introduced according to the arrow in the lower left-hand corner), up to 20 000-30 000 K. This heated gas, called a 'plasma" has a very high speed (up to 1000km/h). A ceramic powder, introduced in this fast-streaming and hot plasma, can thus be deposited in a (partially) molten state onto a metallic or nonmetallic substrate. In this way, ceramic coatings of up to several hundred/am can be obtained.
365
in which the coated device has been implanted. Since this environment in turn may depend on species, site of implantation, specific tissue, age of animal, shape and surface morphology of the implant, and operation technique (which is partially governed by shape of the implant) it is obvious that differences in degradation rate may be found if the experimental conditions are not the same. But, even taking this into consideration, we found that implants with coatings that are the same in terms of (i) thickness, (ii) degree of crystallinity, and (iii) adhesive strength (the currently accepted parameters for characterizing an HA coating) and that are implanted in the same animal species degraded within six months in one experiment and showed little degradation in another. A possible explanation may be as follows. 'Degree of crystallinity' is not a fully descriptive parameter of the morphology of a coating. A given degree ofcrystallinity can be found by different types of morphology, an example being that the coating consists either of a continuous amorphous phase with discrete crystalline particles or a continuous crystalline phase with discrete amorphous areas. In the first situation, degradation occurs by rapid dissolution of the relatively soluble amorphous phase, followed by loosening of crystalline particles, whereas a coating of the second type would degrade at a rate governed by dissolution of the rather insoluble crystalline HA. It is obvious from this example that the 'degree of crystallinity' cannot be used to predict the degradation rate, that at the same degree of crystallinity different degradation rates may be observed, and that characterization of a coating in terms of thickness, degree of crystallinity, and adhesive strength is not sufficient. Recent results have shown indeed that, with the same degree of crystallinity, different resorption rates can be found. Klein et al. ~1 found that the behaviour of two coated implants having the same degree of crystallinity but obtained with different plasma-spray conditions was quite different:" one degraded significantly faster than the other.
C L I N I C A L USE In previous sections, it has been shown that bulk ceramics of calcium phosphate are strong in compression, weak in tension, and not resistant towards fatigue failure. Hence bulk implants can only be used in instances where no mechanical loads other than compression (or no loads at all) are present. This limits the use of bulk bioceramics of calcium phosphate--dense or porous--to small implants, such as granulate to repair or augment
Klaas de Groot
366 Table 5. Clinical applications in oral, head, and neck surgery
(1) Craniofacial applications (A) Augmentation Ridge Mandibular Zygomatic Chin
(B) Reconstruction Periodontal Mandibular Orthognatic Bone grafting Cranioplastry
with biodegradable bioceramics. Coatings o n metallic and c o m p o s i t e s u b s t r a t e s are o f great interest: they allow o s s e o i n t e g r a t i o n , even if the c o a t i n g s t h e m selves d i s a p p e a r a f t e r o n e o r t w o years. It can be c o n c l u d e d that b i o a c t i v e b i o c e r a m i c s h a v e a considerable f u t u r e in surgical disciplines that are c o n c e r n e d with replacing, r e c o n s t r u c t i n g , o r augm e n t i n g parts o f the h u m a n skeleton. REFERENCES
(2) Prosthetic implants (A) Subperiosteal (B) Endosteal Endosseous Endodontic Orthodontic
(C) Transosseous implants Transmandibular
(3) Otological applications (A) Ossicular
(B) Canal wall
small b o n y defects, and i m p l a n t s to replace middleear ossicles. H y d r o x y l a p a t i t e - c o a t e d i m p l a n t s are s t r o n g e r : they can be m a d e bigger a n d used f o r l o a d - b e a r i n g sites. A very successful e x a m p l e is a t i t a n i u m artificial hip joint, c o a t e d with H A . T a b l e 5 gives a s u m m a r y o f clinical applications, based o n the c o n s i d e r a t i o n s o f the p r e v i o u s paragraphs,~ 2 especially for head and neck surgery. CONCLUDING
REMARKS
Bioceramics o f calcium p h o s p h a t e are bioactive, which m e a n s that they c a n b o n d with b o n y tissues. T h e y can be p r o d u c e d either as ' p e r m a n e n t ' i m p l a n t s - - m e a n i n g that such i m p l a n t s will stay for at least five to ten y e a r s - - o r as " d e g r a d a b l e ' i m p l a n t s - - d i s a p p e a r i n g within a few m o n t h s , Deg r a d a b l e implants can be used for repair only, whereas p e r m a n e n t i m p l a n t s are suitable for b o t h repair o f b o n e and p e r m a n e n t a u g m e n t a t i o n . T h i s is o b v i o u s if one considers that a u g m e n t i n g b o n e where it was not b e f o r e will be a t e m p o r a r y success
1. DE GROOT, K., Bioceramics consisting of calcium phosphate salts. Biomaterials, ! (1980)47-50. 2. DE GROOT, K., Medical applications of calcium phosphate bioceramics. J. Ceram. Soc, Japan, 99 (1991) 943-53. 3. AMERICAN CHEMICAL SOCIETY, Phase Diagrams .for Ceramists, 5 (1983) 320-3. 4. KLEIN, C. P. A. T., DE BLIECK-HOGERVORST, J. M. A., WOLKE, J. G. C. & DE GROOT, K., Studies of the solubility ofdiffcrent calcium phosphate ceramic particles #t l'ilro. Biomaterials, I 1 (1990) 509-12. 5. KLEIN, C. P. A. T., DRIESSEN, A. A., DE GROOT, K. & VAN DEN HOOFF, A., Biodegradation behaviour of various calcium phosphate materials in bone tissue. J. Biomed. Mater. Res., 17 11983)769-84. 6. VAN BLITTERSWlJK, C. A., GROTE, J. J,, KUYPERS, W., DAEMS, W. T. H. & DE GROOT, K., Macropore tissue ingrowth: a quantitative and qualitative study on hydroxyapatite ceramic. Biomaterials, 1 (1986) 137-43. 7. DE PUTTER, C.. DE GROOT, K. & SILLEVIS SMITT, P. A. E., Transmucosal implants of dense hydroxylapatite. J'. Prosthet. Dent., 49 (1983) 87-96. 8. GEESINK, R. G. T., Experimental and clinical experience with hydroxylapatite-coated hip implants. Orthopaedics, 12k (1989) 1239-42. 9. G EESINK, R. G. T., DE GROOT, K. & KLEIN, C. P. A. T., Chemical implant fixation using hydroxyapatite coatings: the development of a human total hip prosthesis for chemical fixation to bone using HA coatings on titanium substrates. Clin. Orthop., 225 (19871 147-70. 10. DHERT, W. J. A., KLEIN, C. P. A. T., WOLKE, J. G. C., VAN DER VELDE, E. A., DE GROOT, K. & ROZING, P. M., A mechanical investigation of fluorapatite, magnesium whitlockite, and hydroxylapatite plasma-sprayed coatings in goats. J. Biomed. Mater. Res., 25 (1991) 1183-200. II. KLEIN, C. P. A. T., WOLKE, J. G. C., DE BLIECKHOGERVORST, J. M. A. & DE GROOT, K., Heal treatment influencc plasma-sprayed Ca/P coatings. Paper presented at Third World Conference on Biornaterials, Bcrlin, 1992. 12. DE GROOT, K., Effect of porosity and physicochemical properties on the stability, resorption, and strength of ca!cium phosphate ceramics. Ann. N Y A cad. Sci., 523 (1988) 227-33.
Review Paper
Clinical Applications of Calcium Phosphate Biomaterials: A Review
phosphates at room temperature, among them being several that are thermodynamically stable only at high temperatures. A few calcium phosphate salts have been sintered into a bioceramic, sintering being a heat treatment involving the transition of a very fine powder (particle size about 1 Ibm or less) into a solid or porous form. Table 1 presents two c o m m o n synthetic routes, and Table 2 shows the classification in terms of porosity. This classification is of interest for both mechanical properties and behaviour in terms of biodegradation. The solubilities of various calcium phosphates differ greatly: not only are the chemical crystallographical structures themselves important, but variables such as pH, the specific nature of the buffer, temperature, porosities (or, better, specific surfaces) are also significant. Table 3 shows the differences clearly for three variables: crystallographic structure, pH, and specific nature of the buffer. 4
Klaas de G r o o t Biomaterials Research Group, Department of Biomaterials, School of Medicine, Leiden University, Building 55, Rijnsburgcrweg 10, 2333 AA Lcidcn, The Netherlands (Received 28 May 1992; accepted 12 September 1992) Abstract: It has been known for more than twenty years that ceramics made of calcium phosphate salts can be used successfully for replacing and augmenting bone tissue, t'2 This can be understood from the structure of bone itself: it is a composite structure, consisting of a continuous phase (made of collagenous proteins and other biological polymers, and physiological fluid), in which small calcium phosphate crystals are dispersed. Thus bioceramics made of calcium phosphate are not foreign and hence are biologically compatible with living bone. In this paper, several aspects of bioceramics of calcium phosphates are discussed, namely, their physical-chemical structure, mechanical properties, and biodegradation, their use as coatings on mechanically strong substrates, and their clinical
COMPATIBILITY OF C A L C I U M PHOSPHATE SURFACES
uses.
For two materials, HA and fl-tricalcium phosphate, biodegradation after implantation has been studied by Klein e t al., 5 and by van Blitterswijk e t al. 6 Table
PHASE D I A G R A M S OF THE SYSTEM CaO-PaOs-H20
Table 1. S y n t h e t i c
Phase diagrams clearly show that the combination of CaO, P205, and H20 allows the existence of many calcium phosphate salts. Not only is water essential in defining thermodynamically stable salts, but the temperature also has a large effect. As illustration, several phase diagrams are shown in Figs 1, 2, 3 and 4. 3 They are explained in the captions. If one takes into account that some solidstate transitions may occur slowly (the time ranging from days to many years), then it is obvious that, by selected heat treatments, under chosen vapour pressures, it is possible to obtain numerous calcium
routes for calcium materials
phosphate
(1) Starting powder of the required stoichiometry (Cal0(PO4)e(OH)2 for apatite; Ca3(PO~) 2 for tricalcium phosphate), either bought commercially or synthesized in the laboratory, results, after sintering at 1200-1300' C for several hours, in a solid shape. Micropores and macropores can be introduced as described in Ref. 1. (2) Calcium carbonate shapes (for example: coral) can be transformed into calcium phosphate shapes after heating in phosphate-rich aqueous solutions under pressure at 300-400' C. (Example: Interpore.)
363 Ceramics h~ternational 0272-8842/93/$6.00 ((" 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain
364
KlaasdeGroot
17OO
C3P -I- C4P ~
Li( uid o.CaP+ Liq
m
~O
1600
m__
1500
Ap
C
a
O C4P
CaO
+
~
Ap
"_3
1400
C3P
P205
C a O -I.- C4P
c:P
13OO
P205 (*1.) Fig. 1. Phasediagram of CaO/P205 mixtures.
T (*C) 1600 1500 1400 I I
4
-r F: 2 - C 4 o
Ap + CaO
P +
o.z 1 m Liq
, =c:P
oO0 -
J
I
5.0
I 5.5
I
I 6-0 I041T (K)
I.
I 6.5
65 / 60 55\ 5O C4P C3P CaO (wt *1.)
Fig. 4. Phase diagram of CaO/P20 s mixtures (water pressure 500 mm Hg).
1300 I
\
N
1200 70
7"0
4 shows that biodegradation increases with porosity and that H A degrades hardly at all. Van Blitterswijk studied H A in more detail and found that the surface dissolves at a rate o f about 20-30/lm per year. Thus only for small particles (less than several hundred /am) is this effect visible. In all cases, a direct bond between bone and ceramic has been found by many authors.
Fig. 2. Phase diagram of CaO/P20 s mixtures showing the influence of water pressure.
Table 2. Porosities Micropores: diameter < 5 pm
Liquid 0"C31~+ Liq
_
Macropores: diameter > 100 l~m Dense: total volume of micro- and macropores: < 5%
17OO
16OC
C4P-
15OO
Table 3. Solubility of several calcium phosphates as
eL'CaP+ ~'C3P
-CaO+ C4P
14OO -
1300 -
120070
+Ap ="
.
/
~ / ,~1"
C4P +Ap + i 136C? ~- Liq + n CaO + Ap U O,C 3 P + .~ C2P /5 /i 60 I A/p 55\ I I 50 (n
c4P
%P
CaO (wt*l.)
Fig. 3. Phase diagram of CaO/P205 mixtures.
function of pH and specific buffer in relative units Buffer
HA"
TetraCPb
TCPc
pH
Lactate Citrate Michaelis Gomoris Lactate Citrate Michaelis Gomoris Aq. dest
27 81 14 9 27 38 12 8 2, 3
163 337 94 140 100 86 100 108 4, 7
142 245 30 37 93 91 15 14 2, 7
6, 2 6, 2 6, 2 6, 2 7, 2 7, 2 7,2 7, 2 --
aHydroxylapatite. bTetracalcium phosphate. CTricalcium phosphate.
Clinical applications of cak'ium phosphate hiomaterials
Table 4. Biodegradation of HA and TCP (in rabbits) Material
Biodegradation (loss/3 months) O%
HA, 30vo1% macropores HA, 30 vol% macropores + 40 vol% micropores TCP, 30vo1% macropores TCP, 30vo1% macropores + 40 vol% micropores
0% 15+_6% 30 + 4%
CALCIUM PHOSPHATE COATINGS As mentioned in the literature, 7 the fatigue behaviour of bulk calcium phosphate ceramics is poor. Clinical use (see the next section) is therefore limited. An increasingly popular way to circumvent this problem is to coat a strong substrate with a thin layer of calcium phosphate. A combination is then obtained that not only has good biocompatibility but also possesses resistance towards fatigue failure. The method used to put a coating onto a substrate is plasma-spraying, the principle of which is depicted in Fig. 5. Literature on calcium-phosphate-coated implants clearly shows that, compared with uncoated implants, their clinical performance is better. For example, clinical studies on hydroxylapatitecoated titanium hip implants resulted in higher Harris-scores than uncoated hip implants. 8 Although the general consensus now is that hydroxylapatite coatings degrade after implantation, the quantitative results of i n - v i v o degradationrate studies are not always reproducible. Degradation is not only a property of the coating itself but also depends on the specific biological environment Plasma
spraying ~t Powder"
Anode m
lil
-\-
--
)
Plasma
Fig, 5. Schematic drawing of plasma-spray process. Between anode and cathode, a continuous electric discharge heats a gas (introduced according to the arrow in the lower left-hand corner), up to 20 000-30 000 K. This heated gas, called a 'plasma" has a very high speed (up to 1000km/h). A ceramic powder, introduced in this fast-streaming and hot plasma, can thus be deposited in a (partially) molten state onto a metallic or nonmetallic substrate. In this way, ceramic coatings of up to several hundred/am can be obtained.
365
in which the coated device has been implanted. Since this environment in turn may depend on species, site of implantation, specific tissue, age of animal, shape and surface morphology of the implant, and operation technique (which is partially governed by shape of the implant) it is obvious that differences in degradation rate may be found if the experimental conditions are not the same. But, even taking this into consideration, we found that implants with coatings that are the same in terms of (i) thickness, (ii) degree of crystallinity, and (iii) adhesive strength (the currently accepted parameters for characterizing an HA coating) and that are implanted in the same animal species degraded within six months in one experiment and showed little degradation in another. A possible explanation may be as follows. 'Degree of crystallinity' is not a fully descriptive parameter of the morphology of a coating. A given degree ofcrystallinity can be found by different types of morphology, an example being that the coating consists either of a continuous amorphous phase with discrete crystalline particles or a continuous crystalline phase with discrete amorphous areas. In the first situation, degradation occurs by rapid dissolution of the relatively soluble amorphous phase, followed by loosening of crystalline particles, whereas a coating of the second type would degrade at a rate governed by dissolution of the rather insoluble crystalline HA. It is obvious from this example that the 'degree of crystallinity' cannot be used to predict the degradation rate, that at the same degree of crystallinity different degradation rates may be observed, and that characterization of a coating in terms of thickness, degree of crystallinity, and adhesive strength is not sufficient. Recent results have shown indeed that, with the same degree of crystallinity, different resorption rates can be found. Klein et al. ~1 found that the behaviour of two coated implants having the same degree of crystallinity but obtained with different plasma-spray conditions was quite different:" one degraded significantly faster than the other.
C L I N I C A L USE In previous sections, it has been shown that bulk ceramics of calcium phosphate are strong in compression, weak in tension, and not resistant towards fatigue failure. Hence bulk implants can only be used in instances where no mechanical loads other than compression (or no loads at all) are present. This limits the use of bulk bioceramics of calcium phosphate--dense or porous--to small implants, such as granulate to repair or augment
Klaas de Groot
366 Table 5. Clinical applications in oral, head, and neck surgery
(1) Craniofacial applications (A) Augmentation Ridge Mandibular Zygomatic Chin
(B) Reconstruction Periodontal Mandibular Orthognatic Bone grafting Cranioplastry
with biodegradable bioceramics. Coatings o n metallic and c o m p o s i t e s u b s t r a t e s are o f great interest: they allow o s s e o i n t e g r a t i o n , even if the c o a t i n g s t h e m selves d i s a p p e a r a f t e r o n e o r t w o years. It can be c o n c l u d e d that b i o a c t i v e b i o c e r a m i c s h a v e a considerable f u t u r e in surgical disciplines that are c o n c e r n e d with replacing, r e c o n s t r u c t i n g , o r augm e n t i n g parts o f the h u m a n skeleton. REFERENCES
(2) Prosthetic implants (A) Subperiosteal (B) Endosteal Endosseous Endodontic Orthodontic
(C) Transosseous implants Transmandibular
(3) Otological applications (A) Ossicular
(B) Canal wall
small b o n y defects, and i m p l a n t s to replace middleear ossicles. H y d r o x y l a p a t i t e - c o a t e d i m p l a n t s are s t r o n g e r : they can be m a d e bigger a n d used f o r l o a d - b e a r i n g sites. A very successful e x a m p l e is a t i t a n i u m artificial hip joint, c o a t e d with H A . T a b l e 5 gives a s u m m a r y o f clinical applications, based o n the c o n s i d e r a t i o n s o f the p r e v i o u s paragraphs,~ 2 especially for head and neck surgery. CONCLUDING
REMARKS
Bioceramics o f calcium p h o s p h a t e are bioactive, which m e a n s that they c a n b o n d with b o n y tissues. T h e y can be p r o d u c e d either as ' p e r m a n e n t ' i m p l a n t s - - m e a n i n g that such i m p l a n t s will stay for at least five to ten y e a r s - - o r as " d e g r a d a b l e ' i m p l a n t s - - d i s a p p e a r i n g within a few m o n t h s , Deg r a d a b l e implants can be used for repair only, whereas p e r m a n e n t i m p l a n t s are suitable for b o t h repair o f b o n e and p e r m a n e n t a u g m e n t a t i o n . T h i s is o b v i o u s if one considers that a u g m e n t i n g b o n e where it was not b e f o r e will be a t e m p o r a r y success
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