Histological and histomorphometrical comparative study of the degradation and osteoconductive characteristics of α- and β-tricalcium phosphate in block grafts

Histological and Histomorphometrical Comparative Studyof the Degradation and Osteoconductive Characteristicsof a- and b-Tricalcium Phosphate in Block Grafts

Masahiro Yamada,1 Makoto Shiota,1 Yasuo Yamashita,2 Shohei Kasugai1

1 Oral Implantology and Regenerative Dental Medicine, Tokyo Medical and Dental University,1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan

2 Maxillofacial Anatomy, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan

Received 31 January 2006; revised 7 July 2006; accepted 22 August 2006Published online 14 November 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.30715

Abstract: The purpose of the present study was to compare a- and b-tricalcium phosphate

(TCP) as bone graft material for augmenting highly resorbed alveolar ridges. The cranial

bones of 15 rabbits were used. Three titanium chambers filled with porous blocks of a-TCP, b-

TCP, or blood clots were placed in each slit. The two TCP blocks had similar inner/outer

structures and purities. Animals were sacrificed after 2, 4, and 8 weeks. Specimens were

embedded in polyester resin as nondecalcified specimens, and evaluated both histologically

and histomorphometrically. In both TCP groups, blocks had hardly degraded at 2 weeks while

in the a-TCP group, the block had notably started degrading after 4 weeks. In the b-TCP

group, degradation began at 4 weeks and this degradation had increased just slightly after 8

weeks. The a-TCP block degraded significantly more than the b-TCP block. Residual a-TCP

particles surrounded by newly formed bone decreased over time, and both particles and newly

formed bone were simultaneously absorbed by osteoclast-like cells. These observations suggest

that residual a-TCP particles surrounded by newly formed bone may disappear progressively

from bone and could be incorporated into the bone remodeling cycle in combination with

newly formed bone. ' 2006 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 82B: 139–

148, 2007

Keywords: animal model; osteogenesis; calcium phosphate(s); bone remodeling; biodegra-

dation

INTRODUCTION

Alveolar ridge augmentation is fundamental for implant

dentistry. Although autogenous bone is considered to be

the best material for augmentation, it requires harvesting

surgery with notable associated risks,1,2 such as swelling at

the donor site or devitalization of teeth, abnormal sensa-

tion, etc. A wide variety of graft materials has therefore

been developed. Despite their clinical efficacy, allografts

and xenografts are subject to tight restrictions because of

religious sensibilities, value judgments or state regulations,

which arise mainly from anxiety about infection transmis-

sion from graft materials to recipients.3,4

As a result, alloplasts comprised of calcium phosphate

ceramics and bioactive glass have been studied.5–7 In par-

ticular, synthetic hydroxyapatite (HA), a calcium phosphate

ceramic, has a similar composition to hard tissues such as

teeth and bones8 and has been applied in the medical and

dental fields because of its high biocompatibility and osteo-

conductive properties.9,10

However, because of its high crystallinity and chemical

stability, HA is poorly absorbed or dissolved by multinu-

clear cells or tissue fluids and is poorly displaced by new

bone tissue.11–14 Furthermore, HA has the potential to be

easily adsorbed by bacteria and epithelial cells due to its

high surface energy,15 and may be easily infected and

encapsulated.16,17 Therefore, it is necessary to carefully

consider the application of HA.

Recently, studies have shown that tricalcium phosphate

(TCP) materials are biodegradable ceramics. In particular,

b-TCP has been advanced by both in vitro and in vivostudies.18–26 Because of its favorable osteoconduction and

biodegradation, b-TCP has been applied as a bone substi-

tute material for alveolar ridge augmentation at implanta-

tion sites.27–29

Although a-TCP has a different crystal structure from b-TCP, it has also been reported to show similar favorable

osteoconduction and biodegradation.30,31 Moreover, because

Correspondence to: M. Yamada (e-mail: [email protected])

' 2006 Wiley Periodicals, Inc.

139

of its greater solubility than HA and b-TCP,32 a-TCP easily

allows dissolution of the calcium and phosphate ions that

might otherwise advance osteogenesis.33–35 However, there

are very few in vivo studies comparing a-TCP with b-TCPand their reported differences may not reflect their crystal-

lographic discrepancy due to the extreme differences in

inner or outer shape and porosity36–39 of the tested materi-

als. Recently, Kihara et al.40 reported that osteogenesis

showed good progress following the application of a-TCPand that the residual a-TCP particles were degraded with-

out decreasing the volume of the transplantation region.

These findings suggested the possibility of using a-TCP as

a material for block grafts for the alveolar ridge.

In the present study, we compared the use of a- and b-TCP for augmentation of highly resorbed alveolar ridges

both histologically and histomorphometrically using com-

bined titanium chambers (Ti-chambers).

MATERIALS AND METHODS

Experimental Device

Turned chambers of commercially pure titanium (Node

Iron Works, Tokyo, Japan) with an inner diameter of 8 mm

and an inner height of 4.5 mm were used for standardizing

the experimental spaces and eliminating soft tissue (GBR

efficacy) (Figure 1).

a-TCP and b-TCP Blocks

a-TCP and b-TCP porous blocks (Figure 1) were provided

by BrainBase Corporation (Tokyo, Japan). Prebaked TCPs

(a-TCP, 11358C; b-TCP, 960–10008C) were crushed into

small particles (particle size under 0.85 mm in diameter),

molded with a binder, and sintered (a-TCP, 1150–12008C;

b-TCP, 960–10008C) into porous blocks with almost identi-

cal inner/outer structures (Table I). However, because of

processing difficulties the blocks could not be adjusted for

pore size or inner continuous conditions. The high purity of

the blocks were confirmed by X-ray diffractometry (Mini-

Flex, Rigaku, Tokyo, Japan) in powdering conditions to

crush again a sintered block sample. Just before use, both

TCP blocks were impregnated with rabbit blood using a

vibrator.

Surgery

A total of 15 male adult (6-months-old) Japanese white

rabbits, weighing 3–4 kg, were used in this study. All

experiments were approved by The Committee for Animal

Studies at Tokyo Medical and Dental University. After the

vertex cranium had been shaved and disinfected, general

anesthesia was induced by intramuscular injection of veteri-

nary ketamine hydrochloride (50 mg/mL; Ketalar1; San-

kyo, Tokyo, Japan) and thiopental sodium (25 mg/mL;

Ravonal1; Tanabe, Osaka, Japan). In addition, a local an-

esthetic (20 mg/mL; Xylocaine1; Astra, Sodertalje, Swe-

den) was subcutaneously injected around the operative

field. A full-thickness flap was raised to expose the cra-

nium. Using a trephine (7 mm in diameter), three 0.5-mm

deep circular slits, that crossed the cranial sutures and were

evenly spaced, were prepared at the parietal and frontal

bones. Within these slits, the cortical bone plate on both

sites was perforated with 9 evenly distributed holes using a

small round bur. Fixing screws were placed around the slits

at even spaced intervals.

Three Ti-chambers were filled with the test materials

were placed in each slit and attached to the bone with a

dental adhesive resin cement (Super-Bond C & B1 Clear;

Sun Medical, Shiga, Japan) and fixing screws. The bottom

of each chamber and the material it contained were in

direct contact with the bone without interposition of resin

cement (Figure 2). Each rabbit received 3 chambers: 1 neg-

ative control, 1 positive control, and 1 test chamber.

Negative Control Group: Autologous blood clots only,

that consisted of rabbit’s autologous venous blood þ throm-

Figure 1. Commercial pure titanium chamber (left) and tricalcium-

phosphate porous block used in this study.

TABLE I. Conformation of a-TCP and b-TCP Porous Blocksa

a-TCP b-TCP

Diameter (mm) 6.8 6.7

Thickness (mm) 3.2 3.1

Density (g/mm2) 0.16 0.17

Relative density (%) 53 56

a Both blocks have almost identical inner/outer structure and purity.

Figure 2. Ti-chambers (TC) were placed and attached to the bonewith dental adhesive resin cement involving fixing screws.

140 YAMADA ET AL.

Journal of Biomedical Materials Research Part B: Applied BiomaterialsDOI 10.1002/jbmb

bin (Mitsubishi Pharma Corporation, Osaka, Japan) þ liquid

calcium chloride.

Positive Control Group: b-TCP block containing blood

clots.

Test Group: a-TCP block containing blood clots.

The position of the chambers was randomly chosen for

each rabbit. The skin flaps were sutured with 3-0 surgical

silk. No attempt was made to cover the cylinders with the

periosteum.

Specimen Preparation

The animals were sacrificed after a healing period of 2, 4,

and 8 weeks by excessive intravascular administration of

pentobarbital sodium (50 mg/mL; Nembutal1; Dainippon

Pharmaceutical, Osaka, Japan). The soft tissue of the exper-

imental area was removed and biopsy specimens of the pa-

rietal-frontal bones comprising the three Ti-chambers were

taken en bloc and fixed by immersion in 10% formalin.

The specimens were dehydrated through a graded series of

acetone, embedded in polyester resin (Rigolac1; Showa

Highpolymer, Tokyo, Japan) and sectioned vertically

through the center of each chamber using a sawing and

grinding system (Exakt System; Exakt Apparatebau, Nor-

derstedt, Germany). A total of 7 sections/specimen were

prepared. Each section was ground to a thickness of about

30 mm and stained with 4% toluidine blue.

Analyses

Histological and histomorphometrical analysis were per-

formed using a light microscope digital camera (Olympus1

DP12; Olympus, Tokyo, Japan) and digital image editing

software (Adobe Photoshop CS; Adobe, San Jose, CA).

Morphometric measurements were made on 3 vertical sec-

tions/specimen closest to the center of the specimen. The

following measurements were made (Figure 3).

Area of Residual Implant: Area of residual TCP in the

block (%).

Area of Noninvaded Implant: Area of residual TCP not

invaded by new bone tissue into the block (%).

Each measurement indicated accounted for a certain per-

centage of the area of the original block.

Statistical Analyses

Nonparametric analyses were used because blocks were

grafted on the same individual and because the experimen-

tal groups were not large enough to apply parametric analy-

sis. The Wilcoxon t test was run for intergroup analyses (avs. b) of the measurements. The Kruskal–Wallis H test was

used for comparisons within groups and the Mann–Whitney

U test with the Bonferroni correction was used post hoc to

determine the location of significant differences. The funda-

mental significance for the analyses was set to p < 0.05.

RESULTS

Histological Analysis

Control (Blood Clots Only)

2 Weeks. Connective tissue and a small quantity of

newly formed bone with a high staining intensity were

observed on the surface of the existing bone within the Ti-

chamber (Figure 4).

4 Weeks. Newly formed bone on the surface of the host

bone was increased compared with 2 weeks and proceeded

along the inner walls of the Ti-chamber. Furthermore,

staining intensity was generally decreased compared with 2

weeks (Figure 5).

8 Weeks. Newly formed bone had progressed along the

inner wall of the tectorium of the Ti-chamber along the

inner walls and was observed up to one-third of the height

Figure 3. The digital image by editing software used for histmorphometric analysis. Area of theremained block parts (Area of Residual Implant) is represented as black and checked. Area of the

nonintruded block parts (Area of Noninvaded Implant) is represented as black. [Color figure can be

viewed in the online issue, which is available at www.interscience.wiley.com.]

141CHARACTERISTICS OF a- AND b-TCP POROUS BLOCKS


of the Ti-chamber at the central area, which showed the

least bone formation [Figure 6(a)].

The staining intensity of the newly formed trabeculae

along the inner walls of the Ti-chamber was decreased

compared with 4 weeks, and the thickness was increased.

However, on the upper side of the Ti-chamber, densely

stained bone was observed [Figure 6(b)].

b-TCP

2 Weeks. The surface of the b-TCP block showed a

smooth circumference. Between the host bone and the block,

newly formed immature bone was present that had a high

staining intensity and was continuous with the host bone

(Figure 7).

4 Weeks. Concavities were observed on the bottom sur-

face of the block. Newly formed bone continuing to the

host bone was increased slightly compared to 2 weeks

[Figure 8(a)].

At high magnification, new bone formation was ob-

served that corresponded to the concavities on the bottom

surface of the block. Although the newly formed bone was

partially in contact with the surface of the b-TCP block, it

never invaded the concavities. The trabeculae ran parallel

to the surface of the block [Figure 8(b)].

Figure 6. (a) Blood clots only group at 8 weeks. Newly formed bone

(NB) is visible up to one-third of the height of the titanium chamber at thecentral area. (Original magnification 35). (b) Blood clots only group at 8

week in moderate magnification. Low stainable newly formed bone (NB)

existed along the inner walls of TC. High stainable NB existed on the

upper side of TC. (Original magnification310). [Color figure can be viewedin the online issue, which is available at www.interscience.wiley.com.]

Figure 7. Toluidine blue staining of b-TCP group in nondecalcified

specimens at 2 weeks. The surface of b-TCP block is circumferentially

smooth (Original magnification 35). [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

Figure 4. Toluidine blue staining of blood clots only group in nonde-

calcified specimens at 2 weeks. There is no much newly formedbone (NB) visible. (Original magnification 35) HB, host bone; TC, ti-

tanium chamber. [Color figure can be viewed in the online issue,

which is available at www.interscience.wiley.com.]

Figure 5. Blood clots only group at 4 weeks. Newly formed bone

(NB) has progressed along with the inner wall of titanium chamber

(TC). (Original magnification 35). [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

142 YAMADA ET AL.


8 Weeks. The concavities on the bottom surface of the

block had progressed slightly compared with 4 weeks. The

amount of newly formed bone between the block and the

host bone and along the inner walls of the Ti-chamber was

increased compared with 4 weeks [Figure 9(a)].

The newly formed trabeculae between the block and the

host bone were thicker and showed a slightly lower stain-

ing intensity than at 4 weeks. Newly formed immature

bone with a high staining intensity was found on relatively

larger concavities of the block [Figure 9(b)].

a-TCP

2 Weeks. The surface of the a-TCP block showed a

smooth circumference, similar to b-TCP. Between the

block and the host bone, newly formed immature bone

with a high staining intensity was observed. It continued to

the host bone and was partly in contact with the smooth

surface of the block (Figure 10).

4 Weeks. Unlike b-TCP, many inner cavities of the bot-

tom angles were observed, as well as concavities on the

bottom surface of the block. These inner cavities proceeded

to deep parts of the block. Slight concavities were also

observed around the surface of the block that was in con-

tact with the connective tissue. The quantity and maturity

of the newly formed bone were increased compared with 2

weeks [Figure 11(a)].

Newly formed bone invaded in the region of concavities

and cavities. In this region, degraded and fractionated a-TCP particles were frequently surrounded by newly formed

bone [Figure 11(b)].

Some multinuclear giant cells containing phagocytosed

a-TCP particles were observed as well [Figure 11(c)].

8 Weeks. The inner concavities of the a-TCP block had

proceeded remarkably not only in the region of the angles

Figure 9. (a) b-TCP group at 8 weeks (original magnification 35).

Concavities (triangle) on the bottom surface of the block are slightlyincreased compared to 4 weeks. (b) b-TCP group at 8 weeks in

moderate magnification (original magnification 310). Newly formed

bone (NB) (double arrow) is found on the relatively larger concavities

(triangle) of the block. [Color figure can be viewed in the onlineissue, which is available at www.interscience.wiley.com.]

Figure 8. (a) b-TCP group at 4 weeks (original magnification 35).

Concavities (triangle) are observed on the bottom surface of theblock. (b) b-TCP group at 4 weeks (original magnification 310).

Newly formed bone (NB) is never observed to intrude into the con-

cavities (triangle). [Color figure can be viewed in the online issue,

which is available at www.interscience.wiley.com.]



of the block bottom, but also in the upper part of the side

of the block and deep parts of the inner block. The block

had almost completely lost its original shape. The inner

concavities of the block had approximately progressed to

half the block height [Figure 12(a)].

Newly formed bone had invaded almost the entire

region of the inner concavities. The newly formed trabecu-

lae were thicker and showed a lower staining intensity than

at 4 weeks. Less residual a-TCP particles were surrounded

by the new bone in the same region at 4 weeks as is shown

in Figure 12(b).

Osteoblasts were observed on the surface of the newly

formed bone surrounding the a-TCP particles, as well as

osteocytes [Figure 12(c)]. In addition, both newly formed

bone and a-TCP particles were uniquely absorbed by osteo-

clast-like cells [Figure 12(d)].

Histomorphometrical Analysis (Figure 13 and 14)

Intragroup Comparisons. Area of Residual Implant

and Area of Noninvaded Implant showed notable signifi-

cant differences (p < 0.01) for 2 weeks/8 weeks in the b-TCP group, and 2 weeks/4 weeks and 2 weeks/8 weeks in

the a-TCP group. No other significant differences were

found.

a-TCP vs. b-TCP. Area of Residual Implant and Area

of Noninvaded Implant showed significant differences (p ¼0.044) at 4 weeks and 8 weeks. At 2 weeks, there was no

significant difference.

DISCUSSION

Bone graft materials are categorized as autografts, allo-

grafts, xenografts, and alloplasts. Their fundamental use in

clinical practice is considered to be as scaffolds for osseous

regeneration.41,42 TCPs are recognized as alloplastic materi-

als that have no associated hazards of infecting the recipi-

ents with unknown viruses and are gradually displaced by

new bone in vivo.In the present study, Ti-chambers were used to standard-

ize experimental conditions and borders with the host bone

were sealed circumferentially with dental resin cement. The

biotolerance and border seal-ability of the dental resin

cement were confirmed in previous studies43 and our pilot

study (data not shown). In earlier studies,19,44,45 bone forma-

tion in the chamber was reported to be facilitated by border

sealing around the chamber, similar to guided bone regener-

ation. In the negative control (blood clots only), connective

tissue and newly formed bone originating in the host sutures

and bone marrow were mainly observed along the Ti-cham-

ber side walls and the height of the new bone increased pro-

gressively. At 8 weeks, new bone grew at the lowest point

to one-third of the Ti-chamber height at the central area.

This observation further confirmed previous studies.

In the present study, direct contact and invasion into the

blocks of newly formed bone was observed, corresponding

to concavities on the block surface and to cavities in the

block for both TCP groups. In particular, newly formed

bone surrounded the residual particles in the a-TCP group.

These observations demonstrate the biocompatibility and

biodegradation of the TCPs, which were absorbed and

replaced by new bone. In both groups, the surface of the

blocks showed a smooth circumference and degradation of

the blocks was hardly observed at 2 weeks. However, in

the a-TCP group, many inner cavities in parts of the bot-

tom angles near the host bone were observed at 4 weeks.

At 8 weeks, these regions of inner cavities had proceeded

into the deeper parts of the blocks and to half the block

height at their lowest point. In the b-TCP group, some

minor concavities were found on the bottom surface of the

block at 4 weeks and their number and size had only

increased slightly at 8 weeks. The degradation of b-TCPwas generally lower than a-TCP, where almost no original

block remained at 8 weeks. In the histomorphometrical

analysis, both Area of Residual Implant and Area of Nonin-

vaded Implant differed significantly between the two TCP

groups at 4 weeks and 8 weeks.

Absorption of alloplasts in vivo has been recognized to

occur through two processes, namely cellular absorption by

multinuclear giant cells and dissolution by tissue fluids.26,32

The most different feature between a-TCP and b-TCP is the

solubility in liquid solution. Eggli et al.44 suggested that

absorption of TCPs is initiated via phagocytosis by multinu-

clear giant cells. On the other hand, Schliephake et al.45 and

Fujita et al.24 suggested that their absorption begins with dis-

solution by tissue fluids, and is then followed by cellular

absorption. Furthermore, Zerbo et al.29 and Lu et al.26 sug-

gested that the initial absorption of TCPs is influenced not

only by cellular absorption but also by dissolution. In the

present study, multinuclear giant cells did not contact the a-TCP blocks directly. Instead, the blocks crumbled and

released particles that were phagocytosed at 4 weeks. On the

other hand, there was no direct contact of multinuclear giant

cells with the blocks and phagocytosis of particles was not

Figure 10. Toluidine blue staining of the a-TCP porous block (AB)

group in nondecalcified specimens at 2 weeks. The surface of thea-TCP block is circumferentially smooth (original magnification 35).

[Color figure can be viewed in the online issue, which is available at

www.interscience.wiley.com.]

144 YAMADA ET AL.


observed in the b-TCP group. Accordingly, it is suggested

that the differences in degradation observed in vivo between

the a-TCP and b-TCP porous blocks reflect a higher solubil-

ity of a-TCP in liquid solution, as was shown in a previous

in vitro study32 (dominant dissolution by tissue fluids), and

these differences in degradation are hardly influenced by cel-

lular absorption.

Absorption by osteoclast-like cells of the residual a-TCPparticles surrounded by newly formed bone was observed

at 8 weeks. Additionally, in the same region there were

less residual a-TCP particles surrounded by newly formed

bone at 8 weeks than at 4 weeks. These observations sug-

gest that the residual a-TCP particles surrounded by newly

formed bone might disappear progressively from new bone,

at least in part by cellular absorption.

The Area of Noninvaded Implant, which represents the

area of the block not invaded by regenerating tissue, includ-

ing newly formed bone, was significantly lower for a-TCPthan b-TCP at both 4 weeks and 8 weeks. These findings

indicate that a large amount of new bone formation invaded

the a-TCP block with notable degradation. An earlier study

reported that intrusion of osteoblasts or osteoclasts required

spaces of more than 50 mm, while new bone formation

required spaces of more than 100 mm.12 Consequently, the

Figure 11. (a) a-TCP group at 4 weeks (original magnification 35). Many inner cavities of the bot-

tom angles are visible. (b) a-TCP group at 4 weeks (original magnification 310). Newly formed

bone (NB) intrudes into the inner cavities of the block. Degraded and fractionated a-TCP particles

(asterisk) are surrounded by NB. (c) a-TCP group at 4 weeks at high magnification (original magnifi-cation 340). Multinuclear giant cell (double arrow) phagocytose a-TCP particles. [Color figure can

be viewed in the online issue, which is available at www.interscience.wiley.com.]



formation of different spaces in the two types of blocks fol-

lowing degradation may cause the discrepancy in the ranges

of newly formed bone invasion into the blocks.

In an earlier study, it was reported that increases in the

local Ca and P ion concentrations as a consequence of deg-

radation of TCPs stimulated new bone formation. Sugimoto

et al.46 and Honda et al.47 showed that high concentrations

of Ca ions in vivo accelerated the activation and differen-

tiation of osteoblasts. Aoki et al. speculated that the local

pH showed a trend toward the basic side due to the release

of Ca ions and that this chemical change proved that the

circumjacent bone undergoes some kind of activation. Fur-

thermore, recent studies21,30,33–35 have shown that dissolu-

tion of Ca and P ions around TCPs accelerates new bone

formation via reseparation and the formation of amorphous

apatite layers around the residual particles. In particular,

Fusimi et al.30 observed by transmission electron micros-

copy that a collagen-free layer created from hydroxyapatite

was present around a-TCP particles surrounded by newly

formed bone and that new bone was formed circumferen-

tially on this apatite layer. Furthermore, Habibovic et al.35

concluded in a goat study that biomaterials containing Ca

Figure 12. (a) a-TCP group at 8 weeks (original magnification 35). The inner cavities of the blockprogress to one-third of the Ti-chamber height at the lowest point. Asterisk show a-TCP particles

surrounded by newly formed bone. (b) a-TCP group at 8 weeks (original magnification 310). Newly

formed bone (NB) intrudes almost the entire inner cavities of the blocks. Less residual a-TCP par-

ticles (asterisk) are surrounded by the new bone in the same region at 4 weeks. (c) a-TCP group at8 weeks at high magnification (original magnification 350). Osteoblasts (double arrow) are visible

on the surface of newly formed bone (NB) surrounding TCP particles (asterisk). Arrow shows multi-

nuclear giant cell. (d) a-TCP group at 8 weeks at high magnification (original magnification 350).

Both newly formed bone (NB) and a-TCP particles (asterisk) are absorbed by osteoclast-likecell (double arrow). [Color figure can be viewed in the online issue, which is available at www.

interscience.wiley.com.]

146 YAMADA ET AL.


and P accelerated the differentiation of osteoblasts and the

formation of new bone via biological apatite layers formed

by redeposition of Ca and P and coprecipitated endogenous

proteins such as BMPs.

No apatite layers were observed around the residual a-TCP particles in the present study. It is possible that the

newly formed bone surrounding the residual particles at 4

weeks and 8 weeks could be generated by Ca and P ions,

as observed in previous studies. However, some studies

have suggested that excessive local concentrations of Ca

interrupt new bone formation.11,37,39,48 Although good bone

formation was observed in the a-TCP porous blocks used

in the present study, the same results may not be observed

for a-TCP porous blocks that have a higher solubility and

therefore elevate the local Ca and P concentrations.37,39

In previous studies,25,48 it has been suggested that TCP

particles embedded in newly formed bone are subject to

cellular absorption and become incorporated into the bone

remodeling process. In the present study, these findings

were never observed in the b-TCP group because there was

little new bone formation inside the blocks. However, in

the a-TCP group, bone formation by osteoblasts, and the

resorption of both the residual particles and newly formed

bone by osteoclast-like cells were observed at 8 weeks.

These observations may indicate that the newly formed

bone around a-TCP shows features and homeostasis that

are similar to normal bone, such as the bone remodeling

cycle for functional loading.

CONCLUSIONS

a-TCP porous blocks showed higher initial solubility invivo than b-TCP blocks. Additionally, the residual a-TCPparticles surrounded by newly formed bone appeared to be

dissolved by tissue fluids and absorbed by osteoclast-like

cells. Furthermore, the degradation of a-TCP seemed to be

aligned with bone formation, while the newly formed bone

surrounding a-TCP became incorporated into the bone

remodeling cycle with the residual particles.

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Figure 13. No mark, p > 0.05. The a-TCP group is significantly

lower than the b-TCP group at 4 weeks and 8 weeks. There are sig-

nificant differences at 2 weeks/8 weeks in the b-TCP group, and 2weeks/4 weeks and 2 weeks/8 weeks in the a-TCP group. Compari-

son between two groups – Wilcoxon t test, *p ¼ 0.044; comparison

within each group – Kruskal–Wallis H test and Mann–Whitney U test

with Bonferroni correction, #1p ¼ 0.007, #2p ¼ 0.008.

Figure 14. No mark, p > 0.05. The a-TCP group is significantly

lower than the b-TCP group at 4 weeks and 8 weeks. There are sig-nificant differences for 2 weeks/8 weeks in the b-TCP group, and 2

weeks/4 weeks and 2 weeks/8 weeks in the a-TCP group. Compari-

son between two groups – Wilcoxon t test, *p ¼ 0.044; comparison

within each group – Kruskal–Wallis H test and Mann–Whitney U testwith Bonferroni correction, #3p ¼ 0.006, #4p ¼ 0.009.



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Histological and histomorphometrical comparative study of the degradation and osteoconductive characteristics of α- and β-tricalcium phosphate in block grafts