American Journal of Neuroradiology 24:1540-1544, September 2003
© 2003 American Society of Neuroradiology
INTERVENTIONAL
Hydroxyapatite Ceramics As a Particulate Embolic Material: Report of the Physical Properties of the Hydroxyapatite Particles and the Animal Study
Michiya Kuboa,
Naoya Kuwayamaa,
Yutaka Hirashimaa,
Akira Takakua,
Tetsuro Ogawab and
Shunro Endoa
a Department of Neurosurgery, Toyama Medical and Pharmaceutical University, Toyama, Japan
b New Ceramics Division, Life Care Business Headquarters, PENTAX Corporation, Tokyo, Japan
Address reprint requests to Michiya Kubo, MD, Department of Neurosurgery, Toyama Medical & Pharmaceutical University, 2630 Sugitani, Toyama City, Toyama, 930-0194, Japan
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Abstract
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BACKGROUND AND PURPOSE: Hydroxyapatite ceramic microparticles
are newly developed embolic materials. The purpose of this study
was to evaluate the characteristics, embolic properties of hydroxyapatite
ceramic microparticles, and effects of transcatheter embolization
from an angiographic standpoint and to histopathologically evaluate
the results of animal experimentation.
METHODS: After physical evaluation of the characteristics and embolic properties of hydroxyapatite microparticles (100–250 µm in diameter), microcatheter superselective renal artery embolization was performed in rabbits by using this material. Follow-up angiography and histopathologic study of 18 rabbits were performed at different intervals within 24 weeks after embolization.
RESULTS: No microcatheter clogging was observed because of excellent visibility of this material in the suspension and because of its excellent navigability due to its nearly oval shape with no "spines." Angiographic renal arterial devascularization was consistently obtained without unexpected proximal occlusions; follow-up angiography showed persistent occlusion. Histopathologic evaluation confirmed satisfactory embolization of peripheral arteries. Chronic inflammatory cells were observed in several vessels containing hydroxyapatite microparticles. No evidence of angionecrosis, hemorrhage, or extraluminal migration was noted in any sections.
CONCLUSION: Hydroxyapatite microparticles are excellent biocompatible embolic materials with good visibility during injection control, capable of producing effective occlusion of the distal arteriocapillary bed.
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Introduction
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Among embolic materials for hypervascular tumors, particles
are preferred to liquid, if used in appropriate size, because
of ease of handling and safety. Various solid embolic agents
have been developed, including Gelfoam (
1), microfibrillar collagen
(
2), collagen microbeads (
3), dextran (
4), polyvinyl alcohol
particles (
5), and trisacryl gelatin (
6). Several types of particulate
embolic materials are currently available, but they are not
necessarily satisfactory for clinical use, because of their
reported complications (
3,
7–
18) and because of the frequent
incidence of microcatheter clogging due to their irregular shape
and poor particle visibility in the contrast medium suspension.
A few newer embolic agents have improved their poor visibility
by the use of coloring and do not have the disadvantage of spines.
Hydroxyapatite (Ca
10[PO
4]
6(OH)
2) is a form of calcium phosphate
with a microporous structure. Synthetic hydroxyapatite is a
ceramic that is stable and biocompatible (
19); it has been widely
applied clinically in prosthetic grafts in orthopaedic surgery,
plastic and reconstructive surgery, spinal surgery, cranial
reconstruction, and oral surgery. To our knowledge, the properties
and characteristics of hydroxyapatite as an embolic agent have
never been investigated.
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Methods
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Medical grade nanocrystalline stoichiometric (Ca/P = 1.67) hydroxyapatite
suspension was synthesized through wet reaction by adding phosphoric
acid into calcium hydroxide suspension. The suspension was dried
into the spherical powder (15 µm in diameter) by using
a spray drying technique. The powder was calcined at 700°C
and jet-milled for conditioning. The conditioned powder was
again mixed with water and dried in a mold to obtain a porous
block (50% porosity). The porous block was crushed into small
microspheres by a granulator machine. The particle size of microspheres
was precisely calibrated to be 100 to 250 µm in diameter
by screen mesh, and finally, the microsphere was sintered at
800°C. The microsphere was pure hydroxyapatite according
to chemical analysis, including X-ray powder diffraction. Sterilization
was obtained by using ethylene oxide gas during 24 hr before
use. The suspension of hydroxyapatite in contrast media was
made in various concentrations, and we used hydroxyapatite suspension
with 10 mg of hydroxyapatite particles for 10 mL of 80% contrast
media. One-milliliter syringes were mainly used. Visibility,
sedimentation rate, and incidence of aggregation were visually
examined and compared with those of polyvinyl alcohol particles
of the same size (100–250 µm in diameter) and concentration.
Controllability of injection through a 2.7-French microcatheter
and frequency of clogging phenomenon were also studied. The
renal arteries of rabbits, all of which were fed a standard
laboratory diet, were chosen for evaluating this embolic material.
Eighteen rabbits weighing 2.5 to 4.0 kg were anesthetized with
intramuscular administration of ketamine (50 mg/kg) and atropine
sulfate (1 mg/kg). They were transbronchially intubated, and
no rabbits needed mechanical ventilation. After surgical exposure
of the femoral artery, a 2.7-French microcatheter (Target Therapeutics,
Japan) was advanced into the renal artery by using a microguidewire
under fluoroscopic guidance. Selective angiography of the renal
artery was performed before embolization. Hydroxyapatite microparticles
were mixed with contrast medium at a concentration of 320 mgI/mL
(Hexabrix 320; Tokyo Tanabe Co. Ltd., Tokyo, Japan) and slowly
injected by using 1.0-mL graded syringes until flow stagnation
was observed under fluoroscopy. After embolization, superselective
angiography was again performed. The punctured femoral artery
was compressed manually for hemostasis, and the skin was closed.
The rabbits were divided into six groups of three rabbits each
according to interval after embolization: immediately after
embolization and 1 day, 1 week, 3 weeks, 12 weeks, and 24 weeks
after embolization. Follow-up angiography was performed in the
latter five groups. All the animals were killed by using a standard
method (overdose of anesthetic agents). Both kidneys (one side,
embolized; contralateral, control) with surrounding tissues
and vessels were removed at autopsy after perfusion fixation
with 10% formaldehyde solution under approximately 100 cm H
2O
pressure through the catheter in the thoracic aorta. After paraffin
embedding, the kidneys were sectioned and examined under the
light microscope with hematoxylin and eosin staining.
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Results
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Scanning electron micrographs show that hydroxyapatite particles
have a nearly oval shape with no spines and a microporous structure
(
Fig 1). The visibility of particles in the contrast medium
suspension is significantly superior to that of polyvinyl alcohol
particles of the same diameter and concentration (
Fig. 2). The
diffusion of hydroxyapatite particles in the suspension was
also good because of its slow sedimentation rate in the contrast
media. Injection through a microcatheter was always easy, and
no microcatheter clogging was experienced because of the easy
visibility of particles in the suspension. We could easily control
the injection rate under good visual monitoring so that we were
able to avoid particles cluster at the microcatheter hub and
injection syringe. All the rabbits except three well tolerated
the anesthetic and endovascular procedures. Of the three rabbits,
two had infection of the femoral wound and one had circulatory
and respiratory system dysfunction after additional administration
of anesthetic agents. The endovascular procedure was successfully
performed in all animals. Angiography showed all embolized renal
arteries to be occluded after gradual blood flow reduction with
10 to 30 mg of the particles (
Fig 3A and B); follow-up angiography
showed persistence of occlusion in all the rabbits during a
period of 1 day to 24 weeks (
Fig 4). Macroscopic findings of
embolized kidney were as follows: renal arteries were thrombosed,
and kidneys were decreased in size in proportion to the interval
between embolization and resection. On histologic sections,
hydroxyapatite microparticles appeared slightly basophilic.
No material fragmentation was observed on any of the sections
examined. The microparticles reached the peripheral renal medullary
arteries within 100 to 250 µm in diameter (
Fig. 5A). No
arterial wall necrosis or hemorrhagic changes around the occluded
vessels were found in any of the sections studied. A few small
round cells and multinuclear cells were observed in the thrombus
or organized tissues among the embolic materials. All sections,
prepared in every interval
24 weeks after embolization, showed
complete vascular occlusion with no recanalization and no extraluminal
migration, and hydroxyapatite microparticles were still found
with no resorption. Histologic findings in larger arteries with
a diameter of approximately 400 µm were nearly the same
as those in smaller vessels (
Fig 5B).
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FIG 1. Scanning electron microscopic image shows the oval shape of hydroxyapatite ceramic microparticles. Surface has microporous structure with no spine (original magnification, x300).
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FIG 2. Visibility of the suspension of hydroxyapatite ceramic microparticles in 80% contrast media (left) is comparatively superior to that of polyvinyl alcohol particles of the same size (right) contained in the 1.0-mL graded syringes.
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FIG 3. Selective right renal arterial angiogram of a rabbit.
A, Before embolization.
B, Immediately after embolization with hydroxyapatite microparticles. The renal artery is occluded with hydroxyapatite microparticles after embolization.
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FIG 4. Twenty-four weeks after embolization, persistent occlusion of the embolized right renal artery can be seen. The right kidney seems to have atrophic change.
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FIG 5. Photomicrographs.
A, Three weeks after embolization, kidney shows hydroxyapatite microparticles (arrow) reaching the peripheral renal medullary arteries of approximately 200 µm in diameter. No arterial wall necrosis, extraluminal migration, or hemorrhagic changes are found. Several small round cells can be seen in the thrombus (hematoxylin and eosin; original magnification, x100).
B, Histologic findings in larger arteries are nearly the same as those in smaller vessels (hematoxylin and eosin; original magnification, x100).
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Discussion
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Hydroxyapatite ceramics share numerous microstructural similarities
with human hydroxyapatite (
19), which accounts for 65% of bones.
Excellent biocompatibility and usefulness of hydroxyapatite
ceramics have been consistently reported in bone, soft tissues,
and other organs (
19–
21). It has already been used in
clinical practice, mainly as a bone substitute (
20). Little
is known, however, regarding intravascular use (
22), especially
concerning safety of use and effectiveness as an embolic material
to produce intra-arterial occlusion.
Role of Manufacturing Process
The microporous structure of hydroxyapatite ceramics is related to thermal distribution during the manufacturing process. The surface area of hydroxyapatite ceramics is 0.5 m2/g at 1200°C, 22 m2/g at 800°C, and 47 m2/g at 400°C. It is relatively easy to select the adequate temperature for a particular purpose during the process (21). It is also known that apatite structure is degraded by phosphorylation if produced at temperatures >1300°C and that a strong inflammatory response to a foreign body develops if produced at temperatures <700°C (23). As a bone substitute, the characteristics of hydroxyapatite ceramics should be governed during the manufacturing process primarily by its hardness, which leads to less fragility, and also by the appropriate selection of size of microporous structure for osteoblastic induction (24, 25). Hydroxyapatite ceramics produced at approximately 1100°C were reported to have the best performance for these purposes. However, as microparticles for embolization, adequate specific gravity suitable for good suspension with contrast media is required in addition to minimal brittleness. The microparticles that we produced at 800°C well fulfilled all these conditions.
Requirements for Clinical Use
Turjman et al (3) described the requirements for the ideal particulate embolic material: easiness of passing through a microcatheter, effectiveness in producing vascular occlusion, homogeneous embolization, easy availability, biocompatibility, relatively benign inflammatory response, and no carcinogenesis. The easiness of passing through a microcatheter is mainly related to the shape of the particles; nearly spheroid or oval particles with a smooth surface and no spine are ideal for this purpose. Shape also has an effect on the response of the surrounding tissues. Misiek et al (26) implanted hydroxyapatite particles of two different shapes—one smooth and rounded and the other irregular and sharp—in buccal soft tissue and reported that both induced a very mild inflammatory response, although the resolution of the inflammatory response proceeded at a faster rate with the smooth and rounded particles. One additional requirement to help prevent clogging of microcatheters is clear visibility of particles in the suspension of contrast media. We often encounter clogging of microcatheters during embolization with currently available embolic agents, which can be disappointing because successful placement of microcatheters can prove very difficult. It is thought that clogging is primarily related to poor visibility of particles, which keeps the operator unaware of their focal attenuation in the microcatheter and cluster formation, which occurs most commonly at the microcatheter hub. No such clogging occurred with our material. To obtain a homogeneous embolization result, in addition to shape and particulate surface, lack of fragmentation is another criterion that helps during embolization. Our particles, produced at 800°C, were not easily breakable under normal conditions during embolization procedures; also, we were able to show that the particles retained their shape without fragmentation in our histopathologic study. Moreover, hydroxyapatite ceramics are easily available because they have already been in clinical use for some time in several countries. Carcinogenesis has not been reported to date. Application to humans has been approved by the Food and Drug Administration. The remaining two requirements—effectiveness in producing vascular occlusion and a relatively mild and benign inflammatory response—were shown in our experimental animal study. Embolization was complete and persisted for 24 weeks without recanalization. A mild inflammatory response, without acute angionecrosis and focal hemorrhage, was observed in the examined sections and was considered to play a synergistic role with the embolic materials to ensure effective thrombosis (3). The results of our experimental study are the first reported results to indicate the usefulness and safety of hydroxyapatite ceramics as an embolic material.
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Conclusion
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In conclusion, hydroxyapatite ceramic microparticles are an
excellent biocompatible embolic material with good visibility
during injection control, capable of producing effective occlusion
of distal arteriocapillary beds with a resulting mild benign
inflammatory response and preservation of the arterial feeders.
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Received March 25, 2002;
accepted after revision April 4, 2003.
TOP
Abstract
Introduction