Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
2002-07-02
2004-05-11
Wood, Elizabeth D. (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S691000
Reexamination Certificate
active
06733582
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention concerns a cement for surgical purposes, a method for stabilizing a brushite cement used as temporary bone replacement material and the temporary bone replacement material obtained by said method.
A number of such hydraulic cements based on calcium phosphates for use in surgery are known in the prior art; they are prepared from two components (powder/liquid) by mixing them intraoperatively and applying them in pasteous consistency to the appropriate site where they harden in situ. The disadvantages of the prior art hydraulic cements based calcium phosphates are:
a) impracticable short setting times which do not allow their use for elaborate surgical procedures;
b) poor injectability, i.e. the fresh cement paste tends to clog the injection needle, and/or disintegrates in contact with physiological liquids, which prevents its implantation by minimal invasive surgery procedures;
c) low compacity, i.e. current hydraulic cements need larger amounts of mixing water in order to have them injectable or to confer them a convenient setting time, which results in very low ultimate mechanical strength after hardening; and
d) too fast resorption, i.e. the cement resorbs faster than the bone growth rate, resulting in a non-osseous gap between bone and cement which is detrimental to the mechanical properties of the cement.
In the U.S. Pat. No. 4,880,610 CONSTANTZ a method is disclosed for making an in situ calcium phosphate mineral composition by combining water-free phosphoric acid crystals with a calcium source which leads to a hydroxyapatite. It is clear that the use of 100% phosphoric acid in the operating room and the application of a paste containing 100% phosphoric acid in the human body must be considered a not ideal procedure which requires improvement.
In U.S. Pat. No. 5,129,905 CONSTANTZ—in order to reduce the problem—phosphoric acid crystals were replaced by monocalcium phosphate monohydrate (MCPM) or monocalcium phosphate (MCP). However, the goal was again to obtain a hydroxyapatite material, which has a long resorption period. A long resorption period is not commensurate to the rate of the bone remodelling. The disadvantage of prolonged resorption is that the bone treated by cement will remain for a prolonged time in abnormal biomechanical situation, which may develop secondary post-operational problems. Furthermore, the unresorbed cement may still break down in pieces or fragments after prolonged mechanical loading, which increases the probability of post-operational complications, e.g. aseptic inflammatory reactions. The resorption rate of the ideal cement should match as closely as possible the spontaneous rate of new bone formation which is around 20 micrometers per day. A too fast resorption rate is also not wanted. Certain studies done with plaster of Paris and calcium phosphate cement have shown that the resorption rate is faster than the bone growth rate, leading to a gap between bone and cement. This is obviously detrimental to the mechanical stability of the defect site.
From U.S. Pat. No. 5,605,713 BOLTONG a calcium phosphate composition is known which may contain (among others) &bgr;-TCP, MCPM, water and magnesium salts. However, the invention is limited to pH values in the range of 6.5 to 8.0, range in which brushite does not precipitate. A pH below 6,5 preferably below 4 is needed to obtain brushite. In the pH range of 6.5 to 8.0, octocalcium phosphate and hydroxyapatite are the phases precipitating. However, these phases are much less soluble than brushite at neutral pH and thus lead to too slow resorption rates.
From PCT/EP98/06330 a calcium phosphate composition is known which contains brushite (dicalcium phosphate dihydrate; CaHPO
4
.2H
2
O) as end-product of the setting reaction. This cement has however a too fast resorption rate in vivo, leading to mechanical instabilities and inflammatory reactions.
SUMMARY OF THE INVENTION
The invention as claimed aims at solving the above described problems. The present invention provides a cement for surgical purposes, a method for producing a temporary bone replacement material as defined and a temporary bone replacement material.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to an forming part of this disclosure. For the better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be done to the accompanying examples in which preferred embodiments of the invention are illustrated in detail.
The first component of the cement according to The invention comprises a basic calcium phosphate, preferably &bgr;-tricalcium phosphate [&bgr;-Ca
3
(PO
4
)
2
; &bgr;-TCP], &agr;-tricalcium phosphate [&agr;-Ca
3
(PO
4
)
2
; &agr;-TCP], tetracalcium phopshate Ca
4
(PO
4
)
2
O; TetCP], oxyapatite Ca
10
(PO
4
)
6
O; OXA], hydroxyapatite [Ca
5
(PO
4
)
3
OH; HA], or calcium-deficient hydroxyapatite [Ca
10−x
(HPO
4
)
x
(PO
4
)
6−x
(OH)
2−x
; CDHA] powder. It can also be a mixture of two or three of the latter compounds. &bgr;-TCP is the preferred compound.
The second component of the cement according to the invention comprises an acidic calcium phosphate, preferably monocalcium phosphate monohydrate [Ca(H
2
PO
4
)
2
·H
2
O; MCPM], monocalcium phosphate [Ca(H
2
PO
4
)
2
; MCP], or phosphoric acid [H
3
PO
4
] powder. It can also be a mixture of two or three of the latter compounds. MCPM is the preferred compound.
The third component of the cement according to the invention comprises water.
The fourth component of the cement according to the invention comprises a source of magnesium used to stabilize the end-product of the setting reaction.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The setting reaction is characterized by several partial reaction: dissolution of the first component, dissolution of the second component and precipitation of the end-product of the setting reaction, i.e. brushite (dicalcium phosphate dihydrate; CaHPO
4
·H
2
O). Normally, the dissolution reaction of the second component is much faster than that of the first component. As the second component is acidic, the cement paste reaches pH values of 2 to 4 depending on the cement composition and particle size distribution. When the second component is completely dissolved, the ongoing dissolution of the first component, which is basic, provokes an increase of the pH value of the cement paste. However, the pH of the cement according to the invention at the end of the setting reaction is always in the range of 2 to 6. It is also in this range that brushite preferentially precipitates. At higher pH values (6 and higher), brushite does not precipitate: octocalcium phosphate and hydroxyapatite are the phases precipitating. However, these phases are much less soluble than brushite at neutral pH and would thus lead to too slow resorption rates.
The particular size distribution and the mean specific surface area of the solid components has a large influence on the physico-chemical properties of the cement, in particular the setting time, the mechanical properties, and the workability. Generally speaking, powders with a high specific surface area lead to short setting times, high mechanical-properties, and good workability. However, this rule is no more valid when the powders are agglomerated: a large amount of mixing liquid is required to water the powders, hence leading to poor mechanical properties. Therefore, powders should preferably be desagglomerated.
A good workability depends very much on the application. In some cases, a rather liquid paste is desired (reinforcement of osteoporotic bones). In other cases, a very thick paste may be the most adequate (e.g. plastic surgery). A powder with a small average particle size possess only a very small range in which a paste can be formed with water the paste is either solid and breakable, or very liquid. Generally, a thick paste is preferred, because
Bohner Marc
Matter Sandro
Dr. H. C. Robert Mathys Stiftung
Rankin, Hill Porter & Clark LLP
Wood Elizabeth D.
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