Bone replacement part made of glass ionomer cement

Bone replacement parts consisting of glass ionomer cement are superior to those made of ceramics especially since they may easily be produced and machined using common grinding and milling tools. The required prosthesis part may therefore be formed of freshly mixed glass ionomer cement during the respective operation, or an industrially prefabricated formed body approximating the idealized shape of the bone part to be replaced may be adapted to the anatomic conditions given.

BACKGROUND OF THE INVENTION 
Bone structures or bone parts in the human body may be destroyed due to 
inflammatory processes, malignant tumors or traumatic events; they may be 
replaced by suitable prostheses. Apart from metals, which are used to 
replace highly strained bone structures, ceramic prostheses serve for 
replacing less strained bone structures, such as in the region of the 
head. Ceramic materials are used in this region for reconstructing, e.g., 
parts of the auditory ossicle chain, of the auditory walls or even of the 
jaw. 
Prosthetic parts must be adapted to the given individual dimensions and 
conditions. This is difficult to be done with ceramic prostheses due to 
their complicated workability. Consequently, dimensional adjustability is 
a chief requirement in the design of ceramic protheses, whereas given 
natural anatomic conditions can take into account only to a minor extent. 
A single-piece middle ear prosthesis designed according to this concept is 
described, e.g., in German Patent Specification 2,905,183. 
Heterologous bone grafts, which are equally employed, are not always 
available and have proven to be problematic in view of possible HIV 
infections. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide bone structures, 
specifically bone replacement parts, made of body-tolerated material which 
are readily manufactured extracorporeally to the shape and size of the 
bone part to be replaced or fitted from prefabricated parts. 
This object is met by a bone replacement part, which is made of a 
non-foamed glass ionomer cement. 
The glass ionomer cement may specifically include the following 
constituents: 
(a) a calcium and/or strontium aluminium fluorosilicate glass powder 
containing 20 to 60 wt-% SiO.sub.2, to 50 wt-% Al.sub.2 O.sub.3,0 to 40 
wt-% CaO, 0 to 40 wt-% F, 0 to 10 wt-% Na.sub.2 O and 0 to 10 wt-% P.sub.2 
O.sub.5, with a minimum of 1 wt-% CaO and/or SrO, 
(b) a polycarboxylic acid having an average molecular weight of 1.000 to 
20.000, in a concentration of 5 to 50 wt-% related to constituent (a), 
(c) water, and optionally 
(d) tartaric acid as a chelate-former. 
In a preferred embodiment, constituent (a) contains 25 to 50 wt-% 
SiO.sub.2, 10 to 40 wt-% Al.sub.2 O.sub.3, 0 to 35 wt-% CaO, 0 to 35 wt-% 
SrO, 5 to 30 wt-% F, 0 to 8 wt-% Na.sub.2 O and 1 to 10 wt-% P.sub.2 
O.sub.5, with a minimum of 10 wt-% CaO and/or SrO. Specifically preferred 
are contents of 25 to 45 wt-% SiO.sub.2, 20 to 40 wt-% Al.sub.2 O.sub.3, 
10 to 30 wt-% CaO, 10 to 30 wt-% F, 1 to 8 wt-% Na.sub.2 O and 1 to 10 
wt-% P.sub.2 O.sub.5. 
The bone replacement structures according to the invention consist of a 
compact, i.e. non-foamed, glass ionomer cement and are formed outside the 
body, which is in contrast to the conventional dental use of glass ionomer 
cements. It is thus possible to shape the bone replacement part 
intra-operatively from freshly mixed glass ionomer cement, let it cure 
outside the body and subsequently implant the cured part. According to an 
method, industrially prefabricated formed bodies having a shape 
approximating that of the bone structure to be replaced in an idealizing 
manner may also be adapted to the anatomic conditions. In contrast to 
ceramic material, no problems arise in finishing glass ionomer cement 
formed bodies by means of the usual cutting methods. 
The replacement parts according to the invention may be readily obtained by 
plastically deforming a mixed cement mass which will cure within a few 
minutes to form a rigid part and may then be machined mechanically by 
means of usual grinding or milling instruments. Moreover, the cured part 
will chemically combine with the freshly mixed and still plastic cement so 
that replacement parts in accordance with the invention may easily be 
secured in situ. It as an advantage that glass ionomer cements combine to 
form a chemical bond with the body's hard tissues such as bones. 
Moreover, the replacement parts according to the invention are very 
bio-compatible or bio-active, i.e. they are not enclosed by connective 
tissue. Instead, new bone growth is facilitated in direct bone contact due 
to the presence of a replacement part according to the invention. 
The easy formability and workability permits individual shaping so that the 
bone replacement structures are capable of reproducing the respective 
natural bone in an idealized shape. 
The term "formed body" used in this specification will be understood to 
include also granulates which are implanted to fill a bone defect.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The bio-mimetic bone structures formed extra-corporeally from non-foamed 
glass ionomer cement in accordance with the invention are suited for the 
following applications: 
(1) Ear 
Outer ear 
auricular frame replacement 
Middle ear 
idealized incus 
idealized malleus 
idealized stapes 
TORP (Total Ossicular Replacement Prosthesis) 
PORP (Partial Ossicular Replacement Prosthesis) 
crescent-shaped structure for reconstructing the tympanic frame 
partial or total replacement of the posterior auditory conduit wall 
utilization in mastoid obliterations (closure of the temporal bone). 
(2) Lateral base of the cranium 
Covering a defect of the middle and posterior cranial fossa. 
(3) Cranium 
Replacement in case of calotte defects. 
(4) Frontobase 
Reconstruction of bony frontobase defects inclusive of the posterior wall 
of the frontal sinus and dural lesions. 
(5) Replacement of cranial bones especially in 
cranium base defects and 
cranium dome defects 
replacement of facial bone defects in the middle face, e.g. of the bony 
nose frame, frontal bone, frontal sinus wall, nasal septum, orita base, 
orita dome, and front wall of the maxillary sinus 
general replacement of bone substance and stabilization of middle face 
bridges, with a possible combination with conventionally utilized plates. 
(6) Larynx 
Implants for stabilizing and replacing the trachea and the larynx 
(7) Jaw surgery 
alveolar appendix 
hard palate 
replacement of jaw parts, particularly in the lower jaw 
replacement of bone defects, for stabilization and osteosynthesis in LeFort 
fractures 
as facial bone pads in plastic surgery 
Glass ionomer cements substantially consist of the following constituents: 
(a) a glass or metal oxide which forms, by acid decomposition, metal ions 
causing cross-linkage of (b), 
(b) a polymer poly acid, with the acid functions being sulphonic, 
phosphonic or carboxylic acids, 
(c) water, and optionally 
(d) a chelate-former. 
In addition, stabilizers, disinfectants, pigments, X-ray contrast media and 
other fillers may be contained. 
The glass ionomer cements are available as mixtures of glass and a polymer 
poly acid, on the one hand, and water, on the other hand, with the 
chelate-former being optionally admixed to one of the two constituents. It 
is equally possible to dissolve the polymer poly acid in water, admix the 
optional chelate-former and mix this solution with the glass. 
In addition to glass powders containing calcium, magnesium or lanthanum as 
specified in German Offenlegungsschriften 2,061,513 and 3,248,357, and 
glass powders containing strontium according to Published European Patent 
Application 0,241,277, glass powders comprising other cations may be 
employed. Calcium- and/or strontium-fluorosilicate glasses are prefered so 
that the aluminium fluorosilicate glass powders may comprise the following 
constituents in addition to oxygen: 
______________________________________ 
constituent calculated as 
weight percent 
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Si SiO.sub.2 20 to 60 
Al Al.sub.2 O.sub.3 
10 to 50 
Ca CaO 0 to 40 
Sr SrO 0 to 40 
F F 1 to 40 
Na Na.sub.2 O 0 to 10 
P P.sub.2 O.sub.5 
0 to 10 
______________________________________ 
At least 1 wt-% CaO and/or SrO must be contained. Further, a total of 0 to 
20 wt-%, calculated as oxides, of B, Bi, Zn, Mg, Sn, Ti, Zr, La or other 
trivalent lanthanides, K, W, Ge as well as other additives may be 
contained which do not impair the properties and are physiologically 
harmless. The glasses may be made visible in X-rays by addition of 10 to 
20 wt-% of La.sub.2 O.sub.3. 
The powder particles preferably consist of 
______________________________________ 
Si as SiO.sub.2 
25 to 50 wt % 
Al as Al.sub.2 O.sub.3 
10 to 40 wt % 
Ca as CaO 0 to 35 wt % 
Sr as SrO 0 to 35 wt % 
F 5 to 30 wt % 
Na as Na.sub.2 O 
0 to 8 wt % 
P as P.sub.2 O.sub.5 
1 to 10 wt % 
______________________________________ 
At least 10 wt-% Ca (calculated as CaO) and/or Sr (calculated as SrO) must 
be contained. Further, 0 to 10 wt-% of B.sub.2 O.sub.3, Bi.sub.2 O.sub.3, 
ZnO, MgO, SnO.sub.2, TiO.sub.2, ZrO, La.sub.2 O.sub.3 or other oxides of 
trivalent lanthanides, K.sub.2 O, WO.sub.3, GeO.sub.2 as well as other 
additives are possible which do not impair the properties and are 
physiologically harmless. 
Particularly preferred powders contain: 
______________________________________ 
Si as SiO.sub.2 25 to 45 wt % 
Al as Al.sub.2 O.sub.3 
20 to 40 wt % 
Ca as CaO 10 to 30 wt % 
F 10 to 30 wt % 
Na as Na.sub.2 O 1 to 8 wt % 
P as P.sub.2 O.sub.5 
1 to 10 wt % 
______________________________________ 
Examples of the preferred compositions are listed in the following TABLE: 
TABLE 
______________________________________ 
(wt %) 
A B C D 
______________________________________ 
Si as SiO.sub.2 
35.0 27.6 29.0 45.4 
Al as Al.sub.2 O.sub.3 
30.4 26.0 25.1 35.0 
Ca as CaO 14.9 28.8 24.6 10.1 
F 17.7 17.0 23.0 10.3 
Na as Na.sub.2 O 
2.7 2.1 2.2 6.9 
P as P.sub.2 O.sub.5 
6.9 8.3 5.8 2.4 
______________________________________ 
The glass powder particles utilized in accordance with the invention may be 
calcium or strontium depleted at their surfaces, as described for calcium 
in European Patent Application 0,023,013. 
The glass powders employed in accordance with the invention have an average 
grain size (weight average) of at least 1 .mu.m, preferably 3 .mu.m at 
least. The average grain size (weight average) is 1 to 20 .mu.m, 
preferably 3 to 15 .mu.m, especially preferably 3 to 10 .mu.m. The 
particles have a maximum grain size of 150 .mu.m, preferably 100 .mu.m, 
especially preferably 60 .mu.m. A not too narrow grain size distribution 
is favourable for attaining good mechanical properties, the distribution 
being obtained by milling and removing the coarse parts by screening. 
The polymer poly acids used as constituent (b) may be polycarboxylic acids, 
e.g. polymaleic acid, polyacrylic acid, polyitaconic acid as well as 
mixtures thereof or copolymers, particularly the maleic-itaconic acid 
copolymers and/or the acrylic-itaconic acid copolymers known from European 
Patent application 0,024,056 as known in the production of glass ionomer 
cement powders. The average molecular weight of the polycarboxylic acids 
used is more than 500. An average molecular weight is preferably between 
1.000 and 20.000, the range of 3.000 to 10.000 being especially preferred. 
The polycarboxylic acid is preferably employed in a concentration of 5 to 
50 wt-% related to constituent (a). 
Known chelate-forming additives (cf. German Offenlegungsschrift 2,319,715) 
may be used as constituent (d) in the glass ionomer cement in accordance 
with the invention. Tartaric acid is preferably employed as a 
chelate-former. 
EXAMPLE 1 
250 parts by weight of a calcium aluminium fluorosilicate glass powder 
having the composition A of the above TABLE are mixed with 100 parts by 
weight of a solution consisting of 37 parts of a copolymer (1:1) of 
acrylic acid and maleic acid, 9 parts tartaric acid and 54 parts water. 
A bone structure for replacing a posterior auditory wall may be formed 
manually from the pasty material thus obtained. The replacement part is 
completely cured after 10 minutes and may be applied in situ with freshly 
mixed and still plastic cement. 
The bone replacement will be incorporated with no problem three weeks after 
the operation and there will be no gaps in the structure. 
EXAMPLE 2 
Formed bodies of 15 mm.times.20 mm.times.5 mm are produced from the 
material mixed according to Example 1 and implanted in the left tibia of a 
baboon. The implant does not differ from the bone material on X-ray 
images. Two weeks later, marked bone formation activity at the implant 
edge appears, and after another four weeks, the implant has been 
completely surrounded by newly formed bone material and the spot does no 
longer differ from the surrounding bone material as. 
EXAMPLE 3 
An idealized auditory ossicle (incus) having a rounded shape as shown in 
the attached drawing is produced from the mixed cement of Example 1. 
Rounded shapes are more bio-compatible than the sharp-edged bodies such as 
formed of ceramics. The danger of perforating the tympanum is minimized; 
epithelial cells preferably grow over round shapes. 
EXAMPLE 4 
The formed body according to Example 2 is worked by means of milling 
instruments common in ENT-surgery, and an idealized incus is formed 
thereof (cf. the drawing). It may easily be worked without causing cracks, 
chippings or fractures in the formed body.