A method is described for adhering bone to the surface of a rigid substrate such as a metal or resin prosthesis using an improved surgical bone cement. The bone cement has mechanical properties more nearly matched to those of animal bone and thermal curing characteristics which result in less traumatization of body tissues and comprises a dispersion of short high modulus graphite fibers within a bonder composition including polymer dissolved in reactive monomer such as polymethylmethacrylate dissolved in methylmethacrylate monomer

Knoell, A. C.

Maxwell, H. G.

English

NASA Technical Reports Server

United States Patent [19] 
Fletcher et al. 
[54] METHOD OF ADHERING BONE TO A 
RIGID SUBSTRATE USING A GRAPHJTE 
FIBER REINFORCED BONE CEMENT 
[76] Inventors: James C. Fletcher, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of; Albert C. Knoell, La 
Crescenta, Hugh G. Maxwell, La 
Canada, both of Calif. 
[21] Appl. No.: 674,194 
[22] Filed: Apr. 6, 1976 
[51] Int. Cl.2 ................................................ A61F 1/24 
[52] U.S. Cl. ...................................... 3/1.9; 128/92 e; 
128/92 G 260/42.17 
[58] Field of Search .................................. 3A.9-1.913; 
128/92 C, 92 CA, 92 R, 92 G, 26W42.17; 
32/15, 10A 
~561 References Cited 
U.S. PATENT DOCUMENTS 
3,848,273 11/1974 Frey ...................................... 3A.913 
OTHER PUBLICATIONS 
“Medical & Dental Applications of Cements” by D. C. 
V 
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cy 
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60 
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11 11 4,064,566 
[45] Dee. 27, 1977 
Smith, Journal of Biomedical Materials Research Sym- 
posium, vol. 1, 1971, pp. 189-205. 
“The Selection of Implant Materials”, Chapter 6, by D. 
F. Williams, Implants in Surgery, (book), by Williams & 
Roaf, W. B. Saunders Co. Ltd., (publisher) July 1973, 
“Quartz and Graphite Filament Reinforced Polymer 
Composites for Orthopedic Surgical Application” by S. 
Musikant, Journal of Biomedical Materials Research 
Symposium, vol. 1, 1971, pp. 225-235. 
Primary Examiner-Ronald L. Frinks 
Attorney, Agent, or Firm-Monte F. Mott; Wilfred 
Grifka; John R. Manning 
pp. 323-326. 
[571 ABsTRAa 
A method of adhering bone to the surface of a rigid 
substrate such as a metal or resin prosthesis using an 
improved surgical bone cement. The bone cement has 
mechanical properties more nearly matched to those of 
animal bone and thermal curing characteristics which 
result in less traumatization of body tissues and com- 
prises a dispersion of short high modulus graphite fibers 
within a bonder composition including polymer dis- 
solved in reactive monomer such as polymethylmeth- 
acrylate dissolved in methylmethacrylate monomer. 
7 Claims, 1 Drawing Figure 
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https://ntrs.nasa.gov/search.jsp?R=19780009272 2020-03-20T12:47:26+00:00Z
U.S. Patent Dec. 27, 1977 4,064,566 
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20 40 60 
TIME, mln 
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4,064,5 66 
1 
METHOD OF ADHERING BONE TO A RIGID 
SUBSTRATE USING A GRAPHITE FIBER 
REINFORCED BONE CEMENT 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
jected to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 83-568 10 
(72 Stat. 435; 42 USC 2457). 
5 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to improved surgical 15 
bone cement compositions and, more particularly, to a 
graphite fiber reinforced bone cement of the acrylic 
type. 
2. Description of the Prior Art 
The bio-compatible surgical bone cements previously 20 
available for use in orthopedic surgery have several 
drawbacks. The strength of these materials is marginal 
and curing produces a relatively high exothermic tem- 
perature which has a traumatizing effect on body tis- 
sues. Fiber reinforced resin composites have previously 25 
been used as components of skeletal prosthetics. How- 
ever, such materials are not suitable as bone cements for 
directly adhering bone to bone or for adhering metal or 
resin surfaces of the prosthesis to the existing bone. 
fairly stiff bone material and the opposing bone surface 
or the fairly rigid exterior of the prosthesis. The pres- 
ently available- bone cements cure to form a fairly soft, 
pliable material which does not maintain the prosthesis 
firmly in place and is subject to separation at either 35 
interface due to mechanical stress. 
The bone cement is present at the interface between 30 
SUMMARY OF THE INVENTION 
An improved surgical bone cement is provided in 
accordance with the invention which has mechanical 
properties more nearly matched to those of bone and 
thermal curing characteristics which result in less trau- 
matization of body tissues during curing. The surgical 
bone cement of this invention has improved strength 
and stiffness, a reduced thermal expansion coefficient 
and sustains a lower exothermic temperature reaction 
during curing which is found to be about half of that 
experienced with the prior materials. 
The bone cement of the invention cures to a high 
strength stiff material providing a stiff interface more 
nearly matched to that of bone and offers the potential 
for good adherence to the bone and prosthesis thus 
providing for improved performance of the total im- 
plant system. 
The bone cement composition of the invention com- 
prises a dispersion of from 2 to 12% by weight of short 
fine high modulus graphite fibers within a solution of 
biocompatible polymer dissolved in reactive monomer. 
The polymer content of the composition is preferably 
from 50 to 70% by weight and the monomer from 20 to 
40% by weight. The composition may also contain 
cross-linking agents, free radical catalysts and activa- 
tors therefor and other additives such as plasticizers, 
chain transfer agents and inhibitors. 
The composition is prepared by first forming a slurry 
of the solid polymer in the liquid monomer and then 
adding the graphite fibers and the curing agents to the 
mixture and intimately dispersing them therein. The pot 
40 
45 
50 
55 
60 
65 
life or cure time of the composition will depend on 
curing agents and polymeric and monomeric ingredi- 
ents. The viscous, pasty cement is then applied to the 
bone and/or prosthesis interfaces during surgical use 
and allowed to cure. After cure, a firm, adherant inter- 
facial bond will be formed with the bone and prosthesis 
and the cement will be a stiff flexural material capable 
of withstanding stresses encountered in the skeletal 
system. The addition of fiber to the monomer polymer 
system serves to preserve material isotropy during clini- 
cal applications. 
These and many other attendant advantages of the 
invention will become apparent as the invention be- 
comes better understood by reference to the following 
detailed description when considered in conjunction 
with the accompanying drawing. 
BRIEF DESCRIPTION OF THE DRAWING 
The FIGURE is a set of graphical exothermic cure 
data for unfilled bone cements and graphite filled bone 
cements in accordance with the invention. 
DESCRIPTION OF THE PREFERRED 
EMBODIMENTS 
The graphite fibers utilized in the invention are high 
modulus materials prepared by converting an organic 
precursor into a high strength, high modulus fiber. The 
precursor is converted by a first stage conversion to a 
carbon fiber during which the material is heated usually 
in the presence of oxygen and usually under tension. 
Under applications of heat typically up to a temperature 
of about 450" F, the polymer cyclizes, that is forms a 
6-member hexagon ring similar to that found in graph- 
ite. Heating in an oxygen-containing atmosphere allows 
oxygen to diffuse into the structure of the fiber which 
forms cross-links or chemical bonds between the poly- 
mer chains. The fiber is then subjected to firing and 
graphitization at a temperature above 2000" C up to 
about 3000" C in an inert atmosphere. Typical graphite 
fibers have tensile strengths exceeding about 150 psi X 
103, a tensile modulus of about 25-30 psi X 106 and a 
density of about 2. 
Organic precursors are generally resinous fibers such 
as acrylic polymers, polyvinyl alcohol, regenerated 
cellulose, pitch materials including petroleum resideus, 
asphalt and coal tars. Highly oriented, synthetic poly- 
mer precursors such as acrylic polymers and regener- 
ated cellulose provide higher modulus products. How- 
ever, such ultimate high strength properties are not 
necessary in the invention. Lower modulus products 
having the desired inert clinical behavior and excellent 
coxpatability function satisfactorily in the bone cement 
composition of the invention. 
The form of the graphite fiber is important for proper 
functioning of the bone cement adhesive of the inven- 
tion. Very fine filaments having a diameter below about 
50 microns, typically 5 to 30 microns are very feathery. 
If the filaments adhere to each other, the material is 
feathery, difficult to handle, difficult to uniformly dis- 
perse throughout the composition and does not provide 
cured materials having the desired physical properties. 
Furthermore, the average length of the fibers should be 
between about 0.1 to 15 mm, preferably from about 4 to 
10 mm in average length. When the average length is 
less than about 0.1 mm the reinforcing effect is not 
sufficient and when the length is more than 15 mm the 
viscosity of the composition in the cement stage is too 
low to be easily workable, and the long fibers tend to 
3 
4,064,566 
protrude from the surface causing poorer adhesion at 
the interfaces and creating potential sides for irritation, 
inflamation and unwanted tissue growth. In addition, 
the fibers tend to become preferentially oriented 
thereby destroying the isotropy of the composite mate- 5 
rial. The fibers as received from the manufacturer are 
usually continuous monofilaments which are chopped 
or cut into desired length such as 6 mm length fibers 
from Celanese GY-70 graphite material (individual, 
The polymer constituent of the composition can be 
any methylmethacrylate polymer such as methylmeth- 
acrylate homopolymers and copolymers of methyl- 
methacrylate with alphaethylenically unsaturated com- 
pounds such as vinyl acetate, alkyl acrylates, alkyl 15 
methacrylates and multi-functional acrylic monomers 
such as alkylene dimethacrylate and alkylene diacry- 
lates and triacrylates. These polymers generally have a 
molecular weight between 500,000 and 2,000,000. 
vating action at room temperature for most organic 
polymers. The polymer in monomer composition can be 
derived from a polymerization process containing ex- 
cess methylmethacrylate monomer and may contain 
residual free radical agents such as 0.1-0.5% by weight 25 
of benzoyl peroxide and may also include a cross-link- 
ing agent such as 1 to 5% of ethylene dimethacrylate. 
The polymer in monomer syrup may be prepared by 
the methods described in U.S. Pat. No. 3,154,600 by 
heating a small amount of polymerization initiator in 30 
solution in the methacrylic ester and in the presence of 
a chain transfer agent having the suitable pressure and 
temperature. Heating is continued until the solution 
reaches a predetermined viscosity. Thereafter the hot 
solution is quenched by the addition of cold monomer 35 
containing a polymerization inhibitor. 
Curing of the bone cement composition of the inven- 
tion can be accomplished by any suitable initiator sys- 
tem such.as from about 0.1 to about 3% by weight, 
vreferablv about 0.6% of a conventional free radical 40 
non-adhering filaments, 8 micron diameter). 10 
Pure liquid methylmethacrylate has a very high sol- 20 
ktiator. The initiator can be a peroxy compound of an 
azo compound. For purposes of biocompatability ben- 
zoyl peroxide is a very suitable free radical initiator. 
The curing temperature can be reduced to room tem- 
perature, e.g., about 25" to 30" C by inclusion in the 
formulation of an activator for the peroxide catalyst 
which causes more rapid decomposition of the peroxide 
to form free radicals. Suitable peroxide catalysts are 
benzoyl peroxide, 2,4dichlorobenzoyl peroxide and 
Cchlorobenzoyl peroxide. Activators or accelerators 
for these catalysts are N,N-dialkyl anilines or N,N-dial- 
kyl toluidines generally employed in amounts ranging 
from about 0.1 to 1% based on the weight of monomer 
present. A preferred activator is N,N-di(2-hydroxye- 
thy1)-p-toluidine. In order to provide longer shelf life 
for the compositions of the invention, the composition 
may be stored in a closed container at cold temperature. 
Bone cements containing both activator and peroxide 
should be made as two-part compositions in which the 
45 
4 
activator and monomer and peroxide and polymer com- 
ponent are packaged in separate containers. 
The suitability of the graphite fiber reinforced surgi- 
cal bone cement was investigated in the following ex- 
periments. The bone cement compositions were based 
on polymethacrylate (PPM) polymer dissolved in meth- 
ylmethacrylate (MMA) monomer in which was dis- 
persed celanese GY-70 fibers cut into approximately 6 
millimeter length, 
The polymer was dissolved in liquid monomer and 
grpahite fibers were then added to form a slurry. After 
considerable experimentation with fiber types, process- 
ing variables and an amount of each additive the selec- 
tion of the fiber diameter length and ratio of monomer 
to polymer as discussed above was determined. It was 
found that a mixture of approximately 30% monomer, 
60% polymer and 10% by weight graphite fiber gave 
the best results. This ratio of polymer to monomer was 
used in the following experiments with the ratio of 
graphite fibers being 10% by weight unless otherwise 
indicated. 
Both fiber reinforced and unreinforced beam and 
cylinder specimens were fabricated and tested. The 
unreinforced specimens were geometrically similar to 
their fiber reinforced counterparts and were tested to 
provide baseline data. Several batches of each type of 
specimen were used in the investigation. 
The unreinforced beam specimens were made by 
casting the mixed PMM between parallel aluminum 
plates shimmed to a thickness of approximately 3 mm. 
Individual specimens were machined to the approxi- 
mate dimensions 25 X 100 mm, then measured to the 
nearest 0.01 mm. The specimens were tested as simply 
supported beams in three point bending in accordance 
with ASTM Specification D790. 
The unreinforced cylinder specimens were made by 
mixing and casting the bone cement in 18 mm diameter 
glass tubing approximately 200 mm long. Specimens 
were machined to the approximate dimensions, 13 mm 
diameter by 25 mm long, then measured to the nearest 
0.01 mm. The specimens were tested as compression 
samples in accordance with ASTM Specification D695. 
Flexural fiber reinforced specimens were prepared by 
pressing the material to the required thickness with 
considerable force. The specimens were band sawed 
from the cured material which had several surface ir- 
regularities. 
Compression fiber reinforced specimens were made 
by compressing the material into glass tubes using 
50 tightly fitting rams. Use of more loosely fitting rams 
was unsuccessful because liquid was squeezed out of the 
mix. The resulting specimens were porous and lacks 
uniformity of batch control. 
Static tests were run to determine flexural and com- 
55 pressive mechanical material properties on the beam 
and cylinder specimens, respectively. All specimens 
were tested in an Instron machine at a loading rate of 
1.3 &minute. 
Average test data for the unreinforced baseline speci- 
60 mens are given in Table I. 
TABLE I 
AVERAGE AVG. MODULUS 
BATCH NO.0F DATE UNTIMATE STRENGTH OF ELASTICITY 
NO. SPECIMENS TYPE (N/m2 x 10-4 fN/m2 x 10-4 . .~ .. .. , 
1 6 Compression 85 - 
2 3 Compression 82 - 
3 3 Compression 84 - 
4 3 Flexure 52 2300 
5 3 Flexure 49 2100 
4,064,566 
5 
TABLE I-continued 
AVERAGE AVG. MODULUS 
BATCH NO.OF DATE UNTIMATE STRENGTH OF ELASTICITY 
NO. SPECIMENS TYPE (N/mZx 10-6) ( N / d  x 10-6) 
6 3 Flexure 48 2100 
6 
As can be seen from these data, the specimens were well 
behaved and gave repeatable results. The data for 
graphite reinforced specimens is presented in Table 11. 
On the basis of this data, it appears that use of graph- 
ite fiber additives improves certain of the mechanical 
and thermal properties of surgical bone cement. 
TABLE I1 
AVERAGE AVERAGE 
NO. OF ULT. MODULUS OF 
BATCH SPECI- DATA FILLER RATIO STRENGTH ELASTICITY 
NO. MENS TYPE (% BY WEIGHT) (N/m2 x 10-6) (N/m2 x 10-4 
- 12 8 3 Compression 10 
9 3 Compression 10 23 
1 3 Flexure 1 48 2300 
2 3 Flexure 2 51 2500 
3 2 Flexure 3 48 4100 
4 2 Flexure 10 49 4800 
5 2 Flexure 10 43 4700 
6 3 Flexure 10 51 4400 
7 3 Flexure 10 40 4900 
- 
It is to be realized that only preferred embodiments of 
As indicated by the data of Table 11, considerable the invention have been disclosed and that numerous 
variation and low values of compressive strength were 25 substitutions, alterations and modifications are all per- 
obtained. The flexural modulus and strength values missible without departing from the spirit and =ope of 
were fairly repeatable, as can be seen from the data of the invention as defined in the following claims. 
batches 4 through 7. Batches 1,2 and 3 were run to test 
the effect of variations in fiber content on specimen 
response. 
The exothermic temperature-time profiles of the 
baseline and fiber reinforced materials are shown in 
FIG. 1. The data were collected by means of a thermo- 
couple embedded in a sample of each material approxi- 
mately 13 mm thick by 38 mm in diameter contained in 35 
a plastic dish. 
A summary of the overall average experimental re- 
sults obtained from Tables I, I1 and FIG. 1 is given in 
Table 111. 
TABLE I11 
What is claimed is: 
1. A method of adhering bone to the surface of a rigid 
applying to the bone and surface a surgical bone 
cement composition comprising a dispersion of 2 to 
12% by weight of high modulus graphite fibers 
having a diameter from 1 to 50 microns and a 
length from 0.1 mm to 15 mm within a biocompati- 
ble polymer dissolved in a biocompatible reactive 
monomer; and 
curing the cement to form an adherent flexural bond. 
2. A method according to claim 1 in which the fibers 
3. A method according to claim 1 in which the mono- 
mer is an acrylic monomer, present from 20 to 40% by 
Flexuralstren h 50 47 weight and the polymer is an acrylic polymer soluble in 
the monomer and is present in an amount from 50 to Compressive strength 
(N/m* x 10-6) 
4. A method according to claim 3 in which the mono- Modulus of Elasticity 2200 4600 ( N / d  x 10-6) 
Max. Temperature 104 52 mer is methylmethacrylate and the polymer is poly- 
exotherm C) methylmethacrylate. 
5. A method according to claim 4 in which 10% by 
As can be seen from the data summarized in Table 50 weight of the graphite fibers are present, 30% by 
111, graphite fiber additives and particularly at 10% weight of monomer are present and 60% by weight of 
level resulted in a significant increase in stiffness (modu- polymer are present in the composition. 
lus of elasticity) without compromising the flexural 6. A method according to claim 3 in which the com- 
strength of the material. The compressive strength de- position further includes from 0.1 to 3% of a biocompat- 
creased significantly, however, as a result of poor speci- 55 ible free radical catalyst and 0 to 1% by weight of an 
men quality and the presence of voids. The exotherm activator for the catalyst. 
also decreased significantly which is due primarily to 7. A method according to claim 1 in which the rigid 
the removal of reactants from the filled material and the substrate is a prosthetic device and the bone is living 
inhibiting effect of the graphite fibers on the rate of animal bone. 
reaction. 
30 substrate comprising the steps of: 
4.0 are monofiamentary and non-adhering. 
UNREINFORCED =INFoRcED PARAMETER 
84 17 
(N/m2 x .IO- i? ) 
45 70% by weight. 
60 * * * * *  
65 


Method of adhering bone to a rigid substrate using a graphite fiber reinforced bone cement

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