The mechanical properties of bone cements A. J. C. Lee BSc, PhD, R. S. M. Ling FRCS and S. S. Vangala BTech Department of Engineering Science, University of Exeter, Applied Science Building, North Park Road, Exeter EX4 4QF, UK; ana Princess Elizabeth Orthopaedic Hospital, Wonford Road, Exeter, UK.
J Med Eng Technol Downloaded from informahealthcare.com by Nyu Medical Center on 06/03/15 For personal use only.
The mechanicalproperties of a number of commercially available bone cements have been investigafed.Tests were carriea out on specimens in compression, in bending and in tension. Using the compression test as a standard, the effects of the following variables were studied: the addition of antibiotics, strain rate, environmental temperature, and age. It waA concluded that age, temperature and rate of straining have a marked effect on the strength of the cement, while the addition of small quantities of antibiotics only marginally weakens the cement.
Polymethylmethacrylate bone cement is very widely used in orthopaedic practice for the fixation of implants in bone. Though some of its mechanical properties have been studied extensi~ely,l-~ there are several variables affecting its mechanical behaviour which have received relatively little attention. This paper describes experimental work aimed at clarifying the effects of a number of these variables. All surgeons involved in joint replacement work are concerned about deep infection and its disastrous consequences. Ultra-clean air, body exhaust systems and prophylactic systemic antibiotics have been used separately and in combination in an attempt to minimise infection with varying success.4. A more recently introduced additional precaution is the incorporation of antibiotics within bone cement. Such antibiotics have been shown to leech out into the surrounding tissues in therapeutically significant quantitiese and may help to reduce the danger of infection around implants fixed with bone cement. The effect of the addition of the antibiotics to bone cement on the mechanical properties of the latter is considered in this paper together with the effects of variable strain rates, variable environmental temperature and ageing of the cement. Table 1 gives details of various antibiotic cements used.
Experimental procedure
Table 1. Antibiotic cements Product name
Cement
AKZ
Surgical 0.5g Erythromicin Pre-mixed Simplex RO 0.4g Colistin
S u f i 6 with Nebacetin
Sulfix 6
Palacos R with Palacos R Ref0bacin
Antibiotics*
Form available
0.5g neomycin+ 0.5g bacitracin
Not pre-mixed
0.5 gentamycin
Pre-mixed
*All the above antibiotics are mixed with 40g of polymer powder
down on fine water-lubricated silicone carbide paper tc complete their preparation. They were stored under the same conditions as the compression and tension test specimens.
Compression tests Tests at strain rates of up to 0.1 s-l were carried out on a Mayes Universal Electronic Testing Machine. The crosshead movement of the machine was controlled to give strain rates of 0.003 s-l, 0.012 s-l, 0.031 s-l and 0.093 s-’. Results were obtained in the form of an automatically plotted load deflec. tion curve. Prior to each test, the specimen size was accurately determined by measurement with a micrometer, the ends were then smeared with a very thin layer of colloidal graphite to reduce platen/specimen friction (and hence to help preveni
Sample preparation The way in which a bone cement is mixed can have a significant effect on the mechanical properties of the polymerised product, and data from different laboratories are difficult to compare due to differing preparation procedures. The data presented here were obtained from specimens prepared by the same personnel using the same equipment in the same Table 2. Mixing details for the various cements environment-controlled laboratory throughout. All mixing was carried out according to the manufacturers’ instructions Mixing time Time taken Time inserted (see Table 2) in a polyethylene bowl using a spatula and a (secs) into gloved into moulds mixing rate of approximately 260 cycles/min. The laboratory Cement hands (mins) (mins) was controlled to a temperature of 21°C and a relative humidity of 40 %. Surgical Compression test specimens and tensile test specimens Simplex P; 150 3 4.5 were prepared in the same way. Cylindrical specimens with Surgical a length to diameter ratio of 3 to 1 (length 27 mm, diameter Simplex RO 9mm) were prepared by “thumbing in” cement to poly- AKZ tetrafluoroethylene (PTFE) moulds, and pressurising to Palacos R; 2 atmospheres for 15 seconds. Immediately after polymerisa- Palacos R+ 40 2 2.5 tion, the specimens were removed from the moulds and Refobacin machined to size. They were then stored at 37°C in isotonic saline solution until required for testing. All tests were Sulfix 6 ; Sulfix 6 + 60 4 5.5 carried out at a temperature of 21°C unless otherwise stated. Bending test specimens were prepared by laying cement Nebacetin into grooves cut in a metal bottom plate. A flat top plate was clamped down onto the bottom plate pressurising the cement CMW with 60 2 2.5 and forming the specimens. The beam size was 4mmx Barium sulphate 10 mm x 75 mm, and the specimens only needed to be rubbed May, 1977
137
J Med Eng Technol Downloaded from informahealthcare.com by Nyu Medical Center on 06/03/15 For personal use only.
Fig. 1 . Photograph of high Apeed test rig.
Fig. 2. Photograph of across diameter tensile test.
Jarrelling) and the test completed. From the curve produced :he ultimate compressive stress and the 0.1% proof stress were calculated. The Young's modulus value was obtained i o m a tangent to the load deflection curve at the origin where the cement closely approximates to linear elastic Jehaviour. Results at higher strain rates were obtained using a servo:ontrolled hydraulic cylinder to apply the load, and a load :ell coupled to an oscilloscope or ultraviolet recorder to .cord the load. A deflection transducer was built onto the iydraulic cylinder to enable load deflection curves to be lbtained (Figure 1).
weight hanger fixed to a knife edge, which rested on tht mid span point of the beam, and a further set of tests wa! carried out using the hydraulic cylinder to apply the load The hand tests gave a deflection rate of the applied loac of approximately 0.007 mm s-l (i.e. quasi-static); the hy draulic test equipment applied a central deflection to tht beam of 8 mm s-l. It is difficult to define strain rate undei these circumstances, but the first set can be regarded as ver) low strain rate, the second as high strain rate. Failure of tht beam always occurred on the tensile side and may be regardec as giving the tensile failure stress for the material. A loac deflection curve was obtained for each test from which thc ultimate tensile stress (u) and Young's modulus (E) wert calculated using the equations 3PI
rension tests rension tests were carried out on specimens of the same :ylindrical shape and size as the compression tests. The ipecimens were subjected to diametral compression which gives rise to a transverse tensile failure (Figure 2). The tensile ailure occurs in a fairly brittle manner and the only result )btainable was the ultimate tensile stress. Once again, the Mayes machine was used for the low strain rate tests and the iydraulic machine for the high strain rate tests. 3ending tests 4 beam-shaped specimen was used for the bending tests. The beam was simply supported at each end and the load ipplied in the centre of the span. A set of tests was carried )ut by hand, loading the beam by placing weights onto a ULTlnRlE
LDnPRE5WV€
(T=-
2bd2 where P is the load at failure, 1the span of beam, b the breadtk of beam and d the depth of beam k13 E=4bd3 where k is the slope of the load-deflection curve in the elastic region.
Results and discussion The results of these tests of mechanical properties are sum. marised here in Figures 3 to 10. Where the results are drawn as
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i :ig. 3. Compression test: ultimate compressive stress in 7-day-old pecimens at a strain rate of 0.003 s-I.
138
Fig. 4 . Compression test: 0.1 % proof stress in 7-day-old specimens at a strain rate of 0.003 s-l.
Journal of Medical Engineering and Technology
J Med Eng Technol Downloaded from informahealthcare.com by Nyu Medical Center on 06/03/15 For personal use only.
L
2
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3
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Po
b *
b
5WFn
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Fig. 5 . Compression test: Young's modiilits of elasticity in 7-cia.v-old specimens at a strain rate of 0.003s-'.
Fig. 6 . Bending test: ultimate tensile stress in 2-day-old specimens. Strain: platen movement=0.007 mrn s-'.
histograms, the standard deviations of the means are plotted as vertical bars.
stated that the largest value is best, the optimum elasticit) for bone cement has yet to be determined. The stiffest cemeni is not necessarily the best. Work is continuing to determinr the optimum properties for various applications.
Comparative results The compression test is the easiest to control accurately, and has been used as a basis for comparing strengths of the various cements. For the sake of comparison, results obtained kom specimens that were 7 days old and tested at the low strain rate 0.003-1 are used. The general trends shown are dalid for samples of varying ages tested over the full strain -ate range. Figure 3 shows that all cements tested have a fairly similar iltimate compressive stress. Of the antibiotic cements AKZ ind Sulfix 6 with Nebacetin are rather better than Palacos R with Refobacin. Similarly, it can be seen from Figure 4 that all cements have L similar 0.1 % proof stress. As with the ultimate compressive :tress there is no significant difference between the opaque iimplex RO and AKZ; AKZ being formed by adding antiJiotics to Simplex RO. A 10% variation between the highest and lowest values of (oung's modulus is recorded in Figure 5 . Unlike the ultimate :ompressive stress and 0.1 % proof stress where it can be RENPINL.
RLTL
%JNC&
Three point bending tests The results indicate that Sulphix 6+nebacetin has the highesl bending strength of the antibiotic cements tested (Figures C and 7). AKZ had a bending strength about 12% below thal of Simplex RO. The bending strength and Young's modulur of Palacos R with Refobacin was significantly below that of the other cements. The effect of strain rate All polymethylmethacrylate bone cements are viscoelastic materials, and exhibit a progressive increase in stiffness, tensile and compressive strengths with an increase in strain rate. It is likely that strain rates of up to 1 s-l will be common in vivo. Vigorous activity may lead to higher strain rates and it is therefore important to know how a cement behaves at these higher strain rates. AKZ cement has been studied as a typical antibiotic cement and exhibits a 50% increase in strength at a strain rate of 1.8 s-l (Figure 8). At "impact"
UODUWS
4
ig. 7. Bending test : Young's modulris af elasticity in 2-day-old iecimens. Strain: platen movement=0.007 mm s-'. y , 1977
.,z..eo.
...
Y.
Fig. 8. Efect of strain rate on material properties of A K Z cement.
139
I
J Med Eng Technol Downloaded from informahealthcare.com by Nyu Medical Center on 06/03/15 For personal use only.
1
Fig. 9. Effect of temperature on mechanical properties. Average values are plotted fiom Simplex RO and A KZ tested at a strain race of 0.093 s-'.
rates of 5.2 s-l, a 67 % increase in strength was observed. Young's modulus increased in a similar fashion to the ultimate compressive stress. The effect of the increases to Young's modulus and ultimate compressive stress gave a large increase in 0.1 % proof stress for small increases in strain rate. The effect of temperature All testing detailed above was carried out in an environment at 21°C. Since bone cement has to function in vivo at 37"C, tests were made to determine the temperature dependancy of the material. The results are shown in Figure 9. It can be seen that there is a significant change in material properties For both ordinary opaque cement and antibiotic cement. The decrease in material properties was similar for both types and amounted to about 4 % on Young's modulus, 10 % on ultimate Eompressive stress and 16 % on 0.1 % proof stress. The effect of ageing The effect of time on the mechanical properties of bone Eements is illustrated by the histogram of Figure 10, which looks at ultimate compressive stress as a typical parameter For the cements. All cements tended to gain strength between mixing and seven days, but when specimens were tested six months and one year after mixing it was found that a fall off in strength had occurred, amounting to about 7 % at six months and about 8%% at one year. At this point the rate of rall off in material properties is slowing and is not expected to produce more than a 10% fall off in the long term. These specimens were all stored in isotonic saline at 37°C in the laboratory; a small set of Simplex P specimens recovered from % patient 7% years after implantation showed an average iltimate compressive stress of 76 Nmm-2. This is a very 300d result, as the techniques of implantation employed when !he original operation was carried out were such as to produce ;pecimens that were likely to be weaker than our current laboratory specimens. Further specimens remain in storage 'or long-term tests.
Conclusions The admixture of antibiotics to conventional bone cements ias a marginal effect on the strength of the cements, 3mounting to about 4 % for ultimate compressive stress.
I
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5
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b
2
Slurur I(o
b
5uLPin
L + N&bnc&mw
3
aKZ
1
PnLacos R
P
4 C M W + brniun SUUIIR s h L n c o r e + fkmnacr*. Fig. 10. Eflect of ageing on ultimate compressive stress at a strain rate of 0.003 s-I.
Increasing the strain rate increases the mechanical properties of the cements. i.e. the cements are stiffer and fail at higher loads. A 50% increase in strength at in vivo strain rates as compared with conventional laboratory quasi-static strain rates may be obtained. Cements working at the in viva temperature of 37°C will have strengths about 10% less than the strength obtaining at room temperature, while a fall off in properties amounting to about 10% can be expected in the long-term, i.e. over a period of years.
AUTHOR'SNOTE Complete tables of results of all mechanical property tests carried out are available from the author (AJCL). ACKNOWLEDGEMENT
Grateful thanks are given to Howmedica (UK) Ltd., who obtained and provided all the cements used in this investigation. REFERENCES 1. Lautenschlager, E. P., Marshall, H. W., Marks, K. E., Schwartz J. and Nelson, C. L. (1976) Mechanical strength of acrylic bone cements impregnated with antibiotics Journal of Biomedical Materials Research, 10,837. 2. Lee, A. J. C., Ling,R. S. M. and Wrighton, J. D. (1973) The preparation of polymethylmethacrylate for use in orthopaedic surgery. Clinical Orthopaedics and Related Research, 95 28 1-287. 3. Wilde, A. H. and Greenwald, A. S. (1975) Shear strength o
self curing acrylic cement. Clinical Orthopaedics and Relate( Research, 106, 126. 4. Charnley, J. and Eftekhar, N. (1969) Postoperative infection ii total prosthetic replacement arthroplasty of the hip joini British Journal of Surgery, 56, 9,642-649. 5. Cook, R. and Boyd, N . A. (1971) Reduction of the microbia contamination of surgical wound areas by sterile laminar ail flow. British Journal of Surgery, 58, 1, 48-52. 6. Buchholz, H. W. and Engelbrecht, E. (1970) On the sustainec
release of some antibiotics when mixed with Palacos resir Chirug, 41, 511. Journal of Medical Engineering and Technolc
.O -.
.
-
J Med Eng Technol Downloaded from informahealthcare.com by Nyu Medical Center on 06/03/15 For personal use only.
The mechanicalproperties of a number of commercially available bone cements have been investigafed.Tests were carriea out on specimens in compression, in bending and in tension. Using the compression test as a standard, the effects of the following variables were studied: the addition of antibiotics, strain rate, environmental temperature, and age. It waA concluded that age, temperature and rate of straining have a marked effect on the strength of the cement, while the addition of small quantities of antibiotics only marginally weakens the cement.
Polymethylmethacrylate bone cement is very widely used in orthopaedic practice for the fixation of implants in bone. Though some of its mechanical properties have been studied extensi~ely,l-~ there are several variables affecting its mechanical behaviour which have received relatively little attention. This paper describes experimental work aimed at clarifying the effects of a number of these variables. All surgeons involved in joint replacement work are concerned about deep infection and its disastrous consequences. Ultra-clean air, body exhaust systems and prophylactic systemic antibiotics have been used separately and in combination in an attempt to minimise infection with varying success.4. A more recently introduced additional precaution is the incorporation of antibiotics within bone cement. Such antibiotics have been shown to leech out into the surrounding tissues in therapeutically significant quantitiese and may help to reduce the danger of infection around implants fixed with bone cement. The effect of the addition of the antibiotics to bone cement on the mechanical properties of the latter is considered in this paper together with the effects of variable strain rates, variable environmental temperature and ageing of the cement. Table 1 gives details of various antibiotic cements used.
Experimental procedure
Table 1. Antibiotic cements Product name
Cement
AKZ
Surgical 0.5g Erythromicin Pre-mixed Simplex RO 0.4g Colistin
S u f i 6 with Nebacetin
Sulfix 6
Palacos R with Palacos R Ref0bacin
Antibiotics*
Form available
0.5g neomycin+ 0.5g bacitracin
Not pre-mixed
0.5 gentamycin
Pre-mixed
*All the above antibiotics are mixed with 40g of polymer powder
down on fine water-lubricated silicone carbide paper tc complete their preparation. They were stored under the same conditions as the compression and tension test specimens.
Compression tests Tests at strain rates of up to 0.1 s-l were carried out on a Mayes Universal Electronic Testing Machine. The crosshead movement of the machine was controlled to give strain rates of 0.003 s-l, 0.012 s-l, 0.031 s-l and 0.093 s-’. Results were obtained in the form of an automatically plotted load deflec. tion curve. Prior to each test, the specimen size was accurately determined by measurement with a micrometer, the ends were then smeared with a very thin layer of colloidal graphite to reduce platen/specimen friction (and hence to help preveni
Sample preparation The way in which a bone cement is mixed can have a significant effect on the mechanical properties of the polymerised product, and data from different laboratories are difficult to compare due to differing preparation procedures. The data presented here were obtained from specimens prepared by the same personnel using the same equipment in the same Table 2. Mixing details for the various cements environment-controlled laboratory throughout. All mixing was carried out according to the manufacturers’ instructions Mixing time Time taken Time inserted (see Table 2) in a polyethylene bowl using a spatula and a (secs) into gloved into moulds mixing rate of approximately 260 cycles/min. The laboratory Cement hands (mins) (mins) was controlled to a temperature of 21°C and a relative humidity of 40 %. Surgical Compression test specimens and tensile test specimens Simplex P; 150 3 4.5 were prepared in the same way. Cylindrical specimens with Surgical a length to diameter ratio of 3 to 1 (length 27 mm, diameter Simplex RO 9mm) were prepared by “thumbing in” cement to poly- AKZ tetrafluoroethylene (PTFE) moulds, and pressurising to Palacos R; 2 atmospheres for 15 seconds. Immediately after polymerisa- Palacos R+ 40 2 2.5 tion, the specimens were removed from the moulds and Refobacin machined to size. They were then stored at 37°C in isotonic saline solution until required for testing. All tests were Sulfix 6 ; Sulfix 6 + 60 4 5.5 carried out at a temperature of 21°C unless otherwise stated. Bending test specimens were prepared by laying cement Nebacetin into grooves cut in a metal bottom plate. A flat top plate was clamped down onto the bottom plate pressurising the cement CMW with 60 2 2.5 and forming the specimens. The beam size was 4mmx Barium sulphate 10 mm x 75 mm, and the specimens only needed to be rubbed May, 1977
137
J Med Eng Technol Downloaded from informahealthcare.com by Nyu Medical Center on 06/03/15 For personal use only.
Fig. 1 . Photograph of high Apeed test rig.
Fig. 2. Photograph of across diameter tensile test.
Jarrelling) and the test completed. From the curve produced :he ultimate compressive stress and the 0.1% proof stress were calculated. The Young's modulus value was obtained i o m a tangent to the load deflection curve at the origin where the cement closely approximates to linear elastic Jehaviour. Results at higher strain rates were obtained using a servo:ontrolled hydraulic cylinder to apply the load, and a load :ell coupled to an oscilloscope or ultraviolet recorder to .cord the load. A deflection transducer was built onto the iydraulic cylinder to enable load deflection curves to be lbtained (Figure 1).
weight hanger fixed to a knife edge, which rested on tht mid span point of the beam, and a further set of tests wa! carried out using the hydraulic cylinder to apply the load The hand tests gave a deflection rate of the applied loac of approximately 0.007 mm s-l (i.e. quasi-static); the hy draulic test equipment applied a central deflection to tht beam of 8 mm s-l. It is difficult to define strain rate undei these circumstances, but the first set can be regarded as ver) low strain rate, the second as high strain rate. Failure of tht beam always occurred on the tensile side and may be regardec as giving the tensile failure stress for the material. A loac deflection curve was obtained for each test from which thc ultimate tensile stress (u) and Young's modulus (E) wert calculated using the equations 3PI
rension tests rension tests were carried out on specimens of the same :ylindrical shape and size as the compression tests. The ipecimens were subjected to diametral compression which gives rise to a transverse tensile failure (Figure 2). The tensile ailure occurs in a fairly brittle manner and the only result )btainable was the ultimate tensile stress. Once again, the Mayes machine was used for the low strain rate tests and the iydraulic machine for the high strain rate tests. 3ending tests 4 beam-shaped specimen was used for the bending tests. The beam was simply supported at each end and the load ipplied in the centre of the span. A set of tests was carried )ut by hand, loading the beam by placing weights onto a ULTlnRlE
LDnPRE5WV€
(T=-
2bd2 where P is the load at failure, 1the span of beam, b the breadtk of beam and d the depth of beam k13 E=4bd3 where k is the slope of the load-deflection curve in the elastic region.
Results and discussion The results of these tests of mechanical properties are sum. marised here in Figures 3 to 10. Where the results are drawn as
5TUE55
0.1
I
7 onv
'L
DLP
PROOF
5TREM
~PECAUENS
5Ts01N
r?*7a
*-
Y L
i :ig. 3. Compression test: ultimate compressive stress in 7-day-old pecimens at a strain rate of 0.003 s-I.
138
Fig. 4 . Compression test: 0.1 % proof stress in 7-day-old specimens at a strain rate of 0.003 s-l.
Journal of Medical Engineering and Technology
J Med Eng Technol Downloaded from informahealthcare.com by Nyu Medical Center on 06/03/15 For personal use only.
L
2
5IMftEx
3
RHL
Po
b *
b
5WFn
8
PaLaco5 It PLFJeacrN
Nt6RC€TN
Fig. 5 . Compression test: Young's modiilits of elasticity in 7-cia.v-old specimens at a strain rate of 0.003s-'.
Fig. 6 . Bending test: ultimate tensile stress in 2-day-old specimens. Strain: platen movement=0.007 mrn s-'.
histograms, the standard deviations of the means are plotted as vertical bars.
stated that the largest value is best, the optimum elasticit) for bone cement has yet to be determined. The stiffest cemeni is not necessarily the best. Work is continuing to determinr the optimum properties for various applications.
Comparative results The compression test is the easiest to control accurately, and has been used as a basis for comparing strengths of the various cements. For the sake of comparison, results obtained kom specimens that were 7 days old and tested at the low strain rate 0.003-1 are used. The general trends shown are dalid for samples of varying ages tested over the full strain -ate range. Figure 3 shows that all cements tested have a fairly similar iltimate compressive stress. Of the antibiotic cements AKZ ind Sulfix 6 with Nebacetin are rather better than Palacos R with Refobacin. Similarly, it can be seen from Figure 4 that all cements have L similar 0.1 % proof stress. As with the ultimate compressive :tress there is no significant difference between the opaque iimplex RO and AKZ; AKZ being formed by adding antiJiotics to Simplex RO. A 10% variation between the highest and lowest values of (oung's modulus is recorded in Figure 5 . Unlike the ultimate :ompressive stress and 0.1 % proof stress where it can be RENPINL.
RLTL
%JNC&
Three point bending tests The results indicate that Sulphix 6+nebacetin has the highesl bending strength of the antibiotic cements tested (Figures C and 7). AKZ had a bending strength about 12% below thal of Simplex RO. The bending strength and Young's modulur of Palacos R with Refobacin was significantly below that of the other cements. The effect of strain rate All polymethylmethacrylate bone cements are viscoelastic materials, and exhibit a progressive increase in stiffness, tensile and compressive strengths with an increase in strain rate. It is likely that strain rates of up to 1 s-l will be common in vivo. Vigorous activity may lead to higher strain rates and it is therefore important to know how a cement behaves at these higher strain rates. AKZ cement has been studied as a typical antibiotic cement and exhibits a 50% increase in strength at a strain rate of 1.8 s-l (Figure 8). At "impact"
UODUWS
4
ig. 7. Bending test : Young's modulris af elasticity in 2-day-old iecimens. Strain: platen movement=0.007 mm s-'. y , 1977
.,z..eo.
...
Y.
Fig. 8. Efect of strain rate on material properties of A K Z cement.
139
I
J Med Eng Technol Downloaded from informahealthcare.com by Nyu Medical Center on 06/03/15 For personal use only.
1
Fig. 9. Effect of temperature on mechanical properties. Average values are plotted fiom Simplex RO and A KZ tested at a strain race of 0.093 s-'.
rates of 5.2 s-l, a 67 % increase in strength was observed. Young's modulus increased in a similar fashion to the ultimate compressive stress. The effect of the increases to Young's modulus and ultimate compressive stress gave a large increase in 0.1 % proof stress for small increases in strain rate. The effect of temperature All testing detailed above was carried out in an environment at 21°C. Since bone cement has to function in vivo at 37"C, tests were made to determine the temperature dependancy of the material. The results are shown in Figure 9. It can be seen that there is a significant change in material properties For both ordinary opaque cement and antibiotic cement. The decrease in material properties was similar for both types and amounted to about 4 % on Young's modulus, 10 % on ultimate Eompressive stress and 16 % on 0.1 % proof stress. The effect of ageing The effect of time on the mechanical properties of bone Eements is illustrated by the histogram of Figure 10, which looks at ultimate compressive stress as a typical parameter For the cements. All cements tended to gain strength between mixing and seven days, but when specimens were tested six months and one year after mixing it was found that a fall off in strength had occurred, amounting to about 7 % at six months and about 8%% at one year. At this point the rate of rall off in material properties is slowing and is not expected to produce more than a 10% fall off in the long term. These specimens were all stored in isotonic saline at 37°C in the laboratory; a small set of Simplex P specimens recovered from % patient 7% years after implantation showed an average iltimate compressive stress of 76 Nmm-2. This is a very 300d result, as the techniques of implantation employed when !he original operation was carried out were such as to produce ;pecimens that were likely to be weaker than our current laboratory specimens. Further specimens remain in storage 'or long-term tests.
Conclusions The admixture of antibiotics to conventional bone cements ias a marginal effect on the strength of the cements, 3mounting to about 4 % for ultimate compressive stress.
I
5 l M W
5
%FIX
b
2
Slurur I(o
b
5uLPin
L + N&bnc&mw
3
aKZ
1
PnLacos R
P
4 C M W + brniun SUUIIR s h L n c o r e + fkmnacr*. Fig. 10. Eflect of ageing on ultimate compressive stress at a strain rate of 0.003 s-I.
Increasing the strain rate increases the mechanical properties of the cements. i.e. the cements are stiffer and fail at higher loads. A 50% increase in strength at in vivo strain rates as compared with conventional laboratory quasi-static strain rates may be obtained. Cements working at the in viva temperature of 37°C will have strengths about 10% less than the strength obtaining at room temperature, while a fall off in properties amounting to about 10% can be expected in the long-term, i.e. over a period of years.
AUTHOR'SNOTE Complete tables of results of all mechanical property tests carried out are available from the author (AJCL). ACKNOWLEDGEMENT
Grateful thanks are given to Howmedica (UK) Ltd., who obtained and provided all the cements used in this investigation. REFERENCES 1. Lautenschlager, E. P., Marshall, H. W., Marks, K. E., Schwartz J. and Nelson, C. L. (1976) Mechanical strength of acrylic bone cements impregnated with antibiotics Journal of Biomedical Materials Research, 10,837. 2. Lee, A. J. C., Ling,R. S. M. and Wrighton, J. D. (1973) The preparation of polymethylmethacrylate for use in orthopaedic surgery. Clinical Orthopaedics and Related Research, 95 28 1-287. 3. Wilde, A. H. and Greenwald, A. S. (1975) Shear strength o
self curing acrylic cement. Clinical Orthopaedics and Relate( Research, 106, 126. 4. Charnley, J. and Eftekhar, N. (1969) Postoperative infection ii total prosthetic replacement arthroplasty of the hip joini British Journal of Surgery, 56, 9,642-649. 5. Cook, R. and Boyd, N . A. (1971) Reduction of the microbia contamination of surgical wound areas by sterile laminar ail flow. British Journal of Surgery, 58, 1, 48-52. 6. Buchholz, H. W. and Engelbrecht, E. (1970) On the sustainec
release of some antibiotics when mixed with Palacos resir Chirug, 41, 511. Journal of Medical Engineering and Technolc
.O -.
.
-
