Archives of Orthopaedic and Traumatic Surgery

Arch Orthop Traumat Surg 92, 1-18 (1978)

© J F Bergmann Verlag 1978

Some Clinically Relevant Variables Affecting the Mechanical Behaviour of Bone Cement* A J C Lee', R S M Ling2 , and S S Vangala' 'Department of Engineering Science, University of Exeter, Exeter, Devon, England 2 Princess

Elizabeth Orthopaedic Hospital, Wonford Road, Exeter, Devon, England

Summary The effects of 12 clinically relevant variables upon the basic mechanical properties of acrylic cement are reported Attention is drawn to the facts that these variables may at times coexist to lead to serious reductions in the strength of the cement, and that the operating surgeon may exercise a substantial influence on the effective mechanical properties of the cement he is using. Zusammenfassung Es wird fiber die Wirkung von 12 klinisch relevanten Gr 6 13en mit Einflul auf die grundliegenden mechanischen Eigenschaften von Knochenzement berichtet. Die Aufmerksamkeit wird auf die Tatsache gelenkt, da B diese Gro Ben gelegentlich zusammentreffen und damit zu ausgepragter Schwachung des Knochenzementes fiihren sowie die Tatsache, da B der Operateur einen erheblichen Einflu B auf die wirksamen mechanischen Eigenschaften des Zementes, den er benutzt, ausiben kann.

effective mechanical properties of the cement he is using Most of the mechanical testing upon which this paper is based was carried out in the Department of Engineering Science at the University of Exeter, though material from other sources is included and freely acknowledged The results are presented in the form of percentage changes in cement strength The absolute figures and details of the mechanical testing methods are set out in the appendix. The variables to be considered can be divided into three groups: A Variables totally beyond the control of the surgeon. B Variables entirely within the control of the surgeon. C Variables partially within the control of the surgeon.

Acrylic cement has been in extensive use in orthopaedic surgery for well over a decade and the basic mechanical properties of the material are well known. It is the purpose of this paper to give an account of the effects of a number of clinically relevant variables upon the basic mechanical properties of acrylic cement Subsequently attention is drawn to the facts that these variables may at times coexist to lead to serious reductions in the effective mechanical properties of bone cement and that furthermore the operating surgeon may exercise a substantial influence on the

I Environmental Temperature of 37 C before, during and after Polymerisation

* This investigation was supported by: Howmedica International Ltd , North Hill Plastics Division, 622 Western Avenue, Park Royal, London, W3 OTF Offprint requests to: R S M Ling (address see above)

A Variables Totally Beyond the Control of the Surgeon

Conventional laboratory tests (Lee et al , 1973) of the mechanical properties of bone cement have generally been performed at temperatures of 20-21 °C on cement specimens mixed and moulded in environments of similar temperature In vivo, conventional techniques involve mixing the cement at the ambient operating theatre temperature The dough is then taken into the surgeon's hands where the environmental temperature rises above the ambient theatre temperature, and inserted into the relevant boney cavity or surface Thereafter it is exposed to an environmental temperature approaching 37°C prior, during and subsequent to polymerisation Figure 1

0344-8444/78/0092/0001/$ 3 60

A J C Lee et al : Some Clinically Relevant Variables Affecting

2

Average of Simplex RO & AKZ at strain rate of 093 s' rnn U ll .

I

_

I

_

-Youngs Modulus Percentage of Strength at

I I

90

0

_-

-ultimate Lomp.

Stress

I

20 C

.17 Proof Stress

80

Fig 1 Relationship of environmental temperature to mechanical properties of cement

70 -20

35

25 °

Temperature

37

C

Fig 2 Relationship of strain rate to mechanical properties of cement 8 Strain

IStrain

Rate

s'

rate = 003 s)

_90

|

o

_

_

A_

_

_

Ultimate Compressive Stress

x

N/rmm

.\

\

70

tI

2h T

2day

2 hrs

70

S

N

50

10

Log (Time)

I

I 100

hours

M

ac

I

7day

lyS

-· b 00

10 uu

5000

Ia.

10000

4-. 2

I

yrs

Fig 3 Relationship of ultimate compressive strength of cement to time after curing

A J C Lee et al : Some Clinically Relevant Variables Affecting demonstrates that this produces approximately a 10% reduction in the ultimate compressive strength of the subsequently cured resin. II Equilibrium Moisture Content Acrylic cement specimens polymerised in air and then stored in water or saline gradually increase in weight due to the absorption of fluid In the laboratory, cement specimens measuring 9 mm in diameter x 27 mm in length and stored in saline gain weight steadily over a period of 18-21 days at 20 °C (Vangala, 1977) before equilibrium moisture content is established Larger specimens will obviously take longer to equilibrate In vivo, the establishment of equilibrium moisture content is likely to take longer still and with very large specimens may in fact never be achieved Equilibrium moisture content leads on average to a 3% reduction in ultimate compressive strength in comparison with dry specimens. III Strain Rate Bone cement, in common with other polymers, is viscoelastic and thus stiffer and stronger at high strain rates than at low Figure 2 illustrates the relationship between strain rate, Young's modulus and ultimate compressive stress: At impact rates (5 18 s- ') there is a 67 % increase in U C S and a 50 % increase in Young's modulus in comparison with quasi static testing. Strain rates of 1 s- 1 or more are likely in clinical practice at the hip (Unsworth, 1975).

IV

Ageing and Fatigue

a) Ageing Jaffe, Rose and Radin (1974) found no deterioration in the mechanical properties of Simplex and CMW specimens stored in bovine serum at 37°C for up to 2 years Wagner and his colleagues (1972) however found a reduction in tensile strength and Young's modulus together with an increase in compressive strength of specimens stored in blood at 37 ° C for 6 months Figure 3 depicts results obtained in our laboratory and suggests approximately a 10% reduction in U C S over 10 years, though there was always an increase in U C S during the first week after polymerisation Of more importance were the results of tests performed on specimens machined from cement obtained from a total replacement arthroplasty of the hip which had been in situ for 72 years These showed an insignificant reduction in U C S ( 1 3 %) and Young's modulus ( 5 7 %) though in a test specimen containing obvious laminations the U C S was reduced by 29 2 % and Young's modulus by 11%. These specimens are of especial interest because they had not only been exposed to the body environ-

3

ment in vivo but had also been cyclically loaded for 71/2 years with the normal activity of the patient, and are thus relevant to the fatigue behaviour of cement as well as to the effects of ageing Cement polymerising within bone cavities may to a greater or lesser degree be in contact with fat during and after polymerisation. Shadbolt ( 1974) has reported, experimentally, stronger cement when polymerisation and subsequent storage are carried out with the cement specimen surrounded by fat than when polymerisation and storage take place in an environment of saline or water. Monomer is soluble in fat and thus such specimens are likely to lose more monomer to their surroundings than specimens stored in saline or serum Residual monomer acts as a plasticiser and so removing it is likely to produce a stronger cement This may account in part for the findings of Wagner et al and for the maintenance of the strength of the specimens, mentioned above, removed from a patient at 71/2 years It is also a probable explanation for the lower residual monomer reported by Charnley (Charnley, 1970, Page 32) in cement specimens removed after 2 years in the femur of a patient, in comparison with the levelfound in specimens stored under room conditions for 8 years. b) Fatigue Though fatigue is included in this section, it is probably not entirely correct to suggest that it is uninfluenced by surgical performance Fatigue phenomena in acrylic cement have not to date been extensively investigated. Jaffe, Rose and Radin ( 1974) studied a relatively small number of entirely unconstrained specimens in rotating bending tests, leading to the conclusion that the endurance limit for compression fatigue was between 14 and 17 N/mm2 Fatigue failure of cement used in a fully constrained situation in a rig designed for the fatigue testing of femoral prostheses was not encountered by Lee and Ling (1977) even though each test involved between 107 and 108 cycles at loads of 6 k N each cycle Moreover, at the Centre for Hip Surgery, there are now patients with a follow up exceeding 13 years since total hip replacement and as yet, fatigue phenomena in acrylic cement have not been of clinical significance in this group What the future holds with regard to the effects of fatigue in this material remains a matter for conjecture.

B Variables Entirely within the Control of the Surgeon I Radio-opaqueFillers Produce small but statistically insignificant reductions in U C S averaging 5%. II Antibiotics Largely due to the pioneering work of Buchholz (1970) and his colleagues, there is now great interest in the use of cement in which various anti-

4

A J C Lee et al : Some Clinically Relevant Variables Affecting

biotics have been added to the polymer powder The antibiotics, being water soluble, slowly diffuse out of the polymerised cement down a concentration gradient into the surrounding tissue A large amount of work in several centres (Lautenschlager et al , 1976; Lee et al , 1977) has been devoted to the effects of the incorporation of antibiotics on the mechanical properties of cement. It is clear that provided not more than 1 gram of antibiotic is added to 40 grams of polymer, and that mixing is thorough, the average reduction in the U.C S does not exceed 4% Thorough mixing of antibiotic and polymer powders is important to avoid localised areas of weakness As the amount of added antibiotic increases, there is a gradual reduction in the U.C S of cement, reaching 20-25 % with the addition of 5 grams of antibiotic (see appendix) The addition of antibiotic in liquid form has a catastrophic effect and must be avoided (Marks et al , 1976).

a) Delay During and after mixing, the viscosity of the cement dough increases, slowly at first, and then more rapidly as polymerisation approaches In the early stages, the viscosity is such that the cement dough is uncontrollable in the surgeon's hand During this period, there is no tendency for laminations produced by folding to persist As the viscosity increases, so laminations produced by folding are progressively less easy to obliterate and may persist as the source of mechanical weakness after polymerisation (Lee et al , 1973) Cement specimens formed 7/2 minutes after the beginning of mixing show a 43% reduction in U C S. in comparison with cements formed 2/2 minutes after the beginning of mixing (see appendix) Gruen and his colleagues (1976) have demonstrated a 54% reduction in tensile lamination interface strength of two part specimens with a delay of 6/2 minutes. b) Pressure Reduction in lamination and porosity of the cured resin can be achieved by the application of pressure to the cement dough before and during curing. De Wijn and his colleagues ( 1974) demonstrated a 30 % increase in the U C S of the cured resin after the application of two atmospheres pressure before and during curing Wienstein and his colleagues (1976) found a 25 % increase in mechanical properties under the influence of 6 atmospheres of pressure In the author's laboratory, the application of 2 atmospheres pressure to the cement dough for as little as 15 seconds immediately after the dough was inserted into the specimen mould resulted in an 11 % increase in U C S. (see appendix) Bayne et al (1975) pointed out that the application of pressure to the cement during curing exerted a relatively greater effect on the ultimate tensile strength than on the U C S of the cured resin.

III Variations in Mixing Technique Relate first to the frequency of beating during mixing, and second to the duration of mixing Lee et al (1973) demonstrated that the U C S of test specimens made from cement dough beaten at 4 33 Hz was on average 10% less than the U C S of test specimens made from cement dough mixed at 1 Hz Cement specimens prepared from dough placed in specimen moulds after 2/2 minutes mixing show on average an 11 % reduction in U C S in comparison with specimens placed in the moulds after 11/2 minutes mixing (see appendix) The explanation for these differences is not certain During mixing, monomer is lost from the dough by evaporation and it is known that the extent of this loss is proportional to the beating frequency and to its duration (Lee et al , 1973) Thus specimens mixed for longer periods and at higher frequencies will contain less monomer at polymerisation than those in which the mixing period is short and the frequency low If residual monomer acts as a plasticiser, the cements mixed at high frequency and for longer periods might be expected to be stronger, not weaker, than those mixed at low frequency and for shorter periods. It is more likely that the effects of beating frequency and mix duration are exerted through the porosity of the subsequently cured resin (Haas, Brauer and Dixon, 1975) rather than through changes in monomer content Bayne and his colleagues (1975) have demonstrated a relationship between the porosity and mechanical properties of cured cement. IV Variations in the technique of insertion of cement by the surgeon relate first to the influence of delay in insertion and second to the influence of pressure.

C Variables Partially under the Control of the Surgeon I The Inclusion of Blood and Tissue Debris is a common occurence in clinical practice and its effects are important and may be profound Cements deliberately prepared to contain blood showed a reduction of U C S of between 8 and 16% but, perhaps more important, there were very high standard deviations in this group, emphasising the unpredictable behaviour of such specimens (see appendix) Gruen and his colleagues (1976) demonstrated a 77 % reduction in tensile strength and a 69 % reduction in shear strength in specimens containing blood Test specimens formed from cement dough which is left over from the operation site demonstrate the dual effects of delay (i e laminations) and blood entrapment Such specimens showed a reduction of U C S of between 26 and 32% (see appendix) Homsy (1972) demon-

A J C Lee et al : Some Clinically Relevant Variables Affecting

5

Failure at 11k N

No failure at 45 kN

No failure at 91 k N

Fig 6 Diagram shewing circular sectioned tapers in acrylic cement with variable constraint Fig 4 Triangular sectioned taper and surrounding cement cracked along sharp edges of taper

Fig 5 Comparison of stress concentration at sharp corner (A) with more even distribution of stress at the radiused corner (B) demonstrated by photoelastic fringes

strated a reduction of just over 50 % in the tensile strength of specimens under similar cirumstances. II Exposure of Cement to Stress Raising Structural Features on the Implant and on Bone Acrylic cement is a somewhat brittle material showing relatively little plastic deformation before failure Thus, exposing the polymerised cement to local stress raising situations, such as sharp edges and corners, or creating sharp reentrant angles on the surfaces of the cement, may be dangerous, especially at high strain rates and under tensile stresses This is convincingly demonstrated in the laboratory by two simple types of experiment. a) Stainless steel tapers, of triangular and cylindrical section respectively, were driven accurately through the middle of cylindrical moulds containing acrylic cement dough until the distal surfaces of the tapers reached the ends of the cylinders The cement was allowed to polymerise and then removed from the cylinders with the tapers still in situ The tapers were then loaded in a fixture in a Mayes Universal testing

Fig 7 Circular sectioned tapers in acrylic cement with variable constraint

machine in which a load deflection curve is plotted automatically, and the load at failure of the cement was thus noted The load at failure of the cement containing the triangular taper was below the lower limit of resolution of the machine whereas with the circular taper the average was 11 k N In every case it was possible to see that the cement cracking around the triangular taper was related to the sharp edges of the taper (Fig 4). b) Photo elastic techniques show clearly the stress concentrating effects of sharp corners and edges (Fig. 5). III Variable Physical Constraintfor the Acrylic Cement This may well be one of the most important relevant variables in the clinical use of acrylic The use of unconstrained cement may increase the practical significance of creep, i e time dependent deformation under constant load The effect of constraint may be demonstrated in a simple experiment again involving the loading of tapers in acrylic cement cylinders, the

6

A J C Lee et al : Some Clinically Relevant Variables Affecting

Table 1 Effects of variables tested Variable

Possible % change in strength UCS

A

B

Environmental temperature 37 C Equilibrium moisture content Strain rate impact rates

10%

Fatigue

uncertain

Mixing technique Insertion technique

uncertain

5% 4% 21 %

delay pressure

C

10 % 3% + 67 %

Ageing

Radio opaque fillers Antibiotics (1 g A B /40 g polymer)

UTS

Inclusion of blood and tissue debris Stress raisers Constraint Cement thickness

40 %

54 %

+ 20 %

16 % profound profound uncertain

70% profound profound uncertain

cylinders being respectively unconstrained, constrained mildly, or heavily (Fig 6) The unconstrained specimen failed entirely at 11 kN, and though the cement in the constrained specimens cracked, load was still transmitted to very high levels (45 kN and 91 kN) without failure of the total load bearing system, i e the cement plus the constraints (Fig 7) This astonishing capacity for cement to carry load within the constraint after cement cracking may well be due to the fact that, once it has cracked, it is being loaded more in compression than in tension and is stronger in the former mode (Elloy, 1977) The effect of constraint becomes even more important in the light of the work of the late Professor Stachiewicz and his colleagues on the generation of hoop stress by polymerisation shrinkage of acrylic cement (Stachiewicz, Miller and Burke, 1976) These authors have shown that such hoop stress may exceed the ultimate tensile stress of acrylic cement and so lead to cracking of the cement, this situation being aggravated of course by stress raisers on the surface of the implant. IV Cement Thickness Though this clearly is at least in part under the control of the surgeon, little is known concerning the optimum thickness of cement in its various applications In general, a thin layer of cement will not withstand as great a load as a thick layer A crack will require less energy to propogate through a thin layer and the effects of stress concentration, polymerisation shrinkage and viscoelastic behaviour may be more marked with thin layers A layer which is too thick, however, may well deform ex-

cessively because of its low modulus and thermal dimensional changes may be greater Thus there may be an optimal range of allowable cement thickness but as yet this is not established.

Discussion The practical importance of these variables (Table 1) lies in the fact that they may at times act in varying degree in cumulative fashion so as seriously to reduce the effective mechanical strength of acrylic cement in vivo Excessive body weight and illogically vigorous physical activity may produce a sharp increase in the loads to be carried and in strain rates and so further compromise acrylic which is already mechanically defective It is thus clear that circumstances under which acrylic may fail mechanically can arise in vivo and that the surgeon may unintentionally play a part in creating such situations Four examples of this phenomenon follow: 1 Figure 8 is an x-ray of the stem of a total hip supported in its upper 2 cm by entirely unconstrained acrylic cement The lack of constraint has led to failure of the cement and subsequent failure of the implant. 2 Figure 9 is a radiograph of a femoral component supported in its upper 5 cm by cement which is entirely unconstrained The inevitable result has been cracking and failure of the unconstrained cement, (Fig 10) leading to fracture of the implant In at least one of the stem fractures and one of the loosenings

A J C Lee et al : Some Clinically Relevant Variables Affecting

7

Fig 8 X-Ray of femoral component supported by entirely unconstrained cement over its upper 2 cm, with failure of the unconstrained cement and consequent stem fracture

Fig 10 X-Ray of the same hip shewing mechanical failure of the unconstrained cement with subsequent stem fracture

Fig 9 X-Ray of femoral component supported over its upper 5 cm by totally unconstrained cement

Fig 11 Cement fracture produced by stress raising feature on implant

8

Fig 12 Implant shewing sharp edge which was responsible for the cement fracture shewn in Figure 11

Fig 13 Cement grossly fractured beneath the femoral component of a surface replacement at the knee

described by Collis ( 1977), a similar mechanism, i e. progressive failure of unconstrained cement, appears to have been responsible. 3 Figure 11 shows an example of cement fracture produced by stress raising features on the implant (Fig 12) which was removed from the broken cement bed.

A J C Lee et al : Some Clinically Relevant Variables Affecting

Fig 14 X-Ray of McKee-Farrar total hip

Fig 15 X-Ray of same case 7 years later shewing gross bone destruction from repititive movement of cement against and within bone

4 Figure 13 shows an example of cement grossly fractured beneath the femoral component of a surface replacement at the knee. The Effect of Mechanical Failure of Acrylic Cement. Mechanical failure of acrylic may lead to total clinical failure of the implant as in the examples above On the

A J C Lee et al : Some Clinically Relevant Variables Affecting

9

repititive micro movement of cement particles under the influence of the cyclical loads of activity.

Fig 16 X-Ray shewing destruction of calcar beneath collar of implant

The Practical Use of Acrylic Cement How can the surgeon improve the effective mechanical properties of the cement he is using in an effort to prevent prosthetic failure? The work described in this paper suggests that the following measures are likely to contribute to this objective: 1 The beating frequency during mixing should be low. 2 The mixing time should not be prolonged. 3 Laminations, porosities and the admixture of blood and tissue debris should be minimised bya) thorough washing, cleansing and drying of the bone surface. b) pressurising cement before placement.

Fig 17 Bone lavage instrument

other hand, it is perfectly clear that at times acrylic fracture can occur without any significant clinical accompaniment, at least in the short and medium term This has been emphasised by Charnley and Weber (1974) McKee and Watson Farrar ( 1966) drew attention in their original paper on the McKee-Farrar implant in 1966 to another aspect of acrylic failure and an important phenomenon in connection with the clinical use of acrylic cement: once acrylic cement starts to move repititively against or within bone, bone is destroyed Harris and his colleagues (1976) have more recently re-emphasised this An instructive example is shown in Figures 14-15 Of particular interest is the preservation and even hypertrophy of bone adjacent to the intact and uncracked cement at the tip of the implant in comparison with the gross destruction of bone higher up where the cement had cracked and had been moving within bone A similar example on a smaller scale is shown in Figure 16 where the destruction of the calcar beneath the collar of the implant has presumably occurred by a similar mechanism, i e cracking of the very thin layer of cement under the collar of the implant followed by

c) placing the cement relatively early (when its viscosity is relatively low) and accurately. d) pressurising the cement after placement, the technique depending upon the site. 4 Full mechanical constraint should be provided for the cement by bone of 'good' quality This will help to minimise the practical effect of cement cracking. 5 The cement should not be exposed to stress raising structural features on the implant or the bone. Washing and cleansing of the bone surface is conveniently carried out using a pulsating saline jet from an instrument' of the type shown in Figures 17 and 18, different nozzles being used for different sites. Miller and his colleagues (1978) have demonstrated in addition that the use of a brush is effective The exclusion of blood from the cement and the relatively early use of cement which has already been pressurised are best achieved by the use of a cement syringe, which also facilitates the accurate placement of the cement. Pressurisation of cement after placement requires different techniques according to the site A technique Manufactured by Howmedica U K Ltd.

A J C Lee et al : Some Clinically Relevant Variables Affecting

10

ure if it does occur Four of the recommendations made for the surgeon to follow have been shown by Halawa et al (1978) to exert a substantial influence on the shear strength of the cement bone interface in vitro Thus not only the behaviour of the cement itself but also the strength of the fixation between the cement and bone may be substantially influenced by surgical technique A fuller appreciation of the significance of surgical technique in implant surgery may help to reduce the incidence of mechanical failure and for the immediate future, refinements in the technical use of cement are likely to prove more fruitful than changes in the cement itself. Fig 18 Lavage instrument nozzles: A: for acetabulum B: for plane surfaces C: for intramedullary canals

Appendix

Al Specimens and Types of Test for the acetabulum of the hip was described by Lee and Ling (1974) and has been in successful use for 6 years The measures suggested above for the improved use of acrylic cement in clinical practice are aimed at reducing the incidence of mechanical failure of cement and minimising the consequences of mechanical fail-

In all the testing reported below, the mixing technique outlined in Table 2 was adopted Mixing was carried out in a polyethylene bowl using a stainless-steel spatula at a beating frequency of 4 Hz Pressurization of the cement specimen normally took place in the sixth minute and lasted 15 s The cements and the mixing apparatus were thermally equilibrated with the

Table 2 Mixing protocol Manufacturer

Mixing duration (s)

Taken into hands at (s)

Inserted into Pressurized at moulds at (s) (s)

150

165

240

360

60

90

180

300

CMW Laboratories, Blackpool, England

Sulfix 6

100

150

240

360

Sulzer Bros , Winterthur, Switzerland

Palacos R Palacos R (Refobacin)

100

150

240

Cement

Simplex P Simplex P (R O ) AKZ CMW

North Hill Plastics, London, England

Kulzer Bros , Bad Homburg, FDR

Table 3 Temperature dependency of cement strength (Standard deviation in brackets) Temp oC

Number Mix test Young's modulus of interval (E) (k Nmm -2) specimens (days)

20 25 30 35 37

9 9 9 8 8

2 2 2 2 2

(Simplex P (R O )) (Strain rate = 0 093 s- ')

2 2 2 2 2

945 905 874 860 854

(0 (0 (0 (0 (0

020) 101) 087) 097) 043)

Ultimate compressive str (Nmm-2 ) 122 629 112 130 102 854 101 099 100 101

(2 680) (1 080) (2 180) (2 640) (3 970)

0 1 % proof stress (Nmm -2 ) 86 85 79 77 76

364 549 816 748 554

(1 131) (4 670) (7 200) (5 300) (1 180)

Il

A J C Lee et al : Some Clinically Relevant Variables Affecting

Stres

1% Pofre St'e

PMMA

Fig 19 Stress-strain curve for bone cement (not to scale)

Str

Strain Steel

Fig 20 Stress-strain curve for steel (not to scale)

Fig 21 Moulds used for compression specimens Fig 22 Applicator for pressurisation of cement specimens

room before mixing The room was maintained at a temperature of 20 ±+IC and 50 ± 10 per cent relative humidity. The three types of test carried out are outlined below. a) Compression Compression tests were carried out on specimens 27 mm long x 9 mm diameter, compressed axially A plot of compressive load versus specimen deformation was

Fig 23 Mayes Universal Testing Machine

automatically plotted by the machine A typical curve (not to scale) for PMMA bone cement is shown in Figure 19, with a curve for steel shown for comparison in Figure 20. The gradient of the straight, elastic portion of the curve is normally defined as Young's Modulus (E) Since PMMA does not have a true elastic region, the Young's Modulus is obtained from the tangent AB to the curve Similarly, to replace the Yield shown in a steel specimen, a 0 1 % Proof Stress, defined as that stress that, when released, yields a permanent strain of 0 1%, is used. The moulds shown in Figure 21 were used to fabricate the samples To make the best use of cement stocks and to make the samples iniform a packet of powder and monomer was divided and mixed in the proportions 20 g (powder):10 ml (monomer). The samples were pressurized to 0 2 Nmm -2 for 15 S using the applicator shown (Fig 22) On polymerization, the specimens were tapped out of the moulds and the ends turned down to a length of 27 mm Thus the nominal dimensions for the compression test samples were 27 mm long x 9 mm diameter. In order to minimize platen-specimen friction leading to paralleling of the specimen, the specimen ends were smeared with colloidal graphite 2 prior to testing The apparatus used was a Mayes Universal Testing Machine3 (Fig 23) with a hydraulic high-speed test attachment The strain rates obtainable on this apparatus were in the range quasi-static to approximately 6 s- ' on a gauge length of 27 mm Compression tests are the easiest to perform, monitor and control, hence most of the testing reported below has been carried out in this mode. b) Tension A conventional tension test involves using a tensiometer in which the specimen is gripped at both ends and is axially loaded in tension This method is unsuitable in the case of PMMA since the material is brittle and generally fails in or near the grips Hence, the diametral tension test has been employed When a cylinder is loaded in compression along a diameter, it can be shown that there is an almost uniform tensile stress field perpendicular to it (Fig 24). The tensile failure occurs in a brittle manner and the only parameter obtainable in this test is the Ultimate Tensile Stress. Using the hydraulic high-speed test apparatus, strain rates in excess of 20 s' are obtainable. c) Bending A three point bending test rig was designed and the set up is shown schematically in Figure 25 Specimens 10 mm wide and 2 mm thick were obtained using stainless steel moulds Aquadag® Manufactured by Cheson Industries, Plymouth, England Manufactured by Mayes & Sons, Windsor, England 2

12

A J C Lee et al : Some Clinically Relevant Variables Affecting

Tension

Compression

D)

O'

2P TrLD

Fig 24 Diagram of diametral tension test

II Equilibrium Moisture Content The polymerized acrylic absorbs water and tissue fluids from a moist environment To determine the amount of uptake and the variables affecting it, 2 types of sample were used. With the first, 18 cylindrical samples 27 mm long x 9 mm diameter (nominal) were observed over a period of 10 days in a dessicator to ensure that they had attained constant weight. 0 Nine were placed in 0 15 % saline at 20 C and nine in water at 20 ° C The weight of the samples was measured daily and no significant difference in the amount of water uptake or the time to achieve moisture equilibrium (about 21 days) was detected. However, when samples were placed in O 15 % saline at 37 C, the time to achieve moisture equilibrium was reduced to 15 days though the amount of water taken up did not differ significantly with those stored at the lower temperature A 3 % drop in

recording apparatus

Fig 25 Diagram of three point bending test

I

nperature-Controlled ter IN

Water

Fig 26 Diagram of temperature controlled environment test apparatus (attachment to Mayes Universal Testing Machine)

and trimmed to 60 mm length In a three point bending test, strain rate is rather a difficult parameter to define since the gauge length is not very clear However, for the purposes of this test, the gauge length was taken to be the thickness of the sample. Meaningful results were not obtainable at strain rates in excess of 25 s .

strength in the hydrated sample, when compared to the strength of a sample stored dry, was noted. All of the second type of sample weighed 12 g but had varying surface areas The dimensions of the samples were varied to yield disks, tall cylinders and squat cylinders The disk, which was only 1 mm thick, achieved equilibrium in 7 days while the other samples achieved equilibrium in about 49 days. The amount of uptake in all three samples which were all in 0.15 % saline did not differ appreciably.

A2 Clinically Relevant Variables

III Strain Rate The apparatus used for the investigation enables strain rates from quasi-static to 0 1 S ' to be obtained on the Mayes Machine while the hydraulic high speed test apparatus was capable of obtaining strain rates from quasi-static to 6 s- ' (on a gauge length of 27 mm) Figure 2 shows a graph of strain rate v's ultimate compressive stress, Young's Modulus and 0 1 % proof stress.

1 Variables Totally Beyond the Control of the Surgeon I Environmental Temperature of 370 C The temperature controlled environment testing apparatus (Fig 26) which enabled tests to be carried out in an environment controlled to 200 C, 25 °C, 30 °C, 35 °C and 37 °C was utilized All specimens and apparatus were equilibrated at the test temperature before testing The results are shown in Table 3.

IV Ageing and Fatigue The cement samples were tested at 2 hours, 2 days and 7 days after the mixing period Some samples

A J C Lee et al : Some Clinically Relevant Variables Affecting Table 4 Effect of antibiotic deviations in brackets)

additions

13

(Standard

Cement

Antibiotic added (g)

Number of samples

Ultimate compressive stress (Nmm -2)

Simplex P

0

9

87 7 (2 0)

Simplex P

1

5

82 0 (2 3)

Simplex P

2

5

73 0 (1 9)

Simplex P

5

4

70 9 (4 6)

CMW

0

9

87 25 (2 45)

CMW

1

5

80 17 (2 03)

CMW

2

6

71 98 (3 74)

CMW

5

5

64 32 (8 47)

All samples 2 days old Strain rate = 0 003 s- ' Antibiotic = Nebacetin

Table 5 Influence of delay in forming cement specimens (Standard deviations in brackets) Mixing time (min)

Inserted Simplex P (R O ) into moulds No E at (min) (k Nmm 2 )

CMW UCS -

No

(Nmm 2 )

-

E

UCS

(kNmm -2 )

(Nmm 2)

2;30

2;30

9

2 54 (0 03)

82 99 (0 9)

8

2 53 (0 08)

83 47 (2 6)

2;30

5;00

6

2 17 (0 09)

74 63 (3 2)

4

2 41 (0 17)

72 22 (4 01)

2;30

7;30

4

2 00 (0 13)

55 63 (4 31)

4

1 98 (0 41)

45 62 (9 73)

5;00

7;30

3

2 02 (0 43)

50 34 (O 11)

1 ;30

2

46 81 (13 63) 75 84 (4 2)

2

1 ;30

1 87 (0 23) 2 31 (0 06)

Strain rate = 0 003 s'

were stored for periods of up to 2 years to enable studies on the effects of protracted storage periods to be determined The results are shown in Table 6 and a representative cement's strength with ageing shown in Figure 3.

2

Variables Wholly within the Control of the Surgeon

Brand of Cement The various brands tested showed no significant differences as regards their strength. Radio Opaque Fillers In quantities normally recommended by cement manufacturers, they render cement weaker (by about 8 %) However, these radio-opacifiers must be added by the manufacturer to avoid localized concentrations This tendancy was prevalent in C M W with 5 g of Ba 50 4 and was reflected in the high scatter of the strength results In fact spicules of Ba SO4 could be distinguished on the surface of the cement with the naked eye. Antibiotics The addition of antibiotics weaken the cement and when added by hand, the standard deviations are large The

percentage weakening of cement with varying amounts of a typical antibiotic Nebacetin are seen in Table 4. Delay The results are seen in Table 5. Pressure Cement was subjected to a pressure of 0 2 Nmm -2 for 15 S during the moulding stage This led to an increase of 12% in U.C S when compared to those samples which were unpressurized Longer times of application were not possible owing to experimental difficulties Higher pressures were unrealistic since it is most unlikely that these can be obtained in the clinical situation Refer Table 6 for the results.

3 Those Variables Partially within the Control of the Surgeon. The Variable Inclusion of Blood and Tissue Debris 1 ml of blood was added to the cement as it was being moulded and the samples thus obtained tested at 2 days and 7 days. Specimens were also made from cement dough "left over" from the operation site during surgery (refer Table 6).

A J C Lee et al : Some Clinically Relevant Variables Affecting

14

Table 6 Complete test results (Standard deviations in brackets) Cement

Simplex P (R O )

Mix test interval

Young's modulus (E) k Nmm 2

Ultimate compressive stress (UCS) Nmm-2

O 1% proof stress Nmm -2

2 hours

9 9 10 9

3 12 30 90

2 477 2 731 2 762 2 785

(0 011) (0 036) (0 087) (0 033)

80 326 83 893 101 223 102 140

(1 641) (1 234) (1 671) (2 650)

52 57 73 80

293 798 394 530

(2 252) (3 527) (1 977) (2 038)

2 days

9 9 9 9

3 12 30 90

2 609 2 783 2 854 2 945

(0 024) (0 031) (0 019) (0 025)

90 010 97 451 113 149 122 629

(1 (1 (2 (2

488) 365) 028) 680)

57 492 71 151 77 064 86 364

(1 865) (1 773) (2 232) (1 131)

7 days

7 9 8 10

3 12 30 90

2 610 2 844 2 884 2 986

(0 102) (0 030) (0 067) (0 057)

90 723 98 165 120 285 122 324

(3 343) (3 985) (2 120) (1 997)

60 004 71 355 78 491 82 772

(1 467) (4 016) (1 926) (2 507)

7 6 9

3 90 3

2 493 (0 029) 2 627 (0 090) 2 410 (0 019)

2 hours

9 8 7 10

3 12 30 90

2 434 2 487 2 721 2 795

2 days

9 9 10 9

3 12 30 90

7 days

9 9 9 9

12 months 24 months Simplex P

Strain Number of samples rate x 10-3 s

12 months

83 072 (2 189) 101 327 (3 027) 80 737 (0 0210)

56 721 (1 690) 73 029 (4 061) 58 321 (2 360)

(0 021) (0 011) (0 014) (0 012)

80 677 88 267 99 325 101 038

(2 008) (3 465) (1 131) (1 702)

53 109 62 385 74 209 81 447

(1 559) (2 966) (3 363) (1 264)

2 446 2 558 2 844 2 895

(0 012) (0 011) (0 016) (0 013)

87 767 101 325 112 334 127 726

(2 008) (1 457) (2 212) (2 875)

58 511 66 156 77 573 89 092

(1 753) (1 936) (3 557) (2 201)

3 12 30 90

2 446 2 629 2 935 3 003

(0 (0 (0 (0

88 685 113 353 123 649 129 025

(3 230) (2 354) (1 457) (2 008)

59 123 72 579 80 632 90 010

(2 232) (1 896) (1 671) (4 311)

11 6

3 90

2 511 (0 009) 2 723 (0 092)

89 093 (1 177) 100 803 (2 846)

024) 016) 013) 085)

63 851 (2 620) 87 223 (4 719)

Simplex P (R O ) + I ml blood

2 days 7 days

9 8

3 3

2 226 (0 197) 2 319 (0 084)

81 264(11 381) 84 761 (6 294)

-

Simplex P (R O ) theatre

2 days 7 days

4 3

3 3

1 901 (0 113) 1 944 (0 121)

64 714 (8 423) 63 618 (9 774)

-

AKZ

2 hours

9 11 8 9

3 12 30 90

2 456 2 711 2 752 2 845

(0 010) (0 082) (0 039) (0 021)

79.714 91.845 101 732 102 956

(1 121) (1 533) (0 846) (1 223)

51.070 60.958 73.190 81.855

(1 763) (1 213) (2 375) (2 008)

2 days

10 9 7 9

3 12 30 90

2 538 2 791 2 884 2 945

(0 038) (0 040) (0 152) (0 011)

83.588 95.412 106 829 122 436

(0 996) (1 302) (1 529) (0 897)

57.084 62.487 74.311 87.971

(1 (2 (1 (1

7 days

9 9

3 12

2 538 (0 027) 2 782 (0 031)

90.329 (1 143) 95.825 (0 985)

712) 232) 967) 610)

57.696 (1 151) 65.443 (2 048)

15

A J C Lee et al : Some Clinically Relevant Variables Affecting Table 6 (continued) Cement

CMW

Palacos R

Palacos R + Refobacin

Strain rate Young's modulus Number (E) of samples x 10 3s kNmm 2

Ultimate compressive stress (UCS) Nmm -2

0 1 % proof stress Nmm - 2

9 10

30 90

2 884 (0 067) 2 986 (0 055)

107 033 (1 428) 116 209 (1 268)

76 004 (0 927) 82 566 (1 467)

12 months

9

3

2 246 (0 018)

83 180 (1 121)

61 467 (2 528)

24 months

6 4

3 90

2 229 (0 037) 2 339 (0 208)

81 092 (2 930) 98 971 (4 306)

59 927 (3 025) 82 337 (0 836)

2 hours

9 9 12 9

3 12 30 90

2 2 2 2

487 766 895 915

(0 021) (0 046) (0 009) (0 098)

79 510 82 874 90 723 101 936

(2 456) (3 527) (1 671) (1 141)

51 070 55 963 71 253 75 433

(3 477) (3 041) (3 284) (3 997)

2 days

9 9 9 9

3 12 30 90

2 629 2 884 2 935 2 966

(0 094) (0 091) (0 024) (0 087)

87 257 98 051 111 926 120 183

(2 456) (2 264) (2 457) (2 731)

56 982 59 734 68 297 73 394

(2 680) (2 456) (3 537) (3 170)

7 days

9 9 9 9

3 12 30 90

2 579 2 905 2 956 2 996

(0 019) (0 064) (0 011) (0 001)

90 519 96 378 112 538 124 872

(2 030) (2 140) (1 763) (3 465)

59 70 76 81

(2 181) (2 956) (1 314) (2 813)

12 months

7

3

2 438 (0 092)

83 317 (2 170)

50 080 (3 510)

24 months

5

3

2 217 (0 108)

80 001 (3 280)

48 422 (5 736)

2 hours

12 11 9 9

3 12 30 90

2 344 2 436 2 528 2 721

(0 013) (0 028) (0 131) (0 087)

78 389 85 932 94 393 101 936

(2 538) (1 875) (3 710) (3 904)

52 61 72 76

2 days

9 9 8 9

3 12 30 90

2 456 2 579 2 742 2 854

(0 011) (0 084) (0 055) (0 032)

85 629 93 577 103 363 111 722

(1 304) (2 895) (2 242) (1 939)

55 963 63 608 71 457 80 835

(1 (2 (4 (1

7 days

7 9 8 7

3 12 30 90

2 446 2 589 2 793 2 844

(0 018) (0 011) (0 085) (0 138)

87 767 101 325 104 689 117 635

(4 923) (2 140) (1 712) (3 037)

57 186 68 603 76 248 83 282

(1 630) (2 925) (1 824) (3 486)

24 months

7 5

3 30

1 918 (0 017) 2 129 (0 037)

2 hours

6 7 7 7

3 12 30 90

2 283 2 385 2 435 2 558

(0 012) (0 109) (0 238) (0 083)

76.656 84.505 92.354 101 223

(3 486) (2 018) (2 252) (1 814)

51.478 59.938 71.967 75.739

(1 756) (2 028) (3 547) (3 088)

2 days

9 9 10 9

3 12 30 90

2 436 2 548 2 660 2 742

(0 072) (0 093) (0 020) (0 037)

83.995 90.417 99.082 103 669

(1 131) (1 936) (2 793) (1 529)

54.638 61.467 70.132 78.695

(1 936) (2 252) (1 314) (3 476)

7 days

10

3

Mix test interval

2 589 (0 021)

73 342 (3 595) 91 856 (1 803)

88.888 ( 1 916)

225 642 452 651

191 162 273 930

(2 385) (2 629) (4 933) (3 251) 977) 099) 199) 733)

58 190 (5 276) 74 797 (1 923)

58 409 (2 232)

16

A J C Lee et al : Some Clinically Relevant Variables Affecting

Table 6 (continued) Cement

Sulfix 6

Sulfix 6 +Nebacetin

Mix test interval

Number Strain rate Young's modulus of samples x 10-3 s- ' (E) kNmm -2

Ultimat compressive stress (UCS) Nmm - 2

O 1% proof stress Nmm 2

10 10 9

12 30 90

2 609 (0 065) 2 731 (0 096) 2 793 (0 072)

100 917 (3 465) 102 140 (2 354) 115 800 (1 997)

68 287 (1 794) 74 617 (3 068) 82 161 (2 793)

24 months

6

3

2 219 (0 097)

71 323 (2 840)

43 680 (4 780)

2 hours

15 9 9 9

3 12 30 90

2 456 2 691 2 784 2 823

(0 008) (0 013) (0 034) (0 087)

80 632 84 199 99 083 103 669

(1 824) (2 680) (0 835) (3 465)

53 618 58 409 74 821 81 651

(2 864) (3 741) (1 213) (3 180)

2 days

6 9 8 9

3 12 30 90

2 629 2 772 2 864 2 956

(0 071) (0 023) (0 047) (0 031)

89 398 97 043 102 242 103 343

(2 008) (1 661) (2 466) (3 027)

58 71 74 81

002 457 719 753

(2 028) (3 893) (1 223) (4 709)

7 days

7 8 10 7

3 12 30 90

2 630 2 864 2 915 2 947

(0 079) (0 048) (0 115) (0 065)

90 316 101 121 102 324 103 823

(3 527) (1 549) (1 121) (2 018)

60 448 72 477 80 122 80 927

(1 763) (3 614) (2 018) (2 915)

12 months

6

3

2 hours

9 9 10 9

3 12 30 90

2 446 2 660 2 782 2 803

(0 118) (0 080) (0 094) (0 083)

80 122 83 486 98 980 102 854

(2 021) (4 750) (2 956) (1 131)

53 109 57 492 73 394 81 141

(3 934) (4 994) (5 484) (1 743)

2 days

10 10 10 6

3 12 30 90

2 640 2 823 2 884 2 956

(0 072) (0 038) (0 072) (0 010)

89 704 97 966 103 771 102 324

(1 896) (2 385) (2 140) (1 814)

57 900 73 496 75 229 83 466

(1 182) (2 956) (3 710) (3 068)

7 days

8 8 7 9

3 12 30 90

2 660 2 884 2 895 2 917

(0 014) (0 010) (0 193) (0 032)

90 723 101 019 104 897 10 804

(2 345) (1 906) (3 307) (4 077)

61 264 74 311 78 389 91 743

(1 896) (1 457) (2 273) (1 732)

6

3

2 423 (0 009)

80 910 (2 803)

58 752 (3 707)

10 9

3 3

2 500 (0 160) 2 530 (0 128)

80 100 (4 700) 76 010 (3 613)

56 900 (3 000) 52 403 (6 821)

12 months Sulfix 6 + Nebacetin (unpressurized)

2 days 7 days

2 368 (0 116)

87 001 (3 234)

58 540 (4 040)

A J C Lee et al : Some Clinically Relevant Variables Affecting

17

Table 6 (continued)

Tension Cement

Mix test interval

AKZ

2 hours

Simplex P (R O )

Number of samples

Strain rate x 10 s- '

Ultimate tensile stress Nmm -2

6 11

33 16 6

24 658 (1 144) 37 522 (2 180)

2 days

7 9

33 16 6

26 731 (3 627) 49 011 (7 910)

7 days

4

33

28 174 (4 901)

8

16 6

52 935 (6 72)

7

33

24 460 (1 907)

6

16 6 33 3

30 197 (1 838) 31 600 (2 810)

2 hours

8 2 days

7 days

12 months

7

33

4

16 6

(2 013) 45 933 (2 157)

6

33 3

59 750 (4 882)

9

33

38 730 (1 287)

8

16 6

9

33 3

45 581 (2 773) 46 890 (3 382)

8

3 3

36 438 (2 591)

36 69

Bending Cement

Mix test interval

Number of samples

Strain rate s-'

Ultimate tensile stress (UTS) Nmm-2

Young's modulus (E) k Nmm 2

AKZ

2 hours 2 days

8 9

26 26

70 198 (7 200) 73 010 (4 374)

2 881 (0 128) 3 001 (0 086)

7 days

Simplex P (R O )

10

26

73 980 (5 910)

2 998 (0 272)

12 months

7

26

61 007 (2 191)

2 522 (0 018)

2 hours 2 days

7 9

26 26

79 199 (3 103) 74 728 (2 872)

2 906 (0 019) 2 944 (0 128)

10

26

7

26

75 334 (4 496) 60 829 (2 109)

2 872 (0 108) 2 686 (0 031)

7 days 12 months

References Bayne, S C , Lautenschlager, E P , Compere, C L , Wilkes, R : Degree of Polymerisation of Acrylic Bone Cement J. Biomed Mat Res 9, 27 (1975) Bucholz, H W , Englebrecht, E : On the sustained release of some antibiotics when mixed with Palacos Resin Chirug 41, 511 (1970) Charnley, J : Acrylic Cement in Orthopaedic Surgery LondonEdinburgh: Livingstone 1970 Collis, D K : Femoral stem failure in total hip replacement J. Bone Jt Surg 59A, 1033 (1977) DeWijn, J R , Slooff, T J J H , Driessens, F C M : Mechanical Properties of bone cements in vitro and in vivo. In: The knee Joint-Proceedings of the International Congress Amsterdam: Excerpta Medica 1974 Elloy, M A : Personal communication 1977

Gruen, T A , Markolf, K L , Amstutz, H C : Effects of Lamination and blood entrapment on the strength of acrylic bone cement Clin Orthop 119, 250 (1976) Haas, S S , Brauer, G M , Dickson, G : A characterisation of polymethylmethacrylate bone cement J Bone Jt Surg 57A, 380 (1975) Halawa, M , Lee, A J C , Ling, R S M , Vangala, S S : The shear strength of trabecular bone and some factors affecting the shear strength of the cement-bone interface (In press, 1978) Harris, W H , Schiller, A L , Scholler, J M , Freiberg, R A , Scott, R : Extensive localised bone resorption in the femur following total hip replacement J Bone Jt Surg 58 A, 612 (1976) Homsy, C A , Tullos, H S , Anderson, M S , Differante, N M , King, J W : Some physiological aspects of prosthesis

18 stabilisation with acrylic polymer Clin Orthop 83, 317 (1972) Jaffe, W L , Rose, R M , Radin, E L : On the stability of the mechanical properties of self-curing acrylic bone cement. J Bone Jt Surg 56 A, 1711 (1974) Lautenschlager, E P , Marshall, H W , Marks, K E , Schwartz, J., Nelson, C L : Mechanical strength of acrylic bone cements impregnated with antibiotics J Biomed Mat Res. 10, 837 (1976) Lee, A J C , Ling, R S M , Wrighton, J D : Some properties of polymethylmethacrylate with reference to its use in orthopaedic surgery Clin Orthop 95, 281 (1973) Lee, A J C , Ling, R S M : A device to improve the extrusion of bone cement into the bone of the acetabulum in the replacement of the hip joint Biomed Eng 9, 1 (1974) Lee, A J C , Ling, R S M , Vangala, S S : The mechanical properties of bone cements J Med Eng Technol 1, 137 (1977) Lee, A J C , Ling, R S M : Problems associated with the fatigue testing of implants In: The Evaluation of Artificial Joints D Downson, V Wright (eds ) London: The Biological Engineering Society 1977 McKee, G K , Watson-Farrar, J : Replacement of Arthritic hips by the McKee-Farrar prosthesis J Bone Jt Surg 48B, 245 (1966) Marks, K E , Nelson, C L , Lautenschlager, E P : Antibiotic impregnated bone cement J Bone Jt Surg 58 A, 358 (1976)

A J C Lee et al : Some Clinically Relevant Variables Affecting Miller, J , Burke, D L , Krause, W , Ahmed, A W , Kelebay, L. C.: Blood & surgical debris on the surface of the medullary canal bone as a factor in the loosening of hip arthroplasty components Paper read at the 24th Annual Meeting of the Orthopaedic Research Society, Dallas 1978 Shadbolt, L E : Personal communication 1974 Stachiewicz, J W , Miller, J , Burke, D L : Hoop stress generated by shrinkage of polymethylmethacrylate as a source of prosthetic loosening Paper read at the th International Conference on Medical and Biological Engineering, Ottawa 1976 Unsworth, A : Personal communication 1975 Vangala, S S : Unpublished work 1977 Wagner, J , Hermans, M , Bourgois, R : Etude en durimetrie sur de plastique acrylique retire de hanches humaines Acta Orthop Belge 38, (Supp 1) 111 (1972) Weber, F A , Charnley, J : A radiological study of fractures of acrylic cement in relation to the stem of a femoral head prosthesis J Bone Jt Surg 57B, 297 (1975) Weinstein, A M , Bingham, D N , Barry, W , Sauer, D V M , Lunceford, E M : The effect of high pressure insertion & antibiotic inclusions upon the mechanical properties of polymethylmethacrylate Clin Orthop 121, 67 (1976)

Received March 25, 1978