The fracture toughness of titanium-fiber-reinforced bone cement L.D. Timmie Topoleski' University of Pennsylvania, Department of Bioengineering, School of Engineering and Applied Science, Philadelphia, Pennsylvania 19104 Paul Ducheyne University of Pennsylvania, Department of Bioengineering, School of Engineering and Applied Science; and Deprtment of Orthopedics, School of Medicine, Philadelphia, Pennsylvania 19104 John M. Cuckler University of Pennsylvania, Department of Orthopedics, School of Medicine, Philadelphia, Pennsylvania I9104 Fracture of the poly(methy1 methacrylate) bone cement mantle can lead to the loosening and ultimate failure of cemented total joint prostheses. The addition of fibers to the bone cement increases fracture resistance and may reduce, if not eliminate, in vivo fracturing. This study discusses the effect of incorporating titanium (Ti) fibers on fracture toughness. Essential characteristics of the composite bone cement included a homogeneous and uniform fiber distribution, and a minimal increase in apparent viscosity of the polymerizing cement. Ti fiber contents of 1%, 2%,and 5%by volume increased the fracture toughness over non-reinforced bone cement by up to 56%.Bone cements of two different viscosities were used as matrix material, but when reinforced with the same fiber type and content, they showed no difference in fracture toughness. Four

different fiber aspect ratios (68,125,227, 417) were tested. At 5% fiber content, there was no statistically significant dependence of fracture toughness on fiber aspect ratio. Scanning electron microscopy revealed important toughening mechanisms such as fiber/matrix debonding, local fracture path alteration, and ductile fiber deformation and fracture. Fiber fracture was evidence that the critical fiber length was exceeded. The surfaces of the Ti fibers were rough and irregular, indicating that a high degree of mechanical interlock between matrix and fiber was likely. The energy absorption contribution of plastic deformation and ductile fracture is absent in brittle fibers, like carbon, but is a distinction of the Ti fibers used in this study. Q 1992 J o h n Wiley & Sons, Inc.

INTRODUCTION

Poly (methyl methacrylate) (PMMA) bone cement has been perceived as the weak link in maintaining the mechanical integrity of cemented total joint arthroplasties.' Failure or fracture of the PMMA mantle can lead to loosening and ultimate failure of the prosthesis.2Improvement of the mechanical prop'To whom correspondence should be addressed at Department of Mechanical Engineering, University of Maryland Baltimore County, Baltimore, MD 21228. Journal of Biomedical Materials Research, Vol. 26, 1599-1617 (1992) C CC 0021-9304/92/121599-19 8 1992 John Wiley & Sons, Inc.

1600

TOPOLESKI, DUCHEYNE, AND CUCKLER

erties of bone cement, specifically increasing the resistance to fracture, remains essential for increasing the longevity of cemented total joint arthroplasties. Fatigue failure has been shown to be a predominant in vivo failure mode of bone cement; thus modifications of bone cement initiated to increase in vivo longevity must address the improvement of fatigue resistance. Several investigators have studied the fatigue behavior of bone cement, but because of the intrinsic sensitivity of fatigue to variables like environment, loading range, loading frequency, and specimen design, determining the fatigue characteristics of PMMA bone cement is still ongoing? It is important to appreciate that fatigue testing does not yield values for a unique material property, but that the data are related to both the material and the fatigue testing protocol. A fundamental gauge of fracture resistance is essential for evaluating the performance of different bone cements. Despite the clinical relevance of fatigue failure, and because of the uncertainties inherent to fatigue testing and the absence of an unambiguous, basic material property associated with fatigue, investigators studying modifications of bone cement for enhanced fracture performance have, invariably, first determined fracture toughness (it should be noted that almost all of the fracture property determinations were performed before the physical evidence was established to indict fatigue as the in vivo failure mechanism). Fracture toughness is indeed a fundamental material property independent of methodology, expressing in a quantifiable way a material’s ability to resist crack propagation: Moreover, fracture toughness can be easily measured. The fracture resistance of PMMA can be enhanced by chemical modification,5b or the introduction of a reinforcing pha~e.~-” It is well known that greater failure resistance can be achieved by fiber reinforcement of a matrix than by the matrix material alone. The use of a fiber phase is attractive because fibers can be incorporated into bone cements that are in current clinical use. In most of the previous studies on fiber-reinforced bone cement, changes in the overall mechanical properties due to the addition of fibers have been evaluated in terms of ultimate tensile strength or modulus of elasticity; although important, these properties are not necessarily predictive of fracture. Determination of the fracture characteristics of fiber-reinforced bone cement is essential to assess its effectiveness. Some investigators did measure the fracture toughness after fiber reinforcement, and reported increases through the addition of Ke~lar,””*’~ polyethylene (PE),17J8 and carbonI2fibers. Despite the documented increases in mechanical properties and fracture resistance, reinforced bone cement has not yet been accepted in current clinical practice, primarily because of limitations in the clinical environment. The addition of fibers to bone cement increases the apparent viscosity and severely decreases the workability and deli~erability.””~~ Furthermore, a uniform distribution of fibers within the in situ delivered PMMA is difficult, if not impossible, to obtain. To realize the great potential for improving the fracture resistance of bone cement via fiber reinforcement there was the need to develop methodologies to overcome the problems of nonuniform fiber distribution, mixing, and ultimate delivery limitations of the reinforced cement within clinical constraints. A new method to incorporate short fibers into

FRACTURE TOUGHNESS/REINFORCED BONE CEMENT

1601

currently used bone cements was recently described.” The result is a uniform fiber distribution and reduced mixing and delivery complications. Alternative fiber materials deserve continued attention. The use of ductile metal fibers to reinforce a brittle matrix material, like PMMA, can be advantageous over reinforcement with brittle fibers. Ductile fibers increase the amount of energy dissipated during crack growth, in contrast to brittle fibers, especially in short fiber composites.m When a brittle fiber breaks, strain energy not required for the fracture is, by definition of “brittle’; elastic strain energy and is recovered by the matrix, available for additional crack propagation. A ductile fiber, on the other hand, deforms plastically as well as elastically prior to fracture. Plastic deformation requires permanent energy absorption by the fiber. When a ductile fiber fractures, only the elastic portion of the strain energy is recovered; energy of plastic deformation is lost. This energy absorption mechanism of a ductile metal fiber makes it attractive as a reinforcing phase. There is only one published study of PMMA reinforced with short metallic fibers,’ and the fracture toughness of PMMA reinforced with metal fibers has never been reported. The use of ductile metal fibers to reinforce PMMA exploits two advantages over previously used reinforcing fibers: the fibers are ductile as opposed to brittle carbon or glass; and there is no restriction in the maximum fiber diameter, since the metal fibers are manufactured by a drawing process. The potential advantages of an increased fiber diameter include an increase in fiber debonding resistance and an increase in the work of fiber pull-out.mThe advantages must be tempered, however, with potential disadvantages, such as a decrease in fiber strength with increasing diameter. Obviously, optimum fiber dimensions must be determined, but it is a definite benefit to eliminate restrictions on an essential parameter such as fiber diameter. In this study, a high-strength (860 MPa), high-toughness (40 MPa(m)’”), and biocompatible” 26 material, titanium (Ti), was incorporated as short fibers into PMMA bone cement. Fracture toughness of Ti-fiber-reinforced bone cement (R-PMMA) was determined as a function of fiber content, fiber dimensions, and bone cement matrix, and evaluated against nonreinforced bone cement controls. Scanning electron microscopy (SEM) was used to examine the fracture surfaces of specimens and identify the predominant fiber reinforcing mechanisms.

MATERIALS A N D METHODS

Two commercially available bone cements were used as the matrix materials for the Ti fiber reinforced PMMA (Rr-PMMA), designated Type A (Simplex-P, Howmedica, Inc., Rutherford, NJ) and Type B (Zimmer LVC, Zimmer, Inc., Warsaw, IN). The bone cements were used as packaged for clinical use; no alterations were made to the components. Two fiber lengths and two fiber diameters were incorporated into the bone cement. The fiber diameters were 12 pm, because it was the smallest diameter available from the supplier (Bekaert, Inc., Zwevegem, Belgium), and 22 pm.

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TOPOLESKI, DUCHEYNE, AND CUCKLER

The fiber lengths were 1.5 mm, the smallest producible with currently available techniques, and 5.0 mm, which, based on a feasibility experiment, was the longest manageable fiber for fabricating Rf-PMMA. The aspect ratios (fiber length/fiber diameter) of the four fiber types used were: 68, 125, 227, and 417. Fracture toughness was measured on specimens without fibers (PMMA controls for both Type A and B cements), Ti Rf-PMMA with 1%,276, and 5% by volume of 1.5/12 and 5.0/12 Ti fibers and Type A bone cement as the matrix material, Ti Rf-PMMA with 5% by volume of 1-5/22 fibers and Type A bone cement as the matrix, and Ti Rf-PMMA with 5% by volume of 1.5/22 fibers and Type B bone cement as the matrix. Fiber dimensions are reported here as X/Y,where X is the fiber length, in millimeters, and Y is the fiber diameter, in micrometers. The Rf-PMMA preparation was described in detail elsewhere,” and only a summary of the specimen preparation procedures will be reviewed. The Type A and B bone cements were prepared according to the instructions from their respective package inserts prior to fiber addition. All liquid monomer was chilled in ice water at 0°C before mixing with the powder component. The powder-liquid blend was stirred for 1min, at an approximate frequency of 2 beats/s. The appropriate fiber content was added over a 30-s interval and the resulting Rf-PMMA was stirred for an additional 90 s to disperse the fibers uniformly throughout the cement mass. The total preparation time for the Rf-PMMA was approximately 3 min. The Rf-PMMA was injected into a polyethylene mold to form rectangular prismatic specimens using a commercial bone cement injection system (Miller Cement Injection System, Zimmer, Inc., Warsaw, IN). The control PMMA was also mixed for 3 min prior to injecting. Aluminum plates and a mass of approximately 2 kg were placed over the specimens. Figure l(a) shows a cross section of a specimen reinforced with 5% fibers. The fibers are well distributed throughout the cement mass. Figure l(b) shows a transmitted light micrograph illustrating the random orientation of the fibers within the cement.

Figure 1. (a) Light micrograph of a cross section of a fracture toughness specimen, showing uniform distribution of fibers. Fiber cross sections appear as the light spots on the dark PMMA background. (b) Transmission light micrograph of a section of a fracture toughness specimen, showing the random orientation of the embedded fibers.

FRACTURE TOUGHNESS/REINFORCED BONE CEMENT

1603

All of the 1%and 2% Rr-PMMA with the Type A bone cement specimens were centrifuged prior to molding to reduce the porosity inherent in coldcured bone cement. An additional 2 min of handling time was necessary to complete the process of centrifugation (which included 30 s of actual centrifugation). Because standard fracture toughness tests do not bring out improvements of PMMA due to ~entrifugation,2’~ centrifugation was considered an unnecessary step and subsequent mixtures of 5% Rr-PMMA were not centrifuged, thus maintaining the total preparation time at 3 min. A three-point bending specimen geometry was used for the fracture toughness (Kk)testing. Because there are no published standards for the fracture toughness testing of polymers, a geometry was adapted from ASTM E 399-83, a standard for fracture toughness testing of metals. PMMA acts as a brittle material during rapid fracture. Because it does not form a large plastic zone in front of the crack tip, i.e., fails by brittle fracture, the relative specimen dimensions dictated by ASTM E 399-83 should provide a valid measure of Kk for Rv~MA.’~ The specimen geometry is illustrated in Figure 2. A 1.0-mm-wide notch was machined in each specimen with a low-speed diamond saw, and then a groove was cut into the tip of the notch with a scalpel. A fatigue crack was propagated from the notch in order to create a sharp crack in the specimen prior to the K,, testing. Prefatiguing was performed on either an Instron (Canton, MA) 1125 electromechanical testing machine or an Instron 1331 servohydraulic mechanical testing machine. Specimens were prefatigued at a cyclic loading rate of 2 to 5 Hz, at room temperature, and in laboratory air. Nonreinforced specimens required from 200 to 3,500 cycles at a loading range of 0-130 N, and reinforced specimens required from 6,000 to 50,000 cycles at a loading range of 0-200 N to grow the requisite sharp crack. Crack growth was monitored visually. When the prefatigued crack was approximately 2 mm long, the specimen was removed from the load frame, and a colored ink was injected into the open notch to allow subsequent identification of the initial pre-crack length, and to facilitate initial pre-crack length measurements necessary for the fracture toughness measurements.

T

1omm

Figure 2. Geometry of the three-point bending specimen used for the fracture toughness determination for the PMMA and Rr-PMMA.

TOPOLESKI, DUCHEYNE, AND CUCKLER

1604

Specimens were loaded using stroke (displacement) control on an Instron 1331 at a constant cross-head rate of 0.085 mm/s until catastrophic fracture occurred. Load-displacement data was recorded on an X-Y plotter. The validity of applying fracture toughness analysis, and then the value of fracture toughness, was determined for each test as prescribed by ASTM E 399-83. Three to six samples of each specimen type were tested. A public domain statistical software package, EPISTAT, was used for all statistical analysis. Means and standard deviations of the fracture toughness values were calculated for each specimen group. Analysis of variants (ANOVA) was performed to determine whether a statistical difference existed between the means of the different groups. A Student’s t test was performed on the fracture toughness data to determine whether there was a statistical effect due to: (a) varying fiber content (vol %) for a given fiber or cement type; and (b) varying fiber or cement type for a given fiber content. The fracture surfaces of randomly selected reinforced and nonreinforced specimens were examined using the SEM (Phillips500, Eindhoven, the Netherlands). Specimens were sputter coated with gold prior to observation. RESULTS

A comprehensive summary of the fracture toughness testing data is listed in Table I. A histogram of the mean fracture toughness vs. the volume percent fiber content, for the 1.5/12 fibers only, is presented in Figure 3. With only 1%fiber addition, Kk increased from the control value of 1.52 MPa(m)ln to 1.77 MPa(m)In,an increase of 16%.With 2% fibers KI, increased to 2.01 MPa, an increase of 32%. The addition of 5% fibers resulted in the largest increase in Kk,to 2.09 MPa(m)In, 37% greater than the control. A histogram of the mean fracture toughness values from the 5.0/12 fibers, together with the 1.5/12 fiber values, is shown in Figure 4. The fracture toughness of the cement reinforced with the 5.0/12 fibers was greater than with the 1.5/12 fibers TABLE 1 Summary of the Fracture Toughness Data (K,) ~~

Bone Cement Matrix A A A A A A

A A A

B B

Fiber Type

Volume Percent

Mean Kk (MPa(rn)lR)

Standard Deviation

n

-

0 1

1.524 1.77 1.905 2.01 2.135 2.092 2.302 2.372 2.242 1.226 2.278

0.222 0.088 0.157 0.043 0.084 0.162 0.147 0.258 0.090 0.055 0.316

8 3 4 4 4 5 5 6 6 5 5

1.5112 5.0112 1.5112 5.0112 1.5112 5.0112 5.0122 1.5122 1.5122

1 2 2 5 5 5 5 0 5

1605

FRACTURE TOUGHNESS/REINFORCED BONE CEMENT

-

5

m

2.75

6 l 1.5/12

2.00

m 1.75 $ I

1.50

3

1.25

5

1.00

c,

z

0.75

c

0.50

5

0.25 0.00

0 (control)

1

2

3

4

5

Ti fiber content (percent by volume) Figure 3. Fracture toughness of PMMA (Type A cement matrix) reinforced with 1%,2%,and 5% 1.5/12 fibers, showing the increase in fracture toughness with increasing fiber content. A maximum increase in fracture toughness of 37% was obtained with a 5% fiber content. Error bars represent one standard deviation.

-

> N

h

2.75

-

-

Type A h9 15/12 5.0/12

0 (control)

1

2

3

4

5

Ti fiber content (percent by volume) Figure 4. Fracture toughness of PMMA (Type A cement matrix) reinforced with 1%, 2%, and 5% 5.0/12 fibers, together with the results for the 1.5/ 12 fibers, showing the increase in fracture toughness with increasing fiber content. The maximum increase in fracture toughness of 51% was achieved with a 5% fiber content. Error bars represent one standard deviation.

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TOPOLESKI, DUCHEYNE, AND CUCKLER

at each of the fiber volume fractions tested. At a 5% fiber volume, the 5.0/12 fibers produced an increase over the control of 51% (2.30 MPa(m)’/2). The difference in fracture toughness between 1.5/12 and 5.0/12 fibers was statistically significant for 5% and 2% fiber content (p < O.l), but the fracture toughness values were not statistically significant for 1% fiber content. For both fiber types, there was a significant increase in fracture toughness from the 1%to the 2% fiber content; for the 5.0/12 fibers only, there was a significant increase in Kk from 2% to 5% fiber content. There was no statistical difference between the 2% and 5% volume contents for the 1.5/12 fibers. Fracture toughness results for the Type A cement reinforced with 5% of either 1.5/22 or 5.0/22 fibers, compared to the control, are given in Figure 5. For each fiber type, the fracture toughness of the reinforced cement was significantly greater than the control, 2.24 and 2.37 MPa(m)’”, respectively, representing increases of 47% and 56%. To contrast the effect of fiber addition on the two different bone cement matrices, the histogram of the KI, values for the Type B cement control and 5%-1.5/22 fiber reinforcement, together with the corresponding Type A cement control and Type A bone cement with 5%-1.5/22 fiber reinforcement, is given in Figure 6. Nonreinforced Type A bone cement showed a statistically significantly superior fracture toughness (1.52 MPa(m)In) to the nonreinforced Type B bone cement (1.22 MPa(m)In).Type B bone cement reinforced with 5%-1.5/22 fibers had a fracture toughness (2.28 MPa(m)In)significantly greater than either the control Type A or B, with a 50% fracture toughness

T

0 (control)

1

2

3

4

5

Ti fiber content (percent by volume) Figure 5. Fracture toughness of PMMA (Type A cement matrix) reinforced with 5%-1.5/12 or 5.0122 fibers. Both Rf-PMMAsshowed an increase over the control of approximately 50% (47% and 56%, respectively). There was no statistical difference in fracture toughness due to fiber length for the 22-~m-diameterfibers. Error bars represent one standard deviation.

FRACTURE TOUGHNESS/REINFORCED BONE CEMENT

-E

N-

v

a

-

3.00 2.75

I

2.50

CL

2.25

rn

2.00

E

1607

rZa 1.5/22

L

01.5/22

rn

9 0 +I

1.75 1.50

T

1.25

P)

kj

1.00

4

2

c

4

0.75 0.50 0.25

0.00 0 (control)

1

2

3

4

5

Ti fiber content (percent b y volume)

Figure 6. Fracture toughness of PMMA (both Type A and Type B cement matrices) reinforced with 5%-1.5/22 fibers. There was a significant difference between the fracture toughness of the nonreinforced bone cements. Both reinforced cements showed, again, an improvement of approximately 50% over the Type A control (the greater of the two nonreinforced cements). There was no difference, however, between the fracture toughnesses of the two cements reinforced with 5% of the same fiber type, indicating that the reinforcing effect may be independent of cement type. Error bars represent one standard deviation.

increase over the Type A, and an 86% increase over the Type B. In summary, each of the 5% RA'MMAs lead to an increase in fracture toughness ranging from 37% to 56%. DISCUSSION

The experimental data indicate that there is a trend of increasing fracture toughness with increasing fiber content for each of the fiber types tested. This variation is not surprising based on other studies where fiber volume fraction was an independent The statistical analysis validates the increases apparent in Figures 3, 4, and 5. It also appears that there is less of an increase from 2% to 5% fiber content than from 0% (control) to 2% fiber content. The values of fracture toughness for both the Type A and Type B controls are comparable to the results obtained by other investigators.1228 There was no difference in the fracture toughness of the reinforced cements at the 5% volume fraction, indicating that the reinforcement effect was independent of the bone cement matrix. The lower-viscosity matrix may, however, eventually allow the addition of greater fiber volume fractions.

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TOPOLESKI, DUCHEYNE, AND CUCKLER

The effect of fiber length on the fracture toughness of Rr-PMMA can be evaluated from the 12-pm-diameter fiber data for the two different fiber lengths: 1.5 and 5.0 mm. There was no statistically significant difference in the mean fracture toughness at 1%fiber content. Conversely, at 2% fiber content, there was a significant difference (p < 0.05); the 5.0-mm fibers produced a greater fracture toughness (2.14 MPa(m)v2) than the 1.5 mm fibers (2.01 MPa(m)m).At 5% fiber content the 5.0 mm fibers again produced a significantly greater fracture toughness (2.30 MPa(m)m)than the 1.5 mm fibers (2.09 MPa(m)lR).For the 22-pm-diameter fibers there was no difference due to fiber length. Because of the production and handling difficulties of RrPMMA in a clinical setting, finding that the 1.5/22 fibers reinforced the PMMA as well as either the 5.0/12 or the 5.0/22 fibers is advantageous for the application of Rr-PMMA; the shorter fibers are easier to incorporate. All specimens were fabricated without complication using standard clinical cement delivery systems. There are two important features of fiber length that are relevant to reinforced bone cement: critical fiber length (or aspect ratio), and handling impediment. The critical length of the fiber can be defined as the length below which further fiber fracture cannot occur. One potential reinforcing mechanism of the Ti fibers is ductile fiber fracture, and it is necessary that the fibers are initially longer than the critical length to produce fiber fracture. If fiber fracture cannot occur, then one advantage of the ductile Ti fibers has not been realized. It is not necessary for the fibers to be greater than the critical length or aspect ratio to achieve a reinforcing effect, however. The fracture toughness of bone cement can be increased by adding glass spheres (a sphere has an aspect ratio of one)." The fracture toughness increase in that study was most likely due to bead/matrix debonding. By exploiting fiber fracture, energy is absorbed in addition to fibedmatrix debonding. The critical fiber length, I,, is given by?

where D is the fiber diameter, ufis the fiber failure stress, and T is the shear yield stress of the matrix or interface. The ratio I,/D is the critical aspect ratio. Stress is transferred from the matrix to the fiber through shear at the fibedmatrix interface. The tensile stress in the fibers increases linearly from the fiber ends, and thus if the fiber is long enough, the tensile strength will reach the local fracture strength of the fiber at a flaw, and the fiber will break. The critical fiber length, I,, represents the maximum length of the fiber fragments that remain following a single fiber composite test, and the fiber lengths vary from 1,/2 to I,. The critical length is specific to fiber, matrix, and fibedmatrix interface characteristics. The importance of 1, is that if the initial short fibers are less than the critical length, fiber fracture will not occur. For reinforced bone cement, it is important to use the smallest possible fiber to produce the greatest reinforcing effect, since fibers that are too long lead to

FRACTURE TOUGHN ESS/REINFORCED BONE CEMENT

1609

handling difficulties. A fiber that is too short may not provide the optimum reinforcement to the PMMA. The reinforcing effect seen at 1%, 2% and 5% for both the 5.0-mm- and 1.5mm-long fibers, may be attributed to fiber/ matrix debonding, and does not necessarily imply fiber fracture, and thus that the fibers used were greater than the critical length for 'Ti fibers in bone cement. Fractographic analysis, discussed below, is necessary to identify fiber fracture, and thereby establish conclusively that the critical fiber length requirements were exceeded. The effect of fiber diameter was observed for the 5% fiber content. The 1.5/22 fibers produced a greater fracture toughness (2.24 MPa(m)IR)than the 1.5/12 fibers (2.09 MPa(m)In, p < 0.1). There was no difference due to diameter, however, for the 5.0-mm-long fibers. For the 5.0-mm-long fibers, the reduction in critical aspect ratio due to increasing the fiber diameter from 12 to 22 pm had no apparent effect on the fracture toughness of the Rf-PMMA. For the 1.5mm-long fibers, the reduction in fiber aspect ratio with increasing diameter seems to have increased the fracture toughness, which could be due to the increase in surface area per fiber available for debonding. Figure 7(a) shows a scanning electron micrograph of the fracture surface of a specimen reinforced with loJ0-5.0/12fibers. Several light, spermatoid anomalies stand out against the darker background of the fracture surface. Enlargements of a single anomaly [Figs. 7(b), (c)] reveal that the "head of each is the cross section of a Ti fiber. Cross sections of PMMA beads and the surrounding BaSO4-impregnated matrix are also distinguishable. The "tail" of the anomaly was produced by the crack propagating around the fiber. As the crack front progressed past the fiber, it apparently separated into two crack fronts on different planes. The "tail" formed when the two crack fronts intersected, and disappeared when the crack became uniplanar. The separation of the crack into two planes was a source of energy absorption, and one of the contributions of the fibers to the increased fracture toughness of the RJ'MMA. Figure 8 shows an enlargement of fibers in a large void. The serendipitous presence of the void allowed a unique opportunity to examine unstressed fibers in the bulk of the cement. There is continuous fibedmatrix contact, in contrast to BaS04 particles, which are not in contact with the cement, and act as void nucleation sites.327The surface roughness of the Ti fibers is also clearer. The rough Ti surface is the result of the particular drawing procedure used to manufacture the fibers. The desirable surface roughness of the Ti fibers contrasts with the relatively smooth surfaces illustrated for carbon fibers," Kevlar fibers:' or polyethylene fibers," and may provide a mechanical bond. In fact, it was stated that there was little bonding between carbon fibers and the PMMA m a t r i ~ . ~ ' The apparent mechanical interlock, or bonding, of the fibers and matrix may also be a source of energy absorption, leading to an increase in fracture toughness. In Figure 7(c), for example, there is a gap between the fiber and the adjoining matrix. Because fibedmatrix contact was demonstrated for unstressed fibers, the gaps indicate that the matrix and fibers separated, or debonded, during fracture.

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TOPOLESKI, DUCHEYNE, A N D CUCKLER

Figure 7. A series of micrographs of the Ti fiber cross sections on a fracture surface. (a) The head (h) and tail (t) of one anomaly is indicated (X80); (b) (X320) A single fiber. 'The tail seems to form through the inter-bead matrix (beads are indicated: p on right of tail, q on left of tail); (c) (X1250) enlargement of the fiber showing the debond gap (g) between the fiber and surrounding PMMA. Crack propagation is from top to bottom in each case. Cross sections of beads can be seen around the fiber on the cleaved surface typical of rapid fracture in bone cement.

Other contributions of single fibers are illustrated by an examination of corresponding fiber fragments on conjugate fracture surfaces. Figure 9 shows low-magnification micrographs of two opposite fracture surfaces (2%-1.5/22 fibers) and identifies the two conjugate fragments of a fiber that spanned the crack. The fact that two fragments of a fiber were identified on the conjugate

FRACTURE TOUGHN ESS/REIN FORC E D BONE CEMENT

1611

Figure 8. Unstressed fibers ( f ) embedded in the PMMA matrix, showing the fiber matrix contact (X1250). The combination of Ti fiber roughness and fiber-PMMA contact (c) imply that at a minimum, a mechanical interlock forms as a bonding mechanism between the fiber and the PMMA.

fracture surfaces proves that fiber fracture has occurred. Further enlargement, in Figure 10, showed the “debond” gap at the fibedmatrix interface. This particular fiber has also apparently pulled away from the matrix, in addition to debonding; the fiber surface imprint is clear in the matrix below the fiber. Also obvious are voids at the fiber end, and necking of the fiber in the vicinity of the fiber rupture, indicating plastic deformation of the fiber prior to fracture. The plastic deformation of the fiber is an additional energy absorption mechanism. Fractured carbon fibers show no apparent necking and brittle cleavage fractures.31Little energy is absorbed by a brittle fracture other than the energy required to produce the fracture; all elastic energy stored in the brittle fibers is returned to the crack. In contrast, energy used for plastic deformation is lost to the crack, and is not available for further crack propagation. Ends of fractured fibers were a frequent fractographic observation, as shown in Figure 11,for instance, thereby documenting that fracture energy absorption by fiber failure was common. Fiber fracture is direct evidence that: (a) the critical fiber length has been exceeded, and (b) ”bonding” occurs between the fiber and the matrix. Bonding in this case refers to an interaction between two materials (fiber and matrix) that allows stress transfer from one material to another. It is not known whether that interaction takes place on an atomic scale (e.g., covalent or van der Waals bonding); however, there is clear evidence for mechanical interlock of the fiber and the matrix, as well as for frictional effects, which depend on “micro” bonds at surface irregularities. Impressive increases in fracture toughness have been reported?’’ however, with fibers 10 times as long as the fibers considered in this study. Of the re-

1612

TOPOLESKI, DUCHEYNE, AND CUCKLER

Figure 9. Isolation of a single fiber that spanned the crack plane (X10) on one of the fracture surfaces (a), and located on the conjugate fracture surface (b). The local topography is used to identify the two fiber fragments as parts of the same fiber before fracture. The two surfaces, as they are pictured, fold over onto each other.

ported results with comparable fiber length, only one" achieved a larger value of fracture toughness, using Kevlare fibers. The lower fracture toughness values for bone cement reinforced with polyethylene (PE) fibers of simi' ~ be due to lar length and 2% fiber content," and 1% fiber c ~ n t e n t may negligible PE/PMMA bonding strength. At 2% fiber content, the lower fracture toughness of carbon fiber Rr-PMMA" may be attributed to the absence

FRACTURE TOUGHNESS/REINFORCED BONE CEMENT

1613

Figure 10. A further enlargement of the conjugate fibers (X1250). (a) The debond gap (g) is apparent; also in the conjugate, (b). Fiber necking (n) is evident, and voids (v) are visible in the fiber tips. Both the necking and void formation phenomena indicate plastic deformation of the fiber prior to rupture.

of plastic deformation, that has been shown for the Ti fibers, prior to fracture of the carbon fibers. Fracture toughness values from the current study for various fiber contents are listed, together with a summary of the data from other studies, in Table 11.

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TOPOLESKI, DUCHEYNE, AND CUCKLER

Figure 11. Examples of fiber fracture for each of the 22-pm fiber types; fractured fibers were commonly observed. (a) Enlargement of the fracture site of a 1.5/22 fiber (X2500), showing necking (n) and voids (v). (b) 5.0/22 fiber fracture (X1250). The "tracks" (t) left by the pull-out of the other fragment of the fiber are visible.

In addition to the fundamental scientific aspects of fiber reinforced bone cement failure resistance, practicality of fiber incorporation into bone cement during surgery is an essential consideration. Addition of the fibers used in this study was not difficult; the principal contrast between the delivery of nonreinforced and reinforced bone cement was the increase in apparent vis-

FRACTURE TOUGHNESS/REINFORCED BONE CEMENT

1615

TABLE I1 Summary of Comparable Rf-PMMA Fracture Toughness Tests

Material Ti

Kevlar@ KevlaP Kevlar' Ti

Carbon Ti

PE

Fiber Content (vol%)

Fiber Length (mm)

(MPa(m)*)

Reference

5% 5.17% 5.17% 5.17% -5% 2% 2% 1% 1%

1.5,5.0 50 13 3.2 3.2 1.5 1.5 5.0 6.0

2.09-2.37 4.43 2.85 1.96-2.61 1.81 2.01 1.61-1.88 1.91 1.50-1.55

This study

Kk

8 11 15 18 This study 12 This study 17

cosity inherent in adding an additional phase to the bone cement. Because the fiber-reinforced bone cement was introduced into the specimen molds using standard clinical equipment and practices, the cement appears workable "as is," and should be readily optimized (with respect to the number of fibers per bundle, fiber dimensions, initial bone cement viscosity, etc.) for uncomplicated surgical delivery. The improvement of fracture toughness of bone cement is an important step in extending the functional life of cemented total joint prostheses. Fracture toughness is a material property that is predictive of failure, and is relatively easy to determine. Fracture toughness is a static mechanical property, however, and there is evidence documenting that fatigue failure is a predominant in v i m failure mode.3 Improvements in PMMA bone cement must address critical failure modes, therefore the fatigue performance of the Ti fiber reinforced bone cement also needs to be determined.

CONCLUSIONS

The study of the fracture toughness of Ti fiber reinforced PMMA, and the accompanying statistical analysis leads to the following conclusions: 1. PMMA reinforced with 1%, 2%, or 5% of the 1.5/12, 5.0/12, 1.5/22, or 5.0/22 Ti fibers resulted in a significant increase in fracture toughness over non-reinforced Type A or Type B bone cements. 2. Increasing the fiber content from 1%to 5%produces a corresponding increase in fracture toughness of the Rf-PMMA. 3. Fractured Ti fibers are evidence that the fiber dimensions used are greater than the critical fiber dimensions (length or aspect ratio) necessary to produce a reinforcing effect on PMMA. The fractographic analysis of the fracture toughness specimens revealed several important phenomena related to the reinforcing mechanisms of Tifiber-reinforced PMMA: 1. Unstressed Ti fibers and the PMMA matrix are in direct contact. 2. Ti fibers separate or debond from the matrix during specimen fracture.

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TOPOLESKI, DUCHEYNE, A N D CUCKLEIi

3. Ti fibers sustain plastic deformation and ductile rupture during specimen fracture. 4.Ti fibers change the plane of a crack advancing through the PMMA. The four observations are indicative of the mechanisms of energy absorption inherent in the Ti-fiber-reinforced bone cement. Ti fibers provided the additional energy absorption mechanism of plastic fiber deformation, which is apparently absent in carbon, Kevlarm, or polyethylene. The Authors with to express their gratitude to Howmedica, Inc. and Rekaert, Inc. for donations of some of the materials used in this study. T h e help of Dr. Alex liadin, Director of the LRSM iMaterials Testing Lab, and IMS. Debbie Ricketts-Foot, Supervisor of the LRSM Electron Microscopy Facilily, is greatly appreciated (the LRSM facilities of the University of Pennsylvania a r e supported by NSF Grant #DMR88-19885).

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Received September 24, 1991 Accepted April 24, 1992