Bending Properties of Wire-Reinforced Bone Cement for Applications in Spinal Fixation* SUBRATA SAHA and MATTHEW J. KRAAY,? Biomechanics Laboratory, L S U Medical Center, Shreueport, Louisiana 71130
Summary PMMA beam specimens were tested in four-point bending to determine if‘ the bending strength of acrylic bone cement, as used in posterior spinal fusion, could be improved by metal-wire reinforcement. The result showed that the load-carrying capacities of 1- and 0.5-mm diam stainless-steel-wire-reinforced PMMA specimens in bending were significantly higher than similar unreinforced normal PMMA samples. On an average, steel reinforcement comprising approximately 1%of the cross-sectional area of the PMMA specimens caused a 15%increase in bending strength. Even after the cement fractured, the reinforcing wires still sustained an appreciable amount of bending moment, thus preventing catastrophic failure of cement alone.
INTRODUCTION The use of acrylic bone cement (PMMA) in orthopedic surgery has increased markedly in the last decade. However, its brittle character has restricted its use to areas mainly subjected to compressive forces. For instance, Charnley has mentioned that “It is important that it should always be used under compressive loads and not be exposed to tension or bending.” This limitation in the use of PMMA may be overcome by reinforcing it with metal wires, similar to the use of steel rods in reinforced concrete. Wire-reinforced PMMA has been used by several investigators to obtain immediate stabilization in clinical situations such as posterior spinal fusion.2.3 However, little information is available on the effectiveness of such metal-wire reinforcement in improving the mechanical properties of PMMA. Such biomechanical evaluations are * Presented in part a t the Thirty-first Annual Conference on Engineering in Biology and Medicine, Atlanta, October 1978. + Presently a graduate student a t the University of Michigan, Ann Arbor, Michigan. Journal of Biomedical Materials Research, Vol. 13,443-457 (1979) 0021-9304/79/0013-0443$01.00 01979 John Wiley & Sons, Inc.
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essential before the wire-reinforced bone cement could be used rationally as an adjunct in spinal fixation.2 During flexion and extension, the spinal elements are subjected to considerable amounts of bending and associated tensile and compressive loads; therefore, any wire-reinforced bone-cement construct, used as an adjunct for spinal fixation, will also be subjected to these forces. In a previous study, it has been shown that the tensile strength of PMMA can be significantly improved by the addition of vitallium and stainless-steel wires.4 The objective of this investigation was to examine whether the strength characteristics of bone-cement bending specimens could similarly be improved by metal-wire reinforcement.
MATERIALS AND METHODS Machined Teflon molds, as shown in Figure 1,were used to manufacture standardized rectangular bending specimens of nominal dimensions 15 mm in width, 6 mm in depth and 114 mm in length. The PMMA used was Surgical Simplex P Radiopaque Bone Cement
Fig. 1. Teflon molds for preparing standardized rectangular bending specimens of PMMA. Note that the end piece has slots for accurate positioning of' reinforcing wires.
RENDING PROPERTIES OF BONE CEMENT
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(marketed by Howmedica Medical Division, Rutherford, N.J. 07070). The prepackaged recommended mixtures of 40 g of powdered polymer and 20 ml of liquid monomer were mixed at room temperature (68°F) in a stainless-steel bowl using a tongue blade as recommended by the manufacturer. Surgical-grade stainless-steel 316 wires were used to reinforce the specimens. The following procedure was used in making the beam samples: first, the powder and then the liquid components were placed in the mixing bowl with stirring of the resulting mass continuing until the two components were well blended. With this resultant still capable of flowing, the liquid was poured and worked into the molds, leaving a smooth, flat layer approximately 2 mm from the top of the mold. This was done for all molds before placing any wires. While the cement was being put into other molds, any large air bubbles in the cement previously placed in molds usually rose to the top and these were removed with the help of the tongue blade. The wires used for reinforcement were straightened from spools and then placed in the grooved end pieces. These end pieces maintained uniform wire spacing across the width of the beam and an approximately constant depth of 2 mm from the top of the mold to the center of the wires (Fig. 1). The wires were handled while wearing surgical gloves so that wire-PMMA bonding was not affected by skin oils or perspiration. With the reinforcing wires in place, a covering layer of cement (still workable) was applied over the wires and a cover of Teflon was placed on the mold to flatten the top side of the beam. A 5-lb weight was placed on the cover to obtain a smooth surface and any excess cement was allowed to exit the mold through a gap between the cover and the end piece. Following polymerization of the cement, the beams were removed from the molds and the specimens were immersed in Ringer’s Solution for about 12-15 hr before mechanical testing to simulate the in vivo condition. In addition to unreinforced controls, specimens were reinforced with surgical-grade stainless-steel 316 wires and divided into five additional groups: (1) 1.0-mm-diam one-wire reinforced, (2) 1.0mm-diam two-wire reinforced, (3) 0.5-mm-diam three-wire reinforced, (4)0.5-mm-diam four-wire reinforced, and (5) 0.5-mm-diam five-wire reinforced. The samples were tested in the wet condition, using a floor-model Instron testing machine. Employment of a parallel-plate arrangement, shown in Figure 2, allowed the use of a Tensile Load Cell (type
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Fig. 2. The Instron testing machine and the parallel-plate arrangement for bending test of PMMA specimens.
D) for measurement of bending loads. Prior to testing, the load cell was checked for linearity and calibrated after a warm-up period of 30 min. The samples were tested in four-point bending with a span ( L )of 102 mm and the loading points located at a distance of L14 from each end support (Fig. 3). The four-point bending mode was chosen instead of the three-point bending, so that the central region was subjected to a pure uniform bending moment without any shearing force.5 PMMA is weak under tension, similar to concrete; therefore, as in reinforced concrete,6 the wires were placed on the tension side of the beam (Fig. 3). The specimens were loaded a t a cross-head speed of 12 mmlmin and the load-deformation curves were continuously recorded. Following testing of specimens, the fracture surface of the beams were inspected for significant voids, laminations, and irregularities.
BENDING PROPERTIES OF BONE CEMENT
447
P
Fig. 3. Schematics of the four-point bending test showing detail configuration (specimen width 15 mm).
Voids occurring on the compression side of the neutral axis generally did not contribute to the failure of the beams, since for both the unreinforced PMMA and the reinforced composite, the failure was initiated in tension. Samples with voids greater than 1.5 mm were disregarded from further analysis. Approximately one such sample was rejected from each group of specimens that were tested. Fracture load, as well as the load carried after fracture, was obtained directly from the loading curves. Energy absorption capacity was calculated by measuring the area under the load-deformation curve using a planimeter, and the deflection at the loading supports was calculated from the known cross-head speed and chart speed, assuming no slippage. For calculation of failure stresses, actual dimension of each specimen at the fracture site were measured with a micrometer. The mean depth of the reinforcing wires for each specimen was also determined, using a depth gauge. The mean dimensions for each specimen group were used in the calculation of failure stresses.
RESULTS Typical load-deformation curves for unreinforced and stainlesssteel-wire-reinforced specimens appear in Figures 4 and 5. As shown in these figures, the load increased linearly a t the beginning and a small amount of yielding occurred prior to failure. Figures 4 and 5 also show that, compared to unreinforced control groups, the maximum load and maximum deflection both improved for all groups by
SAHA AND KRAAY
448 100
/--
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-3
300
a U 0
-I
200
2WlRES
I WIRE __
IOC
UNRE INFORCED
1.0
2.0
4.0 5.0 DEFORMATION (m rn)
3.0
6.0
7:O
Fig. 4. Load-deformation curves for unreinforced control and 1.0-mm reinforced specimens.
the addition of metal reinforcement. Moreover, while the normal PMMA specimens failed catastrophically when maximum load was reached, for reinforced samples, the wires continued to sustain a significant portion of the maximum load even after fracture of the cement. This is surely an important advantage of using reinforced bone cement in clinical situations. The mean values and standard deviations of the mechanical properties in bending, as calculated from the load-deformation curves, are summarized in Table I. The bending moment ( M ) resisted by the beam specimens was directly proportional to these load values ( P )and can be calculated by5
M
= PLI8
where A4 = bending moment (Nm); P = load (N) as shown in Table I; and L = span of the beam = 0.102 m. Table I, as well as Figure 4,shows that, for specimens with 1.0mm-diam reinforcing wires, the maximum load or bending moment and maximum deformation both increased significantly ( p < 0.001) when the number of reinforcing wires was increased from one to two. However, for specimens with 0.5-mm-diam reinforcing wires, increasing the number of wires from three to four and five produced the
BENDING PROPERTIES OF BONE CEMENT
449
SO0
400
c
3
30C
0
4 0 J
200 5 WIRES 4 WIRES 3 WIRES
100
ONREINFORCED
- . 1.0
2.0
).O 4:O 5-0 DEFORMATION ( m m )
.o
r.0
Fig. 5. Load-deformation curves for unreinforced control and 0.5-mm reinforced specimens.
surprising result of a decrease in the maximum load (Table I and Fig. 5). To explain this apparently anomalous behavior of 0.5-mm-diam stainless-steel-wire-reinforced specimens, the cross section of each such sample and the relative position of reinforcing wires were measured and maximum bending stresses at the point of fracture were calculated. The results are tabulated in Table 11. This calculation was based on composite beam theory, assuming linear elastic behavior In this calculation, the modfor both PMMA and stainless ~tee1.5.~ ulus of elasticity of reinforcing stainless steel was assumed to be 200 GN/m2 and the flexure modulus for PMMA was assumed to be 2.2 GN/m2, following the work of Knoell et al.7 As shown in Tables I and 11, although the specimens with four and five reinforcing wires sustained lower loads than the specimens with three wires, the differences in the failure stresses were quite small. Thus a large part of the lower load-carrying capacities of the four and five wire specimens could be accounted for by their lower heights. As the strength of a beam depends on its section modulus, which in turn depends on the square of the beam depth, small changes in the depth of a beam could significantly affect the load-carrying capacity of a beam.
TABLE I
0.5 0.5 0.5 1.0 1.o
B C D E F
3 4 5 1 2 11 11 11 11 11
12
Number of Specimens
6.06 f 0.61 6.04 f 0.52 6.43 f 0.65b 6.57 f 0.45" 6.92 f 0.65',*
468.4 f 27.8b 456.4 f 35.6b 428.4 f 35.OC 495.5 f 30.2b3e
5.86 f 0.67
Maximum Deflection" (mm)
495.3 f 43.5b
405.5 f 36.1
Maximum Load (N)
1.57 f 0.24b 1.65 f 0.30h 1.54 f 0.22h 1.88 f 0.29b.e
1.65 f 0.31b
1.26 f 0.30
a
137.08 f 21.63d 163.15 f 35.45 95.64 f 23.40 180.06 f 37.92e
89.65 f 25.11
0
Fracture Energy Load after Fracture (4 (Ni
Deflection of the loading points. p < 0.001 comparing reinforced specimens with unreinforced controls. p < 0.01 comparing reinforced specimens with unreinforced controls. p < 0.001 comparing reinforced specimens with those with one less 0.5-mm strand. e p < 0.001 comparing two I-mm wire-reinforced specimen with that reinforced with one 1-mm wire. p < 0.01 comparing two 1-mm wire-reinforced specimen with that reinforced with one 1-mm wire.
Unreinforced (control)
A
Sample Group
Reinforcement Diam No. of (mm) Wires
Mean f 1Standard Deviation of the Bending Properties of Bone Cement Specimens Reinforced with Stainless-Steel Wires
4
P P
?J
P
z U x
5
4 u1 0
d
C
a b
Grout, Sample
15.1 15.0 15.1 15.5
Width, b 6.3 6.6 6.4 6.1
Height, h
Specimen Cross Section (mm)
Unreinforced 3 4 5
No. of Wires
Depth, Y (Control) 4.55 4.62 4.47
Reinforcement (0.5-mm diam)
35.8
1.8 43.7 37.7
Tensile
51.8 57.2 55.1 56.4
Compressive
Failure Stresses in Bending (MN/m2j
TABLE I1 Calculated Mean Failure Stresses of 0.5-mm-diam Stainless-Steel-Wire-Reinforced Bone Cement Specimens Tested in Bending
c”
4
5
5
m
c)
5
0
W
2
F3 0)
2
n
%
P
cd
s 3
SAHA AND KRAAY
452
The bending strength of the unreinforced control group is compared with the reinforced ones in Figure 6. It shows that addition of one and two 1.0-mm-diam reinforcing wires increased the load-carrying capacities (or bending moments) of normal PMMA samples by 5.1 and 22.2%, respectively. Similarly, addition of three, four, and five 0.5-mm-diam reinforcing wires improved the resisting bending moments (or loads) by 22.2,15.5 and 12.6%,respectively. However, this also indicates that, for a standard-size specimen, too many reinforcing wires may actually cause a decrease in the bending strength, as pointed out before. It is also evident from Figure 6 that even after fracture of the PMMA, a considerable portion of the maximum load was still sustained by the reinforcing wires alone and, for each group, this loadcarrying capacity was directly related to the number of reinforcing wires. For instance, the loads borne by the three, four, and five 0.5-mm wires just after fracture of the cement were 18,29, and 36% of the maximum loads supported by those groups, respectively. Similarly, one or two 1.0-mm-diam wires, after fracture of the cement,
5oot
(1.0mml
Fig. 6. Comparison of the maximum load-carrying capacities of unreinforced (1st bar graph) and stainless-steel-wire-reinforced PMMA specimens in bending. The number of reinforcing wires is shown below the horizontal axis a t the bottom of each bar graph and the diameter of the wires are shown within parentheses. The load supported by the specimens after initial fracture of PMMA is shown by dotted lines. The mean value of the maximum load and the load carried after fracture for each group is shown within the bar graph a t its middle and bottom, respectively.
BENDING PROPERTIES OF BONE CEMENT
453
still sustained 22 and 36% of the maximum loads supported by the intact specimens, respectively (Table I and Fig. 5). Energy absorption capacity or fracture energy is an important criterion in determining the resistance of a material against failure. The fracture energy of PMMA specimens was calculated by measuring the area under the load-deformation curves up to the point of maximum load. The energy absorption capacity of the reinforced samples are compared with the control group in Figure 7. As in the case of maximum load, here also there was a significant increase ( p < 0.001) in the fracture energy of all reinforced specimens compared to the unreinforced ones. The amount of increase was 22.2 and 49.2% for the specimens with one and two strands of 1.0-mm-diam reinforcement, respectively. Similarly, addition of three, four, and five strands of 0.5-mm-diam wires produced 31.0,24.6, and 31.0%increases in the fracture energy, compared to the control group. The fracture energy also increased from 1.54 to 1.88J, a significant increase ( p < 0.01), when the number of 1.0-mm-diam reinforcing wires was increased from one to two. However, for 0.5-mm-diam reinforcing wires, the fracture energy remained the same for three and five wire groups and decreased slightly for the group with four reinforcing wires, although these variations were not significant.
T
!j 0
r: 157 1.65
I
4
5
Fig. 7. Comparison of the energy absorption capacities of the unreinforced and stainless-steel-wire-reinforced bone cement specimens in bending. The mean value of the fracture energy in joules is shown within each bar graph. The number of reinforcing wires for each group is shown below the horizontal axis at the bottom of each bar graph and the diameter of the wires are shown within parentheses.
SAHA AND KRAAY
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Maximum deformation or deflection indicates the pliability or ductility of a beam. A comparison of the maximum deformation of the loading points of the tested specimens at the point of fracture is shown in Figure 8. As shown in this figure and in Table I, there was an increase in the deformation capacity of all reinforced specimens and the increase was highly significant ( p < 0.001) for all reinforced PMMA samples except for the three and four 0.5-mm wire groups.
DISCUSSION As can be seen from Table I, addition of wire reinforcement produced significant improvements in the tolerance limits of polymethyl methacrylate beam specimens. However, more reinforcement did not necessarily increase the strength further. Contrary to expectation, addition of 0.5-mm reinforcement from 89
-
79 -
Fig. 8. Comparison of the maximum deflections of the normal and stainlesssteel-wire-reinforced PMMA beam specimens. The deformation was measured as the deflection of the loading points from its undeformed position. The mean value of the deformation in millimeter is shown within each bar graph. The number of reinforcing wires is shown below the horizontal axis at the bottom of each bar graph and the diameter of the reinforcement is shown within parentheses.
BENDING PROPERTIES OF BONE CEMENT
455
three to four and five wires resulted in a reduction in the maximum load-carrying capacity of the beam specimens. As mentioned before, most of this reduction could be attributed to the reduced depth of specimens with four and five reinforcing wires. The small decrease in the failure stresses (Table 11) may be a result of several factors which are important in the fabrication of the specimens. As the number of reinforcing wires increased, placement of the wires became more difficult and the specimens required additional compaction. During compaction, the 0.5-mm wires, offering little resistance, were easily displaced from their nominal positions. The relatively small size of the specimens and the short working time of the cement compounded these problems to yield beams of reduced uniformity in comparison to the three-wire samples. However, the load carried after fracture was consistent with the expectation of increased load with more reinforcing wires. The results of 0.5-mm-diam reinforced specimens are a t variance with findings of a previous study which indicated that the tensile strength of wire-reinforced cement increased with the increasing number of reinforcing wires.4 This is not surprising because, when tested in tension, the continuous wires and the cement both were subjected to the same longitudinal strain; therefore, the cement and the wires sustained loads in parallel independent of any bond between these two phases. However, when such a composite structure is subjected to bending, the tensile stress in the wire has to be transferred to the cement by means of interfacial shear stress.6 Thus, the strength of the composite beam depends on the bond between the cement and the wire. As the number of wires is increased, the gap between the wires is reduced and the shear-stressed regions of cement surrounding each wire is overlapped, which might cause a decrease in the strength of the beam. This might be a mechanism contributing to the decreased failure stress of specimens with four and five reinforcing wires. Tests using 1.0-mm wires for reinforcement were undertaken so that fewer wires could be used and compaction then would not as easily displace the larger wires. Testing of the 1.0-mm-wire-reinforced samples provided the results expected. Addition of reinforcement increased the strength of the composite material with respect to d of the properties investigated: maximum load, maximum deformation, fracture energy, and load carried after fracture. Note that while the single 1.0-mm wire has the same cross-sectional area
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SAHA AND KRAAY
as the four 0.5-mm wires, the four-wire composite is clearly stronger (Table I). This might be attributed to a possible increase in PMMA metal bonding at the four 0.5-mm wire interfaces, a result of the above having twice the surface area for bonding as does the single 1.0-mm wire. The results of this investigation are in general agreement with the previous studies on the tensile and shear properties of reinforced PMMA.4,s For instance, the shear strength of 0.5-mm-diam metalwire-reinforced bone cement specimens improved when the number of reinforcing wires was increased from two to three, but the strength decreased slightly when the number of reinforcing wires was increased further to four and five strands.8 Assuming linear elastic behavior, the mean ultimate bending stress for the unreinforced control group of PMMA was calculated to be 51.8 MN/m2 (Table 11). This is in agreement with the maximum flexure stress of 50.0 MN/m2 (reported by Knoell et al.7) and 49.3 to 56.6 MN/m2 for Simplex P radiopaque bone cement (determined by ~olmg).
CONCLUSION The result of this study shows that the load-carrying capacity of acrylic bone cement in bending can be improved considerably by addition of reinforcing metal wires. Although there were wide variations, on an average, steel reinforcement comprising approximately 1%of the cross-sectional area of the bone cement specimen increased the bending strength by approximately 15%. The enhanced strength of reinforced bone cement will allow its use in areas in which tensile and bending forces are also dominant, such as in posterior spinal fusion. Of equal importance is the obvious significance of a load carried by the reinforcing wires after fracture of the reinforced cement while the unreinforced polymethyl methacrylate fails catastrophically. For optimal increase in its strength, the wires should be placed where tensile stresses are maximum and as far away from the neutral axis as possible, as is commonly done in reinforced concrete beam design. Although the bending strength increases in the beginning in proportion to the number of reinforcing wires, one should be careful about incorporating too many wires in a specimen, as this may cause difficulty in compaction of the cement and displacement of the wires during compaction, which ultimately may reduce the strength.
BENDING PROPERTIES OF BONE CEMENT
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The authors wish to thank Howmedica, Inc., for supplying the acrylic bone cement. Thanks are also due to the National Science Foundation for supporting this work through their Undergraduate Research Participation program.
References 1. J. Charnley, Acrylic Cement in Orthopaedic Surgery, Williams and Wilkins, Baltimore, Maryland, 1970, p. 93. 2. E. J. Dunn, Spine, 2,15 (1977). 3. W. B. Scoville, A. H. Palmer, K. Samra, and G. Chong, J . Neurosurg., 27, 274 (1967). 4. J. P. Taitsman and S. Saha, J. Bone Jt. Surg., 59-A,419 (1977). 5. S. P. Timoshenko and J. M. Gere, Mechanics of Materials, Van Nostrand, New York, 1972. 6. B. Bresler, Reinjorced Concrete Engineering, Vol. 1& 2, Wiley, New York, 1974. 7. A. Knoell, H. Maxwell, and C. Bechtol, Ann. Biorned. Eng., 3,225 (1975). 8. S. Saha and M. L. Warman, Trans. Orthop. Res. SOC.,24th Ann. Meet., 3, 299 (1978). 9. N. J. Holm, Acta Orthop. Scand., 48,436 (1977).
Received July 31,1978 Revised November 8,1978
Summary PMMA beam specimens were tested in four-point bending to determine if‘ the bending strength of acrylic bone cement, as used in posterior spinal fusion, could be improved by metal-wire reinforcement. The result showed that the load-carrying capacities of 1- and 0.5-mm diam stainless-steel-wire-reinforced PMMA specimens in bending were significantly higher than similar unreinforced normal PMMA samples. On an average, steel reinforcement comprising approximately 1%of the cross-sectional area of the PMMA specimens caused a 15%increase in bending strength. Even after the cement fractured, the reinforcing wires still sustained an appreciable amount of bending moment, thus preventing catastrophic failure of cement alone.
INTRODUCTION The use of acrylic bone cement (PMMA) in orthopedic surgery has increased markedly in the last decade. However, its brittle character has restricted its use to areas mainly subjected to compressive forces. For instance, Charnley has mentioned that “It is important that it should always be used under compressive loads and not be exposed to tension or bending.” This limitation in the use of PMMA may be overcome by reinforcing it with metal wires, similar to the use of steel rods in reinforced concrete. Wire-reinforced PMMA has been used by several investigators to obtain immediate stabilization in clinical situations such as posterior spinal fusion.2.3 However, little information is available on the effectiveness of such metal-wire reinforcement in improving the mechanical properties of PMMA. Such biomechanical evaluations are * Presented in part a t the Thirty-first Annual Conference on Engineering in Biology and Medicine, Atlanta, October 1978. + Presently a graduate student a t the University of Michigan, Ann Arbor, Michigan. Journal of Biomedical Materials Research, Vol. 13,443-457 (1979) 0021-9304/79/0013-0443$01.00 01979 John Wiley & Sons, Inc.
444
SAHA AND KRAAY
essential before the wire-reinforced bone cement could be used rationally as an adjunct in spinal fixation.2 During flexion and extension, the spinal elements are subjected to considerable amounts of bending and associated tensile and compressive loads; therefore, any wire-reinforced bone-cement construct, used as an adjunct for spinal fixation, will also be subjected to these forces. In a previous study, it has been shown that the tensile strength of PMMA can be significantly improved by the addition of vitallium and stainless-steel wires.4 The objective of this investigation was to examine whether the strength characteristics of bone-cement bending specimens could similarly be improved by metal-wire reinforcement.
MATERIALS AND METHODS Machined Teflon molds, as shown in Figure 1,were used to manufacture standardized rectangular bending specimens of nominal dimensions 15 mm in width, 6 mm in depth and 114 mm in length. The PMMA used was Surgical Simplex P Radiopaque Bone Cement
Fig. 1. Teflon molds for preparing standardized rectangular bending specimens of PMMA. Note that the end piece has slots for accurate positioning of' reinforcing wires.
RENDING PROPERTIES OF BONE CEMENT
445
(marketed by Howmedica Medical Division, Rutherford, N.J. 07070). The prepackaged recommended mixtures of 40 g of powdered polymer and 20 ml of liquid monomer were mixed at room temperature (68°F) in a stainless-steel bowl using a tongue blade as recommended by the manufacturer. Surgical-grade stainless-steel 316 wires were used to reinforce the specimens. The following procedure was used in making the beam samples: first, the powder and then the liquid components were placed in the mixing bowl with stirring of the resulting mass continuing until the two components were well blended. With this resultant still capable of flowing, the liquid was poured and worked into the molds, leaving a smooth, flat layer approximately 2 mm from the top of the mold. This was done for all molds before placing any wires. While the cement was being put into other molds, any large air bubbles in the cement previously placed in molds usually rose to the top and these were removed with the help of the tongue blade. The wires used for reinforcement were straightened from spools and then placed in the grooved end pieces. These end pieces maintained uniform wire spacing across the width of the beam and an approximately constant depth of 2 mm from the top of the mold to the center of the wires (Fig. 1). The wires were handled while wearing surgical gloves so that wire-PMMA bonding was not affected by skin oils or perspiration. With the reinforcing wires in place, a covering layer of cement (still workable) was applied over the wires and a cover of Teflon was placed on the mold to flatten the top side of the beam. A 5-lb weight was placed on the cover to obtain a smooth surface and any excess cement was allowed to exit the mold through a gap between the cover and the end piece. Following polymerization of the cement, the beams were removed from the molds and the specimens were immersed in Ringer’s Solution for about 12-15 hr before mechanical testing to simulate the in vivo condition. In addition to unreinforced controls, specimens were reinforced with surgical-grade stainless-steel 316 wires and divided into five additional groups: (1) 1.0-mm-diam one-wire reinforced, (2) 1.0mm-diam two-wire reinforced, (3) 0.5-mm-diam three-wire reinforced, (4)0.5-mm-diam four-wire reinforced, and (5) 0.5-mm-diam five-wire reinforced. The samples were tested in the wet condition, using a floor-model Instron testing machine. Employment of a parallel-plate arrangement, shown in Figure 2, allowed the use of a Tensile Load Cell (type
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SAHA AND KRAAY
Fig. 2. The Instron testing machine and the parallel-plate arrangement for bending test of PMMA specimens.
D) for measurement of bending loads. Prior to testing, the load cell was checked for linearity and calibrated after a warm-up period of 30 min. The samples were tested in four-point bending with a span ( L )of 102 mm and the loading points located at a distance of L14 from each end support (Fig. 3). The four-point bending mode was chosen instead of the three-point bending, so that the central region was subjected to a pure uniform bending moment without any shearing force.5 PMMA is weak under tension, similar to concrete; therefore, as in reinforced concrete,6 the wires were placed on the tension side of the beam (Fig. 3). The specimens were loaded a t a cross-head speed of 12 mmlmin and the load-deformation curves were continuously recorded. Following testing of specimens, the fracture surface of the beams were inspected for significant voids, laminations, and irregularities.
BENDING PROPERTIES OF BONE CEMENT
447
P
Fig. 3. Schematics of the four-point bending test showing detail configuration (specimen width 15 mm).
Voids occurring on the compression side of the neutral axis generally did not contribute to the failure of the beams, since for both the unreinforced PMMA and the reinforced composite, the failure was initiated in tension. Samples with voids greater than 1.5 mm were disregarded from further analysis. Approximately one such sample was rejected from each group of specimens that were tested. Fracture load, as well as the load carried after fracture, was obtained directly from the loading curves. Energy absorption capacity was calculated by measuring the area under the load-deformation curve using a planimeter, and the deflection at the loading supports was calculated from the known cross-head speed and chart speed, assuming no slippage. For calculation of failure stresses, actual dimension of each specimen at the fracture site were measured with a micrometer. The mean depth of the reinforcing wires for each specimen was also determined, using a depth gauge. The mean dimensions for each specimen group were used in the calculation of failure stresses.
RESULTS Typical load-deformation curves for unreinforced and stainlesssteel-wire-reinforced specimens appear in Figures 4 and 5. As shown in these figures, the load increased linearly a t the beginning and a small amount of yielding occurred prior to failure. Figures 4 and 5 also show that, compared to unreinforced control groups, the maximum load and maximum deflection both improved for all groups by
SAHA AND KRAAY
448 100
/--
40C
-3
300
a U 0
-I
200
2WlRES
I WIRE __
IOC
UNRE INFORCED
1.0
2.0
4.0 5.0 DEFORMATION (m rn)
3.0
6.0
7:O
Fig. 4. Load-deformation curves for unreinforced control and 1.0-mm reinforced specimens.
the addition of metal reinforcement. Moreover, while the normal PMMA specimens failed catastrophically when maximum load was reached, for reinforced samples, the wires continued to sustain a significant portion of the maximum load even after fracture of the cement. This is surely an important advantage of using reinforced bone cement in clinical situations. The mean values and standard deviations of the mechanical properties in bending, as calculated from the load-deformation curves, are summarized in Table I. The bending moment ( M ) resisted by the beam specimens was directly proportional to these load values ( P )and can be calculated by5
M
= PLI8
where A4 = bending moment (Nm); P = load (N) as shown in Table I; and L = span of the beam = 0.102 m. Table I, as well as Figure 4,shows that, for specimens with 1.0mm-diam reinforcing wires, the maximum load or bending moment and maximum deformation both increased significantly ( p < 0.001) when the number of reinforcing wires was increased from one to two. However, for specimens with 0.5-mm-diam reinforcing wires, increasing the number of wires from three to four and five produced the
BENDING PROPERTIES OF BONE CEMENT
449
SO0
400
c
3
30C
0
4 0 J
200 5 WIRES 4 WIRES 3 WIRES
100
ONREINFORCED
- . 1.0
2.0
).O 4:O 5-0 DEFORMATION ( m m )
.o
r.0
Fig. 5. Load-deformation curves for unreinforced control and 0.5-mm reinforced specimens.
surprising result of a decrease in the maximum load (Table I and Fig. 5). To explain this apparently anomalous behavior of 0.5-mm-diam stainless-steel-wire-reinforced specimens, the cross section of each such sample and the relative position of reinforcing wires were measured and maximum bending stresses at the point of fracture were calculated. The results are tabulated in Table 11. This calculation was based on composite beam theory, assuming linear elastic behavior In this calculation, the modfor both PMMA and stainless ~tee1.5.~ ulus of elasticity of reinforcing stainless steel was assumed to be 200 GN/m2 and the flexure modulus for PMMA was assumed to be 2.2 GN/m2, following the work of Knoell et al.7 As shown in Tables I and 11, although the specimens with four and five reinforcing wires sustained lower loads than the specimens with three wires, the differences in the failure stresses were quite small. Thus a large part of the lower load-carrying capacities of the four and five wire specimens could be accounted for by their lower heights. As the strength of a beam depends on its section modulus, which in turn depends on the square of the beam depth, small changes in the depth of a beam could significantly affect the load-carrying capacity of a beam.
TABLE I
0.5 0.5 0.5 1.0 1.o
B C D E F
3 4 5 1 2 11 11 11 11 11
12
Number of Specimens
6.06 f 0.61 6.04 f 0.52 6.43 f 0.65b 6.57 f 0.45" 6.92 f 0.65',*
468.4 f 27.8b 456.4 f 35.6b 428.4 f 35.OC 495.5 f 30.2b3e
5.86 f 0.67
Maximum Deflection" (mm)
495.3 f 43.5b
405.5 f 36.1
Maximum Load (N)
1.57 f 0.24b 1.65 f 0.30h 1.54 f 0.22h 1.88 f 0.29b.e
1.65 f 0.31b
1.26 f 0.30
a
137.08 f 21.63d 163.15 f 35.45 95.64 f 23.40 180.06 f 37.92e
89.65 f 25.11
0
Fracture Energy Load after Fracture (4 (Ni
Deflection of the loading points. p < 0.001 comparing reinforced specimens with unreinforced controls. p < 0.01 comparing reinforced specimens with unreinforced controls. p < 0.001 comparing reinforced specimens with those with one less 0.5-mm strand. e p < 0.001 comparing two I-mm wire-reinforced specimen with that reinforced with one 1-mm wire. p < 0.01 comparing two 1-mm wire-reinforced specimen with that reinforced with one 1-mm wire.
Unreinforced (control)
A
Sample Group
Reinforcement Diam No. of (mm) Wires
Mean f 1Standard Deviation of the Bending Properties of Bone Cement Specimens Reinforced with Stainless-Steel Wires
4
P P
?J
P
z U x
5
4 u1 0
d
C
a b
Grout, Sample
15.1 15.0 15.1 15.5
Width, b 6.3 6.6 6.4 6.1
Height, h
Specimen Cross Section (mm)
Unreinforced 3 4 5
No. of Wires
Depth, Y (Control) 4.55 4.62 4.47
Reinforcement (0.5-mm diam)
35.8
1.8 43.7 37.7
Tensile
51.8 57.2 55.1 56.4
Compressive
Failure Stresses in Bending (MN/m2j
TABLE I1 Calculated Mean Failure Stresses of 0.5-mm-diam Stainless-Steel-Wire-Reinforced Bone Cement Specimens Tested in Bending
c”
4
5
5
m
c)
5
0
W
2
F3 0)
2
n
%
P
cd
s 3
SAHA AND KRAAY
452
The bending strength of the unreinforced control group is compared with the reinforced ones in Figure 6. It shows that addition of one and two 1.0-mm-diam reinforcing wires increased the load-carrying capacities (or bending moments) of normal PMMA samples by 5.1 and 22.2%, respectively. Similarly, addition of three, four, and five 0.5-mm-diam reinforcing wires improved the resisting bending moments (or loads) by 22.2,15.5 and 12.6%,respectively. However, this also indicates that, for a standard-size specimen, too many reinforcing wires may actually cause a decrease in the bending strength, as pointed out before. It is also evident from Figure 6 that even after fracture of the PMMA, a considerable portion of the maximum load was still sustained by the reinforcing wires alone and, for each group, this loadcarrying capacity was directly related to the number of reinforcing wires. For instance, the loads borne by the three, four, and five 0.5-mm wires just after fracture of the cement were 18,29, and 36% of the maximum loads supported by those groups, respectively. Similarly, one or two 1.0-mm-diam wires, after fracture of the cement,
5oot
(1.0mml
Fig. 6. Comparison of the maximum load-carrying capacities of unreinforced (1st bar graph) and stainless-steel-wire-reinforced PMMA specimens in bending. The number of reinforcing wires is shown below the horizontal axis a t the bottom of each bar graph and the diameter of the wires are shown within parentheses. The load supported by the specimens after initial fracture of PMMA is shown by dotted lines. The mean value of the maximum load and the load carried after fracture for each group is shown within the bar graph a t its middle and bottom, respectively.
BENDING PROPERTIES OF BONE CEMENT
453
still sustained 22 and 36% of the maximum loads supported by the intact specimens, respectively (Table I and Fig. 5). Energy absorption capacity or fracture energy is an important criterion in determining the resistance of a material against failure. The fracture energy of PMMA specimens was calculated by measuring the area under the load-deformation curves up to the point of maximum load. The energy absorption capacity of the reinforced samples are compared with the control group in Figure 7. As in the case of maximum load, here also there was a significant increase ( p < 0.001) in the fracture energy of all reinforced specimens compared to the unreinforced ones. The amount of increase was 22.2 and 49.2% for the specimens with one and two strands of 1.0-mm-diam reinforcement, respectively. Similarly, addition of three, four, and five strands of 0.5-mm-diam wires produced 31.0,24.6, and 31.0%increases in the fracture energy, compared to the control group. The fracture energy also increased from 1.54 to 1.88J, a significant increase ( p < 0.01), when the number of 1.0-mm-diam reinforcing wires was increased from one to two. However, for 0.5-mm-diam reinforcing wires, the fracture energy remained the same for three and five wire groups and decreased slightly for the group with four reinforcing wires, although these variations were not significant.
T
!j 0
r: 157 1.65
I
4
5
Fig. 7. Comparison of the energy absorption capacities of the unreinforced and stainless-steel-wire-reinforced bone cement specimens in bending. The mean value of the fracture energy in joules is shown within each bar graph. The number of reinforcing wires for each group is shown below the horizontal axis at the bottom of each bar graph and the diameter of the wires are shown within parentheses.
SAHA AND KRAAY
454
Maximum deformation or deflection indicates the pliability or ductility of a beam. A comparison of the maximum deformation of the loading points of the tested specimens at the point of fracture is shown in Figure 8. As shown in this figure and in Table I, there was an increase in the deformation capacity of all reinforced specimens and the increase was highly significant ( p < 0.001) for all reinforced PMMA samples except for the three and four 0.5-mm wire groups.
DISCUSSION As can be seen from Table I, addition of wire reinforcement produced significant improvements in the tolerance limits of polymethyl methacrylate beam specimens. However, more reinforcement did not necessarily increase the strength further. Contrary to expectation, addition of 0.5-mm reinforcement from 89
-
79 -
Fig. 8. Comparison of the maximum deflections of the normal and stainlesssteel-wire-reinforced PMMA beam specimens. The deformation was measured as the deflection of the loading points from its undeformed position. The mean value of the deformation in millimeter is shown within each bar graph. The number of reinforcing wires is shown below the horizontal axis at the bottom of each bar graph and the diameter of the reinforcement is shown within parentheses.
BENDING PROPERTIES OF BONE CEMENT
455
three to four and five wires resulted in a reduction in the maximum load-carrying capacity of the beam specimens. As mentioned before, most of this reduction could be attributed to the reduced depth of specimens with four and five reinforcing wires. The small decrease in the failure stresses (Table 11) may be a result of several factors which are important in the fabrication of the specimens. As the number of reinforcing wires increased, placement of the wires became more difficult and the specimens required additional compaction. During compaction, the 0.5-mm wires, offering little resistance, were easily displaced from their nominal positions. The relatively small size of the specimens and the short working time of the cement compounded these problems to yield beams of reduced uniformity in comparison to the three-wire samples. However, the load carried after fracture was consistent with the expectation of increased load with more reinforcing wires. The results of 0.5-mm-diam reinforced specimens are a t variance with findings of a previous study which indicated that the tensile strength of wire-reinforced cement increased with the increasing number of reinforcing wires.4 This is not surprising because, when tested in tension, the continuous wires and the cement both were subjected to the same longitudinal strain; therefore, the cement and the wires sustained loads in parallel independent of any bond between these two phases. However, when such a composite structure is subjected to bending, the tensile stress in the wire has to be transferred to the cement by means of interfacial shear stress.6 Thus, the strength of the composite beam depends on the bond between the cement and the wire. As the number of wires is increased, the gap between the wires is reduced and the shear-stressed regions of cement surrounding each wire is overlapped, which might cause a decrease in the strength of the beam. This might be a mechanism contributing to the decreased failure stress of specimens with four and five reinforcing wires. Tests using 1.0-mm wires for reinforcement were undertaken so that fewer wires could be used and compaction then would not as easily displace the larger wires. Testing of the 1.0-mm-wire-reinforced samples provided the results expected. Addition of reinforcement increased the strength of the composite material with respect to d of the properties investigated: maximum load, maximum deformation, fracture energy, and load carried after fracture. Note that while the single 1.0-mm wire has the same cross-sectional area
456
SAHA AND KRAAY
as the four 0.5-mm wires, the four-wire composite is clearly stronger (Table I). This might be attributed to a possible increase in PMMA metal bonding at the four 0.5-mm wire interfaces, a result of the above having twice the surface area for bonding as does the single 1.0-mm wire. The results of this investigation are in general agreement with the previous studies on the tensile and shear properties of reinforced PMMA.4,s For instance, the shear strength of 0.5-mm-diam metalwire-reinforced bone cement specimens improved when the number of reinforcing wires was increased from two to three, but the strength decreased slightly when the number of reinforcing wires was increased further to four and five strands.8 Assuming linear elastic behavior, the mean ultimate bending stress for the unreinforced control group of PMMA was calculated to be 51.8 MN/m2 (Table 11). This is in agreement with the maximum flexure stress of 50.0 MN/m2 (reported by Knoell et al.7) and 49.3 to 56.6 MN/m2 for Simplex P radiopaque bone cement (determined by ~olmg).
CONCLUSION The result of this study shows that the load-carrying capacity of acrylic bone cement in bending can be improved considerably by addition of reinforcing metal wires. Although there were wide variations, on an average, steel reinforcement comprising approximately 1%of the cross-sectional area of the bone cement specimen increased the bending strength by approximately 15%. The enhanced strength of reinforced bone cement will allow its use in areas in which tensile and bending forces are also dominant, such as in posterior spinal fusion. Of equal importance is the obvious significance of a load carried by the reinforcing wires after fracture of the reinforced cement while the unreinforced polymethyl methacrylate fails catastrophically. For optimal increase in its strength, the wires should be placed where tensile stresses are maximum and as far away from the neutral axis as possible, as is commonly done in reinforced concrete beam design. Although the bending strength increases in the beginning in proportion to the number of reinforcing wires, one should be careful about incorporating too many wires in a specimen, as this may cause difficulty in compaction of the cement and displacement of the wires during compaction, which ultimately may reduce the strength.
BENDING PROPERTIES OF BONE CEMENT
457
The authors wish to thank Howmedica, Inc., for supplying the acrylic bone cement. Thanks are also due to the National Science Foundation for supporting this work through their Undergraduate Research Participation program.
References 1. J. Charnley, Acrylic Cement in Orthopaedic Surgery, Williams and Wilkins, Baltimore, Maryland, 1970, p. 93. 2. E. J. Dunn, Spine, 2,15 (1977). 3. W. B. Scoville, A. H. Palmer, K. Samra, and G. Chong, J . Neurosurg., 27, 274 (1967). 4. J. P. Taitsman and S. Saha, J. Bone Jt. Surg., 59-A,419 (1977). 5. S. P. Timoshenko and J. M. Gere, Mechanics of Materials, Van Nostrand, New York, 1972. 6. B. Bresler, Reinjorced Concrete Engineering, Vol. 1& 2, Wiley, New York, 1974. 7. A. Knoell, H. Maxwell, and C. Bechtol, Ann. Biorned. Eng., 3,225 (1975). 8. S. Saha and M. L. Warman, Trans. Orthop. Res. SOC.,24th Ann. Meet., 3, 299 (1978). 9. N. J. Holm, Acta Orthop. Scand., 48,436 (1977).
Received July 31,1978 Revised November 8,1978
