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Effect of mixing technique on the properties of acrylic bone–cement
The Journal of Arthroplasty Vol. 15 No. 5 2000
Effect of Mixing Technique on the Properties of Acrylic Bone–Cement A Comparison of Syringe and Bowl Mixing Systems J. M. Wilkinson, FRCS,* R. Eveleigh, PhD,† A. J. Hamer, FRCS (Orth),* A. Milne, DCR,‡ A. W. Miles, MSc (Eng),† and I. Stockley, FRCS*
Abstract: Syringe mixing systems have been introduced, but few data exist regarding the mechanical performance of cement they produce. We compared the properties of polymethyl methacrylate cement produced by these systems with that produced by a multiaxial bowl. Mixtures of cement were prepared using the Optivac, Cemvac, and Summit syringes and the Summit bowl. The mixtures were cured in molds to create casts that were radiographed and analyzed for void content, then cut into strips, weighed, measured, and tested to failure in 4-point bending. Syringe-mixed cement was of greater density, bending modulus, and bending strength than bowl-mixed cement (Mann-Whitney, P ⬍ .01) and contained fewer microvoids and macrovoids (Mann-Whitney, P ⬍ .01). No significant differences between the syringes were found for these variables (Kruskall-Wallis, P ⬎ .05). Key words: acrylic bone– cement, syringe, bowl, mechanical properties, porosity.
Polymethyl methacrylate (PMMA) cement forms an integral part of the construct in a cemented total joint arthroplasty, providing a stable interface between the implant and the surrounding bone. Destabilization of the construct and implant failure may occur in relation to cement failure because of debonding at the prosthesis–cement or cement– bone interface and because of fracture or creep within the cement mantle itself [1]. The long-term survival of a cemented total joint arthroplasty thus depends, in part, on the mechanical properties of its cement.
Mixing method has been shown to have a significant influence on the mechanical performance and porosity of PMMA cement [2–6]. There has been a move toward sealed, combined cement mixing and delivery systems, driven, in part, by concerns regarding atmospheric exposure of operating room staff to PMMA monomer. Some data are available on the effect these mixing systems have on cement porosity, but little is known about the mechanical properties of the cement they produce [7,8]. The aim of this study was to determine whether PMMA cement produced by syringe mixing systems differs in porosity and static mechanical properties from that mixed using an established multiaxial bowl and whether any differences exist between syringe systems with respect to these variables.
From the *Lower Limb Arthroplasty Unit, Department of Orthopaedics, Northern General Hospital, Sheffield; †Department of Mechanical Engineering, Faculty of Engineering and Design, University of Bath; and ‡Department of Radiology, The Essex Nuffield Hospital, Brentwood, Essex, United Kingdom. Submitted October 4, 1999; accepted January 17, 2000. No benefits or funds were received in support of this study. Reprint requests: J. M. Wilkinson, FRCS, Lower Limb Arthroplasty Unit, Department of Orthopaedics, Northern General Hospital, Herries Road, Sheffield, S5 7AU, United Kingdom. Copyright r 2000 by Churchill Livingstonet 0883-5403/00/1505-0017$10.00/0 doi: 10.1054/arth.2000.6620
Materials and Methods Four single mixtures (40 g powdered polymer, 20 mL liquid monomer) of nonprechilled Palacos R
663
664 The Journal of Arthroplasty Vol. 15 No. 5 August 2000 Table 1. Details of Mixing Technique for Each of the Mixing Systems
Mixing System
Duration of Mixing (s)
Optivac
30
Cemvac
30
Summit syringe
35
Summit bowl
30
Mixing Method Longitudinal and rotational Longitudinal and rotational Longitudinal and rotational Rotational
Partial Vacuum Applied Vacuum (Bar*) Collection 0.15
Yes
0.20
Yes
0.25
No
0.60
No
*Bar absolute pressure.
cement with gentamicin (Schering-Plough, Welwyn Garden City, England) were prepared for each of 4 proprietary, vacuumed mixing systems. The systems tested were the Summit (Summit Medical, Bourton on the Water, UK), Optivac (Schering-Plough, Welwyn Garden City, England), and Cemvac (Cemvac System AB, Sweden) syringes and the Summit multiaxial bowl. All mixtures were created with cement from the same batch at a stable room temperature of 22°C by 2 of the authors (J.M.W. and A.J.H.) according to manufacturers’ instructions (Table 1). After each mix, the cement was transferred into a polytetrafluoroethylene mold of 80 ⫻ 100 ⫻ 3 mm internal dimension, and the lid was depressed until an even cement layer was achieved. No further pressurization was applied, and the specimens were then allowed to cure. After curing, the 4 cement casts produced for each system were removed from the molds and radiographed directly on the surface of a Kodak Lanex Fine film cassette. The resulting radiographs were digitized using a Lumisys 200 laser film digitizer (Lumisys, Sunnyvale, CA) and analyzed for number
and size of voids using Optimas Image Analysis software, version 6.1 (Optimas Corp, Washington, DC). The voids were characterized into 2 groups according to their mean diameter. Voids with a diameter of ⱖ1 mm were classified as macropores, and those with a diameter ⬍1 mm were classified as micropores. The casts were then cut into strips 80 ⫻ 10 ⫻ 3 mm using a water-cooled, diamond-tipped, low-speed saw (EXAKT, Mederex, Bath, UK), yielding 32 test strips for the Cemvac and 30 for each of the other systems. After allowing 50 days for complete polymerization of the monomer, the dimensions of the individual strips were measured using vernier calipers, their mass recorded, and their density calculated. Each specimen was loaded at room temperature, in air, on a 4-point bending rig using an Instron testing machine (model 4303, High Wycombe, UK) at a rate of 5 mm/min cross-head speed until failure, according to ISO 5833, part 1 (1992). The deflections of each specimen at 15 and 50 N were measured and used to calculate the bending modulus. The maximum load to failure and the corresponding deflection were also recorded, and the bending strength was calculated. All mechanical testing was performed blind by one author (R.E.). Four-point bending strength and bending modulus were calculated according to the formulae given in ISO 5833 (1992). Shapiro-Wilk W testing found the data to be nonnormally distributed, and nonparametric statistical analyses were performed. The Mann-Whitney test was used to determine whether any differences existed for each variable between the pooled data for all the cement samples prepared using a syringe system compared with those prepared using the multiaxial bowl system. Differences between each of the proprietary syringe systems for each of the variables were explored using the Kruskall-Wallis test. The level of statistical significance was set at P ⬍ .05.
Table 2. Results of Digitized Radiographic Analysis of Cement Porosity and Mechanical Testing* Variable Density (kg/m3 )† Bending modulus (MPa)† Bending strength (MPa)† Microvoids/cm2† Macrovoids/cm2† Percentage void area/cm2‡
Multiaxial Bowl
Syringe Data (Pooled)
1,142 (1,058–1,181) 3,501 (2,831–3,776) 70.85 (56.89–80.23) 0.38 (0.09–0.51) 0.03 (0.21–0.44) 0.74 (0.61–0.87)
1,193 (1,128–1,277) 3,855 (3,129–4,671) 89.70 (72.79–102.91) 0.03 (0.00–0.28) 0.05 (0.00–0.15) 0.33 (0.00–1.04)
*Mann-Whitney test. Median and range (in parentheses) values are given. †Syringe systems ⬎ multiaxial bowl; P ⬍ .01. ‡P ⬎ .05.
Effect of Mixing Technique on Bone–Cement ●
Fig. 1. Graph shows density of cement produced by each system (T-bars, range; shaded box, interquartile range; horizontal bar [shaded box], median).
Results The syringe systems (n ⫽ 92 test samples, pooled data) yielded cement of greater density, bending modulus, and bending strength than the bowl system (n ⫽ 30 test samples) (Mann-Whitney, P ⬍ .01; Table 2). All test strips for the Summit bowl failed in 4-point bending. In 15 of the Optivac, 9 of the Summit syringe, and 2 of the Cemvac test samples, the cement did not fail in 4-point bending. These
Wilkinson et al.
665
Fig. 3. Graph shows tensile bending strength of cement produced by each system (T-bars, range; shaded box, interquartile range; horizontal bar [shaded box], median).
cement samples deflected until they touched the base of the rig before failing in 5-point bending and have been excluded from the calculation of the results for bending strength. The median and range values for cement density, bending modulus, and bending strength for each of the individual cement mixing systems are shown in Figs. 1 to 3. Syringe-mixed cement (n ⫽ 12 casts) had significantly fewer microvoids and macrovoids than bowlmixed cement (n ⫽ 4 casts) (Mann-Whitney, P ⫽ .009 and P ⫽ .004; Table 2). There was no difference between syringe-mixed cement and bowlmixed cement for total percentage of void/cm2 (Mann-Whitney, P ⬎ .05). Fig. 4 shows radiographs of the distribution of voids in a representative cast for each of the mixing systems. There were no differences between the different proprietary syringe mixing systems with respect to any of the above-mentioned variables (Kruskall-Wallis, P ⬎ .05).
Discussion
Fig. 2. Graph shows tensile bending modulus of cement produced by each system (T-bars, range; shaded box, interquartile range; horizontal bar [shaded box], median).
The number and size of voids within PMMA cement have been shown to affect its mechanical properties. Porosity is inversely related to cement density, which, in turn, reduces its tensile and compressive strength [3]. Direct correlations between porosity and bending strength have also been shown [2]. James et al [9] showed a strong negative correlation between porosity and fatigue cycles to failure. In a series of in vitro and in vivo studies,
666 The Journal of Arthroplasty Vol. 15 No. 5 August 2000
Fig. 4. Plain radiographs of cement casts from the 4 proprietary mixing systems show void patterns produced within the cement. (A) Optivac cast. (B) Cemvac cast. (C) Summit syringe cast. (D) Summit bowl cast.
Topoleski et al [10,11] showed that pores may act as nucleation sites for microcracks and that large voids can lead to rapid, unstable cement failure. Conversely, they proposed a role for small pores as crack stoppers by dispersing the energy at the tip of a crack by increasing its radius of curvature [10]. The current consensus is, however, that efforts should be made to reduce the number and size of voids to a minimum [10]. Our study showed that the syringe mixing method reduced the number of macropores and micropores in the cement produced. The finding that there was no statistical difference between the systems for total percentage void area/cm2 did not correlate with simple observations of the plain radiographs of the casts (Fig. 4). It is likely that the sample size of 4 casts in the bowl group had insufficient power to show a genuine difference between the systems.
Our radiographic observations suggested that mixing and collection of the cement under vacuum may be important in reducing porosity. Other workers have noted that large pores removed during mixing under vacuum may be reintroduced into the mix during the collection process if it is not carried out under vacuum [7]. The results of this study showed that syringe mixing did improve the density and mechanical properties of the cement; however, the interquartile ranges for the properties of each system were similar (Figs. 1–3), indicating that the consistency of the cement achieved by the bowl was as good as that for the syringe systems. All the syringe mixing systems produced PMMA cement of a similar density and bending modulus. The results of the bending strength should be interpreted with caution because many syringe samples would not fail in 4-point bend-
Effect of Mixing Technique on Bone–Cement ●
ing. The results for the Optivac and Summit syringes particularly may represent an underestimate of the true bending strength of the cement they produce and bias the statistical analysis of these data toward the null hypothesis that there was no difference between the syringes for bending strength. An essential prerequisite for any bioimplantable material is that it behaves in a consistent and predictable manner. Given that the bending strength and modulus of the prosthesis component of the arthroplasty construct are many times higher than that of the cement, small differences in mechanical properties of the cement produced by each of the systems are probably of secondary importance to uniformity of mechanical properties within an individual mix. Creep properties of acrylic cement may be important in the behavior of certain designs of total hip arthroplasty [13,14]. Double-taper polished designs rely on the stress relaxation properties of PMMA, and it is possible that uniformity of cement properties may improve the performance of such stems by allowing even stress relaxation within the cement mantle [15]. Several other factors influence the mechanical performance of the prosthesis–cement interface and the survival of a total joint arthroplasty, many of which are within the control of the operating surgeon [16]. Active bleeding has been shown to reduce the shear strength of the cement–bone interface by 50% [17], and the shear strength of this interface can vary from 1.9 to 36.1 MPa, depending on the type of bone surface preparation used [18]. Laminations and blood entrapment introduced within the cement during curing also decrease tensile and shear strength of the cement [19]. The mechanical performance of cement must be considered in the context of the whole total hip arthroplasty construct. Attempts should be made to produce cement with optimal mechanical properties. A perfect cement mix, however, cannot compensate for inadequate bone preparation or poor cementing technique.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12. 13.
14.
15.
Acknowledgment We thank Mr. C. Ibbottson, Head of the Tissue Bank, Northern General Hospital, for his technical expertise and help in the preparation of the cement test strips. We also thank Schering Plough, for providing the cement for the study, and all the manufacturers for providing gratis mixing systems.
16.
17.
18.
References 19. 1. Masterson EL, Masri BA, Duncan CP: The cement mantle in the Exeter impaction allografting tech-
Wilkinson et al.
667
nique: a cause for concern. J Arthroplasty 12:759, 1997 Kurdy NM, Hodgkinson JP, Haynes R: Acrylic bone– cement: influence of mixer design and unmixed powder. J Arthroplasty 11:813, 1996 Eyerer P, Jin R: Influence of mixing technique on some properties of PMMA bone cement. J Biomed Mater Res 20:1057, 1986 Jasty M, Davies JP, O’Conner DO, et al: Porosity of various preparations of acrylic bone cement. Clin Orthop 259:122, 1990 Lewis G, Nyman JS, Trieu HH: Effect of mixing method on selected properties of acrylic bone cement. J Biomed Mater Res 38:221, 1997 Linden U: Mechanical properties of bone cement: importance of mixing technique. Clin Orthop 272: 274, 1991 Wang JS, Toksvig-Larsen S, Muller-Wille P, Franzen H: Is there a difference between vacuum mixing systems in reducing bone density? J Biomed Mater Res 33:115, 1996 Smeds S, Goertzen D, Ivarsson I: Influence of temperature and vacuum mixing on bone cement properties. Clin Orthop 334:326, 1997 James SP, Jasty M, Davies J, et al: A fractographic investigation of PMMA bone cement focusing on the relationship between porosity reduction and increased fatigue life. J Biomed Mater Res 26:651, 1992 Topoleski LDT, Ducheyne P, Cuckler JM: Microstructural pathway of fracture in poly(methyl methacrylate) bone cement. Biomaterials 14:1165, 1993 Topoleski LDT, Ducheyne P, Cuckler JM: A fractographic analysis of in vivo poly (methyl methacrylate) bone cement failure mechanisms. J Biomed Mater Res 24:135, 1990 Lewis G: Properties of acrylic bone cement: state of the art review. J Biomed Mater Res 38:155, 1997 Verdonschot N, Huiskes R: Dynamic creep behaviour of acrylic bone cement. J Biomed Mater Res 29:575, 1995 Verdonschot N, Huiskes R: Subsidence of THA stems due to acrylic cement creep is extremely sensitive to interface friction. J Biomech 29:1569, 1996 Lu Z, McKellop H: Effects of cement creep on stem subsidence and stresses in the cement mantle of a total hip replacement. J Biomed Mater Res 34:221, 1997 Lee AJ, Ling RS, Vangala SS: Some clinically relevant variables affecting the mechanical behaviour of bone cement. Arch Orthop Trauma 92:1, 1978 Majkowski RS, Bannister GC, Miles AW: The effect of bleeding on the cement–bone interface: an experimental study. Clin Orthop 299:293, 1994 Majkowski RS, Miles AW, Bannister GC, et al: Bone surface preparation in cemented joint replacement. J Bone Joint Surg Br 75:459, 1993 Gruen TA, Markolf KL, Amstutz HC: Effects of laminations and blood entrapment on the strength of acrylic bone cement. Clin Orthop 119:250, 1976
Effect of Mixing Technique on the Properties of Acrylic Bone–Cement A Comparison of Syringe and Bowl Mixing Systems J. M. Wilkinson, FRCS,* R. Eveleigh, PhD,† A. J. Hamer, FRCS (Orth),* A. Milne, DCR,‡ A. W. Miles, MSc (Eng),† and I. Stockley, FRCS*
Abstract: Syringe mixing systems have been introduced, but few data exist regarding the mechanical performance of cement they produce. We compared the properties of polymethyl methacrylate cement produced by these systems with that produced by a multiaxial bowl. Mixtures of cement were prepared using the Optivac, Cemvac, and Summit syringes and the Summit bowl. The mixtures were cured in molds to create casts that were radiographed and analyzed for void content, then cut into strips, weighed, measured, and tested to failure in 4-point bending. Syringe-mixed cement was of greater density, bending modulus, and bending strength than bowl-mixed cement (Mann-Whitney, P ⬍ .01) and contained fewer microvoids and macrovoids (Mann-Whitney, P ⬍ .01). No significant differences between the syringes were found for these variables (Kruskall-Wallis, P ⬎ .05). Key words: acrylic bone– cement, syringe, bowl, mechanical properties, porosity.
Polymethyl methacrylate (PMMA) cement forms an integral part of the construct in a cemented total joint arthroplasty, providing a stable interface between the implant and the surrounding bone. Destabilization of the construct and implant failure may occur in relation to cement failure because of debonding at the prosthesis–cement or cement– bone interface and because of fracture or creep within the cement mantle itself [1]. The long-term survival of a cemented total joint arthroplasty thus depends, in part, on the mechanical properties of its cement.
Mixing method has been shown to have a significant influence on the mechanical performance and porosity of PMMA cement [2–6]. There has been a move toward sealed, combined cement mixing and delivery systems, driven, in part, by concerns regarding atmospheric exposure of operating room staff to PMMA monomer. Some data are available on the effect these mixing systems have on cement porosity, but little is known about the mechanical properties of the cement they produce [7,8]. The aim of this study was to determine whether PMMA cement produced by syringe mixing systems differs in porosity and static mechanical properties from that mixed using an established multiaxial bowl and whether any differences exist between syringe systems with respect to these variables.
From the *Lower Limb Arthroplasty Unit, Department of Orthopaedics, Northern General Hospital, Sheffield; †Department of Mechanical Engineering, Faculty of Engineering and Design, University of Bath; and ‡Department of Radiology, The Essex Nuffield Hospital, Brentwood, Essex, United Kingdom. Submitted October 4, 1999; accepted January 17, 2000. No benefits or funds were received in support of this study. Reprint requests: J. M. Wilkinson, FRCS, Lower Limb Arthroplasty Unit, Department of Orthopaedics, Northern General Hospital, Herries Road, Sheffield, S5 7AU, United Kingdom. Copyright r 2000 by Churchill Livingstonet 0883-5403/00/1505-0017$10.00/0 doi: 10.1054/arth.2000.6620
Materials and Methods Four single mixtures (40 g powdered polymer, 20 mL liquid monomer) of nonprechilled Palacos R
663
664 The Journal of Arthroplasty Vol. 15 No. 5 August 2000 Table 1. Details of Mixing Technique for Each of the Mixing Systems
Mixing System
Duration of Mixing (s)
Optivac
30
Cemvac
30
Summit syringe
35
Summit bowl
30
Mixing Method Longitudinal and rotational Longitudinal and rotational Longitudinal and rotational Rotational
Partial Vacuum Applied Vacuum (Bar*) Collection 0.15
Yes
0.20
Yes
0.25
No
0.60
No
*Bar absolute pressure.
cement with gentamicin (Schering-Plough, Welwyn Garden City, England) were prepared for each of 4 proprietary, vacuumed mixing systems. The systems tested were the Summit (Summit Medical, Bourton on the Water, UK), Optivac (Schering-Plough, Welwyn Garden City, England), and Cemvac (Cemvac System AB, Sweden) syringes and the Summit multiaxial bowl. All mixtures were created with cement from the same batch at a stable room temperature of 22°C by 2 of the authors (J.M.W. and A.J.H.) according to manufacturers’ instructions (Table 1). After each mix, the cement was transferred into a polytetrafluoroethylene mold of 80 ⫻ 100 ⫻ 3 mm internal dimension, and the lid was depressed until an even cement layer was achieved. No further pressurization was applied, and the specimens were then allowed to cure. After curing, the 4 cement casts produced for each system were removed from the molds and radiographed directly on the surface of a Kodak Lanex Fine film cassette. The resulting radiographs were digitized using a Lumisys 200 laser film digitizer (Lumisys, Sunnyvale, CA) and analyzed for number
and size of voids using Optimas Image Analysis software, version 6.1 (Optimas Corp, Washington, DC). The voids were characterized into 2 groups according to their mean diameter. Voids with a diameter of ⱖ1 mm were classified as macropores, and those with a diameter ⬍1 mm were classified as micropores. The casts were then cut into strips 80 ⫻ 10 ⫻ 3 mm using a water-cooled, diamond-tipped, low-speed saw (EXAKT, Mederex, Bath, UK), yielding 32 test strips for the Cemvac and 30 for each of the other systems. After allowing 50 days for complete polymerization of the monomer, the dimensions of the individual strips were measured using vernier calipers, their mass recorded, and their density calculated. Each specimen was loaded at room temperature, in air, on a 4-point bending rig using an Instron testing machine (model 4303, High Wycombe, UK) at a rate of 5 mm/min cross-head speed until failure, according to ISO 5833, part 1 (1992). The deflections of each specimen at 15 and 50 N were measured and used to calculate the bending modulus. The maximum load to failure and the corresponding deflection were also recorded, and the bending strength was calculated. All mechanical testing was performed blind by one author (R.E.). Four-point bending strength and bending modulus were calculated according to the formulae given in ISO 5833 (1992). Shapiro-Wilk W testing found the data to be nonnormally distributed, and nonparametric statistical analyses were performed. The Mann-Whitney test was used to determine whether any differences existed for each variable between the pooled data for all the cement samples prepared using a syringe system compared with those prepared using the multiaxial bowl system. Differences between each of the proprietary syringe systems for each of the variables were explored using the Kruskall-Wallis test. The level of statistical significance was set at P ⬍ .05.
Table 2. Results of Digitized Radiographic Analysis of Cement Porosity and Mechanical Testing* Variable Density (kg/m3 )† Bending modulus (MPa)† Bending strength (MPa)† Microvoids/cm2† Macrovoids/cm2† Percentage void area/cm2‡
Multiaxial Bowl
Syringe Data (Pooled)
1,142 (1,058–1,181) 3,501 (2,831–3,776) 70.85 (56.89–80.23) 0.38 (0.09–0.51) 0.03 (0.21–0.44) 0.74 (0.61–0.87)
1,193 (1,128–1,277) 3,855 (3,129–4,671) 89.70 (72.79–102.91) 0.03 (0.00–0.28) 0.05 (0.00–0.15) 0.33 (0.00–1.04)
*Mann-Whitney test. Median and range (in parentheses) values are given. †Syringe systems ⬎ multiaxial bowl; P ⬍ .01. ‡P ⬎ .05.
Effect of Mixing Technique on Bone–Cement ●
Fig. 1. Graph shows density of cement produced by each system (T-bars, range; shaded box, interquartile range; horizontal bar [shaded box], median).
Results The syringe systems (n ⫽ 92 test samples, pooled data) yielded cement of greater density, bending modulus, and bending strength than the bowl system (n ⫽ 30 test samples) (Mann-Whitney, P ⬍ .01; Table 2). All test strips for the Summit bowl failed in 4-point bending. In 15 of the Optivac, 9 of the Summit syringe, and 2 of the Cemvac test samples, the cement did not fail in 4-point bending. These
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665
Fig. 3. Graph shows tensile bending strength of cement produced by each system (T-bars, range; shaded box, interquartile range; horizontal bar [shaded box], median).
cement samples deflected until they touched the base of the rig before failing in 5-point bending and have been excluded from the calculation of the results for bending strength. The median and range values for cement density, bending modulus, and bending strength for each of the individual cement mixing systems are shown in Figs. 1 to 3. Syringe-mixed cement (n ⫽ 12 casts) had significantly fewer microvoids and macrovoids than bowlmixed cement (n ⫽ 4 casts) (Mann-Whitney, P ⫽ .009 and P ⫽ .004; Table 2). There was no difference between syringe-mixed cement and bowlmixed cement for total percentage of void/cm2 (Mann-Whitney, P ⬎ .05). Fig. 4 shows radiographs of the distribution of voids in a representative cast for each of the mixing systems. There were no differences between the different proprietary syringe mixing systems with respect to any of the above-mentioned variables (Kruskall-Wallis, P ⬎ .05).
Discussion
Fig. 2. Graph shows tensile bending modulus of cement produced by each system (T-bars, range; shaded box, interquartile range; horizontal bar [shaded box], median).
The number and size of voids within PMMA cement have been shown to affect its mechanical properties. Porosity is inversely related to cement density, which, in turn, reduces its tensile and compressive strength [3]. Direct correlations between porosity and bending strength have also been shown [2]. James et al [9] showed a strong negative correlation between porosity and fatigue cycles to failure. In a series of in vitro and in vivo studies,
666 The Journal of Arthroplasty Vol. 15 No. 5 August 2000
Fig. 4. Plain radiographs of cement casts from the 4 proprietary mixing systems show void patterns produced within the cement. (A) Optivac cast. (B) Cemvac cast. (C) Summit syringe cast. (D) Summit bowl cast.
Topoleski et al [10,11] showed that pores may act as nucleation sites for microcracks and that large voids can lead to rapid, unstable cement failure. Conversely, they proposed a role for small pores as crack stoppers by dispersing the energy at the tip of a crack by increasing its radius of curvature [10]. The current consensus is, however, that efforts should be made to reduce the number and size of voids to a minimum [10]. Our study showed that the syringe mixing method reduced the number of macropores and micropores in the cement produced. The finding that there was no statistical difference between the systems for total percentage void area/cm2 did not correlate with simple observations of the plain radiographs of the casts (Fig. 4). It is likely that the sample size of 4 casts in the bowl group had insufficient power to show a genuine difference between the systems.
Our radiographic observations suggested that mixing and collection of the cement under vacuum may be important in reducing porosity. Other workers have noted that large pores removed during mixing under vacuum may be reintroduced into the mix during the collection process if it is not carried out under vacuum [7]. The results of this study showed that syringe mixing did improve the density and mechanical properties of the cement; however, the interquartile ranges for the properties of each system were similar (Figs. 1–3), indicating that the consistency of the cement achieved by the bowl was as good as that for the syringe systems. All the syringe mixing systems produced PMMA cement of a similar density and bending modulus. The results of the bending strength should be interpreted with caution because many syringe samples would not fail in 4-point bend-
Effect of Mixing Technique on Bone–Cement ●
ing. The results for the Optivac and Summit syringes particularly may represent an underestimate of the true bending strength of the cement they produce and bias the statistical analysis of these data toward the null hypothesis that there was no difference between the syringes for bending strength. An essential prerequisite for any bioimplantable material is that it behaves in a consistent and predictable manner. Given that the bending strength and modulus of the prosthesis component of the arthroplasty construct are many times higher than that of the cement, small differences in mechanical properties of the cement produced by each of the systems are probably of secondary importance to uniformity of mechanical properties within an individual mix. Creep properties of acrylic cement may be important in the behavior of certain designs of total hip arthroplasty [13,14]. Double-taper polished designs rely on the stress relaxation properties of PMMA, and it is possible that uniformity of cement properties may improve the performance of such stems by allowing even stress relaxation within the cement mantle [15]. Several other factors influence the mechanical performance of the prosthesis–cement interface and the survival of a total joint arthroplasty, many of which are within the control of the operating surgeon [16]. Active bleeding has been shown to reduce the shear strength of the cement–bone interface by 50% [17], and the shear strength of this interface can vary from 1.9 to 36.1 MPa, depending on the type of bone surface preparation used [18]. Laminations and blood entrapment introduced within the cement during curing also decrease tensile and shear strength of the cement [19]. The mechanical performance of cement must be considered in the context of the whole total hip arthroplasty construct. Attempts should be made to produce cement with optimal mechanical properties. A perfect cement mix, however, cannot compensate for inadequate bone preparation or poor cementing technique.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12. 13.
14.
15.
Acknowledgment We thank Mr. C. Ibbottson, Head of the Tissue Bank, Northern General Hospital, for his technical expertise and help in the preparation of the cement test strips. We also thank Schering Plough, for providing the cement for the study, and all the manufacturers for providing gratis mixing systems.
16.
17.
18.
References 19. 1. Masterson EL, Masri BA, Duncan CP: The cement mantle in the Exeter impaction allografting tech-
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