ORIGINAL ARTICLE
J Appl Biomater Funct Mater 2014 ; 12 (2): 90- 96 DOI: 10.5301/JABFM.5000177
Interfacial sliding properties of bone screw materials and their effect on screw fixation strength Arto P. Koistinen1,2, Hannu Korhonen1, Heikki Kröger3, Reijo Lappalainen1,2 Department of Applied Physics, University of Eastern Finland, Kuopio - Finland SIB Labs, University of Eastern Finland, Kuopio - Finland 3 Department of Orthopaedics, Traumatology and Handsurgery, Kuopio University Hospital, Kuopio - Finland 1 2
Department of Applied Physics, University of Eastern Finland, Kuopio - Finland and SIB Labs, University of Eastern Finland, Kuopio - Finland Department of Applied Physics, University of Eastern Finland, Kuopio - Finland Department of Orthopaedics, Traumatology and Handsurgery, Kuopio University Hospital, Kuopio - Finland Department of Applied Physics, University of Eastern Finland, Kuopio - Finland and SIB Labs, University of Eastern Finland, Kuopio - Finland
Abstract Background: This study examined the effect of interfacial sliding and test material properties on the fixation strength and insertional properties of self-tapping bone screws. Various substitute materials (polyacetal [POM], poly(methyl methacrylate) [PMMA] and E-glass-filled Epoxy [Sawbones®]) for human bone were evaluated, and the results were compared with the findings for cadaver bone. Methods: Initial coefficient of friction (CoF) of the screw material stainless steel AISI316 was tested using a pin-on-disk apparatus, and the screws were exposed to pullout tests after insertion torque tests. The effect of a smooth diamond-like carbon (DLC) coating was studied by applying the coating on both CoF test balls and bone screws. Results: Mechanical properties of test blocks strongly correlated to both pullout strength and insertion torque of the screws: for noncoated 2.7-mm screws, tensile strength correlated to pullout strength and insertion torque, with Pearson correlation coefficients r=0.977 and r=0.738, respectively. In contrast, CoF correlated strongly to screw insertion torque but not to pullout strength in bone substitute materials (for noncoated 2.7-mm screws, r=0.652 and r=0.248, respectively). There were no significant differences in CoF using noncoated and DLC-coated screw materials against bone substitutes. Conclusions: Proper materials for in vitro testing help in evaluating the biomechanics of the implants in advance. However, choosing the material needs attention, as their ability to model human bone depends on test type. Key words: Bone screws, DLC coatings, Fracture fixation, In vitro testing Accepted: December 8, 2012
Introduction Tribology and interfacial chemistry play major roles in implant–tissue interaction, biomechanical behavior and survival of implants in the human body. Insufficient screw anchorage is a serious problem in bone that has become mechanically weak e.g., due to osteoporosis. Implant materials and design, various coatings and surface patterning have been developed to improve bone–implant interfacial behavior. Cortical bone screws are commonly used implants for fracture fixation and also for plate fixation. Nowadays, locking plate and compression plate systems are used to secure fracture fixation stability. Compression plate fixation relies on friction between the screw head and the plate, generated by tensional forces due to tightening of the screw in bone. In the locking plate system, the screw is mechanically attached to the plate by the threads in both the screw and the plate. In both cases, the screw is not supposed to take significant bending loads, to avoid 90
shear failure. Pullout testing of screws in the axial direction is derived from their principal mode of operation in compression plate fixation; the screws are axially preloaded to generate compression between bone and plate. This compression should result in sufficient friction to transfer loads that act orthogonal to the screw axis. In addition to pullout tests (1-4), also screw pushout tests have been performed (5). In both cases, the ultimate holding power of the fixation system is determined. However, only a little effort has been devoted to the measurement and reporting of insertional torque (6-8), although those are relevant parameters as well. Experimental setups are often used to evaluate fixation strength and to develop the function of the fixation devices. For several reasons, bone substitutes are typically used in in vitro experiments instead of cadaver bones. These reasons include, for example, biomechanical, ethical and practical considerations. Most critical concerns are related to the preservation of the specimens and variation in the mechanical properties of the cadaver bones.
© 2014 Società Italiana Biomateriali - eISSN 2280-8000
Koistinen et al
Thus, in many cases, synthetic materials or other natural materials (1, 2, 8-14) have been used in biomechanical research. The improvement of the interface between bone and orthopedic or dental implants is still an unresolved problem. For different applications in orthopedics, metallic, ceramic or biological coatings have been widely investigated. One trend is to improve implant osseointegration. Of all of the factors investigated, the surface structure, biomechanical factors and biological response have been demonstrated to have the greatest influence on implant osseointegration (5). For this purpose, typical methods for surface modification include sandblasting, plasma-spraying and acid-etching, which all alter topography and increase active surface area. On the other hand, coatings can prevent bacterial adhesion, decrease friction or minimize wear on sliding surfaces (15, 16). To meet these demands, coatings need to be hard, inert and smooth. Coatings with the required properties can be applied on implant materials e.g., by physical vapor deposition methods. Diamond-like carbon (DLC) has outstanding tribological properties and is, in addition, tolerated well by the body. Thus, a lot of effort has been put into research of DLC coatings in biomedical applications. The main reason for the success of DLC coatings in different applications is their low coefficient of friction (CoF) in combination with a high wear resistance (16). DLC has been shown to also have excellent hemocompatibility, wear resistance and hardness (17). Thus DLC coatings have good potential also in orthopedics, such as in reducing the insertion torque of fracture fixation screws (11, 18). However, several interfacial factors affect both in vivo and in vitro findings, and there are still some unclear factors regarding fixation stability. The aim of this study was to see whether selected substitute materials could generally model human bone in fixation strength studies in vitro. Secondly, the effect of DLC coating on fixation strength and frictional properties of screws in different types of substitute materials was studied, and the results were compared with the findings in cadaver bone.
Three synthetic test materials were used: poly(methyl methacrylate) (PMMA), polyacetal (POM) and E-glassfilled epoxy (Sawbones®; Pacific Research Laboratories Inc., Vashon, WA, USA). These materials were used to mimic the compact bone and have been used in previous studies (13, 19). See the detailed list of material properties in Table I, comparing synthetic materials with human bone. Human cadaver bone was used only in insertion torque tests, the detailed results of which were reported earlier (18). Bone mineral density (BMD) is reported to significantly affect screw fixation strength (22, 23). Therefore, BMD was previously determined for the cadaver bones used in this study with high-resolution peripheral quantitative computed tomography (18). Commercial cortical bone screws (stainless steel AISI 316L; Zimmer Inc., Warsaw, USA) with an outer diameter of 2.7 mm (inner 2.0 mm) and 3.5 mm (inner 2.5 mm) were inserted and pulled out from the test blocks. Each of the test blocks was of 75-80 mm length, 25-30 mm width and 4 mm thickness, and 3 screws were inserted in each block (see Fig. 1). The total number of experiments in each material/coating combination was 6-9. Pilot holes were drilled in the test blocks following manufacturer-specified size recommendations. A constant speed of 5 rpm for the insertion was achieved by an electric motor attached to the servohydraulic test equipment (Instron 8874; Instron, Norwood, MA, USA) maintaining a constant axial load (20 N) on the screws during the insertion. A 10 Nm load cell with an accuracy ± 1% of reading was used to measure insertion and removal torque. Screw insertion was carried out in diluted bovine serum with a total protein content of 24 mg/mL, using standard additives to prevent microbial activity. The additives included penicillin-streptomycin solution (EuroClone S.p.A., Siziano, Italy) and ethylenediaminetetraacetic acid (EDTA)disodium salt (Merck KGaA, Darmstadt, Germany). After insertion, the screws were pulled out from the test block with a constant displacement speed of 5 mm/min. Servohydraulic test equipment (model 8874; Instron Co,
Materials and methods
TABLE I - MECHANICAL PROPERTIES OF TEST BLOCKS USED IN THE PRESENT STUDY
Synthetic bone blocks of various materials were used to find a more consistent model compared with cadaveric bone. In contrast to cadaveric bones, synthetic test specimens have low variance in mechanical properties and are readily available with well-specified properties. Furthermore, there is no risk of contamination or decomposition with the synthetic materials. The use of human bone samples in the present study received the permission of the Finnish National Authority for Medicolegal Affairs (TEO, 1781/32/200/01).
Screw insertion and pullout tests
Material
Ultimate tensile Elastic modulus Elongation Density strength σ (MPa) E (GPa) ε (%) ρ (g/cm3)
Human cortical bone
133*
10-15*
1-3*
1.2†
POM
65‡
3.0‡
>30‡
1.4‡
PMMA
74
3.0-3.3
5
1.1‡
12.4
0.85
Sawbones
®
‡
90
‡
‡
‡
‡ ‡
1.7‡
PMMA=poly(methyl methacrylate); POM=polyacetal. *Data adopted from (20). † Data adopted from (18). ‡ Data adopted from (21).
© 2014 Società Italiana Biomateriali - eISSN 2280-8000
91
Sliding properties affecting screw fixation
diluted bovine serum with standard additives as described above. DLC coating was applied on screws and balls for interfacial sliding tests by ultrashort pulsed laser ablation techniques (ColdabTM) developed by Picodeon Ltd. (Helsinki, Finland). High-quality DLC coatings have a high fraction of sp3 bonding, with up to 85% achieved using several deposition techniques (25). Coating thickness in this application was about 400 nm as estimated by interference techniques. Statistical analysis One-way ANOVA was used to evaluate differences in insertion torque and pullout strength for different materials. Student’s t-test was used for comparison of the effect of surface modification. Pearson correlation analysis (2-tailed) was performed to evaluate correlations between different parameters. Results Screw insertion and pullout
Fig. 1 - Test setup for inserting the screws and for torque measurements.
Norwood, MA, USA) with a 1 kN load cell (accuracy ± 1% of reading) and a custom-made pullout clamp was used in the testing. Interfacial sliding tests Friction between test blocks and stainless steel or DLC-coated stainless steel was tested by using a pinon-disk apparatus described in standard ASTM G99-95 (24). For the pin, AISI 316 stainless steel balls 6 mm in diameter (density: 7.96 g/cm3, Brinell hardness: 160-190) were used mimicking the screw material (Goodfellow Cambridge Ltd., Huntingdon, UK). In the tests, the sample was placed on a rotated holder while the ball was attached to a lever with a dead weight (load). The digital force signal from a strain gauge was recorded, and the CoF was determined. The diameter of the sliding track was 4.5 mm, and a sliding speed of 3.5 mm/s with an applied load of 9.8-79.0 g was used. A relatively slow sliding speed was used to simulate in vitro testing conditions for torque measurements as per standard ASTM F543-07, and only friction during the initial 50 revolutions (sliding distance 71 cm) was determined for each axial load. Tests were carried out in 92
For both screw sizes, the highest insertion torque values were recorded for Sawbones® and the lowest for human cadaver bone (see Figs. 2 and 3). Insertion torque was significantly higher (P
J Appl Biomater Funct Mater 2014 ; 12 (2): 90- 96 DOI: 10.5301/JABFM.5000177
Interfacial sliding properties of bone screw materials and their effect on screw fixation strength Arto P. Koistinen1,2, Hannu Korhonen1, Heikki Kröger3, Reijo Lappalainen1,2 Department of Applied Physics, University of Eastern Finland, Kuopio - Finland SIB Labs, University of Eastern Finland, Kuopio - Finland 3 Department of Orthopaedics, Traumatology and Handsurgery, Kuopio University Hospital, Kuopio - Finland 1 2
Department of Applied Physics, University of Eastern Finland, Kuopio - Finland and SIB Labs, University of Eastern Finland, Kuopio - Finland Department of Applied Physics, University of Eastern Finland, Kuopio - Finland Department of Orthopaedics, Traumatology and Handsurgery, Kuopio University Hospital, Kuopio - Finland Department of Applied Physics, University of Eastern Finland, Kuopio - Finland and SIB Labs, University of Eastern Finland, Kuopio - Finland
Abstract Background: This study examined the effect of interfacial sliding and test material properties on the fixation strength and insertional properties of self-tapping bone screws. Various substitute materials (polyacetal [POM], poly(methyl methacrylate) [PMMA] and E-glass-filled Epoxy [Sawbones®]) for human bone were evaluated, and the results were compared with the findings for cadaver bone. Methods: Initial coefficient of friction (CoF) of the screw material stainless steel AISI316 was tested using a pin-on-disk apparatus, and the screws were exposed to pullout tests after insertion torque tests. The effect of a smooth diamond-like carbon (DLC) coating was studied by applying the coating on both CoF test balls and bone screws. Results: Mechanical properties of test blocks strongly correlated to both pullout strength and insertion torque of the screws: for noncoated 2.7-mm screws, tensile strength correlated to pullout strength and insertion torque, with Pearson correlation coefficients r=0.977 and r=0.738, respectively. In contrast, CoF correlated strongly to screw insertion torque but not to pullout strength in bone substitute materials (for noncoated 2.7-mm screws, r=0.652 and r=0.248, respectively). There were no significant differences in CoF using noncoated and DLC-coated screw materials against bone substitutes. Conclusions: Proper materials for in vitro testing help in evaluating the biomechanics of the implants in advance. However, choosing the material needs attention, as their ability to model human bone depends on test type. Key words: Bone screws, DLC coatings, Fracture fixation, In vitro testing Accepted: December 8, 2012
Introduction Tribology and interfacial chemistry play major roles in implant–tissue interaction, biomechanical behavior and survival of implants in the human body. Insufficient screw anchorage is a serious problem in bone that has become mechanically weak e.g., due to osteoporosis. Implant materials and design, various coatings and surface patterning have been developed to improve bone–implant interfacial behavior. Cortical bone screws are commonly used implants for fracture fixation and also for plate fixation. Nowadays, locking plate and compression plate systems are used to secure fracture fixation stability. Compression plate fixation relies on friction between the screw head and the plate, generated by tensional forces due to tightening of the screw in bone. In the locking plate system, the screw is mechanically attached to the plate by the threads in both the screw and the plate. In both cases, the screw is not supposed to take significant bending loads, to avoid 90
shear failure. Pullout testing of screws in the axial direction is derived from their principal mode of operation in compression plate fixation; the screws are axially preloaded to generate compression between bone and plate. This compression should result in sufficient friction to transfer loads that act orthogonal to the screw axis. In addition to pullout tests (1-4), also screw pushout tests have been performed (5). In both cases, the ultimate holding power of the fixation system is determined. However, only a little effort has been devoted to the measurement and reporting of insertional torque (6-8), although those are relevant parameters as well. Experimental setups are often used to evaluate fixation strength and to develop the function of the fixation devices. For several reasons, bone substitutes are typically used in in vitro experiments instead of cadaver bones. These reasons include, for example, biomechanical, ethical and practical considerations. Most critical concerns are related to the preservation of the specimens and variation in the mechanical properties of the cadaver bones.
© 2014 Società Italiana Biomateriali - eISSN 2280-8000
Koistinen et al
Thus, in many cases, synthetic materials or other natural materials (1, 2, 8-14) have been used in biomechanical research. The improvement of the interface between bone and orthopedic or dental implants is still an unresolved problem. For different applications in orthopedics, metallic, ceramic or biological coatings have been widely investigated. One trend is to improve implant osseointegration. Of all of the factors investigated, the surface structure, biomechanical factors and biological response have been demonstrated to have the greatest influence on implant osseointegration (5). For this purpose, typical methods for surface modification include sandblasting, plasma-spraying and acid-etching, which all alter topography and increase active surface area. On the other hand, coatings can prevent bacterial adhesion, decrease friction or minimize wear on sliding surfaces (15, 16). To meet these demands, coatings need to be hard, inert and smooth. Coatings with the required properties can be applied on implant materials e.g., by physical vapor deposition methods. Diamond-like carbon (DLC) has outstanding tribological properties and is, in addition, tolerated well by the body. Thus, a lot of effort has been put into research of DLC coatings in biomedical applications. The main reason for the success of DLC coatings in different applications is their low coefficient of friction (CoF) in combination with a high wear resistance (16). DLC has been shown to also have excellent hemocompatibility, wear resistance and hardness (17). Thus DLC coatings have good potential also in orthopedics, such as in reducing the insertion torque of fracture fixation screws (11, 18). However, several interfacial factors affect both in vivo and in vitro findings, and there are still some unclear factors regarding fixation stability. The aim of this study was to see whether selected substitute materials could generally model human bone in fixation strength studies in vitro. Secondly, the effect of DLC coating on fixation strength and frictional properties of screws in different types of substitute materials was studied, and the results were compared with the findings in cadaver bone.
Three synthetic test materials were used: poly(methyl methacrylate) (PMMA), polyacetal (POM) and E-glassfilled epoxy (Sawbones®; Pacific Research Laboratories Inc., Vashon, WA, USA). These materials were used to mimic the compact bone and have been used in previous studies (13, 19). See the detailed list of material properties in Table I, comparing synthetic materials with human bone. Human cadaver bone was used only in insertion torque tests, the detailed results of which were reported earlier (18). Bone mineral density (BMD) is reported to significantly affect screw fixation strength (22, 23). Therefore, BMD was previously determined for the cadaver bones used in this study with high-resolution peripheral quantitative computed tomography (18). Commercial cortical bone screws (stainless steel AISI 316L; Zimmer Inc., Warsaw, USA) with an outer diameter of 2.7 mm (inner 2.0 mm) and 3.5 mm (inner 2.5 mm) were inserted and pulled out from the test blocks. Each of the test blocks was of 75-80 mm length, 25-30 mm width and 4 mm thickness, and 3 screws were inserted in each block (see Fig. 1). The total number of experiments in each material/coating combination was 6-9. Pilot holes were drilled in the test blocks following manufacturer-specified size recommendations. A constant speed of 5 rpm for the insertion was achieved by an electric motor attached to the servohydraulic test equipment (Instron 8874; Instron, Norwood, MA, USA) maintaining a constant axial load (20 N) on the screws during the insertion. A 10 Nm load cell with an accuracy ± 1% of reading was used to measure insertion and removal torque. Screw insertion was carried out in diluted bovine serum with a total protein content of 24 mg/mL, using standard additives to prevent microbial activity. The additives included penicillin-streptomycin solution (EuroClone S.p.A., Siziano, Italy) and ethylenediaminetetraacetic acid (EDTA)disodium salt (Merck KGaA, Darmstadt, Germany). After insertion, the screws were pulled out from the test block with a constant displacement speed of 5 mm/min. Servohydraulic test equipment (model 8874; Instron Co,
Materials and methods
TABLE I - MECHANICAL PROPERTIES OF TEST BLOCKS USED IN THE PRESENT STUDY
Synthetic bone blocks of various materials were used to find a more consistent model compared with cadaveric bone. In contrast to cadaveric bones, synthetic test specimens have low variance in mechanical properties and are readily available with well-specified properties. Furthermore, there is no risk of contamination or decomposition with the synthetic materials. The use of human bone samples in the present study received the permission of the Finnish National Authority for Medicolegal Affairs (TEO, 1781/32/200/01).
Screw insertion and pullout tests
Material
Ultimate tensile Elastic modulus Elongation Density strength σ (MPa) E (GPa) ε (%) ρ (g/cm3)
Human cortical bone
133*
10-15*
1-3*
1.2†
POM
65‡
3.0‡
>30‡
1.4‡
PMMA
74
3.0-3.3
5
1.1‡
12.4
0.85
Sawbones
®
‡
90
‡
‡
‡
‡ ‡
1.7‡
PMMA=poly(methyl methacrylate); POM=polyacetal. *Data adopted from (20). † Data adopted from (18). ‡ Data adopted from (21).
© 2014 Società Italiana Biomateriali - eISSN 2280-8000
91
Sliding properties affecting screw fixation
diluted bovine serum with standard additives as described above. DLC coating was applied on screws and balls for interfacial sliding tests by ultrashort pulsed laser ablation techniques (ColdabTM) developed by Picodeon Ltd. (Helsinki, Finland). High-quality DLC coatings have a high fraction of sp3 bonding, with up to 85% achieved using several deposition techniques (25). Coating thickness in this application was about 400 nm as estimated by interference techniques. Statistical analysis One-way ANOVA was used to evaluate differences in insertion torque and pullout strength for different materials. Student’s t-test was used for comparison of the effect of surface modification. Pearson correlation analysis (2-tailed) was performed to evaluate correlations between different parameters. Results Screw insertion and pullout
Fig. 1 - Test setup for inserting the screws and for torque measurements.
Norwood, MA, USA) with a 1 kN load cell (accuracy ± 1% of reading) and a custom-made pullout clamp was used in the testing. Interfacial sliding tests Friction between test blocks and stainless steel or DLC-coated stainless steel was tested by using a pinon-disk apparatus described in standard ASTM G99-95 (24). For the pin, AISI 316 stainless steel balls 6 mm in diameter (density: 7.96 g/cm3, Brinell hardness: 160-190) were used mimicking the screw material (Goodfellow Cambridge Ltd., Huntingdon, UK). In the tests, the sample was placed on a rotated holder while the ball was attached to a lever with a dead weight (load). The digital force signal from a strain gauge was recorded, and the CoF was determined. The diameter of the sliding track was 4.5 mm, and a sliding speed of 3.5 mm/s with an applied load of 9.8-79.0 g was used. A relatively slow sliding speed was used to simulate in vitro testing conditions for torque measurements as per standard ASTM F543-07, and only friction during the initial 50 revolutions (sliding distance 71 cm) was determined for each axial load. Tests were carried out in 92
For both screw sizes, the highest insertion torque values were recorded for Sawbones® and the lowest for human cadaver bone (see Figs. 2 and 3). Insertion torque was significantly higher (P
