Biomechanical and finite element analyses of bone cement-injectable cannulated pedicle screw fixation in osteoporotic bone Yaoyao Liu,1† Jianzhong Xu,2† Dong Sun,2 Fei Luo,2 Zehua Zhang,2 Fei Dai2* 1
Department of Spine Surgery, Daping Hospital, The Third Military Medical University, Chongqing 400042, People’s Republic of China 2 Department of Orthopedics, Southwest Hospital, The Third Military Medical University, Chongqing 404100, People’s Republic of China Received 1 October 2014; revised 11 March 2015; accepted 27 March 2015 Published online 15 May 2015 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.33424 Abstract: The objectives of this study were to investigate the safety and biomechanical stability of a polymethylmethacrylate (PMMA)-augmented bone cement-injectable cannulated pedicle screw (CICPS) in cancellous bone model, and to analyze the stress distribution at the screw-cement-bone interface. The OMEGA cannulated pedicle screw (OPS) and conventional pedicle screw (CPS) were used as control groups. Safety of the CICPS was evaluated by the static bending and bending fatigue tests. Biomechanical stability was analyzed by the maximum axial pullout strength and maximum torque tests. Stress distribution at the screw–cement– bone interface was analyzed by the finite element (FE) method. The CICPS and CPS produced statistically similar values for bending stiffness, bending structural stiffness, and
bending yield moment. The maximum pullout force was 53.47 6 8.65 N in CPS group, compared to 130.82 6 7.32 N and 175.45 6 43.01 N in the PMMA-augmented OPS and CICPS groups, respectively (p < 0.05). The CICPS had a significantly greater torque than the OPS and CPS. The FE model did not reveal excessive stress at the screw-cement-bone interface in the CICPS group. In conclusion, PMMAaugmentation with CICPS may be a potentially useful method to increase the stability of pedicle screws in patients with C 2015 Wiley Periodicals, Inc. J Biomed Mater Res osteoporosis. V Part B: Appl Biomater, 104B: 960–967, 2016.
Key Words: PMMA, osteoporosis, finite element modeling, biomechanics of spine, screw augmentation
How to cite this article: Liu Y, Xu J, Sun D, Luo F, Zhang Z, Dai F. 2016. Biomechanical and finite element analyses of bone cement-injectable cannulated pedicle screw fixation in osteoporotic bone. J Biomed Mater Res Part B 2016:104B:960–967.
INTRODUCTION
Pedicle screw instrumentation is widely used to obtain rigid internal fixation for the treatment of degenerative spinal diseases with osteoporosis, such as spondylolisthesis, intervertebral disc protrusion, spinal canal stenosis, and vertebral compression fractures. However, enhancing the screw fixation strength in patients with osteoporosis can be a challenge for spinal surgeons because of the low bone mineral density.1,2 Applying pedicle screws in osteoporotic bone has many potential complications, such as screw loosening, migration, and pullout.3 Several techniques have been proposed to increase the strength of pedicle screw fixation, such as increasing the diameter or length of the pedicle screw,4–7 using a bone cement-augmented pedicle screw,8–14 and improving the design of the screw-rod.15–18 Currently, polymethylmethacrylate (PMMA)-augmented screw fixation is considered to be an efficient method.19–23 However, the PMMA hardening reaction is exothermic, which can be problematic when it occurs in close proximity to neural elements.24,25 In more serious cases, pulmonary embolism,26
paraplegia,27 or even death28 could occur due to PMMA leakage. To prevent the above-mentioned complications, a novel type of cannulated pedicle screw, called the bone cementinjectable cannulated pedicle screw (CICPS), has been designed. The purposes of this study were to investigate the safty of the CICPS and to determine whether PMMA augmentation can improve the fixation strength of the CICPS compared to an existing product (OMEGA pedicle screw [OPS]) or a conventional pedicle screw (CPS). And stress distribution at the screw-cement-bone interface was analyzed by the finite element (FE) method. MATERIAL AND METHODS
Screw design The CICPS is a barrel-shaped screw with a 3-mm pitch and an outer diameter and length of various specifications. The cannulation diameter of the CICPS is 2.2 mm. Three side holes (round, 2-mm diameter; oval, 3 mm 3 2 mm; and Ushaped, 4 mm 3 2 mm) are arranged in increasing size
† Both authors contributed equally to this work. *Correspondence to: F. Dai; e-mail: [email protected] Contract grant sponsor: Chongqing City Project; contract grant number: CSTC2012 gg-yyjs10015
960
C 2015 WILEY PERIODICALS, INC. V
ORIGINAL RESEARCH REPORT
FIGURE 1. (A) Photograph of the bone cement-injectable cannulated pedicle screw (CICPS). (B) Schematic drawing showing the PMMA outflow channel and three side holes (arrows), which were arranged in increasing size at the distal end of the screw. (C) Photograph of the specially designed syringe, which has a degree scale for precisely measuring the amount of bone cement.
along the distal end of the screw [Figure 1(A,B)]. After insertion of the CICPS, bone cement is injected with a specially designed adapter and a finely graduated syringe [Figure 1C].
(Siemens SOMATOM Emotion 16, slice thickness of 5 mm, reconstruction interval of 2 mm, pitch of 1.25 mm, and tube current of 175 mA) were performed to observe the distribution of cement in the synthetic specimen.
Osteoporotic polyurethane foam specimens Commercially available synthetic bone (1522–505#, Sawbones, Pacific Research Laboratory, Vashon Island, WA) was used to simulate osteoporotic spinal bone. Test specimens (6 cm 3 6.5 cm 3 4 cm) were extracted with a fretsaw from cellular polyurethane blocks (13 cm 3 18 cm 3 4 cm) and used as the biomechanical testing material. Open-cell rigid polyurethane foam was used as the synthetic test specimen (density 5 0.09 g cm23). The foam has uniform and consistent mechanical properties that make it ideal for comparing pedicle screws, as well as for use in other medical devices and instruments.29
Mechanical testing Biomechanical tests were performed at the Technology Transfer Center of Chung Yuan Christian University. The load cell (model no. 662.20D-05) of the material testing machine (model no. MTS MINI BIONIX 858II) had a maximum axial load of 25 kN and maximum torsional load of 250 N m21. Ten samples in each of the three groups (CPS, OPS, and CICPS) were tested. All of the pedicle screws measured 6.5 mm in diameter by 45 mm in length. A specially designed fixture frame, including a stainless steel anchored fixture and loading bar, was used in the static bending and bending fatigue tests [Figure 2(A)]. A cuboid fixture was designed for the pullout and torque tests [Figure 2(B)].
Screw placement technique and imaging observation Three screw types, requiring or not requiring PMMA augmentation, were investigated in this study: the CICPS, OPS (SmartLoc Spinal Fixation System, A-Spine Asia, Taiwan), and CPS (Kanghui Medical Devices, Jiangsu, China). To ensure that all screws were inserted under the same conditions, screw insertions were performed by the same experienced orthopedic surgeon. Pilot holes (3-mm-diameter) parallel to the 6.5 cm edge were drilled into the test samples before screw insertion. Pedicle screws were tightened until the screw head met the surface of the block. For the groups with PMMA augmentation (CICPS and OPS), after the screws were inserted into the specimens, bone cement (Simplex P, Stryker, USA) was mixed according to the manufacturer’s instructions. The time window for injection was 1–1.5 min after bone cement mixing. Once the cement reached the injection time, it was immediately poured into the syringe. Then, 1.5 mL of PMMA was precisely injected into each of the screws for augmentation. Lateral radiographic (Philips X-ray Machine) and threedimensional computed tomography (3D-CT) reconstruction
Static bending test. For this test, the cantilever beam bending mode of the material testing machine was used at a loading rate of 25 mm min21. Each specimen was loaded with a continuous force until the load-displacement curve entered the plastic zone (i.e., when the load began to decrease constantly). The computer automatically recorded the position and mode of failure. Bending fatigue test. The maximum load for this test was set as 600, 1050, or 1500 N. The loading wave was a sine wave with an R value of 0.1 and frequency of 10 Hz. Each specimen was subjected to the dynamic test until failure (i.e., any part of the screw experienced permanent deformation affecting its function, and the loading wave changed to a straight line) or until 2.5 3 106 cycles. Maximum axial pullout strength and maximum torque tests. Rather than seeking to determine the exact strength value for pedicle screws in osteoporotic bone, these tests aimed to compare the screws in different groups in terms of
JOURNAL OF BIOMEDICAL MATERIALS RESEARCH B: APPLIED BIOMATERIALS | JUL 2016 VOL 104B, ISSUE 5
961
FIGURE 2. Experimental setups for the static bending/fatigue tests (A) and pullout/torque tests (B). Photographs show the test specimens with screw insertion during testing. The test block was placed on a specially designed universal fixture and clamped on the lower side of the MTS MINI BIONIX testing machine.
their resistance to maximal intensity. After the PMMA had solidified under ambient temperature, the specimen unit was placed in a specially designed fixture frame. The screw head was fixed in a wire, which was clamped to the testing machine. For the maximum axial pullout test, a straight axial pullout force was applied at a constant crosshead rate of 2 mm min21.30 The relationship between the load and displacement was recorded. The maximum pullout force was defined as the peak load before the load abruptly decreased. For the maximum torque test, the screw was rotated counterclockwise at a constant rate of 1 s21 along the direction of pedicle screw axis. The prestress value was 10 N. The load was applied until the torsion value significantly decreased. The acquisition system automatically recorded the load signal and generated the torque-angle curve. Finite element analysis The 3D geometries of the block with a threaded hole and the screw were separately created in the Pro/Engineer soft-
ware (PTC, Needham, MA) and imported into the ANSYS Workbench (v11.0) for mesh generation. Mesh refinement was used to represent the stress distribution around the cancellous bone tissue in the threaded region.31,32 A convergence test was conducted to determine the minimum number of meshed elements in the simulation by examining the total strain energy. Results of the convergence test showed that further refinement would not alter the magnitude of the total strain energy by 2% or more [Figure 3(A)] when 62,622 tetrahedral elements are assigned to the FE model [Figure 3(B,C)]. The meshed screw and block models were assembled to generate the final model. To simulate the interactions between the screw, cement, and bone at their interface, the surface-to-surface contact relationships were modeled. Contact pairs were defined between the screw, cement, and bone. The frictional coefficient was set to 0.3.33 Materials were assumed to be perfectly elastic, homogeneous, and isotropic. The boundary condition was that the nail head is exposed on the surface of the bone model. The other five surfaces were fixed, to
FIGURE 3. (A) Results of the convergence test for determining the appropriate number of elements for the finite element model. (B,C) Finite element models of the screw-cement-bone complex (B) and the screw-bone unit (C).
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STABILITY OF BONE CICPS FIXATION
ORIGINAL RESEARCH REPORT
TABLE I. Properties of Materials Used in this Studya Material Pedicle screw (Ti alloy) PMMA Cancellous bone model (rigid polyurethane foam) a
E (Young’s Modulus)
v (Poisson’s Ratio)
110 GPa 3500 MPa 53 MPa
0.3 0.3 0.25
Material properties were adopted from the literature.33,34
achieve a stable effect. Table I reports the material properties assigned to all components.34,35 Stress distribution at the screw-cement-bone interface was evaluated during simulations of the axial pullout strength and torque tests. The maximum axial pullout force was set as 1000 N. The maximum torque was set as 5 N m21 in the clockwise direction (the counterclockwise direction could not be analyzed because the screw would slip out). Magnitudes of the aforementioned loads were defined to confirm the theoretical pullout force and to obtain the failure torque. The failure criterion of the test block was defined as a maximum load of 0.67 MPa, as described in published information on the material (sawbone open cell test block 15 pcf, http://www.sawbones. com/Catalog/Biomechanical/Biomechanical%20Test%20Mat erials/1522-524). Statistical analysis Statistical comparisons were made with SPSS for Windows version 13.0 (SPSS, Chicago, IL). Descriptive statistical results were expressed as the mean 6 standard deviation (SD). Multiple comparisons were used to compare among the CPS, OPS, and CICPS groups. A p values < 0.05 was considered statistically significant. RESULTS
Biomechanical study Static bending and bending fatigue tests. Results of the static bending test are shown in Table II. The CICPS and CPS had similar bending moments (p > 0.05). The bending stiffness and bending structural stiffness values of the CICPS were higher than those of the CPS (1693.68 6 306.45 N mm21 vs. 1416.30 6 185.99 N mm21 and 0.56 6 0.10 N m22 vs. 0.47 6 0.06 N m22), and the bending yield moment was lower (19.10 6 2.63 N m21 vs. 21.23 6 1.87
N m21). The ultimate displacement of the CPS was significantly greater than that of the CICPS, indicating that the ductility of the CPS was higher than that of the CICPS. During the bending fatigue test, screws that broke or became permanently twisted were considered to be destroyed. Under 600 N (approximately equal to the upper body weight of a human adult), no damage to the CPS, CICPS, or OPS was observed after 2.5 3 106 cycles of dynamic loading. However, fatigue related damage was found in all three groups under 1050 or 1500 N [Figure 4(A–C)]. The CPS endured more loading cycles (21,430 6 3542 cycles) before fatigue than the CICPS (19,017 6 2465 cycles) or the OPS (6179 6 2593 cycles). Maximum axial pullout strength and maximum torque tests. The X-ray and 3D-CT scanning results showed that the PMMA distribution was conical in the CICPS and globular in the OPS. The area of PMMA distribution along the screw axis in the CICPS was broader than that in the OPS (Figure 5). All samples were loaded to failure in the biomechanical machine, and then the screw/cement complex was completely pulled out from the cancellous bone model (Figure 6). Diagrams of the pullout and torque tests are shown in Figure 7(A,B), respectively. The mean maximum force at pullout was significantly lower for the CPS (53.47 6 8.65 N) compared to the force for the PMMA-augmented OPS or CICPS (130.50 6 7.32 N and 175.45 6 43.01 N, respectively). Although the average maximum pullout force for CICPS was 45 N greater than that for the OPS, this difference was not significant (p 5 0.06; Figure 8). The mean maximum torque values for CPS, CICPS, and OPS were 102.17 6 22.58 N m21, 863.38 6 156.05 N m21, and 800.24 6 57.33 N m21, respectively (p < 0.01 for CPS vs. CICPS or OPS by Student’s t test). The mean torque for the CICPS was 63 N m21 greater than that of the OPS (p 5 0.0351; Figure 9). Compared to the nonaugmented CPS, the PMMA-augmented CICPS and OPS exhibited significantly increased values of pullout strength (by 328.1 and 244.7%, respectively) and torque (by 845.0 and 783.2%, respectively). Finite element analysis The stress distribution of the CICPS at the screw–cement– bone interface was observed while simulating the maximum axial pullout strength and maximum torque tests. In the
TABLE II. Comparison of the Static Test Results for the CICPS and CPS Parameter 21
Bending stiffness (N mm ) Bending structural stiffness (N m22) Bending yield moment (N m21) Ultimate load (N) Bending ultimate moment (N m21) Ultimate displacement (mm) a
CPS
CICPS
p Value
1416.30 6 185.99 0.47 6 0.06 21.23 6 1.87 3242.19 6 146.08 32.42 6 1.46 6.06 6 0.72
1693.68 6 306.45 0.56 6 0.10 19.10 6 2.63 3100.47 6 175.72 31.00 6 1.76 4.54 6 1.32
0.060 0.057 0.088 0.101 0.101 0.026a
Statistically significant difference (p < 0.05).
JOURNAL OF BIOMEDICAL MATERIALS RESEARCH B: APPLIED BIOMATERIALS | JUL 2016 VOL 104B, ISSUE 5
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FIGURE 4. Fatigue related damage was found in the CICPS (A), OMEGA pedicle screw (OPS, B), and conventional pedicle screw (CPS, C) under loads of 1500 N. All fractures occurred at the head-rod connecting part between two threads (indicated by arrows).
pullout test, the highest von Mises stress was 6.83 MPa under a pullout force of 1000 N. Around the bone interface, most of the simulated load was
Department of Spine Surgery, Daping Hospital, The Third Military Medical University, Chongqing 400042, People’s Republic of China 2 Department of Orthopedics, Southwest Hospital, The Third Military Medical University, Chongqing 404100, People’s Republic of China Received 1 October 2014; revised 11 March 2015; accepted 27 March 2015 Published online 15 May 2015 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.33424 Abstract: The objectives of this study were to investigate the safety and biomechanical stability of a polymethylmethacrylate (PMMA)-augmented bone cement-injectable cannulated pedicle screw (CICPS) in cancellous bone model, and to analyze the stress distribution at the screw-cement-bone interface. The OMEGA cannulated pedicle screw (OPS) and conventional pedicle screw (CPS) were used as control groups. Safety of the CICPS was evaluated by the static bending and bending fatigue tests. Biomechanical stability was analyzed by the maximum axial pullout strength and maximum torque tests. Stress distribution at the screw–cement– bone interface was analyzed by the finite element (FE) method. The CICPS and CPS produced statistically similar values for bending stiffness, bending structural stiffness, and
bending yield moment. The maximum pullout force was 53.47 6 8.65 N in CPS group, compared to 130.82 6 7.32 N and 175.45 6 43.01 N in the PMMA-augmented OPS and CICPS groups, respectively (p < 0.05). The CICPS had a significantly greater torque than the OPS and CPS. The FE model did not reveal excessive stress at the screw-cement-bone interface in the CICPS group. In conclusion, PMMAaugmentation with CICPS may be a potentially useful method to increase the stability of pedicle screws in patients with C 2015 Wiley Periodicals, Inc. J Biomed Mater Res osteoporosis. V Part B: Appl Biomater, 104B: 960–967, 2016.
Key Words: PMMA, osteoporosis, finite element modeling, biomechanics of spine, screw augmentation
How to cite this article: Liu Y, Xu J, Sun D, Luo F, Zhang Z, Dai F. 2016. Biomechanical and finite element analyses of bone cement-injectable cannulated pedicle screw fixation in osteoporotic bone. J Biomed Mater Res Part B 2016:104B:960–967.
INTRODUCTION
Pedicle screw instrumentation is widely used to obtain rigid internal fixation for the treatment of degenerative spinal diseases with osteoporosis, such as spondylolisthesis, intervertebral disc protrusion, spinal canal stenosis, and vertebral compression fractures. However, enhancing the screw fixation strength in patients with osteoporosis can be a challenge for spinal surgeons because of the low bone mineral density.1,2 Applying pedicle screws in osteoporotic bone has many potential complications, such as screw loosening, migration, and pullout.3 Several techniques have been proposed to increase the strength of pedicle screw fixation, such as increasing the diameter or length of the pedicle screw,4–7 using a bone cement-augmented pedicle screw,8–14 and improving the design of the screw-rod.15–18 Currently, polymethylmethacrylate (PMMA)-augmented screw fixation is considered to be an efficient method.19–23 However, the PMMA hardening reaction is exothermic, which can be problematic when it occurs in close proximity to neural elements.24,25 In more serious cases, pulmonary embolism,26
paraplegia,27 or even death28 could occur due to PMMA leakage. To prevent the above-mentioned complications, a novel type of cannulated pedicle screw, called the bone cementinjectable cannulated pedicle screw (CICPS), has been designed. The purposes of this study were to investigate the safty of the CICPS and to determine whether PMMA augmentation can improve the fixation strength of the CICPS compared to an existing product (OMEGA pedicle screw [OPS]) or a conventional pedicle screw (CPS). And stress distribution at the screw-cement-bone interface was analyzed by the finite element (FE) method. MATERIAL AND METHODS
Screw design The CICPS is a barrel-shaped screw with a 3-mm pitch and an outer diameter and length of various specifications. The cannulation diameter of the CICPS is 2.2 mm. Three side holes (round, 2-mm diameter; oval, 3 mm 3 2 mm; and Ushaped, 4 mm 3 2 mm) are arranged in increasing size
† Both authors contributed equally to this work. *Correspondence to: F. Dai; e-mail: [email protected] Contract grant sponsor: Chongqing City Project; contract grant number: CSTC2012 gg-yyjs10015
960
C 2015 WILEY PERIODICALS, INC. V
ORIGINAL RESEARCH REPORT
FIGURE 1. (A) Photograph of the bone cement-injectable cannulated pedicle screw (CICPS). (B) Schematic drawing showing the PMMA outflow channel and three side holes (arrows), which were arranged in increasing size at the distal end of the screw. (C) Photograph of the specially designed syringe, which has a degree scale for precisely measuring the amount of bone cement.
along the distal end of the screw [Figure 1(A,B)]. After insertion of the CICPS, bone cement is injected with a specially designed adapter and a finely graduated syringe [Figure 1C].
(Siemens SOMATOM Emotion 16, slice thickness of 5 mm, reconstruction interval of 2 mm, pitch of 1.25 mm, and tube current of 175 mA) were performed to observe the distribution of cement in the synthetic specimen.
Osteoporotic polyurethane foam specimens Commercially available synthetic bone (1522–505#, Sawbones, Pacific Research Laboratory, Vashon Island, WA) was used to simulate osteoporotic spinal bone. Test specimens (6 cm 3 6.5 cm 3 4 cm) were extracted with a fretsaw from cellular polyurethane blocks (13 cm 3 18 cm 3 4 cm) and used as the biomechanical testing material. Open-cell rigid polyurethane foam was used as the synthetic test specimen (density 5 0.09 g cm23). The foam has uniform and consistent mechanical properties that make it ideal for comparing pedicle screws, as well as for use in other medical devices and instruments.29
Mechanical testing Biomechanical tests were performed at the Technology Transfer Center of Chung Yuan Christian University. The load cell (model no. 662.20D-05) of the material testing machine (model no. MTS MINI BIONIX 858II) had a maximum axial load of 25 kN and maximum torsional load of 250 N m21. Ten samples in each of the three groups (CPS, OPS, and CICPS) were tested. All of the pedicle screws measured 6.5 mm in diameter by 45 mm in length. A specially designed fixture frame, including a stainless steel anchored fixture and loading bar, was used in the static bending and bending fatigue tests [Figure 2(A)]. A cuboid fixture was designed for the pullout and torque tests [Figure 2(B)].
Screw placement technique and imaging observation Three screw types, requiring or not requiring PMMA augmentation, were investigated in this study: the CICPS, OPS (SmartLoc Spinal Fixation System, A-Spine Asia, Taiwan), and CPS (Kanghui Medical Devices, Jiangsu, China). To ensure that all screws were inserted under the same conditions, screw insertions were performed by the same experienced orthopedic surgeon. Pilot holes (3-mm-diameter) parallel to the 6.5 cm edge were drilled into the test samples before screw insertion. Pedicle screws were tightened until the screw head met the surface of the block. For the groups with PMMA augmentation (CICPS and OPS), after the screws were inserted into the specimens, bone cement (Simplex P, Stryker, USA) was mixed according to the manufacturer’s instructions. The time window for injection was 1–1.5 min after bone cement mixing. Once the cement reached the injection time, it was immediately poured into the syringe. Then, 1.5 mL of PMMA was precisely injected into each of the screws for augmentation. Lateral radiographic (Philips X-ray Machine) and threedimensional computed tomography (3D-CT) reconstruction
Static bending test. For this test, the cantilever beam bending mode of the material testing machine was used at a loading rate of 25 mm min21. Each specimen was loaded with a continuous force until the load-displacement curve entered the plastic zone (i.e., when the load began to decrease constantly). The computer automatically recorded the position and mode of failure. Bending fatigue test. The maximum load for this test was set as 600, 1050, or 1500 N. The loading wave was a sine wave with an R value of 0.1 and frequency of 10 Hz. Each specimen was subjected to the dynamic test until failure (i.e., any part of the screw experienced permanent deformation affecting its function, and the loading wave changed to a straight line) or until 2.5 3 106 cycles. Maximum axial pullout strength and maximum torque tests. Rather than seeking to determine the exact strength value for pedicle screws in osteoporotic bone, these tests aimed to compare the screws in different groups in terms of
JOURNAL OF BIOMEDICAL MATERIALS RESEARCH B: APPLIED BIOMATERIALS | JUL 2016 VOL 104B, ISSUE 5
961
FIGURE 2. Experimental setups for the static bending/fatigue tests (A) and pullout/torque tests (B). Photographs show the test specimens with screw insertion during testing. The test block was placed on a specially designed universal fixture and clamped on the lower side of the MTS MINI BIONIX testing machine.
their resistance to maximal intensity. After the PMMA had solidified under ambient temperature, the specimen unit was placed in a specially designed fixture frame. The screw head was fixed in a wire, which was clamped to the testing machine. For the maximum axial pullout test, a straight axial pullout force was applied at a constant crosshead rate of 2 mm min21.30 The relationship between the load and displacement was recorded. The maximum pullout force was defined as the peak load before the load abruptly decreased. For the maximum torque test, the screw was rotated counterclockwise at a constant rate of 1 s21 along the direction of pedicle screw axis. The prestress value was 10 N. The load was applied until the torsion value significantly decreased. The acquisition system automatically recorded the load signal and generated the torque-angle curve. Finite element analysis The 3D geometries of the block with a threaded hole and the screw were separately created in the Pro/Engineer soft-
ware (PTC, Needham, MA) and imported into the ANSYS Workbench (v11.0) for mesh generation. Mesh refinement was used to represent the stress distribution around the cancellous bone tissue in the threaded region.31,32 A convergence test was conducted to determine the minimum number of meshed elements in the simulation by examining the total strain energy. Results of the convergence test showed that further refinement would not alter the magnitude of the total strain energy by 2% or more [Figure 3(A)] when 62,622 tetrahedral elements are assigned to the FE model [Figure 3(B,C)]. The meshed screw and block models were assembled to generate the final model. To simulate the interactions between the screw, cement, and bone at their interface, the surface-to-surface contact relationships were modeled. Contact pairs were defined between the screw, cement, and bone. The frictional coefficient was set to 0.3.33 Materials were assumed to be perfectly elastic, homogeneous, and isotropic. The boundary condition was that the nail head is exposed on the surface of the bone model. The other five surfaces were fixed, to
FIGURE 3. (A) Results of the convergence test for determining the appropriate number of elements for the finite element model. (B,C) Finite element models of the screw-cement-bone complex (B) and the screw-bone unit (C).
962
LIU ET AL.
STABILITY OF BONE CICPS FIXATION
ORIGINAL RESEARCH REPORT
TABLE I. Properties of Materials Used in this Studya Material Pedicle screw (Ti alloy) PMMA Cancellous bone model (rigid polyurethane foam) a
E (Young’s Modulus)
v (Poisson’s Ratio)
110 GPa 3500 MPa 53 MPa
0.3 0.3 0.25
Material properties were adopted from the literature.33,34
achieve a stable effect. Table I reports the material properties assigned to all components.34,35 Stress distribution at the screw-cement-bone interface was evaluated during simulations of the axial pullout strength and torque tests. The maximum axial pullout force was set as 1000 N. The maximum torque was set as 5 N m21 in the clockwise direction (the counterclockwise direction could not be analyzed because the screw would slip out). Magnitudes of the aforementioned loads were defined to confirm the theoretical pullout force and to obtain the failure torque. The failure criterion of the test block was defined as a maximum load of 0.67 MPa, as described in published information on the material (sawbone open cell test block 15 pcf, http://www.sawbones. com/Catalog/Biomechanical/Biomechanical%20Test%20Mat erials/1522-524). Statistical analysis Statistical comparisons were made with SPSS for Windows version 13.0 (SPSS, Chicago, IL). Descriptive statistical results were expressed as the mean 6 standard deviation (SD). Multiple comparisons were used to compare among the CPS, OPS, and CICPS groups. A p values < 0.05 was considered statistically significant. RESULTS
Biomechanical study Static bending and bending fatigue tests. Results of the static bending test are shown in Table II. The CICPS and CPS had similar bending moments (p > 0.05). The bending stiffness and bending structural stiffness values of the CICPS were higher than those of the CPS (1693.68 6 306.45 N mm21 vs. 1416.30 6 185.99 N mm21 and 0.56 6 0.10 N m22 vs. 0.47 6 0.06 N m22), and the bending yield moment was lower (19.10 6 2.63 N m21 vs. 21.23 6 1.87
N m21). The ultimate displacement of the CPS was significantly greater than that of the CICPS, indicating that the ductility of the CPS was higher than that of the CICPS. During the bending fatigue test, screws that broke or became permanently twisted were considered to be destroyed. Under 600 N (approximately equal to the upper body weight of a human adult), no damage to the CPS, CICPS, or OPS was observed after 2.5 3 106 cycles of dynamic loading. However, fatigue related damage was found in all three groups under 1050 or 1500 N [Figure 4(A–C)]. The CPS endured more loading cycles (21,430 6 3542 cycles) before fatigue than the CICPS (19,017 6 2465 cycles) or the OPS (6179 6 2593 cycles). Maximum axial pullout strength and maximum torque tests. The X-ray and 3D-CT scanning results showed that the PMMA distribution was conical in the CICPS and globular in the OPS. The area of PMMA distribution along the screw axis in the CICPS was broader than that in the OPS (Figure 5). All samples were loaded to failure in the biomechanical machine, and then the screw/cement complex was completely pulled out from the cancellous bone model (Figure 6). Diagrams of the pullout and torque tests are shown in Figure 7(A,B), respectively. The mean maximum force at pullout was significantly lower for the CPS (53.47 6 8.65 N) compared to the force for the PMMA-augmented OPS or CICPS (130.50 6 7.32 N and 175.45 6 43.01 N, respectively). Although the average maximum pullout force for CICPS was 45 N greater than that for the OPS, this difference was not significant (p 5 0.06; Figure 8). The mean maximum torque values for CPS, CICPS, and OPS were 102.17 6 22.58 N m21, 863.38 6 156.05 N m21, and 800.24 6 57.33 N m21, respectively (p < 0.01 for CPS vs. CICPS or OPS by Student’s t test). The mean torque for the CICPS was 63 N m21 greater than that of the OPS (p 5 0.0351; Figure 9). Compared to the nonaugmented CPS, the PMMA-augmented CICPS and OPS exhibited significantly increased values of pullout strength (by 328.1 and 244.7%, respectively) and torque (by 845.0 and 783.2%, respectively). Finite element analysis The stress distribution of the CICPS at the screw–cement– bone interface was observed while simulating the maximum axial pullout strength and maximum torque tests. In the
TABLE II. Comparison of the Static Test Results for the CICPS and CPS Parameter 21
Bending stiffness (N mm ) Bending structural stiffness (N m22) Bending yield moment (N m21) Ultimate load (N) Bending ultimate moment (N m21) Ultimate displacement (mm) a
CPS
CICPS
p Value
1416.30 6 185.99 0.47 6 0.06 21.23 6 1.87 3242.19 6 146.08 32.42 6 1.46 6.06 6 0.72
1693.68 6 306.45 0.56 6 0.10 19.10 6 2.63 3100.47 6 175.72 31.00 6 1.76 4.54 6 1.32
0.060 0.057 0.088 0.101 0.101 0.026a
Statistically significant difference (p < 0.05).
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FIGURE 4. Fatigue related damage was found in the CICPS (A), OMEGA pedicle screw (OPS, B), and conventional pedicle screw (CPS, C) under loads of 1500 N. All fractures occurred at the head-rod connecting part between two threads (indicated by arrows).
pullout test, the highest von Mises stress was 6.83 MPa under a pullout force of 1000 N. Around the bone interface, most of the simulated load was
