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Fatigue failure of an orthopedic implant – A locking compression plate
Engineering Failure Analysis 15 (2008) 521–530 www.elsevier.com/locate/engfailanal
Fatigue failure of an orthopedic implant – A locking compression plate C. Kanchanomai a
a,*
, V. Phiphobmongkol b, P. Muanjan
a
Department of Mechanical Engineering, Faculty of Engineering, Thammasat University, Klong-Luang, Pathumthani 12120, Thailand b Department of Orthopedic Surgery, Bhumibol Adulyadej Hospital, Royal Thai Air Force, Bangkok 10210, Thailand Received 15 March 2007; accepted 14 April 2007 Available online 27 April 2007
Abstract In the present work, the fatigue failure of a locking compression plate (LCP) fixed across a transverse fracture (8-mm gap) at the midshaft of femur was experimentally evaluated. The complete fracture of LCP occurred after 42,000 cycles of loading, i.e. equivalence to about 8 days of walking. The fatigue failure of LCP was possible before the adequate healing of fracture, and the full load of walking should not be allowed for the patient with the present fracture condition. The fatigue crack firstly initiated from a subsurface inclusion embedded under the surface of compression hole. After some cycles of loading, another fatigue crack also initiated from the surface of locking hole, and then both cracks propagated inside the LCP. As an evidence of the propagation of fatigue crack, the striations were observed on the fracture surface of the LCP. The striation spacing was long when observed far from the crack initiation site, and became shorter when observed around the crack initiation site. Based on the striation spacing, the number of cycles for the propagation of fatigue crack from the initiation site to the bottom part of LCP was estimated to be approximately 5000 cycles. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Locking compression plate; Femur; Fatigue; Crack initiation; Crack propagation
1. Introduction The dynamic compression plate (DCP) is one of the most commonly used implants for internal fixation. In regular DCP system, the forces acting on femur are bypassed across the fracture area, thus the fracture site is protected and the alignment is maintained throughout the healing process [1]. In order to obtain a stable fixation, the screws on the DCP must be pressed or tightened into the DCP holes, which means the plate compressed the surface of bone. High amount of load will be transmitted from the screws through the bone-plate interface. Thus, the stability of bone and plate complex is achieved. However, the existing compression force between bone and plate may cause the vascular damage to the undersurface bone tissue, which results in unfavorable conditions for bone healing under the plate. Recently, the locking compression plate (LCP) has been *
Corresponding author. Tel.: +66 02 564 3001; fax: +66 02 564 3010. E-mail address: [email protected] (C. Kanchanomai).
1350-6307/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2007.04.001
522
C. Kanchanomai et al. / Engineering Failure Analysis 15 (2008) 521–530
introduced by an association for the study of osteosynthesis/an association for the study of internal fixation (AO/ASIF). This new LCP has combined the advantages of compression hole of DCP with the advantages of threaded hole by introducing combination holes [2–4], as shown in Fig. 1a. The threaded part of the combination hole was designed to use with a locking head screw (Fig. 1b). If the locking head screws are fastened through the conical thread holes of LCP, the load is transmitted through screw-plate system without compression between plate and bone surface, which maintains space and preserve vascular supply to the injured bone. On the other hand, regular screw can be used through the conventional compression hole located on the other side of the combination hole to function as regular DCP. The LCP system is therefore possible to serve for both purposes, i.e. compression plate system or locking plate system. In addition to biological compatibility of LCP to treat femoral fracture, the endurance of the LCP is also one of the crucial considerations. In fractured femur, various types of forces act on the implants and the femur, especially cyclic load acting on implants during walking. The fatigue damage, which is a process of defect accumulation, crack initiation, and crack propagation with number of load cycles, is likely to occur even under low magnitude of cycling load. There were many reports related to the fatigue failure of DCPs, while few researches have been done for LCPs. Van Meeteren et al. [5] studied 40 consecutive patients with subtrochanteric femoral fractures treated with an AO 95° condylar blade plate and found a patient developed a delayed union, which ultimately resulted in repeated plate fractures due to fatigue. Sivakumar et al. [6] studied a man sustained a fracture of the right femur, and a six-holes tubular compression plate (316L stainless steel) was implanted. Eight months later the implanted plate fractured at a screw site adjacent to the site of the original bone fracture. Microscopic examination of the plate fracture surface revealed fatigue striations and beach marks. Azevedo [7] studied the failure of a titanium reconstruction plate for osteosynthesis, and found the corrosion on the surface of the implant, which were in contact with body fluids. The results indicated that the premature fracture of the plate was caused by a corrosion-fatigue mechanism. Sudhakar [8] observed the fracture surface of vitallium (Co–Cr–Mo alloy) plate, and found the evidences of corrosion-fatigue at the interface between implant plate and screw. As the treatment of fractures using LCP increased, more clinical studies have been reported and the advantages of this system have been confirmed. Treatment of femoral shaft fracture with LCP is then an alternative, and has become more popular in treating some indicated cases. However, very few reports regarding fatigue
Fig. 1. (a) Combination hole of LCP; (b) locking head screw [4] and (c) LCP (dimension in mm).
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failure of LCP are available. This study was therefore designed to experimentally evaluate the fatigue failure an LCP (316L stainless steel) when fixed on a diaphyseal-fractured femur (8-mm interfragmentary gap). 2. Material and experimental procedure 2.1. Locking compression plate (LCP) Fourteen holes broad LCP (18 mm-width and 250 mm-length) and 4 locking head screws (4 mm-diameter and 45 mm-length) were used for the present study. The LCP and locking head screw are made from 316L stainless steel. Based on the ASTM recommendation [9], a bar of UNS S31673 or 316L stainless steel was cut, forged, and machined to obtain the geometry as shown in Fig. 1c. Consequently, it was annealed, electropolished, passivated, and then ultrasonically cleaned. To reveal the microstructure (Fig. 2), the LCP was sectioned using an electric discharged machine (EDM), polished, etched with 20 g of picric acid and 100 ml of HCl, and then observed in an scanning electron microscope (SEM). The average grain was approximately 25 lm or ASTM No. 8, i.e. complied with the ASTM recommendation [9]. The hardness of the LCP was 294 HB. The composition of the present LCP was analyzed by an emission spectrometer (Baird: Spectovac 2000 Arc/Spark), as summarized in Table 1. The composition of LCP corresponded to that recommended by the ASTM standard [10]. 2.2. Monotonic loading In order to avoid the inter-specimen variations of the human femurs in different cadavers, the composite large left femurs (Third-generation femur – 3306, Pacific Research Laboratories, Inc., WA) were used for this study. With similar geometry and mechanical properties to those of young human femur, this composite femur has been successfully used in many biomechanical researches [11–13]. A transverse fracture (8-mm gap) on the midshaft of composite femur was fixed with an LCP using 2 locking screws above and below fracture site, i.e
Fig. 2. SEM micrograph of LCP (316L stainless steel).
Table 1 The composition of LCP Element
C
Mn
P
S
Si
Cr
Ni
Mo
LCP 316L Stainless steel [10]
0.018 0.030max
1.84 2.00max
0.025 0.025max
0.009 0.010max
0.305 0.750max
17.82 17–19
14.29 13–15
2.71 – 2.25–3.00 0.100max
N
Cu
Fe
0.073 0.500max
Bal. Bal.
524
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fixation in hole number 1, 6 and 9, 14. The technique for fixation of the locking screws and the LCPs to the composite femurs were the standard procedure used in femoral fixation of human femur [4]. Loading model (Fig. 3a) used in the present work was similar to the model proposed by Cordey et al. [14], which took into account with the forces acting through the ilio-tibial tract in the frontal plane and those forces acting on the femoral condyles in the sagittal plane. A distal femur was supported by a pin and ball bearing in order to prevent undesirable moment and torque. Epoxy resin was used to stabilize the tested composite femur to the loading model. Loadings were performed using a servo-hydraulic fatigue machine (Instron 8801) under 25 °C temperature and 55% relative humidity. The experimental system is shown in Fig. 3b. To simulate the maximum load acting on he femur during walking, the femur was compressed from 0 N to 600 N under 60 N/s loading rate, and held at 600 N for 15 min. Then it was unloaded to 0 N under 60 N/s loading rate. To avoid complication from residue deformation, the femur was left at zero load for 15 min before the next loading was started. The 7 strain gages with 0.3-mm gage length (TML, FLA-03-11) were set along the surface (P1–P7) of an LCP (Fig. 1c). During the test, load as well as deformation of composite femur were recorded by the controller of servo-hydraulic fatigue machine, while the strains on the LCP were collected using computerized data acquisition system (National Instrument: PCI-6013 and LabVIEW 7.0). The loadings were repeated 5 times, and the average values of strain were calculated. 2.3. Cyclic loading Since, the composite femur was made from short E-glass fibers/epoxy resin and solid rigid polyurethane foam, and it was not designed to withstand the cyclic loading. Therefore, fatigue test of LCP was carried out using a 4-point bending specimen, i.e. an LCP fixed on two cylinders (polyvinyl chloride pipe reinforced with epoxy resin), as shown in Fig. 4. The maximum strains distributed on the surface of LCP during fatigue
Fig. 3. (a) Monotonic loading model for composite femur and (b) Experimental system for monotonic loading of composite femur.
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Fig. 4. (a) 4-Point bending specimen and (b) Experimental system for cyclic loading of 4-point bending specimen.
test were controlled to match with the strains distributions of LCP-composite femur model under monotonic loading by adjusting the load positions (L1–L4) and the vertical movement of 4-point bending device, as shown in Fig. 4a. The experimental system is shown in Fig. 4b. In order to maintain contact between specimen and 4-point bending device, the minimum strains distributed on the surface of LCP were controlled to be 5% of the maximum strains. The displacement-controlled fatigue tests (sinusoidal waveform with 3 Hz frequency) were performed by using a servo-hydraulic fatigue machine at 25 °C temperature and 55% relative humidity. The fatigue failure was defined as a complete fracture of LCP. During fatigue test, the applied load, movement of a 4-point bending device, strain distribution on the surface of LCP, and time were simultaneously recorded 100 times in each cycle with computer-controlled data acquisition. After fatigue test, the screw torque was checked for of screw loosing. The fatigue tests in every test condition were performed twice in order to check the repetitiveness. Fracture surfaces of LCP were observed in an SEM (JEOL: JSM-5410), and the mechanisms of fatigue were discussed. 3. Results and discussion 3.1. Strain distribution Under monotonic loading, the variation of strains on an LCP fixed on a composite femur from 5 repeated tests was less than 5%, and the average strains were determined. The tensile strains on LCP surface predominantly distributed between locations P3–P5 with the maximum tensile strain of 2450 le (location P4), as shown in Fig. 5. It should be noted that the distribution of tensile strains obtained in the present work was only used for the simulation the cyclic strain acting on LCP during walking. The actual location of maximum strain on LCP was likely to be the surface of combination hole [3,15], and its magnitude could be numerically determined using the finite element calculation. Under cyclic loading, the peaks of tensile strains on the surface of LCP were compared with those under monotonic loading, as shown in Fig. 5. Only marginal differences (<5%) were observed. It was confirmed that the LCP fixed on a 4-point bending device was under the cyclic strain similar to the strain on LCP during walking.
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Fig. 5. The strains on the surface of LCP.
3.2. Fatigue behavior Relationships between the maximum compression load applied on 4-point bending device and number of cycles are shown in Fig. 6. The plot could be divided into three stages; an initial stage (stage I) of rapid decrease in compression load, a second steady-stage (stage II) in which the rate of decrease in compression load was fairly constant, and a third stage (stage III) in which the value of compression load decreased rapidly. It is known that the polymer materials showed cyclic softening behavior. Therefore, the rapid decrease in compression load (stage I) at the beginning of fatigue test was likely to be the cyclic softening of the 4-point bending device, i.e. polyvinyl chloride pipe reinforced with epoxy resin. After a certain number of cycles, the fatigue crack initiated at the surface of combination hole, and the maximum compression load that maintained stable maximum movement of 4-point bending device decreased. The initiation of fatigue crack corresponded to the second steady-stage (stage II) which covered most of the fatigue life of LCP. As fatigue crack propagated longer, the load bearing area of the LCP decreased, and the stress increased. At this stage, the crack propagated rapidly (stage II), and the complete fracture finally occurred. After fatigue tests, the screw torque was checked and no screw loosing was detected. The fatigue tests were performed twice, and the repetitiveness of the results was confirmed. For the present work, it was assumed that one load cycle per leg takes 2 s, the patient walks 3 h a day, and the adequate healing of fracture to sustain full load of walking without walking aid occurs within 6 months. Once the femur is fully healed, the LCP is no longer under severe loading. Based on this assumption, an LCP
Fig. 6. Relationship between the maximum compression load applied on 4-point bending specimen and number of cycles.
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will be cyclically loaded about 106 cycles before healing. According to the present results, the complete fracture of LCP occurred after 42,000 cycles of loading, i.e. equivalence to about 8 days of walking. Therefore, the fatigue failure of LCP was possible before the adequate healing of fracture, and the full load of walking should not be allowed for the patient with the present fracture condition (a transverse fracture with 8-mm interfragmentary gap at the middle of diaphysis). 3.3. Fatigue mechanisms After fatigue test, the cracks were detected at the middle of LCP, as shown in Fig. 7. Since, the load bearing area near the compression hole was smaller than that near the locking hole, while the similar deflection was applied on both areas. Therefore, fatigue crack firstly initiated from the surface of compression hole, i.e. crack A. The fracture surface of area A was observed using optical microscope, and the location of crack initiation was indicated, as shown in Fig. 8. After some cycles of loading, another fatigue crack (crack B) also initiated from the surface of locking hole, and then both cracks propagated inside the LCP. The initiation of crack A and crack B corresponded to the steady reduction of compression load (stage II of Fig. 6). To reveal the area of fatigue crack initiation, the fracture surface of the crack at the surface of compression hole (crack A) was observed in an SEM, as shown in Fig. 9. A subsurface inclusion was observed at the area of
Fig. 7. Fatigue cracks at the middle of LCP.
Fig. 8. Fracture surface near the compression hole of LCP.
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Fig. 9. Subsurface inclusion on the fracture surface near the compression hole of LCP.
crack initiation. The composition of the inclusion was analyzed using an energy dispersive spectroscopic technique (EDS, Oxford INCA 300), as shown in Fig. 10. The main element of the inclusion was found to be carbon. According to the ASTM recommendation [16], the surface of the implants should be cleaned to minimize the iron particles, ceramic media, and other foreign particles. These particles may imbedded into the surface of the implants during processing operations, e.g. forming, machining, tumbling, bead blasting. Unfortunately, the cleaning was unable to remove some of the foreign particles imbedded below the surface of LCP (subsurface inclusions). Since, the interface strength between subsurface inclusion – matrix was weak and the stress concentration was severe around the surface of the compression hole, the fatigue crack was likely to initiate from this subsurface inclusion (Fig. 9). As an evidence of the propagation of fatigue crack, the striations were observed on the fracture surface (Fig. 11). The striation spacing was long when observed far from the crack initiation site, and became shorter when observed around the crack initiation site. Based on the striation spacing at the middle of fracture surface (1 lm) and the thickness of LCP (5 mm), the number of cycles for the propagation of fatigue crack from the initiation site to the bottom part of LCP was estimated to be approximately 5000 cycles. This estimation was in accordance with the number of cycles spent during the stage III of Fig. 6.
Fig. 10. Composition of the subsurface inclusion analyzed by the energy dispersive spectroscopic technique.
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Fig. 11. Fatigue striations on the fracture surface near the compression hole of LCP.
4. Conclusions The fatigue failure of a locking compression plate (LCP) fixed across a transverse fracture (8-mm gap) at the midshaft of femur was experimentally evaluated. The main findings are summarized as follows. 1. The complete fracture of LCP occurred after 42,000 cycles of loading, i.e. equivalence to about 8 days of walking. The fatigue failure of LCP was possible before the adequate healing of fracture, and the full load of walking should not be allowed for the patient with the present fracture condition. 2. The fatigue crack firstly initiated from a subsurface inclusion embedded under the surface of compression hole. After some cycles of loading, another fatigue crack also initiated from the surface of locking hole, and then both cracks propagated inside the LCP. 3. As an evidence of the propagation of fatigue crack, the striations were observed on the fracture surface of the LCP. The striation spacing was long when observed far from the crack initiation site, and became shorter when observed around the crack initiation site. Based on the striation spacing, the number of cycles for the propagation of fatigue crack from the initiation site to the bottom part of LCP was estimated to be approximately 5000 cycles.
Acknowledgements The authors would like to acknowledge the supports from the Thailand Research Fund (TRF), the National Research Council of Thailand (NRCT), and the National Metal and Materials Technology Center (MTEC). References [1] Thakur AJ. The elements of fracture fixation. Glasgrow: Churchill Livingstone; 1997. [2] Aslam N, Hazarika S, Nagarajah K, McNab I. AO 2 mm locking compression plate for arthrodesis of the proximal interphalangeal joint. Injury Extra 2005;36(10):428–31. [3] Frigg R. Development of the locking compression plate. Injury 2003;34(Suppl. 2):6–10. [4] www.synthes.com. Technique guide – Large fragment locking compression plate (LCP), Synthes, 2005. [5] Van Meeteren MC, Van Riet YEA, Van Der Werken CHR, Roukema JA. Condylar plate fixation of subtrochanteric femoral fractures. Injury 1996;27(10):715–7. [6] Sivakumar M, Kamachi Mudali U, Rajeswari S. Investigation of failures in stainless steel orthopaedic implant devices: fatigue failure due to improper fixation of a compression bone plate. J Mater Sci Lett 1994;13(2):142–5. [7] Azevedo CRF. Failure analysis of a commercially pure titanium plate for osteosynthesis. Eng Fail Anal 2003;10(2):153–64. [8] Sudhakar KV. Investigation of failure mechanism in vitallium 2000 implant. Eng Fail Anal 2005;12(2):257–62.
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[9] ASTM F621-02: Standard Specification for Stainless Steel Forgings for Surgical Implants, vol. 13.01, in Annual Book of ASTM Standards, 2002. [10] ASTM F138-03: Standard Specification for Wrought 18Chromium–14Nickel–2.5Molybdenum Stainless Steel Bar and Wire for Surgical Implants (UNS S31673), vol. 13.01, in Annual Book of ASTM Standards, 2005. [11] Cristofolini L, Cappello A, Viceconti M, Toni A. Mechanical validation of whole bone composite femur models. J Biomech 1996;29(4):525–35. [12] Heiner AD, Brown TD. Structural properties of a new design of composite replicate femurs and tibias. J Biomech 2001;34(6):773–81. [13] Waide V, Cristofolini L, Stolk J, Verdonschot N, Toni A. Experimental investigation of bone remodeling using composite femurs. Clin Biomech 2003;18(6):523–36. [14] Cordey J, Borgeaud M, Frankle M, Harder Y, Martinet O. Loading model for the human femur taking the tension band effect of the ilio-tibial tract into account. Injury 1999;30(Suppl. 1):SA26–30. [15] Stoffel K, Dieter U, Stachowiak G, Gachter A, Kuster MS. Biomechanical testing of the LCP – how can stability in locked internal fixators be controlled? Injury 2003;34(Suppl. 2):11–9. [16] ASTM F86-04: Standard Practice for Surface Preparation and Marking of Metallic Surgical Implants, vol. 13.01, in Annual Book of ASTM Standards, 2005.
Fatigue failure of an orthopedic implant – A locking compression plate C. Kanchanomai a
a,*
, V. Phiphobmongkol b, P. Muanjan
a
Department of Mechanical Engineering, Faculty of Engineering, Thammasat University, Klong-Luang, Pathumthani 12120, Thailand b Department of Orthopedic Surgery, Bhumibol Adulyadej Hospital, Royal Thai Air Force, Bangkok 10210, Thailand Received 15 March 2007; accepted 14 April 2007 Available online 27 April 2007
Abstract In the present work, the fatigue failure of a locking compression plate (LCP) fixed across a transverse fracture (8-mm gap) at the midshaft of femur was experimentally evaluated. The complete fracture of LCP occurred after 42,000 cycles of loading, i.e. equivalence to about 8 days of walking. The fatigue failure of LCP was possible before the adequate healing of fracture, and the full load of walking should not be allowed for the patient with the present fracture condition. The fatigue crack firstly initiated from a subsurface inclusion embedded under the surface of compression hole. After some cycles of loading, another fatigue crack also initiated from the surface of locking hole, and then both cracks propagated inside the LCP. As an evidence of the propagation of fatigue crack, the striations were observed on the fracture surface of the LCP. The striation spacing was long when observed far from the crack initiation site, and became shorter when observed around the crack initiation site. Based on the striation spacing, the number of cycles for the propagation of fatigue crack from the initiation site to the bottom part of LCP was estimated to be approximately 5000 cycles. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Locking compression plate; Femur; Fatigue; Crack initiation; Crack propagation
1. Introduction The dynamic compression plate (DCP) is one of the most commonly used implants for internal fixation. In regular DCP system, the forces acting on femur are bypassed across the fracture area, thus the fracture site is protected and the alignment is maintained throughout the healing process [1]. In order to obtain a stable fixation, the screws on the DCP must be pressed or tightened into the DCP holes, which means the plate compressed the surface of bone. High amount of load will be transmitted from the screws through the bone-plate interface. Thus, the stability of bone and plate complex is achieved. However, the existing compression force between bone and plate may cause the vascular damage to the undersurface bone tissue, which results in unfavorable conditions for bone healing under the plate. Recently, the locking compression plate (LCP) has been *
Corresponding author. Tel.: +66 02 564 3001; fax: +66 02 564 3010. E-mail address: [email protected] (C. Kanchanomai).
1350-6307/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2007.04.001
522
C. Kanchanomai et al. / Engineering Failure Analysis 15 (2008) 521–530
introduced by an association for the study of osteosynthesis/an association for the study of internal fixation (AO/ASIF). This new LCP has combined the advantages of compression hole of DCP with the advantages of threaded hole by introducing combination holes [2–4], as shown in Fig. 1a. The threaded part of the combination hole was designed to use with a locking head screw (Fig. 1b). If the locking head screws are fastened through the conical thread holes of LCP, the load is transmitted through screw-plate system without compression between plate and bone surface, which maintains space and preserve vascular supply to the injured bone. On the other hand, regular screw can be used through the conventional compression hole located on the other side of the combination hole to function as regular DCP. The LCP system is therefore possible to serve for both purposes, i.e. compression plate system or locking plate system. In addition to biological compatibility of LCP to treat femoral fracture, the endurance of the LCP is also one of the crucial considerations. In fractured femur, various types of forces act on the implants and the femur, especially cyclic load acting on implants during walking. The fatigue damage, which is a process of defect accumulation, crack initiation, and crack propagation with number of load cycles, is likely to occur even under low magnitude of cycling load. There were many reports related to the fatigue failure of DCPs, while few researches have been done for LCPs. Van Meeteren et al. [5] studied 40 consecutive patients with subtrochanteric femoral fractures treated with an AO 95° condylar blade plate and found a patient developed a delayed union, which ultimately resulted in repeated plate fractures due to fatigue. Sivakumar et al. [6] studied a man sustained a fracture of the right femur, and a six-holes tubular compression plate (316L stainless steel) was implanted. Eight months later the implanted plate fractured at a screw site adjacent to the site of the original bone fracture. Microscopic examination of the plate fracture surface revealed fatigue striations and beach marks. Azevedo [7] studied the failure of a titanium reconstruction plate for osteosynthesis, and found the corrosion on the surface of the implant, which were in contact with body fluids. The results indicated that the premature fracture of the plate was caused by a corrosion-fatigue mechanism. Sudhakar [8] observed the fracture surface of vitallium (Co–Cr–Mo alloy) plate, and found the evidences of corrosion-fatigue at the interface between implant plate and screw. As the treatment of fractures using LCP increased, more clinical studies have been reported and the advantages of this system have been confirmed. Treatment of femoral shaft fracture with LCP is then an alternative, and has become more popular in treating some indicated cases. However, very few reports regarding fatigue
Fig. 1. (a) Combination hole of LCP; (b) locking head screw [4] and (c) LCP (dimension in mm).
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523
failure of LCP are available. This study was therefore designed to experimentally evaluate the fatigue failure an LCP (316L stainless steel) when fixed on a diaphyseal-fractured femur (8-mm interfragmentary gap). 2. Material and experimental procedure 2.1. Locking compression plate (LCP) Fourteen holes broad LCP (18 mm-width and 250 mm-length) and 4 locking head screws (4 mm-diameter and 45 mm-length) were used for the present study. The LCP and locking head screw are made from 316L stainless steel. Based on the ASTM recommendation [9], a bar of UNS S31673 or 316L stainless steel was cut, forged, and machined to obtain the geometry as shown in Fig. 1c. Consequently, it was annealed, electropolished, passivated, and then ultrasonically cleaned. To reveal the microstructure (Fig. 2), the LCP was sectioned using an electric discharged machine (EDM), polished, etched with 20 g of picric acid and 100 ml of HCl, and then observed in an scanning electron microscope (SEM). The average grain was approximately 25 lm or ASTM No. 8, i.e. complied with the ASTM recommendation [9]. The hardness of the LCP was 294 HB. The composition of the present LCP was analyzed by an emission spectrometer (Baird: Spectovac 2000 Arc/Spark), as summarized in Table 1. The composition of LCP corresponded to that recommended by the ASTM standard [10]. 2.2. Monotonic loading In order to avoid the inter-specimen variations of the human femurs in different cadavers, the composite large left femurs (Third-generation femur – 3306, Pacific Research Laboratories, Inc., WA) were used for this study. With similar geometry and mechanical properties to those of young human femur, this composite femur has been successfully used in many biomechanical researches [11–13]. A transverse fracture (8-mm gap) on the midshaft of composite femur was fixed with an LCP using 2 locking screws above and below fracture site, i.e
Fig. 2. SEM micrograph of LCP (316L stainless steel).
Table 1 The composition of LCP Element
C
Mn
P
S
Si
Cr
Ni
Mo
LCP 316L Stainless steel [10]
0.018 0.030max
1.84 2.00max
0.025 0.025max
0.009 0.010max
0.305 0.750max
17.82 17–19
14.29 13–15
2.71 – 2.25–3.00 0.100max
N
Cu
Fe
0.073 0.500max
Bal. Bal.
524
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fixation in hole number 1, 6 and 9, 14. The technique for fixation of the locking screws and the LCPs to the composite femurs were the standard procedure used in femoral fixation of human femur [4]. Loading model (Fig. 3a) used in the present work was similar to the model proposed by Cordey et al. [14], which took into account with the forces acting through the ilio-tibial tract in the frontal plane and those forces acting on the femoral condyles in the sagittal plane. A distal femur was supported by a pin and ball bearing in order to prevent undesirable moment and torque. Epoxy resin was used to stabilize the tested composite femur to the loading model. Loadings were performed using a servo-hydraulic fatigue machine (Instron 8801) under 25 °C temperature and 55% relative humidity. The experimental system is shown in Fig. 3b. To simulate the maximum load acting on he femur during walking, the femur was compressed from 0 N to 600 N under 60 N/s loading rate, and held at 600 N for 15 min. Then it was unloaded to 0 N under 60 N/s loading rate. To avoid complication from residue deformation, the femur was left at zero load for 15 min before the next loading was started. The 7 strain gages with 0.3-mm gage length (TML, FLA-03-11) were set along the surface (P1–P7) of an LCP (Fig. 1c). During the test, load as well as deformation of composite femur were recorded by the controller of servo-hydraulic fatigue machine, while the strains on the LCP were collected using computerized data acquisition system (National Instrument: PCI-6013 and LabVIEW 7.0). The loadings were repeated 5 times, and the average values of strain were calculated. 2.3. Cyclic loading Since, the composite femur was made from short E-glass fibers/epoxy resin and solid rigid polyurethane foam, and it was not designed to withstand the cyclic loading. Therefore, fatigue test of LCP was carried out using a 4-point bending specimen, i.e. an LCP fixed on two cylinders (polyvinyl chloride pipe reinforced with epoxy resin), as shown in Fig. 4. The maximum strains distributed on the surface of LCP during fatigue
Fig. 3. (a) Monotonic loading model for composite femur and (b) Experimental system for monotonic loading of composite femur.
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Fig. 4. (a) 4-Point bending specimen and (b) Experimental system for cyclic loading of 4-point bending specimen.
test were controlled to match with the strains distributions of LCP-composite femur model under monotonic loading by adjusting the load positions (L1–L4) and the vertical movement of 4-point bending device, as shown in Fig. 4a. The experimental system is shown in Fig. 4b. In order to maintain contact between specimen and 4-point bending device, the minimum strains distributed on the surface of LCP were controlled to be 5% of the maximum strains. The displacement-controlled fatigue tests (sinusoidal waveform with 3 Hz frequency) were performed by using a servo-hydraulic fatigue machine at 25 °C temperature and 55% relative humidity. The fatigue failure was defined as a complete fracture of LCP. During fatigue test, the applied load, movement of a 4-point bending device, strain distribution on the surface of LCP, and time were simultaneously recorded 100 times in each cycle with computer-controlled data acquisition. After fatigue test, the screw torque was checked for of screw loosing. The fatigue tests in every test condition were performed twice in order to check the repetitiveness. Fracture surfaces of LCP were observed in an SEM (JEOL: JSM-5410), and the mechanisms of fatigue were discussed. 3. Results and discussion 3.1. Strain distribution Under monotonic loading, the variation of strains on an LCP fixed on a composite femur from 5 repeated tests was less than 5%, and the average strains were determined. The tensile strains on LCP surface predominantly distributed between locations P3–P5 with the maximum tensile strain of 2450 le (location P4), as shown in Fig. 5. It should be noted that the distribution of tensile strains obtained in the present work was only used for the simulation the cyclic strain acting on LCP during walking. The actual location of maximum strain on LCP was likely to be the surface of combination hole [3,15], and its magnitude could be numerically determined using the finite element calculation. Under cyclic loading, the peaks of tensile strains on the surface of LCP were compared with those under monotonic loading, as shown in Fig. 5. Only marginal differences (<5%) were observed. It was confirmed that the LCP fixed on a 4-point bending device was under the cyclic strain similar to the strain on LCP during walking.
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Fig. 5. The strains on the surface of LCP.
3.2. Fatigue behavior Relationships between the maximum compression load applied on 4-point bending device and number of cycles are shown in Fig. 6. The plot could be divided into three stages; an initial stage (stage I) of rapid decrease in compression load, a second steady-stage (stage II) in which the rate of decrease in compression load was fairly constant, and a third stage (stage III) in which the value of compression load decreased rapidly. It is known that the polymer materials showed cyclic softening behavior. Therefore, the rapid decrease in compression load (stage I) at the beginning of fatigue test was likely to be the cyclic softening of the 4-point bending device, i.e. polyvinyl chloride pipe reinforced with epoxy resin. After a certain number of cycles, the fatigue crack initiated at the surface of combination hole, and the maximum compression load that maintained stable maximum movement of 4-point bending device decreased. The initiation of fatigue crack corresponded to the second steady-stage (stage II) which covered most of the fatigue life of LCP. As fatigue crack propagated longer, the load bearing area of the LCP decreased, and the stress increased. At this stage, the crack propagated rapidly (stage II), and the complete fracture finally occurred. After fatigue tests, the screw torque was checked and no screw loosing was detected. The fatigue tests were performed twice, and the repetitiveness of the results was confirmed. For the present work, it was assumed that one load cycle per leg takes 2 s, the patient walks 3 h a day, and the adequate healing of fracture to sustain full load of walking without walking aid occurs within 6 months. Once the femur is fully healed, the LCP is no longer under severe loading. Based on this assumption, an LCP
Fig. 6. Relationship between the maximum compression load applied on 4-point bending specimen and number of cycles.
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will be cyclically loaded about 106 cycles before healing. According to the present results, the complete fracture of LCP occurred after 42,000 cycles of loading, i.e. equivalence to about 8 days of walking. Therefore, the fatigue failure of LCP was possible before the adequate healing of fracture, and the full load of walking should not be allowed for the patient with the present fracture condition (a transverse fracture with 8-mm interfragmentary gap at the middle of diaphysis). 3.3. Fatigue mechanisms After fatigue test, the cracks were detected at the middle of LCP, as shown in Fig. 7. Since, the load bearing area near the compression hole was smaller than that near the locking hole, while the similar deflection was applied on both areas. Therefore, fatigue crack firstly initiated from the surface of compression hole, i.e. crack A. The fracture surface of area A was observed using optical microscope, and the location of crack initiation was indicated, as shown in Fig. 8. After some cycles of loading, another fatigue crack (crack B) also initiated from the surface of locking hole, and then both cracks propagated inside the LCP. The initiation of crack A and crack B corresponded to the steady reduction of compression load (stage II of Fig. 6). To reveal the area of fatigue crack initiation, the fracture surface of the crack at the surface of compression hole (crack A) was observed in an SEM, as shown in Fig. 9. A subsurface inclusion was observed at the area of
Fig. 7. Fatigue cracks at the middle of LCP.
Fig. 8. Fracture surface near the compression hole of LCP.
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Fig. 9. Subsurface inclusion on the fracture surface near the compression hole of LCP.
crack initiation. The composition of the inclusion was analyzed using an energy dispersive spectroscopic technique (EDS, Oxford INCA 300), as shown in Fig. 10. The main element of the inclusion was found to be carbon. According to the ASTM recommendation [16], the surface of the implants should be cleaned to minimize the iron particles, ceramic media, and other foreign particles. These particles may imbedded into the surface of the implants during processing operations, e.g. forming, machining, tumbling, bead blasting. Unfortunately, the cleaning was unable to remove some of the foreign particles imbedded below the surface of LCP (subsurface inclusions). Since, the interface strength between subsurface inclusion – matrix was weak and the stress concentration was severe around the surface of the compression hole, the fatigue crack was likely to initiate from this subsurface inclusion (Fig. 9). As an evidence of the propagation of fatigue crack, the striations were observed on the fracture surface (Fig. 11). The striation spacing was long when observed far from the crack initiation site, and became shorter when observed around the crack initiation site. Based on the striation spacing at the middle of fracture surface (1 lm) and the thickness of LCP (5 mm), the number of cycles for the propagation of fatigue crack from the initiation site to the bottom part of LCP was estimated to be approximately 5000 cycles. This estimation was in accordance with the number of cycles spent during the stage III of Fig. 6.
Fig. 10. Composition of the subsurface inclusion analyzed by the energy dispersive spectroscopic technique.
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Fig. 11. Fatigue striations on the fracture surface near the compression hole of LCP.
4. Conclusions The fatigue failure of a locking compression plate (LCP) fixed across a transverse fracture (8-mm gap) at the midshaft of femur was experimentally evaluated. The main findings are summarized as follows. 1. The complete fracture of LCP occurred after 42,000 cycles of loading, i.e. equivalence to about 8 days of walking. The fatigue failure of LCP was possible before the adequate healing of fracture, and the full load of walking should not be allowed for the patient with the present fracture condition. 2. The fatigue crack firstly initiated from a subsurface inclusion embedded under the surface of compression hole. After some cycles of loading, another fatigue crack also initiated from the surface of locking hole, and then both cracks propagated inside the LCP. 3. As an evidence of the propagation of fatigue crack, the striations were observed on the fracture surface of the LCP. The striation spacing was long when observed far from the crack initiation site, and became shorter when observed around the crack initiation site. Based on the striation spacing, the number of cycles for the propagation of fatigue crack from the initiation site to the bottom part of LCP was estimated to be approximately 5000 cycles.
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