In-body Corrosion Fatigue Failure of a Stainless Steel

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Fatigue failure of an orthopedic implant – Alocking compression plate

C. Kanchanomai a,*, V. Phiphobmongkol b, P. Muanjan a

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 2007Available online 27 April 2007

Abstract

In the present work, the fatigue failure of a locking compression plate (LCP) fixed across a transverse fracture (8-mmgap) 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 fatiguecrack 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 theLCP. 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 aroundthe crack initiation site. Based on the striation spacing, the number of cycles for the propagation of fatigue crack from theinitiation site to the bottom part of LCP was estimated to be approximately 5000 cycles. 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. Inregular DCP system, the forces acting on femur are bypassed across the fracture area, thus the fracture site isprotected and the alignment is maintained throughout the healing process [1]. In order to obtain a stable fix-ation, the screws on the DCP must be pressed or tightened into the DCP holes, which means the plate com-pressed the surface of bone. High amount of load will be transmitted from the screws through the bone-plateinterface. Thus, the stability of bone and plate complex is achieved. However, the existing compression forcebetween bone and plate may cause the vascular damage to the undersurface bone tissue, which results in unfa-vorable conditions for bone healing under the plate. Recently, the locking compression plate (LCP) has been

1350-6307/$ - see front matter 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.engfailanal.2007.04.001

* Corresponding author. Tel.: +66 02 564 3001; fax: +66 02 564 3010.E-mail address: [email protected] (C. Kanchanomai).

Engineering Failure Analysis 15 (2008) 521–530

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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 combi-nation hole was designed to use with a locking head screw ( Fig. 1b). If the locking head screws are fastenedthrough the conical thread holes of LCP, the load is transmitted through screw-plate system without compres-

sion 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 otherside of the combination hole to function as regular DCP. The LCP system is therefore possible to serve forboth 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 alsoone of the crucial considerations. In fractured femur, various types of forces act on the implants and thefemur, 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 occureven 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 withsubtrochanteric femoral fractures treated with an AO 95 condylar blade plate and found a patient developeda delayed union, which ultimately resulted in repeated plate fractures due to fatigue. Sivakumar et al. [6] stud-

ied a man sustained a fracture of the right femur, and a six-holes tubular compression plate (316L stainlesssteel) 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 striationsand beach marks. Azevedo [7] studied the failure of a titanium reconstruction plate for osteosynthesis, andfound the corrosion on the surface of the implant, which were in contact with body fluids. The results indi-cated 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-fati-gue at the interface between implant plate and screw.

As the treatment of fractures using LCP increased, more clinical studies have been reported and the advan-tages 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 failurean 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-diameterand 45 mm-length) were used for the present study. The LCP and locking head screw are made from 316Lstainless steel. Based on the ASTM recommendation [9], a bar of UNS S31673 or 316L stainless steel wascut, forged, and machined to obtain the geometry as shown in Fig. 1c. Consequently, it was annealed, elec-tropolished, passivated, and then ultrasonically cleaned. To reveal the microstructure (Fig. 2), the LCP wassectioned using an electric discharged machine (EDM), polished, etched with 20 g of picric acid and 100 mlof HCl, and then observed in an scanning electron microscope (SEM). The average grain was approximately25 lm or ASTM No. 8, i.e. complied with the ASTM recommendation [9]. The hardness of the LCP was294 HB. The composition of the present LCP was analyzed by an emission spectrometer (Baird: Spectovac2000 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 compositelarge left femurs (Third-generation femur – 3306, Pacific Research Laboratories, Inc., WA) were used for thisstudy. With similar geometry and mechanical properties to those of young human femur, this composite femurhas been successfully used in many biomechanical researches [11–13]. A transverse fracture (8-mm gap) on themidshaft 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 1The composition of LCP

Element C Mn P S Si Cr Ni Mo N Cu Fe

LCP 0.018 1.84 0.025 0.009 0.305 17.82 14.29 2.71 – 0.073 Bal.316L Stainless

steel [10]

0.030max 2.00max 0.025max 0.010max 0.750max 17–19 13–15 2.25–3.00 0.100max 0.500max Bal.

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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 thecomposite 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 forcesacting 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 femurto the loading model. Loadings were performed using a servo-hydraulic fatigue machine (Instron 8801) under25 C temperature and 55% relative humidity. The experimental system is shown in Fig. 3b. To simulate themaximum load acting on he femur during walking, the femur was compressed from 0 N to 600 N under 60 N/sloading rate, and held at 600 N for 15 min. Then it was unloaded to 0 N under 60 N/s loading rate. To avoidcomplication from residue deformation, the femur was left at zero load for 15 min before the next loading wasstarted. 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 con-troller of servo-hydraulic fatigue machine, while the strains on the LCP were collected using computerizeddata acquisition system (National Instrument: PCI-6013 and LabVIEW 7.0). The loadings were repeated 5times, 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 polyurethanefoam, and it was not designed to withstand the cyclic loading. Therefore, fatigue test of LCP was carriedout using a 4-point bending specimen, i.e. an LCP fixed on two cylinders (polyvinyl chloride pipe reinforcedwith 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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test were controlled to match with the strains distributions of LCP-composite femur model under monotonicloading by adjusting the load positions (L1–L4) and the vertical movement of 4-point bending device, asshown in Fig. 4a. The experimental system is shown in Fig. 4b. In order to maintain contact between specimenand 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 failurewas defined as a complete fracture of LCP. During fatigue test, the applied load, movement of a 4-point bend-ing device, strain distribution on the surface of LCP, and time were simultaneously recorded 100 times in eachcycle with computer-controlled data acquisition. After fatigue test, the screw torque was checked for of screwloosing. 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 werediscussed.

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 repeatedtests was less than 5%, and the average strains were determined. The tensile strains on LCP surface predom-inantly distributed between locations P3–P5 with the maximum tensile strain of 2450 le (location P4), asshown in Fig. 5. It should be noted that the distribution of tensile strains obtained in the present work wasonly used for the simulation the cyclic strain acting on LCP during walking. The actual location of maximumstrain on LCP was likely to be the surface of combination hole [3,15], and its magnitude could be numericallydetermined using the finite element calculation. Under cyclic loading, the peaks of tensile strains on the surfaceof LCP were compared with those under monotonic loading, as shown in Fig. 5. Only marginal differences(


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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 rapiddecrease in compression load, a second steady-stage (stage II) in which the rate of decrease in compressionload 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 com-pression load (stage I) at the beginning of fatigue test was likely to be the cyclic softening of the 4-point bend-ing device, i.e. polyvinyl chloride pipe reinforced with epoxy resin. After a certain number of cycles, the fatiguecrack initiated at the surface of combination hole, and the maximum compression load that maintained stablemaximum 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 propagatedlonger, the load bearing area of the LCP decreased, and the stress increased. At this stage, the crack propa-gated rapidly (stage II), and the complete fracture finally occurred. After fatigue tests, the screw torque waschecked 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, andthe 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. 5. The strains on the surface of 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 fractureof LCP occurred after 42,000 cycles of loading, i.e. equivalence to about 8 days of walking. Therefore, thefatigue failure of LCP was possible before the adequate healing of fracture, and the full load of walking shouldnot be allowed for the patient with the present fracture condition (a transverse fracture with 8-mm interfrag-mentary 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 bearingarea near the compression hole was smaller than that near the locking hole, while the similar deflection wasapplied on both areas. Therefore, fatigue crack firstly initiated from the surface of compression hole, i.e. crackA. The fracture surface of area A was observed using optical microscope, and the location of crack initiationwas indicated, as shown in Fig. 8. After some cycles of loading, another fatigue crack (crack B) also initiatedfrom the surface of locking hole, and then both cracks propagated inside the LCP. The initiation of crack Aand 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 compressionhole (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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crack initiation. The composition of the inclusion was analyzed using an energy dispersive spectroscopic tech-nique (EDS, Oxford INCA 300), as shown in Fig. 10. The main element of the inclusion was found to be car-bon. According to the ASTM recommendation [16], the surface of the implants should be cleaned to minimizethe iron particles, ceramic media, and other foreign particles. These particles may imbedded into the surfaceof the implants during processing operations, e.g. forming, machining, tumbling, bead blasting. Unfortu-nately, 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 weakand the stress concentration was severe around the surface of the compression hole, the fatigue crack waslikely 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 observedfar from the crack initiation site, and became shorter when observed around the crack initiation site. Based onthe 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 tobe approximately 5000 cycles. This estimation was in accordance with the number of cycles spent during thestage III of Fig. 6.

Fig. 10. Composition of the subsurface inclusion analyzed by the energy dispersive spectroscopic technique.

Fig. 9. Subsurface inclusion on the fracture surface near the compression hole of LCP.



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4. Conclusions

The fatigue failure of a locking compression plate (LCP) fixed across a transverse fracture (8-mm gap) atthe 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 loadof 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 compressionhole. After some cycles of loading, another fatigue crack also initiated from the surface of locking hole, andthen 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 becameshorter when observed around the crack initiation site. Based on the striation spacing, the number of cyclesfor the propagation of fatigue crack from the initiation site to the bottom part of LCP was estimated to beapproximately 5000 cycles.

Acknowledgements

The authors would like to acknowledge the supports from the Thailand Research Fund (TRF), the Na-tional Research Council of Thailand (NRCT), and the National Metal and Materials Technology Center(MTEC).

References

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Fig. 11. Fatigue striations on the fracture surface near the compression hole of LCP.


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In-body Corrosion Fatigue Failure of a Stainless Steel

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