- Email: [email protected]
PEEK composite compression bone plates
Biomaterials 24 (2003) 2661–2667
Performance study of braided carbon/PEEK composite compression bone plates K. Fujiharaa,*, Zheng-Ming Huanga,b, S. Ramakrishnaa, K. Satknananthamc, H. Hamadad a
Division of Bioengineering, Biomaterials Laboratory, The National University of Singapore, 9 Engineering Drive 1 Singapore 117576 b Department of Engineering Mechanics, Tongji University, 1239 Siping Road, Shanghai 200092, People’s Republic of China c Department of Orthopedic Surgery, The National University of Singapore, 9 Engineering Drive 1 Singapore 117576 d Kyoto Institute of Technology, Division of Advanced Fibro Science, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Received 4 October 2002; accepted 20 January 2003
Abstract In addition to unidirectional laminates and short fiber reinforcements for compression bone plate developments in the literature, we have proposed using a textile structure, i.e. braid preform, for this purpose. In the present paper, the influence of braiding angles and plate thicknesses on the bending performance of the braided composite bone plates is investigated. As a result, the influence of the braiding angle, varied in a certain range, on the plate bending properties is not significant when the plate thickness is thin. This influence becomes higher with an increase in the plate thickness. A 10 braiding angle has been seen to be appropriate for all the cases under consideration. The present study indicates that the braided composite plate with 2.6 mm thickness can be suitable for forearm treatment whereas the braided composite plate of 3.2 mm thickness is applicable to femur or tibia fixation. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Compression bone plate; Micro-braiding fabrication; Braided carbon/PEEK composite; Bending behavior
1. Introduction Patients with diaphyseal fracture of a long bone are treated by the internal fixation device, such as a compression bone plate. The conventional compression plates are made of metal materials that have 5–10 times higher modulus than cortical bone. Using metal plates, it normally takes 1–2 years to obtain a complete bone healing. During this implant period, the damaged bone is restored according to the surrounded mechanical environment (known as Wolff’s law) [1]. The mismatch between the moduli of the metal plates and the cortical bone leads to a situation where the majority of the load is transferred by the plate rather than by the underlying bone. This phenomenon is widely recognized as ‘‘stressshielding effect’’ or ‘‘stress-protection’’ [2]. As a result, micromotion to enhance bone healing is restricted. Bone is a living tissue with metabolic reaction. A decrease in tissue strain of bone regularly results in bone resorption. Thus, a proper amount of strain is necessary for the *Corresponding author. Fax: +65-6874-6593. E-mail address: [email protected] (K. Fujihara).
growth of the fractured bone during its healing [3–5]. The metal plates provoke the decrease of bone mineral mass [6–12] and occasionally cause bone refracture after the plate removal [13,14]. In order to avoid ‘‘stressshielding effect’’, it is desirable to use plates whose mechanical properties are close to those of cortical bone. In addition to their over high modulus, metal materials may show relatively weak strength under cyclic loading. Compression bone plates are normally subjected to extremely high cyclic loads. The metal plates have a possibility to be fractured during implant under relatively small stresses [15,16]. These examples suggest that the materials proposed for compression bone plates must also possess sufficiently high fatigue strength. Another shortcoming of metal plates is that the healing status of the fractured bone fixed beneath the plate may not be correctly identified due to metal’s radio opaque, which in some cases results in undesirable artifacts in X-ray radiography [17]. In order to resolve these problems, polymer based composite materials, which have less stiffness, high fatigue strength, and good radiolucency, have been proposed for bone plate fixations as alternative of metal
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materials. However, the composite plates developed in the past mainly adapted UD (unidirectional) laminates [18–32] and discontinuous short fibers [33] as reinforcement. Environment of a compression bone plate is greatly severe because of high fatigue loading in body fluids. Although composite plates made of discontinuous short fibers have the advantage of easy fabrication by injection molding, the plates need large thickness to avoid fracture. On the other hand, compression plates made of UD laminates have to be drilled to make screw holes, resulting in a reduction of their load carrying capacity due to the breaking down of the fiber continuity. With regard to these drawbacks, the present authors have suggested using textile reinforcement to make composite compression bone plates in recent years [34,35]. The advantage of the textile reinforcement is that the plates can sustain multi-directional loads as a drilling process to make screw holes is unnecessary and hence the yarn continuity can be retained. In an initial effort [34], the effectiveness of braided fabric reinforcement was recognized by using a carbon/epoxy material system. However, since epoxy resins have a possibility to give harmful influence to human body, the matrix of braided composite compression plates was later replaced with a thermoplastic PEEK (poly ether–ether-ketone) material [35], which is known to be well biocompatible [36–38]. In our previous work [35], a micro-braiding fabrication method to gain high and consistent impregnation of matrix into reinforcing fibers was introduced to fabricate the braided carbon/PEEK composite compression bone plates. Four-point bending properties of the composite bone plates were investigated with three different braiding angles. Although the bending properties of braided carbon/PEEK composites showed promising potential for bone plate application, detailed influence of plate thickness and other parameters on the bending behavior was not recognized. In this work, the braided carbon/PEEK bone plates of three different plate thicknesses each with three different braiding angles were comparatively studied in terms of their bending performance. Furthermore, comparison with other composite bone plates and implant feasibility of the developed composite compression plate are addressed in the paper.
2. Experimental procedure 2.1. Materials and fabrication In this study, the micro-braided yarn, which contained PEEK matrix and reinforcement carbon fibers, was used to fabricate braided carbon/PEEK compression bone plates. The unique feature of this yarn is that the reinforcing and matrix fibers are easily mixed using a simple braiding technique. The detailed process of the
Fig. 1. Photograph of a flat braided fabric made of micro-braided yarns (y indicates a braiding angle and a braided fabric have certain range of braiding angle with constant preform width).
Fig. 2. Schematic drawing of insertion way of flat braided fabric. A pin can be inserted into a fabric without damages of micro-braided yarns.
micro-braiding was shown in our previous publication [35]. Flat braided fabrics were preformed using microbraided yarns, as indicated in Fig. 1. In terms of a hot press machine, a composite bone plate was obtained by placing multiple layers of the flat braided fabrics in a stainless-steel mould to which pins were attached to form screw holes without breaking the yarn continuity (Fig. 2). Braided fabrics with three different braiding angles, i.e., 5 , 10 and 15 , were used to investigate the influence of the braiding angles on the bending performance of the composite plates. In order to further investigate such influence, three different plate
K. Fujihara et al. / Biomaterials 24 (2003) 2661–2667
thicknesses, 2.6, 3.2 and 3.8 mm, were achieved using different fabric layers. Table 1 lists detailed information of the composite bone plates made in this study. In our previous work, the composite bone plates were fabricated at 400 C for 60 min under pressure [35]. Although specimens of good quality were obtained through that fabrication condition, matrix degradation was later recognized. Hence, in this study, the plate fabrication was conducted at 380 C for 20 min with a different pressure-temperature relationship history as given in Fig. 3. Fig. 4 shows photos of the fabricated braided composite bone plates and their cross-sectional views. At hole part of a plate (section A–A), carbon fibers are fully packed and no voids have been seen. Table 1 Specimen types of braided carbon/PEEK composite compression bone plates Preform
Braiding angle of preform
Number of layers
Thickness (mm)
Braided fabric (diamond structure)
5 , 10 , 15
8
2.6
5 , 10 , 15 5 , 10 , 15
10 12
3.2 3.8
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There are some voids at a non-hole cross-section (section B–B). These voids, however, are negligibly small. The fabricated braided carbon/PEEK composite bone plates had an averaged volume fraction of 53% in all types of the specimens. A stainless-steel narrow dynamic compression plate (DCP) of AO Institute, which is used widely in surgery, was also tested for comparative purpose. The geometry of the AO plate is 103 mm long, 13 mm wide and 3.8 mm thick. 2.2. Four-point bending test Bending behavior of a bone plate is one of the most critical mechanical properties from an application viewpoint, and is generally evaluated by maximum bending moment and bending stiffness calculated from the initial linear moment against the total bending angle (angulation) of the plate. In this study, static four-point bending tests were conducted with a cross-head speed of 1.0 mm/min at room temperature. Specimen geometry is indicated in Fig. 5. The developed composite bone plate has six holes and curvature geometry to fix it to diapheseal part of a long bone. Upper and lower span lengths are 41 and 73 mm, respectively.
3. Results Temp.
3.1. Stainless-steel dynamic compression plate 380°
Average = -5°C / min.
25 ° 40
10
0
0
20
71 11
Time (min) Pressure (kN)
Fig. 3. Pressure and thermal history applied to the molding.
The stainless-steel plate was tested for comparison purpose. As shown in Fig. 6, this plate indicated a rapid increase of bending moment at small angulation. After the bending moment reached 13 N m, the curve still exhibits an increased bending moment-angulation relationship, but becomes much more ductile. Since the stainless-steel plate permanently deformed without a fracture during the test, the load was finally released to zero. Bending stiffness determined by the initial slope of
Fig. 4. Photographs of braided carbon/PEEK composite compression plates and cross-section photos of screw hole part (A–A) and non-hole part (B–B).
7
16
16
25
16
16
7 15
D = 8.1 D = 4.5 P/2
r = 13.7
P/2 41
103
(a)
73
Bending moment (Nm)
2
P/2
P/2
Unit: mm
Fig. 5. Specimen geometry of braided carbon/PEEK composite compression plate.
Proof load = 21Nm
10 5 degree
8
10 degree
6
15 degre
4 2 0
2.6 mm thickness
0
16
2
10 13Nm
5 0 0
5
7°
10
15
20
25
30
Angulation (degree)
Fig. 6. Bending moment-angulation curve of AO stainless-steel compression plate. Angulation is an angle determined by lines of A–A and B–B.
the curve was 1.4 N m/deg. According to ISO9585 standard [39] (implants for surgery—determination of bending strength and stiffness of bone plates), when a plate shows this kind of testing curve, the maximum moment is replaced with the proof moment which is determined by the plate span lengths. In the present case, the proof moment is defined at the cross-point between the initial slope shifted with 7 angulation and the moment curve. The proof moment thus obtained was 21 N m. 3.2. Braided composite compression plates Fig. 7 shows bending moment—angulation curves of the braided composite compression plates with three different thicknesses. In the case of the specimens with 2.6 mm thickness, all of them showed linear increment of bending moment versus angulation regardless of different braiding angles. When the bending moment reached around 6 N m, some plastic deformation began to occur and all the specimens finally fractured at the third hole near a loading point. On the other hand, all the other specimens with larger thicknesses (i.e., 3.2 and 3.8 mm)
(c)
10
10 degree 15 degree
8 4 3.2 mm thickness
0
0
2
(b) 15
4 6 8 Angulation (degree)
5 degree
12
20
Bending moment (Nm)
Bending moment (Nm)
25
Bending moment (Nm)
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4 6 8 Angulation (degree)
10
20 5 degree
16
10 degree 15 degree
12 8 4 0
3.8 mm thickness
0
2
4
6
8
10
Angulation (degree)
Fig. 7. Bending moment-angulation curves of braided composite compression plates with three different thicknesses.
did not show a significant plastic behavior on their loadangulation curves. In other words, the bending moment increased essentially linearly until rupture and all the specimens finally fractured in the same way as those with the minimum thickness. 3.3. Performance comparison The measured bending results are plotted against plate thickness, as shown in Fig. 8, in which the percentage values are relative to those of the stainlesssteel plate. Thus, the bending performance of the stainless-steel plate was taken as 100%. The braided composite plates showed an increase in the maximum bending moment with the increase of plate thickness for all the braiding angles. The plates which have the minimum thickness showed only 8% difference in the bending moment among different braiding angle specimens. This difference was more distinct for the thicker specimens, i.e., 18% for the 3.2 mm and 19% for the 3.8 mm thick specimens among different braiding angles. The yield bending moment of the stainless-steel
K. Fujihara et al. / Biomaterials 24 (2003) 2661–2667
Maximum bending moment (Nm)
25
2665
Bone specimen beneath of a plate
Braiding (5 degree) Braiding (10 degree)
20
Braiding (15 degree)
15
67~86%
100%
Bone specimen opposite of a plate
Stainless-Steel
43~61%
10 33~41%
5 0 2
2.5
3
3.5
4
Compression bone plate
4.5
Plate thickness (mm)
Fig. 9. Two different bone specimens treated by a compression bone plate.
100~131%
15 61~96% 500
100%
10 46~49%
Braiding (5 degree) Braiding (10 degree) Braiding (15 degree) Stainless-steel
5
0 2
2.5
3
3.5
4
4.5
Plate thickness (mm)
Bending stiffness (Nm/deg)
1.6 Braiding (5 degree) Braiding (10 degree)
1.2
100%
88~96%
Ultimate bone srength treated by carbon/polysulfone composite compression bone plate (%)
Yield bending moment (Nm)
20
Opposite of a plate 400
Beneath of a plate
300
200
100
0 0
Braiding (15 degree) Stainless-Steel
63~68%
0.8
20
40
60
80
100
120
Ultimate stiffness of carbon/polysulfone composite compression bone plate (%)
Fig. 10. The shown date is referred from the work of Bradley et al. [22]. The strength of dog femur was measured after the treatment of stainless-steel compression plate and carbon/polysulfone UD laminated composite plate with various stiffnesses. The values of 100% were treated by stainless-steel compression plate.
39~40%
0.4
0 2
2.5
3
3.5
4
4.5
Plate thickness (mm) Fig. 8. Bending data comparison against plate thickness.
2.6 mm thick specimens with the three different braiding angles. This was not applicable, however, to the specimens with larger thicknesses.
4. Discussion plate was relatively lower than its maximum bending moment. However, the braided composite plates did not show drastic yield moment decrease as compared with their maximum value. It is noted that the braided composite plates of 3.2 mm thickness showed almost the same yield moment as that of the stainless-steel plate which was 3.8 mm thick. The bending stiffness of the braided composite plates increased with the increase in thickness, and the specimens of 3.8 mm thickness indicated a quite close stiffness to that of the stainlesssteel plate. There was no stiffness variation for the
A lot of efforts have been made in the past to develop composite compression plates for possible replacement of metal plates, but mainly focused on UD laminates and discontinuous short fibers as reinforcement. Although some of those composite plates have achieved some success in animal and clinical trials, there are still a number of critical issues which need to be addressed on the plate design with required mechanical property. Clinically, a thinner compression bone plate is preferred because of the limited space between muscle and
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Table 2 Bending properties of UD laminated composite bone plates used for human clinical trials Ref.
[32] [29]
[27]
Plate type
Implant site
Bending stiffness (N m/deg) Stainlesssteel
Composite
Maximum bending moment (N m) Composite
Carbon/epoxy (6-hole narrow DCP) Carbon/epoxy (8-hole broad DCP)
Human forearm
1.25
0.377 (30%)
9.1
Human tibia
3.8
—
Carbon/epoxy (8-hole broad DCP)
Human tibia
1.88
0.94–2.22 (25–58%) 2.0 (53%) was recommended 0.8 (43%)
fragmented bones. However, this requirement may lead to fatigue fracture of the plate [25] before bone is healed. It may be considered that the previously developed composite compression plates could not achieve thinner size because of shortcomings of the reinforcing methods used. Therefore, the concern would be whether our developed braided composite compression plates are applicable to clinical usage. Since bending stiffness of a compression plate has a close relationship with bone healing process, the previous animal and clinical trials have suggested an appropriate choice for the plate stiffness to achieve good bone healing. One of the examples discussed the relationship between the plate stiffness and the bone strength after a fixation treatment was given by Bradley et al. [22]. In their work, fractured dog femurs were treated using a stainless-steel compression plate and a carbon/polysulfone UD laminate plate, respectively. Then, the bone pieces both opposite to and beneath the plate (see Fig. 9) were tested under bending loading. The results shown in Fig. 10 tell that when the plate stiffness is around 10–25% value of stainless-steel, the healed bone strengths are much higher than those treated by even stiffer composite plates. However since the stress environment of the dog femur is not as severe as that of human femur or tibia due to the weight and bone site differences, the stiffness values of composite plates for human femur and tibia should be higher (Table 2). For instance, 53% [29] and 43% [27] of stainless-steel plate stiffness were recommended to obtain a quick human bone healing on human tibia. In the case of human forearm where less stress environment than that of femur and tibia is expected, the appropriate stiffness of composite plates is around 30% of that of the stainless-steel plate. The braided composite compression bone plate developed in this study showed 40% stiffness value with 2.6 mm thickness together with the maximum bending moment, which is comparable with the result of Ali et al. [32]. Thus, this plate can be appropriate for forearm fixation. On the other hand, since the braided composite bone plates with 3.2 mm thickness showed 63–68% stiffness of the
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stainless-steel plate, these composite plates may be used to fix femur and tibia bones. It is noted that in both the cases, the current braided composite bone plates are thinner than previously developed composite plates and even a clinically used stainless-steel compression plate which is 3.8 mm thick.
5. Conclusion Although several composite bone plates were developed using UD laminates and discontinuous short fibers to serve as alternatives for the conventional stainlesssteel AO compression plates, there still remain a number of improvements to be addressed from a mechanical viewpoint. In this regard, we have proposed using braided carbon/PEEK fabric composites as a new material system for bone plate development. In this work, the influence of braiding angle and plate thickness on the bending properties of the braided composite bone plates is further investigated. As a result, the influence of the braiding angle, varied in a certain range, on the plate bending properties is not significant when the plate thickness is thin. This influence is higher when the plate becomes thicker. A 10 braiding angle can be recommended for all the cases under consideration. It is considered that the braided composite plate with 2.6 mm thickness can be suitable for forearm treatment whereas the braided composite plate of 3.2 mm thickness is applicable to femur or tibia fixation.
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Performance study of braided carbon/PEEK composite compression bone plates K. Fujiharaa,*, Zheng-Ming Huanga,b, S. Ramakrishnaa, K. Satknananthamc, H. Hamadad a
Division of Bioengineering, Biomaterials Laboratory, The National University of Singapore, 9 Engineering Drive 1 Singapore 117576 b Department of Engineering Mechanics, Tongji University, 1239 Siping Road, Shanghai 200092, People’s Republic of China c Department of Orthopedic Surgery, The National University of Singapore, 9 Engineering Drive 1 Singapore 117576 d Kyoto Institute of Technology, Division of Advanced Fibro Science, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Received 4 October 2002; accepted 20 January 2003
Abstract In addition to unidirectional laminates and short fiber reinforcements for compression bone plate developments in the literature, we have proposed using a textile structure, i.e. braid preform, for this purpose. In the present paper, the influence of braiding angles and plate thicknesses on the bending performance of the braided composite bone plates is investigated. As a result, the influence of the braiding angle, varied in a certain range, on the plate bending properties is not significant when the plate thickness is thin. This influence becomes higher with an increase in the plate thickness. A 10 braiding angle has been seen to be appropriate for all the cases under consideration. The present study indicates that the braided composite plate with 2.6 mm thickness can be suitable for forearm treatment whereas the braided composite plate of 3.2 mm thickness is applicable to femur or tibia fixation. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Compression bone plate; Micro-braiding fabrication; Braided carbon/PEEK composite; Bending behavior
1. Introduction Patients with diaphyseal fracture of a long bone are treated by the internal fixation device, such as a compression bone plate. The conventional compression plates are made of metal materials that have 5–10 times higher modulus than cortical bone. Using metal plates, it normally takes 1–2 years to obtain a complete bone healing. During this implant period, the damaged bone is restored according to the surrounded mechanical environment (known as Wolff’s law) [1]. The mismatch between the moduli of the metal plates and the cortical bone leads to a situation where the majority of the load is transferred by the plate rather than by the underlying bone. This phenomenon is widely recognized as ‘‘stressshielding effect’’ or ‘‘stress-protection’’ [2]. As a result, micromotion to enhance bone healing is restricted. Bone is a living tissue with metabolic reaction. A decrease in tissue strain of bone regularly results in bone resorption. Thus, a proper amount of strain is necessary for the *Corresponding author. Fax: +65-6874-6593. E-mail address: [email protected] (K. Fujihara).
growth of the fractured bone during its healing [3–5]. The metal plates provoke the decrease of bone mineral mass [6–12] and occasionally cause bone refracture after the plate removal [13,14]. In order to avoid ‘‘stressshielding effect’’, it is desirable to use plates whose mechanical properties are close to those of cortical bone. In addition to their over high modulus, metal materials may show relatively weak strength under cyclic loading. Compression bone plates are normally subjected to extremely high cyclic loads. The metal plates have a possibility to be fractured during implant under relatively small stresses [15,16]. These examples suggest that the materials proposed for compression bone plates must also possess sufficiently high fatigue strength. Another shortcoming of metal plates is that the healing status of the fractured bone fixed beneath the plate may not be correctly identified due to metal’s radio opaque, which in some cases results in undesirable artifacts in X-ray radiography [17]. In order to resolve these problems, polymer based composite materials, which have less stiffness, high fatigue strength, and good radiolucency, have been proposed for bone plate fixations as alternative of metal
0142-9612/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0142-9612(03)00065-6
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materials. However, the composite plates developed in the past mainly adapted UD (unidirectional) laminates [18–32] and discontinuous short fibers [33] as reinforcement. Environment of a compression bone plate is greatly severe because of high fatigue loading in body fluids. Although composite plates made of discontinuous short fibers have the advantage of easy fabrication by injection molding, the plates need large thickness to avoid fracture. On the other hand, compression plates made of UD laminates have to be drilled to make screw holes, resulting in a reduction of their load carrying capacity due to the breaking down of the fiber continuity. With regard to these drawbacks, the present authors have suggested using textile reinforcement to make composite compression bone plates in recent years [34,35]. The advantage of the textile reinforcement is that the plates can sustain multi-directional loads as a drilling process to make screw holes is unnecessary and hence the yarn continuity can be retained. In an initial effort [34], the effectiveness of braided fabric reinforcement was recognized by using a carbon/epoxy material system. However, since epoxy resins have a possibility to give harmful influence to human body, the matrix of braided composite compression plates was later replaced with a thermoplastic PEEK (poly ether–ether-ketone) material [35], which is known to be well biocompatible [36–38]. In our previous work [35], a micro-braiding fabrication method to gain high and consistent impregnation of matrix into reinforcing fibers was introduced to fabricate the braided carbon/PEEK composite compression bone plates. Four-point bending properties of the composite bone plates were investigated with three different braiding angles. Although the bending properties of braided carbon/PEEK composites showed promising potential for bone plate application, detailed influence of plate thickness and other parameters on the bending behavior was not recognized. In this work, the braided carbon/PEEK bone plates of three different plate thicknesses each with three different braiding angles were comparatively studied in terms of their bending performance. Furthermore, comparison with other composite bone plates and implant feasibility of the developed composite compression plate are addressed in the paper.
2. Experimental procedure 2.1. Materials and fabrication In this study, the micro-braided yarn, which contained PEEK matrix and reinforcement carbon fibers, was used to fabricate braided carbon/PEEK compression bone plates. The unique feature of this yarn is that the reinforcing and matrix fibers are easily mixed using a simple braiding technique. The detailed process of the
Fig. 1. Photograph of a flat braided fabric made of micro-braided yarns (y indicates a braiding angle and a braided fabric have certain range of braiding angle with constant preform width).
Fig. 2. Schematic drawing of insertion way of flat braided fabric. A pin can be inserted into a fabric without damages of micro-braided yarns.
micro-braiding was shown in our previous publication [35]. Flat braided fabrics were preformed using microbraided yarns, as indicated in Fig. 1. In terms of a hot press machine, a composite bone plate was obtained by placing multiple layers of the flat braided fabrics in a stainless-steel mould to which pins were attached to form screw holes without breaking the yarn continuity (Fig. 2). Braided fabrics with three different braiding angles, i.e., 5 , 10 and 15 , were used to investigate the influence of the braiding angles on the bending performance of the composite plates. In order to further investigate such influence, three different plate
K. Fujihara et al. / Biomaterials 24 (2003) 2661–2667
thicknesses, 2.6, 3.2 and 3.8 mm, were achieved using different fabric layers. Table 1 lists detailed information of the composite bone plates made in this study. In our previous work, the composite bone plates were fabricated at 400 C for 60 min under pressure [35]. Although specimens of good quality were obtained through that fabrication condition, matrix degradation was later recognized. Hence, in this study, the plate fabrication was conducted at 380 C for 20 min with a different pressure-temperature relationship history as given in Fig. 3. Fig. 4 shows photos of the fabricated braided composite bone plates and their cross-sectional views. At hole part of a plate (section A–A), carbon fibers are fully packed and no voids have been seen. Table 1 Specimen types of braided carbon/PEEK composite compression bone plates Preform
Braiding angle of preform
Number of layers
Thickness (mm)
Braided fabric (diamond structure)
5 , 10 , 15
8
2.6
5 , 10 , 15 5 , 10 , 15
10 12
3.2 3.8
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There are some voids at a non-hole cross-section (section B–B). These voids, however, are negligibly small. The fabricated braided carbon/PEEK composite bone plates had an averaged volume fraction of 53% in all types of the specimens. A stainless-steel narrow dynamic compression plate (DCP) of AO Institute, which is used widely in surgery, was also tested for comparative purpose. The geometry of the AO plate is 103 mm long, 13 mm wide and 3.8 mm thick. 2.2. Four-point bending test Bending behavior of a bone plate is one of the most critical mechanical properties from an application viewpoint, and is generally evaluated by maximum bending moment and bending stiffness calculated from the initial linear moment against the total bending angle (angulation) of the plate. In this study, static four-point bending tests were conducted with a cross-head speed of 1.0 mm/min at room temperature. Specimen geometry is indicated in Fig. 5. The developed composite bone plate has six holes and curvature geometry to fix it to diapheseal part of a long bone. Upper and lower span lengths are 41 and 73 mm, respectively.
3. Results Temp.
3.1. Stainless-steel dynamic compression plate 380°
Average = -5°C / min.
25 ° 40
10
0
0
20
71 11
Time (min) Pressure (kN)
Fig. 3. Pressure and thermal history applied to the molding.
The stainless-steel plate was tested for comparison purpose. As shown in Fig. 6, this plate indicated a rapid increase of bending moment at small angulation. After the bending moment reached 13 N m, the curve still exhibits an increased bending moment-angulation relationship, but becomes much more ductile. Since the stainless-steel plate permanently deformed without a fracture during the test, the load was finally released to zero. Bending stiffness determined by the initial slope of
Fig. 4. Photographs of braided carbon/PEEK composite compression plates and cross-section photos of screw hole part (A–A) and non-hole part (B–B).
7
16
16
25
16
16
7 15
D = 8.1 D = 4.5 P/2
r = 13.7
P/2 41
103
(a)
73
Bending moment (Nm)
2
P/2
P/2
Unit: mm
Fig. 5. Specimen geometry of braided carbon/PEEK composite compression plate.
Proof load = 21Nm
10 5 degree
8
10 degree
6
15 degre
4 2 0
2.6 mm thickness
0
16
2
10 13Nm
5 0 0
5
7°
10
15
20
25
30
Angulation (degree)
Fig. 6. Bending moment-angulation curve of AO stainless-steel compression plate. Angulation is an angle determined by lines of A–A and B–B.
the curve was 1.4 N m/deg. According to ISO9585 standard [39] (implants for surgery—determination of bending strength and stiffness of bone plates), when a plate shows this kind of testing curve, the maximum moment is replaced with the proof moment which is determined by the plate span lengths. In the present case, the proof moment is defined at the cross-point between the initial slope shifted with 7 angulation and the moment curve. The proof moment thus obtained was 21 N m. 3.2. Braided composite compression plates Fig. 7 shows bending moment—angulation curves of the braided composite compression plates with three different thicknesses. In the case of the specimens with 2.6 mm thickness, all of them showed linear increment of bending moment versus angulation regardless of different braiding angles. When the bending moment reached around 6 N m, some plastic deformation began to occur and all the specimens finally fractured at the third hole near a loading point. On the other hand, all the other specimens with larger thicknesses (i.e., 3.2 and 3.8 mm)
(c)
10
10 degree 15 degree
8 4 3.2 mm thickness
0
0
2
(b) 15
4 6 8 Angulation (degree)
5 degree
12
20
Bending moment (Nm)
Bending moment (Nm)
25
Bending moment (Nm)
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4 6 8 Angulation (degree)
10
20 5 degree
16
10 degree 15 degree
12 8 4 0
3.8 mm thickness
0
2
4
6
8
10
Angulation (degree)
Fig. 7. Bending moment-angulation curves of braided composite compression plates with three different thicknesses.
did not show a significant plastic behavior on their loadangulation curves. In other words, the bending moment increased essentially linearly until rupture and all the specimens finally fractured in the same way as those with the minimum thickness. 3.3. Performance comparison The measured bending results are plotted against plate thickness, as shown in Fig. 8, in which the percentage values are relative to those of the stainlesssteel plate. Thus, the bending performance of the stainless-steel plate was taken as 100%. The braided composite plates showed an increase in the maximum bending moment with the increase of plate thickness for all the braiding angles. The plates which have the minimum thickness showed only 8% difference in the bending moment among different braiding angle specimens. This difference was more distinct for the thicker specimens, i.e., 18% for the 3.2 mm and 19% for the 3.8 mm thick specimens among different braiding angles. The yield bending moment of the stainless-steel
K. Fujihara et al. / Biomaterials 24 (2003) 2661–2667
Maximum bending moment (Nm)
25
2665
Bone specimen beneath of a plate
Braiding (5 degree) Braiding (10 degree)
20
Braiding (15 degree)
15
67~86%
100%
Bone specimen opposite of a plate
Stainless-Steel
43~61%
10 33~41%
5 0 2
2.5
3
3.5
4
Compression bone plate
4.5
Plate thickness (mm)
Fig. 9. Two different bone specimens treated by a compression bone plate.
100~131%
15 61~96% 500
100%
10 46~49%
Braiding (5 degree) Braiding (10 degree) Braiding (15 degree) Stainless-steel
5
0 2
2.5
3
3.5
4
4.5
Plate thickness (mm)
Bending stiffness (Nm/deg)
1.6 Braiding (5 degree) Braiding (10 degree)
1.2
100%
88~96%
Ultimate bone srength treated by carbon/polysulfone composite compression bone plate (%)
Yield bending moment (Nm)
20
Opposite of a plate 400
Beneath of a plate
300
200
100
0 0
Braiding (15 degree) Stainless-Steel
63~68%
0.8
20
40
60
80
100
120
Ultimate stiffness of carbon/polysulfone composite compression bone plate (%)
Fig. 10. The shown date is referred from the work of Bradley et al. [22]. The strength of dog femur was measured after the treatment of stainless-steel compression plate and carbon/polysulfone UD laminated composite plate with various stiffnesses. The values of 100% were treated by stainless-steel compression plate.
39~40%
0.4
0 2
2.5
3
3.5
4
4.5
Plate thickness (mm) Fig. 8. Bending data comparison against plate thickness.
2.6 mm thick specimens with the three different braiding angles. This was not applicable, however, to the specimens with larger thicknesses.
4. Discussion plate was relatively lower than its maximum bending moment. However, the braided composite plates did not show drastic yield moment decrease as compared with their maximum value. It is noted that the braided composite plates of 3.2 mm thickness showed almost the same yield moment as that of the stainless-steel plate which was 3.8 mm thick. The bending stiffness of the braided composite plates increased with the increase in thickness, and the specimens of 3.8 mm thickness indicated a quite close stiffness to that of the stainlesssteel plate. There was no stiffness variation for the
A lot of efforts have been made in the past to develop composite compression plates for possible replacement of metal plates, but mainly focused on UD laminates and discontinuous short fibers as reinforcement. Although some of those composite plates have achieved some success in animal and clinical trials, there are still a number of critical issues which need to be addressed on the plate design with required mechanical property. Clinically, a thinner compression bone plate is preferred because of the limited space between muscle and
K. Fujihara et al. / Biomaterials 24 (2003) 2661–2667
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Table 2 Bending properties of UD laminated composite bone plates used for human clinical trials Ref.
[32] [29]
[27]
Plate type
Implant site
Bending stiffness (N m/deg) Stainlesssteel
Composite
Maximum bending moment (N m) Composite
Carbon/epoxy (6-hole narrow DCP) Carbon/epoxy (8-hole broad DCP)
Human forearm
1.25
0.377 (30%)
9.1
Human tibia
3.8
—
Carbon/epoxy (8-hole broad DCP)
Human tibia
1.88
0.94–2.22 (25–58%) 2.0 (53%) was recommended 0.8 (43%)
fragmented bones. However, this requirement may lead to fatigue fracture of the plate [25] before bone is healed. It may be considered that the previously developed composite compression plates could not achieve thinner size because of shortcomings of the reinforcing methods used. Therefore, the concern would be whether our developed braided composite compression plates are applicable to clinical usage. Since bending stiffness of a compression plate has a close relationship with bone healing process, the previous animal and clinical trials have suggested an appropriate choice for the plate stiffness to achieve good bone healing. One of the examples discussed the relationship between the plate stiffness and the bone strength after a fixation treatment was given by Bradley et al. [22]. In their work, fractured dog femurs were treated using a stainless-steel compression plate and a carbon/polysulfone UD laminate plate, respectively. Then, the bone pieces both opposite to and beneath the plate (see Fig. 9) were tested under bending loading. The results shown in Fig. 10 tell that when the plate stiffness is around 10–25% value of stainless-steel, the healed bone strengths are much higher than those treated by even stiffer composite plates. However since the stress environment of the dog femur is not as severe as that of human femur or tibia due to the weight and bone site differences, the stiffness values of composite plates for human femur and tibia should be higher (Table 2). For instance, 53% [29] and 43% [27] of stainless-steel plate stiffness were recommended to obtain a quick human bone healing on human tibia. In the case of human forearm where less stress environment than that of femur and tibia is expected, the appropriate stiffness of composite plates is around 30% of that of the stainless-steel plate. The braided composite compression bone plate developed in this study showed 40% stiffness value with 2.6 mm thickness together with the maximum bending moment, which is comparable with the result of Ali et al. [32]. Thus, this plate can be appropriate for forearm fixation. On the other hand, since the braided composite bone plates with 3.2 mm thickness showed 63–68% stiffness of the
24
stainless-steel plate, these composite plates may be used to fix femur and tibia bones. It is noted that in both the cases, the current braided composite bone plates are thinner than previously developed composite plates and even a clinically used stainless-steel compression plate which is 3.8 mm thick.
5. Conclusion Although several composite bone plates were developed using UD laminates and discontinuous short fibers to serve as alternatives for the conventional stainlesssteel AO compression plates, there still remain a number of improvements to be addressed from a mechanical viewpoint. In this regard, we have proposed using braided carbon/PEEK fabric composites as a new material system for bone plate development. In this work, the influence of braiding angle and plate thickness on the bending properties of the braided composite bone plates is further investigated. As a result, the influence of the braiding angle, varied in a certain range, on the plate bending properties is not significant when the plate thickness is thin. This influence is higher when the plate becomes thicker. A 10 braiding angle can be recommended for all the cases under consideration. It is considered that the braided composite plate with 2.6 mm thickness can be suitable for forearm treatment whereas the braided composite plate of 3.2 mm thickness is applicable to femur or tibia fixation.
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