- Email: [email protected]
A novel, inexpensive and easy to use tendon clamp for in vitro biomechanical testing
Medical Engineering & Physics 34 (2012) 516–520
Contents lists available at SciVerse ScienceDirect
Medical Engineering & Physics journal homepage: www.elsevier.com/locate/medengphy
Technical note
A novel, inexpensive and easy to use tendon clamp for in vitro biomechanical testing DuFang Shi a , DongMei Wang a,∗ , ChengTao Wang a , Anmin Liu b a b
Institute of Biomedical Manufacturing and Life Quality, School of Mechanical and Power Engineering, ShangHai JiaoTong University, Shanghai, PR China School of Health, Sport and Rehabilitation Sciences, University of Salford, Manchester M6 6PU, UK
a r t i c l e
i n f o
Article history: Received 5 May 2011 Received in revised form 23 October 2011 Accepted 20 November 2011 Keywords: Biomechanical testing Non-frozen clamp Ligaments clamp Asymmetry-triangle-teeth Creep deformation
a b s t r a c t Frozen clamps can hold tendons and ligaments tightly and transmit high loads, from 4 kN to 13 kN, without slippage, yet they are complex and expensive. The existing non-frozen serrated jaw clamp is simple to fabricate and use, but the maximal tensile force it can sustain is only about 2.5 kN, which is not enough in many biomechanical tests. In this study, a new type of non-frozen clamp, which has lateral block boards and asymmetrical teeth jaws, was designed. The lateral block boards made of titanium alloy were used to prevent the soft tissues from being squeezed out during compressing, while the asymmetrical teeth jaws made of nylon were used to grip and keep holding soft tissues. The capability of this new type of clamp was tested by stretching five cattle tendons to failure on the tensile and compression testing machine, none of them displayed any slippage before rupture, the maximum tension force was 6.87 kN. This non-frozen asymmetrical teeth jaw clamp was designed for gripping tendons in foot and ankle dynamic simulation test, but it can also be applied to other in vitro tests, such as hip and knee dynamic tests. © 2011 IPEM. Published by Elsevier Ltd. All rights reserved.
1. Introduction Biomechanical testing on cadaver specimens has always raised the question of a reliable linkage of an actuator to either tendons or ligaments [12]. Because of their viscoelastic characteristics and low friction between the clamp material and wet soft tissues, it is difficult to hold them rigidly at in vitro loads and loading speed. Excessive compression on the soft tissue will elevate stress around the contact area, which leads to rupture before target loads are achieved, too little compression will result in slippage [10]. To solve this problem, many different clamp designs have been proposed, such as using self-tightening clamp, sandpaper or other high friction surfaces, however slippage still cannot be avoided [8]. Since Riemersa and Schamhardt [10], first described the ‘cryojaw’ clamp, which showed excellent capability in transferring very high loads, about 10,200 N, without causing slippage, it has been considered as the gold standard for high load mechanical testing of tendons or ligaments, and has been adapted by many [12,8,11]. But due to its complex configuration (freezing equipment), usually large size and high costs, other groups have sought pure mechanical solutions [9,2–5]. For reducing the containing water in tendons which is a major factor attribute to slippage, blotting papers had been used by A. Probst and his coworkers [9], but they were only
∗ Corresponding author. Tel.: +86 21 3420 6798. E-mail address: [email protected] (D. Wang).
applied on small size of tendon, which ruptured within 10 N. To increase the friction between the materials of the devices and wet soft collagenous tissues a non-frozen serrated jaw clamp was used by Cheung and Zhang [3]. This had the advantage of simple fabrication and adjustment and small size, but the maximum tensile force which it could sustain was only 2.5 kN, which is below in vitro loads for some lower limb tendons. Also, a non-negligible portion of tendon was squeezed out of the jaw at both sides. In addition, for reducing large deformations produced on the ends of the tendons by compression, the method of entwining the tendon ends with thread or copper wire was applied [2,4,5], but the central part of fibers could not solely provide all the load bearing function of the tendon, so significantly reduced their origin tensile resistance capability. The main problem with a tendon clamp is that it is hard to maintain the high pressure that provides enough friction force between the tendon and the clamp to resist the large tensile load, and at the same time to reduce the cutting effect as much as possible. Maintaining high clamping pressure and avoiding cutting off the tendon have become the two fundamental requirements for the design of a tendon clamp. When a tendon is subject to compression force, it would be markedly deformed into a thin layer of fibers that results in rapid damage [10], or the loss of strength due to part of the tendon being squeezed out of the clamp at both sides [3]. The tests on several preliminary failed designs in the study proved that it was the main reason for failing to produce high tensile load. Therefore the key mechanical problem to solve is to prevent the tissue from expansion under high clamp pressure. Obviously, the design of a
1350-4533/$ – see front matter © 2011 IPEM. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.medengphy.2011.11.019
D. Shi et al. / Medical Engineering & Physics 34 (2012) 516–520
517
Fig. 1. Illustration of the non-frozen tendon clamps with titanium-alloy clamp bodies and asymmetrical triangle-teeth nylon jaws.
clamp that can contain the tendon or tissue would be ideal for the two fundamental requirements. In this study, the objective was to design a tendon clamp that could withstand 4.0 kN tensile force for the dynamic cadaver foot and ankle simulation test rig being developed. The 4.0 kN target was set for the clamp used for the Achilles tendon [1,6]. Based on the serrated jaw clamp [3] and the preliminary failed prototypes, a new non-frozen type jaw clamp was developed through step by step optimization and renovation to the initial design. The shape and arrangement of the teeth in the jaws were specially designed so the maximum compression force could be sustained, and block structures were designed to prevent the soft tissue from expansion laterally. Reduced lateral expansion would increase the pressure within the clamp at wet interface and maintain a certain depth of tendon to resist any risk of cutting the tendon.
the tooth and the tendon pulling force acting line (Fig. 1). Since the tooth angle was chosen as right angle, the clip-angle should have a range of 45–90◦ . Fourthly, the tooth depth and the total thickness of clamp should be well balanced. Although it was clear that larger tooth would improve the capacity of the clamp to maintain the soft tissue and tendon within the clamp, the clamp itself could not be excessively large in size due to the limited space, as a compromise the tooth depth was chosen as 1.6 mm. Then according to the formula relationship among clip-angle, pitch value and tooth depth (pitch value × cos(clip-angle) × sin(clip-angle) = tooth depth), the clip-angle, 63◦ , was determined. The jaw was designed as peak to peak instead of peak to valley, which was the novel feature differentiate the design from others (Fig. 1). This design reduced the risk of the expansion and creep of the tendon or tissue. The width of this clamp was chosen as the
2. Materials and methods Before the final design was achieved, several different prototypes were developed and tested. The common problem with these designs was that much of the tendon was squeezed out while fastening the clamp due to the flat structure of each clamp plate. The instant high pressure generated by fastening the clamp could not be maintained and the clamp could not successfully hold the tendon firmly until the two clamp plates met together with only a little potion of tendon tissue between them. So during tests only partial fibers of the tendon or ligament could transfer the load, and the maximum force of the peak loads was less than 2 kN, which was far less than the target. All the preliminary prototypes failed because they could not prevent tendon from slipping and could not contain the tissue within the area of the jaws of the clamp. To address the problems experienced in the early designs a lateral board was added to the clamp design (Fig. 1). Initial tested showed that the soft tissue could be prevented from being squeezed out at both sides and that a high pressure inside the clamp was maintained as the clamp was closed (Fig. 2). The new design of the clamp had two components, each of which had an asymmetrical nylon teeth jaw and an alloyed titanium clamp body with lateral block boards, to prevent tissues from being extruded (Figs. 1 and 2). The pitch of the nylon rack was 4 mm with the indentations of 1.6 mm deep at an angle of 27◦ in one side and 63◦ in the other side (Fig. 1). The parameters of the nylon teeth jaw met four principles. Firstly, the tooth was chosen as right angle so that it would be easy for machining. Secondly, at least five teeth along the nylon plate could be fabricated, so that it would be better for the distribution of tensile force, since the total length of the plate was only 20 mm, so the pitch value, 4 mm, was determined. Thirdly, anti-slippage and anti-cutting should be well balanced, which were mainly affected by the clip-angle, the angle between one edge of
Fig. 2. (a) The translucent peak-to-peak end of the tendon in the jaw comparing to the valley-to-valley end of tendon and the uncompressed middle portion. (b) The clamp with tendon fixed in.
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D. Shi et al. / Medical Engineering & Physics 34 (2012) 516–520
Table 1 The parameters of three types of new non-frozen clamp. Clamp I Width
Clamp II D-sha
D-jhb
40 M4 Ø6 Suitable for tendons whose width is around 16–24 mm a b
Width
Clamp III D-sha
D-jhb
32 M4 Ø5 Suitable for tendons whose width is around 12–20 mm
Width
D-sha
D-jhb
24 M3 Ø5 Suitable for tendons whose width is around 9–15 mm
D-sh, diameter of screw hole. D-jh, diameter of joint hole.
twice the width of the tendon. Four M6 screws were used to fix the two pieces of the clamp together while left a hole of Ø 6 for installation to the test rig (Fig. 1). For clipping different sizes of tendons in dynamic cadaver foot and ankle simulation test, three types of clamps with different width and different mounting hole and screws were developed (Table 1, Fig. 3). A ZWick/Roell Z010 material testing machine with a 100 kN load cell was used to test the maximum tensile force that the asymmetrical teeth jaw clamp could sustain before slippage. Due to cost reason, fresh cattle tendons with similar section size to human Achilles tendon were chosen to test the clamp (clamp I). The cattle tendon suited the study very well because it was the closest material to human cadaver tendon in testing the friction between it and the clamp. In the test, the cattle tendon were gripped in both ends with the same type of clamp and tested at a speed of 200 mm/min [7]. The displacement between the two clamps was recorded together with tensile load. The tendons were loaded to failure, which was judged by either visually large slippage occurred or the tendon ruptured. The test on the clamp was recognized successful when a maximum load of 4.0 kN without visual slippage
even certain slippage did exist when it was judged based on more scientific calculation on load–displacement results. The slippage could also be judged by the outcome of the gripping ends after testing. As it is shown in Fig. 4, the slipped part of the gripping end was pressed to the same thickness, while the ruptured part of the tendon in the clamp was still in the neat wave-shape that was formed by the clamping force. 3. Results Totally five cattle tendons were tested with the clamp on the ZWick/Roell Z010 material testing machine. The test results were presented by the column chart (Fig. 5a) and the displacement–tension curves (Fig. 5b), which indicated that the non-frozen asymmetrical teeth jaw clamp worked effectively; it could sustain 2.50 kN with negligible slippage. All the tests on the five chosen cattle tendons were ended due to the rupture of the tendon rather than slippage. The maximum loads achieved on four of the specimens were beyond 4.0 kN, which satisfied the target of the design. The maximum tensile load recorded was from 2.50 kN to 6.87 kN with an average of 4.85 ± 1.22 kN, and no visual slippage happened before the target load of 4.0 kN was achieved in four tests and even before ruptured, which could be clearly confirmed from the tension versus displacement cures of the five tests. 4. Discussion
Fig. 3. (a) The inner side of a pair of clamps I; (b) three types of clamps.
This study has aimed at a none-frozen clamp that can sustain at least 4.0 kN tensile force, the test results have shown this object has achieved. The results from the tensile loading tests have shown that the lateral blocks adopted in the design are a unique feature of this type of clamp, which could successfully prevent the compressed tissue or tendon from expansion at both sides. The combination of the nylon asymmetrical teeth jaw, the peak-to-peak arrangement and the lateral blocks demonstrated the increased ability of high compression force and therefore high capacity of sustaining tensile force more than 4.0 kN. The application of nylon as the material of the teeth has reduced the risk of the tendon being cut off. The reason why this design of clamp could work well can be explained by the observation achieved during testing, the gripping portion (peak-to-peak part) of the tendon was turned to be translucent and stiff after being clamped (Fig. 2a). During fixation of tendons, the liquid was squeezed out from the tendon by clamping force, therefore the peak-to-peak part of tendon turned to be less wet, much thinner, more translucent and hardened than valley-to-valley part of tendon, which helped increase the friction force in two aspects. Firstly, the peak-to-peak part of tendon is the major part to sustain compression, as it became more hardened and less wet and was better for sustaining higher compression and increasing friction coefficient so to increase the friction force. This effect was similar to the frozen tissue in cryo-jaw which also hardens the soft tissues. Secondly, since the valley-to-valley part of tendon, which was much thicker, contained more water, and was closed in the airtight valley-to-valley space secured by the lateral blocks, it was easier to maintain the high pressure within the space so to prevent the tendon from stretching and slipping.
D. Shi et al. / Medical Engineering & Physics 34 (2012) 516–520
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Fig. 4. Comparing of the outcomes of the slipped part and the ruptured part of the gripping ends after testing.
Comparing to the frozen type of clamp [12,8,11], the only short coming of the described non-frozen asymmetrical teeth jaw clamps was that it could not be used to test the maximum strength of the tendon because the frontal metal edge caused a stress concentration and resulted in the rupture at the metal clamping area. However, in many biomechanical experiments, such as foot and ankle, shoulder, elbow, knee and hip dynamic simulation tests, it is not always necessary that the tendons are loaded as high as they reach their strength limit [11]. Additional advantages of these clamps were the low cost in fabrication and application. The maximum size of this clamp was only 42 mm × 40 mm × 16 mm as shown in Fig. 1. Production of a pair
of this clamps required approximately 6 hours of labor. The material cost was near to those of serrated jaw clamp [3], and much cheaper than the cost of the ‘cryo-jaw’ clamp due to the complexity of the clamp itself and the need of the cooling system that is used to frozen the tendon. And furthermore this new type of clamp can be easily used and the technical requirement to the operator is low. For many in vitro biomechanical tests, with such clamps it avoids the umbilical freezing medium supplying pipeline, which reduces the experiment costs and simplifies the test rig as well. For the tendon strength test, the design of the clamp needs to be improved so that the stress concentration effect can be reduced and
Fig. 5. (a) The peak load of the five specimens. (b) The tension versus displacement curves.
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D. Shi et al. / Medical Engineering & Physics 34 (2012) 516–520
the tensile load can be increased to as high as its biological strength limit without causing rupture at the clamping edge. 5. Conclusions The new type of non-frozen tendon clamps could sustain the targeted tensile force of 4.0 kN by holding the tendon firmly without any slippage, the maximal tensile force it could sustain was as high as 6.87 kN. This new type of clamp is easy to manufacture and use, small in size, cheap in cost. It can be a good alternative to complex frozen clamps in biological simulation tests of muscular activity in musculoskeletal systems. Acknowledgments This work was supported by the Natural Science Foundation of China through the research project (No. 81071234), the International Cooperation and Exchange Foundation of China through the research project (No. 30810103908) and the Open Project Foundation of State Key Laboratory of Mechanical System and Vibration (No. MSV-2009-17). The authors would like to acknowledge the help and technical support of Mr. Yuanchao Li and Ms. Feng Sun. Conflict of interest None.
References [1] Arndt AN, Kom PV, Briiggemann G-P, Lukkariniem J. Individual muscle contributions to the in vivo Achilles tendon force. Clinical Biomechanics 1998;13(1998):532–41. [2] Benedict JV, Walker LB, Harris EH. Stress–strain characteristics and tensile strength of unembalmed human tendon. Journal of Biomechanics 1968;1:53–63. [3] Cheung JTM, Zhang M. A serrated jaw clamp for tendon gripping. Medical Engineering & Physics 2006;28(4):379–82. [4] Cohen RE, Hooley CJ, McCrum NG. Visco-elastic creep of collagenous tissue. Journal of Biomechanics 1976;9:175–84. [5] Elliott DH. The biomechanical properties of tendon in relation to muscular strength. Annals of Physical Medicine 1967;9:1–7. [6] Fröberg Å, Komi P, Ishikawa M, Movin T, Arndt A. Force in the Achilles tendon during walking with ankle foot orthosis. The American Journal of Sports Medicine 2009;37(6). [7] Johnson GA, Tramaglini DM, Levine RE, Ohno K, Choi NY, Woo SLY. Tensile and visco-elastic properties of human patellar tendon. Journal of Orthopaedic Research 1994;12:796–803. [8] Kiss MO, Hagemeister N, Levasseur A, Fernandes J, Lussier B, Petit Y. A lowcost thermoelectrically cooled tissue clamp for in vitro cyclic loading and load-to-failure testing of muscles and tendons. Medical Engineering & Physics 2009;31(9):1182–6. [9] Probst A, Palmes D, Freise H, Langer M, Joist A, Spiegel HU. A new clamping technique for biomechanical testing of tendons in small animals. Journal of Investigative Surgery 2000;13:313–8. [10] Riemersa DJ, Schamhardt HC. The cryo-jaw, a clamp designed for in vitro rheology studies of horse digital flexor tendons. Journal of Biomechanics 1982;15(8):619–20. [11] Sharkey NA, Smith TS, Lundmark DC. Frezze clamping musculotendinous junctions for in-vitro simulation of joint mechanics. Journal of Biomechanics 1995;28(5):631–5. [12] Wieloch P, Buchmann G, Roth W, Rickert M. A cryo-jaw designed for in vitro tensile testing of the healing Achilles tendons in rats. Journal of Biomechanics 2004;37(11):1719–22.
Contents lists available at SciVerse ScienceDirect
Medical Engineering & Physics journal homepage: www.elsevier.com/locate/medengphy
Technical note
A novel, inexpensive and easy to use tendon clamp for in vitro biomechanical testing DuFang Shi a , DongMei Wang a,∗ , ChengTao Wang a , Anmin Liu b a b
Institute of Biomedical Manufacturing and Life Quality, School of Mechanical and Power Engineering, ShangHai JiaoTong University, Shanghai, PR China School of Health, Sport and Rehabilitation Sciences, University of Salford, Manchester M6 6PU, UK
a r t i c l e
i n f o
Article history: Received 5 May 2011 Received in revised form 23 October 2011 Accepted 20 November 2011 Keywords: Biomechanical testing Non-frozen clamp Ligaments clamp Asymmetry-triangle-teeth Creep deformation
a b s t r a c t Frozen clamps can hold tendons and ligaments tightly and transmit high loads, from 4 kN to 13 kN, without slippage, yet they are complex and expensive. The existing non-frozen serrated jaw clamp is simple to fabricate and use, but the maximal tensile force it can sustain is only about 2.5 kN, which is not enough in many biomechanical tests. In this study, a new type of non-frozen clamp, which has lateral block boards and asymmetrical teeth jaws, was designed. The lateral block boards made of titanium alloy were used to prevent the soft tissues from being squeezed out during compressing, while the asymmetrical teeth jaws made of nylon were used to grip and keep holding soft tissues. The capability of this new type of clamp was tested by stretching five cattle tendons to failure on the tensile and compression testing machine, none of them displayed any slippage before rupture, the maximum tension force was 6.87 kN. This non-frozen asymmetrical teeth jaw clamp was designed for gripping tendons in foot and ankle dynamic simulation test, but it can also be applied to other in vitro tests, such as hip and knee dynamic tests. © 2011 IPEM. Published by Elsevier Ltd. All rights reserved.
1. Introduction Biomechanical testing on cadaver specimens has always raised the question of a reliable linkage of an actuator to either tendons or ligaments [12]. Because of their viscoelastic characteristics and low friction between the clamp material and wet soft tissues, it is difficult to hold them rigidly at in vitro loads and loading speed. Excessive compression on the soft tissue will elevate stress around the contact area, which leads to rupture before target loads are achieved, too little compression will result in slippage [10]. To solve this problem, many different clamp designs have been proposed, such as using self-tightening clamp, sandpaper or other high friction surfaces, however slippage still cannot be avoided [8]. Since Riemersa and Schamhardt [10], first described the ‘cryojaw’ clamp, which showed excellent capability in transferring very high loads, about 10,200 N, without causing slippage, it has been considered as the gold standard for high load mechanical testing of tendons or ligaments, and has been adapted by many [12,8,11]. But due to its complex configuration (freezing equipment), usually large size and high costs, other groups have sought pure mechanical solutions [9,2–5]. For reducing the containing water in tendons which is a major factor attribute to slippage, blotting papers had been used by A. Probst and his coworkers [9], but they were only
∗ Corresponding author. Tel.: +86 21 3420 6798. E-mail address: [email protected] (D. Wang).
applied on small size of tendon, which ruptured within 10 N. To increase the friction between the materials of the devices and wet soft collagenous tissues a non-frozen serrated jaw clamp was used by Cheung and Zhang [3]. This had the advantage of simple fabrication and adjustment and small size, but the maximum tensile force which it could sustain was only 2.5 kN, which is below in vitro loads for some lower limb tendons. Also, a non-negligible portion of tendon was squeezed out of the jaw at both sides. In addition, for reducing large deformations produced on the ends of the tendons by compression, the method of entwining the tendon ends with thread or copper wire was applied [2,4,5], but the central part of fibers could not solely provide all the load bearing function of the tendon, so significantly reduced their origin tensile resistance capability. The main problem with a tendon clamp is that it is hard to maintain the high pressure that provides enough friction force between the tendon and the clamp to resist the large tensile load, and at the same time to reduce the cutting effect as much as possible. Maintaining high clamping pressure and avoiding cutting off the tendon have become the two fundamental requirements for the design of a tendon clamp. When a tendon is subject to compression force, it would be markedly deformed into a thin layer of fibers that results in rapid damage [10], or the loss of strength due to part of the tendon being squeezed out of the clamp at both sides [3]. The tests on several preliminary failed designs in the study proved that it was the main reason for failing to produce high tensile load. Therefore the key mechanical problem to solve is to prevent the tissue from expansion under high clamp pressure. Obviously, the design of a
1350-4533/$ – see front matter © 2011 IPEM. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.medengphy.2011.11.019
D. Shi et al. / Medical Engineering & Physics 34 (2012) 516–520
517
Fig. 1. Illustration of the non-frozen tendon clamps with titanium-alloy clamp bodies and asymmetrical triangle-teeth nylon jaws.
clamp that can contain the tendon or tissue would be ideal for the two fundamental requirements. In this study, the objective was to design a tendon clamp that could withstand 4.0 kN tensile force for the dynamic cadaver foot and ankle simulation test rig being developed. The 4.0 kN target was set for the clamp used for the Achilles tendon [1,6]. Based on the serrated jaw clamp [3] and the preliminary failed prototypes, a new non-frozen type jaw clamp was developed through step by step optimization and renovation to the initial design. The shape and arrangement of the teeth in the jaws were specially designed so the maximum compression force could be sustained, and block structures were designed to prevent the soft tissue from expansion laterally. Reduced lateral expansion would increase the pressure within the clamp at wet interface and maintain a certain depth of tendon to resist any risk of cutting the tendon.
the tooth and the tendon pulling force acting line (Fig. 1). Since the tooth angle was chosen as right angle, the clip-angle should have a range of 45–90◦ . Fourthly, the tooth depth and the total thickness of clamp should be well balanced. Although it was clear that larger tooth would improve the capacity of the clamp to maintain the soft tissue and tendon within the clamp, the clamp itself could not be excessively large in size due to the limited space, as a compromise the tooth depth was chosen as 1.6 mm. Then according to the formula relationship among clip-angle, pitch value and tooth depth (pitch value × cos(clip-angle) × sin(clip-angle) = tooth depth), the clip-angle, 63◦ , was determined. The jaw was designed as peak to peak instead of peak to valley, which was the novel feature differentiate the design from others (Fig. 1). This design reduced the risk of the expansion and creep of the tendon or tissue. The width of this clamp was chosen as the
2. Materials and methods Before the final design was achieved, several different prototypes were developed and tested. The common problem with these designs was that much of the tendon was squeezed out while fastening the clamp due to the flat structure of each clamp plate. The instant high pressure generated by fastening the clamp could not be maintained and the clamp could not successfully hold the tendon firmly until the two clamp plates met together with only a little potion of tendon tissue between them. So during tests only partial fibers of the tendon or ligament could transfer the load, and the maximum force of the peak loads was less than 2 kN, which was far less than the target. All the preliminary prototypes failed because they could not prevent tendon from slipping and could not contain the tissue within the area of the jaws of the clamp. To address the problems experienced in the early designs a lateral board was added to the clamp design (Fig. 1). Initial tested showed that the soft tissue could be prevented from being squeezed out at both sides and that a high pressure inside the clamp was maintained as the clamp was closed (Fig. 2). The new design of the clamp had two components, each of which had an asymmetrical nylon teeth jaw and an alloyed titanium clamp body with lateral block boards, to prevent tissues from being extruded (Figs. 1 and 2). The pitch of the nylon rack was 4 mm with the indentations of 1.6 mm deep at an angle of 27◦ in one side and 63◦ in the other side (Fig. 1). The parameters of the nylon teeth jaw met four principles. Firstly, the tooth was chosen as right angle so that it would be easy for machining. Secondly, at least five teeth along the nylon plate could be fabricated, so that it would be better for the distribution of tensile force, since the total length of the plate was only 20 mm, so the pitch value, 4 mm, was determined. Thirdly, anti-slippage and anti-cutting should be well balanced, which were mainly affected by the clip-angle, the angle between one edge of
Fig. 2. (a) The translucent peak-to-peak end of the tendon in the jaw comparing to the valley-to-valley end of tendon and the uncompressed middle portion. (b) The clamp with tendon fixed in.
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D. Shi et al. / Medical Engineering & Physics 34 (2012) 516–520
Table 1 The parameters of three types of new non-frozen clamp. Clamp I Width
Clamp II D-sha
D-jhb
40 M4 Ø6 Suitable for tendons whose width is around 16–24 mm a b
Width
Clamp III D-sha
D-jhb
32 M4 Ø5 Suitable for tendons whose width is around 12–20 mm
Width
D-sha
D-jhb
24 M3 Ø5 Suitable for tendons whose width is around 9–15 mm
D-sh, diameter of screw hole. D-jh, diameter of joint hole.
twice the width of the tendon. Four M6 screws were used to fix the two pieces of the clamp together while left a hole of Ø 6 for installation to the test rig (Fig. 1). For clipping different sizes of tendons in dynamic cadaver foot and ankle simulation test, three types of clamps with different width and different mounting hole and screws were developed (Table 1, Fig. 3). A ZWick/Roell Z010 material testing machine with a 100 kN load cell was used to test the maximum tensile force that the asymmetrical teeth jaw clamp could sustain before slippage. Due to cost reason, fresh cattle tendons with similar section size to human Achilles tendon were chosen to test the clamp (clamp I). The cattle tendon suited the study very well because it was the closest material to human cadaver tendon in testing the friction between it and the clamp. In the test, the cattle tendon were gripped in both ends with the same type of clamp and tested at a speed of 200 mm/min [7]. The displacement between the two clamps was recorded together with tensile load. The tendons were loaded to failure, which was judged by either visually large slippage occurred or the tendon ruptured. The test on the clamp was recognized successful when a maximum load of 4.0 kN without visual slippage
even certain slippage did exist when it was judged based on more scientific calculation on load–displacement results. The slippage could also be judged by the outcome of the gripping ends after testing. As it is shown in Fig. 4, the slipped part of the gripping end was pressed to the same thickness, while the ruptured part of the tendon in the clamp was still in the neat wave-shape that was formed by the clamping force. 3. Results Totally five cattle tendons were tested with the clamp on the ZWick/Roell Z010 material testing machine. The test results were presented by the column chart (Fig. 5a) and the displacement–tension curves (Fig. 5b), which indicated that the non-frozen asymmetrical teeth jaw clamp worked effectively; it could sustain 2.50 kN with negligible slippage. All the tests on the five chosen cattle tendons were ended due to the rupture of the tendon rather than slippage. The maximum loads achieved on four of the specimens were beyond 4.0 kN, which satisfied the target of the design. The maximum tensile load recorded was from 2.50 kN to 6.87 kN with an average of 4.85 ± 1.22 kN, and no visual slippage happened before the target load of 4.0 kN was achieved in four tests and even before ruptured, which could be clearly confirmed from the tension versus displacement cures of the five tests. 4. Discussion
Fig. 3. (a) The inner side of a pair of clamps I; (b) three types of clamps.
This study has aimed at a none-frozen clamp that can sustain at least 4.0 kN tensile force, the test results have shown this object has achieved. The results from the tensile loading tests have shown that the lateral blocks adopted in the design are a unique feature of this type of clamp, which could successfully prevent the compressed tissue or tendon from expansion at both sides. The combination of the nylon asymmetrical teeth jaw, the peak-to-peak arrangement and the lateral blocks demonstrated the increased ability of high compression force and therefore high capacity of sustaining tensile force more than 4.0 kN. The application of nylon as the material of the teeth has reduced the risk of the tendon being cut off. The reason why this design of clamp could work well can be explained by the observation achieved during testing, the gripping portion (peak-to-peak part) of the tendon was turned to be translucent and stiff after being clamped (Fig. 2a). During fixation of tendons, the liquid was squeezed out from the tendon by clamping force, therefore the peak-to-peak part of tendon turned to be less wet, much thinner, more translucent and hardened than valley-to-valley part of tendon, which helped increase the friction force in two aspects. Firstly, the peak-to-peak part of tendon is the major part to sustain compression, as it became more hardened and less wet and was better for sustaining higher compression and increasing friction coefficient so to increase the friction force. This effect was similar to the frozen tissue in cryo-jaw which also hardens the soft tissues. Secondly, since the valley-to-valley part of tendon, which was much thicker, contained more water, and was closed in the airtight valley-to-valley space secured by the lateral blocks, it was easier to maintain the high pressure within the space so to prevent the tendon from stretching and slipping.
D. Shi et al. / Medical Engineering & Physics 34 (2012) 516–520
519
Fig. 4. Comparing of the outcomes of the slipped part and the ruptured part of the gripping ends after testing.
Comparing to the frozen type of clamp [12,8,11], the only short coming of the described non-frozen asymmetrical teeth jaw clamps was that it could not be used to test the maximum strength of the tendon because the frontal metal edge caused a stress concentration and resulted in the rupture at the metal clamping area. However, in many biomechanical experiments, such as foot and ankle, shoulder, elbow, knee and hip dynamic simulation tests, it is not always necessary that the tendons are loaded as high as they reach their strength limit [11]. Additional advantages of these clamps were the low cost in fabrication and application. The maximum size of this clamp was only 42 mm × 40 mm × 16 mm as shown in Fig. 1. Production of a pair
of this clamps required approximately 6 hours of labor. The material cost was near to those of serrated jaw clamp [3], and much cheaper than the cost of the ‘cryo-jaw’ clamp due to the complexity of the clamp itself and the need of the cooling system that is used to frozen the tendon. And furthermore this new type of clamp can be easily used and the technical requirement to the operator is low. For many in vitro biomechanical tests, with such clamps it avoids the umbilical freezing medium supplying pipeline, which reduces the experiment costs and simplifies the test rig as well. For the tendon strength test, the design of the clamp needs to be improved so that the stress concentration effect can be reduced and
Fig. 5. (a) The peak load of the five specimens. (b) The tension versus displacement curves.
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the tensile load can be increased to as high as its biological strength limit without causing rupture at the clamping edge. 5. Conclusions The new type of non-frozen tendon clamps could sustain the targeted tensile force of 4.0 kN by holding the tendon firmly without any slippage, the maximal tensile force it could sustain was as high as 6.87 kN. This new type of clamp is easy to manufacture and use, small in size, cheap in cost. It can be a good alternative to complex frozen clamps in biological simulation tests of muscular activity in musculoskeletal systems. Acknowledgments This work was supported by the Natural Science Foundation of China through the research project (No. 81071234), the International Cooperation and Exchange Foundation of China through the research project (No. 30810103908) and the Open Project Foundation of State Key Laboratory of Mechanical System and Vibration (No. MSV-2009-17). The authors would like to acknowledge the help and technical support of Mr. Yuanchao Li and Ms. Feng Sun. Conflict of interest None.
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