Al-khwarizmiii EEE ngineering Journal Al-Khwarizmi Engineering Journal, Vol.3, No.1, pp 40-47, (2007) ٤٠ An experimental comparative study between polypropylene and laminated lower limb prosthetic socket Dr. Muhsin J. Jweeg Dr. Samira K. Radhi Haider F. Neama, M.Sc. Mechanical Eng.Department Mechanical Eng.Department Mechanical Eng.Department Nahrain University Al-Mustansiriya University Baghdad University (Received 21 August 2006; accepted 19 December 2006) Abstract: Most researchers concentrate their studies on the design, stress and pressure distributions of the prosthetic socket. A little attention is considered for the stiffness of the various materials of the prosthetic sockets. Prosthetic laminated sockets in Iraq are costly to be manufactured while polypropylene socket is relatively cheap in comparing with the laminates. Experimental study is conducted to compare the stiffness of five prosthetic sockets made of different materials. Compression, three point flexural and tensile tests are implemented by the Testometric machine. The laminate sockets give better results in compression than polypropylene. Polypropylene gives good results in bending compared with the laminate sockets. When the socket loads are mainly in compression i.e. the low activity level patients, it seems that any of the tested sockets could be used, however, when the load will be not only in compression but in flexion as well i.e. high activity patients, socket No.1 and 5 could be used. Keywords::: Prosthetic, Socket, Laminate, Polypropylene and Stiffness. 1-Introduction The studies concerning the prosthetic socket are always about the pressure distribution of the socket, ignoring the materials of it. Appoldt and Bennett[1], found the loading on an above-knee fiberglass socket by building the socket with the pressure transducers incorporated. Unfortunately their results are only accurate for the single socket used in the experiment. This is due to all modern sockets having different geometries and external loads due to differences in the amputees. Bielefeldt and Schreck[2], investigated the difference in loading of four different material sockets, during stance phase, for the same patient. Their sockets were built with transducers incorporated. Joshua et. al.[3], developed a new rapid prototyping method for fabrication of prosthetic socket. The system, referred to as SQUIRT shape, fabricates sockets by extruding a continuous bead of molten plastic and laying it down in the desired socket form. This technique eliminates intermediary steps (e.g., fabrication of plaster blanks and carving of socket positives) used in contemporary CAD/CAM of prosthetics, and enables the socket to be fabricated in a single operation. Ross Stewart[4], devised an experiment to quantify the effectiveness of various suspension system. A transtibial limb was constructed to have similar anatomical feature and skin interface characteristics as an anatomical residual... To do this, vacuum, strap and anatomical suspension transtibial prosthetic socket could be done. This page was created using Nitro PDF trial software. To purchase, go to http://www.nitropdf.com/ http://www.nitropdf.com/ Dr.Muhsin J. Jweeg\Al-khwarizmi Engineering Journal ,Vol.3, No. 1 PP 40-46(2007) ٤١ By using a tensile tester, a graph of the tensile force versus displacement was obtained. This work can be achieved to test the strength and stiffness of the material of the prosthetic socket. Matt Fleming[5], conducted a similar experiment to Stewart’s experiment, but in this case a real transtibial prosthetic patient is tested. The subject was cast and fitted with four different suspension systems. In the experiment each socket was donned by the subject and the displacement of the socket/liner was measured with various weights added to the set-up. Arthur F.T. et. al.[6], reviewed the recent research literature on socket biomechanics, including socket pressure measurement, friction, computation modeling, and limb tissue responses to external loads and other physical conditions. It was noted that an understanding of comfort and optimal load transfer as patterns of socket interface stress could culminate in socket design. Winson C.C. et. al.[7], perform a finite element analysis to determine the effect of monolimb flexibility on structural strength and interaction between residual limb and prosthetic socket. 2- Experimental Programme 1 Prosthetic Socket Manufacturing -2 The polypropylene and laminations are manufactured on plaster mold of cylindrical shape (100mm in diameter and 500mm in length). The mold is fastened to a vaccum device and covered with a nylon stockinette (woman socks). A heated sheet of polypropylene is fitted over the mold, while for laminations a PVA is pulled on the mold. The layup materials are then pulled onto the mold and placed under tension. Once all layup materials has been pulled onto the pipe, another PVA is pulled over the layup and tightened below the vaccum. At this point the layup is ready to laminate. The resin is added, then the vaccum is turned on. Once polypropylene and laminations are completed, they are cut to test size, as shown in Fig.1. For the compression test a 50mm length of polypropylene and lamination is used whilst a 140mm length is used for the flexural test. The compression test gives an indication of comparative socket stiffness whilst the flexural test provides us the results which can be related to the clinical situation of socket loading. For tensile test, polypropylene material with 12 layers of laminate are chosen to compare between thermoplastic and composite material. Table 1 shows the sockets for the testing purposes. 3- Experimental Procedure Testing of polypropylene and lamination pieces are completed with the use of a Testometric testing machine. The compression and flexural test pieces are positioned in the testometric on the crosshead by suitable grips. 25000 N full scale load and 10mm/min crosshead speed are set. Fig.2 and Fig.3 shows the tested specimens used for compression and bending tests, while Fig.4 shows the position of the sample for the flexural test. Calculations of the flexural strengths of the tested sockets are shown in the Appendix. D638(ASTM standard) is used for sizing the polypropylene and laminated samples for tensile test. Fig.5 shows the sizes for polypropylene and No.5 socket (12-layers of laminate). For polypropylene material, the necking of the test piece should be occurred during 100 seconds. Therefore, trial pieces were tested at different cross head speeds to get the correct results. For the laminate piece, the cross head speed is 5mm/min.The tensile tested pieces for No.1 and No.5 sockets are positioned on the testometric machine by a suitable grips. 4-Results From Table 2 and Fig.6, it can be noticed that socket No.5 has the largest compressive stiffness, followed by sockets No.4, No.3, No.1 and No.2 respectively. The difference in compressive stiffness between the laminated sockets is not significant, while there is relatively apparent difference between sockets No.5 and No.1. This is due to the fibers strength in the direction of the load. For the fracture compression stresses , there are differences in this aspect. Sockets No.1, No.4 and No.5 do not fracture, and they tolerate larger than 25000 N as compression This page was created using Nitro PDF trial software. To purchase, go to http://www.nitropdf.com/ http://www.nitropdf.com/ Dr.Muhsin J. Jweeg\Al-khwarizmi Engineering Journal ,Vol.3, No. 1 PP 40-46(2007) ٤٢ loads. While No.2 and No.3 sockets tolerate 21000 N and 23000 N respectively. There is a difference in the nature of the fracture of polypropylene and laminated socket. For polypropylene, failure starts with stress whitening phenomenon in the socket with final fracture in the bonding region, While for laminated socket, fracture starts with resin failure. For flexural bending test, polypropylene sample deflects to more than 4mm and after unloading the bending load, the sample seems to return to its original shape with small plastic deformation. For laminates samples, failure is apparent by fracturing the resin. From Table 2 and Fig.7, sockets No.5 and No.1 have the largest flexion stiffness, followed by No.4, No.3 and No.2 sockets. Fig.8 shows the experimental results of the tensile test for socket No.1 and No.2. For polypropylene material (socket No.1), the material properties are,E=1.235 GPa, σult. =33.5 MPa, σy = 25 MPa. The polypropylene does not have a well-defined yield point, and consequently the graphical method of offset method is used to define the yield point. Normally, a 0.2 % strain is chosen, and from this point on the strain axis, a line parallel to the initial straight line portion of the stress- strain diagram is drawn. The point where this line intersects the curve defines the yield strength. It is found that polypropylene is an extended material under tensile load or No.5 socket, the material properties are, E= 2.092 GPa, σult. =48.97 MPa. The socket No.5 (laminated socket) undergoes a little deformation before its final fracture because of its brittleness. 5-Discussion The prosthetists consider laminated sockets stronger than polypropylene in all mechanical properties. Through the tests, it is found that compressive stiffness of lamination is larger than polypropylene with not very high difference. From Fig.5 it should be noted that the socket No.1&2 (polypropylene) have more deflection than the rest sockets(laminations), and this means that the ductility of polypropylene is greater than laminations. This is a useful factor if we take into considerations the pressure relief regions of patient's stump(truncated part of his leg). For the bending test, it should be noted from Fig.6 that all the sockets in the bending test pass through three stages: 1. Linear behavior at the first stage. 2. Appearance of the effect of the sample slipping over the supports as the deflection increases at the second stage. It should be noted that the sample slipping does not affect the stiffness which is evaluated at the first stage. 3. Then, failure of the sample by the effect of the maximum load. Socket No.1(polypropylene,5mm) has better flexural stiffness than the tested laminated sockets except socket No.5 which has a very little higher stiffness than socket No.1 socket. In the bending test, the stiffness of laminated sockets depends upon the types and the amount of the resin in addition to the fibers. The problem is that the manufacturers in the medical rehabilitation centers do not use the sufficient amount of resin because of its high cost. While the modulus(stiffness) of a laminate depends upon fiber and resin volume fractions. It should be noted that stiffness of sockets No.5 and No.1 are approximate. 6-Conclusions 1. When the socket loads are mainly in compression i.e. the low activity level patients, it seems that any of the tested sockets could be used, however, when the load will be not only in compression but in flexion as well i.e. high activity patients, socket No.1 could be used. 2. In Iraq, polypropylene is good selected material compared with laminates, because of its good mechanical properties and cost. References [1] Appoldt and Bennet;" A Preliminary Report on Dynamic Socket Pressures"; Bull Prosthetic Res.; 1967. [2] Bielefeldt and Schreck; "The Altered Alignment Influence on above Knee Prosthesis Socket Pressure Distribution"; This page was created using Nitro PDF trial software. To purchase, go to http://www.nitropdf.com/ http://www.nitropdf.com/ Dr.Muhsin J. Jweeg\Al-khwarizmi Engineering Journal ,Vol.3, No. 1 PP 40-46(2007) 3 International Series on Biomechanics VIIa; 1981: 387-393. [3] Joshua S. Rovick & Dudley S. Childress;" An Additive Fabrication Technique for the CAM of Prosthetic Sockets"; (VA Project Number: A711-DA); 1995. [4] Matt Flamming;" Clinical Evaluation of Trans-tibial Suspension System"; Monash Rehabilitation Technology Research Unit; 1998. [5] Ross Stewart;" Tran-tibial Syspension System", Monash Rehabilitation Technology Research Unit; 1998. [6] Arthur F.T. Mak,PhD,Ming Zhang,PhD & David A.Boon,"State-of-the-are research in lower limb prosthetic biomechanics socket interface",JRRD,Vol.38 No.2 March/April 2003. [7] Winson C.C.Lee,BSc,Ming Zhang,PhD,"Finite element analysis to determine effect of monolimb flexibility on structural strength and interaction between residual limb and prosthetic socket", JRRD,Vol.41,No.6A,Nov.2004. [8] R.C. Hibbler;" Mechanics of Materials"; Prentice Hall International, Inc. Appendix Calculations Concerning Flexural Tests Fig.9 shows the cross-sectional of the bending sample. The equations used in calculation of the bending strenfths are as follows: Xc= (2/3)(R)(sinα/α) (1-t/R+(1/(2-t/R))) x1= R-Xc x2= Xc-(R)(cosα) Iy=(R4-r4)/8(2α+sin2α) Ic=Iy-(R2-r2)(α)(Xc2) σt= (M)(x1/Ic) σc= (M)(x2/Ic) Table 1 The test sockets Socket No. Material 1 5mm polypropylene 2 2mm polypropylene 3 Perlon (10-layers)with orthocryl resin. 4 Nyglass(8-layers)with orthocryl resin. 5 Perlon(2-layers),nyglass(2-layers),fiberglass(2-layers), nyglass(2-layers),fiberglass(2-layers) and perlon(2-layers)with polyster resin. Table 2 Summary of results for experimental tests Socket No. Thickness mm Compression test Flexural bending test Cross sectional area mm2 Compressive stiffness(Kc)N/mm Flexural stiffness(Kb)N/mm σt MPa σc MPa 1 5 1884.956 8333 32.4 30.17 39.429 2 2 779.114 7222 16.67 27.11 41.73 3 2 779.114 9000 18.5 24 36.96 4 3 1178.097 9524 20 32.58 46.63 5 3.5 1280.98 10364 33.33 44.43 62.38 This page was created using Nitro PDF trial software. To purchase, go to http://www.nitropdf.com/ http://www.nitropdf.com/ Dr.Muhsin J. Jweeg\Al-khwarizmi Engineering Journal ,Vol.3, No. 1 PP 40-46(2007) 4 Table 3 The parameters used in finding tensile & compression bending stresses for bending test Parameter Socket No.1 Socket No.2 Socket No.3 Socket No.4 Socket No.5 R mm 62.5 63 63 64 60 r mm 57.5 61 61 61 56.5 α degree 20.609º 20.438º 20.438º 20.105º 21.51º Xc 58.748 60.698 60.698 61.237 56.908 x1 3.75 2.3 2.3 2.763 3.091 x2 4.9 3.54 3.54 3.954 4.34 Ic 714.576 148.45 148.45 264.961 373.935 Fmax N 230 70 62 125 215 M N.m 5750 1750 1550 3125 5375 Fig.1 The socket and the test pieces Fig.3 Bending test specimen Fig.2 Compression test specimen Fig.4 Bending test of socket This page was created using Nitro PDF trial software. To purchase, go to http://www.nitropdf.com/ http://www.nitropdf.com/ Dr.Muhsin J. Jweeg\Al-khwarizmi Engineering Journal ,Vol.3, No. 1 PP 40-46(2007) 5 Fig.5 Dimension of tensile specimen(D638) 0.00 2.00 4.00 6.00 8.00 Deflection(mm) 0.00 40.00 80.00 120.00 160.00 L o a d (N ) Socket No.1 Socket No.2 Socket No.3 Socket No.4 Socket No.5 Fig.6 Compression graphs for all sockets 0.00 5.00 10.00 15.00 20.00 25.00 strain(%) 0.00 10.00 20.00 30.00 40.00 50.00 s tr e s s (M P a ) socket No.1 socket No.5 Fig.8 tensile test for socket No.1&5 0.00 2.00 4.00 6.00 Deflection(mm) 0.00 5000.00 10000.00 15000.00 20000.00 25000.00 L o a d (N ) Socket No.1 Socket No.2 Socket No.3 Socket No.4 Socket No.5 Fig.7 Flexural bending graphs for all sockets Fig.9 Cross section of the bending sample This page was created using Nitro PDF trial software. To purchase, go to http://www.nitropdf.com/ http://www.nitropdf.com/ Dr.Muhsin J. Jweeg\Al-khwarizmi Engineering Journal ,Vol.3, No. 1 PP 40-46(2007) 6 دراسة تجریبیة لمقارنة بین وقب البولي بروبلین والمواد المركبة للطرف ).الصناعي( السفلي البدیل حیدر فانوس نعمة. سمیرة كریم راضي أ. محسن جبر جویج د. د كلیة الھندسة قسم المیكانیك كلیة الھندسة قسم المیكانیك كلیة الھندسة قسم المیكانیك جامعة النھرین الجامعة المستنصریة جامعة بغداد :الخالصة م الب ز معظ دیل یرك ناعي الب ب الص غوط للوق ادات و الض ع االجھ میم و توزی ى تص اتھم عل ان (احثین دراس رف االنس ن ط ع ن (ان سعر االوقاب الطبقیة. المواد المصنوع منھا الوقب البدیل (stiffness)،و یكون االھتمام قلیًال بجساءة) المبتور المصنوعة م .قارنة بالوقب المصنوع من مادة البولي بروبلین الذي یكون سعره زھیدًاالبدیلة یكون غالیًا جداًً م)عدة طبقات من االلیاف ة أ واد مختلف ن م غط، . جریت دراسة عملیة لغرض مقارنة جساءة خمسة أنواع من االوقاب البدیلة مصنوعة م ار الض ري اختب أج ب الطبق ئمن خالل النتا. Testometricواختبار االنحناء و الشّد بماكنة ل يج وجد أنھ الوق ن الحم اتج م وه الن ة للتش ك مقاوم یمتل روبلین (االنضغاطي اكبر من تلك للبولي بروبلین ولي ب ك للب ن تل ى م غاط اعل ر ) جساءة الوقب الطبقي في حالة االنض و أن األخی م ب رق دا الوق ي ع ب الطبق واع الوق ل ان ن ك ر م خاص . ٥یمتلك مقاومة للتشوه الناتج من الحمل أالنثنائي اكب ة ان االش ذوي فعالی غاطیة اءة االنض یم الجس ھ ق اب الن ن االوق تخدام اي م ن اس ذا یمك غاطي وبھ ب انض ى الوق المشي الواطئة یكون اغلب التحمیل عل ب ى الوق ل عل ب التحمی ون اغل ذي یك ة ال ة العالی خاص ذوي الفعالی ا االش ي، بینم ب الطبق لیة للوق ع افض ة م ا متقارب ًا م نوع .ملم٥ووقب البولي بروبیلین ذو سمك ) الذي یمتلك اعلى الطبقات( ٥دام الوقب الطبقي رقم یتم استخ(flexural)انثنائي This page was created using Nitro PDF trial software. To purchase, go to http://www.nitropdf.com/ http://www.nitropdf.com/