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Influence of test temperature and test speed on the mechanical strength of absorbable suture anchors
Influence of Test Temperature and Test Speed on the Mechanical Strength of Absorbable Suture Anchors Dominik Christoph Meyer, M.D., Evelyne Felix, M.D., Kurt Ruffieux, Ph.D., and Christian Gerber, M.D.
Purpose: Absorbable implant materials offer various advantages but are mechanically far weaker than metals. Despite known temperature dependence of the biomechanical properties of these materials, mechanical testing has almost exclusively been performed at room temperature in the literature. In this study, the difference in mechanical performance at room and body temperature was assessed in vitro at different test speeds. Type of Study: Biomechanical bench study. Methods: Five absorbable suture anchor models were held in a metallic holder and loaded under tension using 0.5-mm steel wires until failure. Testing temperature was 20°C ⫾ 1°C or 37°C ⫾ 1°C, test speed was 50 mm/min or 5 mm/min. Tensile load at failure and failure mode were recorded. To test creep behavior, a constant load of 100 N was applied, and time to failure was recorded at both temperatures. Results: Both raising the temperature and decreasing test speed significantly (P ⬍ .0001) impaired the mechanical performance of the tested implants. Increase of temperature (20°C to 37°C) resulted in a decrease of the maximal failure strength by up to 40% and decreased time to failure by up to 98% under static load. At 37°, decreasing the test speed from 50 to 5 mm/min lowered the load to failure by up to 18%. Failure of the anchors always occurred by eyelet cutout of the wire. Conclusions: The lower the test speed, the higher is the influence of the testing temperature. Testing of implants at room temperature instead of body temperature may falsely improve test results by a factor of up to 50 under static load. Therefore, testing absorbable implants at body temperature seems mandatory, preferably at slow test speeds. Key Words: Absorbable suture anchor—Test temperature—Test speed—Failure load.
A
bsorbable implant materials offer great advantages, because they do not interfere with radiologic imaging and may not require later implant removal. Besides minor problems regarding acidic degradation products,1 the main drawback of these materials are their poorer mechanical properties as compared with titanium.2,3 Therefore, mechanical testing of absorbable implants is important, and nu-
From the Department of Orthopaedics (D.C.M., E.F., C.G.), University of Z¨urich; and the Department of Biomaterials Science and Engineering (K.R.), Swiss Federal Institute of Technology, Z¨urich, Switzerland. Address correspondence and reprint requests to Christian Gerber, M.D., Department of Orthopaedics, University of Z¨urich, Balgrist, Forchstrasse 340, 8008 Z¨urich, Switzerland. E-mail: [email protected] © 2004 by the Arthroscopy Association of North America 0749-8063/04/2002-3607$30.00/0 doi:10.1053/S0749-8063(03)01113-7
merous publications report the results of such tests. The temperature dependence of the mechanical performance of any polymeric material is generally known.4 However, when analyzing the first 26 MEDLINE-referenced medical publications reporting results of mechanical tests of absorbable implants until 2001,5-30 we found that tests were performed at body temperature in only 1 study.5 In 13 publications, room temperature was chosen and in 12 papers, the test temperature was not indicated. Test speed ranged from 124 to 300 mm/min22 and was not mentioned in 2 papers. Therefore, the purpose of this study was to evaluate the hypothesis that test temperature and loading speed have an effect on the mechanical performance of absorbable implants. A further hypothesis was that the mechanical properties of highly crystalline absorbable implants are less influenced by changing test temper-
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 20, No 2 (February), 2004: pp 185-190
185
186
D. C. MEYER ET AL. TABLE 1. Specifications of the Anchor Models
Anchor Model
Manufacturer
Material
Eyelet Suture
Crystallinity
Weight (g)
Bio-Corkscrew 5.0 mm Experimental prototype Panalok 3.5 mm Bio-Anchor Tag Wedge 3.7 mm
Arthrex Degradable Solutions Mitek Linvatek Acufex Microsurgical
PLA PLDLA PLA PLLA PGA
Polyester — — — —
0.0% 0.0% 20.0% 44.7% 51.6%
0.161 0.091 0.036 0.06 0.072
NOTE. Anchors are sorted according to their crystal content. Indicated are: Anchor model, manufacturer, type of material, material of the eyelet, crystallinity,32 and anchor weight without suture.
ature as compared with amorphous ones. As test samples, absorbable suture anchors have been used. METHODS We assessed testing conditions with 5 types of suture anchors (Table 1, Fig 1), with 22 specimens of each type. Commercially available anchors were provided by 4 manufacturers (Arthrex, Naples, FL; Mitek Products, Westwood, MA; Linvatec, Largo, FL; Acufex Microsurgical, Mansfield, MA). All anchors are designed for use with USP No. 2 suture material. Testing Temperature and Medium Mechanical testing was performed in a water bath buffered with 0.1 mol/L Gomorri-phosphate buffer to pH 7.4,31 thermostatically adjusted to either room temperature (20°C ⫾ 1°C) or body temperature (37°C ⫾ 1°C). All probes were conditioned to test temperature for 10 minutes before testing, as in a previous report.32
FIGURE 1. Intact suture anchors without suture material, from top left to bottom right: Bio-Corkscrew 5.0, Experimental Prototype, Panalok 3.5, Bio-Anchor, and Tag Wedge 3.7 mm. Ruler indicates millimeters.
Weight Measurements and Material Crystal Content The weight of the dry anchors was measured after removing all suture materials using a balance with an accuracy of ⬍ 0.001 g. The correlation of crystal content of the polymers, as measured in a previous study,32 with the temperature and test speed dependence, was analyzed. Mechanical Testing A protocol was designed to test the influence of test temperature (20°C v 37°C) and loading condition (50 mm/min, 5 mm/min, and static load of 100 N) on the mechanical performance of absorbable suture anchors. Preloaded suture material was removed and replaced by stainless steel wire 0.5 mm in diameter. To test only the structural strength of the implants and to exclude possible variations resulting from the environment, the anchors were implanted in special holders that were constructed for each anchor model. The holders were steel disks 2.5 cm thick with 2 concentric drill holes. The smaller hole was the diameter of the drill hole through which each anchor would be inserted into the bone. The larger hole corresponded to the greatest diameter of the anchor and was only 2 cm long, leaving a 0.5-cm-long narrow portion of the smaller diameter. The anchors were inserted backward through the larger diameter and found purchase at the smaller diameter, through which the steel wire loop was passed. In this configuration, the situation in which the anchor finds purchase against a hole in cortical bone is simulated. The holders with the anchors were always held in the water bath 5 cm below surface. Loading of the steel wires was always in line with the anchor axis, perpendicular to the disk. The steel wires were clamped to the cross-head of the testing machine (single pull tests) or attached to longer ropes for the static loading.
TEST TEMPERATURE
187
Static Loading (Creep Test) Time to failure under static load of 100 N32 was measured to analyze the creep behavior of the implants at 20°C or 37°C, as displayed in Fig 2. At each temperature, 3 anchors were tested per anchor model. Time to failure and failure mode were again recorded. The time limit for the experiment was 300 hours. Statistical Evaluation
FIGURE 2. Static loading in water bath. In the buffered (pH 7.4) water bath with 20°C ⫾ 1°C or 37°C ⫾ 1°C, the holder with the anchor is clamped down. The wire loop inserted in the anchor is connected to a rope, which is led around 2 polished bars and loaded with 10 kp (100 N). At failure of the suture anchor, the weight falls to the ground, stopping the stopwatch.
Differences in mechanical performance at different test temperatures and test velocities were analyzed using analysis of variance (ANOVA) and post-hoc Bonferroni comparison. For static tests, logarithmic transformation was performed. Correlation of performances of the anchor models in tensile and creep tests with crystal content and weight were calculated using Spearman rank correlation.
RESULTS Single-Pull Tests
This setup was chosen to compare the mechanical performance of each anchor type under varied conditions with high reproducibility. It does not reproduce the functionality of the anchor but allows us to show the strength of the weakest component, the eyelet. Single-Pull Tests Tensile single-pull tests until anchor failure were performed using an Instron materials-testing machine (Instron Limited, High Wycombe, U.K.). Cross-head displacement rate was either 50 or 5 mm/min, both at 20°C or 37°C. For each test, 4 anchors per model were used. Load at failure and failure mode were recorded.
In all tests, the anchor eyelet was the weakest link and failed. Failure loads (Table 2) were always significantly lower (up to 40%; P ⬍ .0001) at 37°C than at 20°C and significantly lower (up to 15%; P ⬍ .0001) at 5 mm/min than at 50 mm/min. The BioCorkscrew always failed at 20°C by tear of the eyelet suture, and at 37°C the eyelet suture was always pulled out from the anchor body (Fig 3). The proximal part of the Bio-Anchor was torn off the anchor body. In the other 3 models, the wire always cut through the anchor at both temperatures. Lowering the test speed at 37°C results in a decrease of mechanical performance of 15% for Bio-Corkscrew, 7% for an experi-
TABLE 2. Performance of the Anchor Models in Single-Pull Test Load to Failure at 50 mm/min
Load to Failure at 5 mm/min
Anchor Model
20°C N ⫾ SD
37°C N ⫾ SD
P
Change
20°C N ⫾ SD
37°C N ⫾ SD
P
Change
Bio-Corkscrew 5.0 mm Experimental prototype Panalok 3.5 mm Bio-Anchor Tag Wedge 3.7 mm
267 ⫾ 17 356 ⫾ 18 189 ⫾ 7 268 ⫾ 6 344 ⫾ 10
232 ⫾ 14 247 ⫾ 4 155 ⫾ 3 236 ⫾ 4 237 ⫾ 20
⬍.0001 ⬍.001 ⬍.0001 ⬍.0001 ⬍.0001
⫺13% ⫺31% ⫺18% ⫺12% ⫺31%
276 ⫾ 4 336 ⫾ 10 178 ⫾ 5 262 ⫾ 7 323 ⫾ 21
198 ⫾ 15 229 ⫾ 7 141 ⫾ 6 214 ⫾ 2 194 ⫾ 11
⬍.0001 ⬍.001 ⬍.0001 ⬍.0001 ⬍.0001
⫺28% ⫺32% ⫺21% ⫺18% ⫺40%
NOTE. Anchors are sorted as in Table 1. Indicated are: Mean load to failure ⫾ standard deviation in single-pull testing at 20°C and 37°C, at 50 mm/min and 5 mm/min (n ⫽ 4 for each test), P value for the difference between the 2 values, and percent difference in failure load at the 2 temperatures.
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D. C. MEYER ET AL.
FIGURE 3. Suture anchors as in Fig 1 after single pull testing. Top row shows failure modes at 37°C, bottom row at 20°C: At 37°C, the Bio-Corkscrew 5.0 fails by pullout of the eyelet-suture, which fails at 20°C at a higher value. The other specimens show more deformation of the anchor body despite lower failure load. Ruler indicates millimeters.
mental prototype, 9% for Panalok 3.5 mm, 9% for Bio-Anchor, and 18% for Tag Wedge. Static Loading (Creep Tests) Time to failure in static loading is indicated in Table 3. Again, a clear influence of the test temperature was seen, with a decrease of 98% in time to failure in Panalok 3.5, which failed at both temperatures before 300 hours. Two anchor models (Bio-Corkscrew, Tag Wedge) failed at 37°C before 300 hours but stayed intact at 20°C, and 2 anchor models (experimental prototype and Bio-Anchor) stayed intact for the entire period of 300 hours at 20°C and 37°C. Still, the experimental prototype showed marked deformity of the anchor eyelet at 37°C and no alterations at 20°C. At 37°C, the Bio-Corkscrew always failed by eyelet suture pullout. The other anchors failed by eyelet cutout. Because only the Panalok 3.5 failed within the time limit, it was the only model to be statistically evaluated (P ⫽ .004).
able implants. Such testing is important, because the goal of any implant design should be to use as little material as possible for sufficient mechanical strength. Benchmarks may be the tensile strength of sutures in the case of suture anchors or the tensile strength of meniscus suture stitches in case of meniscal repair devices. If plates and screws are designed, the performance of the corresponding metallic implant may be the gold standard. Performing tests with polymeric materials under physiologic conditions is generally established in materials science literature.4 However, testing conditions obtain hardly any attention in medical journals. Of 26 publications analyzed,5-30 in 12, the testing temperature is not even mentioned, and the overall testing speed varies by the factor 300. The results from the present study show that testing conditions have a decisive influence on the mechanical performance of degradable implants. The explanation is that at the glass transition temperature (typically 63°C), absorbable implant materials become ductile, comparable to chewing gum. With lower temperatures, these materials become increasingly harder and more brittle.2 Because body temperature is relatively close to the critical glass transition temperature on an absolute scale (degrees Kelvin), temperature changes in that region have a proportionally large influence on mechanical properties. In single-pull tests, raising the temperature from 20°C to 37°C may decrease the test results by 40%. In static creep tests, the performance may be decreased by up to 98%. The softening of the material at 37°C also resulted in altered failure modes. The Bio-Corkscrew failed by debonding of the eyelet-suture, which remained stable in the anchor at 20°C. The other specimens showed more deformation at failure despite decreased failure strength (Fig 3, Table 2). TABLE 3. Performance of the Anchor Models in Static Loading With 100 N Time to Failure With 100 N Static Load
Influence of Anchor Weight and Crystal Content No significant correlation of anchor weight or crystal content was found with the effect of altered temperature or test speed on the mechanical performance of the anchors. DISCUSSION 5-30
have been performed to assess Numerous studies and optimize the mechanical performance of absorb-
Anchor Model
20°C h ⫾ SD
37°C h ⫾ SD
P
Change
Bio-Corkscrew 5.0 mm Experimental prototype Panalok 3.5 mm Bio-Anchor Tag Wedge 3.7 mm
⬎300 ⬎300 31 ⫾ 3 ⬎300 ⬎300
48 ⫾ 58 ⬎300 0.65 ⫾ 0.07 ⬎300 174 ⫾ 67
— — .004 — —
⫺84% — ⫺98% — ⫺42%
NOTE. Anchors are sorted as in Table 1. Indicated are: Mean time to failure ⫾ Standard deviation at 20°C and 37°C (n ⫽ 3 for each test), P value for the difference between the two values, and percent difference in time to failure at the two temperatures.
TEST TEMPERATURE Researchers may speculate that when thawed bone samples are used in combination with absorbable implants, sometimes thawing may not be complete and the desired test temperature was not been reached at time of testing, which may result in even larger error. The fact that room temperature is the usually preferred testing temperature may be in part because of unawareness of the problem and in part because of the technical difficulties with performing the experiments in a thermostatically regulated environment. To allow for comparison with test results obtained at room temperature, both temperatures may need to be considered. Lowering the test speed also resulted in a decrease of the mechanical performance, more pronounced at high temperature (up to 18%). Therefore, to simulate the worst case, slow (1-5 mm/min) cross-head displacement rates seem preferable. If implants are subjected to significant static load, creep tests may also be an important test component. Unexpectedly, the crystal content of the material had no detectable influence on the decrease of the performance at altered temperature. Implant design and other material properties seem to be more important, even though in this study determining one decisive variable was not possible. The experiment was designed to isolate and quantify the influence of the testing conditions on the performance of absorbable implant materials using absorbable suture anchors. Because the anchors were held in metal holders, the results do not reflect the performance of the implants in a clinical setting. However, we can presume that the hold of an absorbable implant in bone will depend on the mechanical properties of the material. Test temperature and test speed have been shown to affect the strength of absorbable implants. Because the influence of these factors seems to be rather constant, the conclusions from studies comparing the “relative” strength of various implants may not necessarily need to be changed. However, studies designed to assess the “absolute” values would need to be conducted under conditions that better replicate in vivo conditions. Unfortunately, this will raise the complexity and expense of such studies. Acknowledgment: The authors recognize the help of Burkhart Seifert, Ph.D., with the statistical evaluation of the data.
REFERENCES 1. Bergsma EJ, Rozema FR, Bos RR, de Bruijn WC. Foreign body reactions to resorbable poly(L-lactide) bone plates and
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screws used for the fixation of unstable zygomatic fractures. J Oral Maxillofac Surg 1993;51:666-670. Wintermantel E, Ha S. Biokompatible Werkstoffe und Bauweisen. Heidelberg: Springer, 1998. Ruffieux K. Degradables Osteosynthesensystem aus Polylactid f¨ur die maxillofaciale Chirurgie: Ein Beitrag zur Werkstoffund Prozessentwicklung. In: Institut f¨ur Biokompatible Werkstoffe und Bauweisen. Z¨urich: Eidgen¨ossische Technische Hochschule, 1997;145. Domke W. Werkstoffkunde und Werkstoffpr¨ufung. D¨usseldorf: Cornelsen-Girardet, 1986. Merloz P, Minfelde R, Schelp C, et al. [In vitro study of the properties of bioresorbable lactic acid polymer materials]. Rev Chir Orthop Reparatrice Appar Mot 1995;81:433-444. Goradia VK, Mullen DJ, Boucher HR, et al. Cyclic loading of rotator cuff repairs: A comparison of bioabsorbable tacks with metal suture anchors and transosseous sutures. Arthroscopy 2001;17:360-364. Kousa P, Jarvinen TL, Kannus P, Jarvinen M. Initial fixation strength of bioabsorbable and titanium interference screws in anterior cruciate ligament reconstruction: Biomechanical evaluation by single cycle and cyclic loading. Am J Sports Med 2001;29:420-425. Araujo MM, Waite PD, Lemons JE. Strength analysis of Le Fort I osteotomy fixation: Titanium versus resorbable plates. J Oral Maxillofac Surg 2001;59:1034-1039. Bozic KJ, Perez LE, Wilson DR, et al. Mechanical testing of bioresorbable implants for use in metacarpal fracture fixation. J Hand Surg [Am] 2001;26:755-761. Kousa P, Jarvinen TL, Kannus P, et al. A bioabsorbable plug in bone-tendon-bone reconstruction of the anterior cruciate ligament: Introduction of a novel fixation technique. Arthroscopy 2001;17:144-150. Becker R, Schroder M, Starke C, et al. Biomechanical investigations of different meniscal repair implants in comparison with horizontal sutures on human meniscus. Arthroscopy 2001;17:439-444. Matsushita T, Nakamura K, Shiro R, et al. Initial strength of highpressed extrusion poly-L-lactide screw. Biomed Mater Eng 2000;10:199-204. Demirhan M, Kilicoglu O, Akpinar S, et al. Time-dependent reduction in load to failure of wedge-type polyglyconate suture anchors. Arthroscopy 2000;16:383-390. Barber FA, Herbert MA. Meniscal repair devices. Arthroscopy 2000;16:613-618. Furukawa T, Matsusue Y, Yasunaga T, et al. Bone bonding ability of a new biodegradable composite for internal fixation of bone fractures. Clin Orthop 2000;379:247-258. Barber F, Herber M. Suture anchors update 1999. Arthroscopy 1999;15:719-725. Song EK, Lee KB. Biomechanical test comparing the load to failure of the biodegradable meniscus arrow versus meniscal suture. Arthroscopy 1999;15:726-732. Boenisch UW, Faber KJ, Ciarelli M, et al. Pull-out strength and stiffness of meniscal repair using absorbable arrows or Ti-Cron vertical and horizontal loop sutures. Am J Sports Med 1999;27:626-631. Pirhonen E, Kaikkonen A, Helevirta P, et al. Development of a bioabsorbable suture anchor. J Biomed Mater Res 1999;48: 875-880. Rupp S, Krauss PW, Fritsch EW. Fixation strength of a biodegradable interference screw and a press-fit technique in anterior cruciate ligament reconstruction with a BPTB graft. Arthroscopy 1997;13:61-65. Rupp S, Seil R, Schneider A, Kohn DM. Ligament graft initial fixation strength using biodegradable interference screws. J Biomed Mater Res 1999;48:70-74. Seil R, Rupp S, Krauss PW, et al. Comparison of initial fixation strength between biodegradable and metallic interfer-
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ence screws and a press-fit fixation technique in a porcine model. Am J Sports Med 1998;26:815-819. van der Elst M, Bramer JA, Klein CP, et al. Biodegradable interlocking nails for fracture fixation. Clin Orthop 1998;357: 192-204. An YH, Friedman RJ, Powers DL, et al. Fixation of osteotomies using bioabsorbable screws in the canine femur. Clin Orthop 1998;355:300-311. Barber FA, Herber MA, Click JN. Internal fixation strength of suture anchors: Update 1997. Arthroscopy 1997;13:355-362. Albrecht-Olsen P, Lind T, Kristensen G, Falkenberg B. Failure strength of a new meniscus arrow repair technique: Biomechanical comparison with horizontal suture. Arthroscopy 1997; 13:183-187. Shall LM, Cawley PW. Soft tissue reconstruction in the shoulder: Comparison of suture anchors, absorbable staples, and absorbable tacks. Am J Sports Med 1994;22:715-718.
28. Carpenter JE, Fish DN, Huston LJ, Goldstein SA. Pull-out strength of five suture anchors. Arthroscopy 1993;9:109-113. 29. Walsh SP, Evans S, O’Doherty DM, Barlow IW. Failure strengths of suture vs. biodegradable arrow and staple for meniscal repair: an in vitro study. The Knee 2001;8:151156. 30. Balch OK, Collier MA, DeBault LE, Johnson LL. Bioabsorbable suture anchor (co-polymer 85/15 D,L lactide/glycolide) implanted in bone: correlation of physical/mechanical properties, magnetic resonance imaging, and histological response. Arthroscopy 1999;15:691-708. 31. Sambrook J, Russell D. Appendix 1.5. In: Molecular cloning, a laboratory manual. Vol 3. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press, 2001. 32. Meyer DM, Fucentese SF, Ruffieux K, Jacob HA, Gerber C. Mechanical testing of absorbable suture anchors. Arthroscopy 2003;19:188-193.
Purpose: Absorbable implant materials offer various advantages but are mechanically far weaker than metals. Despite known temperature dependence of the biomechanical properties of these materials, mechanical testing has almost exclusively been performed at room temperature in the literature. In this study, the difference in mechanical performance at room and body temperature was assessed in vitro at different test speeds. Type of Study: Biomechanical bench study. Methods: Five absorbable suture anchor models were held in a metallic holder and loaded under tension using 0.5-mm steel wires until failure. Testing temperature was 20°C ⫾ 1°C or 37°C ⫾ 1°C, test speed was 50 mm/min or 5 mm/min. Tensile load at failure and failure mode were recorded. To test creep behavior, a constant load of 100 N was applied, and time to failure was recorded at both temperatures. Results: Both raising the temperature and decreasing test speed significantly (P ⬍ .0001) impaired the mechanical performance of the tested implants. Increase of temperature (20°C to 37°C) resulted in a decrease of the maximal failure strength by up to 40% and decreased time to failure by up to 98% under static load. At 37°, decreasing the test speed from 50 to 5 mm/min lowered the load to failure by up to 18%. Failure of the anchors always occurred by eyelet cutout of the wire. Conclusions: The lower the test speed, the higher is the influence of the testing temperature. Testing of implants at room temperature instead of body temperature may falsely improve test results by a factor of up to 50 under static load. Therefore, testing absorbable implants at body temperature seems mandatory, preferably at slow test speeds. Key Words: Absorbable suture anchor—Test temperature—Test speed—Failure load.
A
bsorbable implant materials offer great advantages, because they do not interfere with radiologic imaging and may not require later implant removal. Besides minor problems regarding acidic degradation products,1 the main drawback of these materials are their poorer mechanical properties as compared with titanium.2,3 Therefore, mechanical testing of absorbable implants is important, and nu-
From the Department of Orthopaedics (D.C.M., E.F., C.G.), University of Z¨urich; and the Department of Biomaterials Science and Engineering (K.R.), Swiss Federal Institute of Technology, Z¨urich, Switzerland. Address correspondence and reprint requests to Christian Gerber, M.D., Department of Orthopaedics, University of Z¨urich, Balgrist, Forchstrasse 340, 8008 Z¨urich, Switzerland. E-mail: [email protected] © 2004 by the Arthroscopy Association of North America 0749-8063/04/2002-3607$30.00/0 doi:10.1053/S0749-8063(03)01113-7
merous publications report the results of such tests. The temperature dependence of the mechanical performance of any polymeric material is generally known.4 However, when analyzing the first 26 MEDLINE-referenced medical publications reporting results of mechanical tests of absorbable implants until 2001,5-30 we found that tests were performed at body temperature in only 1 study.5 In 13 publications, room temperature was chosen and in 12 papers, the test temperature was not indicated. Test speed ranged from 124 to 300 mm/min22 and was not mentioned in 2 papers. Therefore, the purpose of this study was to evaluate the hypothesis that test temperature and loading speed have an effect on the mechanical performance of absorbable implants. A further hypothesis was that the mechanical properties of highly crystalline absorbable implants are less influenced by changing test temper-
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 20, No 2 (February), 2004: pp 185-190
185
186
D. C. MEYER ET AL. TABLE 1. Specifications of the Anchor Models
Anchor Model
Manufacturer
Material
Eyelet Suture
Crystallinity
Weight (g)
Bio-Corkscrew 5.0 mm Experimental prototype Panalok 3.5 mm Bio-Anchor Tag Wedge 3.7 mm
Arthrex Degradable Solutions Mitek Linvatek Acufex Microsurgical
PLA PLDLA PLA PLLA PGA
Polyester — — — —
0.0% 0.0% 20.0% 44.7% 51.6%
0.161 0.091 0.036 0.06 0.072
NOTE. Anchors are sorted according to their crystal content. Indicated are: Anchor model, manufacturer, type of material, material of the eyelet, crystallinity,32 and anchor weight without suture.
ature as compared with amorphous ones. As test samples, absorbable suture anchors have been used. METHODS We assessed testing conditions with 5 types of suture anchors (Table 1, Fig 1), with 22 specimens of each type. Commercially available anchors were provided by 4 manufacturers (Arthrex, Naples, FL; Mitek Products, Westwood, MA; Linvatec, Largo, FL; Acufex Microsurgical, Mansfield, MA). All anchors are designed for use with USP No. 2 suture material. Testing Temperature and Medium Mechanical testing was performed in a water bath buffered with 0.1 mol/L Gomorri-phosphate buffer to pH 7.4,31 thermostatically adjusted to either room temperature (20°C ⫾ 1°C) or body temperature (37°C ⫾ 1°C). All probes were conditioned to test temperature for 10 minutes before testing, as in a previous report.32
FIGURE 1. Intact suture anchors without suture material, from top left to bottom right: Bio-Corkscrew 5.0, Experimental Prototype, Panalok 3.5, Bio-Anchor, and Tag Wedge 3.7 mm. Ruler indicates millimeters.
Weight Measurements and Material Crystal Content The weight of the dry anchors was measured after removing all suture materials using a balance with an accuracy of ⬍ 0.001 g. The correlation of crystal content of the polymers, as measured in a previous study,32 with the temperature and test speed dependence, was analyzed. Mechanical Testing A protocol was designed to test the influence of test temperature (20°C v 37°C) and loading condition (50 mm/min, 5 mm/min, and static load of 100 N) on the mechanical performance of absorbable suture anchors. Preloaded suture material was removed and replaced by stainless steel wire 0.5 mm in diameter. To test only the structural strength of the implants and to exclude possible variations resulting from the environment, the anchors were implanted in special holders that were constructed for each anchor model. The holders were steel disks 2.5 cm thick with 2 concentric drill holes. The smaller hole was the diameter of the drill hole through which each anchor would be inserted into the bone. The larger hole corresponded to the greatest diameter of the anchor and was only 2 cm long, leaving a 0.5-cm-long narrow portion of the smaller diameter. The anchors were inserted backward through the larger diameter and found purchase at the smaller diameter, through which the steel wire loop was passed. In this configuration, the situation in which the anchor finds purchase against a hole in cortical bone is simulated. The holders with the anchors were always held in the water bath 5 cm below surface. Loading of the steel wires was always in line with the anchor axis, perpendicular to the disk. The steel wires were clamped to the cross-head of the testing machine (single pull tests) or attached to longer ropes for the static loading.
TEST TEMPERATURE
187
Static Loading (Creep Test) Time to failure under static load of 100 N32 was measured to analyze the creep behavior of the implants at 20°C or 37°C, as displayed in Fig 2. At each temperature, 3 anchors were tested per anchor model. Time to failure and failure mode were again recorded. The time limit for the experiment was 300 hours. Statistical Evaluation
FIGURE 2. Static loading in water bath. In the buffered (pH 7.4) water bath with 20°C ⫾ 1°C or 37°C ⫾ 1°C, the holder with the anchor is clamped down. The wire loop inserted in the anchor is connected to a rope, which is led around 2 polished bars and loaded with 10 kp (100 N). At failure of the suture anchor, the weight falls to the ground, stopping the stopwatch.
Differences in mechanical performance at different test temperatures and test velocities were analyzed using analysis of variance (ANOVA) and post-hoc Bonferroni comparison. For static tests, logarithmic transformation was performed. Correlation of performances of the anchor models in tensile and creep tests with crystal content and weight were calculated using Spearman rank correlation.
RESULTS Single-Pull Tests
This setup was chosen to compare the mechanical performance of each anchor type under varied conditions with high reproducibility. It does not reproduce the functionality of the anchor but allows us to show the strength of the weakest component, the eyelet. Single-Pull Tests Tensile single-pull tests until anchor failure were performed using an Instron materials-testing machine (Instron Limited, High Wycombe, U.K.). Cross-head displacement rate was either 50 or 5 mm/min, both at 20°C or 37°C. For each test, 4 anchors per model were used. Load at failure and failure mode were recorded.
In all tests, the anchor eyelet was the weakest link and failed. Failure loads (Table 2) were always significantly lower (up to 40%; P ⬍ .0001) at 37°C than at 20°C and significantly lower (up to 15%; P ⬍ .0001) at 5 mm/min than at 50 mm/min. The BioCorkscrew always failed at 20°C by tear of the eyelet suture, and at 37°C the eyelet suture was always pulled out from the anchor body (Fig 3). The proximal part of the Bio-Anchor was torn off the anchor body. In the other 3 models, the wire always cut through the anchor at both temperatures. Lowering the test speed at 37°C results in a decrease of mechanical performance of 15% for Bio-Corkscrew, 7% for an experi-
TABLE 2. Performance of the Anchor Models in Single-Pull Test Load to Failure at 50 mm/min
Load to Failure at 5 mm/min
Anchor Model
20°C N ⫾ SD
37°C N ⫾ SD
P
Change
20°C N ⫾ SD
37°C N ⫾ SD
P
Change
Bio-Corkscrew 5.0 mm Experimental prototype Panalok 3.5 mm Bio-Anchor Tag Wedge 3.7 mm
267 ⫾ 17 356 ⫾ 18 189 ⫾ 7 268 ⫾ 6 344 ⫾ 10
232 ⫾ 14 247 ⫾ 4 155 ⫾ 3 236 ⫾ 4 237 ⫾ 20
⬍.0001 ⬍.001 ⬍.0001 ⬍.0001 ⬍.0001
⫺13% ⫺31% ⫺18% ⫺12% ⫺31%
276 ⫾ 4 336 ⫾ 10 178 ⫾ 5 262 ⫾ 7 323 ⫾ 21
198 ⫾ 15 229 ⫾ 7 141 ⫾ 6 214 ⫾ 2 194 ⫾ 11
⬍.0001 ⬍.001 ⬍.0001 ⬍.0001 ⬍.0001
⫺28% ⫺32% ⫺21% ⫺18% ⫺40%
NOTE. Anchors are sorted as in Table 1. Indicated are: Mean load to failure ⫾ standard deviation in single-pull testing at 20°C and 37°C, at 50 mm/min and 5 mm/min (n ⫽ 4 for each test), P value for the difference between the 2 values, and percent difference in failure load at the 2 temperatures.
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FIGURE 3. Suture anchors as in Fig 1 after single pull testing. Top row shows failure modes at 37°C, bottom row at 20°C: At 37°C, the Bio-Corkscrew 5.0 fails by pullout of the eyelet-suture, which fails at 20°C at a higher value. The other specimens show more deformation of the anchor body despite lower failure load. Ruler indicates millimeters.
mental prototype, 9% for Panalok 3.5 mm, 9% for Bio-Anchor, and 18% for Tag Wedge. Static Loading (Creep Tests) Time to failure in static loading is indicated in Table 3. Again, a clear influence of the test temperature was seen, with a decrease of 98% in time to failure in Panalok 3.5, which failed at both temperatures before 300 hours. Two anchor models (Bio-Corkscrew, Tag Wedge) failed at 37°C before 300 hours but stayed intact at 20°C, and 2 anchor models (experimental prototype and Bio-Anchor) stayed intact for the entire period of 300 hours at 20°C and 37°C. Still, the experimental prototype showed marked deformity of the anchor eyelet at 37°C and no alterations at 20°C. At 37°C, the Bio-Corkscrew always failed by eyelet suture pullout. The other anchors failed by eyelet cutout. Because only the Panalok 3.5 failed within the time limit, it was the only model to be statistically evaluated (P ⫽ .004).
able implants. Such testing is important, because the goal of any implant design should be to use as little material as possible for sufficient mechanical strength. Benchmarks may be the tensile strength of sutures in the case of suture anchors or the tensile strength of meniscus suture stitches in case of meniscal repair devices. If plates and screws are designed, the performance of the corresponding metallic implant may be the gold standard. Performing tests with polymeric materials under physiologic conditions is generally established in materials science literature.4 However, testing conditions obtain hardly any attention in medical journals. Of 26 publications analyzed,5-30 in 12, the testing temperature is not even mentioned, and the overall testing speed varies by the factor 300. The results from the present study show that testing conditions have a decisive influence on the mechanical performance of degradable implants. The explanation is that at the glass transition temperature (typically 63°C), absorbable implant materials become ductile, comparable to chewing gum. With lower temperatures, these materials become increasingly harder and more brittle.2 Because body temperature is relatively close to the critical glass transition temperature on an absolute scale (degrees Kelvin), temperature changes in that region have a proportionally large influence on mechanical properties. In single-pull tests, raising the temperature from 20°C to 37°C may decrease the test results by 40%. In static creep tests, the performance may be decreased by up to 98%. The softening of the material at 37°C also resulted in altered failure modes. The Bio-Corkscrew failed by debonding of the eyelet-suture, which remained stable in the anchor at 20°C. The other specimens showed more deformation at failure despite decreased failure strength (Fig 3, Table 2). TABLE 3. Performance of the Anchor Models in Static Loading With 100 N Time to Failure With 100 N Static Load
Influence of Anchor Weight and Crystal Content No significant correlation of anchor weight or crystal content was found with the effect of altered temperature or test speed on the mechanical performance of the anchors. DISCUSSION 5-30
have been performed to assess Numerous studies and optimize the mechanical performance of absorb-
Anchor Model
20°C h ⫾ SD
37°C h ⫾ SD
P
Change
Bio-Corkscrew 5.0 mm Experimental prototype Panalok 3.5 mm Bio-Anchor Tag Wedge 3.7 mm
⬎300 ⬎300 31 ⫾ 3 ⬎300 ⬎300
48 ⫾ 58 ⬎300 0.65 ⫾ 0.07 ⬎300 174 ⫾ 67
— — .004 — —
⫺84% — ⫺98% — ⫺42%
NOTE. Anchors are sorted as in Table 1. Indicated are: Mean time to failure ⫾ Standard deviation at 20°C and 37°C (n ⫽ 3 for each test), P value for the difference between the two values, and percent difference in time to failure at the two temperatures.
TEST TEMPERATURE Researchers may speculate that when thawed bone samples are used in combination with absorbable implants, sometimes thawing may not be complete and the desired test temperature was not been reached at time of testing, which may result in even larger error. The fact that room temperature is the usually preferred testing temperature may be in part because of unawareness of the problem and in part because of the technical difficulties with performing the experiments in a thermostatically regulated environment. To allow for comparison with test results obtained at room temperature, both temperatures may need to be considered. Lowering the test speed also resulted in a decrease of the mechanical performance, more pronounced at high temperature (up to 18%). Therefore, to simulate the worst case, slow (1-5 mm/min) cross-head displacement rates seem preferable. If implants are subjected to significant static load, creep tests may also be an important test component. Unexpectedly, the crystal content of the material had no detectable influence on the decrease of the performance at altered temperature. Implant design and other material properties seem to be more important, even though in this study determining one decisive variable was not possible. The experiment was designed to isolate and quantify the influence of the testing conditions on the performance of absorbable implant materials using absorbable suture anchors. Because the anchors were held in metal holders, the results do not reflect the performance of the implants in a clinical setting. However, we can presume that the hold of an absorbable implant in bone will depend on the mechanical properties of the material. Test temperature and test speed have been shown to affect the strength of absorbable implants. Because the influence of these factors seems to be rather constant, the conclusions from studies comparing the “relative” strength of various implants may not necessarily need to be changed. However, studies designed to assess the “absolute” values would need to be conducted under conditions that better replicate in vivo conditions. Unfortunately, this will raise the complexity and expense of such studies. Acknowledgment: The authors recognize the help of Burkhart Seifert, Ph.D., with the statistical evaluation of the data.
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