TABLE 1 TightRope Displacement Testing (in Millimeters)
Femoral Suspension Devices for Anterior Cruciate Ligament Reconstruction: Letter to the Editor
DeBerardino Sample No. 1 2 3 4 5 Average Standard deviation
DOI: 10.1177/0363546513518297
Dear Editor: We write this letter in regard to ‘‘Femoral Suspension Devices for Anterior Cruciate Ligament Reconstruction, Do Adjustable Loops Lengthen?’’ by Barrow et al.2 We feel that there is a technical error with the 42 mm of total displacement the authors report for the TightRope, as this is not consistent with other findings involving the TightRope or with our clinical experience. While the authors use a ramped cyclic loading profile that is different from that used in the majority of the literature, the last portion of the regimen is 500 cycles from 10 to 250 N. This portion of the regimen is similar to previously reported methodologies and can be used to evaluate the validity of their results. The authors report approximately 7 mm of displacement for the TightRope during the last 500 cycles from 10 to 250 N. Petre et al3 performed testing of the same device using similar materials and methods and reported 1.1 6 0.2 mm of displacement. White and Baer4 also conducted a mechanical study similar to that of Petre et al in which TightRopes were loaded from 50 to 250 N for 1000 cycles; they reported cyclic displacement of 0.34 6 0.07 mm. There is a large discrepancy between the Barrow et al results compared with those reported by these studies. This large discrepancy seems to suggest the Barrow et al results are an outlier in terms of displacement, as all 3 studies reported similar ultimate loads (809 6 53 N, 841 6 55 N, and 814 6 76 N, respectively) suggesting no structural difference in the implant after cyclic loading. We would respectfully suggest that this disparity warrants more explanation than currently provided by the authors. While the authors reference the Petre et al3 article, they do not elaborate on the wide disparity between their results and those reported by Petre. The authors also reference an Arthrex white paper1 that reports 1.13 6 0.1 mm of displacement for the TightRope after 500 cycles from 50 to 250 N; however, the discrepancy was again not addressed. The displacement results for the TightRope presented by White and Baer4 were not referenced by the authors. Because of the disparity in the Barrow et al2 study compared with 3 previous studies, our clinical experiences, the lack of explanation for the disparity, the potential clinical ramifications, and the relative simplicity of testing, we
Knotted
Unknotted
Cook/Smith Unknotted
1.24 1.03 1.03 1.2 — 1.13 0.10
1.42 2.16 1.85 2.02 — 1.87 0.32
2.8 3.1 3.2 2.4 2.6 2.8 0.33
the undersigned agreed to independently evaluate the TightRope using the protocol reported by Barrow et al. Our 2 laboratories independently conducted materials testing, with the results shown in Table 1. These results from 2 independent laboratories are significantly different from those reported by Barrow et al2 and are much more in line with those reported by Petre et al,3 White and Baer,4 and Arthrex.1 To us, this suggests an unaccounted-for technical error during testing by Barrow et al. While the cause of the error is unknown, possible hypotheses are improper loading of the device, improper manipulation of the device, poor calibration of their testing machine, or inaccurate interpretation of cyclic displacement from the raw load versus displacement data. The questionable results are further exacerbated by technical errors in the manuscript such as the ACL TightRope being described as a 2 3 10–mm stainless steel button instead of a 3.4 3 13–mm titanium button and the EndoButton originally being described as a 25-mm continuous loop, then later as a 42-mm loop. In an attempt to elucidate the potential technical error, we conducted additional testing to further evaluate the previously mentioned hypotheses and to attempt to replicate the results reported by Barrow et al.2 We found that by improperly manipulating the loops so that the tensioning portion of the loop (marked by the red and blue colors in Figure 1) were both on the same side of the metal rod would increase cyclic displacement to 12.8 mm after cyclic loading using the profile described by Barrow et al. Furthermore, ‘‘un-tuning’’ the machine so that minimum load during cycling was 0 N instead of 10 N as reported by Barrow et al further increased cyclic displacement. Using the combination of loop manipulation and minimum loads of 0 N during cyclic loading, we tested 3 constructs, with cyclic displacement results of 41.1 6 6.6 mm. These results are similar to those reported by Barrow et al, and loop manipulation and poor tuning of the testing machine may be an explanation for the disparity of their results with those previously reported.
Note: This letter refers to the prepublication version. Based on the concerns raised, the authors have revised the paper for final publication. The American Journal of Sports Medicine, Vol. 42, No. 2 Ó 2014 The Author(s)
NP15
NP16 Letter to the Editor
The American Journal of Sports Medicine
Address correspondence to Thomas M. DeBerardino, MD (e-mail: [email protected]). One or more of the authors has declared the following potential conflict of interest or source of funding: T.M.D. is contracted with Arthrex Inc to receive royalties for a device unrelated to the TightRope device; all authors are consultants for Arthrex. Arthrex Inc supplied the ACL TightRope RT devices used for testing performed independently in the research labs at UCONN Health Center and the University of Missouri.
REFERENCES 1. Arthrex Inc. Arthrex ACL TightRope and Biomet ZipLoop with ToggleLoc: Mechanical Testing. Vol LA0179A. Naples, Florida: Arthrex Inc; 2010. 2. Barrow AE, Pilia M, Guda T, Kadrmas WR, Burns TC. Femoral suspension devices for anterior cruciate ligament reconstruction: do adjustable loops lengthen? [published online ahead of print October 24, 2013. Am J Sports Med. doi: 10.1177/0363546513507769. 3. Petre BM, Smith SD, Jansson KS, et al. Femoral cortical suspension devices for soft tissue anterior cruciate ligament reconstruction; a comparative biomechanical study. Am J Sports Med. 2013;41(2): 416-422. 4. White MJ, Baer GS. Suspensory ACL fixation: a biomechanical study of fixed and adjustable length implants. Presented at the 2011 Annual AANA Meeting, San Francisco, CA. http://www.aana.org/VideoEduca tionLibrary/CourseListings/Knee/ACL/SuspensoryACLFixation/tabid/ 787/Default.aspx.
Figure 1. Marks indicate tensioning sutures. (A) Proper orientation of the TightRope as recommended by the manufacturer. (B) Manipulation of the TightRope so that both tensioning sutures are on the same side. While not peer reviewed, our clinical experiences are also divergent from the cyclic displacement results reported by the authors. Forty-two millimeters of cyclic displacement would most certainly result in clinical failure; however, we have collectively used the TightRope in approximately 550 cases. To date, we are extremely satisfied with our clinical results, and to further bolster our clinical experience, Dr Smith is conducting a prospective study involving the TightRope for ACL fixation on both the femoral and tibial side of soft tissue grafts. While the study is in the initial stages, KT-1000 arthrometer data have been collected for 7 subjects at 6 weeks postoperative. The laxity of the operated limb was equal to that of the nonoperated uninjured limb in 4 subjects, 1 mm greater in the operated limb in 2 subjects, and 1 mm less in the operated limb for 1 subject. While we certainly appreciate mechanical studies, we are concerned with validity of the results reported by Barrow et al and the potential improper conclusions that could be drawn. We encourage both the authors and AJSM to closely examine the results and determine if the results are appropriate to remain in publication.
Thomas M. DeBerardino, MD Avon, Connecticut, USA Patrick A. Smith, MD James L. Cook, DVM, PhD Columbia, Missouri, USA
Femoral Suspension Devices for Anterior Cruciate Ligament Reconstruction: Response DOI: 10.1177/0363546514522958
Authors’ Response: Thank you for your interest in our manuscript titled ‘‘Femoral Suspension Devices for Anterior Cruciate Ligament Reconstruction, Do Adjustable Loops Lengthen?’’ As you mentioned, we found a significant increase in displacement of the Arthrex TightRope in our study compared to previous studies by White and Baer5 and Petre et al.3 We were asked by the editors to comment on this difference prior to print, and this was added to the discussion in our manuscript. We agree with you that multiple studies have shown the adjustable loop devices to have minimal lengthening when cycled between 50 and 250 N.3,5 We believe that the difference in our results was not due to technical error or inaccurate interpretation of data but to a difference in the cyclic load parameters. In the supplemental data to the article, we provide the load ranges for mean, minimum, and maximum loads during each load block, which show the minimum loads approach 0 N. To the best of our knowledge, the ability of the Arthrex TightRope (and other suspensory devices) to resist slippage when the minimum load approaches 0 N has not been previously reported.
Vol. 42, No. 2, 2014
Letter to the Editor
NP17
TABLE 2 Comparison of Results (in Millimeters)a
Smith & Nephew EndoButton Biomet ToggleLoc Arthrex TightRope
Our Results Through 1000 Cycles (Load Protocol: 10-75 Nb)
Petre et al3 Results: 1000 Cycles (Load Protocol: 50-250 N)
0.33 6 0.02 0.95 6 0.06 3.22 6 1.33
0.42 6 0.08 2.18 6 0.31 1.10 6 0.20
a
Values are expressed as mean 6 standard deviation. Testing performed in load control mode and the actual cycle minimum and maximum loads are listed in the supplementary online material to our article. b
The important question is, which loading parameters more precisely replicate the in vivo forces experienced by the ACL and/or femoral fixation? While the exact load range is unknown (and likely varies based on tunnel position), several studies reveal midflexion low load or ‘‘slack’’ in the ACL.1,2,4 Cadaveric studies by More et al2 and Wascher et al4 show graft tension approaches 0 N between 30° and 120° of knee motion. A cadaveric study by Arnold et al1 evaluated ACL tension based on femoral tunnel position. The authors found that ‘‘9 o’clock’’ tunnels most closely replicated the normal ACL tension curve, but all tunnels had ‘‘slack in mid-flexion.’’ Considering these forces were measured at time zero and may even decrease further with cycling, minimum load parameters close to 0 N seems the most clinically appropriate. Comparing our results with prior studies3 head-to-head is difficult since those studies tested to a total of 1000 cycles over a single load range, unlike our study, which evaluated multiple increasing load ranges with 500 cycles in each ‘‘block.’’ Through 1000 cycles of our testing protocol we actually had less displacement for the Biomet ToggleLoc and the Smith &Nephew EndoButton, likely due to our loading protocol only going to 75 N to that point. However, the Arthrex TightRope showed increased lengthening even at this low maximum load, which we hypothesize is due to the ‘‘unlocking’’ of the friction mechanism as minimum loads approach 0 N during cyclic loading. This slippage was not demonstrated in prior studies, as the minimum load in all of the studies referenced was 50 N (see Table 2). Thank you for illustrating the importance of the loop configuration, as it is an important technical point for surgeons implanting these devices. The Arthrex TightRope comes packaged with the loops oriented correctly, which is how we tested the devices. With the limited data provided in your independent testing it is difficult to determine the difference in results. It is likely related to the load ranges of each load block and may be related to the minimum load applied during cyclic testing, which can be compared to ours in the supplementary material to our article. The absolute amount of displacement of the adjustable loop devices does not seem as important as the principle that they lengthen under cyclic loading. Since the Arthrex TightRope continued to lengthen with cyclic loading under our loading parameters, the lengthening could be any value dependent on number of times cycled. We appreciate
Figure 2. (A) Screen capture of Arthrex TightRope at beginning of cyclic loading protocol. (B) Screen capture of same TightRope at end of cyclic loading protocol before load to failure. Full video available as a Video Supplement to the article. your clinical experience and anticipate the outcome of your clinical trial with tibial and femoral adjustable loop fixation. It is difficult to draw conclusions from 6-week follow-up data, but this information is pertinent and appreciated. We are similarly evaluating our clinical results comparing adjustable-loop to fixed-loop fixation. We recognized the error in the description of the implants in the manuscript and this was corrected prior to print. While we regret the error, it was simply a switching of the manufacturer descriptions while writing the manuscript and had nothing to do with nomenclature during data collection, analysis, or results. This should be evident by viewing the videos in our supplemental data, where the Arthrex TightRope can be observed lengthening over the test, and in Figure 2, showing the device at the initiation and conclusion of cyclic loading. Caution should always be observed when translating mechanical data to in vivo performance due to the numerous other variables involved in surgical outcome. However, the Arthrex TightRope lengthens in vitro with cyclic loading when tension goes to 0 N, and that is an important consideration for surgeons using the product.
Travis C. Burns, MD Aaron E. Barrow, MD Marcello Pilia, PhD Teja Guda, PhD Warren R. Kadrmas, MD San Antonio, Texas, USA
NP18 Letter to the Editor
Address correspondence to Travis C. Burns, MD (e-mail [email protected]). One or more of the authors has declared the following potential conflict of interest or source of funding: Smith & Nephew, Arthrex Inc, and Biomet provided the devices used for testing in this study.
REFERENCES 1. Arnold MP, Verdonschot N, van Kampen A. ACL graft can replicate the normal ligament’s tension curve. Knee Surg Sports Traumatol Arthrosc. 2005:13:625-631.
The American Journal of Sports Medicine
2. More RC, Karras BT, Neiman R, et al. Hamstrings—an anterior cruciate ligament protagonist: an in vitro study. Am J Sports Med. 1993;21:231-237. 3. Petre BM, Smith SD, Jansson KS, et al. Femoral cortical suspension devices for soft tissue anterior cruciate ligament reconstruction: a comparative biomechanical study. Am J Sports Med. 2012;41: 416-422. 4. Wascher DC, Markolf KL, Shapiro MS, et al. Direct in vitro measurement of forces in the cruciate ligaments, part 1: the effect of multiplane loading in the intact knee. J Bone Joint Surg Am. 1993;75:377-386. 5. White MJ, Baer GS. Suspensory ACL Fixation: A Biomechanical Study of Fixed and Adjustable Length Implants, 2011 Annual AANA Meeting (San Francisco). http://www.aana.org/VideoEducationLibrary/Course Listings/Knee/ACL/SuspensoryACLFixation/tabid/787/Default.aspx.
For reprints and permission queries, please visit SAGE’s Web site at http://www.sagepub.com/journalsPermissions.nav
Femoral Suspension Devices for Anterior Cruciate Ligament Reconstruction: Letter to the Editor
DeBerardino Sample No. 1 2 3 4 5 Average Standard deviation
DOI: 10.1177/0363546513518297
Dear Editor: We write this letter in regard to ‘‘Femoral Suspension Devices for Anterior Cruciate Ligament Reconstruction, Do Adjustable Loops Lengthen?’’ by Barrow et al.2 We feel that there is a technical error with the 42 mm of total displacement the authors report for the TightRope, as this is not consistent with other findings involving the TightRope or with our clinical experience. While the authors use a ramped cyclic loading profile that is different from that used in the majority of the literature, the last portion of the regimen is 500 cycles from 10 to 250 N. This portion of the regimen is similar to previously reported methodologies and can be used to evaluate the validity of their results. The authors report approximately 7 mm of displacement for the TightRope during the last 500 cycles from 10 to 250 N. Petre et al3 performed testing of the same device using similar materials and methods and reported 1.1 6 0.2 mm of displacement. White and Baer4 also conducted a mechanical study similar to that of Petre et al in which TightRopes were loaded from 50 to 250 N for 1000 cycles; they reported cyclic displacement of 0.34 6 0.07 mm. There is a large discrepancy between the Barrow et al results compared with those reported by these studies. This large discrepancy seems to suggest the Barrow et al results are an outlier in terms of displacement, as all 3 studies reported similar ultimate loads (809 6 53 N, 841 6 55 N, and 814 6 76 N, respectively) suggesting no structural difference in the implant after cyclic loading. We would respectfully suggest that this disparity warrants more explanation than currently provided by the authors. While the authors reference the Petre et al3 article, they do not elaborate on the wide disparity between their results and those reported by Petre. The authors also reference an Arthrex white paper1 that reports 1.13 6 0.1 mm of displacement for the TightRope after 500 cycles from 50 to 250 N; however, the discrepancy was again not addressed. The displacement results for the TightRope presented by White and Baer4 were not referenced by the authors. Because of the disparity in the Barrow et al2 study compared with 3 previous studies, our clinical experiences, the lack of explanation for the disparity, the potential clinical ramifications, and the relative simplicity of testing, we
Knotted
Unknotted
Cook/Smith Unknotted
1.24 1.03 1.03 1.2 — 1.13 0.10
1.42 2.16 1.85 2.02 — 1.87 0.32
2.8 3.1 3.2 2.4 2.6 2.8 0.33
the undersigned agreed to independently evaluate the TightRope using the protocol reported by Barrow et al. Our 2 laboratories independently conducted materials testing, with the results shown in Table 1. These results from 2 independent laboratories are significantly different from those reported by Barrow et al2 and are much more in line with those reported by Petre et al,3 White and Baer,4 and Arthrex.1 To us, this suggests an unaccounted-for technical error during testing by Barrow et al. While the cause of the error is unknown, possible hypotheses are improper loading of the device, improper manipulation of the device, poor calibration of their testing machine, or inaccurate interpretation of cyclic displacement from the raw load versus displacement data. The questionable results are further exacerbated by technical errors in the manuscript such as the ACL TightRope being described as a 2 3 10–mm stainless steel button instead of a 3.4 3 13–mm titanium button and the EndoButton originally being described as a 25-mm continuous loop, then later as a 42-mm loop. In an attempt to elucidate the potential technical error, we conducted additional testing to further evaluate the previously mentioned hypotheses and to attempt to replicate the results reported by Barrow et al.2 We found that by improperly manipulating the loops so that the tensioning portion of the loop (marked by the red and blue colors in Figure 1) were both on the same side of the metal rod would increase cyclic displacement to 12.8 mm after cyclic loading using the profile described by Barrow et al. Furthermore, ‘‘un-tuning’’ the machine so that minimum load during cycling was 0 N instead of 10 N as reported by Barrow et al further increased cyclic displacement. Using the combination of loop manipulation and minimum loads of 0 N during cyclic loading, we tested 3 constructs, with cyclic displacement results of 41.1 6 6.6 mm. These results are similar to those reported by Barrow et al, and loop manipulation and poor tuning of the testing machine may be an explanation for the disparity of their results with those previously reported.
Note: This letter refers to the prepublication version. Based on the concerns raised, the authors have revised the paper for final publication. The American Journal of Sports Medicine, Vol. 42, No. 2 Ó 2014 The Author(s)
NP15
NP16 Letter to the Editor
The American Journal of Sports Medicine
Address correspondence to Thomas M. DeBerardino, MD (e-mail: [email protected]). One or more of the authors has declared the following potential conflict of interest or source of funding: T.M.D. is contracted with Arthrex Inc to receive royalties for a device unrelated to the TightRope device; all authors are consultants for Arthrex. Arthrex Inc supplied the ACL TightRope RT devices used for testing performed independently in the research labs at UCONN Health Center and the University of Missouri.
REFERENCES 1. Arthrex Inc. Arthrex ACL TightRope and Biomet ZipLoop with ToggleLoc: Mechanical Testing. Vol LA0179A. Naples, Florida: Arthrex Inc; 2010. 2. Barrow AE, Pilia M, Guda T, Kadrmas WR, Burns TC. Femoral suspension devices for anterior cruciate ligament reconstruction: do adjustable loops lengthen? [published online ahead of print October 24, 2013. Am J Sports Med. doi: 10.1177/0363546513507769. 3. Petre BM, Smith SD, Jansson KS, et al. Femoral cortical suspension devices for soft tissue anterior cruciate ligament reconstruction; a comparative biomechanical study. Am J Sports Med. 2013;41(2): 416-422. 4. White MJ, Baer GS. Suspensory ACL fixation: a biomechanical study of fixed and adjustable length implants. Presented at the 2011 Annual AANA Meeting, San Francisco, CA. http://www.aana.org/VideoEduca tionLibrary/CourseListings/Knee/ACL/SuspensoryACLFixation/tabid/ 787/Default.aspx.
Figure 1. Marks indicate tensioning sutures. (A) Proper orientation of the TightRope as recommended by the manufacturer. (B) Manipulation of the TightRope so that both tensioning sutures are on the same side. While not peer reviewed, our clinical experiences are also divergent from the cyclic displacement results reported by the authors. Forty-two millimeters of cyclic displacement would most certainly result in clinical failure; however, we have collectively used the TightRope in approximately 550 cases. To date, we are extremely satisfied with our clinical results, and to further bolster our clinical experience, Dr Smith is conducting a prospective study involving the TightRope for ACL fixation on both the femoral and tibial side of soft tissue grafts. While the study is in the initial stages, KT-1000 arthrometer data have been collected for 7 subjects at 6 weeks postoperative. The laxity of the operated limb was equal to that of the nonoperated uninjured limb in 4 subjects, 1 mm greater in the operated limb in 2 subjects, and 1 mm less in the operated limb for 1 subject. While we certainly appreciate mechanical studies, we are concerned with validity of the results reported by Barrow et al and the potential improper conclusions that could be drawn. We encourage both the authors and AJSM to closely examine the results and determine if the results are appropriate to remain in publication.
Thomas M. DeBerardino, MD Avon, Connecticut, USA Patrick A. Smith, MD James L. Cook, DVM, PhD Columbia, Missouri, USA
Femoral Suspension Devices for Anterior Cruciate Ligament Reconstruction: Response DOI: 10.1177/0363546514522958
Authors’ Response: Thank you for your interest in our manuscript titled ‘‘Femoral Suspension Devices for Anterior Cruciate Ligament Reconstruction, Do Adjustable Loops Lengthen?’’ As you mentioned, we found a significant increase in displacement of the Arthrex TightRope in our study compared to previous studies by White and Baer5 and Petre et al.3 We were asked by the editors to comment on this difference prior to print, and this was added to the discussion in our manuscript. We agree with you that multiple studies have shown the adjustable loop devices to have minimal lengthening when cycled between 50 and 250 N.3,5 We believe that the difference in our results was not due to technical error or inaccurate interpretation of data but to a difference in the cyclic load parameters. In the supplemental data to the article, we provide the load ranges for mean, minimum, and maximum loads during each load block, which show the minimum loads approach 0 N. To the best of our knowledge, the ability of the Arthrex TightRope (and other suspensory devices) to resist slippage when the minimum load approaches 0 N has not been previously reported.
Vol. 42, No. 2, 2014
Letter to the Editor
NP17
TABLE 2 Comparison of Results (in Millimeters)a
Smith & Nephew EndoButton Biomet ToggleLoc Arthrex TightRope
Our Results Through 1000 Cycles (Load Protocol: 10-75 Nb)
Petre et al3 Results: 1000 Cycles (Load Protocol: 50-250 N)
0.33 6 0.02 0.95 6 0.06 3.22 6 1.33
0.42 6 0.08 2.18 6 0.31 1.10 6 0.20
a
Values are expressed as mean 6 standard deviation. Testing performed in load control mode and the actual cycle minimum and maximum loads are listed in the supplementary online material to our article. b
The important question is, which loading parameters more precisely replicate the in vivo forces experienced by the ACL and/or femoral fixation? While the exact load range is unknown (and likely varies based on tunnel position), several studies reveal midflexion low load or ‘‘slack’’ in the ACL.1,2,4 Cadaveric studies by More et al2 and Wascher et al4 show graft tension approaches 0 N between 30° and 120° of knee motion. A cadaveric study by Arnold et al1 evaluated ACL tension based on femoral tunnel position. The authors found that ‘‘9 o’clock’’ tunnels most closely replicated the normal ACL tension curve, but all tunnels had ‘‘slack in mid-flexion.’’ Considering these forces were measured at time zero and may even decrease further with cycling, minimum load parameters close to 0 N seems the most clinically appropriate. Comparing our results with prior studies3 head-to-head is difficult since those studies tested to a total of 1000 cycles over a single load range, unlike our study, which evaluated multiple increasing load ranges with 500 cycles in each ‘‘block.’’ Through 1000 cycles of our testing protocol we actually had less displacement for the Biomet ToggleLoc and the Smith &Nephew EndoButton, likely due to our loading protocol only going to 75 N to that point. However, the Arthrex TightRope showed increased lengthening even at this low maximum load, which we hypothesize is due to the ‘‘unlocking’’ of the friction mechanism as minimum loads approach 0 N during cyclic loading. This slippage was not demonstrated in prior studies, as the minimum load in all of the studies referenced was 50 N (see Table 2). Thank you for illustrating the importance of the loop configuration, as it is an important technical point for surgeons implanting these devices. The Arthrex TightRope comes packaged with the loops oriented correctly, which is how we tested the devices. With the limited data provided in your independent testing it is difficult to determine the difference in results. It is likely related to the load ranges of each load block and may be related to the minimum load applied during cyclic testing, which can be compared to ours in the supplementary material to our article. The absolute amount of displacement of the adjustable loop devices does not seem as important as the principle that they lengthen under cyclic loading. Since the Arthrex TightRope continued to lengthen with cyclic loading under our loading parameters, the lengthening could be any value dependent on number of times cycled. We appreciate
Figure 2. (A) Screen capture of Arthrex TightRope at beginning of cyclic loading protocol. (B) Screen capture of same TightRope at end of cyclic loading protocol before load to failure. Full video available as a Video Supplement to the article. your clinical experience and anticipate the outcome of your clinical trial with tibial and femoral adjustable loop fixation. It is difficult to draw conclusions from 6-week follow-up data, but this information is pertinent and appreciated. We are similarly evaluating our clinical results comparing adjustable-loop to fixed-loop fixation. We recognized the error in the description of the implants in the manuscript and this was corrected prior to print. While we regret the error, it was simply a switching of the manufacturer descriptions while writing the manuscript and had nothing to do with nomenclature during data collection, analysis, or results. This should be evident by viewing the videos in our supplemental data, where the Arthrex TightRope can be observed lengthening over the test, and in Figure 2, showing the device at the initiation and conclusion of cyclic loading. Caution should always be observed when translating mechanical data to in vivo performance due to the numerous other variables involved in surgical outcome. However, the Arthrex TightRope lengthens in vitro with cyclic loading when tension goes to 0 N, and that is an important consideration for surgeons using the product.
Travis C. Burns, MD Aaron E. Barrow, MD Marcello Pilia, PhD Teja Guda, PhD Warren R. Kadrmas, MD San Antonio, Texas, USA
NP18 Letter to the Editor
Address correspondence to Travis C. Burns, MD (e-mail [email protected]). One or more of the authors has declared the following potential conflict of interest or source of funding: Smith & Nephew, Arthrex Inc, and Biomet provided the devices used for testing in this study.
REFERENCES 1. Arnold MP, Verdonschot N, van Kampen A. ACL graft can replicate the normal ligament’s tension curve. Knee Surg Sports Traumatol Arthrosc. 2005:13:625-631.
The American Journal of Sports Medicine
2. More RC, Karras BT, Neiman R, et al. Hamstrings—an anterior cruciate ligament protagonist: an in vitro study. Am J Sports Med. 1993;21:231-237. 3. Petre BM, Smith SD, Jansson KS, et al. Femoral cortical suspension devices for soft tissue anterior cruciate ligament reconstruction: a comparative biomechanical study. Am J Sports Med. 2012;41: 416-422. 4. Wascher DC, Markolf KL, Shapiro MS, et al. Direct in vitro measurement of forces in the cruciate ligaments, part 1: the effect of multiplane loading in the intact knee. J Bone Joint Surg Am. 1993;75:377-386. 5. White MJ, Baer GS. Suspensory ACL Fixation: A Biomechanical Study of Fixed and Adjustable Length Implants, 2011 Annual AANA Meeting (San Francisco). http://www.aana.org/VideoEducationLibrary/Course Listings/Knee/ACL/SuspensoryACLFixation/tabid/787/Default.aspx.
For reprints and permission queries, please visit SAGE’s Web site at http://www.sagepub.com/journalsPermissions.nav
