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Displacement of a Fixed Versus Adjustable Suspensory Fixation Device for Anterior Cruciate Ligament ReconstructionMike Perriello MS1, Rod Berube MS1, Chris Moore MS2

1. Smith & Nephew, Inc., Andover, MA, USA

2. Smith & Nephew, Inc., Memphis, TN, USA

Summary

Suspensory fi xation devices in anatomic anterior cruciate ligament (ACL) reconstruction commonly feature a button that rests on the cortex of the femur, and a loop that holds the graft in position until healing can occur. While this loop can be closed and fi xed in length, some contemporary designs feature a loop that is adjustable. The purpose of this study was to assess the displacement of the fi xed ENDOBUTTON™ CL ULTRA (Smith & Nephew, Inc., Andover, MA, USA) and TightRope® (Arthrex, Inc., Naples, FL, USA) fi xation devices. All loop samples were securely fi xed to a metal plate and axial loading apparatus. Next, a zero to 60N static load was applied to each sample a total of ten times. Following completion of the loading regimen,

loop displacement and fi nal distance were measured. Results of this study demonstrated signifi cantly less displacement for the ENDOBUTTON CL ULTRA device vs the TightRope device (0.58mm versus 9.95mm; p < 0.05). Furthermore, the mean dis-tance from the button to the top of the hook for the devices was 8.93mm (ENDOBUTTON) and 14.63mm (TightRope), respectively (p < 0.05). The results of this study suggest that the adjustable TightRope loop can slip and elongate under load after it has been adjusted to its minimum length. It also suggests that ENDOBUTTON CL Ultra may allow for more graft in the femoral tunnel following ACL reconstruction.

Vol 03, No 7 - July 2012

Short femoral tunnel length is recognized as a problem during anatomic anterior cruciate ligament (ACL) reconstruction. Specifi cally, a short tunnel can effectively reduce graft length, potentially reducing the strength of the graft-bone tunnel construct [1-3]. Furthermore, reduced ACL graft length is thought to adversely affect healing following reconstruction [4]. This has led to the introduction of suspensory fi xation devices capable of maximizing the amount of graft in the femoral tunnel, thereby improving the outcome of ACL reconstruction [1, 4].

Suspensory devices commonly feature a button that rests on the cortex of the femur, and a loop that holds the folded soft tissue ACL graft in position until healing can occur. The ENDOBUTTON™

CL ULTRA (Smith & Nephew, Inc., Andover, MA, USA) is a fi xation device that features a closed, fi xed length loop. However, adjustable loop devices have recently been introduced to the market. The TightRope (Arthrex, Inc., Naples, FL, USA) is one such cortical fi xation device, featuring two sutures that can be pulled to enable intraoperative adjustment of graft tension.

While there are clear benefi ts to suspensory fi xation during anatomic ACL reconstruction, the rigidity of adjustable loop devices has not been fully established. Therefore, the purpose of the current study was to assess displacement and fi nal distance following loading of the ENDOBUTTON CL ULTRA and TightRope devices.

Introduction

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Bone&JointScience Displacement of a Fixed Versus Adjustable Suspensory Fixation Device for Anterior Cruciate Ligament Reconstruction

Vol 03, No 7 - July 2012

Twenty ENDOBUTTON™ CL ULTRA, 10mm and twenty TightRope® sample loops were tested utilizing an Insight® 30 axial loading apparatus (MTS Systems Corp., Eden Prairie, MN, USA). Each button was fixed securely to a metal plate and vise construct, with the loop attached to the loading apparatus using a metal hook (Figure 1). Prior to testing, all ENDOBUTTON CL ULTRA loops were held taught with a minimum load of 5 Newton’s (N). For the TightRope device, all loops were tightened so that the loading hook was in contact (or nearly in contact) with the construct plate at the start of loading. This simulated the optimal surgical situation where the surgeon is able to adjust the device so that the ACL graft is touching the cortex of the femoral tunnel.

The Insight 30 apparatus was then used to apply a zero to 60N static load to each sample a total of ten times. This load is approximate to the force applied by the surgeon during intraoperative tensioning of an ACL graft [5]. Following the completion of the loading regimen, displacement of the hook was assessed. Furthermore, the length of each loop was determined by using a 1 kilogram mass to hold each loop taut, at which point the distance from the top of the loading hook to the construct plate was measured.

A statistical comparison of mean displacement and final distance between device groups was performed using a two-sample t-test with a significance level of p < 0.05.

Study results for displacement and final distance are illustrated in Figures 2 and 3, respectively. The average displacement for ENDOBUTTON CL ULTRA was significantly less than that observed for TightRope (0.58mm versus 9.95mm; p < 0.05). Displacement of the TightRope loop is illustrated in Figure 4. Furthermore, the average measurement from the bottom of the plate to the top of the hook for ENDOBUTTON CL ULTRA was significantly shorter than that of TightRope (4.91mm versus 10.61mm; p < 0.05). Taking the thickness of the plate (4.02mm) into account, the mean distance from the button to the top of the hook for the devices was 8.93mm (ENDOBUTTON) and 14.61mm (TightRope), respectively (p<0.05).

Figure 1: Loading apparatus

Materials and Methods Results

Final Construct Length (mm)

TightRope® ENDOBUTTON™ CL ULTRA

18

15

12

9

6

3

0

Figure 3: Mean distance for TightRope® and ENDOBUTTON™ CL ULTRA plate, device, and hook construct

Displacement (mm)

TightRope® ENDOBUTTON™ CL ULTRA

12

10

8

6

4

2

0

Figure 2: Mean displacement for TightRope® and ENDOBUTTON™ CL ULTRA

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Bone&JointScience Displacement of a Fixed Versus Adjustable Suspensory Fixation Device for Anterior Cruciate Ligament Reconstruction

Vol 03, No 7 - July 2012

The results of this study suggest that the adjustable TightRope® loop can slip and elongate under load after it has been adjusted to its minimum length. This is evidenced by a statistically significant increase in mean displacement and distance following loading, as compared to the fixed loop ENDOBUTTON™ CL ULTRA device. Furthermore, the minimum length of the TightRope device after loading was longer than that of ENDOBUTTON CL ULTRA under a 1 kilogram load.

The excellent loading performance of the ENDOBUTTON CL ULTRA loop corroborates a recent report from Conner et al [6], who evaluated the in vitro performance of suspensory fixation devices in a porcine ACL reconstruction model. Cyclic testing was performed under loads ranging from 50N to 450N, which represents the range of in vivo ACL loads during human activities of daily living [6-9]. Following 2,000 loading cycles, a first generation ENDOBUTTON CL device demonstrated significantly less graft-implant elongation, as compared to competitive femoral fixation devices (p = 0.005). These results are noteworthy, as the efficacy of suspensory fixation devices depends upon their ability to maintain graft tension [4, 10]. It also suggests that ENDOBUTTON CL Ultra may allow for more graft in the femoral tunnel following ACL reconstruction.

Discussion and Conclusion

Figure 4: TightRope® displacement from initial (A) to final position (B). Illustration of displacement distance (C)

References1. Chang CB, Choi JY, Koh IJ, Lee KJ, Lee KH, Kim TK. Comparisons of femoral tunnel

position and length in anterior cruciate ligament reconstruction: modified transtibial versus anteromedial portal techniques. Arthroscopy 27(10): 1389, 2011

2. Wang JH, Kim JG, Lee DK, Lim HC, Ahn JH. Comparison of femoral graft bending angle and tunnel length between transtibial technique and transportal technique in anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc, 2011

3. Zantop T, Ferretti M, Bell KM, Brucker PU, Gilbertson L, Fu FH. Effect of tunnel-graft length on the biomechanics of anterior cruciate ligament-reconstructed knees: intra-articular study in a goat model. Am J Sports Med 36(11): 2158, 2008

4. Ekdahl M, Wang JH, Ronga M, Fu FH. Graft healing in anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 16(10): 935, 2008

5. Amis AA, Jakob RP. Anterior cruciate ligament graft positioning, tensioning and twisting. Knee Surg Sports Traumatol Arthrosc 6 Suppl 1: S2, 1998

6. Conner CS, Perez BA, Morris RP, Buckner JW, Buford WL, Jr., Ivey FM. Three femoral fixation devices for anterior cruciate ligament reconstruction: comparison of fixation on the lateral cortex versus the anterior cortex. Arthroscopy 26(6): 796, 2010

7. Heijne A, Fleming BC, Renstrom PA, Peura GD, Beynnon BD, Werner S. Strain on the anterior cruciate ligament during closed kinetic chain exercises. Med Sci Sports Exerc 36(6): 935, 2004

8. Kvist J, Gillquist J. Sagittal plane knee translation and electromyographic activity during closed and open kinetic chain exercises in anterior cruciate ligament-deficient patients and control subjects. Am J Sports Med 29(1): 72, 2001

9. Morrison JB. Function of the knee joint in various activities. Biomed Eng 4(12): 573, 1969

10. Hoher J, Livesay GA, Ma CB, Withrow JD, Fu FH, Woo SL. Hamstring graft motion in the femoral bone tunnel when using titanium button/polyester tape fixation. Knee Surg Sports Traumatol Arthrosc 7(4): 215, 1999

Study results originally reported in Smith & Nephew test report #15001530.

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