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Knee Surg Sports Traumatol ArthroscDOI 10.1007/s00167-015-3711-8

KNEE

A comparison of three adjustable cortical button ACL fixation devices

Meghan Pasquali1 · Matthew J. Plante2 · Keith O. Monchik2 · David B. Spenciner1

Received: 26 January 2015 / Accepted: 13 July 2015 © European Society of Sports Traumatology, Knee Surgery, Arthroscopy (ESSKA) 2015

strength. The clinical relevance of this study is that cyclic displacement results from bench-top tests may correlate with post-operative laxity, which ideally should be kept to a minimum.

Keywords Cyclic displacement · Laxity · Adjustable cortical button · Cortical fixation · ACL reconstruction

Introduction

Anterior cruciate ligament (ACL) tears are one of the most common soft tissue knee injuries [2]. When an ACL tear occurs, anterior tibial translation can result and rotational stability may be lost [14] causing pain and a reduction in activity level. While an ACL reconstruction is not always required, it is desirable for young and/or active patients who want to return to their previous activity level [5, 9]. A variety of techniques and fixation devices may be used to reconstruct the ACL. The goal of any fixation is to secure the new graft without contributing to laxity via displace-ment [10]. The performance criteria have been quantified by researchers as a maximum displacement of 3 mm [5] and minimum ultimate strength of 250 N [7, 11, 12]. Since there is no clinical consensus regarding which type of fixa-tion is optimal, researchers continue to study biomechani-cal properties to understand the performance and potential impact on patient outcomes.

Cortical fixation has become increasingly popular since the initial release of Smith and Nephew’s Endobutton in the early 1990s [4], as it offered strong and rigid fixation. The evolution of this “fixed” loop technology leads to the devel-opment of “adjustable” loop buttons that provided sufficient performance and allowed more graft to be pulled into short femoral tunnels. This supports the increasingly popular

Abstract Purpose Adjustable cortical fixation has gained popu-larity recently for ACL reconstruction; however, one con-cern with these types of devices is the potential for laxity occurring post-operatively. An indicator of clinical laxity is cyclic displacement during bench-top mechanical testing. The hypothesis was that the cyclic displacement and maxi-mum strength of different adjustable buttons currently on the market would vary depending on their mechanism of fixation.Methods Three devices were studied: Biomet’s Toggle-Loc with ZipLoop Technology (ZL), Arthrex’s TightRope RT (TR), and DePuy Mitek’s RIGIDLOOP Adjustable (RLA). Each was tested in isolation on a servohydraulic test machine. The implants were pre-conditioned from 5 to 67 N for 10 cycles and then tested from 50 to 250 N for 1000 cycles at 1 Hz. Following cyclic loading, a load to failure test was conducted at a rate of 20 mm/min. Total dis-placement was calculated from the 1st to the 1000th cycle, and ultimate strength and failure mode were recorded.Results Results showed that the RLA had significantly lower displacement versus both the TR (p = 0.012) and the ZL (p < 0.001). The TR also showed significantly lower displacement than the ZL (p < 0.001). The RLA and the ZL were both significantly stronger than the TR (p < 0.001).Conclusions Both the RLA and TR showed clinically acceptable amounts of cyclic displacement and maximum

* David B. Spenciner dspencin@its.jnj.com

1 DePuy Synthes Mitek Sports Medicine, 325 Paramount Drive, Raynham, MA 02767, USA

2 Foundry Orthopedics and Sports Medicine, 285 Promenade Street, Providence, RI 02908, USA

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“anteromedial” technique where surgeons are attempting to achieve a more anatomic repair with low anteromedial portals. A common concern with adjustable buttons, how-ever, is the construct laxity that may occur post-operatively, with some data supporting a difference between fixed loop and adjustable loop devices [1] and other data suggesting equivalence [3]. Since the various adjustable devices use different loop adjustment mechanisms, the clinical ration-ale was to better understand the effect of these mechanisms on cyclic displacement. This parameter serves as an indi-cator for in vivo laxity. Therefore, the hypothesis was that the cyclic displacement and maximum strength of differ-ent adjustable buttons currently on the market would vary depending on their mechanism of fixation. To our knowl-edge, the three commercially available adjustable loop but-ton constructs have not been compared in a single study previously.

Materials and methods

The three devices tested were the ToggleLoc with ZipLoop Technology (Biomet Warsaw, ID) (ZL), TightRope RT (Arthrex Naples, FL) (TR), and RIGIDLOOP Adjustable (DePuy Mitek Raynham, MA) (RLA) (Fig. 1). A test method similar to what has been reported in the literature [6, 13] was used to evaluate the biomechanical performance of each device in isolation (i.e. to measure implant performance inde-pendent of tissue). In short, each implant (six samples per device type) was loaded onto a servohydraulic test machine (Instron 8871, Norwood, MA) through a metal test plate sub-merged in a 37 °C saline bath. A shackle hook was placed through the implant loops and secured to the test fixture. Each implant was then adjusted to a 20-mm loop length per the manufacturer’s prescribed technique. The implants were pre-cycled ten times from 5 to 67 N to simulate intra-oper-ative cycling of the knee before cutting the adjusting suture tails with a standard suture cutter. Next the implants were cyclically loaded from 50 to 250 N at 1 Hz for 1000 cycles before pulling each to failure at a rate of 20 mm/min. Cyclic displacement was calculated as the total amount of slip that resulted in each device from the 1st to the 1,000th cycle. The maximum strength and failure mode were also recorded.

Statistical analysis

The total displacement and strength data were compared using a one-way ANOVA, and a student’s two-sample t test was used for direct comparison (p = 0.05 set a priori).

Using a sample size calculator for a two-sample t test (MiniTab Inc., State College PA), it was estimated (with

80 % power) that six samples per group were sufficient to detect a difference of 0.3-mm cyclic displacement. This represents 10 % of the estimated clinical limit of laxity.

Results

The data for all three groups are summarized in Table 1. One ZL sample was unable to be tested.

For cyclic displacement, there was a statistically signifi-cant difference among the groups (p < 0.001). Specifically, RLA was significantly lower than both TR (p = 0.012) and TL (p < 0.001). Similarly, the TR had significantly less displacement than the ZL (p < 0.001). Interestingly, both TR and ZL utilize one type of fixation mechanism (finger traps), while RLA utilizes a second type (knot). For ulti-mate strength, all implants exceeded the clinical require-ment by at least threefold. Both RLA and ZL were sig-nificantly stronger than TR (p < 0.001). Failure modes included suture break, tail slip, and knot slip.

Fig. 1 Image showing each adjustable cortical button (untested). From left to right: TightRope RT, ToggleLoc with ZipLoop Technol-ogy, and RIGIDLOOP Adjustable

Table 1 Summary of cyclic displacement and strength data for RIG-IDLOOP Adjustable (RLA), TightRope RT (TR), and ToggleLoc with ZipLoop Technology (ZL)

Implant (sample size) Average cyclic dis-placement (stdev)

Average strength (stdev)

RLA (n = 6) 0.88 mm (0.14) 1743.2 N (54.8)

TR (n = 6) 1.13 mm (0.15) 790.2 N (100.0)

ZL (n = 5) 2.12 mm (0.16) 1161.7 N (98.2)

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Discussion

The most significant finding of this study was realization that the mechanism of action by which these devices limit laxity may influence the amount of cyclic displacement observed during testing. However, it is unknown whether isolated cyclic displacement tests are a direct indicator of post-operative laxity.

The results reported here (cyclic displacement and strength) as well as the described failure modes are in agreement with data reported elsewhere that showed con-tinued displacement of TR, even beyond the 1000 cycles that were reported in this study [6]. The cyclic displace-ment and strength values in this study are also quite simi-lar to those reported by Petre et al. [13] who compared the performance of fixed loop constructs to that of adjust-able implants. In that study, it was found that the TR had cyclic displacement of 1.10 ± 0.20 mm and that the ZL had cyclic displacement of 2.18 ± 0.17 mm, both of which were more than two times the total displacement of the Endobutton (fixed loop). Additionally, Petre et al. found that the ultimate strengths of the TR and ZL were 841 ± 55 and 1561 ± 112 N, respectively. Johnson et al. [8] tested both fixed loop and adjustable loop designs by cycling at higher loads. Without retensioning the adjust-ment mechanisms, the total cyclic displacement for TR and ZL were 2.20 ± 0.62 and 3.69 ± 2.39 mm, respectively. The fact that those values exceed the displacement val-ues in this study is not surprising due to the higher maxi-mum loads employed by Johnson; however, the rank order remains the same. Additionally, Johnson et al. found that the ultimate strengths of the TR and ZL were 784 ± 45 and 1995 ± 217 N, which is consistent with the results of this study.

Both ZL and TR devices rely on a “finger-trap” lock-ing mechanism where one end of the adjusting suture is spliced through the centre of the other suture end. When a load is applied in the opposing direction, the finger trap collapses around the inner suture, catching it and holding it in place. Failure of this type of mechanism may occur from small micro-motions that gradually disengage the col-lapsed finger trap. In contrast, RLA devices are designed with a one-way sliding–locking knot that wedges into the metal implant to resist disengagement. The tighter the knot is cinched, the better the performance. Failure of this mech-anism may therefore occur if the knot is left loose.

It is widely recognized that low cyclic displacement and high ultimate strength are necessary features of any device used to secure a reconstructed ACL. While adjustable cor-tical buttons are increasingly popular for ACL reconstruc-tion due to their performance and ease of use with shorter femoral tunnels (resulting from low anteromedial portals), post-operative laxity remains a concern for some users. The

results from this study showed that the “knot-based” design outperforms that of the “finger-trap” designs for both cyclic displacement and strength.

One limitation of this study was that the test set-up did not include tissue. While isolated device testing is necessary to compare different designs directly, a more clinically relevant test method would include the use of either human or animal tissue (for both bone and grafts) in order to observe confounding effects. Additionally during this study, the orientation of the button relative to the direction of loading was always perpendicular. Again, this is not necessarily clinically relevant as the direc-tion of loading in vivo is dependent on factors such as surgical technique, tunnel orientation, and bone surface geometry.

Conclusion

Although a knot-based adjustment mechanism seems to offer mechanical advantages over finger trap-based adjust-ment mechanisms, both the RIGIDLOOP Adjustable and TightRope RT showed clinically acceptable amounts of cyclic displacement and maximum strength. The clinical relevance is that cyclic displacement may correlate with post-operative laxity, which ideally should be kept to a minimum.

Acknowledgments Adam Gustafson for conducting implant testing.

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