The most important finding of this study was that the improved cortical button was better able to retain suture breaking strength and modulus of rigidity, regardless of the number of load cycles, in comparison to the original cortical button. ACL fixation is associated with these post-operative complications [6, 11]. Successful fixation of the ACL depends on whether the reconstructed ACL was fixed using appropriate tension and whether the strength of the fixture was adequate.
A literature review on the outcomes of suspensory fixation and interference screw fixation reported that the side-to-side difference (evaluated using the KT-1000) was smaller in patients who received suspensory fixation, that ligament tears were more frequent in those who received interference screw fixation and that there was no difference in the clinical International Knee Documentation Committee (IKDC) scores between the two types of fixation . In one study, histological evaluation of bone holes revealed that suspensory fixation provided more favorable tendon-bone healing than interference screw fixation .
On the other hand, Lubowitz et al. reported that there was no clinical difference between the two types of fixation when the all-inside technique was used . A study in dogs reported that suspensory fixation provided better tendon-to-bone fixation than interference screw fixation . A previous mechanical study revealed that strong fixation can be obtained by a combination of techniques in which the ACL is directly fixed with an interference screw and the sutures attached to the reconstructed ACL are fixed using suspensory fixation . However, all of these studies reported no clinically relevant difference between the fixation types.
While there are various types of suspensory fixation, there has recently been increased clinical use of an adjustable-loop device, a type of cortical suspensory device [15,16,17]. In procedures using suspensory fixation, the possible causes of ACL loosening include ACL fixation with low tension and slipping of sutures off the fixture over time.
Some mechanical studies that compared adjustable-loop devices and fixed-loop devices reported that the latter showed higher breaking strength and less displacement [18,19,20,21,22,23,24,25]. On the other hand, other studies reported no difference in either parameter [26, 27]. One clinical study reported no differences between the two types of devices in evaluations using the KT-1000 .
The EndoButton is a basic cortical suspensory apparatus that is categorized as a fixed-loop device, and the consensus based on mechanical studies is that such devices provide favorable fixation. Adjustable-loop and fixed-loop devices have the same shape but the artificial ligament component differs in terms of spinning methods and materials.
To ensure stable post-operative outcomes, it is important to establish a fixation technique that provides a high breaking strength and modulus of rigidity and reduces displacement. Hopefully, such a technique will enable all surgeons to fix a reconstructed ACL with the same degree of tension regardless of their skill and experience.
Therefore, considering that it is important to allow for ligation with tension applied to the suture in order to ensure tension of the reconstructed ACL, we developed a modified EndoButton whose structure prevents sutures from slipping on the button when a large load is applied on the ACL after surgery (Fig. 2b). The improved EndoButton has a specific feature that allows temporary fixation of the reconstructed ACL by simply applying a transverse force to sutures after they are passed through the button holes (Fig. 1b). Since the button holes are obliquely angled in relation to the button surface, temporary fixation can be obtained by contact between the suture and button. Since the button facilitates suturing while tension is maintained, it can reduce postoperative loosening of the ACL.
While the moduli of rigidity of the sutures of the improved EndoButton were almost the same regardless of the number of loading cycles, those of the sutures of the original EndoButton increased with higher numbers of load cycles. The test results show that the original EndoButton most likely causes slipping and loosening of sutures in the early postoperative period which represents after 500 loading cycles, resulting in loosening of the ACL, but the improved EndoButton demonstrates increased ability to retain sutures in the early postoperative period.
It has been reported that adjustable-loop devices have the advantage of providing all-inside fixation, but their mechanical strength is slightly inferior to that of fixed-loop devices [19, 25, 26]. We plan to evaluate and compare the mechanical strength of our improved Endobutton in combination with an adjustable-loop device and fixed devices.
In clinical settings, we should consider using a quantitative tension device for temporary fixation of reconstructed ACLs, and perform ligation after removing the tension device.
Since the improved EndoButton was associated with more favorable breaking strength and modulus of rigidity than the original one, we consider that it can be used clinically not only for ACL fixation on the tibial side during ACL reconstruction but also for fixation or ligation of other ligaments under tension during reconstruction procedures.
Regarding the strength of this study, we developed a new EndoButton that can fix a reconstructed ligament easily and securely by ligation while maintaining the appropriate ACL tension. The improved EndoButton was superior to the original EndoButton in terms of both the breaking strength of the sutures after 1000 loading cycles, and the modulus of rigidity of the sutures after 500 loading cycles.
This study could not directly predict clinical outcomes because it used a simplified model consisting of a suture and a fixation device, and did not use any biological bones or ligaments for mechanical evaluation. Future mechanical studies need to be conducted using femurs, tibiae and tendons of the lower extremities of animals such as swine to simulate ACL reconstruction procedures.
The potential clinical relevance of the present study is that the improved cortical button is a beneficial and easy-to-use ligament fixture for ACL reconstruction.