To the best of our knowledge, this is the first study to investigate and compare the outcomes of modified hybrid tibial fixation (combining adjustable suspensory device and bioabsorbable interference screw) in ACL reconstruction. A major highlight of this study was that patients who underwent modified hybrid tibial fixation showed less anterior knee irritation and tibial tunnel widening in the LAT view radiographs compared with patients who underwent traditional hybrid fixation. Moreover, the functional scores and clinical examinations of the patients in both the groups showed similar results. This confirmed our hypothesis.
In terms of development of TW, it was considered to be multifactorial in previous studies. Micromotion between the graft and bone interface, early rehabilitation, synovial fluid infiltration, and misplaced graft could all lead to a higher incidence of TW [15–18]. The type of fixation was considered one of the most important factors for tibial TW, and thus previous studies have compared all types of fixations to determine the optimal type [15–18]. With regard to suspensory devices, two commonly observed phenomena with fixed suspensory devices were the “bungee cord effect” and the “windshield wiper effect,” secondary to the longitudinal motion and transverse movement created by the gap between the graft and the fixation, respectively [15, 19]. Many studies have reported that a greater gap would lead to a greater TW, and therefore adjustable suspensory devices were introduced to overcome this deficit [6, 20, 21]. Although, in theory, adjustable suspensory devices could diminish the disadvantage of fixed suspensory devices, Choi et al. reported no significant difference between these two types of devices in terms of tunnel enlargement as well as clinical outcomes . In addition, Bressy et al. reported insufficiency of tibial graft stability when only adjustable suspensory devices were used .
In traditional hybrid fixation, interference screws present some well-known disadvantages such as migration, loosening, cyst formation and TW. These might be attributable to the less dense structure of the proximal tibia [8, 19]. Thus, the cortical screw post was frequently applied to augment the stability and strength. Indeed, hybrid tibial fixation did result in stronger initial fixation and less knee laxity compared with interference screw alone; yet, this method did not yield significantly better clinical results [9, 23]. Our modified method was proposed to afford the advantages of both fixation methods and reduce the subsequent complications. As the interference screws had been reported to be associated with graft migration and loosening, we secured the graft by adding adjustable suspensory device to the tibial side, which could reduce the possibility of graft migration; furthermore, the “bungee cord effect” and the “windshield wiper effect” might be decreased owing to less direct graft-to-bone contact and micro-movement owing to the barrier created by the surrounding interference screw.
TW is more apparent in the femoral tunnel than in the tibial tunnel, as reported in previous studies [19–21]; however, the current study revealed that the evident TW could happen in the tibial tunnel, even with hybrid fixation. As the development of TW is multifactorial, not yet fully clarified, and inevitable in most cases , we emphasize the importance of tibial hybrid fixation for its double guarantee and safety for accelerated rehabilitation. Kawaguchi et al. reported that anatomic double-bundle ACL reconstruction resulted in less TW in the femoral tunnel , and our study found that anatomic single-bundle ACL reconstruction using the anteromedial trans-portal technique also yielded stable outcomes in most cases.
Further, we observed that three out of the four patients with TW from the traditional hybrid fixation group were female (age range, 40–48 years), and none of them reported that they were exercising regularly in the past. Although a previous study has reported that the transtibial technique could cause more damage to the bony structure than the inside-out method could , all patients included in this study had received the same transtibial technique, which potentially eliminated this concern. Thus, the lifestyle and the natural process of bone loss among middle-age women might weaken the structure in the proximal tibia, presumably leading to greater percentage of TW compared with other patients. Meanwhile, we attributed the percentage of TW to the loosening of the suture. This could have resulted from the cutting off by the sharp margin of the cortical screw or even the tibial tunnel opening, consequently leading to instability between the graft and the interference screw [10, 11]. By contrast, the modified method seemed to overcome this issue by replacing the cortical screw post with an adjustable suspensory device. Moreover, considering the routine usage of four stranded autografts with gracilis and semitendinosus, this technique can be used to obtain grafts of sufficient diameter but possess the potential risk of notch impingement for the narrower notch, especially in female patients. Some studies have reported that the notch impingement could account for the TW [12, 25]; hence, more attention should be paid to notchplasty during the procedure. The visualization of TW only in the LAT view might be attributable to the application of a more anterior translation force than the rotatory force for the tibia under the weight-bearing activity after anatomic single-bundle ACL reconstruction.
Despite the statistical difference in percentage TW, we observed no significant difference in clinical outcomes such as functional scores and clinical examinations between patients who underwent modified and traditional fixation. Our results were compatible with those of many other studies [4–7, 9], indicating that TW had no clear correlation with clinical outcomes. Nevertheless, patients who underwent modified hybrid fixation showed less anterior knee irritation. Previous studies have reported anterior knee irritation with the use of cortical screw post in traditional hybrid tibial fixation [10, 11]. Thus, our modified hybrid fixation method could be considered as an option to improve the quality of life of patients scheduled to undergo ACL reconstruction in the future.
This study has some limitations. Firstly, the patients included in the study were not randomized. The composition of patients might potentially confound the results. As there was no blinding in the study, the preference of examiner and the expectation of patients could influence the assessment. We had had the examiners blinded for physical examination and performed repeatability test for measurement of tunnel widening in order to decrease the influence. In addition, sex distribution showed a significant difference between the groups. Most of the patients in the traditional fixation group as well as most patients who experienced TW were female. Whether there was any correlation between sex and percentage of TW was unclear.
In addition, we had only documented most of our functional scores and clinical examinations after the operation. Any significant difference in these results failed to reflect improvement or regression in each patient but could only indicate the postoperative difference between the groups resulting from the different operative methods used, as no previous measurement could be referred to. Moreover, we only documented the TW right after the operation and at least 1 year after the operation. Hence, we could not determine the long-term influence of each operative technique on TW. Nonetheless, as stated in previous research, our course of follow-up yielded adequate results, as the majority of tunnel change occurred within 6 months after the operation [13, 14].
Another limitation was that we did not use the KT-1000 or KT-2000 arthrometer for objective evaluation. We focused on postoperative TW, and previous studies have also recommended that the KT-1000 arthrometer be used as a diagnostic tool only, as it is unsuitable for use as an outcome tool .
Moreover, radiography was used instead of computed tomography (CT) in this study to measure TW. A previous study reported that CT could provide more accurate and reliable measurements of TW ; yet, several studies have reported that radiography could yield acceptable results [6, 28, 29]. Further, postoperative CT was not a routine clinical practice allowed by Taiwan National Health Insurance for follow-up of ACL reconstruction. Consequently, we decided to evaluate the imaging changes through radiographs instead.