To the best of our knowledge, this is the first study to investigate the outcomes of ASIS for hybrid tibial fixation in ACL reconstruction. After adjustment for gender, a major highlight of this study was that patients in ASIS group showed less tibial tunnel widening in the LAT view radiographs compared with patients in CSIS group. Meanwhile, we noticed the post-operative functional scores were lower in ASIS group which was different from our hypothesis. Concerning physical exams and anterior knee pain, both groups showcased similar results.
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, selection of grafts and misplaced graft could all lead to a higher incidence of TW [18,19,20,21,22,23]. 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 [18,19,20,21]. 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 [18, 24]. 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, 25, 26]. 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 [6]. In addition, Bressy et al. reported insufficiency of tibial graft stability when only adjustable suspensory devices were used [27].
In hybrid tibial fixation using CSIS, 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, 24]. 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, 28]. As the development of TW is multifactorial, not yet fully clarified, and inevitable in most cases [8], we emphasize the importance of tibial hybrid fixation for its double guarantee and safety for accelerated rehabilitation. The ASIS 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.
Furthermore, we observed that three out of the four patients with TW from the CSIS group were female (age range, 40–48 years). Perimenopausal women were reported to possess lower bone mineral density [29, 30]. Perhaps osteopenia or even osteoporosis could be one of the risk factors which in turn lead to TW. Additionally, none of these patients with TW were reported to be regularly exercising in the past. 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. Furthermore, a previous study has reported that the transtibial technique could cause more damage to the bony structure than the inside-out method [31]; nonetheless, all patients included in this study had received the same transtibial technique, which potentially eliminated this concern. Nevertheless, we did not document the bone mineral density in our patients after operation as a routine practice. The correlation between bone density and TW would need further study to clarify.
Interestingly, women were more prone to ACL rupture, as stated in previous studies [32,33,34]. A recent meta-analysis conducted by Mok et al had evaluated what gender could play on the outcomes of ACL reconstruction and demonstrated that functional scores were better post-operatively in men while re-rupture rate was lower in women [35]. Nevertheless, the incidence of TW was not compared in this study. In the meantime, our sample size was not enough to generate a concrete conclusion. Further study would be necessary to clarify the correlation between gender and TW in the future.
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 ASIS 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 [15, 36]; 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.
Aside from the statistical difference in percentage TW, we had found significant difference in post-operative functional scores such as SANE score and Tegner activity level scale between the groups after adjustment for gender. We could also see the similar trend in IKDC score. Our results were similar to those of many other studies [4,5,6,7, 9] which stated TW had no clear correlation with clinical outcomes. Nevertheless, improvement in clinical outcomes within each group before and after the operation was discovered. This finding indicated both methods had provided adequate stability and strength for patients. In addition, we had discovered lower degrees of knee flexion prior to operation in ASIS group. The difference in pre-operative status might affect the post-operative results. As for anterior knee pain, despite the lower risk of developing pain in ASIS group, we did not observe significant difference between the groups. Previous studies have reported anterior knee pain owing to the use of cortical screw post in hybrid tibial fixation [10, 11]. Whether the use of ASIS in hybrid tibial fixation could relieve the pain caused by impingement was still unclear. Further studies are warranted to clarify the relationship.
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. We had applied regression models to cope with the confounding effect of the gender.
In addition, we had 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 [16, 17].
Another limitation was that we did not use the KT-1000 or KT-2000 arthrometer for objective evaluation. Physical examination such as Lachman test could not be precisely graded when there was no measurement instrument applied. Nevertheless, 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 [37].
With regard to the choice of imaging, 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 [18, 38]; yet, several studies have reported that radiography could yield acceptable results [6, 39, 40]. On the other hand, we did not conduct MRI on every patient after the operation. Complication such as re-rupture mainly relies on MRI for diagnosis. As a result, some patients with re-rupture might be missed. Our study could not clarify the correlation between TW and re-rupture. Despite these limitations, the present study may still contribute to provide an alternative choice for the tibial hybrid fixation in ACL reconstruction.