Background
Among the current methods of anterior cruciate ligament reconstruction (ACLR), single-bundle(SB) reconstruction is performed by most surgeons [
1,
2]. During single-bundle ACLR, femoral tunnel location plays an important role in restoring the intact knee mechanisms, whereas malplacement of the tunnel was cited as the most common cause of knee instability [
3‐
5]. As a result, the best location of the femoral tunnel during single-bundle ACLR is subject to extensive exploration with the development of anatomic studies [
6‐
9]. There are mainly two methods for femoral tunnel creation: transtibial versus anteromedial portal techniques. The current femoral tunnel preparation focus has shifted from the TT method(with femoral tunnel location at the “over-the-top” position approximately 11 o’clock in the femoral notch of the right knee) toward the independent drilling method(with the femoral tunnel location at the center of the AM bundle of the ACL footprint approximately 9 o’clock in the femoral notch of the right knee) with restoration of the native ACL knee kinematics. The use of the AM portal drilling technique has increased in recent years from 68% of surgeons using this technique in 2013 [
1] to 89.6% in 2016 [
2]. Although the anatomic single-bundle ACLR procedure is currently in use, it remains controversial whether the AM technique is biomechanically superior compared with the TT method. Investigators focusing on the femoral tunnel position have shown improvements in knee stability by placing the femoral tunnel into the native femoral footprint [
5,
10‐
12]. However, not all of the results supported the advantages of anatomical reconstruction. Other studies have demonstrated that no significant knee kinematic changes were found between the TT versus AM portal drilling techniques [
13‐
15].In addition, two meta-analyses showed that there were no significant clinical differences found between the two techniques [
13,
16]. As a result of these conflicting outcomes, the best technique of femoral tunnel creation for restoring intact knee kinematics remains unclear. Therefore, the purposes of this study were (1) to quantify the effect of two femoral tunnel creation methods on the tibiofemoral joint contact area and stress after single-bundle ACL reconstruction, (2) to identify the optimal femoral tunnel creation method, and (3) to give new evidence to the present conflicting results. The hypothesis was that SB ACLR by the use of the AM portal method would better restore the intact tibiofemoral contact area and stress compared with the TT method.
Discussion
In this study, the tibiofemoral joint contact area and stress of the knees after ACL reconstruction by the AM portal and TT techniques were measured and compared. Specifically, the experimental data collected from the same cadaveric knee specimen under different experimental conditions (ACL-intact and ACL-reconstructed with TT and AMP methods) reduced the effect of interspecimen variation [
5]. The results supported that SB ACL reconstruction via the AM portal technique restored the tibiofemoral joint contact area and stress more closely to the intact knee than SB ACL reconstruction via the TT technique. Until now, there have been few studies comparing the changes in tibiofemoral contact mechanics between TT and AM portal ACLR groups to the authors’ knowledge. In one prior study, Lee et al. [
25] investigated the contact area and stress in knees with serial posterior medial meniscectomies. In Lee et al., the outcomes of contact area, mean contact stress, and peak contact stress, which were measured in intact knees, were similar to our studies. Another prior study has evaluated the effects of SB ACLR and double-bundle ACLR. In that study, Morimoto et al. [
26] tested knees after undergoing SB ACLR and double-bundle ACLR and pointed out that SB ACL reconstruction resulted in a significantly smaller tibiofemoral contact area and higher stress. The peak stress and contact areas measured in their study were also comparable to our data in intact ACL knees, while the mean contact stress reported is higher than our outcomes. This difference may be explained by the type of stress-sensor used in the joint space. And the various experimental conditions and methods for measuring knee contact stress made the comparisons between studies complicated. In their study, the TT or AM portal method was used for femoral tunnel placing. However, in our experience, the femoral tunnel position cannot be placed at the center of the AM bundle position via the TT method. Nevertheless, our study confirms the observation that the SB ACLR condition resulted in decreasing contact area and increasing mean tibiofemoral contact stress and peak contact stress compared with the intact knee.
The alternation of the tibiofemoral joint contact area and stress in reconstructed knees may be caused by the mismatch of the tibiofemoral joint during knee movement procedures compared with intact knees. And the reconstructed ACL, which cannot provide original biomechanics compared with the original ACL, may have resulted in the mismatch of the tibiofemoral joint. Anatomic studies of the ACL indicated that the ligament consists of two grossly distinguishable components: the anteromedial (AM) bundle and posterolateral (PL) bundle [
27,
28]. Comparing the in situ forces between the two bundles, the PL bundle has higher in situ forces from full extension to 30° of flexion, whereas the AM bundle has higher in situ forces from 30° of flexion to further flexion under anterior tibial loads [
29]. The PL bundle also shows an important role, especially at lower flexion angles under rotatory loads [
30]. Such anatomic complexity of the ACL cannot be restored by non-anatomic SB ACLR, which may alter the tibiofemoral joint matching relationship during knee movement procedures and results in a significant alternation of the tibiofemoral joint contact area and stress.
The alternation of the tibiofemoral joint contact area and stress between SB ACL- reconstructed knees via TT versus AM portal drilling techniques may be caused by different femoral tunnel positions. Previous studies indicated that the tunnel location plays an important role in ACL reconstruction, and small variations in the femoral tunnel placement significantly influence the resulting knee kinematics and clinical outcomes [
10,
22,
31‐
33]. Biomechanical studies using cadavers indicated that the AM portal technique placed the femoral tunnel more closely to the anatomic femoral footprint [
24,
34,
35], and this may be the reason why SB ACLR by the use of the AM portal method more closely restores the intact tibiofemoral contact area and stress compared with the TT method. There are numerous studies that indicated that the AM portal technique reconstruction provided better rotatory stability at low flexion angles [
36‐
38] without sacrificing anteroposterior stability compared with TT ACL reconstruction [
10,
39‐
42]. Guler et al. [
34] and Lee et al. [
43] evaluated the femoral tunnel positions created by the AM portal or TT technique in their study and indicated that the AM portal technique is superior to the TT technique in terms of anatomical graft positioning. In a meta-analysis, Riboh et al. [
16] reported that there are biomechanical data suggesting improved knee stability and more anatomic graft placement with independent drilling. This literature may also help to explain the superiority of the AM portal technique reconstruction, which better restored the normal knee kinematics, resulting in closer normal contact area and stress, when compared with TT technique reconstruction.
Other studies also have shown that the ACL reconstruction by the use of the TT technique could not effectively prevent the prevalence of secondary knee OA [
44‐
48]. Leiter et al. [
49] shown in their meta-analysis that ACL-reconstructed knees using the TT technique had a higher incidence of normal and serious OA than control knees, especially in patients combined with medial meniscus repair or excision. Hart et al. [
48] reported in their article that the patellar tender ACL reconstruction using the TT method did not lead to prevention of the occurrence of radiological OA after 10 years by the use of the Kellgren and Fairbank classifications. Janssen et al. [
50] found that the radiological signs of OA were detected in 53.5% of the patients with transtibial ACL reconstruction using four-strand hamstring autograft at the 10-year follow-up. However, all of the studies mentioned above were based on the TT technique of ACL reconstruction. With regard to ACL reconstruction using the AM portal method, there are some studies that indicated that anatomic ACL reconstruction showed favorable results regarding OA [
51,
52]. Alentorn-Geli et al. [
13] indicated in their study that patients in the TT ACLR group had greater long-term knee osteoarthritic changes (greater space narrowing) compared with the AM portal ACLR group when the radiographic parameters were statistically analyzed with a KT-1000 arthrometer. According to Wolff’s law, osteoclasia and bone resorption may be triggered by a low bone stress and an overloading bone stress [
53]. Therefore, the ACL reconstructed knee with altered contact area and stress may result in undesirable bone remodeling and predisposition of the knee joint, which finally lead to the occurrence of OA. In this study, ACL reconstruction using the AM portal technique better restored the normal tibial-femoral contact area and stress when compared with the TT technique and may help to explain the favorable results regarding the OA after AM portal technique ACL reconstruction. However, there are too few studies to confirm whether the ACL reconstruction using the AM portal technique will better prevent the occurrence of knee arthritis, and long-term clinical follow-up studies are necessary to verify our hypothesis.
Limitations
As for the limitations of this study, the number of cadaveric knees used in the experiment was relatively limited, and the donor age and specimen tissue quality were variable. Another limitation of this study is the reuse of the specimens, both the grafts and cemented femoral condyles. Reusing the graft after it has already been tested and fixated on the tibia via screws may have led to some compromising of the tissue itself. The additional cycling will also introduce some additional creep between the tests. Besides, due to the lack of freedom of the varus/valgus moment, a slight deviation of varus/valgus positioning may have resulted in an unequal load between the medial and lateral compartments when putting and positioning the knee into the knee simulator. Moreover, this controlled laboratory experiment cannot simulate muscle load, and we conducted an extensive soft tissue dissection to the posteromedial capsule in order to insert the Tekscan stress sensors, which may be different from the in vivo research. Although all of the conditions mentioned above may affect the knee joint contact area and stress, the conclusions of this study remain valid; the main purpose was to observe the biomechanical variations of the two ACLR conditions within each specimen.