The most important finding of this study was that anatomical ACLR exhibited significant higher peak knee flexion and extension torques under the velocity of both 180°/s and 60°/s, as well as isometric flexion torque, as compared with non-anatomical ACLR at 6 months postoperatively, but no significant differences for all parameters were detected between the two techniques at 12 months postoperatively. Also, both groups showed no significant difference in clinical outcomes regarding of knee stability and subjective knee function.
In our study, the results implied that anatomical ACLR showed a superior muscle strength as compared with non-anatomical ACLR at 6 months after surgery. However, there were no differences on muscle strength between the two groups at 12 months after surgery. The possible reasons may be as follows. Knees with non-anatomical ACLR potentially led to asymmetric knee kinematics and alteration of cartilage contact pattern [24, 25]. Yan et al.  compared anatomical and non-anatomic ACLR on gait kinematics with minimal 6-month follow-up, finding that operated knees with non-anatomical ACLR exhibited significant range of motion of anterior-posterior translation by approximately 0.5 cm than contralateral knees. Graft healing may be affected by tunnel position as well. Oshima et al.  reported low femoral tunnel was one of the factors significantly associated with high graft signal/noise quotient value, which indicated inferior graft healing. On the other hand, the study of Novaretti et al.  proved that deficit of quadriceps strength did not predict return to preinjury level of sport at 6 months postoperatively, which were consistent with the outcomes of 12 months after surgery in our study.
Strength recovery after ACLR is of great importance for patients who want to return to sport, especially athletes [14, 15]. Muscle strength may also have correlations with knee function. Wang et al.  follow 88 patients who underwent double-bundle hamstring ACLR and performed a second-look arthroscopy at an average of 24 month postoperatively, finding that greater than 80% recovery of quadriceps strength after ACLR is associated with less severe patellar cartilage damage. In the study of Palmieri-Smith et al. , 73 patients were tested at the time they were cleared for return to activity after ACLR. The results indicated that patients with high and moderate quadriceps strength symmetry had larger central activation ratios as well as greater limb symmetry indices on the hop for distance compared with patients with low quadriceps strength symmetry. Similarly, knee flexion angle and external moment symmetry were higher in the patients with high and moderate quadriceps symmetry compared with those with low symmetry. However, Thomeé et al.  believed that muscle function tests were not demanding enough or not sensitive enough to identify differences between injured and non-injured sides. More studies with long-term follow-up are required to validate the influence of muscle strength after ACLR.
Tunnel preparation is the most important procedure in ACLR, especially in femoral side. According to a multicenter study with the largest collected data of ACL revision, the malposition of the tunnel socket accounts for most of technique errors, which are the main cause of atraumatic ACLR failure . Femoral tunnel malposition is 3 times more frequent than tibial tunnel malposition . In our study, the femoral tunnel position was measured with the use of Bernard quadrant method, which was applied in several studies [5, 27]. The anatomic position of the femoral tunnel socket for single-bundle ACLR is defined in line with the study of Xu et al. . They systemically reviewed 13 studies of the ACL femoral footprint position and combined data, concluding that the standard area of femoral footprint of the ACL as a whole bundle is a circle with a center of 27.53%, 35.85% (x, y), and a radius of 4.58%, 9.2% (x, y), respectively. Different measurements were used in other researches [34, 35]. Forsythe et al.  measured the femoral tunnel aperture from posterior to anterior position (percentages of the distance from the line running through the posterior border of the medial wall of the lateral condyle to the line running through the most anterior point of the notch) and proximal to distal position (percentages of the distance from the line running through the proximal border of the notch to the line running through the distal point of the notch roof), finding that anatomic range of femoral tunnel aperture center fell between 8.9% and 36.4% of the distance from posterior to anterior position and 20.1% and 73.1% from proximal to distal position. Future studies with uniform measurement are required to make a convincing result.
As compared with traditional TT drilling technique, AM drilling technique for ACLR was more likely to achieve an anatomical femoral tunnel. However, in this study, about 54.17% (39/72) included patients had non-anatomical femoral tunnel position. Jaecker et al.  analyzed 101 cases of ACLR failure and found 37 (36.6%) used AM technique. There were 73% non-anatomical femoral tunnel positions and 35.1% non-anatomical tibial tunnel positions in those patients with AM drilling techniques. Yan et al.  studied the gait kinematics of ACLR with anatomical femoral tunnel and non-anatomical femoral tunnel. They enrolled 34 patients undergoing ACLR with AM drilling technique, finding 21 (61.76%) had non-anatomical femoral tunnel positions. Despite the potential of achieving more anatomical tunnel position, AM drilling technique may lead to quite a few non-anatomical tunnel positions.
Several studies research the correlation of tunnel positioning with short-term functional outcomes after ACLR. Fernandes et al.  prospectively studied 86 athletes who underwent ACLR between anteromedial footprint and high anteromedial position. They observed that tunnel projection along Blumensaat's line was correlated with functional outcomes on Tegner scale at 6 and 12 months and IKDC subjective at 12 months. There was a significant difference in mean tunnel projection along Blumensaat's line when analyzing return to sports. No differences were found on coronal view. Biswal et al.  found that more posterior positioning of femoral tunnel resulted in better clinical scores. On the other hand, Seo et al.  classified 102 patients into the anterior group, center group or posterior group depending on the location of the femoral tunnel. Center group was defined as location of femoral tunnel was 29.3% ± 3.5% on anteroposterior plane. They found no correlation was found between the location of femoral tunnel and postoperative stability and clinical outcomes.
The current study has several limitations. First, the sample size was small, and the length of follow-up was relative short, which limited assessment on long-term complications and secondary treatment. Second, reported standard area for anatomical ACL footprint rather than the contralateral normal ACL footprint was used for the determination of tunnel placement. Third, only knee flexion and extension torques at velocities of 60°/s and 180°/s were studied. Advanced Isokinetic test under different movements of knee and velocities should be further evaluated. In addition, the study failed to randomize the groups initially as the grouping was performed after the surgery, which increased the confounding risk of patient selection. Lastly, tibial tunnel position, graft sizes, conditions of meniscal injuries and their treatment manners may have affected the outcomes as well.