Demographic data for participants and meniscus quality

The participants’ characteristics are presented in Table 1. The mean waiting period was 114.8 ± 55.8 days. For patients in the ACL group, the HKAA was 0.6 ± 1.9° at baseline. A few restricted ranges of motion were observed at baseline and postoperatively (extension: -2.6 ± 5.2° preoperatively, -1.7 ± 2.8° postoperatively; flexion: 134.7 ± 4.8° preoperatively, 138.2 ± 3.4° postoperatively).

Regarding meniscal quality, seven patients experienced complications of medial meniscal injury. Six complications were longitudinal tears in the posterior section and one was in the middle section of the medial meniscus. Seven patients experienced complications due to lateral meniscal injury. Four patients had longitudinal tears in the posterior section, and three patients had lateral tears in the middle or posterior horn of the lateral meniscus. Therefore, there was no patient with meniscus posterior root tear.

Meniscus extrusion in the supine position on MRI and ultrasonography

On MRI, MME did not significantly differ between examinations (1.7 ± 1.2 mm preoperatively, 1.8 ± 1.5 mm postoperatively). Moreover, LME also showed no significant difference (-1.7 ± 2.3 mm preoperatively, -1.8 ± 2.1 mm postoperatively).

On ultrasonography, MMEs and LMEs did not show significant differences between the groups or pre- and postoperatively on each side (MME: 1.9 ± 1.4 mm preoperatively, 2.1 ± 1.1 mm postoperatively, 1.5 ± 0.8 mm for control; LME: -0.3 ± 1.3 mm preoperatively, 0.3 ± 1.1 mm postoperatively, 0.2 ± 0.9 mm for control) (Figs. 4, 5a).

Fig. 4

The extrusion of medial meniscus parameters on ultrasound. Preoperative and postoperative values are shown. MME on ultrasound images in the supine a, standing b, and walking c positions. The meniscal extrusion behavior during walking is shown (d). These values are presented as mean ± standard deviation, with * denoting a significant difference between groups or at follow-up (p < 0.05)

Fig. 5

The extrusion of lateral meniscus parameters on the ultrasound. Preoperative and postoperative values are shown. LME on ultrasound images in the supine (a), standing (b), and walking (c) positions. The meniscal extrusion behavior during walking is shown (d). These values are presented as mean ± standard deviation, with * denoting a significant difference between follow-up visits (p < 0.05)

Standing and walking meniscus extrusion using ultrasonography

Under the standing condition, postoperative MME was significantly higher than that at baseline, but not that in the control group (MME: 2.1 ± 1.4 mm preoperatively, 2.6 ± 1.3 mm postoperatively, 1.9 ± 0.7 mm for control; preoperative vs postoperative: p = 0.015) (Fig. 4b). LME was not significantly different between groups or at follow-up (– 0.5 ± 1.5 mm preoperatively, – 0.6 ± 1.7 mm postoperatively, 0.1 ± 0.9 mm for control) (Fig. 5b).

Under the walking condition, postoperative maximum MME was significantly higher than that at baseline, but not that in the control group (2.2 ± 1.0 mm preoperatively, 2.7 ± 1.2 mm postoperatively, 2.3 ± 0.6 mm for control; preoperative vs postoperative: p = 0.015). The postoperative behavior of the MME was significantly better than that at baseline and in the control group. Moreover, the behavior of the MME at baseline in the ACL group was significantly higher than that of the control group. (0.9 ± 0.3 mm preoperatively, 1.2 ± 0.4 mm postoperatively, 0.5 ± 0.2 mm for control; preoperative vs postoperative: p = 0.019; preoperative vs control: p = 0.001; postoperative vs. control: p = 0.001) (Fig. 4c, d). In contrast, the maximum postoperative LME was significantly lower than that at baseline, but not that in the control group (0.8 ± 1.3 mm preoperatively, 0.2 ± 1.4 mm postoperatively, 0.3 ± 0.9 mm for control; preoperative vs postoperative: p = 0.03). The behavior of LME was not significantly different between groups or at postoperative follow-up (0.5 ± 0.4 mm preoperatively, 0.4 ± 0.3 postoperatively, 0.3 ± 0.5 mm for control) (Fig. 5c, d).

Time-course change in kinematics and kinematics during walking

In the ACL group, the knee angles, moments, and GRF did not show significant differences between ACL examinations (Table 2).

Table 2 Between-group comparisons of kinetic and kinematic parameters during walking

At both baseline and postoperatively, the knee flexion angles were significantly higher than those in the control group (15.2 ± 6.4°preoperatively, 17.9 ± 3.8°postoperatively, 9.5 ± 4.7 for control; preoperative vs control: p = 0.007; postoperative vs control: p = 0.001). The knee extension angle was significantly lower than those in the control group (knee extension angle: – 11.0 ± 7.7°preoperatively, – 9.0 ± 5.4°postoperatively, – 1.4 ± 3.8° for control; preoperative vs control: p = 0.001; postoperative vs control: p = 0.001). The knee extension moment at baseline was significantly lower than that in the control group (knee extension moment: – 0.1 ± 0.2 Nm/kg preoperatively, – 0.1 ± 0.2 Nm/kg postoperatively, 0.01 ± 0.2 Nm/kg for control; preoperative vs control: p = 0.037; postoperative vs control: p = 0.057). (Table 2).

Comparison of meniscus dynamics between subgroups with meniscus repair

In the subgroup with medial meniscal repair, no significant difference was observed in MME parameters between baseline and postoperative values. On the other hand, in the subgroup without meniscus repair, the behavior of MME only during walking was higher postoperatively than that at baseline (0.8 ± 0.2 mm preoperatively, 1.2 ± 0.3 mm postoperatively, p = 0.001). Regarding LME, in the subgroup without meniscus repair, the behavior of LME only during walking was lower postoperatively than that at baseline (0.6 ± 0.3 mm preoperatively, 0.3 ± 0.3 mm postoperatively, p = 0.016).

Time-course change in kinematics and kinematics during walking in meniscus repair subgroup

In the subgroup without meniscus repair, initial and maximum knee flexion angles were greater postoperatively than those at baseline (initial knee flexion: 6.0 ± 7.3° preoperatively, 11.4 ± 4.7° postoperatively, preoperative vs postoperative: p = 0.014; maximum knee flexion: 13.4 ± 6.7° preoperatively, 17.4 ± 4.2° postoperatively, preoperative vs postoperative: p = 0.021), but not those in the meniscus repair subgroup.

Kinematics and kinematic parameters did not differ significantly between the subgroups at baseline. On the other hand, postoperatively in the subgroup without medial meniscus repair, initial knee flexion angle was greater than that in the subgroup with meniscus repair (without meniscus repair: 11.4 ± 4.7°, with meniscus repair: 6.7 ± 3.7°, p = 0.015), and the range of motion occurring in the early stance phase also exhibited a lower value than that in the ACL reconstruction group with meniscus repair (without meniscus repair: 5.9 ± 1.8°, with meniscus repair: 12.0 ± 3.0°, p = 0.002) (Table 3).

Table 3 Comparison of kinetic and kinematic parameters during walking among subgroups