ACL rupture is now mostly advocated to be reconstructed arthroscopically using autologous or allograft tendons. After reconstruction, the graft underwent a biological healing process to become a mature graft . Graft healing consists of two regions of healing: intra-bone tunnel healing and intra-articular healing, in which there are three distinct stages of the intra-articular healing process of the graft, namely, early cellular necrosis, cellular regeneration and hematologic reconstruction, and graft remodeling [19, 20]. Early necrosis occurs within four weeks postoperatively and is characterized by graft necrosis and an abnormal decrease in cell numbers [21, 22]. Cellular regeneration and hematologic reconstitution begin at postoperative 4th –12th week  when the greatest changes in cellular activity and extracellular matrix occur. A certain degree of rehabilitation load is helpful to stimulate cellular regeneration and hematologic reconstitution. Still, due to the knowledge regarding mechanism underlying of the graft are limit at this stage, and excessive load should be avoided to damage the graft . The remodeling phase begins during the postoperative 12th week, and this process is a continuous transformation of graft morphology and mechanical strength to normal ACL.
Many factors influence the biological healing process of these grafts. The commonly known factors include the preservation of the ACL stump, preservation of the popliteal tendon stop, or the location of the bone tunnel-opening pattern, the type of graft, and bending angle. Some studies have concluded that the retention of the stump has a positive effect on graft synovialization and thus promoting graft maturity . However, it has been suggested that stump can increase the intra-articular volume causing intercondylar socket impingement, which causes an increased incidence of postoperative limited knee extension , so whether to preserve the stump for ACL reconstruction is controversial and requires further clinical validation. Liu et al  found that graft MRI signal intensity was relatively low and unchanged after ACL reconstruction with preservation of the popliteal stop, and Ruffilli et al  also concluded that grafts with stop preservation, had good morphology and better maturity. This may be because the retention of the stop can somewhat bypass the first two stages of biological healing and thus enhance the ligamentization process, but there are still relatively few studies, and the long-term efficacy needs to be demonstrated further. Graft bending angle can also influence early maturity , where the anatomical positioning of the femoral tunnel is an essential factor influencing the graft bending-angle.
In fact, the tunnel structure, graft fixation, and rehabilitation strategies may also have a critical combined effect on graft maturity. This study showed that the maturity of the middle region of the intra-articular graft after all-inside and anatomical reconstruction performed the worst, followed by the distal region. It may since the middle region of the graft is the main part subjected to distraction forces, and the distal region of the graft is close to the tibial tunnel that easily affected by the tibial tunnel. Because the tibial tunnel is the main difference between all-inside and anatomical reconstruction, and some studies believe that all-inside reconstruction cortical suspension fixation is prone to produce mechanical effects such as “bungee” and “wiper”  in the early stage, while the anatomical reconstruction with interference screw fixation is relatively firmer in the early stage, which may be the reason why all-inside reconstruction graft maturity is worse than anatomical reconstruction in the first six months after surgery. However, the unique “bone socket” structure used in all-inside reconstruction may be the major reason for the trend of getting worse first then getting better of graft maturity. Because the blind end of the “bone socket” structure is a good barrier to the bone destruction by bioerosive factors such as joint fluid and promotes tendon-bone healing, increases the stability of graft attachment in the tunnel, inhibits the mechanical effects of pulling and friction, thus, accelerating graft maturity.
It is worth noting that even though graft maturity after all-inside reconstruction improved well in the late follow-up period, it was found that knee stability after all-inside reconstruction was not better than that of anatomical reconstruction, which may be related to returning to exercise prematurely when the graft was immature after all-inside reconstruction. Suppose the difference in graft maturity between the two techniques is ignored, and the same rehabilitation intensity is used, the graft of all-inside reconstruction cannot withstand the load and anatomical reconstruction, which will cause laxity, severe ones leads to dislocation and fracture, which will eventually require revision surgery. Darren et al  mentioned in a systematic evaluation that 61.5% of the studies on all-inside techniques allowed jogging at 2–4 months postoperatively, and 69.2% allowed cutting and rotational movements to resume at 6–9 months postoperatively. The fact that so many studies chose to allow patients to return to exercise up to six months postoperatively may be because they made rehabilitation decisions based solely on postoperative functional recovery and ignored the role of graft maturity. This study concluded that good improvement in knee function was obtained after all-inside reconstruction, but there was no correlation between functional recovery and graft maturity. Good functional recovery did not mean that the graft was well matured, which is consistent with the studies of Marcus and Zhang et al [31, 32], so it is unreasonable to make rehabilitation decisions based on postoperative functional recovery alone, and we still need to consider the graft maturity.
In clinical practice, the appropriate time to restore exercise status after all-inside reconstruction is often based on the rehabilitation strategy of anatomical reconstruction. Sill, this study shows that there is a significant difference in graft maturity between all-inside reconstruction and anatomical reconstruction during postoperative 6th month, only after that there is a gradual improvement, and Insignificant difference in graft maturity between the two groups at one year after surgery. Therefore, it is necessary to adopt a more conservative rehabilitation strategy after all-inside reconstruction than anatomical reconstruction, mainly to avoid premature and overly aggressive rehabilitation training around postoperative 6th month. Follow-up, further research is required to develop scientific rehabilitation strategies that match the graft maturity of the all-inside reconstruction so that a better recovery of knee stability can be achieved after all-inside reconstruction.
This study had several constraints. First, the small sample size resulted in relatively low statistical power, and a study with a larger sample size is required to find more reliable conclusions. Second, data on clinical outcomes and SNQ values follow-up time were short, so the exact change tendency at 2–5 years remains unclear. Third, the SNQ values based on MRI mostly reflected the water content in the graft, which is still indirect evidence for graft maturity.