Anterior cruciate ligament (ACL) injury has been increasing among young athletes owing to early sports participation and specialization, as well as the excessive load of intensive training [1,2]. The best method for treating ACL injury in young athletes remains controversial. On the one hand, ACL reconstruction in younger patients is considered to prevent limitations in physical activity and secondary meniscal tears or chondral injuries due to persistent joint instability [3,4]. On the other hand, even with bracing and physical therapy, conservative therapy is often ineffective in active young patients [5].

There have been concerns regarding growth disturbance (0.81–4.2%) or angular deformities (0–3.7%) in pediatric patients following ACL reconstruction [6,7]. Several studies have reported a significantly higher graft failure risk in younger patients [8,9]. Furthermore, many factors such as higher activity levels, lower compliance to rehabilitation protocols, and higher anxiety levels have been suggested for higher failure rates in younger patients [8,10].

The semitendinosus (ST) tendon is the main choice of graft selection for ACL reconstruction in skeletally immature patients because a graft with bone block, such as a bone–tendon–bone graft, is not appropriate owing to the open growth plates [11]. Asai et al. found a lower density of tenocytes and smaller collagen fibril diameter in ST graft tissues of skeletally immature patients than in those of mature patients who underwent ACL reconstruction or medial patellofemoral ligament reconstruction [12]. In skeletally immature patients, the morphological and mechanical properties of ST tendon as a transplanted graft may dramatically change with growth and major changes in bone morphology. To the best of our knowledge, only one study has explored the biomechanical properties of tendon grafts in skeletally immature cadavers [13]. The mean age of cadavers in that study was 9.2 years, and the study did not include a control group. No research has investigated the properties of tendon grafts during ACL reconstruction within a narrow age range with and without growth plate closure.

Hence, this study aimed to investigate the microstructural and mechanical properties of ST tendon graft tissues during ACL reconstruction and the clinical outcomes in skeletally immature and mature patients. We hypothesized that the tendons in skeletally immature patients would be weaker and more elongated that those in skeletally mature patients.