RESUMEN
BACKGROUND: The anterior cruciate ligament (ACL) injury often occurs in young athletes, but it also occurs in middle-aged individuals and the elderly during recreational sports activities. Clinical outcomes after the ACL reconstruction depend on postoperative recovery of muscular strength. The current study aimed to evaluate the recovery of knee extension and flexion strength after ACL reconstruction surgery and to examine the relationship between preoperative and postoperative muscle strength by age and the type of graft used. METHODS: From 2007 to 2010, 32 patients (17 men, 15 women; average age, 31 years; range, 14-66 years) who underwent two-bundle ACL reconstruction surgery using hamstrings, i.e., semitendinous and gracilis tendon (STG) graft, and 25 patients (15 men, 10 women; average age, 28 years; range, 15-59 years) who underwent the ACL reconstruction surgery using bone-patellar tendon-bone (BTB) graft were included in this study. The muscular strength of the knee extension and flexion compared to non-injury side was measured by an isokinetic dynamometer at a velocity at 60°/s preoperatively, and postoperative measurements were performed at 6, 9 months, and 1 year after the ACL reconstruction surgery. RESULTS: Covariates that influenced the outcome of the force of knee extension at 12 months were the preoperative muscular strength [p = 0.045, odds ratio (OR): 1.105, 95% confidence interval (CI): 1.002-1.219] and muscular strength at 6 months after surgery (p = 0.040, OR: 1.155, 95% CI: 1.006-1.326). Only muscle strength at 6 months after surgery influenced the outcome of the force of the knee flexion at 1 year after surgery. In sub-analysis, muscular strength of the knee extension and flexion was greater in the STG group than in the BTB group at 6 months after surgery although there was no difference between muscular strength of the knee extension in the STG group and that in BTB group at 1 year. CONCLUSIONS: Recovery of knee extension strength at a year after ACL reconstruction was significantly associated with preoperative muscular strength and muscle recovery at 6 months. Age and graft type might be related to the muscle strength recovery. Preoperative and early postoperative strength training could improve the recovery of knee extension strength, which would support an earlier return to sports after ACL reconstruction.