reported the occurrence of contralateral ACL rupture after ACL reconstruction in 45 patients (4.1%) of 1120 patients who underwent ACL reconstruction. Salmon
 examined 675 patients who underwent ACL reconstruction. They conducted a phone survey 5 years after reconstruction surgery. There were 35 patients (5.7%) who developed a contralateral ACL rupture. Wright
 conducted a survey for 2 years after ACL reconstruction and reported that 3% of their subjects had a contralateral ACL injury. Contralateral ACL injury is one of the most serious complications after ACL reconstruction for patients and surgeons
. Recently, Wright
 reported systematic review for contaralateral ACL rupture at five years or more following ACL reconstruction. The systematic review demonstrates that the risk of ACL tear in the contralateral knee was 11.8%.
Previous studies have evaluated bilateral ACL injuries to try to determine their risk factors
[7, 17, 18]. Souryal
 reported that the risk factor of contralateral ACL injury was young age at the time of initial injury. Furthermore, Pinczewski
 showed that individuals who are young at the time of ACL reconstruction have a greater risk of a contralateral ACL injury during follow up. In our study, the age at the time of initial injury in contralateral group trended to be low compare with control group. Patients with a ruptured ACL are likely to have a higher risk of having various intrinsic factors that make patients more susceptible to an ACL injury. Young patients are probably more likely to return to sports activity than older individuals with first-time ACL injury.
 analyzed the type of primary ACL injury (contact or non-contact injury), activity level according to the International Knee Documentation Committee (IKDC) scale, gender, graft type, family history of ACL injury, articular surface damage, presence of meniscal injury, history of meniscectomy, and correlation to contralateral ACL injury. The only significant predictor for a contralateral ACL injury was a return to sports activity of level 1 or 2. In our study, most of the cases had undergone reconstruction to return to playing competitive sports. Only a small number professional athletes were in our study, which could have resulted in an insignificant difference in Tegner activity score.
For graft choice, a significant difference has been demonstrated between patients who have undergone ACL reconstruction with hamstring tendon autograft compared to patients who have undergone ACL reconstruction with patellar tendon, in terms of contralateral ACL injury rate. There were significantly more contralateral ACL injuries in the patellar tendon group at 10-year follow up
There has not been any report on the examination of knee laxity and muscle strength of reconstructed legs in patients with contralateral ACL injuries after ACL reconstruction. At first, our study was begun with the hypothesis that the risk factors for contralateral ACL injury are knee laxity and insufficient muscle strength of the reconstructed knee. However, our results indicated that when the patients returned to playing sports, there was no significant difference between the contralateral group and the control group in the knee laxity and strength of the knee extensor and flexor muscles of the reconstructed legs.
We also focused on the injury situation. We found that most of the cases of contralateral ACL injuries were non-contact injuries at the time of initial and re-injury. The re-injuries of approximately 70% of the cases occurred under similar situations as their initial injuries. These results may suggest that the patients with initial non-contact ACL injuries tended to develop non-contact injuries again if neuromuscular control of the legs was not improved. One of the risk factors of contralateral ACL injury is considered to be a non-contact injury at the initial injury. To prevent non-contact injuries, the following are thought to be effective: physiological and kinesiological approaches suitable to each individual and prevention programs for ACL injuries such as neuromuscular training
To our knowledge, there has not been any report on risk factors for contralateral ACL injury focusing on knee laxity and muscle strength of the reconstructed legs. Our study suggest that knee laxity and muscle weakness of the reconstructed legs 6 months following surgery were not individually related to contralateral ACL injury occurring approximately 2 years after surgery.