A number of factors make the knee particularly prone to injury during this period -
Injury due to proprioception deficit
After a tear of the ACL, there may be loss of position sense in the joint, and also loss of some ability to detect passive movement (someone else moving the joint). This phenomenon is called 'proprioception loss'. After an ACL injury proprioception loss may also be experienced in the good leg!
Patients may stumble or trip, or bang their knee more often due to the proprioception loss. After reconstruction, the joint can be retrained with specific exercises and proprioception can bre regained, but meanwhile the joint remains vulnerable to injury.
Injury related to muscle imbalance
After hamstrings harvest there may be residual weakness of the hamstrings muscles, although regeneration of the tendon often occurs. The rehabilitation protocol may also favour the quadriceps muscle group over the hamstrings, creating muscle imbalance.
The muscle weakness may make the knee more prone to injury.
Injury related to returning to activities inappropriate for stage of healing
It is imperative that patients are in the hands of an experienced knee rehabilitation team after an ACL graft, as the physiotherapist needs to understand the issues of the site of graft harvest and the issues of graft fixation as well as the time frames for graft re-vascularisation and ligamentisation.
If the patient is returned to activities inappropriate for the stage of healing then the graft will be under threat of re-injury.