Why is knee extension is difficult after acl reconstruction?

Swelling or scarring: Acutely after acl reconstruction the knee can be swollen and this could hinder knee extension. Physical therapy is started quickly after surgery to regain extension. It is vitally important to regain knee extension asap or scar tissue can form in the front of the knee. Once this scar tissue forms extension maybe very difficult to obtain and surgery my be required.

Swelling.: The swelling following ACL surgery prevents you from fully extending the knee, but keep working on it in physical therapy. Hopefully you regain your full extension.