nic,
Anterior cruciate ligament (ACL) injuries are usually from a noncontact twisting injury. A common scenario is for a football receiver to come down from catching a pass and collapse on the field, without being hit. Or for a basketball player to come down from doing a routine layup and fall to the floor. Almost always it is a twisting injury with the foot planted on the floor and no hit from anyone else.
So, your description of the mechanism of injury in your case is typical of an ACL injury.
It is very common for one of the menisci (cartilage) in the knee to be torn along with an ACL rupture. Since the ACL is no longer providing the stability to the knee, a meniscus is at risk for injury. They can be injured in the same twisting injury as the ACL or later, in one of the patient's instability episodes.
Which is why it is recommended that a young active patient who cannot regain stability from physical therapy, should have the ACL reconstructed. Continued instability episodes put the menisci and other structures at risk for damage.
If the ACL is torn, that should be able to be picked up on a thorough examination by an orthopedic surgeon. An MRI is usually not needed to tell if the ACL is incompetent. However, it is hard to pick up associated injuries of the menisci or articular cartilage. This is usually why an MRI is ordered.
If you happen to have a bucket-handle tear of the meniscus (in which the tear is in the substance of the cartilage and it displaces forward) then you may have difficulty with range of motion of the knee. However, in the classic bucket-handle, the patient cannot completely extend the knee (straighten it).
Tightness in flexion can also be from a torn meniscus, but usually not a bucket-handle. Though it can be, in very rare cases. Usually, it is from a parrot-beak or radial tear of the cartilage. It could also be from a stump of the ACL getting stuck in the notch.
Tightness can also be from contraction of the soft tissues around the joint. The patellar tendon, fat pad, and capsule can all contract after a significant injury leading to arthrofibrosis. This is a very big problem if it occurs, so in most cases, the surgeon wants the patient to have as close to full range of motion as possible, before doing an ACL reconstruction.
So, yes it sounds like you have an ACL injury, probably with a meniscus tear. This is a very common combination. But, there are other reasons for decreased range of motion also, and these can usually be picked up on an MRI.
As to leaving it alone, you usually do not want to put off having a reconstruction done if you are having quite a few episodes of instability. However, ACL reconstructions are not usually done acutely. Most surgeons want the patient to rehabilitate the knee as much as possible. Get as much strength and range of motion and get rid of any inflammation and swelling.
Then, once full strength and range of motion are regained, if the person still is unstable, an ACL reconstruction is done. The ACL cannot be repaired, as it is intrasynovial. Which means that it is inside of the joint, and bathed in synovial fluid all the time. This prevents a repair of the ligament from occurring naturally or by surgery. So, the ligament has to be reconstructed using some other tissue, which replaces the ligament and is fixed to the bones. This can be a graft of your own tissues (patellar tendon, hamstring, etc called an autograft) or from banked cadaver tissue (called an allograft).
During the surgery, any other injuries inside the knee are also addressed. So, if you have a torn meniscus or an articular cartilage defect, these will be treated at the time of surgery. In some cases, a surgeon will do an arthroscopy to assess the joint, take care of a bucket-handle meniscus tear, and trim up the stumps of the ACL, so that the patient can do physical therapy better. Then go back later and do the complete reconstruction.
After surgery, there is a long rehabilitation process to get the reconstructed knee back into shape. So, the better shape the knee is in before the surgery, the easier the rehab after. Also, by doing therapy before the surgery, the patient knows what it is going to be like after the surgery. So, there are no surprises.
You will need to sit down and discuss everything with your surgeon. What exactly is going to be done, when it is going to be done, and what to expect afterwards. Be sure to have the surgeon answer all of your questions.
Good luck.
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