I’m four-weeks post-op following ACL surgery for a ruptured knee. I think I’m coming along but I’m wondering about how much motion I need to get back. My knees were always a little lax before surgery and now the ACL side is close to zero extension. But my other knee can straighten even more past zero. Am I going for zero or more? I don’t want to be back in the situation where the knee is so loose, it’s unstable and likely to blow out again.

This is a very good question and one that physical therapists and surgeons are beginning to address more directly. Let’s start with exploring what is normal knee motion? The standard range for knee motion is from zero (full knee extension) to about 135 (full knee flexion). Some people like yourself naturally have knee extension beyond zero. That condition is called hyperextension.

Restoring knee extension to zero after ACL surgery in someone who has five or 10 degrees of extra extension (or flexion) in the other knee isn’t going to feel “normal.” In most patients, there is a tendency to start favoring the involved knee by putting less weight on it and shifting over to the uninvolved leg. This can create problems of its own. Rehab must continue until both knees have equal amounts of motion. This of course assumes the other knee has not been injured or altered from normal.

With proper measuring, the therapist can identify even small (three to five degree) losses of motion early on. This is important while the graft tissue is still remodeling in order to regain full motion. Waiting too long can result in a stiff, painful, and weak knee. Studies show that small losses of either knee flexion or extension can lead to knee osteoarthritis. This is especially true when there is any damage to the cartilage.

Knee extension is restored first, and then knee flexion. When knee motion on the operative side equals motion on the uninvolved side, then the patient progresses to the next stage of strengthening and motor control. Athletes will advance even further in order to complete exercises that will specifically prepare them for the kind of movements (e.g., pivoting, jumping/landing, switching directions quickly) needed to compete in sports.

There’s one other reason to work toward symmetry (equal amounts of knee motion from side-to-side) and that’s to prevent the development of osteoarthritis. The long-term studies that are available showed a significant increase in the number of patients with loss of knee motion who developed abnormal joint findings as seen on X-rays.

Such changes were observed as early as five years after ACL surgery in patients who had loss of knee motion. On the flip side, patients with known cartilage damage but who maintained normal knee motion were much less likely to develop knee osteoarthritis.

It would probably be a good idea to voice your concerns to both your surgeon and your therapist. Listen to their perspectives and advice and work with your therapist to adjust your rehab program accordingly.