Patient Education · Knee

ACL Reconstruction

We do not repair a torn ACL. We build you a new one. Here is how that works, what the graft choices mean, and the honest timeline.

Dr. Darin W. Allred Darin W. Allred, MD Board-certified orthopedic surgery & sports medicine
~1 hrTypical time for the operation
No braceI want the knee moving with confidence from day one
10–16 wksBefore running, and only if you meet the criteria
9–12 moBack to cutting and pivoting sports

What the ACL does

The anterior cruciate ligament runs diagonally through the middle of your knee, from the back of the thigh bone to the front of the shin bone. It has one job: stop the shin bone from sliding forward and twisting out from under the thigh bone.

Illustration of a bent knee showing the cruciate ligaments
The knee bent, with its ligaments shown. The ACL is the one running diagonally through the centre of the joint.

You can walk in a straight line without one. What you cannot reliably do is plant and turn. That is why an ACL tear ends basketball seasons but does not stop people from going to work.

How it tears, and how you know

Most ACL tears do not involve anyone touching you. You plant a foot and change direction, or land off balance, and the knee twists. Classically:

  • A pop you hear or feel.
  • Swelling within a few hours — that is blood in the joint, and it is a meaningful sign.
  • A sense that the knee gave way, and might again.

Later, once the swelling settles, the story becomes instability: the knee buckles on uneven ground, going down stairs, or turning quickly. Every one of those episodes can damage the meniscus and cartilage further, which is the real argument for fixing it.

Illustration of a healthy knee from the front with the ligaments intact
A healthy knee with its ligaments intact. Compare this ACL with the torn one below.

Why we cannot just sew it back together

The ACL sits bathed in joint fluid, with a poor blood supply, and when it tears the ends shred and retract. Stitching them together simply does not hold — surgeons tried for decades, and it failed often enough that the approach was abandoned.

So we reconstruct instead: we take a piece of strong tendon, pass it through tunnels drilled in the bones along the path the old ligament took, and fix it at both ends. Over the following months your body grows blood vessels and cells into that graft and it becomes a living ligament. That biology is why the timeline is long and why the graft is at its weakest somewhere around two to three months — exactly when you feel great.

Close-up illustration of a torn anterior cruciate ligament inside the knee
A torn ACL, seen from inside the knee. The fibres have pulled apart in the middle. That is why stitching the two ends back together does not hold, and why we build a new ligament instead.

Where the graft comes from

This is a real decision, and we make it together:

  • Your own patellar tendon (a strip from the tendon below your kneecap, with a block of bone at each end). Very strong, heals bone-to-bone, long track record. The trade-off is kneeling discomfort and front-of-knee soreness for some people.
  • Your own hamstring tendons (from the back of the thigh). Smaller incision and less front-of-knee pain. The trade-off is slower healing into the bone tunnels, and we keep resisted hamstring work off the program for eight weeks.
  • Your own quadriceps tendon (from above the kneecap). Strong, increasingly popular, less kneeling trouble than the patellar tendon.
  • Donor tissue (allograft). No second surgical site and an easier early recovery — which makes it attractive for older, less pivot-heavy patients. I am cautious about it in young athletes, where the retear rate is meaningfully higher.
There is another option worth knowing about. For the right patient with the right tear pattern, the BEAR implant lets your own torn ACL heal back together instead of being replaced with a graft — no second surgical site at all. It is not for everyone, and I am candid about the case against it. Here is my full take on it.

How the operation goes

  1. Anesthesia. Usually a general anesthetic with a nerve block, plus numbing medicine in the knee.
  2. Arthroscopy first. A camera goes in through small punctures. Before anything else I look at the whole joint — the menisci and the cartilage — because roughly half of ACL tears come with a meniscus tear, and what I find there can change your recovery.
  3. Harvest the graft (or prepare the donor tissue) and size it.
  4. Drill the tunnels in the thigh bone and the shin bone, positioned where your original ligament attached. Getting that position right is the single most important technical step in the operation.
  5. Pass and fix the graft through the tunnels, tensioned and secured at both ends.
  6. Check it. The knee is tested for stability and full motion before closing.

It is an outpatient operation — you go home the same day.

Why there is no brace

Many surgeons send patients home in a hinged brace. I usually do not. The evidence has not shown that a routine brace protects the graft or improves the outcome, and a braced knee is a knee that moves less, swells more, and takes longer to wake its quadriceps back up.

I want you moving your knee freely and with confidence from the start. If I do give you a brace, it will be for a specific reason — a meniscal repair, a second ligament, a particular concern — and I will tell you exactly how and when to use it.

Recovery

Progress here is earned by meeting targets, not by the calendar. Your therapist moves you to the next phase when your knee is ready, and that varies between people.

  • First 2 weeksGet it straight. Full extension is the top priority — it is far harder to regain later than it is to keep now. Wake the thigh muscle up with quad sets and straight-leg raises. Bend and straighten several times a day. Weight bearing as tolerated; crutches until the leg holds you, ideally gone within two weeks. Keep the incision dry 48 hours, no soaking for three weeks. Aspirin 81 mg twice daily for two weeks. Therapy starts within a few days — call us if it is not scheduled.
  • Weeks 2–4Walk normally without crutches or a limp. Stationary bike. Mini-squats, calf raises, balance work.
  • Weeks 4–6Build strength: step-ups, then step-downs, lunges in a protected range. Bend should be approaching 120 degrees. No jogging yet.
  • Weeks 6–10Heavier strengthening and balance work. Around week 8, direct hamstring work begins if you had a hamstring graft.
  • Weeks 10–16Running — but only once you have full motion, no swelling, sound mechanics, and thigh strength at least 80% of your other leg. Not before. Then jumping and landing work begins.
  • 9–12 monthsBack to cutting and pivoting sports, once strength and control testing say you are ready.

If I also repaired your meniscus, everything above shifts about two weeks later and we keep you from forcing the knee past 90 degrees early on. That repair needs protecting, and it is worth protecting.

The risks, honestly

  • The graft tears again The main risk, and it lands hardest on the youngest, most athletic patients returning to cutting sports — in that group it is roughly 1 in 5 over the following years, counting both the new graft and the other knee. Two things genuinely lower that risk: not returning early, and passing strength testing rather than a date on the calendar.
  • Stiffness or lost extension Uncommon, and largely preventable by getting the knee straight in the first two weeks. Occasionally needs a procedure to release scar tissue.
  • Front-of-knee pain or trouble kneeling Most associated with a patellar tendon graft.
  • Numbness beside the incision Common, from a small skin nerve, and it usually shrinks over a year or two.
  • Infection Rare after arthroscopic surgery.
  • Blood clots Uncommon in this age group, but the reason for the aspirin and for staying active.
  • Arthritis later in life Worth being straight about: an ACL injury raises the odds of knee arthritis decades on, and reconstruction reduces further damage but does not erase that risk — especially when the meniscus or cartilage was hurt at the same time.

Does everyone need this operation?

No. If you are older, not doing pivoting sports, and your knee feels stable in your actual life, a strong knee without an ACL can work well. The people who need reconstruction are the ones whose knee gives way — because every giving-way episode risks the meniscus and cartilage, and those are much harder to replace than a ligament.

This page is general education. It describes how I do this operation and what most patients experience. Graft choice and timeline are individual — if anything here does not match what you were told in clinic, ask.