This is the question people ask quietly, usually some weeks after the injury, and it deserves a straight answer.
The figure
In a 2023 pooled analysis of studies with long follow-up, 37.9% of knees that had been reconstructed and 40.5% of knees treated without surgery showed arthritis on X-ray at a median of about fifteen years after the injury. Surgical technique has improved over the decades the studies covered, and the authors found that the year of surgery made little difference to the figure. For comparison, an uninjured knee in a person in their thirties or forties rarely shows arthritis at all.
Two things soften that number without removing it. Arthritis on an X-ray is not the same as a painful knee, and many knees with X-ray changes work well for decades. And “arthritis” in these studies ranges from minor narrowing to severe change; the proportion with the severe end is much smaller.
What the reconstruction does and does not do
The honest reading of the evidence is that reconstructing the ligament does not, by itself, prevent arthritis on the X-ray. The KANON trial found no difference in arthritis at five years between early reconstruction and rehabilitation first. A 2026 meta-analysis of 18 studies and 33,540 patients found no difference in X-ray arthritis between reconstructed and non-operated knees.
What the reconstruction does is protect the meniscus. The same meta-analysis found that reconstructed patients were much less likely to go on to a knee replacement, about one sixth the risk. A population study found that patients treated without surgery had more than five times the rate of a later meniscal tear and about six times the rate of arthritis compared with those reconstructed early. The ligament’s job, in the long run, is to stop the knee giving way; each episode of giving way is a chance to tear the meniscus or damage the cartilage; and those are what drive the arthritis.
The meniscus is the thing
Across 1,554 reconstructed knees in a pooled analysis, 28% showed arthritis. Among those who had had part of a meniscus removed, it was 50%; among those who had not, 16%. The odds ratio for meniscectomy was 3.5, the largest single factor in the analysis. The MOON cohort’s ten-year data point the same way: lateral meniscectomy and high-grade cartilage damage were among the strongest predictors of a worse knee at ten years, alongside smoking, higher body weight and a lower starting score.
This is why so much of the decision-making in this guide is about the meniscus rather than the ligament. A repairable tear is repaired, even though the recovery is slower. An unrepairable tear is trimmed as little as possible. And a knee that keeps giving way is reconstructed promptly, because each delay beyond three months roughly doubles the rate of medial meniscal tears found at surgery and, after twelve months, raises the rate of high-grade cartilage damage.
The graft does not change it
A meta-analysis of randomised trials found no difference in post-traumatic arthritis between graft types. Whether the new ligament came from your hamstring, your patellar tendon, your quadriceps tendon or a donor, the arthritis risk is set by the injury, the meniscus, the cartilage and what happens in the years afterwards.
What you can do
- Do not play on an unstable knee. The single most avoidable cause of meniscal and cartilage damage.
- If the meniscus can be repaired, let it be. The slower recovery buys decades.
- Keep the quadriceps strong for life. A strong thigh muscle unloads the joint; the program does not stop at nine months.
- Keep your weight in a healthy range. Higher body weight was an independent predictor of a worse knee at ten years in the MOON cohort, and it is the one factor entirely in your hands.
- Do not smoke. Smoking at the time of surgery predicted worse ten-year outcomes.
- Keep up the warm-up program. A second ACL injury to either knee doubles the problem, and structured programs halve the rate.
- Be realistic about impact. A knee that has had an ACL injury and a meniscal operation may not be a knee for high-impact sport into middle age. Cycling, swimming, rowing and the gym are kind to it.
If the knee does develop arthritis in later life, the options are the same as for any knee, and the hip and knee guide covers them, starting with the non-surgical ones.