Hamstring ACL Grafts

Do they grow back? Does it matter if they don't?

One of the questions I get asked most often by patients having ACL surgery with a hamstring graft is: “Mick, will the hamstring tendon grow back?”

It’s a brilliant question.

We know hamstring grafts are a popular choice, often leading to less kneeling pain afterwards compared to other options. But taking tendons out naturally raises concerns about what happens long-term.

So, let’s look at what the science tells us.

Does the tendon regenerate?

And, perhaps just as importantly, does it actually matter for your strength, your function, and your recovery journey?

We'll explore the evidence from several research reviews and studies.

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So, Do Hamstring Tendons Actually Regenerate?

The short answer is yes, mostly… but it's complicated.

• Tissue Regrowth is Common, But Variable: Research reviews, like those led by Suijkerbuijk et al. (using data up to 2014) and later Dhillon et al. (using data up to 2020), consistently show that some form of tissue regrowth happens at the harvest site in the majority of people – often reported above 70%. This regrowth process seems to happen mostly within the first year after surgery.

• But What Is "Regeneration"? A Fuzzy Definition: Here’s a key challenge: researchers don't always agree on what truly counts as "regeneration." Is seeing any new tissue on an MRI scan enough, or does it need to look and behave exactly like the original tendon? As Dhillon and colleagues highlighted, this lack of a clear definition makes comparing studies tricky. MRI scans are useful for seeing this new tissue, but they might struggle to tell us if it's proper tendon or just scar tissue. Ultrasound can be a handy, cost-effective way to track regrowth, while only a biopsy (taking a small sample) can truly confirm the exact tissue type – but that’s invasive.

• Often an Imperfect Copy: When tissue does regrow, it’s often different from the original tendon. Studies show it frequently doesn't reach all the way back to the original attachment point on the shin bone, and it might attach more broadly into the surrounding tissues (fascia) rather than precisely at the native footprint. Sometimes the two harvested tendons (semitendinosus and gracilis) even seem to fuse as they regrow.

• Muscle Changes Too: Harvesting the tendon also affects the muscle it came from. We often see the muscle belly undergo proximal retraction (it pulls upwards slightly) and sometimes muscle atrophy (it can shrink a bit). These changes can affect how the muscle works later on.

So in a nutshell, Yes, tissue typically grows back where the tendons were taken. However, this "regeneration" is highly variable, often doesn't perfectly replicate the original anatomy (in terms of length or attachment), and the muscle itself also undergoes some changes. Plus, the scientific definition of 'regeneration' is still a bit fuzzy.

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Okay, It Regenerates Differently – But Does It Matter?

This is the crucial next question.

If the regrowth isn't perfect, what does that mean for your hamstring strength, your ability to get back to activities, and your long-term knee health?