Why the 6 Phases? The 3 Stages of Graft Healing

Once a graft is placed, it doesn't become a ligament — it has to be biologically rebuilt into one, through a process called ligamentization.² This happens in three stages.

1. Early healing (necrosis). The moment a tendon graft is harvested and placed into the knee, it's cut off from its original blood supply. Without that blood supply, the core of the graft actually dies — this is called necrosis (more precisely, avascular necrosis, tissue death caused by loss of blood flow). It's not a complication; it's an expected, universal part of how a standard, fully detached tendon graft heals. Inflammatory cells move in alongside this to clear the dead tissue and prepare the graft for rebuilding.

2. Proliferation (revascularization). Before real rebuilding can happen, the graft needs a new blood supply. New blood vessels grow in from the surrounding synovium and fat pad — this is revascularization. Once blood flow is restored, new cells move into the graft and start repopulating and remodelling it. This is also the stage where the graft is mechanically weakest — old tissue is being broken down faster than new tissue is being built.

3. Maturation. From here, the graft slowly remodels into tissue that behaves like a ligament, and strength gradually climbs back. It never becomes a perfect copy of your original ACL, but it becomes strong enough to do the job. This stage takes the longest — human biopsy data puts full maturation anywhere from about 1 year to as long as 3 years, depending on graft type.

Syncing This to My 6 Phases

Phase 1 (0–2 weeks): The graft has just entered necrosis — the core is dying back after losing its native blood supply. Mechanically, the graft hasn't hit its weak point yet, so my focus is pure protection: control swelling, protect the fixation, restore motion.

Phase 2 (2–6 weeks): Still in the early healing/necrosis window, but the transition into proliferation begins here — the first new blood vessels start growing in around week 3, and by week 6 the fibres anchoring tendon to bone start forming. This is why I protect the fat pad and synovial lining at surgery — it's the graft's future blood supply.

Phase 3 (6 weeks–3 months): The graft is now in full proliferation. Necrotic tissue is being cleared and replaced, revascularization and cell turnover are at their peak, and this is the graft's weakest mechanical window. A patient can feel strong here well before the tissue actually is — progression has to be deliberate, not symptom-led.

Phase 4 (3–6 months): Proliferation is winding down and strength is climbing, but biopsy data shows many grafts still haven't entered true maturation at this point. This is healing tissue with improving capacity, not mature ligament.

Phase 5 (6 months, start running): Collagen fibres are just beginning to realign, but full maturity is still months away — even the best-case data shows grafts aren't histologically mature until at least 9–12 months. Running starts while the graft is still biologically immature, which is why load has to build gradually, not clear purely because the calendar says 6 months.

Phase 6 (~12 months, return to sport): Even at 1 year, the best human data shows some grafts are still immature at a tissue level; more conservative data puts full maturation at 19–24 months, and the strictest criteria in some studies weren't met until 3 years. Return to sport here is a function-and-strength milestone — not proof the biology is finished.

How a Vascularized Over-the-Top Graft May Bypass Necrosis

A standard graft is a free graft, fully detached at harvest, which is exactly why it must pass through necrosis before it can heal. A vascularized graft — a hamstring left attached at its native tibial insertion (the pes anserinus) rather than fully detached — keeps a blood supply from day one, and anatomy, animal, and human MRI studies consistently show these grafts largely skip the die-off phase and mature earlier and more predictably than detached grafts.¹,³–⁸ Pairing this with the over-the-top technique, which fixes the femoral side against a vascularised soft-tissue envelope instead of inside a bone tunnel, means the graft may avoid losing its blood supply at either end — potentially bypassing necrosis almost entirely. The clinical outcome evidence for this is real but more modest than the biological and imaging evidence,⁷,⁹ so I treat this as a strong biological rationale rather than a proven change to rehab timing.

References

1. Zaffagnini S, et al. Vascularity and neuroreceptors of the pes anserinus: anatomic study. Clin Anat. 2003.
2. Yao S, Fu BSC, Yung PSH. Graft healing after anterior cruciate ligament reconstruction (ACLR). Asia-Pac J Sports Med Arthrosc Rehabil Technol. 2021.
3. Papachristou G, et al. ACL reconstruction with semitendinosus tendon autograft without detachment of its tibial insertion: a histologic study in a rabbit model. Knee Surg Sports Traumatol Arthrosc. 2007.
4. Liu S, et al. Advantages of an attached semitendinosus tendon graft on bone tunnel healing after anterior cruciate ligament reconstruction: a rabbit model. Am J Sports Med. 2018.
5. Liu S, et al. A randomized clinical trial to evaluate attached hamstring anterior cruciate ligament graft maturity with magnetic resonance imaging. Am J Sports Med. 2018.
6. Grassi A, et al. Hamstring grafts for anterior cruciate ligament reconstruction show better magnetic resonance features when tibial insertion is preserved. Knee Surg Sports Traumatol Arthrosc. 2020.
7. Ruffilli A, et al. Hamstring graft tibial insertion preservation versus detachment in anterior cruciate ligament reconstruction. Eur J Orthop Surg Traumatol. 2016.
8. Sinha S, et al. ACL reconstruction with attachment-sparing hamstring autograft results in earlier graft maturation and better short-term clinical outcome in comparison to free graft. Am J Sports Med. 2026.
9. Gupta R, et al. Outcome of hamstring autograft with preserved insertions compared with free hamstring autograft in anterior cruciate ligament surgery at 2-year follow-up. Arthroscopy. 2017.