ACL Graft Healing Timeframes

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Graft healing after anterior cruciate ligament reconstruction (ACLR)

Yao, S., Fu, B.S.C. and Yung, P.S.H., 2021. Graft healing after anterior cruciate ligament reconstruction (ACLR). Asia-Pacific journal of sports medicine, arthroscopy, rehabilitation and technology, 25, pp.8-15.

In today’s letter

• Overview of the timeframes that it takes for the ACL to heal post reconstruction

• A fun infographic for you to save and use in the future

• ACL healing timeframes —> visualised

• Meme of the week: I’m showing you what not to do 😅 

• Rapid Results =

Be mindful with high-intensity loading and aggressive stretching in the first 6 months post-ACLR, as anterior knee laxity tends to peak around this time due to the ongoing graft healing process. Instead, prioritise gradual, controlled strengthening and neuromuscular training to support graft integrity and stability

• Professional takeaway =

The ACL graft healing process is a complex, phased process influenced by inflammation, revascularisation, and collagen remodeling. Preserving native ACL remnants and intra-articular structures during surgery can enhance graft healing (which is not a physios role, but a good to know)

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Did you know?

ACL injuries are common—about 200,000 happen in the U.S. each year, with up to 150,000 requiring reconstruction. But even with advanced techniques, ACL graft failure rates can still reach 13.3%.

This is because healing isn’t straightforward.

The Three Phases of ACL Graft Healing

1. Early Healing Phase (Weeks 1-6)

   • The graft initially undergoes necrosis (cell death) and becomes hypocellular (low cell activity).

   • No significant blood supply yet, which means limited healing.

   • This is why early loading should be controlled—too much stress can weaken the graft.

2. Proliferation Phase (Weeks 6-12)

   • The graft experiences heavy cell infiltration.

   • Some revascularisation begins, but it’s still vulnerable.

   • The tendon starts transitioning into ligament-like tissue, but it’s far from a functional ACL.

3. Maturation Phase (3-12+ months)

   • Slow matrix remodelling takes place.

   • The graft strengthens, but complete tunnel closure and full ossification (bone growth around the graft) are rarely achieved.

   • “Ligamentisation” (turning a tendon graft into a functioning ligament) is incomplete, meaning the graft is still biologically different from an actual ACL.

Why Do ACL Grafts Fail?

Graft failure can result from:

1. Surgical technique – Poor tunnel placement, improper fixation, or graft tensioning errors.

2. Graft incorporation failure – The body struggles to integrate the graft into the bone tunnels and joint.

3. Trauma – A secondary injury before the graft has matured.

How Is The Graft Monitored?

MRI: Tracking Graft Maturity

MRI is the most commonly used tool for tracking graft healing. High MRI signal intensity suggests more water content (edema), which gradually stabilises over 12 months. Higher signal-to-noise quotient values early on indicate a less mature graft.

CT Scans: Assessing Bone Integration

CT scans are useful for assessing bone plug integration in bone-patellar tendon-bone (BTB) grafts. Studies show significant tunnel size changes in the first six weeks post-op, stabilising around one year. Future advancements may allow simultaneous soft tissue and bone assessment.

Second-Look Arthroscopy and Biopsy

Second-look arthroscopy and biopsy provide direct visualisation of the graft and synovial coverage. Research shows hamstring autografts develop better synovial coverage than allografts. Biopsies also show that grafts reach blood flow stability around 18 months post-op.

ACL Healing Timeframes: Breaking Down the Science

ACL reconstruction is a complex process, and the success of a graft depends on how well it heals.

This healing occurs in three key phases: early healing, proliferation, and maturation.

1. The Early Healing Phase: Inflammation and Necrosis

The first response after graft implantation is inflammation. Neutrophils and macrophages rush to the graft, releasing cytokines such as IL-6, TNF-α, and TGF-β. These molecules trigger events that prepare the graft for healing. Meanwhile, matrix metalloproteinases (MMP-1, MMP-13) break down collagen, helping new cells infiltrate the graft.

At the same time, the graft undergoes avascular necrosis, particularly in its central portion. The intra-articular part of the graft is exposed to synovial fluid, which contains enzymes that can interfere with healing. This phase is very importnat, as some surgeons prescribe NSAIDs or COX-2 inhibitors to manage post-op pain and swelling. However, evidence suggests these medications could negatively affect graft healing, meaning their use should be carefully managed.

2. The Proliferation Phase: Revascularisation and Cell Repopulation

Following necrosis, revascularisation is whats needed most. A deficient blood supply can impair cell repopulation, which is necessary for matrix remodelling. Multiple cell sources contribute to this phase, including bone marrow stem cells, fibroblasts from the ACL stump, and mesenchymal cells from articular cartilage.

During this phase, new fibroblasts first appear at the graft’s periphery before migrating inward. This process is influenced by various growth factors, including BMP-2 and BMP-7, which promote bone ingrowth at the bone tunnel junction. Preserving the surrounding knee tissues—like the synovium and infrapatellar fat pad—is essential to maintaining this blood supply.

3. The Maturation Phase: Remodelling and Functional Integration

Over time, the matrix remodels under both mechanical and biochemical influences. Basic fibroblast growth factor (bFGF) contributes heavily, signalling fibroblasts to produce type III collagen. However, the remodelled collagen structure never fully replicates the original ACL, meaning the graft will always have some structural differences from native ligament tissue.

The healing process also differs between the intra-articular portion and the bone tunnels. While the intra-articular graft undergoes ligamentisation, the bone tunnel graft experiences ossification. These regional variations mean that targeted biological interventions could improve specific aspects of healing

Resource to Check Out

Meme of The Week

When You’re Ready to Learn More

We have great resources for you to devourer

1. Killer Articles —> Literally everything you need to know about a specific injury

2. Tendinopathy 4 phase rehab frame work - My book that walks you through a step by step process of treating tendons

3. Tendininopathy Specific Guides - Learn how to diagnose and manage patellar, achilees, elbow and many more tendinopathies

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