How do Ligament Grafts Work?
When a ligament is reconstructed, such as during anterior cruciate ligament reconstruction, the surgeon replaces the torn ligament with a graft, usually a tendon taken from the person’s own body. Although this graft is strong at the time of surgery, it does not behave like a normal ligament straight away. Over time, the body slowly remodels the graft so that it begins to function more like a ligament. This process is commonly called ligamentisation. In simple terms, ligamentisation is the body’s way of turning a piece of tendon into something that can act like a ligament by changing its structure, blood supply, and mechanical behaviour. This transformation is slow, highly organised, and vulnerable to overload, which is why rehabilitation timelines are long and carefully staged.
Ligament graft healing is best understood as a series of overlapping biological stages. Each stage has typical signs, specific risks, and implications for rehabilitation and re-rupture risk. Although these stages apply to all ligament grafts, the examples below focus mainly on ACL reconstruction, with brief comparisons to other ligament injuries where relevant.
Early Post-surgical Phase: Graft Necrosis and Inflammation (Approximately 0–6 weeks)
Process
Immediately after surgery, the graft is fixed into bone tunnels and provides mechanical stability to the joint. However, its blood supply is largely disrupted. During the first few weeks, many of the original graft cells die in a process known as graft necrosis. This sounds alarming but is a normal and necessary part of healing. At the same time, the body mounts an inflammatory response to surgery.
Symptoms
Common signs during this phase include joint swelling, warmth, pain, and stiffness. Muscle inhibition is very noticeable, particularly reduced quadriceps activation after ACL reconstruction. People often report that the knee feels weak or “not connected,” even if it does not feel unstable.
Precautions
The main risks in this phase relate to stiffness, poor muscle activation, and excessive joint irritation. From a graft perspective, the fixation points in the bone are still vulnerable. Sudden twisting, slipping, or aggressive exercises can compromise graft fixation rather than the graft tissue itself.
The risk of graft re-rupture is relatively low in this early phase because most people are not yet exposing the knee to high loads. However, poor movement patterns or ignoring weight-bearing and range-of-motion guidance can create problems that affect later stages.
Physiotherapy
Physiotherapy during this time focuses on swelling control, restoring full knee extension, gentle flexion, muscle activation, and safe walking mechanics. Similar principles apply after posterior cruciate ligament or medial collateral ligament reconstructions, although weight-bearing and brace use may differ.
Early Remodelling and Cell Repopulation Phase (Approximately 6–12 weeks)
Process
As inflammation settles, the body begins to repopulate the graft with new cells and blood vessels. This is sometimes called the proliferative phase. Importantly, despite the person feeling stronger and more mobile, the graft tissue itself is biologically weaker than it was immediately after surgery.
Symptoms
Typical signs at this stage include improving range of motion, reduced swelling, and better muscle control. People often feel encouraged and may believe they are “ahead of schedule.” This mismatch between how the knee feels and how strong the graft actually is represents a key risk period.
Precautions
The greatest danger in this phase is overconfidence. The graft is still fragile at a microscopic level, and excessive loading, especially twisting or pivoting movements, can stretch or damage the graft. Research shows that the graft’s strength temporarily decreases during this stage as old tissue is broken down before stronger ligament-like tissue forms.
Although complete re-rupture is still less common than later on, micro-damage can occur and compromise long-term graft integrity. This is particularly relevant in ACL reconstruction, where early return to running, jumping, or sport-specific drills can overload the healing graft.
Physiotherapy
Rehabilitation emphasises progressive strengthening, controlled closed-chain exercises, and neuromuscular training. Straight-line activities may be introduced cautiously. For shoulder ligament reconstructions, such as labral repairs, a similar phase exists where movement improves faster than tissue healing, increasing the risk of premature loading.
Late Remodelling Phase: Ligamentisation and Structural Organisation (Approximately 3–9 months)
Process
During this phase, the graft gradually begins to resemble a ligament in structure and behaviour. Collagen fibres realign along lines of stress, blood supply improves, and the graft becomes more responsive to load. This is when rehabilitation becomes more demanding and sport-specific.
Symptoms
Signs of this stage include improved strength, better balance, and increased confidence in the joint. Swelling should be minimal, and functional tasks such as running and jumping may feel achievable. However, the graft is still not equivalent to a native ligament.
Precautions
This period carries a high risk of graft re-rupture, particularly for ACL reconstruction. Many people resume running and training during this stage, increasing exposure to pivoting, deceleration, and landing forces. Epidemiological data from Australia show that young athletes returning to sports such as Australian Rules football, netball, and soccer within this window have a significantly increased risk of re-injury.
The risk is heightened by asymmetrical strength, poor landing mechanics, fatigue, and psychological factors such as fear or urgency to return to competition. Importantly, re-rupture often occurs not because the graft is “weak,” but because movement patterns place excessive load on it.
Physiotherapy
Physiotherapy focuses on progressive loading, plyometric training, change-of-direction drills, and sport-specific skills. Objective testing, including strength symmetry and movement quality, is critical at this stage. Similar principles apply to ankle ligament reconstructions, where cutting and uneven surfaces introduce high re-injury risk during late rehabilitation.
Maturation Phase: Long-term Adaptation (9 Months to Several Years)
Process
Even after formal rehabilitation ends, graft healing continues. The graft continues to mature, with gradual improvements in stiffness and load tolerance. Studies show that an ACL graft may take two years or more to approach the structural properties of a native ligament.
Symptoms
Most people report minimal symptoms during this stage, although occasional swelling or soreness can occur with high training loads. Strength and confidence may be excellent, but subtle deficits can persist.
Precautions
The risk of re-rupture decreases compared to earlier phases but does not disappear. Importantly, the risk of injuring the opposite ACL can be as high as, or higher than, re-injuring the reconstructed side, particularly in younger athletes.
Physiotherapy
Ongoing strength training, movement retraining, and load management are essential. This concept also applies to other ligament grafts, including those in the shoulder and elbow, where long-term conditioning is required to maintain joint health and performance.