Why is your knee still swollen after ACL surgery?

Even though surgeons and some healthcare professionals tell patients that it can take 6-8 months to heal from ACL surgery, the reality is that it can take much longer.

Due to the fact that the body views ACL surgeries as an injury, there can be increased swelling that is “normal” for up to a year from surgery.

The ACL will undergo a process of healing and you may be notice swelling that occurs throughout these phases of healing.

What are the healing phases of the ACL?

Typically after a graft is in place, the graft will go through a three part phase that is the body’s attempt to incorporate the tissue. The three phases are “strong” - “weak” - “strong”.

Strong phase 1

The first phase is as the graft is initially place, and can last 6-10 weeks. At this point, the graft is strong, however, you are limited in your activities due to the bone not fixing the screws that are placed in the tibia along with traumatic acute swelling from surgery.

Additionally, your surgeon will usually have restrictions on your movement with a device such as a knee brace, limiting ROM.

Weak phase

The next phase, lasting 10-20 weeks, is when the graft is weaker. At this point, the body starts to deposit collagen, with some elastin as well. This deposition of new connective tissue building blocks is the reason why the graft will be weak, The graft is weak because to lay down new tissue components, the body needs to partially break down the ACL.


These two components, elastin and collagen, are the building blocks for many of our internal fibrous tissues and are necessary for recovery. Additionally, scar tissue may form around the graft.

At this point, the graft is weak and you are at risk for tears. This is the reason why most surgeons tell you to avoid cutting motions and hard running.

Strong phase 2

During the last phase, after 20 weeks, the graft will slowly start to get strong due to the collagen and elastin depositing phase ending. This can take 8 months, and in most people 1 year from surgery.

At this point, the screws have incorporated themselves into the bone and collagen and elastin have helped to make the graft strong and pliable.

Why is my knee still swelling after ACL surgery, it’s been 5 months?

The reason that the knee swells at 5 months is due to the metabolic processes that are involved in the re-depositing of collagen and elastin.

As stated previously, at the 5 month mark, you are still healing. Whenever there is a metabolic process in the body, the body has to swell that area to allow the flow of metabolites and other cellular components to repair the tissue and surrounding joint.

In addition, you are probably anxious to get back to whatever activity or sport you were doing previously. People can be so adamant about getting mobile again that they can sometimes push themselves too hard and engage in too rigorous of activity, which stresses the joint and the surgically repaired tissues, causing increased swelling.

Furthermore, stretching and mobilizing the joint will cause more swelling because you are stretching the scar tissue, which causes an inflammatory response and thereby, swelling.

How can biomechanics affect stress on the knee?

The knee joint is commonly referred to as a “hinge” joint. However, along with motion in the sagittal (forwards and backwards) plane, it can also have some rotational aspects to it.

This torsion or “twisting motion” can put stress on many different structures. Some of these structures include the menisci, MCL, LCL, and of course, the ACL. All of these structures will resist a torsional motion of the knee, however, the ACL is the most important.

Oftentimes when people injury their ACL, it will be because of them cutting, lateral movement, or twisting their knee. The ACL will be injured along with the MCL and medial meniscus, culminating in a terrible triad, which is the rupture and tear of all these structures.

It is due to this bio-mechanical nature of the knee that swelling can become an issue as you start to progress in your therapy.

As you start to perform more complex activities and more rigorous movements, there is a torsional stress on the knee that can start to “test” or stress the ACL. This is a good thing, however, it can lead to increased swelling because the body will respond to this stress by depositing more collagen, elastin, and some other fibrous materials.

This is a normal, expected, and good response of the body because that is how we heal our surgical grafts and get back to our normal activities.

Another anatomical aspect of the body that can have an impact or surgical repair of the ACL is swelling that gets “trapped” or “locked” inside the interstitial tissues of the body. As the ACL swells and is in the joint, it can lead to an inhibition (weakening) of the quads.

Therefore, it can impair the recovery process by decreasing the overall strength of the leg. In this way, the motion of the knee during therapy is what heals and causes stress on the graft.

What is the importance of rehab greater than 5 months after ACL surgery?

As demonstrated in the previous sections, ACL repair is a long, arduous, and sometimes tedious healing process. Unfortunately, we cannot rush biology (yet), and it will often take longer to heal an ACL graft than a patient or family member would expect.

For this reason, it is important to continue your rehab, either 1x or 2x per week, well past 6 months of surgery. Generally, it would be important to continue with rehab 2x per week, until around 8 months.

From there 1x per week, or 1-2x every other week would suffice for up to a year out of surgery. This type of rehab program will allow your physical therapist to address concerns or comments that you may have as well as engage in return to sport protocols and higher level balance, plyometrics, or functional activities.

Specifically, return to sport protocols are very important. There is a chance of re-injuring the operated leg or the non-operated leg after an ACL repair. Return to sport protocols may lower the chance of the re-injury and allow the athlete to better assess how he/she is progressing.

It is also important that you do not let the swelling stay in the leg, because this can regress you in your therapy and prolong your return to sport. It is important that you rehab the knee properly.

What are the risks of not doing enough rehab?

There has been a growing body of evidence that ACLs that are poorly rehabilitated or, return to sport that is done too quickly can result in long term damage of the knee.

In one study, two out of three people have defects in the knee joint including meniscus tears, bony spurs, and cartilage defects after an ACL repair. This includes osteoarthritis and cartilage defects that can lead to pain, immobility, and possible joint replacement. These defects are usually seen on imaging 1-4 years after ACL surgery.

Additionally, improper or insufficient rehab can lead to injuries when the person returns to activity or sport. There is a risk of re-injury and studies have demonstrated that after ACL rehab, some 7% of people will experience re-injury with the ACL repaired leg within the first two years from surgery.

Also, due to the bio-mechanical changes in gait and fear of putting weight on the repaired leg, there is an 8% prevalence in tearing the non-injured knee, this has been supported in a large meta-analysis of randomized controlled trials.

For those people who rehabilitate the injuries, only 61% of individuals will feel that they get back to prior sports activity at the same level before the injury and surgery. This statistic is shocking because one would assume rehab helps a higher percentage of athletes get back to playing sports.

Additionally, there are some people, 19%, who never return back to the sport that they got injured from. Their limitation is mostly due to knee pain, knee instability, and fear of injury.

All of these issues can be alleviated by physical therapy rehabilitation that improves strength, ROM, reduces scar tissue, and alleviates swelling.

For the reasons above, it is very important that you continue your rehabilitation up to a year from your surgery. Without an on-going rehab program, you could be risking injury, decreased participation in athletics, and possibly arthritis.

Other than rehab, what steps you can take to reduce swelling?

It is important that you listen to your individual physical therapist and the plan that he/she will come up with. However, there are some concrete steps that you can take in order to improve your outcomes greater than 5 months post op.

Some simple steps are: maintaining your home exercise regime, listening to activity precautions, icing, wraps or compression sleeves to reduce swelling, and gentle stretching. All of these activities can help prevent the swelling of the knee, and ameliorate the swelling if it does occur.

Final infograhic - reduce acl swelling

How Adam reduced knee pain

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