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Beginners' Guide to ACL Injuries: Phases of Healing and Rehab

Phases of Healing and Rehab

Let’s say you or someone you know has torn the ACL. From reading “How Gender and Contact Influence Injury”, you may even understand why the injury happened. Then there was a surgical reconstruction to give you, or that someone you know, a new ACL. From reading “Surgical Interventions”, you may even have a better understand of what procedure was done. So after all this, what do you do next? And how long is it going to take?

Through this post we will cover the phases of healing, phases of rehab, and the goals for each rehab phase.

Phases of Healing

Once an injury occurs, healing begins immediately. And it follows the same course. This course of healing is comprised of three (3) phases: inflammatory response phase, proliferation phase, and remodeling or maturation phase. Let’s start at the beginning.

Inflammatory Response Phase

The inflammatory response phase spans an average of 0-4 days after injury. This varies with the severity of the injury. It also varies with the type of tissue injured. For example, the ACL does not have great blood supply, if any. And we know that inflammation and swelling occurs as a result of broken blood vessels. The meniscus is the same way. Poor blood supply means minimal inflammatory response. This phase includes a vascular (blood vessels) and cellular response to trauma.

Believe it or not, the inflammation process is designed to protect the area, collect and remove the damaged/bad cells so they don’t travel to other parts of the body, and begin repairing the damage. The goal during this phase is to reduce inflammation, not eliminate it. Some is needed to carry the good cells to the area so the clean up can begin.

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Usually, with inflammation, there are 5 key signs to keep an eye on. They are known as the “Cardinal Signs of Inflammation”: redness, swelling, pain, warmth, and loss of function. Notice that “swelling” is a sign of inflammation, but not always present. Redness is caused by the dilation of blood vessels which brings them close to the surface showing off their red color. The swelling is caused by a “leak” in the blood vessels. We all know about pain. Nerve endings are stimulated at the site of the injury, signals are sent to the brain, and the brain interprets the signals as pain. Remember, there is no such thing as pain receptors. Your nerves can interpret heat, cold, pressure, and vibration, but they can’t interpret pain; your brain does that. Warmth is the result of increased blood flow and chemical activity. Loss of function is the result of all other signs of inflammation.

Think of the construction site of a demolished building. The building, of course, represents the injury. As soon as the building is torn down, the construction crew members arrive. These represent the white blood cells and chemical mediators of the body. They begin directing the big cells for “cleaning duty”. These big cells (macrophages) are the trucks, bulldozers, and backhoes that remove the mess. Once the junk is removed, your body plugs the leak, just like the construction crew puts up a “construction zone” fence. The injured area is closed off. Your body functions the same as construction site; the area is cleared of debris (and sometimes the process takes longer than expected).

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Proliferation (repair) Phase

This next phase of healing attempts to normalize the soft tissues by removing debris and temporarily repairing the structures. The estimated duration is 3-21 days after the inflammation phase ends. There are three (3) parts to this phase: re-vascularization, wound contraction, and wound remodeling. Re-vascularization is growing new blood vessels. The white blood cells produce the tissue needed to create the “outline”, and then collagen fills in the gaps. During wound contraction, the body is removing the blood clot that originally stopped leak, and scar tissue starts to mature and resemble the tissue it is replacing (the damaged tissue). Finally, the wound gets remodeled. Collagen begins to form around the scar tissue giving back elasticity to the area.

Going back to the construction analogy, re-vascularization would be like construction crews placing water pipes, electrical wires, and cables. The collagen that starts in this phase is like the wooden/metal framework for walls of the new building. Right now it is just the basics. The body has to make sure all of the little parts are in place before the grunt work can begin.

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Remodeling (Maturation) Phase

For most minor injuries at the beginning of this phase, the area begins to feel better, and people tend to think everything is back to normal. Just before this phase begins is when most people would try going back to their activity. Like we said before, the proliferation phase can take up to 21 days (or more if there is repeated trauma). The final phase of healing can take up to 1 year to complete! We are not saying that every injury is going to take a year to fully heal, but ACL injuries would fall into that time-frame.

This phase starts out with high fiber building activity to replace the damaged tissues, and slowly decreases that activity until back to pre-injury levels. This phase is the ‘high stress” phase. We mean rehab, physical therapy, exercise, lifting, running, any type of therapeutic activity that progressively overloads the body tissues to elicit a rebuilding response. Over time, the body will gradually strengthen the once damaged tissues to handle more and more stress, decreasing the risk of re-injury.

Again to the construction analogy. In the final phase, the tissues are repaired and takes on the form of normal tissue similar to the framework of the building taking the shape of a normal building. You can see where the windows are, the roof is going on, the walls are being closed. As this phase continues, the tissues become functional, but still need some remodeling with time. This is like the building having desks, some lights, and (of course) an internet connection, but missing doors, paint, and those final touches that bring everything together.



Throughout the healing process, there are different factors that can modify the length of inflammation and repair as well as the quality of repair. These factors include the extent of injury (micro vs macro tears), age, malnutrition, anemia, corticosteroids, anti-inflammatory drugs, immobilization, and muscle spasms. These are just some of the factors!

A more severe injury will obviously take longer to clean and repair. Poor nutrition can make the healing process take longer due to a lack of building materials (look for our new series on nutrition). You would think anti-inflammatory drugs would be a good thing right? Nope! Remember inflammation is the only way for your body to clear out the junk so the good stuff can replace it. If you stop the inflammation phase, the bad stuff gets to stick around and create chaos for the good stuff. As you know from our first blog series (“I Wear A Brace, But It Still Hurts”) immobilization leads to prolonged rest which means decreased blood flow. Blood flow is important to bring the healing cells to the injured area for rebuilding strength of your ligaments and muscles. The takeaway: your recovery will not be a steady improvement. Be prepared for ups and downs.

Phases of Rehab

So we know there are three (3) phases of healing. We know the first phase is pretty quick. The second phase takes about 3-4 times as long as the first phase. And that the third phase will take the longest to complete; especially with a more severe injury like an ACL tear. Because the ACL requires surgery to repair, our phases of healing begin after surgery. The surgery is an addition to the trauma.

As these phases of healing occur, the rehab specialist you are seeing for your recovery will prepare and administer a rehab protocol for you to follow that has been researched and tried to produce the best possible outcome for you. Usually, these protocols contain 5-7 phases for ACL reconstruction. Looking at the traditional rehab model, there are only 4 phases of rehab.

Protective Phase

The first phase of rehab, which occurs during the inflammation phase of healing, is known as the protective phase. This is usually when you will be in an immobilization device to restrict additional tissue damage. As clinicians, our goals are to reduce your pain, reduce muscle spasms, reduce excessive swelling, protect your surgical site, and maintain your conditioning levels. It doesn’t seem like much can happen at this phase, but it would surprise you.

One important, and sometimes lengthy, therapeutic intervention is to teach the proper walking and movement patterns; well, re-teach. Because our body’s primary function is protection, most people limp to prevent additional damage to the area. We help you trust your leg to do the work the right way.

Your rehab professional will also help your minimize muscle loss and optimize the healing process by creating a safe and ideal healing environment. Muscle loss during the protective phase can mean additional time to regain strength and equality between the two sides.

Before you move on to the second phase, you will be required to complete a list of progression criteria. This list will be part of your rehab protocol, and can be different if your doctor has additional items he/she feels are appropriate for your recovery. Most protocols have these items on the list: removal of immobilization device; sufficient reduction in pain and swelling, and spasms reduced to permit safe and effective therapeutic exercise; and acceptable degree of tissue healing to permit therapeutic exercise without injury aggravation (usually based on moving out of the acute inflammatory phase of healing).

Repair (Restorative) Phase

The second phase of rehab is the restorative phase. Your rehab professional will begin prescribing more vigorous therapeutic exercises to restore the ACL and knee to pre-injury functional levels. This phase lines up with the proliferation phase of healing, and bleeds over into the remodeling phase of healing. The goals of this phase are to increase the knee joint’s range of motion; increase the strength of the quadriceps muscles; reduce the scar tissue formed from the surgery; increase the endurance of all muscles in the leg; and reduce pain, muscle spasms, and swelling to be nonexistent.

Treatment during this phase may include quad sets, hip exercises, balancing, hands-on range of motion, pool exercises, and perfecting normal movement. The point is to re-establish normal, full-weight bearing status with normal walking patterns. Your rehab professional will focus on restoring the optimal range of motion for your knee, restore the flexibility of the muscles involved, and help maintain and develop your total body conditioning.

Just like for phase one, there is another list of progression criteria you have to achieve before moving to phase three. That list includes even less pain, less swelling, sufficient wound healing that allows for partial return to functional activity, return of range of motion, normalized flexibility and strength, normalized mechanics, and satisfactory protection from re-injury (bracing, taping, activity modification, etc.).

Maturation (Return to Full Functional Activity) Phase

Phase three is the period of time where sufficient healing and reconditioning of the knee, as well as the whole body, has occurred to allow for a safe return to regular practice or conditioning activities. This phase takes place during the remodeling phase of healing. This is the resistive phase. At this phase, the goal is to have full motion, increase strength and endurance, increase balance and body awareness, and maintain conditioning levels. Your rehab professional will focus on returning you to your sport safely and with confidence.

That is the end goal: to get you to return to sport. Throughout this phase you will most likely perform functional activities that mimic sport specific demands so your body can adapt and prepare for the unpredictability of sport. This included coordination, agility, speed, and balance. But the most important thing is your confidence. If you don’t feel confident in our knee, no amount of training will help you perform at pre-injury levels.

To progress to the final phase, all athletes must have optimal restoration of all physical components involved. You should be able to fully participate in the rehab program without restriction and complete symptom free.

Maintenance Phase

This is it. The final phase of rehab. The maintenance phase. It is exactly what it sounds like: the period of time after complete restoration of the knee (or other body part) and knee function, and return to full participation in sport. This phase is designed to continue specific conditioning or to ensure good mechanics are retained, and that the knee maintains newly gained strength, range of motion, and fitness level.

This phase is particularly relevant for extended rehab programs (like an ACL rehab program!). There is a point around month 6 in the ACL protocol when you physician may release you to begin integrating back into practice and full activity. For most ACL injuries, a return to sport at month 6 could potentially happen, but research tells us that every month after month 6 that return to sport is delayed decreases the risk of re-injury by 51%. Hypothetical math time: you have an 80% risk of re-injuring your ACL at month 6, by month 7 that risk is now 39%, by month 8 that risk is 19%, and by month 9 the risk is reduced to 9%!! Yes, I know that you lose 3 months of playing time, but there is so much you can do to strengthen your knee to prepare for 110% participation by month 9.