The knee is the largest joint in the human body and proper function and health of the knees is required to perform most everyday activities. The knee is made up of the lower end of the femur (thighbone), the patella (kneecap), and the upper end of the tibia (shinbone). Articular cartilage, which is a smooth substance that protects the bones and allows them to move freely, covers the ends of the three bones and acts as the main "shock-absorber". Between the femur and the tibia are two C-shaped cushioning wedges known as the menisci that act as the secondary "shock absorbers". Large ligaments (tough bands of tissues) help hold the femur and the tibia together in order to stabilize the joint by preventing excessive movement. The lining joint is covered by the synovial membrane, which is a thin lining that releases fluid that lubricates the cartilage, reducing the friction within the knee joint and providing nutrition to the cartilage. All of these components work together to facilitate proper function of the knee.
There are four primary ligaments in the knee that act like strong ropes to hold the bones together in order to keep the knee stable. They are medial and lateral collateral ligaments and the anterior and posterior cruciate ligaments. The collateral ligaments are found on the sides of your knee and are responsible for controlling the sideways motion of the knee and brace it against unusual movement. The cruciate ligaments are found inside your knee joint where they form an "X" with the anterior cruciate ligament (ACL) in front and the posterior cruciate ligament (PCL) in back. They are responsible for controlling the forward to backward motion of the knee. The ACL runs diagonally in the middle of the knee and prevents the tibia from sliding out in front of the femur as well as provides rotational stability for the knee.
An ACL injury occurs when the knee is sharply twisted or extended beyond it's normal range of motion. There are between 100,000 and 250,000 ACL ruptures annually in the United States. 70% of ACL tears occur as a result of sport participation with a majority of them occurring in individuals between the ages of 15 to 45. The ACL is one of the most commonly injured ligaments in the knee with 70% of ACL injuries being noncontact - sudden change in direction with a planted foot or rapidly stopping. Female athletes are two to eight times more likely to suffer an ACL injury than male athletes and athletes who suffer an ACL injury are at an increased risk of tearing or injuring the other ACL. High risk sports include basketball, soccer, football, volleyball, and skiing.
Individuals who experience an ACL injury generally have the following signs and symptoms:
- Hearing or feeling a "pop" at the time of injury
- Severe pain, difficulty walking, and an inability to continue activity immediately after the injury
- Immediate onset of swelling around the knee joint that worsens for hours after the injury occurs
- A feeling of instability and bucking ("giving way") in the knee with weight bearing
- Limited or restricted range of motion
This pain can be very severe; therefore most individuals who injure their ACL seek immediate medical attention soon after an injury occurs, especially if they hope to return to sport / activity soon. However, if ignored, the swelling and pain may resolve on its own. The knee may feel fairly normal with daily activities, but the knee will usually feel very unstable when returning to sport. Returning to sports following an ACL injury runs the risk and the risk of causing further damage to the meniscus of the knee.
An ACL injury is typically well diagnosed with a combination of patient history, physical examination, as well as imaging studies. The physician will examine the knee to assess the range of motion, stability, swelling, tenderness, and overall strength and compare that to the uninjured knee. Often the diagnosis can be made on the basis of physical exam alone, especially if there is a complete rupture of the ACL, however, the physician will usually order X-rays to rule out any additional damage and may order an MRI scan to determine the extent of damage.
ACL injuries are considered "sprains" and are typically classified in the following three ways:
Grade 1: Mild ACL Sprain - This involves mild damage of the anterior cruciate ligament where it has been slightly stretched, but is still able to provide support and stability for the knee joint.
Grade 2: Moderate ACL Sprain - This involves loosening of the anterior cruciate ligament and is referred to as a partial tear of the ligament where it needs time and a period of inactivity to heal and mend itself. Sometimes a partial tear is significant enough to cause joint instability that does not resolve on its own.
Grade 3: Severe ACL Sprain - This involves complete disruption of the anterior cruciate ligament and is referred to as a complete tear of the ligament where the ligament has split into two pieces and the knee joint is unstable.
Treatment for an ACL injury or tear will vary depending on the individual based on the severity of the damage, age, and the level of activity they wish to resume. Younger athletes involved in agility-based sports will most likely require a surgery to safely return to the same level of sports, whereas older, less active individuals may be able to return to a less active lifestyle without the need for surgery.
Since an injury to the ACL is considered to be a sprain, the immediate treatment for an ACL injury is the self-care RICE approach:
- Rest - Walking will be painful, so it's best to avoid putting pressure on the injured knee and limiting activity while inflammation persists.
- Ice - Ice should be applied for the first 48 to 72 hours or until the swelling subsides for 10 to 20 minutes no more than once per hour. Use of a barrier, such as a towel, is strongly advised to protect your skin. Heat should be avoided while inflammation is developing; once the swelling goes down, heat can help soothe the pain.
- Compression - Using a compressing wrap can help significantly decrease swelling. Ensure that the wrap is snug; however, if there is numbness, tingling, or swelling above or below the wrap, it's probably on too tight and needs to be loosened.
- Elevation - Raising the knee above the heart level for a few hours a day can aide tremendously in decreasing swelling.
Once the initial swelling goes down with the help of the RICE approach, the patient and the physician will need to decide on the proper treatment plan. A torn ACL will not heal without surgery; however, non-operative treatments may be adequate for older patients who have very low level activity. These treatments may come in the form of a brace placed on the knee to provide stability or crutches to keep weight off the affected knee, or a physical therapy program where the physical therapist will provide specific exercises to restore function to the knee and strengthen the surrounding muscles that support it.
The operative process to address an ACL tear requires a rebuilding of the ligament. A torn ACL can't be successfully sutured or sewn back together, so the ligament is usually replaced with a piece of tendon harvested from another source. Commonly used grafts include: the patellar tendon which runs between the kneecap and the shinbone, hamstring tendons from the back of the thigh, or an allograft from a cadaver donor. The choice of allograft and pros/cons of each option will be discussed with the patient prior to surgery.
Small incisions are made around the joint where surgical instruments and the arthroscope (a small camera) will be inserted and the image will be sent to a video monitor allowing the physician to see inside the joint. Using surgical instruments, the torn ACL is completely removed. The next step is preparation and insertion of the graft:
Patellar Tendon Graft - The central portion of the patellar tendon is removed. The ends of the patellar tendon are attached to pieces or plugs of bones from your patella and your tibia which will act as an anchor for the new ACL. A guide wire is inserted for accuracy and a new tunnel is created in the tibia and the femur using a surgical drill. The new ACL graft is tied to the guide wire and the patellar tendon graft is pulled into place. Screws are then used to secure the plugs of bones into the tunnels. The screws may be made of metal, plastic, or an absorbable material.
Hamstring Graft - A portion of the hamstring tendons are removed using a surgical tool that is specially designed for this purpose. The hamstring graft is then folded over in order to increase strength and both ends are sutured to facilitate passage through the tunnel. The new ACL graft is tied to the guide wire and the hamstring graft is pulled into place. The physician will decide what devices are best to secure the new ACL graft into place - this can include screws, wedges, post and washers, button-like devices, or cross-pins. Over time, the tunnels will fill in with new bone.
Allograft (Cadaver Graft) - Allograft tissue is harvested from a donor and can either have bone plugs similar to a patellar tendon graft or consist entirely of soft tissue. Special tissue processing is used to clean and prepare the new ACL graft. A guide wire is inserted for accuracy and a new tunnel is created between the tibia and the femur for the ACL graft. The new ACL graft is tied to the guide wire and the allograft is pulled into place. Depending on the structure of the allograft (bone plugs vs. pure tissue), appropriate devices will be used to secure the allograft in place. Over time the tunnels/plugs will incorporate into the surrounding bone.
Regardless of the treatment approach taken, patients go through a rehabilitation program which includes physical therapy exercises that are crucial to strengthen your leg muscles and regain knee strength and motion. Each patient is unique, so the therapy program will vary based on his/her level of pain, extent of injury, and desired level of activity. For patients who have undergone surgery, physical therapy occurs in phases:
Phase 1: Range of Motion - This phase will first focus on returning motion to the joint and the surrounding muscles. This phase starts right after surgery and lasts approximately six weeks.
Phase 2: Strengthening - This phase is designed to protect the new ligament and beings after the first phase. Low-impact, cardio exercises such as elliptical trainer or a stair-stepper can be started around eight weeks with weight training starting two to three months post-operation, but only under the guidance of a physical therapist.
Phase 3: Sport-Specific Exercises - In this phase, the physical therapist will work with the individual to develop a tailored rehabilitation program to prepare to return to the desired sport or level of activity. Running is typically allowed somewhere between three and six months with activities that require pivoting and twisting at four to nine months.
Phase 4: Return to Sport - The final phase involves return to sport with significant supervision and usually occurs approximately between six to 12 months.
Typically, most patients return to normal day-to-day activities within three to four months, however, athletic activities and the return of strength can take up to a year; this is something to discuss with the physician as well as the physical therapist. Ultimately, the physician and physical therapist will work together to determine the safest time for the individual to return to their choice of sports. Careful consideration will be used to get the individual back to their chosen lifestyle while minimizing the risk of a re-tear of the ACL.
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