All You Need To Know About ACL Reconstruction

This article provides detailed information on anterior cruciate ligament (ACL) injuries. The following information includes the details of ACL anatomy and the pathophysiology of an ACL tear, treatment options for ACL injuries as well as a description of ACL surgical techniques and rehabilitation, potential complications, and outcomes. This article is meant to help the patient in making a well-informed decision regarding the management of an ACL injury.

Which part of the knee does it concern?

The knee is made up of four main things — the bones, cartilage, ligaments and tendons.

The femur, tibia and patella make up the bone structure of the knee joint. The ACL is one of four main ligaments in the knee that connect the femur to the tibia.

In essence, the knee is a hinged joint that is held together by the medial collateral (MCL), lateral collateral (LCL), anterior cruciate (ACL) and posterior cruciate (PCL) ligaments. The ACL runs diagonally in the centre of the knee, preventing the tibia from sliding out in front of the femur, along with providing rotational stability to the knee.

A layer of articular cartilage covers the weight-bearing surface of the knee. The medial meniscus and lateral meniscus are on either side of the joint, between the cartilage surfaces of the femur and tibia. These menisci function as shock absorbers and work with the cartilage to reduce the stresses between the tibia and femur.

What is an ACL injury?

The anterior cruciate ligament (ACL) is one of the most commonly injured ligaments in the knee. By and large, the incidence of ACL injury is higher in people who take part in high-risk sports such as football, soccer, basketball and skiing.

About half of all ACL injuries occur along with damage to the meniscus, articular cartilage, or other ligaments. In addition, patients may have bruises of the bone beneath the cartilage surface. These bruises may be noticed on a magnetic resonance imaging (MRI) scan and may indicate injury to the overlying articular cartilage.

What causes ACL injuries?

Studies have estimated that most ACL injuries result from non-contact mechanisms, while a smaller fraction occur through direct contact with another object or player.

The mechanism of injury is often associated with pivoting, sidestepping manoeuvres, awkward landings, deceleration coupled with cutting or “out of control” play.

Certain studies have even shown that female athletes have a higher incidence of ACL injury than male athletes in some sports. Experts propose that this is because of the differences in muscular strength, physical conditioning and neuromuscular control. Pelvis and lower extremity (leg) alignment, increased ligamentous laxity and the effects of estrogen on ligament properties are some other hypothesized causes of this gender-related difference in ACL injury rates.


How are ACL injuries examined?

Right after an ACL injury, patients typically experience pain and swelling. The knee might also feel unstable. Within a few hours, there will often be a large amount of knee swelling, a loss of full range of motion, tenderness or pain along the joint line and discomfort while walking.

During the clinical evaluation, the doctor may conduct X-rays to examine for any possible fractures. A magnetic resonance imaging (MRI) scan may also be conducted to evaluate the ACL and check for any evidence of injury to the other knee ligaments, meniscus cartilage or articular cartilage.

On top of performing special tests for identifying meniscus tears and injury to other ligaments of the knee, the physician will usually perform the Lachman’s test to see if the ACL is intact.

If the ACL is torn, the examiner will feel increased forward (upward or anterior) movement of the tibia in relation to the femur. This is especially so when compared to the normal leg. A soft, mushy endpoint will also be felt when this movement ends.

What is the natural history of an ACL injury?

Without surgical intervention, what happens to an ACL injury naturally varies from according to each individual’s activity level, degree of injury and instability symptoms.

The prognosis for a partially torn ACL is usually favourable, with the recovery and rehabilitation period being at least 3 months. However, certain patients with partial ACL tears may still show some instability symptoms. In these cases, close clinical follow-up and a complete course of physical therapy will help identify such patients with unstable knees due to partial ACL tears.

Complete ACL ruptures, on the other hand, have a much less favourable result without surgical intervention. After a complete ACL tear, certain patients are not able to take part in cutting or pivoting-type sports, while others have instability even during normal activities such as walking. However, there are a rare few who can take part in sports without any signs of instability. This variability depends on the severity of the original knee injury and also the physical demands of the patient.

Around 50 per cent of ACL injuries occur along with damage to the meniscus, articular cartilage or other ligaments. Secondary damage may take place in patients who have repeated episodes of instability due to ACL injury. 10 or more years after the initial injury, a large portion of patients with chronic instability is shown to have meniscus damage when reassessed. Likewise, patients who have a 10-year-old ACL deficiency have an increased risk of articular cartilage lesions. 

Nonsurgical treatment

In non-surgical treatment, progressive physical therapy and rehabilitation can restore the knee’s condition close to its pre-injury state. The patient can also be educated on how to prevent instability and made to use a hinged knee brace. However, many who opt out of surgery may experience secondary injury to the knee because of episodes of instability.

Normally, surgical treatment is recommended in managing combined injuries, in which the ACL tears in combination with other injuries in the knee. Although, it is reasonable for certain patients to decide against surgery. Nonsurgical management of isolated ACL tears is likely to be effective or may be advised for patients such as:

  • Those with partial tears and no signs of instability
  • Those with complete tears and no symptoms of knee instability during low-impact sports, who are willing to give up high-impact sports
  • Those who live sedentarily or do light manual work
  • Children, whose growth plates are still open

Surgical treatment

Normally, ACL tears are not repaired using suture to sew it back together, because repaired ACLs have mostly been shown to fail over time. As such, the torn ACL is usually replaced by a substitute graft made of tendon.

  • Patellar tendon autograft (autograft comes from the patient)
  • Hamstring tendon autograft
  • Quadriceps tendon autograft
  • Allograft (taken from a cadaver) patellar tendon, Achilles tendon, semitendinosus, gracilis, or posterior tibialis tendon


Patient considerations

Active adult patients who are involved in sports or jobs that require heavy manual work and turning, pivoting or hard-cutting are encouraged to consider surgical treatment. This is inclusive of older patients who have previously been excluded from consideration for ACL surgery. It is one’s activity level, not age, that should determine if surgical intervention should be considered.

ACL surgery may be delayed in young children or adolescents until they are closer to skeletal maturity. This is because early ACL reconstruction can create a possible risk of growth plate injury, leading to bone growth problems. Alternatively, the surgeon may modify the surgical technique to reduce the risk of growth plate injury.

As for patients with a torn ACL and significant functional instability, they are at a high risk of developing secondary knee damage and should thus consider ACL reconstruction. It is common for there to be ACL injuries combined with damage to the articular cartilage, collateral ligaments, menisci, joint capsule or a combination of the above. Injuries to the ACL, MCL and medial meniscus are known as the “unhappy triad”, frequently seen in skiers and football players.

In the event of combined injuries, surgical treatment may be justified and is usually more effective. As many as half of meniscus tears may be repairable and able to heal better if the repair is done in combination with the ACL reconstruction.


Surgical choices

Patellar tendon autograft
In a patellar tendon autograft, the middle third of the patellar tendon of the patient, a bone plug from the shin and the kneecap are used. Sometimes referred to by surgeons as the “gold standard” for ACL reconstruction, this method is usually recommended for high-demand athletes and patients whose jobs do not require a significant amount of kneeling.

Studies comparing the results of the patellar tendon and hamstring autograft ACL reconstruction have shown that the rate of graft failure was lower in the patellar tendon group. Additionally, most studies show equal or better outcomes in terms of postoperative tests for knee laxity (such as Lachman’s, anterior drawer and instrumented tests) when this graft is compared to others. However, there is a greater incidence of postoperative patellofemoral pain (pain behind the kneecap) complaints and other problems with patellar tendon autografts.

This means that patellar tendon autograft has its downfalls too, such as pain with kneeling, postoperative pain behind the kneecap, slightly increased risk of postoperative stiffness and low risk of a patella fracture.

Hamstring tendon autograft
The semitendinosus hamstring tendon on the inner side of the knee is used in creating the hamstring tendon autograft for ACL reconstruction. An additional tendon, the gracilis, which is attached below the knee in the same area, is used by some surgeons. This creates a two- or four-strand tendon graft. Advocates of this method claim there are fewer problems associated with harvesting of the graft compared to the patellar tendon autograft such as a faster recovery and smaller incisions needed. Furthermore, there are fewer problems with anterior knee pain or kneecap pain after surgery and less postoperative stiffness problems.

Because the graft does not have bone plugs, its function may be limited by the strength and type of fixation in the bone tunnels. There have been contradictory results in research studies as to whether hamstring grafts are slightly more prone to graft elongation (stretching), which may cause increased laxity during objective testing. In recent times, some studies have also found decreased hamstring strength in patients post-surgery.

There is also certain evidence that patients with intrinsic ligamentous laxity and knee hyperextension of 10 degrees or more may face a higher risk of postoperative hamstring graft laxity on clinical exams. As such, some clinicians suggest the use of patellar tendon autografts in these hypermobile patients instead.

Furthermore, since the medial hamstrings frequently provide dynamic support against instability and valgus stress, certain surgeons feel that chronic or residual medial collateral ligament laxity (grade 2 or more) at the time of ACL reconstruction may be a contraindication for use of the patient’s own semitendinosus and gracilis tendons as an ACL graft.

Quadriceps tendon autograft
The quadriceps tendon autograft is normally used for patients who have previously failed ACL reconstruction. The middle third of the patient’s quadriceps tendon and a bone plug from the upper end of the knee cap are used. This yields a larger graft for heavier patients. However, because there is a bone plug on only one side, the fixation is not as solid as for the patellar tendon graft. As a result, there is a high association with postoperative anterior knee pain and a low risk of a patella fracture. On top of that, patients may find that the incision is not cosmetically appealing.

An increasingly popular option, allografts are grafts taken from cadavers. These grafts are used for patients who have previously failed ACL reconstruction and in surgery to repair or reconstruct more than one knee ligament. Some advantages of using allograft tissue include smaller incisions, shorter surgery time and the elimination of pain caused by obtaining the graft from the patient. The patellar tendon allograft allows for strong bony fixation in the tibial and femoral bone tunnels with screws.

Unfortunately, the downside to allografts are that they are associated with a risk of infection, including viral transmissions such as HIV and Hepatitis C, despite careful screening and processing. Several deaths linked to bacterial infection from allograft tissue, due to improper procurement and sterilization techniques, have caused improvements in processing techniques and the testing of allograft tissue. In addition, there are contradictory results from studies as to whether allografts are slightly more prone to graft elongation (stretching), which may cause increased laxity during testing.

There are some pieces of published literature which may state that the use of allografts for ACL reconstruction has a higher failure rate. There have been certain reports of higher failure rates for allografts in young, active patients returning to high-impact sporting activities after ACL reconstruction as compared to with autografts. 

However, the exact reason for this higher failure rate is not known. It could possibly be due to graft material properties such as graft donor age, sterilization processes used and storage of the graft. On the other hand, it could also be due to an ill-advised earlier return to sport by the athlete. This could be because of a faster perceived physiologic recovery, when in fact the graft is not biologically ready to be loaded and stresses during sporting activities. Further research in this area is still ongoing.


How is ACL reconstruction carried out?

Before the start of any surgical procedure, the patient is typically sent for physical therapy. Patients who have a stiff, swollen knee lacking full range of motion at the time of ACL surgery may face significant issues with regaining motion post-surgery. To achieve full range of motion from the time of injury usually takes around three weeks or more. Prior to ACL surgery, it is also advised that some ligament injuries be braced and allowed to heal. 

Next, the patient, surgeon and anesthesiologist will select the anaesthesia used during surgery. An anaesthetic block of the nerves of the leg may benefit the patient by decreasing postoperative pain.

To start off the surgery, the knee is examined while the patient is relaxed from the effects of the anaesthesia. This purpose of this final check is to verify that the ACL is torn and also to check for looseness of other knee ligaments that may need to be repaired during surgery or addressed after the operation.

If this physical examination strongly suggests a tear in the ACL, the selected tendon is then either harvested (for an autograft) or thawed (for an allograft). Afterwards, the graft is prepared to the right size for the patient.

Once the graft has been prepared, the surgeon places an arthroscope into the joint. Tiny incisions of one centimetre, called portals, are made in front of the knee. The arthroscope and the instruments are then inserted into the incision for the surgeon to examine the condition of the knee. The meniscus and cartilage injuries are trimmed or repaired and the torn ACL stump is removed.

In the most common method of ACL reconstruction, bone tunnels are drilled into the tibia and femur to position the ACL graft where the torn ACL previously was. Then, a long needle is passed through the tunnel of the tibia, up through the femoral tunnel, and then out through the skin of the thigh. The sutures of the graft are placed through the eye of the needle and the graft is pulled into place up through the tibial tunnel, and then up into the femoral tunnel. This graft is held under tension as it is fixed in position using interference screws, spiked washers, staples or posts. Generally, these devices that are used to hold the graft in place are not removed.

There are variations to this surgical procedure such as the “over-the-top”, “double-bundle” and “two-incision” types of ACL reconstructions. They may be used according to the surgeon’s preference or special circumstances, such as revision ACL reconstruction or open growth plates.

Right before completing the procedure, the surgeon will probe the graft to ensure it has good tension, verify the knee has a full range of motion and conduct tests such as the Lachman’s test to assess graft stability. Finally, the skin is closed and dressings are applied. Depending on the surgeon’s preference, a postoperative brace and cold therapy device may also be used. Normally, patients are able to go home on the same day of surgery.


How is pain from ACL reconstruction managed?

It is natural to feel some pain after the surgery as this is a normal part of the healing process. The doctor and nurses will help to ease and reduce the pain, which can assist in a speedier recovery.

Typically, medications are prescribed for short-term pain relief post-surgery. There are various types of medications available to help manage pain. They include opioids, local anaesthetics and non-steroidal anti-inflammatory drugs (NSAIDs). A combination of these medicines may be used to improve pain relief and also minimize the need for opioids.

One should be aware that even though opioids are used to help relieve pain after surgery, they are a narcotic acid and can be addictive. In the U.S., opioid dependency and overdose has even become a critical public health concern. It is thus crucial to use opioids only as instructed by a doctor. The usage of opioids should come to a halt once the pain begins to subside. If the pain has not started to improve within a few days after surgery, one should consult a doctor.


What does the rehabilitation period of ACL reconstruction look like?

An essential part of successful ACL surgery is physical therapy, with exercises starting immediately after the operation. A large portion of the success of ACL reconstructive surgery is dependent on the patient’s commitment to rigorous physical therapy. As there are now new surgical techniques and stronger graft fixation, physical therapy nowadays uses an accelerated course of rehabilitation.

Postoperative course
Within the first 10 to 14 days after surgery, the wound is kept dry and clean. Early emphasis is placed on regaining the ability to straighten the knees completely and restore quadriceps control.

The knee will be iced consistently to reduce pain and swelling. Depending on the surgeon, a postoperative brace and machine to move the knee through its range of motion may be used. Other than that, the physician also determines the weight-bearing status, which is the use of crutches to keep some or all of the patient’s weight off of the surgical leg, as well as other injuries addressed during surgery.

The goals of the rehabilitation of ACL reconstruction include regaining full range of motion of the knee, reducing knee swelling, strengthening the quadriceps and hamstring muscles as well as maintaining mobility of the kneecap.

Only when there is no longer pain or swelling, full knee range of motion has been achieved, and when muscle strength, endurance and functional use of the leg have been fully restored can the patient return to sporting activities.

Over the course of 4 to 6 months, the patient’s sense of balance and control of the leg should also be restored through exercises designed to improve neuromuscular control. Generally, the use of a functional brace when returning to sports is not necessary after successful ACL reconstruction. However, some patients may feel more secure by wearing one.

What are the surgical complications of ACL reconstruction?

Although the incidence of infection after arthroscopic ACL reconstruction is very low, there have been reports of deaths linked to bacterial infection from allograft tissue. This is due to improper procurement and sterilization techniques.

Viral transmission
Allografts, in particular, are linked to risks of viral transmission, including HIV and Hepatitis C. This is in spite of careful screening and processing. However, the chances of getting a bone allograft from an HIV-infected donor is said to be less than 1 in a million.

Numbness and bleeding
There are low risks of bleeding from acute injury to the popliteal artery and weakness paralysis of the leg or foot. Although, it is not rare to have numbness of the outer part of the upper leg next to the incision, which may be temporary or permanent.

Blood clots
Even though this is rare, a blood clot in the veins of the calf or thigh is a potentially life-threatening issue. A blood clot may break off into the bloodstream and travel to the lungs, causing a pulmonary embolism. It could also travel to the brain, leading to a stroke.

It might be possible for recurrent instability to occur. This may be due to rupture or stretching of the reconstructed ligament or poor surgical technique.

Stiffness is the feeling of difficulty in motion or the apparent loss of range of motion. It is sometimes accompanied by pain or swelling. Some patients have reported a loss of motion or stiffness in the knee after surgery.

Extensor mechanism failure
Rupture of the patellar tendon (patellar tendon autograft) or patella fracture (patellar tendon or quadriceps tendon autografts) may occur because of weakening at the site of graft harvest.

Growth plate injury
This is particularly applicable to young children or adolescents with ACL tears. Early ACL reconstruction creates a possible risk of growth plate injury, which can cause bone growth problems. The ACL surgery can be delayed until the child is closer to skeletal maturity. Alternatively, the technique of ACL reconstruction may be modified to lower the risk of growth plate injury.

Kneecap pain
Lastly, postoperative anterior knee pain is especially common after patellar tendon autograft ACL reconstruction. The incidence of pain behind the kneecap varies widely in studies, while the incidence of kneeling pain is usually found to be higher after patellar tendon autograft ACL reconstruction.