Anterior Cruciate Ligament Pathology

Updated: Oct 19, 2021
Author: John Maguire, MBBS, MSc, FRACS; Chief Editor: Thomas M DeBerardino, MD, FAAOS, FAOA 



The anterior cruciate ligament (ACL) is one of the most commonly injured ligaments of the knee. Injuries occur predominantly in a young and sports-active population. Many patients are left with significant disability following injury to the ACL. The injury leads to alteration in the mechanics of the knee. This mechanical deficit can lead to an increased risk of meniscal injury. The incidence of osteoarthritis rises sharply when the meniscus is injured. 

Understanding and preventing associated meniscal pathology is the key to management of this condition. Treatment aims to protect the meniscus by modifying activity levels or reconstructing the ACL. This article endeavors to explain the complex nature of the ligament and its injuries and aid the reader in making informed management decisions.

Bonnet first discussed ACL injury in the medical literature in 1845. Further discussion was made by Segond in 1879. Stark made the first reports in the English literature in 1850. In 1900, Battle described surgical intervention when he attempted repair of the ACL. Subsequent operative descriptions include those by Groves and Jones in 1913.

In 1917, Groves described a reconstructive procedure. He used the iliotibial band (ITB) as the graft. In 1918, Smith detailed combined intra-articular and extra-articular procedures. Zur Verth first used the patellar tendon graft in 1933. Campbell used this same donor tissue in 1936. In 1966, Bruckner reported using the patellar tendon as a free graft. Since that time, numerous entries have been made in the literature describing natural history, operative reports, and surgical series.

Future research in the field of ACL surgery will attempt to address a number of features, including the following:

  • Kinematics - True kinematics of the knee have not yet been restored, and research continues in an attempt to accomplish this; more complex three-dimensional (3D) substitutes that accurately match the original ligament always are being considered
  • Fixation - New types of graft fixation are being tested regularly; in particular, anchoring devices that incorporate into the tissues themselves seem to be a good option [1]
  • Graft donor tissue - Much research is focused on new graft tissues; culturing cruciate ligaments on tissue scaffolds may be possible, and this would reduce donor-site morbidity and decrease operative times; currently, a number of laboratories around the world are attempting this [2, 3, 4]
  • Synthetic substitutes - Newer synthetics may emerge to replace those that have been tried in the past; results would have to be substantially better than those of previous attempts
  • Other developments - Morphogenic proteins will be used to aid graft incorporation, vascularization, and fixation; a number of different chemicals and delivery systems presently are in experimental use [4, 5] ; gene therapy also may emerge as an adjunct, and if genetic predisposition can be identified, modification may be useful to alter tissue types to create more resilient ligaments and other soft tissues

Problems of abnormal kinematics and early osteoarthritis continue, despite excellent surgery. Which of the emerging technologies will allow continued gains to be made remains to be seen. Until many of these problems are addressed, physicians can only continue to inform patients and allow them to choose the best management course.

For patient education resources, see Knee Injury and Knee Pain.


The anatomy of the ACL is highly complex. The ligament is intra-articular but extrasynovial. It is described as being composed of the following three main bundles:

  • Anteromedial
  • Posterolateral
  • Intermediate

The ACL really functions as a continuum, with a portion being tight through all ranges of knee flexion.

The ligament courses obliquely, running from the tibia anteriorly and medially to the femur posteriorly, superiorly, and laterally. The broad tibial footprint lies at a point one third to one half the distance between the medial and lateral tibial spines, 5-7 mm anterior to the posterior cruciate ligament (PCL).

On the femoral side, the attachment lies on the medial aspect of the lateral femoral condyle, just anterior to the posterior aspect of the intercondylar notch. An intercruciate ligament joins the ACL to the PCL. This intercruciate ligament may have some role in proprioception and coupling of the two ligaments.

The microstructure of the ligament is composed of collagen fiber bundles, grouped into fascicles. Type I collagen is the predominant collagen type, accounting for more than 90% of total collagen. Types III and VI also are found. Elastin is found in significant amounts and provides some of the elastic properties of the ligament.

The major blood supply for the ACL comes from the synovium and fat pads. The middle geniculate and terminal branches of the inferior medial and lateral geniculate vessels are the vessels involved.

Sensory receptors and nerve fibers have been identified in the ligament and associated feeding blood vessels. This suggests some sensory role and possible proprioceptive function.


The ACL acts as the primary restraint to anterior tibial translation and guides the screw-home mechanism associated with knee extension. It acts secondarily to prevent varus and valgus, particularly in the extended knee. Injury leads to abnormal kinematics of the knee. Subluxation episodes occur, creating abnormal shear forces on the meniscus and articular cartilage. Subsequent meniscal injury therefore is increased significantly.

The authors have found a significant increase in this meniscal pathology when ACL reconstruction is delayed. Associated with this meniscal pathology is an increased incidence of osteoarthritis. A series conducted by the authors demonstrated a 15% incidence of ACL tears in patients undergoing total knee replacement (TKR). This incidence is at least three times the incidence of ACL tears found in the general population.


Approximately 70% of ACL injuries occur through noncontact mechanisms. Patients experience giving way of the knee when attempting to rapidly change direction. This involves deceleration, coupled with a cutting, pivoting, or sidestepping maneuver. The remainder of cases tend to occur through direct contact and often are associated with other ligament injury.


The incidence of ACL injury in the United States is estimated to approach 1 case per 3000 individuals. In the United States alone, more than100,000 injuries occur from snow skiing each year. The estimated cost of management is on the order of 2 billion dollars annually, which is a significant problem. The authors' series demonstrated an incidence of 1.5% of the population of New South Wales, Australia, with males affected twice as often as females.

Females are at higher risk of ACL injury when sports participation numbers are taken into account. This higher risk is believed to be related to both intrinsic factors (increased Q angle, decreased notch width, increased joint laxity, hormonal influences) and extrinsic factors (less muscle strength, different muscle activation patterns, altered cutting and landing patterns). Further investigation is required to fully identify which of these factors are the most important and determine whether any alterations to ACL injury patterns can be made as a result of intervention.


The natural history of ACL injury is an interesting topic. Meniscal preservation and activity levels are the key points to consider. Multiple studies demonstrate an increased incidence of osteoarthritis following meniscectomy. Nonoperative management with adequate rehabilitation can be undertaken in those patients with sedentary lifestyles. Castelyn et al have demonstrated this finding.[6]  In 228 low-demand patients, long-term risk of requiring meniscal surgery or ACL reconstruction was low.

Patients that remain active should undergo reconstruction. Daniel states that more than 50 hours of high-level sports per year is significant. Incidence of meniscal injury increases with time, and therefore, there is an increased incidence of osteoarthritis. The authors' series of over 1000 cases confirms these findings; meniscal injury is time-dependent in those with ACL injury.

ACL reconstruction decreases the incidence of meniscal injury that requires meniscectomy. Normal kinematics are not restored. Osteoarthritis levels are thus reduced by the meniscal preservation associated with the reconstruction; however, these levels of osteoarthritis are not eliminated. Bone bruising may also play some role in the ongoing incidence of osteoarthritis following reconstruction.



History and Physical Examination

Clinical presentation of anterior cruciate ligament (ACL) injury occurs either as an acute injury or as a more chronic problem of recurrent instability.

Acute injury

Patients with acute injury present after an instability episode. More than 50% of these patients state that they felt a pop as the injury occurred. Up to 80% of these patients develop a rapid hemarthrosis (see the image below) over the subsequent 4 hours. Some studies demonstrate that ACL injury may occur with as many as 70% of all hemarthroses and must be excluded in such injuries.

Hemarthrosis of the knee occurs early following in Hemarthrosis of the knee occurs early following injury to the anterior cruciate ligament.

Associated injury is common. Meniscal tears occur in approximately 50% of cases, with a slightly higher incidence of lateral meniscal tears than of medial meniscal injury. Other pathology includes bone bruising in up to 70% of cases (mostly of the lateral femoral condyle), medial collateral ligament injury, and true fractures of the tibial plateaus or femoral condyles.

Clinical signs in the acute phase include a large hemarthrosis, limited range of motion (ROM), and joint-line tenderness. Various special tests can aid in making the diagnosis.

A positive Lachman test (see the first image below), dynamic extension test (see the second and third images below), and anterior draw test (see the fourth image below) assess anterior tibial translation. KT1000 assessment with more than 3 mm of translation as compared with the other knee can quantify the degree of instability.

The Lachman test is a sensitive test for examining The Lachman test is a sensitive test for examining the cruciate-deficient knee.
Dynamic extension test produces anterior tibial tr Dynamic extension test produces anterior tibial translation in the cruciate-deficient knee.
This image demonstrates the anterior tibial transl This image demonstrates the anterior tibial translation compared to that seen in the image above.
The anterior draw test commonly is performed to di The anterior draw test commonly is performed to diagnose anterior cruciate ligament injury.

Pivot-shift and jerk tests (see the image below) assess rotational instability but are difficult to perform, particularly in early postinjury stages. The anterior draw test may be negative in as many as 50% of cases because the posterior horn of the medial meniscus can directly block translation (a feature well described by DeHaven).

The pivot-jerk test is a sensitive test for examin The pivot-jerk test is a sensitive test for examining anterior cruciate ligament integrity.

Chronic ACL instability

Chronic presentations tend to involve repeated instability episodes, particularly when the cutting maneuver is attempted. Presentation also may be associated with mechanical pain related to meniscal tears or early osteoarthrosis. Recurrent effusions and a history of locking also are important points in assessing associated injury.

In chronic presentations, a similar assessment is made. Meniscal pathology should be sought with joint-line palpation and the McMurray maneuver. Pivot-shift and jerk tests are performed more easily and can provide a better indication of the degree of rotatory instability. The Lachman test can be negative in chronic cases. The ACL can scar to the posterior cruciate ligament (PCL), thus limiting anterior translation of the tibia. Because the scarring is nonanatomic, the pivot-shift phenomena persists, as does symptomatic instability. This should be kept in mind in chronic cases.



Laboratory Studies

Routine laboratory studies are not required. However, infection, crystal arthropathy, and inflammatory arthropathy must be excluded in routine fashion when suspected.

Imaging Studies


Radiologic examination should be undertaken in all cases. Radiographs are needed to exclude fracture, tumor, chondrocalcinosis, crystalline arthropathy, and osteoarthrosis. Routine series should include an anteroposterior (AP) view, a lateral view at 30°, notch view, a patellofemoral view, and a weightbearing posteroanterior (PA) view in extension and 45° in chronic cases or in older individuals who may be at risk of osteoarthritis.

Computed tomography

In cases associated with fracture or avulsions of the cruciates, computed tomography (CT) can add valuable information. The location and origin of bony loose bodies can be identified well with this modality.


Magnetic resonance imaging (MRI; see the image below) is a useful tool when the diagnosis is unclear or when nonoperative management of the anterior cruciate ligament (ACL) tear is being considered.[7, 8] Cruciate integrity is well visualized in T1- and T2-weighted images. In the acute phase, when pain and apprehension can cloud clinical signs, an accurate diagnosis can be made in 98% of cases.

MRI is used as an aid to diagnose anterior cruciat MRI is used as an aid to diagnose anterior cruciate ligament injury. MRI has a sensitivity of 95%.

Meniscal pathology also can be identified. With more than 50% of cases having a meniscal tear, some surgeons find this information of use in choosing a management plan. Bone bruising commonly is seen, particularly of the lateral femoral condyle. The long-term influence this may have on the outcome is not yet known.

Radionuclide scanning

Radionuclide scanning is not used routinely. Its major uses in the setting of ACL injury are to identify and quantify degeneration in chronic cases. Radionuclide scanning also is used to help make the diagnosis in cases of suspected postoperative infection.

Other Tests

A number of mechanical devices (eg, KT1000) are available on the market that are used in some centers to assist in diagnosis. Most centers do not use these devices on a routine basis in acute assessment. They are widely used in research to help quantify the degree of tibial translation.

Strength assessment using specific muscle testing units is a tool commonly involved in research. Muscle testing also can be used in the rehabilitation phase to compare strength in opposite legs. This can aid some practitioners in deciding when return to play is possible.


In the postinjury workup, few procedures are used.

Aspiration and drainage of a large hemarthrosis can aid diagnosis and assist in providing pain relief. If crystal arthropathy is suspected, assessment can be made to exclude this. Fracture creates fatty deposits in the hemarthrosis. Therefore, aspiration can aid in making a diagnosis in these cases. The authors do not recommend this as a routine measure.

Some practitioners use arthroscopy to assist in making a diagnosis. If the diagnosis is unclear after other testing, accurate assessment can be made by means of arthroscopy. Meniscal pathology and cruciate stump impingement also can be dealt with at the time of arthroscopy. This may be the only requirement for patients who are not planning to undergo early reconstruction. Results of meniscal repair are improved if the procedure is combined with cruciate reconstruction. Therefore, the authors recommend a single procedure if reconstruction is planned.



Approach Considerations

The aim in treating patients with anterior cruciate ligament (ACL) injuries is to prevent recurrent instability and associated meniscal injury. Once meniscal pathology occurs, there is a much higher incidence of osteoarthritis.

After acute injury, the major indications for surgical reconstruction are related to the degree of instability and level of activity. Activity levels can include sporting or work-related activities. Reviewing the patient profile in the history from the patient is very important in the decision-making process.

Daniel defined patients at highest risk as those who participated in more than 50 hours of high-level activity per year prior to injury and have marked instability.[9, 10]  Instability was defined as having a KT1000 manual max test of greater than 5 mm of anterior translation. In those with translation of greater than 7 mm, this was defined as marked instability. Surgical intervention can be justified in these cases, as these patients were found to be at greatest risk of requiring further surgery.

In the less active group, the decision for surgery becomes more controversial. A nonoperative management plan with extensive physical therapy and activity avoidance can be undertaken. If patients experience ongoing instability or are unwilling to modify activity levels, surgery should be considered.

In chronic cases, the major indication for surgical reconstruction is recurrent instability. Other types of surgical intervention may be required to deal with associated pathology, such as arthroscopy, partial meniscectomy, or meniscal repair. High tibial osteotomy also may be required to correct varus alignment, especially in degenerative cases and complex instabilities.

When surgery is indicated, there are many decisions to make, including the following:

  • Timing of surgery
  • Surgical approach
  • Type of graft
  • Rehabilitation (return to activity and sport)

Early surgery is indicated in the active and professional population because it ensures future stability and negates the possibility of future giving-way episodes, protecting the menisci. Early surgery is thought by some to cause postoperative stiffness and arthrofibrosis. However, early rehabilitation negates these concerns.

Surgical contraindications are limited and include the following:

  • Active infection
  • Soft-tissue abrasion
  • Patient reluctance to participate in the complex rehabilitation required

However, relative contraindications are common and include the following:

  • Patient is less than 2 weeks from injury
  • Low activity levels
  • Preexisting osteoarthrosis
  • Skeletal immaturity
  • Inflammatory arthropathy

Medical Therapy

Medical therapy is used in patients with cruciate injuries to minimize pain and swelling and to regain range of motion (ROM). Nonsteroidal medication combined with simple analgesia can be used in the acute phase for adequate pain relief.

Physical therapy is a very useful adjunct in cruciate injury.[11] In the acute phase, efforts are concentrated on regaining ROM, reducing effusion, and maintaining strength. In chronic cases, maximizing strength (particularly of the hamstring musculature) and retraining proprioceptive function are undertaken to reduce instability episodes.

Postoperatively, the role of the physical therapist or athletic trainer is essential. Most centers concentrate on involved rehabilitation programs that aim to gain early ROM and reduce postoperative swelling. Programs then progress through isometric strengthening and co-contractions of the hamstrings and quadriceps muscles. As time progresses, closed-chain exercises are introduced. Some centers also use protected range, open-chain quadriceps exercises. The next phase involves the introduction of nonimpact strengthening and endurance activities such as swimming. The final phase includes a gradual return to functional activities with sports-specific exercises.[12, 13, 14]

Surgical Options

The aim of surgical therapy is to reduce meniscal injury by decreasing instability episodes (see the image below). Fu stated that normal knee kinematics cannot truly be regained but that the meniscus can be protected.

Anterior cruciate ligament reconstruction aims to Anterior cruciate ligament reconstruction aims to reduce instability episodes in an attempt to preserve the meniscus. When meniscal injury has occurred, the knee becomes degenerate with time.

Arthroscopic techniques have dominated over the past few decades. The placement of the tunnels and the approach to the tunnel drilling remain controversial. Femoral tunnel drilling should be from an anteromedial portal rather than through the tibial tunnel. The anteromedial portal allows more freedom to correctly place the femoral tunnel. This approach has a secondary advantage because it allows the fixation screws to be placed parallel to the graft in the tunnel.[15]

Double-bundle techniques are also popular; however, they have created discussion as to their necessity. Because of its shape, the patella tendon graft usually automatically produces two bundles.

Graft types

Midthird patella tendon grafts are the criterion standard, for many reasons. The fixation of the graft and its incorporation into the tunnels is of primary importance and is assured with the patella tendon graft. The bone blocks on either end of the graft heal in the tunnels, negating tunnel widening. Fixation of the bone plug at the aperture of the femoral tunnel ensures a physiological and biomechanical advantage because it negates the windscreen wiper effect as the graft enters the tunnel. An added is the “block and tackle” effect that can be used on the tibial bone block fixation, ensuring the tension in the graft at full extension can be increased threefold.

Taking a core of bone with a special coring drill provides a suitable bone graft for the defects in the patella and tibia from where the graft was taken. This decreases the kneeling problem that has been a complication of patella tendon grafts and, hence, an argument for hamstring grafts.

A Cochrane study compared the outcomes of ACL reconstruction using patellar tendon (PT) or hamstring tendon (HT) autografts in ACL-deficient patients.[16] After a minimum 2- year follow-up, no statistically significant differences were noted between the 2 techniques for functional assessment, return to activity, Tegner and Lysholm scores, and subjective measures of outcome.

Whereas PT reconstructions resulted in a more statically stable knee, they were also associated with more anterior knee problems, a statistically significant loss of extension ROM, and a trend towards loss of knee extension strength; a trend towards loss of flexion ROM and a statistically significant loss of knee flexion strength were noted in HT reconstructions.[16]

Quadruple hamstring grafts have become popular as an alternative graft, mainly because of the ease of threading them in the tunnels; quintuple-strand grafts have been described as well.[17] However, quadruple grafts have two major problems. Their incorporation into bone is not reliable, as evidenced by tunnel widening and a higher failure rate. In elite athletes, recurrent hamstring tears are more likely, though some healing of the tendons occurs after grafting.

Quadriceps tendon grafts have not achieved universal acceptance but have the advantages of a bone block at one end and lower graft morbidity.

Allografts, prosthetic ligaments, and xenografts have all been tried but do not have the advantages of autografts.

Operative approaches

Multiple surgical procedures have been described for anterior cruciate ligament reconstruction and can be broken down into the following three types:

  • Intra-articular
  • Extra-articular
  • Combined

Intra-articular procedures are more commonly undertaken and usually performed arthroscopically. These procedures involve using harvested donor tissue and inserting and fixing it at the site of the old ACL.

Extra-articular procedures involve transferring tissue insertions in an attempt to stop rotatory instability. Operations such as the Ellison iliotibial band (ITB) transfer have been shown to provide short-term reductions in instability patterns. Reid stated that at 2 years, good results had been obtained in 70% of cases, but by 10 years, good results were found in only 20% of cases.[18]  Extra-articular procedures may be considered for patients who are skeletally immature or who are older and considered to be low-demand

When combined with intra-articular procedures, extra-articular procedures have been found to be of no added benefit. Current recommendations limit their use.

Tissues used in intra-articular procedures

PT (see the first image below), HT (see the second image below), and quadriceps tendon are the most commonly used donor grafts. Current literature suggests that little difference exists in long-term outcomes among the different tissues used.[19] Graft positioning is far more important.

Surgical reconstruction most commonly uses the pat Surgical reconstruction most commonly uses the patella tendon or the hamstrings as donor grafts. This image shows the patella tendon being harvested.
Hamstring tendons also may be used as a donor graf Hamstring tendons also may be used as a donor graft for anterior cruciate ligament reconstruction. This image shows a 4-strand hamstring graft being prepared.

Many papers suggest that there is some mild increase in the incidence of patellofemoral pain with the use of the PT. Patella fracture also has been described.

Increased laxity also has been suggested to occur with hamstring grafts. To date, the length of follow-up is shorter for hamstring grafts, so continued reports are required.

Allografts (Achilles tendon or PT) are expensive and have the risk of disease transmission. Allograft tissues can be useful in revision cases and in multiple ligament reconstruction.

Synthetic grafts (ligament augmentation device [LAD] or Gore-Tex grafts) have been shown to fail and have poor long-term results. Currently, these grafts are not recommended.

Fixation methods

Fixation can be undertaken with a number of devices. These include screws, staples, endoscopically inserted buttons, and transtunnel fixation devices. Little difference in outcome has been found in the literature regarding fixation type. Accurate placement of graft tunnels is far more important. The most common error is to place the femoral tunnel too anterior.

Anterior Cruciate Ligament Reconstruction

Preparation for surgery

Timing of procedure

Shelbourne and DeHaven suggested  that surgery performed within 3 weeks from injury can lead to an increase in arthrofibrosis and subsequent poor results.[13, 20] Noyes demonstrated that meniscal repairs performed within 8 weeks of injury do better than those with a longer delay. Results also are improved if repairs are combined with cruciate reconstruction. Thus, the optimal time for repair would seem to be 3-8 weeks from injury.

On the basis of the senior authors' experience, performing surgery before the third postinjury week does not affect the outcome.

The results of one study noted that increased time to surgery is strongly associated with a higher risk of medial meniscus injury and decreased repair rate. Whereas female patients experienced a lower risk of cartilage injury, increasing age and increasing time to surgery (>12 months) in male patients realized a greater risk.[21]

Meniscal pathology

Meniscal pathology can affect long-term outcome. Meniscal repair should be utilized whenever possible. The repair is best combined with ACL reconstruction. Partial meniscectomy leads to early degeneration of the knee. Various studies demonstrate that following meniscectomy, more rapid degeneration of the knee occurs. Patients need to be aware of this prior to undergoing surgery.

Other ligament injury

ACL injury often is associated with other ligament pathology. The medial collateral ligament (MCL), in particular, often is injured. The current recommendation is to treat the MCL injury nonoperatively in a brace, controlling valgus force but allowing ROM. Time spent in the brace varies with the grade of injury. AFter adequate healing, ACL surgery can be performed. If the MCL remains lax, thus failing nonoperative management, the two procedures can be combined in a single operative session.

After a 3-year follow-up, the data from one study showed that patients with combined lesions of the ACL and MCL who had undergone an arthroscopic double-bundle ACL reconstruction showed a significantly greater mean medial joint opening (1.7 mm) as compared with uninjured knees (0.9 mm). However, no significant difference was noted between anteroposterior laxity and other clinical parameters. Because the data showed that residual valgus laxity did not affect anteroposterior laxity significantly, these results suggest that no additional surgical procedure is necessary for the MCL in combined lesions.[22]

Activity level

Preoperative activity level has been shown to be associated with ongoing instability and further injury. Highly active individuals should undergo early reconstruction.

Skeletal immaturity

In the skeletally immature individual, concern exists regarding ACL reconstruction. Transphyseal drilling has been reported to cause growth arrest. Arnoczky suggested in his laboratory studies that small tunnels less than 6 mm in diameter that cross the physis at right angles and are filled with soft-tissue grafts cause few problems.

Fowler also demonstrated no such problems with his series, when using soft-tissue grafts less than 6 mm in diameter, placed over the top on the femur and centrally on the tibia. Other options include the use of the "over the top" positioned graft, which does not cross the physis, or extra-articular procedures.

One study sought to determine the volume of injury to the physis during double-bundle ACL reconstruction in children.[23] Using computer-generated three-dimensional (3D) models, drill-holes of 6, 7, 8, and 9 mm in diameter were placed to simulate tunnels in the femur; computer-aided software was used to calculate the total physial volume and the volume of physis that was removed by creation of the tunnels.

The results found that the volume of physial damage caused by posterolateral drill holes was greater than that produced by anteromedial drill holes in all subjects, which leads to the conclusion that double-bundle techniques substantially increase the volume of injury to the physis and may increase the risk of abnormal growth in the distal femoral physis.[23]

Operative details

A number of considerations regarding surgery in general are necessary and include the following.

Graft type

The literature at present shows little difference between the commonly used autografts of the PT (see the image below) and those of the hamstrings. These autografts are better options than an allograft, as the graft scaffold fills with native cells more completely and quickly. Synthetic grafts have shown poor results with high failure rates, and significant complications.

Image demonstrating the use of the patella tendon Image demonstrating the use of the patella tendon as a donor graft for anterior cruciate ligament reconstruction

Graft position

Graft positioning is where most technical mistakes are made (see the first image below). The femoral tunnel commonly is placed too far anteriorly (see the second image below), leading to an isometry problem. Likewise, the tibial tunnel often is made too far forward, which can lead to notch impingement of the graft.

Poor tunnel position and improper placement of har Poor tunnel position and improper placement of hardware are common causes of anterior cruciate ligament graft failure.
Anterior femoral tunnel placement is a common erro Anterior femoral tunnel placement is a common error in ACL reconstruction. This image shows a femoral tunnel placed far too anterior, resulting in a nonfunctional graft.

Graft fixation

Fixation types commonly used include intra-articular interference screws (see the image below), endoscopic buttons, post screws, and transfixing devices. Interference screws are the criterion standard, offering good biomechanical fixation. The graft weakens over subsequent months as incorporation is occurring.

Interference screws are the most commonly used for Interference screws are the most commonly used form of anterior cruciate ligament graft fixation.

Fixation can be a problem in the first few weeks following reconstruction. Concern has been raised by certain authors that button-type devices may allow windscreen wiper effects to occur. This allows the tunnels to enlarge. Using shorter button fixation loops may reduce the incidence of this phenomenon. This same problem also has been shown with interference screws. The outcome does not seem to be affected by this occurrence. Time will tell as to whether such problems are real and to what degree they may affect results.

Graft tension

Graft tensioning is a controversial issue. Most authors suggest that the graft should be tensioned in full extension to approximately 20-40 newtons (N). This should prevent overconstraining the graft. Arneja et al did a systematic review of randomized, controlled trials that evaluated graft tension in ACL reconstruction and outcomes. They found a trend of 80 N as the most effective tension with hamstring-polyester graft, but they found no clear tension trend for semitendinosus-gracilis or PT grafts.[24]

Graft isometry

The graft is placed in such a position to ensure that it is isometric. This ensures minimal translation of the graft with flexion and extension. If the correct footprints are used, this should be adequate. Interestingly, the real ACL is not isometric, and therefore, this is a problem with reconstruction and graft protection.

Notchplasty and impingement

Some surgeons perform notchplasty to ensure adequate visualization and reduce graft impingement. If a large notchplasty is required, many surgeons would suggest that the graft position may have been incorrect. Performing a large notchplasty can also make accurate positioning difficult.

Graft augmentation

Augmentation of the ACL grafts described has been found to be of no benefit in routine cases. Graft augmentation may be of value in complex ligament injuries when more than the ACL is injured.

Patient age

Surgery in patients who are skeletally immature must be viewed with caution. The authors prefer to wait until skeletal maturity occurs because of the risk of physeal growth arrest.

Activity level

If activity levels cannot be modified or if the individual competes at an elite level, the authors proceed with surgery. In these cases, the authors tend to use a hamstring graft. Arnoczky has demonstrated that no growth arrest occurs when the physis is crossed with a hamstring graft of less than 6 mm in diameter. This topic, however, remains controversial.


Most surgical cases are performed arthroscopically. The patient is placed supine on the operating table, and a tourniquet is applied. The leg is positioned with footrests and side supports, such that the leg remains in 70-90° of flexion. An arthroscopy is performed to identify any meniscal pathology and to clear the remnant ACL from the femoral notch.

The graft is harvested at the next stage. The two most commonly used grafts are the PT (with bony blocks at either end) and the hamstrings (semitendinosus and gracilis). Bushnell et al found that a 20 mL injection of 0.25% bupivacaine into the hamstring donor site through an arthroscopic shaver sleeve significantly reduces postoperative pain levels.[25]  Grafts then are fashioned so that they can be passed through tunnels in the femur and tibia.

The femoral tunnel commonly is prepared first. The tunnel is placed posterolaterally at the junction of the roof and sidewall (1- to 2-o'clock position [left knee]). The tunnel is enlarged to allow passage of the graft. The tibial tunnel then is prepared. The drill hole emerges at a point one third to one half the distance between the medial and lateral tibial spines, at a point just anterior to the posterior cruciate ligament (PCL).

When using the transtibial technique, the surgeon performs the tibial tunnel first. The femur is then drilled via the tibial tunnel. Correct position is essential to allow correct femoral preparation. The graft is then passed through the tibial tunnel into the femoral tunnel. Fixation can then be performed in a number of ways.

Jarvela et al compared tunnel enlargement with double-bundle (35 patients) and single-bundle (25 patients) ACL reconstruction. There was no significant difference in tunnel enlargement on the femoral side, but tunnel enlargement on the tibial side was greater with single-bundle reconstruction.[26]

The last phase involves assessment of the notch to ensure that the graft does not impinge on the anterior aspect of the femoral notch. Wounds then are closed in routine fashion.

Postoperative Care

In the early postoperative period, the major considerations relate to pain management, wound healing, prevention of neurovascular complications, prevention of deep vein thrombosis (DVT), and rehabilitation.

Pain is best managed with a number of interventions. These include adequate infiltration of local analgesia; cold therapy using ice; and administration of anti-inflammatory medications, oral analgesia, and simple narcotics in sparing doses.

Adequate assessment is required to monitor the development of compartment syndrome and neurological or vascular complications. DVT in the postoperative phase also is well documented, but rare. Prophylaxis may be required in high-risk patients, including those taking the oral contraceptive pill. The authors have a low threshold for investigating those with clinical signs or symptoms and those at risk. Early mobilization is one way of aiding prevention. Infection occurs in approximately 0.5% of cases. Although infection is rare, measures such as antibiotic prophylaxis are recommended.

Early rehabilitation and physical therapy has been one of the greater advances in the postoperative management of ACL surgery. Shelbourne coined the term accelerated rehabilitation for this approach. The authors concentrate on early ROM and mobilization. Patients then proceed through a lengthy protocol, which concentrates on isometric contraction, co-contraction of hamstrings and quadriceps, closed-chain exercises, and finally, functional return to activity. The entire program takes approximately 6-9 months before patients may return to full sporting activity.

A return to full activity prior to 6 months is not recommended. Arnoczky demonstrated in his studies on graft incorporation that the graft weakens following surgery and then undergoes a number of phases before reaching revascularization and ligamentization. The entire process takes as long as 2 years.[27, 28] Graft strength is good, however, at around 6-9 months after surgery.

The techniques of rehabilitation depend on the principles of graft protection and return rapidly to muscle and proprioceptive control. Early graft protection depends on closed chain isometric exercise and, in some cases, early bracing. One needs to realize that full graft incorporation and revascularization is complete 6 months after surgery. Therefore, a graduated program has plenty of time.

Other complications include patella fracture (in grafts harvested from this site) and arthrofibrosis. Timing of surgery should be delayed until the knee has returned to a settled state. Our studies have found the timing is not a problem in relation to arthrofibrosis, but other authors have suggested a concern in surgery prior to 2-3 weeks post injury.


The major complications associated with ACL surgery are as follows:

Infection, neurovascular injury, and DVT have been described previously. Complex regional pain syndrome (CRPS) occurs, but it is rare. CRPS must be recognized early and treated aggressively. Skin mottling, abnormal pain response, hypersensitivity, limb atrophy, abnormal sympathetic response, and regional osteopenia are all features that must be considered. Early motion, adequate analgesia, neurotrophics, regional guanethidine blocks, and sympathectomy may be required.

ACL surgical failure was well described by Fu et al, who listed the following four major subgroups:

  • Arthritis or pain
  • Arthrofibrosis
  • Extensor dysfunction
  • Recurrent instability

A number of considerations exist regarding development of arthritis and pain. Bone bruising occurs in up to 70% of cases. This phenomenon has been detected only with the advent of magnetic resonance imaging (MRI), and long-term effects are unknown. There may be an association with ongoing pain and poor results. Meniscal injury leads to the development of early arthritis. Those with meniscal injury can expect less favorable results than those in whom the meniscus is untouched.

ACL surgery cannot restore the true mechanics of the knee. Incidence of arthritis increases in active individuals, despite surgery. Long term, this development leads to pain and decreased function. Some would suggest that the best protection and long-term results, with reduced arthritis and minimal pain, can be achieved only by sedentary activity. Unfortunately, few patients follow this program. Therefore, awareness that failure can occur despite a technically well-performed operation is required.

In some series, arthrofibrosis has been identified as being related to early surgical intervention or surgery to the MCL. In the authors' experience, arthrofibrosis has not been related to timing of intervention, but this should be considered. Other causes of reduced extension can include graft impingement, associated with an anterior tibial tunnel and/or the presence of a cyclops lesion.

A loss of flexion can be associated with anterior femoral tunnel placement. This can also lead to recurrent instability, as the graft is nonfunctional. This is the most commonly performed error in ACL surgery. Proper tunnel placement in the tibia and femur and adequate debridement of tissue around the tibial tunnel can reduce the occurrence of these problems.

Extensor dysfunction and ongoing patellofemoral pain can be reduced with adequate rehabilitation. In those individuals with preoperative symptoms, hamstring grafts may be more appropriate. An increased incidence of problems when the PT is used has been suggested in the literature. Patella fracture can occur in association with patella tendon harvesting; however, incidence of this problem is low.

Ongoing instability can be related to a number of problems. Poor technique, graft or fixation failure, further injury, varus alignment, and combined ligament laxity (eg, posterolateral corner injury) all have been listed as causes.

Accurate assessment of alignment, correct diagnosis of the ligaments involved, proper surgical technique, and functional rehabilitation can limit the incidence of these problems.

Long-Term Monitoring

Medical follow-up is undertaken at 7-10 days to ensure wound healing and to assess for presence of postoperative complications. Subsequent reviews occur in relation to upgrades in activity levels during the rehabilitation process. These generally are at 6 weeks and 3 months. A final review is undertaken at 6 months, before a return to full activity may take place.

Physical therapy review is more involved. Patients are seen in the perioperative period to begin ROM exercises and reduce swelling. Strengthening programs then begin as previously described. ACL classes with group activity sessions are used in the authors' center and are found to be useful in motivating some patients. A graded program is used, and assessment is required before progress to the next activity level is permitted.