History
The surgical treatment of anterior cruciate ligament (ACL) injuries began in the early 1900s with attempted primary repair. Patients treated with primary repair had poor results, including recurrent instability, which led to the advent of ACL reconstruction in the 1970s. The MacIntosh and Ellison procedures, two of the original reconstruction techniques, used the iliotibial band for extraarticular stabilization of the knee. A high incidence of arthrofibrosis and overall poor clinical outcome led to the development of an anatomic intraarticular reconstruction through an open approach in the 1980s. A variety of synthetic grafts, such as polypropylene and Gore-tex, were used in the early stages. Over time, these grafts stretched and fragmented, leading to high failure rates, sterile effusions, pain, and instability. Tendon autografts and allografts, including hamstrings (HS) and bone–patellar tendon–bone (BPTB), quickly became the standard of care.
The development of arthroscopic instrumentation in the early 1990s facilitated transition to an arthroscopically assisted reconstruction. Initially, femoral and tibial tunnels were drilled independently through a two-incision technique and the grafts were fixed on the anterior tibia and lateral femur. However, in the early 1990s, a single-incision ACL reconstruction in which the femoral tunnel was drilled through the tibial tunnel became popular. In some instances, this approach resulted in a relatively vertical femoral tunnel. Recent biomechanical evidence has suggested that a femoral tunnel placed in the center of the native femoral footprint may convey a biomechanical advantage. Consequently, there has been a heightened awareness of the native ACL anatomy and a renewed focus on techniques that result in more anatomic placement of the graft.
Anatomy and Biomechanics
The ACL originates on the tibial articular surface lateral and anterior to the medial intercondylar spine. Proximally, it courses posterior and lateral to insert on the posteromedial wall of the lateral femoral condyle. Two functional bundles are present, the anteromedial (AM) and the posterolateral (PL), which are named for their tibial insertion sites ( Fig. 98-1 ). The ACL provides rotational stability and resists anterior tibial translation, varus stress, and valgus stress.
The position of the AM and PL bundles varies with flexion and extension of the knee. In extension, the AM and PL bundles are parallel, but as the knee flexes, the bundles cross and the posterolateral bundle moves anteriorly. The PL bundle is tight in extension and the AM bundle is tight in flexion. The femoral attachment of the ACL spans an area of 113 mm 2 and is circular in shape. The ACL attaches on the tibial side over an oval area, comprising approximately 136 mm 2 . ACL fibers do not pass anterior to the cruciate ridge (also referred to as the lateral intercondylar ridge), which runs proximal to distal on the lateral femoral condyle.
Indirect comparison from various studies suggests that ACL strength decreases with age. Under normal walking conditions, the ACL experiences forces of approximately 400 N, whereas passive knee motion produces only 100 N. High-level activities such as cutting, accelerating, and decelerating are estimated to produce forces up to 1700 N, which approaches the average maximal tensile strength of the ligament, 2160 ± 157 N. Despite this narrow window, the ACL works in concert with other stabilizing structures in the knee to resist force and usually requires an abnormally high load to fail. Many other structures in the knee can be injured in conjunction with an ACL rupture, including the menisci, collateral ligaments, articular cartilage, and joint capsule.
Basic Science
Microanatomy
The ACL is composed of longitudinal collagen fibrils that range in diameter from 20 to 170 µm. The fibrils are composed primarily of type I collagen but also contain type III collagen. They arrange to form a unit called the subfascicular unit, which is surrounded by a layer of connective tissue called the endotendineum . These units combine to form the fasciculus, which has an outer layer called the epitendineum . The fasciculus is ensheathed by the paratenon, forming the largest ligamentous unit. The microscopic architecture changes to a more fibrocartilaginous appearance near the bony attachments on the tibia and femur.
The blood supply to the ACL comes from branches of the middle genicular artery and secondarily from branches of the inferior medial and lateral genicular arteries, as well as the infrapatellar fat pad and synovium. The proximal portion of the ACL has better vascularity, because the middle genicular artery gives rise to ligamentous branches proximally and courses distally along the dorsal aspect of the ACL. The largest ligamentous branch is the tibial intercondylar artery, which arises proximally and bifurcates distally at the tibial spine to supply the tibial condyles.
Nerve fibers have been found in all regions of the ACL, primarily running parallel with the vasculature in a longitudinal manner, but also incorporating freely into the connective tissue. The appearance of the nerve fibers suggests a role in vasomotor control. However, the diameter of the nerve fibers in the connective tissue suggests a role in pain or reflex activity. This role is supported by findings of altered proprioception in patients with capsuloligamentous injury and partial restoration of this function with ligamentous reconstruction.
Biology of ACL Injury
The ACL is an intraarticular structure encased by a thin soft tissue envelope formed by the synovial lining. Rupture of the ligament usually causes disruption of this synovial lining and hematoma formation throughout the joint space with very little local reaction. Extraarticular ligaments, such as the medial collateral ligament (MCL), are contained within a robust soft tissue envelope. Injury to these ligaments causes formation of local hematoma and fibrinogen mesh that allows for invasion of inflammatory cells, resulting in healing with granulation tissue and eventually organized fibrous tissue.
Biology of ACL Reconstruction
Epidemiology
ACL injuries comprise 40% to 50% of all ligamentous knee injuries, primarily as a result of sporting activity. Injury to the ACL is most common in young athletes, especially female athletes in their adolescent years. Sports in which athletes are particularly prone to ACL injury are skiing, soccer, basketball, and football. The majority of injuries are from noncontact mechanisms and often occur during landing. Some studies have shown that the maximum strain on the ACL occurs with the knee near extension and a valgus force applied with internal tibial rotation and anterior tibial translation. Females have a higher risk of ACL injury, which is possibly due to gender differences in knee biomechanical forces during landing. Females have increased quadriceps to HS strength and land in a more erect posture, which creates a larger anterior shear force and a greater strain on the ACL.
It is estimated that 200,000 ACL reconstructions are performed annually, with an 8% failure rate. Failure can occur early or late, with early failure occurring within 6 months of reconstruction and late failure occurring after 6 months. Early failure is largely due to technical error in 22% to 79% of cases, with tunnel malposition being the primary culprit. Late failure is most commonly due to repeat trauma to the graft but can also be due to tunnel malposition, most commonly a vertical femoral tunnel.
Basic Science
Microanatomy
The ACL is composed of longitudinal collagen fibrils that range in diameter from 20 to 170 µm. The fibrils are composed primarily of type I collagen but also contain type III collagen. They arrange to form a unit called the subfascicular unit, which is surrounded by a layer of connective tissue called the endotendineum . These units combine to form the fasciculus, which has an outer layer called the epitendineum . The fasciculus is ensheathed by the paratenon, forming the largest ligamentous unit. The microscopic architecture changes to a more fibrocartilaginous appearance near the bony attachments on the tibia and femur.
The blood supply to the ACL comes from branches of the middle genicular artery and secondarily from branches of the inferior medial and lateral genicular arteries, as well as the infrapatellar fat pad and synovium. The proximal portion of the ACL has better vascularity, because the middle genicular artery gives rise to ligamentous branches proximally and courses distally along the dorsal aspect of the ACL. The largest ligamentous branch is the tibial intercondylar artery, which arises proximally and bifurcates distally at the tibial spine to supply the tibial condyles.
Nerve fibers have been found in all regions of the ACL, primarily running parallel with the vasculature in a longitudinal manner, but also incorporating freely into the connective tissue. The appearance of the nerve fibers suggests a role in vasomotor control. However, the diameter of the nerve fibers in the connective tissue suggests a role in pain or reflex activity. This role is supported by findings of altered proprioception in patients with capsuloligamentous injury and partial restoration of this function with ligamentous reconstruction.
Biology of ACL Injury
The ACL is an intraarticular structure encased by a thin soft tissue envelope formed by the synovial lining. Rupture of the ligament usually causes disruption of this synovial lining and hematoma formation throughout the joint space with very little local reaction. Extraarticular ligaments, such as the medial collateral ligament (MCL), are contained within a robust soft tissue envelope. Injury to these ligaments causes formation of local hematoma and fibrinogen mesh that allows for invasion of inflammatory cells, resulting in healing with granulation tissue and eventually organized fibrous tissue.
Biology of ACL Reconstruction
Epidemiology
ACL injuries comprise 40% to 50% of all ligamentous knee injuries, primarily as a result of sporting activity. Injury to the ACL is most common in young athletes, especially female athletes in their adolescent years. Sports in which athletes are particularly prone to ACL injury are skiing, soccer, basketball, and football. The majority of injuries are from noncontact mechanisms and often occur during landing. Some studies have shown that the maximum strain on the ACL occurs with the knee near extension and a valgus force applied with internal tibial rotation and anterior tibial translation. Females have a higher risk of ACL injury, which is possibly due to gender differences in knee biomechanical forces during landing. Females have increased quadriceps to HS strength and land in a more erect posture, which creates a larger anterior shear force and a greater strain on the ACL.
It is estimated that 200,000 ACL reconstructions are performed annually, with an 8% failure rate. Failure can occur early or late, with early failure occurring within 6 months of reconstruction and late failure occurring after 6 months. Early failure is largely due to technical error in 22% to 79% of cases, with tunnel malposition being the primary culprit. Late failure is most commonly due to repeat trauma to the graft but can also be due to tunnel malposition, most commonly a vertical femoral tunnel.
History (Clinical Presentation)
It is important to obtain a detailed patient history, including the injury mechanism and symptoms, as a first step to diagnosing an ACL injury. These injuries generally occur during a rotational movement or deceleration, and only one third of these injuries occur with direct contact. Most patients are unable to recall the exact mechanism of injury, but studies have shown that most acute hemarthroses and ACL injuries occur as a result of twisting injuries of the knee. Many patients report a popping sensation; however, the correlation between a popping sensation and an ACL injury is fairly nonspecific. A large proportion of patients with an ACL rupture experience immediate pain, swelling, and a feeling of instability. Most are unable to return to sport.
The most sensitive marker for an acute ACL injury is a severe effusion within 2 to 12 hours of the injury. Injury to the ligament disrupts the blood supply and causes a large hemarthrosis that is a hallmark of an ACL injury. It must be noted, however, that hemarthrosis can be caused by injury to the menisci or posterior cruciate ligament (PCL) or by osteochondral fractures. Moreover, lack of severe effusion should not exclude injury to the ACL.
Physical Examination
Physical examination is very important in diagnosing an ACL injury. Together with the patient history, the physical examination can often provide enough information for a definitive diagnosis. It is critically important to examine both the affected and unaffected knee to get a baseline measure for each patient. Examining the unaffected knee first calms the patient and helps him or her to relax, which is important when testing for ligamentous stability of the injured knee.
Examination of the acutely injured knee should begin with inspection. Patients who have significant knee injuries, such as an ACL rupture, usually present with a large effusion. The affected knee is often flexed to relieve the increased pressure in the joint caused by the hemarthrosis. If several days or weeks have passed since the injury occurred, the quadriceps may be atrophied compared with the contralateral leg.
Palpation should be performed to evaluate for warmth, degree of effusion, crepitus, and local tenderness. Warmth and effusion indicate inflammation, which correlates with a large hemarthrosis in the setting of an ACL injury. A large majority of acutely injured knees have tenderness to palpation either medially, laterally, or on both sides. Local swelling or tenderness over the lateral or medial aspects of the knee suggests injury to the MCL or lateral collateral ligament (LCL). Focal joint-line tenderness could indicate meniscal or chondral injury. Osteochondral injury may also present with crepitus on range of motion (ROM) testing of the knee.
ROM is restricted in almost 90% of patients with acute ACL injury. Apprehension and guarding are common, and physical examination findings can be more revealing after aspiration or local intraarticular injection. Although it is not commonly performed, aspiration can also provide clues to the diagnosis because a hemarthrosis suggests ligamentous injury, whereas the presence of fat globules suggests a bony injury. Both active and passive ROM should be tested to determine if there is injury to the extensor mechanism or mechanical block from a meniscal tear, loose body, or torn ACL that is obstructing motion.
Ligament Laxity
Although some studies have reported less than 30% sensitivity of the ligamentous examinations in patients with acute ACL injuries, a recent metaanalysis showed that the sensitivity and specificity of a combined ligamentous examination (anterior drawer, pivot shift, and Lachman testing) was 84% and 92%, respectively. The Lachman test showed a sensitivity ranging from 60% to 100% (mean, 84%) and specificity of 100% in a single study. Anterior drawer testing is 9% to 93% sensitive (mean, 62%) for ACL injury, with specificity ranging from 23% to 100% (mean, 67%). The anterior drawer and Lachman test address anteroposterior stability of the knee but not rotational stability. The pivot-shift test addresses rotational stability by combining a valgus stress on the knee with rotation and axial load during knee flexion. A positive test is marked by a palpable clunk produced by reduction of the subluxed lateral tibial plateau by the iliotibial band when moving from full extension into flexion. This test is 27% to 95% sensitive (mean, 38%) and is limited in patients who are awake because of guarding. It is also important to test for varus and valgus stability of the knee to evaluate for injury to the LCL or MCL, which can also be injured in the setting of an ACL rupture.
In one study, arthroscopic examination in athletes with acute knee injuries and hemarthrosis who had no obvious clinical laxity on examination revealed surgical-type injuries in 90% of patients. Seventy-two percent had ACL tears, and two thirds of these patients had associated meniscal tears. Other injuries included isolated major meniscal injuries, osteochondral fractures, and injuries to the PCL. Other studies have confirmed these findings, and the authors recommend arthroscopy in patients who experience tense effusion within 12 hours of injury.
Arthrometers such as the KT-1000 and KT-2000 can be used as an adjunct to manual maneuvers such as the Lachman and anterior drawer tests, but they are not necessary to make the diagnosis of an ACL rupture. Thus they are most commonly used for research purposes.
Imaging
Imaging studies including radiographs and magnetic resonance imaging (MRI) can help confirm the diagnosis of an ACL injury. Radiographic evidence of a lateral capsular avulsion of the proximal tibia (often referred to as a Segond fracture) is pathognomonic for an ACL injury. Standard radiographs can help exclude associated injuries such as loose bodies, tibial eminence avulsion fractures in younger patients, degenerative changes, and acute fractures of the proximal tibia or distal femur.
MRI is the gold standard for diagnosing an ACL injury because it is both highly sensitive and specific in detecting ACL tears ( Fig. 98-2 ). The majority of ACL tears occur in the midsubstance of the ligament and are visualized on MRI as increased signal intensity with discontinuity of the ligamentous fibers. Hemarthrosis is common, and the presence of a bone bruise is observed on MRI in 84% of patients with an ACL rupture, with the highest incidence on the lateral tibial plateau and lateral femoral condyle, at 73% and 68%, respectively. Secondary signs of ACL rupture may include buckling of the PCL and uncovering of the posterior horn of the lateral meniscus on sagittal MRI sequences due to anterior translation of the tibia relative to the femur. Additionally, the LCL, which is oblique in orientation and typically not visualized in its entirety, may be seen from its origin to insertion on a single coronal image. MRI is also useful for evaluating meniscal injury and osteochondral defects. In patients with ACL rupture, injury is observed to the medial meniscus in 51% of patients and to the lateral meniscus in 54% of patients, with injury to both menisci observed in 33% of patients. MCL injury is observed in 23% of patients with an ACL rupture.
Decision-Making Principles
Operative Versus Nonoperative Treatment
The desired activity level of the patient must factor into the decision about whether to pursue nonoperative management of an ACL rupture or ACL reconstruction. The most common complaint of patients with a deficient ACL is recurrent instability and “giving way,” and as a result, fewer than 20% of patients return to their preinjury activity level. No large prospective trials have been conducted to demonstrate the natural history of ACL deficiency and the risk for further injury and osteoarthritis. However, it is accepted that high-level athletes with ACL deficiency have a significantly increased risk of symptomatic instability and meniscal injury without ACL reconstruction, which was demonstrated in a group of East German Olympic athletes who continued to compete after sustaining an ACL rupture. Thirty-five year follow-up after the injury revealed that 18 of the 19 athletes had undergone surgery for at least a partial meniscectomy, and 10 of 19 patients had severe osteoarthritis that was treated with a total knee arthroplasty. Other studies have confirmed the increased incidence of meniscal tears with chronic ACL insufficiency, which is thought to occur as a result of decreased rotational stability. Bray and Dandy showed a significantly higher incidence of meniscal injury in patients who had positive results of a pivot-shift test after ACL reconstruction. Although ACL reconstruction may protect the meniscus, it does not completely guard against subsequent injury, nor does it prevent subsequent osteoarthritis.
Age
A study of middle-aged patients who elected nonoperative treatment revealed that 83% had a satisfactory outcome despite a very high incidence of instability. Eighty-seven percent had little to no change on radiographs at 7-year mean follow-up. However, at 4-year follow-up of patients treated conservatively, only 14% were able to return to unlimited athletic activity. Five-year follow-up in athletic persons showed that 74% were unable to participate in twisting or turning activity and 69% had pain with strenuous activity. More importantly, at 11-year follow-up, 44% of patients had radiographic evidence of moderate to severe degenerative joint disease and moderate to severe disability.
The success of ACL reconstruction is age independent, with 91% of patients older than 40 years reporting excellent or good results at 2-year follow-up, compared with 89% for patients younger than 40 years. Nonoperative management with activity modification produces good to excellent results in 57% of patients older than 40 years. Older patients often have more social and professional obligations that may prevent them from proceeding with ACL reconstruction and successfully completing a rehabilitation program, which highlights the importance of stratifying patients by activity level to determine the indication for ACL reconstruction. The use of an allograft instead of an autograft in the older population decreases morbidity and has been shown to produce comparable results.
Gender
Female athletes are at a two- to eightfold greater risk for ACL injury compared with their male counterparts. Most studies have looked at high school and collegiate athletes in soccer, basketball, and volleyball. The majority of injuries occurred as a result of noncontact mechanisms, which led to investigation and speculation about gender differences that can account for this significant disparity. Possible etiologies have centered on hormonal and neuromuscular differences, environmental conditions, and differences in anatomy, such as alignment or joint laxity.
Anatomic differences that have been evaluated include Q angle, the size and shape of the intercondylar notch, the size of the ACL, material properties of the ACL, foot pronation, body mass index, and generalized ligament laxity. None of these differences alone places females at a greater risk of ACL injury. However, a study of West Point cadets produced a logistic regression model that could predict risk for noncontact ACL injury in 75% of cases, based on a narrow femoral notch, body mass index one standard deviation above the mean, and generalized joint laxity. Some studies have shown that hormonal changes during the menstrual cycle affect the material and mechanical properties of the ACL, which could make it more vulnerable to injury during specific phases of the cycle. However, this effect has not been shown definitively and requires further investigation.
Partial ACL Tears
Diagnosis of a partial ACL tear can be challenging and requires close evaluation of the history, physical examination, and MRI findings, although the gold standard for diagnosis is arthroscopy. Partial tears comprise 10% to 28% of all ACL tears, and if left untreated, 42% will proceed to complete rupture. In addition, a large majority of patients with partial tears are unable to return to their preinjury activity level. KT-2000 arthrometer testing in patients with partial tears has shown only minimal differences in anterior tibial translation. Decision making regarding treatment of partial ACL tears should include evaluation of the patient’s desired activity level, the degree of laxity, and symptomatic instability. Options for conservative management include rehabilitation, focusing on HS strengthening, activity modification, and brace wear during activity. Well-designed prospective clinical trials are needed to accurately compare treatment regimens for partial ACL injuries.
Associated Injuries
Rupture of the ACL is associated with injury to other structures in the knee, including the medial and lateral menisci, MCL and LCL, chondral surfaces, PL corner structures, and fracture of the distal femur and proximal tibia. Many years ago, the phrase “unhappy triad” was coined by O’Donoghue to refer to the constellation of ACL rupture, MCL injury, and tearing of the medial meniscus. Subsequent studies have shown that lateral meniscal tears are equally common with ACL rupture. A review of meniscal injury in the setting of ACL rupture showed a predominance of lateral meniscal tears (56%) in persons with an acutely injured ACL and increased incidence of medial meniscal tears (70%) in persons with chronic ACL insufficiency. In addition, the authors found increased reparability with tears to the medial meniscus and increased likelihood of successful repair with concomitant ACL reconstruction. The likelihood of successful meniscal repair decreased with increased time from injury to surgery. An additional study of pediatric and adolescent patients demonstrated an increased risk of medial meniscal tears with delay in ACL reconstruction.
All associated injuries should be evaluated individually to form an overall management plan. Partial-thickness vertical tears posterior to the popliteus tendon that are stable at the time of ACL reconstruction may respond particularly well to nonoperative management. Injury to the medial meniscus should be addressed aggressively, and it has been shown that repair of stable peripheral tears decreases the risk of postoperative pain and the need for subsequent partial meniscectomy.
MCL injuries are common in the setting of ACL rupture, occurring in approximately 23% of cases. It was previously thought that high-grade MCL injuries may need to be treated operatively in the setting of ACL rupture. However, recent data have shown that nonoperative bracing of MCL injuries after ACL reconstruction results in equivalent clinical outcome as tested by anterior tibial displacement, function, participation in sporting activities, strength, and one-leg–hop testing. Another prospective randomized study found no difference between operative and nonoperative groups for treatment of grade III MCL injury combined with ACL rupture. However, in some persons with severe combined ligamentous injuries, MCL repair may be indicated, although it is often not necessary.
Revision ACL
As the number of ACL reconstructions continues to increase, so does the total number of failures. These failures can typically be categorized as biologic, technical, or traumatic. The majority of failures in the past were due to technical errors, such as improper graft placement, inadequate notchplasty, inadequate graft fixation, improper graft tensioning, use of a graft with inadequate tensile strength, or failure to correct other causes of instability in the knee. However, more recent data have shown that traumatic reinjury, which occurs in 32% of patients, is the primary mode of failure. The technical approach to revision ACL reconstruction has been refined during the past 10 to 20 years; however, the results of revision surgery are worse than those for primary reconstruction. The risk of chondral damage in the lateral compartment and patellofemoral space is increased with revision ACL reconstruction. Moreover, risk of chondral lesions at revision reconstruction increases in the presence of a previous partial meniscectomy. Patients must be counseled regarding the limitations of revision surgery and the potential for future failures.
Treatment Options
Nonoperative Options
Nonoperative management of ACL rupture can lead to recurrent instability and meniscal injury in athletes. For this reason, ACL reconstruction is recommended in patients who are active and require the ability to cut or pivot during physical activity. Persons older than 40 years can do well with a conservative training program but should be advised that a return to their previous activity level is unlikely. Patients should not make a decision regarding surgical management based on their age because studies have shown equivalent outcomes in patients younger and older than 40 years. In a recent randomized controlled trial of young, active adults, early reconstruction versus rehabilitation with the option of delayed reconstruction was evaluated. Patients undergoing delayed reconstruction had outcomes similar to those receiving early reconstruction, and the majority of patients assigned to the rehabilitation group elected to continue with nonoperative management.
Operative Options
ACL reconstruction is commonly recommended for young, active patients. The timing of when to reconstruct ACL injuries has been debated, but most studies have recommended delayed reconstruction. A few studies have shown an increased risk of arthrofibrosis with early reconstruction within the first month. Another study showed that rehabilitation prior to the operation can decrease the risk of arthrofibrosis, suggesting that operative treatment should wait for 2 to 6 weeks when motion returns. However, a metaanalysis of the current literature found no difference in clinical outcome between early reconstruction (performed within 3 weeks of the injury) and late reconstruction (performed more than 6 weeks after the injury).
Loss of terminal extension is the primary difficulty encountered, and patient satisfaction is greatly influenced by stiffness and restricted ROM. Patients who have an effusion, swelling, inflammation, and stiffness beyond 4 weeks after the injury was sustained, and who undergo ACL reconstruction, have an equal likelihood of experiencing arthrofibrosis, suggesting that it is the amount of effusion, stiffness, and inflammation present at the time of surgery that results in an increased risk of the development of arthrofibrosis. Currently the only strong indications for immediate reconstruction are associated injury to the PL corner or a repairable meniscal injury.
Preoperative loss of motion has a significant correlation with postoperative loss of motion. Sixty-seven percent of patients who have restricted ROM after surgery had limited ROM at the time of reconstruction. More recent studies have shown that excellent clinical results can be obtained with acute reconstruction (within 2 to 17 days) if a postoperative rehabilitation protocol emphasizing early ROM and terminal knee extension is used. We believe that the best approach is to allow time for the swelling to resolve and wait for the patient to regain good preoperative ROM prior to surgery.
Graft Selection
The keys to choosing an appropriate graft are that it exhibits properties similar to the native ACL, allows for secure fixation, incorporates into the bone tunnels, and limits donor site morbidity. All tendon autografts (BPTB, quadruple HS, and quadriceps tendon) and BPTB allografts have greater tensile strength and stiffness compared with the native ACL. Synthetic devices are no longer used in the United States because of an unacceptably high rate of complications, including failure and persistent effusion. HS and BPTB autografts are popular, but the choice of graft should be discussed with the patient.
The first choice in decision making is allograft versus autograft. An autograft incorporates into bone earlier, matures more rapidly, and avoids the risk of a host immune reaction, as well as disease transmission. Conversely, the use of an allograft is associated with less morbidity and requires a shorter surgical time. A metaanalysis of BPTB autografts versus allografts showed that patients treated with an autograft had a lower incidence of graft rupture and performed better on hop testing. However, when irradiated and chemically processed allografts were excluded from the analysis, no differences were found between autografts and allografts. Patients who underwent allograft reconstruction more commonly reported a final International Knee Documentation Committee (IKDC) score of A (normal knee). However, if a good result was defined as an IKDC score of A or B, no difference existed between the groups. Patients often prefer use of autografts rather than allografts because of a general aversion to allograft tissue. Physician recommendation also has a significant impact on graft selection, because more than two thirds of patients identified physician recommendation as the primary factor in their decision making. Recent evidence suggests a higher failure rate of ACL allografts in young, active patients. A retrospective cohort study found that patients 25 years and younger undergoing ACL reconstruction had a 29.2% failure rate with allograft tissue compared with an 11.8% failure rate with BPTB autografts. Another study showed that higher activity level after ACL reconstruction and allograft use for reconstruction were risk factors for ACL graft failure.
Allografts sterilized with radiation or ethylene oxide are significantly weakened, but current techniques using cryopreservation maintain the biomechanical properties of the tendon allografts. Disease transmission from tendon allografts has been reported, but the incidence is quite infrequent, occurring on average in less than one patient per year. Stringent guidelines have almost eliminated the risk of hepatitis C or human immunodeficiency virus, but theoretically a risk still exists. These risks are between 1 in 173,000 and 1 in 1 million for human immunodeficiency virus and approximately 1 in 421,000 for hepatitis C. A few reports of bacterial infection from donor grafts have been reported in the past 10 years, although a recent study specific for allograft ACL reconstruction found no increased risk of infection with allografts compared with autografts.
BPTB and HS tissues are most commonly used for ACL reconstruction. A metaanalysis comparing BPTB and HS autografts showed lower rates of graft failure (4.9% vs. 1.9%) and less anterior laxity on arthrometer testing in the BPTB group. However, the use of the patellar tendon is not without complication, because a greater number of patients in that group reported anterior knee pain (17.4% vs. 11.5%) and required manipulation under anesthesia for lysis of adhesions (6.3% vs. 3.3%). Additionally, the risk of patellar tendon rupture, patellar fracture, and quadriceps weakness is increased with a BPTB autograft. The primary morbidity associated with an HS autograft is pain from hardware prominence, which results in a higher rate of hardware removal (5.5% vs. 3.1%). HS autografts are also associated with HS weakness, but this weakness generally resolves within 1 year. A systematic review of current literature found that no single graft source is clearly superior to others. Another systematic review of outcomes using HS or BPTB autografts showed equivalent functional and clinical results but increased anterior knee pain and pain upon kneeling in the BPTB group.
We prefer using autografts rather than allografts in most patients having an index procedure because of the lower failure rate with autografts and the slight risk of disease transmission with allografts. Our preferred choice for an autograft in patients who place a high demand on their knees is BPTB because of earlier incorporation of the bone plugs into the tibial and femoral tunnels and excellent clinical results. We reserve the use of allografts for patients who place a lower demand on their knees, in the revision setting, or in cases of specific patient preference.
Graft Harvest
Harvesting of each type of tendon provides unique technical challenges. Patellar tendon autografts require harvesting bone from the patella and tibial tubercle, which can increase the risk of fracture and damage the articular cartilage of the patella. Many surgeons prefer repairing the paratenon to improve glide and prevent scarring to the overlying tissue. HS harvest requires elevating the sartorius to access the semitendinosus and gracilis and can put the superficial branch of the saphenous nerve at risk. Amputating the tendon prematurely during the harvest is also a risk. The quadriceps tendon is another autograft option.
Graft Tension and Fixation
Graft tension is influenced by the amount of force placed on the graft, as well as the amount of knee flexion and rotation. The graft needs enough tension to stabilize the knee, but too much tension can stretch the graft and lead to failure of the graft itself or failure of fixation. Cadaveric studies have evaluated knee stability under various amounts of tension and showed that 40 N to 60 N with the knee in full extension is ideal for HS and BPTB grafts. Some authors recommend providing tension with the knee in full extension, whereas other authors argue that it is best to provide tension with the knee in 20 to 30 degrees of flexion. It has also been suggested that grafts should be preconditioned prior to implantation to prevent creep. Evaluation of outcome is complicated because most surgeons provide tension manually and in various knee positions. Graft-tensioning boots are used by some surgeons because they eliminate the need for manual provision of tension and allow the surgeon to use both hands for tibial fixation. Further trials are required to provide more comprehensive data regarding provision of graft tension.
Graft fixation should be strong enough to withstand closed-chain exercises for at least 12 weeks until the bone or tendon is able to incorporate into the bone tunnels. Closed-chain exercises produce on average 200 N of force but can produce up to 500 N of force. Poor fixation can cause the graft to slip or the fixation to fail altogether. Interference screw fixation of patellar bone blocks has the highest stiffness and fixation strength, ranging from 423 N to 558 N. Screw placement parallel to the bone block is optimal for maximum pull-out strength, whereas divergence greater than 30 degrees has increased risk of failure from pullout. Screw diameter and length also influence fixation strength. The use of tibial dilators has no effect on fixation strength, nor does the use of bioabsorbable instead of metallic screws.
Many graft fixation devices are commercially available. Soft tissue grafts can be secured with use of interference screws, suture posts, screw and washer constructs, and staples on the tibial side. Similar fixation can be used on the femoral side in addition to cross-pins and buttons. Cross-pins, screw and washer constructs, and buttons all provide indirect fixation, meaning the graft is suspended within the bony tunnel. All others provide direct fixation, which compresses the graft against the side of the bone tunnel. It should be noted that the clinical implications of most biomechanical fixation pull-out studies are limited because they were performed on porcine and bovine specimens at time zero.
Graft Healing
A successful ACL reconstruction relies on incorporation of the graft into the surrounding bone, as well as ligamentization and revascularization of the graft. Bone-to-bone healing is stronger and faster than soft tissue healing to bone. BPTB allografts and autografts heal in a process similar to fracture healing. With soft tissue grafts, the tendon takes 12 weeks to incorporate into the surrounding bone through remodeling of a cellular and fibrous layer formed at the tendon-bone interface. At 12 weeks, collagen fibers form an attachment to bone that resembles Sharpey fibers. At 12 months after ACL reconstruction, all histologic markers of ligamentization and revascularization, including fiber pattern, cellularity, and degree of metaplasia, resemble those of a native ACL. Vascularity and fiber pattern demonstrate no maturation after 6 months, suggesting that tendon autografts may be mature enough at 6 months to proceed with more aggressive rehabilitation and possible return to sport.
Patellar tendon autografts incorporate faster than allografts and have stronger mechanical properties at 6 months. Allografts have a prolonged inflammatory response, decreased strength, and a slower rate of incorporation and tissue remodeling at the 6-month time point.
Revision Options
Determining the cause of failure is the key to successful revision ACL reconstruction. The surgeon should discuss with the patient the expected outcome and the anticipated postoperative activity level. Planning for a revision ACL reconstruction should involve all the steps of a primary reconstruction, including evaluation for associated injuries. In addition, thought should be given to correcting any technical error from the primary surgery and graft selection for revision. The same graft options in primary ACL reconstruction exist for revision surgery, although reharvesting previously harvested graft tissue is not advised.
In one recent study, it was reported that only 54% of patients returned to their preinjury activity level after revision surgery. Patellar tendon autografts and allografts used for revision ACL reconstruction have produced equivalent clinical results and ligamentous stability on arthrometer testing. Overall, revision ACL reconstruction should be viewed largely as a salvage procedure, and patients should be aware that they may never return to their preinjury function and activity level.
A single-bundle ACL reconstruction uses one large graft that is fixed in place at the insertion site of the ACL on the tibia and femur (the so-called footprints of the ACL). If the insertion site is large enough, some surgeons attempt to recreate the AM and PL bundles as two distinct structures using two grafts in a procedure referred to as a double-bundle ACL reconstruction. This section presents a single-bundle ACL reconstruction.
Positioning and Setup
The patient is positioned supine on the operating table. After administration of an appropriate anesthetic, both the operative and uninvolved legs are examined, including ROM and anterior drawer, Lachman, and pivot-shift tests. Special attention is given to varus, valgus, and posterolateral instability because those structures are not assessed arthroscopically. A post is placed proximally and laterally against the thigh to allow for valgus stress on the knee and visualization of the medial compartment.
Graft Harvest
If examination of the anesthetized patient confirms the diagnosis of an ACL tear, we proceed directly to harvesting of the graft. We most commonly use a BPTB autograft. The incision is marked 1 cm medial to the inferior pole of the patella, extending longitudinally 1 cm medial to the tibial tubercle ( Fig. 98-3 ). The skin is incised and sharp dissection is carried down through the skin and subcutaneous tissue to the level of the patellar tendon paratedon. The parateon is incised at the midline and separated from the underlying tendon with use of a scalpel. The knee is slightly flexed and a scalpel is used to harvest the central portion (usually 9 to 10 mm) of the patellar tendon. An oscillating saw is used to make the bone cut on the tibial side. Our goal is to make the tibial bone plug 20 to 25 mm in length and trapezoidal in shape, which is achieved by making a cut perpendicular to the surface of the bone medially and a lateral cut that is angled 20 degrees toward the medial cut. The distal cut is made last, and the bone plug is extracted by hand with use of a 0.5-inch curved osteotome ( Fig. 98-4 ).
The knee is then placed into extension with the inferior two thirds of the patella exposed. The patellar bone cut should be 20 to 25 mm in length and triangular in shape, which is achieved by making medial and lateral bone cuts angled 45 degrees from the bone surface with an oscillating saw. The cuts should be made to a depth of 10 to 12 mm and should meet, allowing for easy extraction.
Graft Preparation
The bone plugs are shaped to fit into a 10-mm tunnel, and any excess bone is reduced to morsels for bone grafting of the patellar defect ( Fig. 98-5 ). Because loss of fixation is more likely in the tibial tunnel, the patellar bone plug (which has a denser architecture) is placed into the tibial tunnel to maximize purchase with the interference screw. Two perpendicular 2-mm drill holes are made at the distal one third of the patellar bone plug, and two drill holes are placed in the tibial bone plug. Heavy nonabsorbable suture is loaded onto a Keith needle and passed through each hole. The bone-tendon junction is marked with a sterile marker to allow for visualization during graft passage. The tendinous portion of the graft is measured with a sterile ruler.
