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Anterior Cruciate Ligament Injuries
History
The anatomy and injury of the anterior cruciate ligament (ACL) has been well-documented in history for centuries, but it was not until the mid-1800s that reports of surgical treatment of ACL injuries began to appear in surgical literature. Although there are some reports of success with early repair, surgeons realized that a more robust reconstruction was necessary to provide adequate knee stability. Ernest William Hey Groves was one of the first to report on ACL reconstruction in the early 1900s. He used a fascia lata graft placed through femoral and tibial tunnels and sutured it to the periosteum. Over time, Hey Groves’ original idea of intra-articular ACL reconstruction has evolved with a number of different simultaneous advancements, including early proponents of extra-articular reinforcements like that described by MacIntosh, which used an iliotibial band around the anterolateral aspect of the knee. Additional attempts to use synthetic grafts such as GORE-TEX or polypropylene were used in the early stages of ACL reconstruction. Due to high failure rates, stretching and fragmentation, resultant knee effusions, pain, and instability, these were quickly replaced by tendon autografts and allografts.
With the development of fiber optics and miniature television cameras in the 1970s arthroscopic assisted ACL surgery became more mainstream. 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 a tibial tunnel became popular. In some instances, this approach resulted in a relatively vertical femoral tunnel. As arthroscopic instrumentation and imaging modalities like computed tomography (CT) and magnetic resonance imaging (MRI) continued to improve, more biomechanics research was performed, and a heightened understanding of ACL anatomy and its influence on knee kinematics and stability showed that a femoral tunnel placed in the center of the native femoral footprint might convey a biomechanical advantage.
Anatomy and Biomechanics
The ACL originates on the tibial articular surface, just lateral and anterior to the medial intercondylar spine. Proximally, it courses posteriorly as well as laterally and inserts 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 PL bundle moves anteriorly. The AM bundle is tight in flexion and the PL bundle is tight in extension. The tibial origin of the ACL is oval and is approximately 136 mm 2 in size. The femoral attachment is circular and spans an average area of 113 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 only produces 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 translational force and rotational torque, and usually requires a high load to rupture. 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.
Though a complete discussion of anatomic structures that influence and/or are affected by ACL injury is beyond the scope of this chapter, the anterolateral ligament (ALL) and the proximal tibial slope (PTS) deserve mention as both have gained newfound interest. Though the specific anatomic description of the ALL location varies slightly by author, the consensus is that it courses from the lateral femoral condyle to an area near the anterolateral meniscus. Human cadaveric and biomechanic studies show that the ALL contributes to internal rotatory stability of the knee. With the resurgence of interest in the anterolateral region of the knee, modifications of early extra-articular ACL reconstructions are being performed in conjunction with modern intra-articular ACL reconstructions, with mixed early results. Further study is needed to clarify if and when these reconstructions are appropriate.
The bony morphology of the proximal tibia can also influence ACL injury. When the knee is axially loaded with an increased PTS, sheer forces are directed anteriorly, which place increased stress on the ACL. With the ACL being the main restraint to anterior translation, the increased PTS is thought to be an anatomic risk factor for injury. The influence of the medial and the lateral tibial slope is currently being investigated, with some studies showing that the medial slope has a greater influence, some showing that the lateral slope has a greater influence, and some showing no association with slope and ACL injury. More research is needed to more clearly define the role of tibial bony anatomy and its influence on ACL injury and ACL reconstruction.
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 are arranged 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 primarily from branches of the middle geniculate artery and secondarily from branches of the inferior medial and lateral geniculate arteries, the infrapatellar fat pad, and synovium. The proximal portion of the ACL has better vascularity because the middle geniculate 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. These fibers primarily run parallel with the vasculature in a longitudinal manner, but also incorporate freely into the connective tissue. The proximity of the nerve fibers and vasculature 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.
Anterior Cruciate Ligament Biology
The ACL is an intra-articular 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. Extra-articular 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.
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 and disproportionally high in female athletes during their adolescent years. Sports in which athletes are particularly prone to ACL injury are skiing, soccer, basketball, and football. Over 70% of ACL injuries occur in noncontact situations. 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 many have suggested is the result of difference in ACL geometry, pelvic tilt, generalized joint laxity, hormonal influences, and differences in muscle reaction time. However, the exact reasons are currently unknown.
The number of ACL reconstructions has increased over the years. Currently, it is estimated that 200,000 ACL reconstructions are performed annually in the United States with a 5% to 15% failure rate. Failure can occur early or late, with early failure occurring within 6 months of reconstruction and late failure occurring after 6 months. Failure can be due to recurrent trauma, nonanatomic placement of tibial or femoral tunnels, and lack of graft incorporation.
Clinical History
A detailed patient history, including the injury mechanism and symptoms, is the first step in diagnosing an ACL rupture. The initial presentation often includes a history of a noncontact, low-velocity, twisting injury with or without an audible pop or snap and immediate knee swelling. Though many patients may be unable to recall the exact mechanism of injury, some studies have demonstrated that as high as one out of every two patients with an acute hemarthrosis has an ACL injury. 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.
A severe knee effusion soon after injury is an indication of an intra-articular pathology. Early arthroscopic studies demonstrated that nearly 75% of patients with acute hemarthrosis of the knee after injury had some degree of disruption of the ACL. Rupture of the ligament disrupts the blood supply and causes this hemarthrosis. Though a high proportion of hemarthroses can be attributed to ACL injuries, it is important to consider other intra-articular pathology such as meniscal or osteochondral injuries, fractures, or posterior cruciate ligament (PCL) ruptures. Additionally, the lack of a large hemarthrosis should not rule out an ACL injury.
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 can calm the patient, set expectations, and help the patient to relax, which will be important when testing for ligamentous stability of the injured knee.
Examination of an acutely injured knee should start with inspection. An effusion can be an obvious clue to injury. The patient's skin should be examined for cuts or abrasions or bruising. The affected knee will often be held in flexion which can relieve intra-articular pressure due to a hemarthrosis. If several days or weeks have passed since the injury occurred, the quadriceps may be atrophied compared with the contralateral leg.
Following inspection, examiners should palpate the knee for warmth, a more subtle effusion, crepitus, and local tenderness. 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 medial or lateral collateral ligament (MCL or LCL). Focal joint-line tenderness could indicate meniscal or chondral injury. Osteochondral injury or the presence of loose bodies may present with crepitus on knee range of motion (ROM) testing although this is rare with most ACL injuries.
Knee ROM is often restricted in patients with acute ACL injuries. Apprehension and guarding are common, and physical examination findings can be more revealing after aspiration or local intra-articular 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 if there is a mechanical block from a meniscal tear, loose body, or ACL fragment that is obstructing motion.
Ligamentous Laxity
There are various physical exam maneuvers that can help examiners diagnose ACL injuries. The Lachman test for anterior laxity testing of the knee is performed by translating the tibia anteriorly while stabilizing the femur at 20 to 30 degrees of knee flexion. The anterior drawer test is performed at 90 degrees of knee flexion but is of little diagnostic value. In a recent meta-analysis comparing physical examination maneuvers in the diagnosis of ACL injuries with and without anesthesia, examination under anesthesia had a higher sensitivity than examination without anesthesia. Without anesthesia, the anterior drawer had a sensitivity and specificity of 38% and 81% compared to a sensitivity and specificity of 81% and 81% for the Lachman test. The ACL not only provides anterior stability, but also provides rotational stability for the knee. The pivot shift test is performed using a combination of valgus stress with rotatory and axial loading 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 as the knee moves from full extension into flexion. The sensitivity and specificity of the pivot shift test in diagnosing an ACL tear without anesthesia is 28% and 81%, and increases to 73% and 98% with anesthesia. In all circumstances (with or without anesthesia, and in the acute or chronic setting), a 2006 study showed that the Lachman test is the most reliable test when considering combined sensitivity and specificity.
Instrumented testing systems such as the KT-1000 and the KT-2000 can be used as an adjunct to manual maneuvers like the Lachman and anterior drawer tests, but are not necessary to diagnose ACL ruptures. They have sensitivities and specificities at maximal manual force of 93% and 93%. These instrumented systems are most commonly used for research purposes.
Imaging
Imaging studies should include standard anterior-posterior and lateral radiographs of the knee. These images 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. Radiographic evidence of a lateral proximal tibia fracture, commonly known as a Segond fracture, is pathognomonic for an ACL injury. Recent literature suggests that the Segond fracture is an avulsion of the ALL.
MRI is the imaging 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 mid-substance of the ligament and are visualized on MRI as increased signal intensity with discontinuity of the ligamentous fibers. Hemarthrosis is common. 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. 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 menisci in 51% of patients with injury to the medial meniscus at a rate of 13.9%, to the lateral meniscus in 24.9% of patients, and injury to both menisci observed in 13.1% 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, “giving way,” and difficulty with cutting sports. No large prospective trials have been conducted to demonstrate the natural history of ACL deficiency and the risk for further injury and osteoarthritis. There have been smaller studies showing that patients treated without ligament reconstruction after ACL tear have a higher risk of meniscal tears, arthritis, and knee arthroplasty compared to normal controls without ACL injury. In patients with ACL injuries treated operatively and nonoperatively, there is a lower risk of secondary meniscal injuries after ACL reconstruction compared to those patients managed conservatively. Although ACL reconstruction may protect the meniscus, it does not completely guard against rerupture, nor does it prevent subsequent osteoarthritis.
Age
Age is an important factor when considering treatment options for patients with ACL injuries. As reported in a cohort study looking at more than 21,000 patients undergoing primary ACL reconstruction, the 5-year risk of revision was 9% in patients less than 21 years of age, 8.3% in those between 20 and 40, and 1.9% in those greater than 40 years of age. The higher rate of revision may be due to the higher activity levels of younger patients.
As youth become more active, there is an increasing frequency of ACL injuries in the skeletally immature. In New York state, one study showed that there has been a threefold increase in ACL reconstruction from 1990 to 2009 in patients less than 20 years old. Historically, skeletally immature patients with ACL injuries were treated nonoperatively. However, a better understanding of the risks of nonoperative management, along with the development of new techniques, has supported a trend for earlier surgical treatment of pediatric patients.
Overall, patients between 20 and 40 years of age do well with ACL reconstructions. Though some argue that nonoperative means should be attempted first, many have attempted to categorize patients with ACL injuries into the copers and the noncopers. Those who are unable to return to high-level sporting activities, or those with symptomatic instability are those who are categorized as noncopers. Those who are able to return to sport and activity are categorized as copers.
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. Ultimately, physiologic age and activity level seem to be more important than chronologic age when deciding between operative and nonoperative management.
Gender
Female athletes are at a two- to eightfold greater risk for ACL injury compared with their male counterparts. Many 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. 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 anatomic characteristics such as a narrow femoral notch, a high body mass index, 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 Anterior Cruciate Ligament Tears
Diagnosis of a partial ACL tear can be challenging and requires close evaluation of the history, physical examination, and MRI findings. 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. 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 hamstring (HS) strengthening, activity modification, and brace wear during activity. Proposed operative treatments include augmentation, partial (selective) ACL reconstruction, or traditional ACL reconstruction. Well-designed prospective clinical trials are needed to accurately compare treatment regimens for partial ACL injuries.
Associated Injuries
Rupture of the ACL can be 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/or 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 recent review demonstrates that increased time from injury to ACL reconstruction increases the risk of intra-articular pathology, which included the trochlea, lateral femoral condyle, medial tibial plateau, and meniscus across all age ranges included. Therefore surgical timing should be considered when deciding to pursue operative reconstruction to reduce associated injuries.
All associated meniscus injuries should be evaluated individually to form an overall management plan. There is some evidence that stable lateral meniscus tears, or partial thickness lateral meniscus tears, 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. A consensus of ACL reconstruction and nonsurgical management of grade I and II MCL injuries is accepted; however, there is controversy in the management of grade III MCL injuries in the setting of ACL injury. 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. However, in some persons with severe combined ligamentous injuries, MCL repair may be indicated, and there is no consensus on the timing of surgical management or reconstruction of the MCL in relation to the ACL.
Revision Anterior Cruciate Ligament
As the number of ACL reconstructions increases, 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
In a 2016 Cochrane review analyzing current research comparing operative and nonoperative treatment of ACL injuries, the authors concluded that there was insufficient evidence to recommend ACL reconstruction based on patient-reported outcomes alone. Some authors in the past have advocated for conservative treatment in lower demand patients. Others have tried to determine if patients will be copers or noncopers to help guide treatment. Conservative management can lead to recurrent instability and meniscal injury in athletes. For this reason, ACL reconstruction is recommended in patients who are active, have symptomatic instability, have failed conservative management, or 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 whether or not to pursue surgical management based on their age alone, because studies have shown equivalent outcomes in patients younger and older than 40 years. In a 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
Timing
The timing of when to reconstruct ACL injuries has been debated in the past, with many studies recommending delayed reconstruction as some patients are able to cope with their injuries with rehabilitation. Additionally, some have argued that there is a risk of arthrofibrosis with early reconstruction within the first month. If postoperative stiffness occurs, 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. The risk of arthrofibrosis with early treatment is concerning, with one study suggesting that operative treatment should wait for 2 to 6 weeks when motion returns. 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.
Other studies have argued for early reconstruction. They cite cost savings and improved quality adjusted life years (QALYs) with early reconstruction within 10 weeks of injury, compared to rehabilitation and optional delayed reconstruction. Additionally, a meta-analysis of the current literature found no difference in knee stiffness when reconstruction was performed between 1 and 20 weeks. 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. The amount of time for this to occur varies considerably from patient to patient.
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