Internal Derangement of the Knee: Ligament Injuries

Prevalence, Epidemiology, and Definitions

Injury to the anterior cruciate ligament (ACL) represents the most important ligament injury around the knee. It is relatively uncommon in the general population but is an important cause of injury in individuals involved in sports. Prevalence data are difficult to acquire, but more than 100,000 ACL reconstructions are performed per year in the United States. This suggests a prevalence of around 1 in 5000. A specific figure is difficult to calculate because many injuries will not undergo reconstruction. The injury is between 2 and 10 times more common in females, although more reconstructions are carried out in males. In a study of Finnish skiers, the prevalence of injury was 1 in 6000.

Anatomy

The femoral origin of the ACL is semicircular and lies on the medial aspect of the lateral femoral condyle. The ligament passes in a spiral course forward and laterally to a fan-shaped insertion anterior to the tibial spines ( eFig. 27-1 ). Its fibers blend with a condensation from the anterior horn of the lateral meniscus. Like the posterior cruciate ligament (PCL), the ACL comprises a number of collagen fiber bundles that are intertwined to form two dominant groups. The more anterior of these, the anteromedial bundle, is more densely packed and more easily depicted on MRI ( eFig. 27-2 ). The posterolateral bundle is more loosely arranged and therefore appears larger and more disorganized and has a higher signal intensity on MRI. The ligament is enclosed in a fibrous connective tissue sheath, which also contains a little fluid. The size of the ACL varies between different populations and between males and females. It averages 4 cm long and 1 cm thick, although it is slightly smaller in women. The femoral notch is also correspondingly smaller in women, and this combination may account for the differences between the sexes in the incidence of ACL injury. The attachment site has, like other entheses, a transitional zone of fibrocartilage and mineralized cartilage. A number of variants in the normal appearance are recognized. Like other ligaments and tendons, separation of the fiber bundles may occur with interposition of fluid. Linear areas of high signal intensity on T2-weighted MR images should not therefore be regarded as abnormal. Because the ligament has a curved configuration, care should also be taken to disregard areas of signal alteration that can arise as a result of the magic angle phenomenon. This is particularly evident on gradient-echo and short echo-time images but less commonly affects the ACL compared with the PCL. The lower third of the ligament can be rather poorly defined, probably as a consequence of splaying of the insertional fibers. This is especially the case in children. Indeed, in very young children, the anteromedial bundle in its entirety may be so poorly demarcated that a misdiagnosis of a rupture may result. As is outlined later, injuries of the ACL in children are relatively uncommon and tend to be associated with bony avulsion. The insertional fibers communicate with fibers attaching to the anterior horn of the lateral meniscus, giving a fibrillated appearance to the lateral meniscus that must not be interpreted as a tear ( eFig. 27-3 ). The ACL receives its blood supply from branches of the lateral geniculate artery and a nerve supply from divisions of the tibial nerve. The ligamentum mucosum or infrapatellar plica runs anterior to the ACL, paralleling the anteromedial bundle. It then turns superiorly within Hoffa's fat pad to attach close to the lower pole of the patella.

Biomechanics

Although flexion and extension are the predominant movements at the knee joint, there is also rotation at full extension that allows the tibia to become more fully engaged with the femoral condyles. Because of this rotation, there is a small component of anteroposterior femoral translation. To function, the cruciate ligaments must be capable of remaining taut throughout this more complex movement. The cruciate ligaments are made up of several bundles that are grouped together into two major groups, and it is this configuration that enables the ligament to remain isotonic throughout the full range of knee motion. The anteromedial band becomes taut in flexion, whereas, in extension, the larger, posterolateral portion is under tension.

ACL injury can occur by a variety of mechanisms but occurs most frequently with tibial internal rotation and abduction. It is an especially common skiing injury, in which, not uncommonly, valgus stress also results in the distraction of the medial joint compartment and impaction of the lateral femoral condyle with the lateral tibia plateau. Skis may also act as a lever before release of bindings, which may augment the injury. Typically, patients will describe an instantaneous moment of injury, where an audible “pop” may occur. Generally, the knee joint does not swell immediately, but hemarthrosis develops slowly over the ensuing hours. The delay is due to a slow drip from the investing blood vessels, sometimes called the leaking faucet . The position of the knee at the time of injury determines whether the anteromedial bundle or the posterolateral bundle is taut and, consequently, which of these structures tears first. This information may be helpful in the evaluation of partial tears of the ACL, although usually the precise details of the position of the knee are either unknown or too complex for reliable evaluation. Occasionally, the pattern of microfracture can provide some additional information.

Although the orthopedic literature often describes a high accuracy in clinical assessment, physical examination of the knee is rendered more difficult by the presence of acute injury. Clinical examination is associated with up to 20% false-negative findings. The classic clinical finding in the ACL rupture is increased anterior motion of the tibia. This is tested clinically by the Lachman test and the anterior drawer test. The clinician places one hand on either side of the knee with the thumbs pressed against the femoral condyles. The examiner's fingers lie in the popliteal fossa and press the tibia forward. These tests can be carried out in various degrees of knee flexion. The Lachman test is carried out at 30 degrees of flexion, and the anterior drawer test is done at 90 degrees of flexion. The Lachman test is thought to be more sensitive to posterolateral bundle tears, and the anterior drawer test is effective for tears of the anteromedial bundle. The quadriceps active test is similar: The knee is placed in 30 degrees of flexion, and, as the patient contracts the quadriceps muscle, the ACL-deficient knee will pull the tibia slightly forward before the lower leg begins to extend. The pivot-shift test is carried out with the knee in full extension. Valgus and internal rotation stress are applied with anterior pressure on the fibula head. Flexion induces anterior tibial translation with a palpable clunk. These tests are more reliable in the chronic phase of injury. In the acute situation, examination of ligament stability is impeded by pain and tense hemarthrosis with a clinical false-negative rate that has been estimated between 12% and 60%. The presence of hemarthrosis itself is a sensitive indicator of ACL rupture but is nonspecific, with ACL rupture accounting for as few as 20% of all hemarthrosis, although the rate is much higher in the context of a sporting injury.

Pathology

The most common pathologic process encountered by far in the ACL is a tear. Tears can be complete or partial; if partial, they may involve the anteromedial or posterolateral bundles. Complete ruptures are divided into avulsion fractures and intrasubstance ligament tears. Avulsions may involve the proximal or distal attachments or the midsubstance of the ligament. In many cases, although rupture is easy to identify, the associated edema makes the precise location of injury difficult to identify. Recognition of an avulsed bony fragment is important. In children, the ACL is commonly disrupted at its insertion into the tibial plateau, where associated bony fragments may be avulsed. Avulsion fractures of the ACL are less common in adults. They are classified into three types. A type 1 fracture shows minimal avulsion from the underlying tibial plateau. A type 2 fracture is more significantly avulsed, but the avulsion predominates on one side, resulting in a hinge-like appearance. Usually, the anterior component is more significantly avulsed in the ACL injury, and the posterior component is more often affected when the PCL is involved. A type 3 fracture is one that is completely separated from the underlying tibial plateau. An avulsion fracture of the head of the fibula may be a sign of posterolateral corner injury associated with an ACL tear.

In adults, different patterns of bony injuries are typically associated with ACL rupture, and these have been divided into three groups. Microfracture in the posterior and lateral aspect of the tibia is most commonly recognized. As the tibia internally rotates with respect to the femur, impaction of the posterior aspect of the lateral tibial plateau occurs against the lateral femoral condyle. The force of impaction is increased when there is associated valgus injury. The relationship between the abnormal biomechanics and the pattern of microfracture is characteristic for ACL rupture. The second type of microfracture that is associated with ACL rupture is a consequence of femoral condylar impaction. This impaction occurs at or close to the anatomic location of the lateral condylar notch ( eFig. 27-4 ). Impaction results in the deepening of the notch, which is an alteration in the normally smooth curvature of its base, edema in the surrounding medulla, or a combination of these.

Microfracture may also be present in the posteromedial tibia, although this is less common than the previously described posterolateral impaction. The injury occurs close to the location of the attachment of the central slip of the semimembranosus tendon, which may become avulsed from its insertion. Yao and Lee suggested that this was a true avulsion, although a similar injury has been described as a result of impaction of the medial tibia during varus and external rotation as it translates anteriorly. The latter scenario has been shown to occur experimentally, but the corresponding impaction injury that might be expected in the medial femoral condyle is not always present.

Rupture of the ACL is commonly associated with injuries to other structures of the knee, including injuries to bone and cartilage and soft tissues, such as menisci and other ligaments. The associated bony contusions have been outlined earlier. The pattern and prevalence of other soft tissue injuries depend on whether the injury is due to direct trauma or related to a sport and on the age and fitness of the patient.

Meniscal tears are commonly associated with ACL rupture and may be found in up to 70% of cases. Once again, the prevalence and pattern are dependent on the nature of the injury. Lateral meniscal injuries are reported as more common after skiing injuries, in which Duncan and colleagues found that the lateral meniscus was affected in 83% of cases. In children, meniscal tears occurred in 37% of cases of ACL rupture, but two thirds involved the medial meniscus. Certain patterns of meniscal injury are particularly associated with ACL rupture. These patterns include a vertical tear of the meniscal periphery and a radial tear involving the posterior portion of the meniscus or its root. With this posterior predominance, particular attention should be paid to the posterior thirds of the menisci, especially the lateral meniscus when a torn ACL is encountered.

From the nature of the injury, ACL ruptures also are frequently accompanied by injuries to the collateral ligaments. Because excessive valgus is a common component of the injury, tears of the medial collateral ligament (MCL) are frequent, although mostly minor. The combination of ACL rupture, medial meniscal tear, and MCL injury is sometimes referred to as the O'Donoghue or unhappy triad. The full tetrad of injuries also includes osseous microfracture. Overall, this combination of injury, despite being well known, is not particularly common. The grade of MCL injury may be helpful in predicting the likelihood of an associated meniscal lesion being present. Shelbourne and Nitz divided patients with ACL rupture and MCL tear according to the grade of MCL injury. Patients with grade 2 MCL injury had more medial meniscus tears than patients with minor sprains or complete MCL rupture. Lateral meniscal tears were more common in both groups. In the grade 2 MCL tear group, medial meniscal injuries were always associated with lateral meniscal injury. In children, MCL injuries are also common and will be present in up to one fourth of cases of ACL rupture.

Injuries to the posterolateral corner are associated with ACL injury, although there is disagreement on both its incidence and importance ( eFig. 27-5 ). Posterolateral instability has been listed as one of the most common causes of graft failure after ACL reconstruction, although not all authors agree, and studies to support this hypothesis are lacking. The prevalence of posterolateral instability in patients with successful ACL reconstruction remains unknown. Despite this uncertainty, a careful examination of the posterolateral corner is recommended in patients with ACL rupture.

Tears of the ligamentum mucosum have been described, but variation in the normal appearances of this plica makes reliable interpretation of the tear difficult. High signal changes may also be found in Hoffa's fat pad in the presence of the tear of the ligamentum, and these need to be distinguished from normal clefts in the fat pad.

Mucoid degeneration and ganglion formation may occur in relation to any ligament, and the cruciate ligaments are no exception. Congenital cruciate deficiency ( eFigs. 27-6 and 27-7 ) has been described in isolation but is more commonly associated with fibular dysplasia and proximal focal femoral deficiency.

Manifestations of the Disease

Magnetic resonance imaging is the most important imaging technique for evaluating the anterior cruciate ligament (ACL). Signs may be present on plain radiographs. CT and, more specifically, CT arthrography are useful adjuncts or when patients cannot undergo MRI.

Magnetic Resonance Imaging

The ACL can be detected on sagittal, axial, and coronal MR images. The sagittal image is the most useful, but corroboration on the coronal and axial sections is helpful in difficult cases or when partial tears are suspected. Slice thickness more than 4 mm may cause partial volume average artifact and give rise to the false-positive diagnosis of a tear. Sections should therefore be kept at least 4 mm in thickness and preferably less. The smaller the slice thickness, the less is the dependency on knee position. The ACL lies 20 to 25 degrees away from the true sagittal plane. A slight external rotation of the knee from the true sagittal position improves visualization of the ligament. The anterolateral margin of the lateral femoral condyle can be used as a guide for the degree of rotation necessary. In practice, with 3- to 4-mm slices on modern imaging equipment, angulation is rarely necessary, although it is useful to show the ACL in a single slice. Sagittal images can be supported by coronal ( Fig. 27-1 ) and axial (see eFig. 27-8 ) sections. These can be particularly helpful in providing alternative visualization of the femoral origin, which can sometimes be difficult to depict on sagittal images.

Primary Signs

The anteromedial bundle is most easily identified on sagittal sections; when taut, it shows as a hypointense line that can be traced from its origin to close to the insertion on the tibia. In the insertional area, the fibers of the anteromedial bundle spread out and may therefore become poorly defined as the space between them is filled with fluid, fat, or connective tissue. This apparent loss of conspicuity of the ligament should not be misinterpreted as an insertional tear. Although the anteromedial bundle becomes less well defined in this insertional area, the individual fibers can usually be traced, and this helps exclude injury. The posterolateral bundle is more poorly defined but can be identified as a number of strands separated by fluid and connective tissue.

The principal finding in acute ACL rupture on sagittal-orientated MR images is failure to identify the normal hypointense low signal line of the anteromedial bundle ( Fig. 27-2 ). This carries a high positive predictive value for injury. Additional signs depend on whether the injury is acute or chronic. In the acute stage, the ligament fibers are grossly disrupted and separated by hemorrhage and edema. Individual fibers of the ligament are difficult to identify, and it is often unclear whether the injury involves the proximal, distal, or midsubstance of the ligament.

Less commonly, the ACL may displace anteriorly within the notch. The presence of the ligamentous mass in the anterior compartment prevents full knee extension, and the patient will present with a locked knee. This locked-knee presentation may mimic a bucket-handle tear of the meniscus. Anterior displacement of the ACL can be difficult to recognize on MRI. Two patterns have been described by Huang and colleagues : The more common type 1 stump shows the ACL as a mass lying in the anterior recess of the joint, and the type 2 stump has the appearance of a tongue-like fold of ACL displacing out of the intercondylar notch into the anterior joint recess. An ACL stump should be considered as a possible cause of knee locking in cases in which a displaced meniscal tear is not identified and the ACL has been shown to have been torn. The differential diagnosis of a locked knee in the absence of a displaced bucket-handle tear also includes a tear of the medial collateral ligament (MCL) or other ligamentous injury, leading to muscle spasm and pseudo locking. True or mechanical locking may be due to a loose intraarticular chondral or osteochondral fragment. Reduced patellar dislocation in association with the tear of the medial retinaculum ( Fig. 27-3 ) and a displaced osteochondral fragment should also be considered as a cause of true locking. Signs of reduced patellar dislocation include an osseous microfracture on the anterolateral aspect of the lateral femoral condyle, microfracture on the medial retropatellar facet with or without an osteochondral defect and fluid-fluid level ( Fig. 27-4 ), and edema medially as a consequence of the medial retinacular injury. Care should be taken not to confuse a prominent oblique intermeniscal ligament for a torn ACL.

The appearances in the chronic stage depend on the response of the ACL to injury, and this can be quite variable. In some cases, it can undergo rapid atrophy such that, within a short number of weeks, the ligament is completely absent. In other instances, as the edema and hemorrhage settle, the torn ligament may reappear, having fallen to the floor of the intercondylar groove ( Fig. 27-5 ). It may also fall backward to lie against the posterior cruciate ligament (PCL). In some cases, it may reattach to either the adjacent PCL or bony margin and derive a blood supply from it ( Fig. 27-6 ). In these cases, the structure of the ACL may be relatively well maintained even having a near-normal configuration. Clinically, however, the ligament is functionally weak and lax to clinical examination.

Although the sagittal plane remains the section of choice for evaluating the ACL, troublesome cases can be helped by reviewing the coronal and axial images. Attention should be directed at the femoral attachment, which provides the most useful information. The normal femoral attachment appears as a near round structure of low signal intensity in the coronal plane. On the axial images, the femoral attachment has an oval configuration with its anteroposterior diameter much larger than the medial to lateral diameter. In both cases, injury to the ligament shows edema and hemorrhage replacing the normal low signal ligamentous structure.

Bony Injuries Associated with Anterior Cruciate Ligament Rupture

There are three major bony injuries associated with ACL rupture. Microfracture in the posterior and lateral aspects of the tibia is most easily identified on coronal-orientated, fat-saturated images ( Fig. 27-7 ; see also eFig. 27-9 ). This pattern arises as a consequence of internal tibial rotation with impaction against the lateral femoral condyle. Indeed, this pattern of microfracture is so typical that, when identified, the patient should be regarded as having torn the ACL until proved otherwise. The presence of microfracture depends on the length of time since the injury. Microfracture is most commonly identified in the acute post-injury phase and can persist for up to 6 months. In general, a microfracture resolves more quickly than this and, therefore, its absence should not be used as a useful sign in predicting an intact ligament. Occasionally, the injury can be seen on the plain radiograph. A posterolateral tibial microfracture may be associated with lateral femoral condylar microfracture in a proportion of patients. Kaplan and colleagues detected isolated occult fractures in 43% of patients with ACL rupture and combined tibial and femoral fractures in 46%. Less common patterns were fractures in the posterior aspect of the medial tibial plateau in 7% and fractures involving all three areas in 2%. In contrast, Murphy and colleagues detected posterolateral microfracture in 94% of 35 patients with a much higher association with microfracture in the femoral condyle, which was present in 91%.

The pattern of microfracture seen in the lateral femoral condyle is somewhat variable. Most typically, impaction against the posterolateral tibial plateau occurs at or close to the anatomic location of the lateral condylar notch (see eFig. 27-10 ). Impaction results in the deepening of the notch, in an alteration in the normally smooth curvature of its base, in edema in the surrounding medulla, or in a combination of these. The most useful finding is microfracture, with the increased signal intensity changes extending in a radiating pattern from the cortex of the lateral femoral notch. Deepening of the notch can be appreciated by measuring from the depth of the bony injury to its surface. A measurement of more than 3 mm is definitely abnormal. Measurements of between 2 and 3 mm should also be regarded as suspicious. The contour of the floor of the notch should also be scrutinized. It is normally smooth; therefore, any angulation or cortical breach indicates fracture. Deepening of the notch and cortical interruption may sometimes be identified on plain radiographs.

Microfracture has also been described in the posteromedial tibia where the central slip of semimembranosus may become avulsed from its insertion. Yao and Lee suggested that this was a true avulsion, although a similar injury has been described as a result of impaction of the medial tibia during varus and external rotation as it translates anteriorly. The latter has been shown to occur experimentally, but the corresponding impaction injury that might be expected in the medial femoral condyle is not always present.

Soft Tissue Secondary Signs

Microfracture is most commonly seen during the acute phase after ACL rupture. In the more chronic phase, the microfracture will resolve and no longer serves as a useful secondary sign. The second group of secondary signs is related to changes within the ACL itself. As has been previously noted, after rupture, the ACL may disappear entirely or may reattach either close to its normal bony insertion site or to an adjacent soft tissue structure, most commonly the PCL. If it fails to attach to an adjacent structure, the ligament may lie along the floor of the joint within the intercondylar notch. The abnormal orientation of the ligament can usually be readily appreciated, but, in some cases, particularly those where it reattaches close to its original insertion, measurement may be required to detect subtle findings. The measurements used are the ACL angle and the Blumensaat angle.

The ACL angle is the angle formed by the intersection of the anterior aspect of the distal portion of the ACL and the most anterior aspect of the intercondylar eminence on a midsagittal MR image. The normal angle is around 55 degrees. An angle of less than 45 degrees is regarded as abnormal and indicative of a torn ACL ( Fig. 27-8 ). Sensitivity and specificity increase with decreasing angle, reaching 100% for both at angles of less than 25 degrees.

The Blumensaat angle is formed by the intersection of a line drawn through the distal portion of the ACL, along its anterior margin, and a line drawn through the intercondylar roof (see Fig. 27-8 ). Because the ACL parallels the intercondylar roof, the Blumensaat angle is normally close to 0 degrees. The angle may form with either the proximal or distal end of the roofline. By convention, angles that form proximally are considered negative, and those that form distally are positive. An angle over 21 degrees positive is strongly associated with an ACL rupture.

Secondary Signs Due to Tibial Translation

An intact ACL prevents forward displacement of the tibia with respect to the femur. Anterior tibial translation is free to occur when the ligament is ruptured, although it is not seen in all patients. It is less likely to occur in younger individuals with good muscle tone or where the posterior portions of the menisci remain intact. In these cases, the meniscus abuts the posterior aspect of the femoral condyle and prevents anterior translation.

Anterior tibial translation can be measured directly or by noting the alteration in the configuration and normal alignment of other structures in and around the knee. The technique for direct measurement uses either the lateral condyle tangent distance or the posterior femoral line.

The lateral condyle tangent distance is calculated by drawing a tangent at the most posterior point of the lateral femoral condyle to form the baseline from which the distance to the tibia will be measured. The section midway between the cortex adjacent to the PCL and the most lateral section containing the femoral condyle is used. Under normal circumstances, the posterior margin of the tibial plateau passes within 5 mm of this line. A distance of greater than 5 mm separating the posterior margin of the tibial plateau from this line indicates anterior tibial translation ( Fig. 27-9 ).

The more complex posterior femoral line is positive when a line drawn at 45 degrees from the posterosuperior corner of the Blumensaat line does not intersect the flat portion of the proximal tibial surface or intersect within 5 mm of its posterior margin.

Several indirect signs of anterior tibial translation have been described, most relying on changes in the configurations of other soft tissue structures. As the tibia translates anteriorly, an alteration occurs in the normal configuration of the PCL. On sagittal images, the PCL normally has an angulated appearance, with a slightly curved proximal third forming an angle between the straighter distal two thirds. With tibial translation, the angle between the proximal and distal portions becomes more exaggerated, and a reverse curve may appear in the distal limb, giving the PCL a sigmoid shape. These changes are readily apparent on visual inspection, but several measures of this PCL laxity have been described. These include the PCL line sign, the PCL angle sign, and the PCL curvature ratio.

The PCL line is drawn along the dorsal aspect of the PCL close to its insertion. The linear area is defined by two points, the more distal being within 3 to 4 mm of the PCL insertion. A line drawn connecting these two points when traced proximally should intersect the medullary cavity of the femur within 5 cm of its most distal point. The sign is positive when the proximally extended line does not intersect the medullary cavity of the femur. The reason for this becomes obvious when the line is drawn along the buckled PCL, as shown in Figure 27-10 .

As the PCL buckles during anterior tibial translation, the angle formed between the proximal and distal parts becomes more acute. This angle is normally greater than 115 degrees and usually greater than 125 degrees. Angles less than 111 degrees and, in some cases, less than 96 degrees have been reported to be associated with ACL rupture. The variation in these findings probably reflects the variation in anterior tibial translation that will be present in the study populations.

The normal configuration of the PCL has also been likened to a bow, with an imaginary line joining the attachments representing the string of the bow. The change in PCL angulation that occurs as the PCL buckles has also been quantified by measuring the amount of bowstringing that has occurred. A perpendicular is dropped from the apex of the PCL to the “string” of the bow. The ratio of the length of this perpendicular to the length of the string is calculated. The more that the PCL is buckled, the larger this ratio becomes. Values over 0.39 have a high specificity for ACL rupture.

Changes in the orientation of other soft tissue structures also occur with anterior tibial translation. The lateral collateral ligament runs in an oblique course posteriorly and inferiorly from its femoral attachment to its attachment into the fibular head. Normally, several sequential coronal sections must be viewed to see the ligament in its entirety. When anterior tibial translation occurs, the orientation of the lateral collateral ligament becomes more vertical and is oriented in a more parallel coronal plane. Depending on the degree of translation, the ligament may then be visualized on a single coronal slice. In extreme circumstances, a considerable proportion of the PCL may also appear on a single coronal slice.

Other indirect signs of ACL rupture that have been described include the posterior synovial bulge sign, an irregular anterior margin of the ACL, rupture of the iliotibial band, severe buckling of the patellar tendon, a shearing injury of Hoffa's fat pad, and posterior displacement of lateral meniscus.

In the majority of cases, the presence of an intact anteromedial bundle confirms an intact ACL, and its absence is a reliable sign of complete rupture. The diagnosis of partial ACL rupture can be more difficult because the findings on MRI are not as clearly defined, the literature is less replete, and those signs that have been suggested are neither as sensitive nor as specific as those indicating complete rupture. A further contributor to the problem of defining reliable signs is the lack of a strict surgical definition. Despite this, the normally consistent appearance of the ACL means that any focal areas of loss of signal, other than at its insertion, kinks, buckles, or loss of parallelism between the ligament and the intercondylar roof should all be regarded as suspicious for a partial tear. Signs that are also moderately sensitive include bowing of the ACL and nonvisualization of the ACL on one MRI sequence with visualization of intact fibers on the other. Lawrence and colleagues, in a retrospective review, proposed four features that helped differentiate partial ACL tears from either complete ACL tears or normal ligaments. These were the appearance of some intact fibers, thinning of the ligament, a wavy or curved ligament, and the presence of an inhomogeneous mass posterolateral to the ACL.

These features have not been tested prospectively. Axial images have also been used to try to differentiate stable from unstable ligaments. Stable ACLs were described as elliptical, attenuated, or showing areas of increased intra­substance signal intensity. Unstable ligaments were more likely to have an isolated ACL bundle, nonvisualization of the ligament, or the presence of a cloud-like mass in place of the ACL.

In the presence of such findings, it is important to look carefully for secondary signs, if present, that are more likely to indicate a high-grade tear. This is especially true for signs of anterior tibial translation. In the absence of secondary signs, the diagnosis is more circumspect. Many clinicians regard a partial ACL tear with less clinical concern if the knee is stable clinically. Clinical correlation, particularly the presence of an anterior draw sign, can therefore be helpful in identifying a more significant ACL injury. A combination of clinical and MRI findings is usually sufficient to allow the correct choice of management.

The ability of MRI to depict the internal anatomy of the knee with great detail has led to increased recognition of intraarticular ganglionic cysts. These are most commonly seen in the anterior joint close to the anterior horn of the lateral meniscus, but they are also recognized as arising from the cruciate ligaments. Indeed, it is likely that these are the same with the anterior lesion representing an extension of the anterior cruciate ganglion. The link between the insertional fibers of the ACL and the anterior horn of the lateral meniscus provides the connection between the two spaces.

Intraarticular ganglia are found in around 1 in 50 knees on MRI. The majority are not associated with any other internal derangement. Pain is described as the most common complaint, worse on activity and in sports participation, but medial joint line tenderness is also described. One fourth of patients give a history of trauma. Only five (20%) of the patients in this group underwent arthroscopic débridement, and four of the five patients had a decrease in symptoms. On this basis, it is difficult to apply a pattern of symptoms to ACL ganglia or to comment on etiology, although a decrease in patient symptomatology has been described in other studies after arthroscopic or CT aspiration.

ACL ganglia typically have two patterns. One is where the ganglion is interspaced between the fibers of the ACL, distending its sheath with posterior bulging. The fibers of the ACL are easily seen within the sheath, although the course of the fibers may be deviated by the mucinous material ( Fig. 27-11 ). The second pattern of ACL ganglia consists of a more cyst-like structure that extends from the ACL sheath, most commonly near its femoral attachment.

Computed Tomography

Computed tomography has been used by a number of investigators to detect ACL injury, although, in general, it has played a subsidiary role to MRI. Its use is most commonly reserved for those with either contraindication to MRI or when there is limited availability. Advanced multislice CT is capable of producing high-resolution images of the cruciate ligaments with a high diagnostic accuracy, and the role of CT in the assessment of internal derangement of the knee continues to develop.

The ACL is best appreciated on reformatted sagittal sections as a soft tissue density structure contrasted to the surrounding fat. There are few studies that define the accuracy of reformatted plain CT in the assessment of ACL injury, but good sensitivities have been reported. Detail in these studies is lacking, and confident sensitivity and specificity data are difficult to calculate.

CT has more proven accuracy in the detection and assessment of ACL avulsion injuries. Two patterns of ACL avulsion fracture have been recognized. Most fractures involve the anteromedial bundle insertion, with one third extending beyond the insertional area. These latter injuries tend to be complete, involving the insertion of both the anteromedial and the posterolateral bundles. It should also be appreciated that many incomplete avulsion fractures are associated with complete ligamentous avulsion, with the fracture line continued through the ligament itself. Therefore, care should be taken in the diagnosis of an incomplete avulsion injury using CT. Although CT is superior to MRI in defining the configuration of ACL avulsion fractures, overall CT does not improve visualization of the degree of comminution, displacement, or extension of the fracture over the information provided by plain films.

CT arthrography provides an excellent depiction of ACL integrity with images and accuracy rivaling those of MRI. The normal ACL shows as a continuous tubular structure with a CT attenuation of soft tissue contrasting sharply to injected contrast material. Like MRI, a straight or slightly concave anterior margin is typical. Variants include linear streaks of contrast material parallel to its long axis. The signs of ACL rupture on CT arthrography have been derived from the MRI findings. Failure to visualize the ACL carries a high positive predictive value for ligament tears. Contrast medium extending into the ACL or loss of the normal contour, bowing, and parallelism with the intercondylar roof has also been used to indicate a tear. Many of the indirect signs of ACL rupture that have been described in the section covering MRI also apply to CT arthrography. Using a combination of these signs, the sensitivity and specificity for the detection of ACL tears were 95% and 99%, respectively. The role of CT arthrography in detecting partial ACL tears has yet to be evaluated.

CT arthrography can also be used to detect ganglion cysts of the ACL as a non–contrast-filled defect. CT can be used to guide aspiration and injection of these lesions.

Ultrasonography

Ultrasonography has also been used to assess injuries to the ACL. Two approaches have been proposed. The earliest described technique is direct visualization of the ACL from an anterior approach with the knee in flexion. More recently, a posterior approach seeking to demonstrate an abnormal femoral attachment has been used. Both methods are technically demanding. The anterior approach requires more than 90 degrees of knee flexion. The described method requires a 30-degree rotation of the probe. Although some have found success with the anterior approach, others have favored the posterior approach, where failure to identify a normal femoral ACL attachment has been interpreted with high reliability as an ACL tear. Skovgaard Larsen and colleagues describe a sensitivity of 88%, specificity of 98%, and positive and negative predictive values of 93% and 96%, respectively, using hematoma at the femoral attachment site as a sign of ACL rupture. Similar results were previously reported by Ptasznik and colleagues (91% sensitivity, 100% specificity, 100% positive predictive value), although in this study of patients with acute hemarthrosis the negative predictive value was lower at 63%. Indirect signs of ACL rupture may also translate on ultrasound. Hawe has used the S-shaped course and the thickening of the PCL to infer ACL rupture, and Fuchs and Chylarecki describe posterior protrusion of the posterior fibrous capsule displacing the soft tissue structures. The latter sign was found to be less reliable, with a sensitivity of 68% and a specificity of 77%. Ultrasonography can also readily detect an associated Segond fracture.

Despite these apparently favorable results, ultrasonography has not established itself as a method of ACL evaluation in everyday practice. There may be several reasons for this. Many of the studies involve single reviewers, and, therefore, interobserver and intraobserver variation has not been firmly established. Variations in the described echogenicity of the cruciate ligaments coupled with some differences in anatomic labeling on published images have added to the uncertainties. Most likely, difficulties in the assessment of associated injuries, specifically meniscal and bony injury, mean that ultrasonography is unlikely to displace MRI.

The dynamic nature of ultrasonography means that it is useful to guide aspiration of cruciate ganglion cysts. Because of the depth of these lesions, however, they can be difficult to visualize in some patients.

Nuclear Medicine

There are several reports of the role of scintigraphy in the assessment of ACL rupture, with most centered on its ability to detect the associated bone injury. In a study of 28 patients with ACL injury, MRI detected microfracture in 64%, but single-photon emission computed tomography (SPECT) demonstrated increased uptake in all. Increased activity is not always associated with symptoms, however, and may be found in the population actively involved in sports. Although SPECT may have some value in detecting more occult bone microfracture, comparative studies overall favor MRI for both cruciate injuries and meniscal tears.

Synopsis of Treatment Options

There is debate in the orthopedic literature regarding the indications for, timing of, and choice of procedure for ACL reconstruction. A consensus appears to suggest that more active patients will benefit, particularly in the younger age groups. Early reconstruction has been associated with an increased risk of arthrofibrosis, although some authors attribute this to the rehabilitation program used rather than the surgery. A variety of grafts has been used. The four-strand hamstring (semitendinosus and gracilis) is the most popular, at present, because it has strength advantages over the patellar tendon graft and is not associated with the same level of postoperative extensor mechanism complications as the patellar tendon graft, which include patellar fracture, patellar tendon rupture, and anterior knee pain.