8 The Knee

Acute knee trauma commonly presents with pain and hemarthrosis. ACL tears are seen in more than 70% of acute hemarthroses.1 Usually, ACL tears are associated with other injuries such as meniscal tears, osteochondral fractures, patellar dislocations, and ligament and tendon tears. MRI is commonly used to discern the etiology of an acute hemarthrosis, especially when the knee is too tender or the patient is too anxious for a thorough physical examination. MRI is especially helpful when conventional radiographs are negative. Typically, acute hemarthrosis appears as fluid within the joint with high signal intensity on T2-weighted images and intermediate signal intensity on T1-weighted images. Similar findings can be seen with inflammatory processes, resulting in a nontraumatic knee effusion, and the two processes may be differentiated by the clinician, based on the history and physical examination. The presence of blood in joint fluid causes a relatively brighter appearance on T1 imaging than that observed with nonhemorrhagic fluid because of the effect of methemoglobin. Hemosiderin deposition within joint fluid, associated with chronic hemorrhage or PVNS, appears dark on all pulse sequences.

Source: From Khanna AJ, Cosgarea AJ, Mont MA, et al. Magnetic resonance imaging of the knee: current techniques and spectrum of disease. J Bone Joint Surg Am 2001;83:128–141. Reprinted by permission.

Meniscal Tears

Meniscal tears are graded according to how they appear on MRI (Table 8.1²; Fig. 8.1), and are best seen on T1-weighted, gradient-echo, and proton-density images. The menisci are low intensity on all sequences.^3–10 Most tears involve the medial meniscus, but most acute tears involve the lateral meniscus (Table 8.2^2,11). Special attention should be given to the menisci in the presence of a chronic ACL tear because associated meniscus tears are common. The medial meniscus is commonly torn in the chronically ACL-deficient knee because the meniscus plays a secondary stabilizing role to anterior tibial translation.

The types of meniscal tear morphologies have been well described (Table 8.2^2,11). In early publications on MRI evaluation of meniscal pathology, detection focused primarily on meniscal signal intensity patterns and their relationship to the meniscal surface.^12,13 Grade I and grade II⁹ meniscal tears referred to increased intrameniscal signal that appeared punctate or linear, respectively, and were particularly evident on T1-weighted images. The grade III designation indicated that the signal intensity pattern communicated with the superior or inferior meniscal surface, and therefore most likely corresponded to an arthroscopically identifiable meniscal tear.⁹ If grade II meniscal pathology was reported in an MRI interpretation as a grade II tear, however, it could create the erroneous impression that meniscal surface pathology existed. Although grade II intrameniscal signal can reflect the presence of a meniscal contusion or an intrameniscal tear, it may also denote myxoid internal meniscal degeneration, artifact (magic angle effect or truncation), or normal meniscal vascularity in a child or young adult.^14,15 This grading system also does not consider morphologic alterations in characterizing meniscal pathology. Because the morphology of a meniscal tear impacts its relative clinical significance and influences surgical planning, a more specific classification system, currently afforded wide acceptance, uses MRI findings to categorize tears as follows¹³:

Horizontal tears divide the meniscus into superior and inferior components, identified on sagittal and coronal imaging series (Fig. 8.2). The tear often communicates with the inferior meniscal surface; sometimes a concomitant meniscal cyst is identified, involving the perimeniscal soft tissue (Fig. 8.3).

Vertical radial tears exhibit a linear signal pattern. They violate the superior and inferior meniscal surfaces and are oriented perpendicular to the meniscal axis of curvature.16 As such, if a vertical radial tear involves the anterior or posterior portion of the meniscus, the tear will show a linear vertical configuration on coronal images (Fig. 8.4A) and an abrupt truncation of the central meniscal contour on sagittal images (Fig. 8.4B). With this type of tear, the change in meniscal morphology, appreciated in the imaging plane oriented parallel to the tear, is equally as important as the surface communication of linear abnormal meniscal signal intensity, noted in the imaging plane perpendicular to the axis of the tear. If the vertical radial tear involves the mid-body of the meniscus, a linear meniscal defect is noted on sagittal images (Fig. 8.5A), and blunting or truncation of the central meniscal contour is observed on coronal images (Fig. 8.5B). A commonly overlooked vertical radial tear occurs at the root of the posterior horn of the medial meniscus (Fig. 8.6). This tear, particularly common in obese patients, destabilizes the meniscus, and medial extrusion of the meniscus is often identified on MRI. Chondromalacia and osseous stress injury are also frequently observed in association with these radial tears of the root of the posterior horn of the medial meniscus.¹⁷

Vertical longitudinal tears also exhibit a vertical signal orientation on MRI, but because the tear is oriented parallel to the axis of meniscal curvature, it is identified as a vertical linear signal intensity interfacing with the superior and inferior meniscal articular surfaces on sagittal and coronal imaging planes. Peripheral vertical longitudinal tears of the posterior horn of the medial meniscus and vertical longitudinal tears of the posterior horn of the lateral meniscus at the attachment of the meniscofemoral ligament are commonly associated with ACL tears (Fig. 8.7). Displacement of a meniscal flap associated with a vertical longitudinal tear may occur, and unstable meniscal tears are more likely to be symptomatic and impair biomechanics; in such circumstances, surgical intervention usually is necessary to restore function.¹⁸ Failure to address surgically the presence of a displaced meniscal flap or fragment represents a cause of persistent joint pain and dysfunction after arthroscopy.¹⁹ Unstable meniscal tears may exhibit central or peripheral flap displacement. A typical bucket-handle tear represents a vertical longitudinal tear with central displacement of meniscal tissue. If the medial meniscus is involved, the flap is displaced inferior to the PCL, creating the double-PCL sign²⁰ (Figs. 8.8 and 8.9). Bucket-handle tears of the lateral meniscus often exhibit anterior central displacement of a flap arising from the posterior horn of the meniscus (Fig. 8.10). Displaced bucket-handle tears are a common cause of locked knee in an athlete. Posterior central flap displacement may also occur with a flap arising from the posterior horn of the medial meniscus and the displaced meniscal tissue positioned in the recess between the distal PCL and the medial meniscus (Fig. 8.11). Meniscal tissue may also displace peripherally into the recess between the capsule and the femoral condyle (Fig. 8.12) or between the capsule and the tibial plateau (Fig. 8.13). Although peripherally displaced tears are less likely than centrally displaced tears to result in locking, they tend to cause adjacent capsular inflammation and osseous stress reaction, contributing to pain related to the meniscal tear. Preoperative identi fication and localization of the displaced meniscal flap with MRI contribute to a more expeditious operative procedure.¹⁰

The designation of a tear as complex, based on MRI findings, implies that the tear exhibits multiple vectors or that the meniscus appears macerated and fragmented. These tears are usually unstable and, if chronic, are associated with chondromalacia and osseous stress reaction, also identified on MRI.

ACL evaluation is one of the primary indications for MRI of the knee.21 The patient usually describes a noncontact, twisting or valgus injury to the knee with a planted foot and often describes sensing a “pop” inside the knee. The patient may experience a subsequent lack of quadriceps function because of the resulting effusion. Physical examination findings include a positive Lachman test and pivot shift. The patient’s anxiety may make the physical examination equivocal, increasing the importance of the MRI.

The adult ACL usually avulses from its femoral attachment or develops an intrasubstance tear. The tendon does not have to dissociate completely to become incompetent.²² The presence of blood at the femoral avulsion can appear as a “pseudomass” because of volume averaging.²³ With skeletally immature patients, the ACL may remain intact and avulse a fragment of bone off of the tibial attachment (Fig. 8.14). Treatment is determined by the degree of displacement, but it usually requires surgical fixation via arthroscopic or open techniques.

The signs of an ACL injury on MRI include abnormal ligament signal and loss of the normal orientation and continuity of the ligament (Fig. 8.15). There are primary and secondary signs of ACL tears as visualized on MRI (summarized in Table 8.3^24–26). MRI findings commonly associated with acute ACL injury include loss of ligament continuity and replacement of the ligament by a poorly marginated pseudomass that shows high signal on T2-weighted imaging and low signal on T1-weighted imaging (Fig. 8.16). With both acute and chronic ACL tears, the ligament may appear abnormal in angulation, typically because of inferior displacement of the proximal portion of the ligament. Acute tears of the ACL are often accompanied by bone bruises involving the posterior aspect of the lateral tibial plateau, the anterolateral aspect of the lateral femoral condyle with impaction of the articular surface notch, and the posterior aspect of the medial tibial plateau (Fig. 8.17). The osseous contusions are related to the initial pivot shift rotational stress (lateral injuries) and the subsequent abrupt return to normal alignment (medial plateau injury). Additional findings include anterior translation of the tibia, resulting in increased PCL flexion, popliteus muscle strain, peripheral tears of the posterior horn of the medial meniscus (Fig. 8.18), and tears of the lateral meniscus associated with the attachment of the meniscofemoral ligament (Fig. 8.19).

Other knee structures are also commonly injured at the time of ACL injury, and it is imperative that the MRI be evaluated for these concomitant injuries. These injuries, such as meniscus tears, bone bruises, and cartilage injuries, not only may need to be addressed surgically, but also aid in the diagnosis when the ACL tear is not obvious.27

A variety of other radiographic findings seen in association with ACL tears are considered secondary signs (Table 8.3^24–26). A relatively small avulsion fracture seen at the lateral tibial cortex (known as a Segond fracture) is caused by avulsion of the middle third of the lateral capsule²⁸ (Fig. 8.20).

When the ACL tears, the posterior tibial plateau translates anteriorly, impacting the anterior aspect of the lateral femoral condyle in the region of the terminal sulcus. The resulting contusion leaves a bone bruise pattern in these two locations, best seen on T2-weighted images (Fig. 8.17). This pathologic contact of the tibial and the femoral condyles may leave an indentation or notch, also called a “deepened terminal sulcus,” particularly when the patient experiences repeated instability episodes. Another sign of chronic ACL insufficiency is buckling of the PCL, which suggests that the tibia is positioned in an anteriorly displaced position. Patients with chronic ACL insufficiency also frequently develop secondary degenerative changes, including partial- and full-thickness chondral lesions, peripheral osteophytes, and peaking of the tibial spines.

When evaluating the MRI study of a patient with ACL injury, multiple factors should be taken into consideration and included in the description of the MRI findings, including the location and degree of disruption of the ACL. In addition, the collateral ligaments, PCL, and secondary restraints (including the menisci) should be carefully reviewed. It is also important to review the status of the osseous structures and articular cartilage because injuries to these structures can also be treated at the time of ACL reconstruction and repair. Lastly, the condition of any tissues that might be used when reconstructing the ACL should be evaluated, and injury to them should be ruled out.

Chronic degeneration of the ACL should be differentiated from an acute ACL injury or tear. Patients with chronic degeneration often present with chronic knee discomfort and a vague, nonspecific knee pain. The process is thought to be secondary to repetitive low-energy microtrauma as opposed to a single high-energy event that might produce an acute ACL tear. Minimal to no instability is seen on physical examination. The MRI may show evidence of advanced degeneration of the ACL with thickening of the ligament and less increase in T2-weighted signal than that which is typically seen with an acute tear (Fig. 8.21). Such patients also often have similar PCL involvement.

The PCL is more robust and more consistently has lower signal on MRI than does the ACL. MRI is very sensitive for tears of the PCL because of its normal low signal (Fig. 8.22). Any increased signal within the PCL on T2-weighted images is suspicious for injury.^29–31 The PCL can be torn partially, indicated by increased signal on T2 images with the ligament in continuity (Fig. 8.23), or completely, visualized as a loss of continuity of the ligament. Bone bruises may be present, depending on the mechanism of injury. A blow to the proximal tibia from a dashboard during a motor vehicle accident can produce soft-tissue edema and a bone contusion involving the tibial plateau, best seen on T2-weighted images. Bone bruises may be present on the anterior tibial plateaus and the femoral condyles if the PCL injury occurs from a hyperextension mechanism.

Related posts:

MRI Safety Essentials of MRI Physics and Pulse Sequences Advanced Techniques in Musculoskeletal MRI The Shoulder A Systematic Approach to the Review of Musculoskeletal MRI Studies The Foot and Ankle

Stay updated, free articles. Join our Telegram channel

Join