Magnetic resonance imaging (MRI) of the knee joint is one of the most commonly requested in general radiological practise examinations and belongs to the core clinical practice in most MRI units along with spinal and brain MRI. Therefore, these examinations are often reported by general radiologists in most institutions and are an important part of routine education of radiology residents in clinical MRI.
Although interpretation of knee MRI seems straightforward in most scenarios, there are a number of pitfalls that may cause common mistakes. The purpose of this pictorial review is to present an overview of those common interpretation errors and pitfalls in MRI of the knee. We will particularly emphasize those pitfalls that are encountered by young residents or less experienced radiologists.
Insufficient Knowledge of Developmental Anatomy and Ossification Variants
Bipartite or multipartite patella is the presence of one or more accessory ossification centers near the patella. In a previous study at our institution, we found a prevalence of 0.53% . It may present as an incidental finding on imaging, but may be symptomatic and cause anterior knee pain. In symptomatic cases, there is often bone marrow edema at the accessory ossification center and the adjacent patella (Figure 1). It should not be confused with a patellar fracture . Correlation with clinical history (acute onset of pain in fracture versus chronic knee pain in symptomatic bipartite or multipartite patella) is useful in the differential diagnosis. Plain films show a sclerotic delineation of the accessory ossification fragments in case of a bipartite or multipartite patella, whereas the borders are sharp in case of a fracture.
Dorsal defect of the patella
Dorsal defect of the patella (DDP) is a well-delineated defect at the articular side of the superolateral aspect of the patella (Figure 2). Retrospectively, this variation was seen in 0.16% of our MRI studies of the knee and may be bilateral . On plain radiographs, DDP presents as a well-defined radiolucent lesion. It should not be confused with osteochondritis dissecans (OCD) of the patella, Brodie abscess or bone tumors. The clues to the correct diagnosis are the typical superolateral location and the intact smooth overlying cartilage. Most patients are asymptomatic, except in cases of overlying cartilage defects .
Cortical avulsive irregularity
Cortical avulsive irregularity is a benign defect located at the posteromedial condyle of the femur. Previously, it has been designated by the misnomer subperiosteal desmoid, which incorrectly suggests that the lesion is of neoplastic origin. Cortical avulsive irregularity is believed to result from chronic traction either of the medial head of the gastrocnemius muscle or of the insertion of the aponeurosis of the adductor magnus muscle at the posteromedial femoral condyle . It is typically seen in young adolescents around 10 to15–years–old and may be bilateral. Usually, patients do not complain of pain. Physical examination rarely reveals a palpable mass . On plain radiographs, it is located at the distal posteromedial femur above the growth plate and presents as a radiolucent lesion often with some reactive surrounding sclerosis (Figure 3D). On MRI, the lesion is of low signal intensity on T1-weighted images (T1-WI). On T2-WI, the lesion is of high signal intensity and is surrounded by a low signal intensity rim. There may be subtle bone and soft tissue edema (Figure 3A, 3B, 3C). The lesion should not be misinterpreted as an aggressive neoplastic lesion .
Femoral ossification variants mimicking osteochondritis dissecans
Ossification variants of the femoral condyles (Figure 4) are common and should not be confused with osteochondritis dissecans (OCD). They are more frequent in boys than in girls . Useful parameters to distinguish femoral ossification variants from OCD are age (peak age 7–10 years for ossification variants versus 11–14 years for OCD), residual physeal cartilage of more than 30%, location (posterior third of the femoral condyle in ossification variants versus inner third in OCD), absence of intercondylar extension, absence of perilesional bone marrow edema and a lesion angle on coronal images of less than 105 degrees .
Failure to Correlate with Clinical Findings
Some imaging findings are incidental (e.g., age-related degeneration) and do not correlate with the patient’s symptoms. Although a systematic approach of all anatomical knee structures is recommended, imaging findings should always be correlated with the patient’s symptoms in order to avoid overdiagnosis and potentially harmful and unnecessary treatment. It is probably appropriate to describe all MRI abnormalities within the radiological report, but to summarize only those findings which have a high probability of clinical significance in the conclusion of the report. Whenever MRI findings are equivocal, this should be clearly emphasized within the report .
Asymptomatic meniscal tears are common, particularly if they are horizontally or obliquely oriented. Borderline or subtle MRI findings that are equivocal for a meniscal tear remain another common problem in daily practice [6, 7]. Radial, vertical, complex, or displaced meniscal tears and abnormalities of the collateral ligaments, pericapsular soft tissues, and bone marrow are more seen in symptomatic patients and thus clinically significant .
As request forms for radiological investigations are often incomplete (e.g., not mentioning the precise location of the patient’s pain nor details on previous surgery), we recommend that every patient undergoing an MRI examination should fill out a screening questionnaire indicating previous clinical history (e.g., trauma, previous surgery to cruciate ligaments, menisci, cartilage, rheumatic diseases), location and duration of pain, aggravating activities, previous imaging, etc. (Figure 5).
Failure to Correlate with Other/Previous Imaging
Most patients that are referred for MRI already underwent previous imaging (plain radiography, ultrasound or MRI). Too often these examinations are disregarded, particularly if they have been performed in an outside institution. We highly recommend looking at previous imaging in the picture archiving and communication system (PACS) or urge the patient to provide the radiologist with examinations done in outside institutions, as comparison with these examinations can be extremely helpful in obtaining the correct diagnosis.
MRI may be less sensitive and specific for identification of calcifications than plain radiographs (Figure 6). In addition, chondrocalcinosis may mimic meniscal tears.
Interpretation errors due to technical artifacts such as magic angle phenomenon on imaging with low echo time (TE) , truncation artefacts on gradient echo imaging , blurring artifact on fast spin echo T2-WI , phase-artifacts and susceptibility artifacts  will not discussed in this short overview.
Satisfaction of Search
Detection of one abnormality may reduce the detectability of another abnormality. Although additional findings are not always clinically significant, some abnormalities may have an impact on the treatment of the patient (Figure 7). A typical example of an undiagnosed finding is a ramp lesion of the medial meniscus, which is often associated with lesions of the anterior cruciate ligament (ACL). A meniscal ramp lesion may either result from disruption of the meniscotibial ligaments of the posterior horn of the medial meniscus, or by disruption of the peripheral attachment of the posterior horn of the medial meniscus . To overcome the phenomenon of satisfaction of search, we recommend a systematic approach in the analysis of all intra- and extra-articular structures of the knee joint.
Intermeniscal connections (e.g., anterior intermeniscal or geniculate ligament) (Figure 8), ligamentous attachments to the medial and lateral meniscus and partial volume effect of the popliteal recess at the posterolateral meniscus  (Figure 9) may be misinterpreted as meniscal tears. The medial posterior femoral recess or the medial gastrocnemius bursa against the posteromedial meniscus are often misinterpreted by the non-experienced reader as meniscocapsular separation. A thorough knowledge of the intra-articular anatomy of the knee is of utmost importance for correct interpretation .
The normal fibrovascular peripheral border (also known as the red zone) of the meniscus should not be confused with a peripheral meniscocapsular separation. The red zone is of intermediate signal intensity on T2-WI. In meniscocapsular separation, an abnormal high T2-signal (similar to fluid) is seen between the meniscus and the capsule or within the peripheral zone of the meniscus. It is often accompanied with irregular meniscal borders and meniscal displacement (Figure 10) [15, 16].
The precise origin of a so-called meniscal ramp lesion either resulting from disruption of the meniscotibial ligaments of the posterior horn of the medial meniscus or rather by disruption of the peripheral attachment of the posterior horn of the medial meniscus, is still debated .
Meniscal root tears
Meniscal root tears are less common than other types of meniscal tears. Nevertheless, they have an important clinical impact. They are often associated with extrusion of the meniscus with respect to the tibial margin. This may alter the biomechanical forces on the medial femorotibial compartment and predisposes to meniscal extrusion, premature cartilage lesion (Figure 11) and subchondral insufficiency fractures (Figure 12) . Posterior medial root tears are most common compared to other root tears .
Radial tears result from a high impact and have a perpendicular course to the long axis of the meniscus. They occur initially at the free edges of the meniscus and may progress through the meniscus, resulting in splitting of the meniscus into separate parts . Radial tears are often subtle and a meticulous analysis of coronal, sagittal and axial images is required to make a confident diagnosis. MRI signs of radial tears are the missing triangle sign, disrupted bow tie or focal absence of the meniscus without displacement of a meniscus fragment (Figure 13).
Posterior horn tears of the lateral meniscus
Tears of the posterior horn of the lateral meniscus are difficult to recognize and often underreported. Careful analysis of thin axial images associated with sagittal MR sections is required to detect this often subtle lesion. The zip sign on axial images is a useful imaging sign encountered in tears of the posterolateral meniscus, extending from the distal insertion of meniscofemoral ligaments (MFLs) to the posterior horn of the lateral meniscus (Figure 14) .
Displaced meniscal fragments
Although complex tears with displaced meniscal fragments are readily identified, the precise displacement of free meniscal fragments is often underreported or disregarded.
A bucket-handle tear consists of a displaced longitudinal tear (Figure 15). MRI signs on sagittal images include the double posterior cruciate ligament (PCL) sign  and absent bow tie sign [20, 21]. MRI signs on coronal images include the fragment-in-notch sign , the flipped meniscus sign  or a centrally located meniscus fragment.
Fragment displacement underneath the collateral capsuloligamentary structures into the meniscal recess is less frequent [24, 25], but these fragments are difficult to identify arthroscopically. Therefore, a correct description provides crucial information for the surgeon . Fragment displacement within the inferior meniscocapsular recess may be accompanied by focal bone marrow edema at the proximal tibia, probably due to menisco-osseous impingement (Figure 16).
A discoid meniscus is a rather rare variant in the shape of the meniscus, usually involving the lateral meniscus and more rarely the medial meniscus . The patient may be asymptomatic or a snapping sound may be present. Discoid meniscus may also predispose to tears of the involved meniscus, causing pain and swelling.
According to the Watanabe classification, discoid meniscus is divided in three types: type I complete; type II, incomplete; and finally type III or Wrisberg-ligament variant, in which the posterior meniscofemoral attachment is absent resulting in an unstable meniscus with hypermobility .
As a rule of thumb, a discoid meniscus should be suspected if three or more contiguous body segments are present or if the size between the free margin and the periphery of the body on coronal images exceeds more than 1.5 cm. More accurate criteria are a ratio of the minimal meniscal width to maximal tibial width (on coronal images) of more than 20% and a ratio of the sum of the width of both lateral horns to the meniscal diameter (on a sagittal slice showing the maximal meniscal diameter) of more than 75% .
The evaluation of recurrent tears after surgery is much more complex than in the native meniscus because the imaging criteria for evaluation of meniscal tears in the native meniscus (abnormal signal on two MR slices and abnormal meniscal shape) are not reliable at the location of the meniscectomy. Detailed correlation with the type of surgery and preoperative imaging is mandatory. Discussion of the postoperative meniscus is beyond the scope of this short review and we refer to a recent review on the topic .
Partial versus complete anterior cruciate ligament tears
MRI evaluation of a partial anterior cruciate ligament (ACL) tear and differentiation from a complete ACL tear, mucoid degeneration or even a normal ACL can be challenging because of overlapping imaging features .
Partial tears of the femoral origin of the ACL may be particularly challenging on sagittal images alone. Therefore, meticulous correlation of axial and coronal images for assessment of the degree of ligament fiber disruption is mandatory .
MRI has an overall moderate accuracy to distinguish stable from unstable ACL tears. ACL discontinuity and abnormal orientation of ACL fibers have an accuracy of 79% and 87% respectively. Although anterior tibial translation, uncovering of the posterior horn of the lateral meniscus, and hyperbuckled PCL are specific signs of an unstable tear, the sensitivity of these signs is as low as 23% . Bone marrow edema around the lateral knee compartment is not a good parameter for predicting stability .
Ganglion cyst and mucoid degeneration of the anterior cruciate ligament
Thickening and increased T2-signal may also be seen in ganglion cysts or mucoid degeneration of cruciate ligaments, but the pattern is often more striated on T2-WI with interspersed intact ligamentous fibers, resembling a celery stalk. There is often associated bone marrow edema and/or intraosseous ganglion cyst formation at the femoral and tibial insertion of the ACL . On T1-WI, the ACL is of intermediate signal and the ligamentous structure has disappeared (Figure 17). Typically, there is no history of previous trauma.
Analysis of Abnormal Bone Marrow Pattern and Determination of its Etiology
Small residual islands of red bone marrow are often correctly characterized as physiological by most radiologists (Figure 18), but larger focal areas of physiological bone marrow reconversion may be more worrisome (Figure 19). Correlation with age, female gender, heavy smoking, sports activities with increases oxygen debt (e.g. distance running, free diving), obesity, diabetes, other diseases causing chronic anemia and finally treatment with hematopoietic growth factors is mandatory .
Abnormal bone marrow about the knee has a variable etiology including acute or chronic trauma , degenerative joint disease  and tumoral causes. Although bone marrow edema (BME) is readily identified, less experienced radiologists often struggle with precise identification of the underlying etiology. BME due to avulsion injuries is often more subtle than bone marrow edema due to compression injuries . Identification of the BME pattern and location helps to identify the mechanism of trauma [37, 38].
Subchondral insufficiency fractures are often not correctly reported by the less experienced radiologist and may be misinterpreted as osteochondral fractures (Figure 20) or avascular necrosis (Figure 21). These lesions result often from altered biomechanics and may be seen in patients with recent meniscectomy on the involved site .
Correct Grading of Cartilage Lesions
Correct grading of cartilage lesions on conventional MRI using the arthroscopic classification grading system remains a difficult task and there is much interobserver variability.
Identification of loose bodies is often disregarded, particularly if they are adjacent to bone of ligamentous structures (Figure 22).
Cystic Lesions about the Knee
There is a variety of cystic lesions and cyst mimickers about the knee of which the terminology is often confused and inappropriately used.
Joint recesses are normal outpouchings of the joint cavity, which may enlarge in case of a joint effusion. The prototype of a joint recess is the medial gastrocnemius-semimembranosus recess, which is located posteromedially in the knee. It has a typical connecting stalk with the knee joint between the medial gastrocnemius muscle and the semimembranosus tendon. Expansion gives rise to a so-called Baker’s cyst (Figure 23).
The popliteus recess is located posterolaterally and may be the source of a pseudotear of the posterolateral meniscus due to volume averaging (Figure 9). Distention of the ligamentum mucosum (infrapatellar plica) anteriorly within Hoffa’s fad pad is rarer (Figure 24). The significance of this finding is not clear. It may result from chronic stress or represent a variant of Hoffa’s disease .
Bursae are synovial-lined structures that decrease friction between moving structures. They may become distended due to (repetitive) trauma, inflammatory or infectious disease (e.g., rheumatoid arthritis, crystal deposition disease) or tumor or tumor-like conditions (e.g., pigmented villonodular synovitis, chondromatosis). Around the knee, bursae are characterized by their specific location, shape and relationship with surrounding structures. The following bursae are often found around the knee: prepatellar bursa (Figure 25), superficial infrapatellar bursa, deep infrapatellar bursa, pes anserinus bursa (Figure 26), medial collateral ligament bursa (Figure 27), semimembranosus – tibial collateral ligament bursa. A fibular collateral ligament – biceps femoris bursa is much rarer.
Synovial cysts and ganglion cysts
Ganglion cysts (Figure 28) may be located anywhere around the joints. They may either have a communicating stalk with the knee joint or rather be remote from the joint without any visible communication. Special forms of ganglion cysts include meniscal cysts, cruciate ligament cysts, intraosseous ganglia, cystic adventitial disease and peri- or intraneural cysts.
Meniscal cysts consist of a collection of synovial fluid, which is extruded through a meniscal tear. Lateral meniscal cysts are usually located at the periphery of the middle third of the meniscus, whereas medial meniscal cysts may present at a distant location from the joint because of the firm attachment of the medial meniscus to the joint capsule. The identification of an associated meniscal tear and communication of the cyst with the tear is the key to the characterization of a meniscal cyst (Figure 29).
Cruciate ligament cysts occur within the fibers or on the surface of the cruciate ligaments (ACL – PCL). They have a similar appearance as mucoid degeneration of the cruciate ligaments (Figure 17).
Intraosseous ganglia (Figure 30) are intraosseous extensions from synovial fluid through the subchondral bone.
Cystic adventitial disease is a ganglion cyst which is located in the wall of vessel (e.g., popliteal artery).
Not all masses which display very high signal intensity on T2-WI are necessarily fluid-filled. Tumors mimicking cysts include both benign (e.g., peripheral nerve sheath tumors, myxomas) and malignant tumors with prominent areas of necrosis or myxoid degeneration (e.g., myxoid liposarcoma).
Intravenous contrast should be administered whenever there is doubt about the cystic or solid nature of the visualized mass. Furthermore, abscesses and vascular masses, such as varices and popliteal artery aneurysms may simulate cystic lesions. For a more detailed discussion of cystic lesion about the knee, we refer to specific articles on this subject [41, 42].
Although potential pitfalls are numerous in the interpretation of MRI of the knee, a thorough knowledge of anatomy and its variations, close correlation with age, previous medical history and symptoms and systematic imaging approach will avoid interpretation errors. Specific recommendations are summarized in the Table 1.