- The knee is the most commonly imaged joint in most practices, particularly in the realm of MR imaging. Achieving a functional comfort level with the relevant anatomy, pathology, and imaging techniques is imperative in order for the radiologist to add value to the diagnostic work-up of the patient with knee pain. Because the literature regarding knee imaging is bountiful, there is at once both an opportunity and an obligation for the radiologist to stay current with the latest techniques and reading strategies, lest they fall behind. This section explores the full range of knee trauma pathology seen in a modern practice, using the latest published data available.
Terminology and Conventions
- Degenerative changes in a tendon are referred to as tendinopathy, and not as tendinitis or tendinosis, in an effort to stay true to the appropriate etymologic meanings of these terms. The term spontaneous osteonecrosis of the knee is not used but is instead covered under the section on osteochondral injury, as this imaging finding is considered by most authors to be due to shear trauma &/or insufficiency fractures.
- The femorotibial (knee) joint is a simple hinge with very little rotational motion occurring at the articulation in normal physiologic motion. A few degrees of external tibial rotation occur in terminal extension, serving to lock the knee and reduce the need of constant muscular action to help hold the knee in this position during standing (this is sometimes referred to as the screw-home mechanism). The popliteus muscle serves to rotate the femur externally during initiation of flexion in order to unlock the knee. The patella is a large sesamoid bone in the extensor tendon complex and articulates with the trochlear groove of the femur in order to increase the fulcrum length of the quadriceps unit and reduce friction between the tendons and the femur.
- The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) serve to balance each other in the full range of flexion and extension of the knee. The ACL prevents anterior translation of the tibia relative to the femur and is taut predominantly in knee extension. The PCL prevents posterior tibial translation; it is taut primarily in knee flexion. These ligaments also complement each other in resisting rotational knee motion. The medial collateral ligament resists valgus forces, and the lateral collateral ligament complex resists varus forces. The posterolateral corner ligament complex is a series of mostly capsular thickenings that serve to stabilize this important part of the joint.
- The menisci are fibrocartilage wedges conforming to the shapes of the tibial articular condyles; they serve to cushion the impact of the femur on the tibia during weight bearing. The medial meniscus is larger and has a larger radius of curvature than its lateral counterpart. The medial meniscus is also more firmly attached to the bones than is the lateral meniscus, allowing more lateral meniscal motion during flexion and extension. The menisci derive their blood supply from a vascular pedicle that enters at the capsular margin of the meniscus. Vascularity within the meniscus becomes progressively more sparse toward the central free edge and diminishes in general in older patients.
- Because of its superficial location, the common peroneal nerve is the only commonly injured nerve in the knee region. It lies close to the skin as it curves around the proximal fibula and can be compressed either by internal processes (ganglia, osteophytes, etc.) or impacted from extrinsic trauma.
- Injury to the knee is a common occurrence across the age spectrum and, thus, results in a high frequency of imaging studies in a typical practice. Radiographs and CT imaging are useful for characterizing osseous trauma, and surgical classification systems have been provided where appropriate. Injury to the knee is often related to sports activity and, as such, may be either acute or the result of chronic repetitive microtrauma. In either case, it is often the ligaments, tendons, and cartilage of the knee that bear the brunt of such injuries. In particular, tears of the menisci of the knee and focal or diffuse articular cartilage defects account for significant disability in modern society, and accurate imaging evaluation of these injuries may serve to target appropriate therapy and, in some cases, may obviate surgical intervention.
- An understanding of the typical injury patterns encountered in the knee will aid the radiologist, both in recognizing common injuries and in anticipating more subtle but clinically relevant findings based on their association with these injury patterns. For instance, the pivot-shift mechanism of knee injury (often due to making a sudden cut while running or due to a clipping injury) not only frequently causes disruption of the ACL but is often associated with vertical longitudinal tears of the posterior horns of the medial and lateral menisci. Occasionally, avulsion of the lateral capsular ligament and injury to the posterolateral corner ligaments occur as well. These associated injuries may be subtle on MR imaging, and anticipating their presence based on pattern of injury will serve the radiologist well. On the other hand, the presence of a small avulsion from the lateral tibial rim on radiography (Segond injury) may appear to be inconsequential at 1st glance, but its very high association with an ACL tear should lead to further evaluation with MR imaging.
- Radiographic evaluation of the knee usually includes 3 standard views but in the setting of trauma may be limited to AP and lateral projections. A cross-table lateral view is useful in the setting of acute trauma, as a large lipohemarthrosis may be visible as a fat-fluid level in the suprapatellar joint recess and serves as an indicator of intraarticular fracture. The axial patellofemoral (sunrise) view is useful for evaluating patellofemoral arthritis and alignment, though less valuable in the setting of acute trauma, except patella fractures.
- MR imaging is the technique of choice in the evaluation of intraarticular pathology of the knee. Injection of dilute gadolinium compounds into the knee prior to MR imaging (MR arthrography) has been shown to provide substantial benefit in certain clinical situations, particularly in the evaluation of the postoperative meniscus; however, utilization for this purpose is very surgeon dependent. Indirect MR arthrography (IV gadolinium injection with delayed MR imaging) has been shown by some authors to be useful in evaluation of postoperative cartilage and meniscus lesions. CT arthrography is an alternative imaging technique in those patients unable to undergo MR imaging for the diagnosis of meniscus tears.
- CT of the knee is commonly used to characterize complex intraarticular fractures, particularly those of the tibial plateau. Because they may be anatomically complex, these fractures are difficult to fully evaluate with routine radiography. MR imaging of intraarticular fractures can be useful in depicting associated soft tissue injury, but MR often lacks the spatial resolution to depict small osseous fragments.
- US of the knee can be valuable in the study of superficial structures of the knee (tendons and collateral ligaments), particularly in the setting of previous arthroplasty or in patients unable to undergo MR imaging. It can also be useful in the evaluation and drainage of cystic collections (popliteal or Baker cysts) or for guidance in joint aspiration or injection.
- MR imaging serves as the mainstay of imaging for internal derangement of the knee. A thorough familiarity with the appearance of normal anatomy, common variants, and pathologic processes in the knee is imperative for the radiologist who interprets these studies. MR imaging of the knee is performed with a rigid volume coil with the knee near isocenter of the magnetic field, serving to maximize both signal-to-noise ratio and field homogeneity. The use of a transmit-receive knee coil further improves on these features. The recent advent of multichannel extremity coils has led to even greater improvements in image quality, and they can also be used to improve time efficiency of imaging. Isotropic volumetric imaging with reconstructions can decrease scan time and may replace the standard knee MR examination in certain settings.
- Imaging of the menisci: special mention of MR imaging strategies as they regard the menisci is warranted. Because they are composed of relatively homogeneous fibrocartilage, the normal menisci should demonstrate low signal on all imaging sequences. Degeneration or tear of a meniscus is diagnosed based on abnormal signal within the meniscus substance. In the absence of prior meniscus surgery, any elevated signal that clearly extends to the superior or inferior articular surface or free edge (central tip) of the meniscus fulfills the MR criteria for a tear. Some authors have argued that such contact with the articular surface of the meniscus should be confirmed on at least 2 MR images. Morphologic changes in the meniscus may also herald a tear; blunting of the free edge, separation or displacement of a portion of the meniscus, or absence of meniscal substance in its expected location are all consistent with a tear in the absence of a history of surgery. In the postoperative meniscus, signal can extend to the surface of the meniscus due to resection of meniscal tissue down to an area of central degeneration, and the presence of fluid or arthrographically injected contrast within the meniscus may be necessary to diagnose a tear in this situation. Some reports of evaluation of meniscus pathology using high-definition US have been published, but this is certainly not in the armamentarium of the typical radiology practice.
- Because of anatomic constraints, no special positioning maneuvers are used commonly. Imaging planes are straightforward with coronal and sagittal images based on the position of the knee in the coil. A useful landmark for the prescription of coronal images is a line drawn across the furthest posterior edges of the femoral condyles on the appropriate axial image. Sagittal images are prescribed perpendicular to the coronals. The use of 3T magnets has increased and with them have come the promise of higher spatial resolution, chemical composition information, and faster imaging times.
- Though the choices of pulse sequences available to perform MR imaging of the knee is vast and growing constantly, certain basic principles are worth bearing in mind. Meniscus imaging is best accomplished using short time of echo (TE) sequences, such as T1- or PD-weighted imaging, in order to maximize signal in a meniscus tear and minimize echo blurring. The use of fast spin-echo sequences to evaluate the menisci is now considered mainstream, as the earlier concerns about echo blur and spatial resolution limitations have been solved by equipment manufacturers. Fluid-sensitive sequences, usually employing spectral fat suppression, are useful for the evaluation of marrow processes, cystic collections, and soft tissue injury. Experienced radiologists tend to find their favorite combinations of planes and sequences, and a typical diagnostic knee MR examination is comprised of 4-6 such sequences, using at least 1 sequence in each major imaging plane (sagittal, coronal, and axial). General rules of thumb include using thin-slice profiles for the evaluation of the menisci (≤ 3-4 mm thickness, with interslice gap minimized to < 1 mm) and the use of short TE sequences in both sagittal and coronal planes. Some authors advocate the addition of spectral fat suppression to these meniscus-sensitive sequences, claiming superior sensitivity for meniscal tears. Broader experience suggests that it may be that the use of fat suppression in this setting is best left simply to subjective preference.
- Optimal visualization of articular cartilage is best accomplished using sequences that emphasize the difference of appearance between fluid and cartilage; many such combinations have been described and are part of the basic armamentarium of imaging sequences employed for routine knee imaging. On the other hand, the evaluation of meniscus and articular cartilage pathology has led to an explosion of specialized MR pulse sequences designed to provide increased sensitivity and specificity for subtle injuries to these important structures. Many have entered the mainstream lexicon of practicing radiologists, while others remain largely experimental. A brief overview of some of these techniques is presented in the chapters on articular cartilage imaging and postoperative cartilage imaging. Unfortunately, each magnet manufacturer identifies their particular versions of these types of sequences with proprietary names or acronyms; thus, one is best served by becoming familiar with the capabilities of the magnets used in their practice setting.
- Baker JC et al: Imaging the postoperative knee meniscus: an evidence-based review. AJR Am J Roentgenol. 211(3):519-27, 2018
- Ariyachaipanich A et al: Update on MRI pulse sequences for the knee: imaging of cartilage, meniscus, tendon, and hardware. Semin Musculoskelet Radiol. 21(2):45-62, 2017
- Nacey NC et al: Magnetic resonance imaging of the knee: an overview and update of conventional and state of the art imaging. J Magn Reson Imaging. 45(5):1257-75, 2017
- Akatsu Y et al: Accuracy of high-resolution ultrasound in the detection of meniscal tears and determination of the visible area of menisci. J Bone Joint Surg Am. 97(10):799-806, 2015
- Draghi F et al: Knee bursitis: a sonographic evaluation. J Ultrasound. 18(3):251-7, 2015
- Foley R et al: Sonography of traumatic quadriceps tendon tears with surgical correlation. J Ultrasound Med. 34(5):805-10, 2015
- Nogueira-Barbosa MH et al: Ultrasound assessment of medial meniscal extrusion: a validation study using MRI as reference standard. AJR Am J Roentgenol. 204(3):584-8, 2015
- Tresley J et al: Sonographically guided posteromedial approach for intra-articular knee injections: a safe, accurate, and efficient method. J Ultrasound Med. 34(4):721-6, 2015
- Xia XP et al: Ultrasonography for meniscal injuries in knee joint: a systematic review and meta-analysis. J Sports Med Phys Fitness. 56(10):1179-87, 2015
- Boutin RD et al: Magnetic resonance imaging of the postoperative meniscus: resection, repair, and replacement. Magn Reson Imaging Clin N Am. 22(4):517-55, 2014
- Cook JL et al: MRI versus ultrasonography to assess meniscal abnormalities in acute knees. J Knee Surg. 27(4):319-24, 2014
- De Maeseneer M et al: Ultrasound of the knee with emphasis on the detailed anatomy of anterior, medial, and lateral structures. Skeletal Radiol. 43(8):1025-39, 2014
- Forney MC et al: Magnetic resonance imaging of cartilage repair procedures. Magn Reson Imaging Clin N Am. 22(4):671-701, 2014
- Gill KG et al: Magnetic resonance imaging of the pediatric knee. Magn Reson Imaging Clin N Am. 22(4):743-63, 2014
- Karrasch C et al: The acutely injured knee. Med Clin North Am. 98(4):719-36, xi, 2014
- Lesniak BP et al: Use of ultrasonography as a diagnostic and therapeutic tool in sports medicine. Arthroscopy. 30(2):260-70, 2014
- Mohankumar R et al: Pitfalls and pearls in MRI of the knee. AJR Am J Roentgenol. 203(3):516-30, 2014
- Naraghi A et al: MR imaging of cruciate ligaments. Magn Reson Imaging Clin N Am. 22(4):557-80, 2014
- Rosas HG: Magnetic resonance imaging of the meniscus. Magn Reson Imaging Clin N Am. 22(4):493-516, 2014
- Samim M et al: MRI of anterior knee pain. Skeletal Radiol. 43(7):875-93, 2014
- Singer A et al: Tip of the iceberg: subtle findings on traumatic knee radiographs portend significant injury. Am J Orthop (Belle Mead NJ). 43(3):E48-56, 2014
- Tan HK et al: Variants and pitfalls in MR imaging of knee injuries. Semin Musculoskelet Radiol. 18(1):45-53, 2014
- Yablon CM et al: Magnetic resonance imaging of the extensor mechanism. Magn Reson Imaging Clin N Am. 22(4):601-20, 2014
- Yablon CM et al: Ultrasound of the knee. AJR Am J Roentgenol. 202(3):W284, 2014
- Zbojniewicz AM: US for diagnosis of musculoskeletal conditions in the young athlete: emphasis on dynamic assessment. Radiographics. 34(5):1145-62, 2014
- Zbojniewicz AM et al: Imaging of osteochondritis dissecans. Clin Sports Med. 33(2):221-50, 2014
- Barber BR et al: Meniscal injuries and imaging the postoperative meniscus. Radiol Clin North Am. 51(3):371-91, 2013
- Ciuffreda P et al: Mechanism of traumatic knee injuries and MRI findings. Musculoskelet Surg. 97 Suppl 2:S127-35, 2013
- Davis KW et al: Magnetic resonance imaging and arthroscopic appearance of the menisci of the knee. Clin Sports Med. 32(3):449-75, 2013
- Farshad-Amacker NA et al: MRI of knee ligament injury and reconstruction. J Magn Reson Imaging. 38(4):757-73, 2013
- Griffin JW et al: MRI of the knee with arthroscopic correlation. Clin Sports Med. 32(3):507-23, 2013
- Ostlere S: The extensor mechanism of the knee. Radiol Clin North Am. 51(3):393-411, 2013
- Rabuck SJ et al: Anterior cruciate ligament healing and advances in imaging. Clin Sports Med. 32(1):13-20, 2013
- Steinbach LS et al: Imaging of cysts and bursae about the knee. Radiol Clin North Am. 51(3):433-54, 2013
- Walker RE et al: Radiologic review of knee dislocation: from diagnosis to repair. AJR Am J Roentgenol. 201(3):483-95, 2013
- Kalke RJ et al: MR and CT arthrography of the knee. Semin Musculoskelet Radiol. 16(1):57-68, 2012
- Klauser AS et al: Clinical indications for musculoskeletal ultrasound: a Delphi-based consensus paper of the European Society of Musculoskeletal Radiology. Eur Radiol. 22(5):1140-8, 2012
- Gnannt R et al: MR imaging of the postoperative knee. J Magn Reson Imaging. 34(5):1007-21, 2011
- Hayter C et al: Magnetic resonance imaging of cartilage repair techniques. J Knee Surg. 24(4):225-40, 2011
- Milewski MD et al: MRI-arthroscopy correlation: the knee. J Bone Joint Surg Am. 93(18):1735-45, 2011
- Sanders TG: Imaging of the postoperative knee. Semin Musculoskelet Radiol. 15(4):383-407, 2011
- Vohra S et al: Normal MR imaging anatomy of the knee. Magn Reson Imaging Clin N Am. 19(3):637-53, ix-x, 2011
- Miller TT: MR imaging of the knee. Sports Med Arthrosc. 17(1):56-67, 2009
- Griffin N et al: High resolution imaging of the knee on 3-Tesla MRI: a pictorial review. Clin Anat. 21(5):374-82, 2008
- Lee MJ et al: Ultrasound of the knee. Semin Musculoskelet Radiol. 11(2):137-48, 2007
- Chung CB et al: MR arthrography of the knee: how, why, when. Radiol Clin North Am. 43(4):733-46, viii-ix, 2005
Related Differential Diagnoses