MRI characteristics of cysts and “cyst-like” lesions in and around the knee: what the radiologist needs to know
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Objectives and Methods
A variety of benign cystic or “cyst-like” lesions may be encountered during a routine magnetic resonance imaging (MRI) of the knee. These lesions comprise a diverse group of entities from benign cysts to complications of underlying diseases. In addition, normal anatomic bursae and recesses may be misdiagnosed as an intra-articular cystic lesion when they are distended. However, the majority of the aforementioned lesions have characteristic MR appearances that allow a confident diagnosis, thus obviating the need for additional imaging or interventional procedures.
This article includes a comprehensive pictorial essay of the characteristic MRI features of common and uncommon benign cysts and “cyst-like” lesions in and around the knee joint.
For accurate assessment of the “cystic structure”, a radiologist should be able to identify typical MRI patterns that contribute in establishing the correct diagnosis and thus guiding specific therapy and avoiding unwarranted interventional procedures such as biopsy or arthroscopy.
• Cystic lesions are common in knee MRI and the commonest, the Baker’s cyst, has an incidence of 38 %.
• Synovial cysts, meniscal cysts, normal knee bursae and recesses have characteristic MR appearances.
• Miscellaneous “cyst-like” lesions may require a more dedicated MR protocol for a correct diagnosis.
KeywordsKnee MRI diagnosis Cyst Bursae Recesses
Given the fact that magnetic resonance imaging (MRI) is being performed more frequently for assessment of the knee joint (e.g. post-traumatic, in sport injuries, in rheumatological disorders, in oncological imaging), the number of incidental cystic and “cyst-like” lesions in and around the knee joint found on routine knee MRI scans has also increased [1, 2, 3, 4]. The vast majority of these lesions are benign, ranging from benign cysts to complications of underlying diseases and many of them demonstrate characteristic features on MRI, thus allowing a confident diagnosis to be made [1, 2, 3, 4, 5, 6]. Knowledge of the common anatomical locations and appearances of bursae, recesses, cysts and ganglia is necessary so that radiologists do not misinterpret these benign entities as soft-tissue tumours [1, 2, 3, 4, 5, 6, 7, 8]. It is of paramount importance for the radiologist to be aware of the MRI features because understanding the spectrum of appearances of the various benign cystic lesions is vital for optimal patient management. This article is intended to be a comprehensive pictorial review of the most common and uncommon benign cystic and “cyst-like” lesions in and around the knee joint. For easier classification purposes, benign cysts were subdivided into categories as following: (1) synovial cysts, (2) ganglion cysts, (3) meniscal cysts and (4) intraosseous cysts. Similarly, “cyst-like” lesions were subclassified into the following: (1) normal knee bursae, (2) normal knee recesses and (3) miscellaneous cyst-like lesions.
Synovial cysts are defined as juxta-articular fluid collections that are lined by synovial cells [1, 2]. The synovial lining is the characteristic histological feature that distinguishes them from other juxta-articular fluid collections [1, 2, 6, 8]. From a pathophysiological point of view a synovial cyst represents a focal extension of joint fluid that may extend in any direction and may, or may not, communicate with the joint. They can be encountered as incidental findings in MR examinations, but regarding their aetiology, they have been associated with other underlying knee disorders such as osteoarthritis, trauma, rheumatoid arthritis, gout, systemic lupus erythematosus and juvenile rheumatoid arthritis [1, 2, 4, 5, 6]. Although usually asymptomatic, they can manifest with pain and swelling. The most common examples of a synovial cyst in the knee are the popliteal cyst (Baker’s cyst) and the proximal tibiofibular joint (PTFJ) synovial cyst.
A ganglion cyst is defined as a benign cystic mass that is surrounded by dense connective tissue, without a synovial lining and is filled with a gelatinous fluid rich in hyaluronic acid and other mucopolysaccharides. Ganglia are traditionally divided into the following categories: intra-articular, extra-articular, intraosseous and (rare) periosteal.
They are extremely rare and are thought to be produced by mucoid degeneration and cyst formation of the periosteum [1, 2, 6, 7, 8, 9]. They are most commonly located in the proximal tibial shaft, in proximity to pes anserinus. On MR images they typically appear as periosteally based, well-defined homogeneous lesions with fluid signal intensity. Superficial cortical erosion and scalloping, as well as reactive new bone formation may be present. Other lesions of periosteal origin, such as periosteal chondroma, subperiosteal hematoma, chronic subperiosteal abscess or malignant soft tissue tumours, mainly when they erode the adjacent bone and cause periosteal reaction, may need differential diagnosis from a periosteal ganglion.
Intraosseous cysts can be classified into intraosseous ganglion cysts, subarticular degenerative cysts (geodes) and insertional (avulsion-traction) cysts.
Intraosseous ganglion cysts
These are covered above in the “Intraosseous ganglia” section.
Degenerative or subarticular cysts (geodes)
Normal knee bursae
Numerous bursae can be encountered around the knee joint and their primary action is to reduce friction between adjacent moving structures, such as tendons, ligaments and bone surfaces [6, 7, 8, 9, 35, 36]. From a histological point of view they are synovium-lined structures and are usually collapsed but may often contain a small amount of synovial fluid. Typically are not visible on MRI, unless they are inflamed from various causes (hence the term bursitis) [6, 7, 8, 9, 35, 36]. In the following classification, an anatomical location-based scheme (anterior-medial-lateral-posterior) is used for descriptional purposes:
Superficial infrapatellar bursitis
The superficial infrapatellar or pretibial bursa is located between the tibial tubercle and the overlying skin. It is an uncommon site for bursitis, but direct trauma or occupational overuse (clergyman’s knee) may result in inflammation and micro-haemorrhage) [1, 2, 6]. The characteristic MRI finding is a focally poorly defined fluid collection anterior to the tibial tubercle.
Deep infrapatellar bursitis
Anserine bursitis (Pes anserinus bursitis)
Medial collateral ligament bursitis (MCL bursitis)
Semimembranosus-tibial collateral ligament bursitis
The iliotibial bursa is located between the distal part of the iliotibial band, near its insertion on Gerdy’s tubercle, and the adjacent tibial surface. It may mimic iliotibial tendinitis and lateral meniscal or lateral collateral ligamentous pathology [1, 2, 6, 8]. On MR images iliotibial bursitis is demonstrated as a well-defined fluid collection between the insertion of the distal iliotibial band and the adjacent bony surface.
Lateral/fibular collateral ligament-biceps femoris bursitis (LCL bursitis)
Gastrocnemius-semimembranosus bursitis (Posterior bursitis)
The posteriorly located gastrocnemius-semimembranosus bursa (popliteal or Baker’s cyst) together with its symptomatology is covered above in the synovial cyst section.
In summary, the MR characteristics of the various bursae that can be encountered around the knee joint have been presented in this section. Above and beyond correct diagnosis the radiologist can also be implicated in the clinical management of these conditions. Treatment with ultrasound-guided aspiration and local injection of long-acting analgesic and steroid may relieve symptoms and represent the optimal therapy in cases of bursitis [43, 44, 45, 46]. Percutaneous-guided treatments have been used successfully for pain management in bursitis and have been proven effective, thus obviating the need for surgical therapy [43, 44, 45, 46].
Normal knee recesses
There are numerous anatomical knee recesses that can be demonstrated in cases of knee effusion and may be misinterpreted as cyst-like lesions [47, 48, 49, 50]. Good knowledge of those spaces is essential in order to avoid pitfalls in MRI.
The posterior femoral recesses (subgastrocnemius recesses) are found posteriorly to both femoral condyles and the deep surface of the lateral and medial heads of gastrocnemius.
The posterior capsular recesses (in the midline) behind the PCL, may be identified as an extension of the medial femorotibial compartment.
The subpopliteal recess is demonstrated between the popliteus tendon and the posterior horn of the lateral meniscus.
The suprahoffatic recess is at the superior part of the Hoffa’s fat pad, close to the inferior border of the patella.
The infrahoffatic recess lies anterior to the inferior portion of the infrapatellar plica (also called ligamentum mucosum).
The anterior tibial recess, is a normal capsular recess immediately anterior to the proximal tibia.
The central synovial recess lies between the patella/patellar ligaments and the anterior aspect of the femur.
Other miscellaneous cyst-like lesions
A variety of “cystic” lesions can be encountered in and around the knee joint that may complicate the differential diagnosis even more [1, 2, 6, 7, 8, 9, 10, 11]. The most common benign non-tumoral are the following:
Benign cystic and “cyst-like” lesions are a common finding inside and around the knee joint. MRI is an excellent method for demonstrating and differentiating these lesions. A radiologist should be able to identify typical MRI patterns that contribute in establishing the correct diagnosis and thus guiding specific therapy and avoiding unwarranted interventional procedures such as biopsy or arthroscopy.
Conflict of interest
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