NORMAL FINDINGS

The synovium is a membrane that lines the knee capsule and attaches to the margins of the articular surfaces and the menisci [2]. Synovial recesses are outpouchings of the synovial membrane between structures, where fluid may accumulate and such distention may lead to cystic appearance and misinterpretation [7]. Recesses may also serve a reservoir for loose bodies, such as cartilaginous or meniscal fragments.

Fig 1: Axial Proton Density Fat Saturated (PD FS) images demonstrating distended synovial recesses of the posterior knee in a patient with an effusion. Note the portions of the posterior recesses beneath the femoral insertions of the gastrocnemius muscle on each side, reffered by some as the ‘subgastrocnemuis bursae’.

Fig 2: Saggital and coronal PD FS images demonstrating distended synovial recesses of the posterior aspect of the joint. Note the subpopliteus recess embedding the proximal tendon of the popliteus musce (arrows).

• The posterior femoral recesses are located between the femoral condyles and deep surface of the heads of the gastrocnemius on each side. Distension of the portions beneath the femoral insertions of the gastrocnemius are referred by some authors as "subgastrocnemius bursae'' [12, 4]. 
  

  Fig 3: PD FS images depicting a distended subgastrocnemius portion of the medial posterior recess, referred by some authors as a subgastrocnemius bursa, appearing cyst-like in this patient.
• The PCL recess is located in relation to the posterior cruciate ligament (PCL). Communication with the medial femorotibial compartment and the absence of capsule help excluding a PCL ganglion [8].
• The subpopliteus recess represents an extension on the synovial space through the popliteus hiatus embedding the proximal popliteus tendon, the latter being intracapsular, but extrasynovial. Loose bodies can migrate into the recess and present as posterolateral knee pain.
  

  Fig 4: PD FS images showing normal appearance of a distended subpopliteal recess embedding the proximal popliteus tendon.
  

  Fig 5: PD FS images showing distended subpopliteus recess with synovial proliferation in a patient with synovitis.
  

  Fig 6: PD FS images depicting a distended subpopliteus recess with multilocular appearance due to multiple septae, containing migrated loose bodies. The complex appearance of the finding makes the differentiation between a popliteus tendon ganglion and synovial recess with chronic changes difficult.

Bursae represent enclosed synovial-lined sacs that help reduce friction between moving structures. Normally, they are collapsed or may contain a small amount of synovial fluid [11]. When a bursa becomes inflamed due to friction, trauma or inflammatory disease, the condition is known as bursitis. There seems to be confusion in the literature between the terms ‘’bursa’’, ‘’recess’’ and ‘’synovial cyst’’, as they are often used interchangeably. According to Giard MC et. al, the term “bursa’ should be reserved to synovium-lined structures not linked to the joint [3].

Popliteal lymph nodes lie between the popliteal artery and the posterior aspect of the knee joint. Lymph nodes with less fatty replacement in young individuals express high T2WI/PD signal and mimic cystic lesions [9].

Fig 7: Popliteal lymph nodes with no fatty replacement appearing cystic in a 20 y-o patient.

CYSTIC LESIONS

Synovial cysts (SC) represent fluid-filled herniations of the synovial membrane through the joint capsule, lined by synoviocites [3]. They are ususally associated with underlying arthopathy and are thought to serve as drainage reservoirs for joint effusion, escaping into the area of least resistance [11]. Examples of SC are the proximal tibiofibular joint synovial cyst and the commonest Baker’s cyst.

• Baker’s cyst is a misnomer since it does not represent a true cyst, but probably a herniation of synovium through the posterior joint capsule. Usually, a well traceable connecting stalk is seen between the tendons of the semimembranosus muscle and the medial head of gastrocnemius, where a valve-like mechanism is being implicated, causing unidirectional movement of joint fluid [11]. There seems to be confusion in the literature in terms of the exact nature of the cyst, as usually gastrocnemius–semimembranosus recess and bursa are used interchangeably, while some authors refer to it as a synovial cyst [3]. Most likely, it represents either a recess, or synovial herniation, since pathology seem to occur simultaneously intra-articularly and in the Baker’s cyst and a well traceable stalk is often observed. Distension is due to any degenerative, inflammatory or traumatic condition and incidence increases with age. Septations, haemorrhage, loose bodies and synovial proliferation may produce a heterogeneous appearance [11]. The cyst can extend in the superior or inferior soft tissues and may rupture causing pain and swelling with diffuse surrounding oedema and fluid tracking along fascial planes [7, 12].
  

  Fig 8: PD FS images demonstrating a Baker’s cyst with synovial proliferation in a patient with osteoarthritis.
  

  Fig 9: A patient with a history of sudden onset of sharp pain in the posterior knee. Note the perifocal oedema and fluid tracking along the fascial planes of a ruptured Baker’s cyst. A well traceable stalk is depicted between the tendons of the medial head of the gastrocnemius and semimembranosus muscles, being diagnostic of a Baker’s cyst (triangle).
  

  Fig 10: A patient complaining of tension, pain and swelling of the calf after a massage session. MR images revealing a complicated haemorrhagic Baker’s cyst. T1 weighted image [A] showing hyperdense periphery consisting with haemoglobin degradation of a subacute intracystic hematoma. Gradient echo image [C] depicting low intracystic signal (triangle) consistent with haemoglobin degradation products.

Ganglions represent cystic lesions arising from a variety of structures like tendons, ligaments, muscles, and nerves. Some authors believe that they result from degeneration of connective tissue since they are located in areas of continuous mechanical stress. Histologically they are delimited by connective tissue, in contrast to SC which are lined by synoviocites [3]. However, the terms are often used interchangeably and both can’t be fully differentiated by imaging alone, thus the final diagnosis stays histological. Ganglion cysts are unilocular or multilocular (“bunch of grapes” appearance) and may show complex morphology with septations, debris, and loose bodies. A stalk leading to an adjacent joint or ligamentous structure may be seen [1, 3, 10]. Special forms of ganglion-like conditions include meniscal cysts, cystic adventitial disease and intraneural ganglions [14]. Most ganglion cysts are asymptomatic, but some may present with pain or impingement syndromes.

• ACL ganglions are associated with degeneration of anterior cruciate ligament (ACL) where they appear as a multiloculated cystic lesions embedded within the ligamentous fibres.
  

  Fig 11: PD FS images showing an ACL ganglion cyst embedded in a degenerated ACL. Note the high signal of the ACL with a ‘celery stalk’ appearance, consisting with mucoid degeneration.
• PCL ganglions are located in relation to the PCL. Awareness should be kept not to misinterpret the PCL ganglion as a recess, as ganglia may be associated with pain, joint line tenderness, clicking sensation or limitation of motion [6].
  

  Fig 12: PD FS weighted image showing a ganglion in relation to the PCL with a stalk embedding into the fibres depicted on the axial image. Note the cystic appearance of the popliteal lymph nodes (triangle).
  

  Fig 13: PD FS images showing a ganglion cyst at the insertion of the lateral head of the gastrocnemius, note the prominent fluid-filled stalk at the insertion of the tendon.
• Popliteus tendon ganglia have been reported in the literature [10]. Multi-lobular appearance and septations favour ganglion over a subpopliteus recess, though such differentiation is often impossible and final diagnosis requires histological confirmation.
• Cystic adventitial disease represents accumulation of mucinous material within an arterial, or rarely a venous wall causing stenosis and claudication in young patients. It has a strong predilection for the popliteal artery, the most accredited theory suggesting synovial fluid from nearby joints tracking along vascular branches to the adventitia of vessels.
  

  Fig 14: A patient with cystic adventitial disease. Computed Tomography Angiography image (A), Volume Rendering image (B) and DSA image (C) showing eccentric stenosis reminiscent of a scimitar (scimitar sign). Notice the prominent collateral vessels on the affected side, while no atherosclerosis is evident.
  

  Fig 15: Saggital PD FS (A) and axial T2 weighted images (B) showing typical characteristics of the lesion (arrow) causing crescentic stenosis of the popliteal artery and communication with the cystic lesion with the joint space in the same patient with cystic adventitial disease [Fig. 14].
  

  Fig 16: Schematic representation of the synovial space, showing fluid propagating along a genicular arterial branch. A propagation to the knee joint along a genicular arterial branch was identified during surgery in the same patient with cystic adventitial disease [Fig 14, 15].
• Intraneural ganglions are located within the epineurium of nerves. According to the more recent theory, they originate from a nearby joint and dissect along an articular branch into the nerve. These lesions commonly affect the peroneal nerve, possibly due to it close proximity with the tibio-fibular joint, and may present with pain, motor weakness and paraesthesia along the distribution of the involved nerve [3, 11].
  

  Fig 17: Saggital PD FS images showing multilocular cystic lesion tracking along the tibial nerve consisting with a intraneural ganglion.
• Parameniscal Cysts are ganglion-like lesions associated with a meniscal tear, called by some authors “fibrocartilaginous tear-related cysts” [3]. Identification of a tear contacting the cyst is diagnostic [5]. Parameniscal recesses should not be mistaken for a meniscal cyst.
  

  Fig 18: PD FS images showing horizontal meniscal tear reaching the femoral articular surface (triangle) associated with a parameniscal cyst (arrow).
  

  Fig 19: PD FS images depicting a large parameniscal cyst adjacent to a horizontal rupture (triangle) of the medial meniscus.

CYSTIC MIMICS

• Popliteal artery aneurysms present on MR images with variable signal intensity depending on flow characteristics and wall thrombus, but show continuity with the popliteal artery.
  

  Fig 20: CTA (A) and PD FS (B,C) images showing a popliteal artery aneurysm with heterogeneous appearance due to mural thrombosis and turbulent flow.
• Popliteal vein conditions such as popliteal vein aneurysms or venous thrombosis may mimic cystic lesions, as continuity with the popliteal vein is usually diagnostic.
  

  Fig 21: PD FS images depicting focal dilatation of the popliteal vein corresponding to a venous aneurysm.
  

  Fig 22: PD FS images depicting cyst-like distension of the popliteal vein and soft tissues edema in a patient with venous thrombosis.
• Abcesses and hematomas may appear as a fluid collections and therefore cystic, though internal signal may vary. There are no imaging criteria that absolutely indicate whether a fluid collection is infected as aspiration is required for definitive diagnosis [5].
  

  Fig 23: A patient with a painful lump at the back of the knee and difficulty in movement. PD FS (A, B), DWI (C) and post-Gad (D,E) images showing a thick-walled collection infiltrating the distal tendon of the semitendinosus muscle with intense wall enhancement subsequently proven to be an abscess collection. Note the resolution of the collection after antibiotic therapy with residual oedema in the affected area (F).
• Tumors - vascular tumors such as arteriovenous malformations may present as a cystic-appearing masses in the popliteal fossa. The presence of flow voids in a high-flow AVN, phleboliths in venous malformation, evidence of a feeding or draining vessel and intense enhancement suggest vascular lesion [1]. Malignant tumours with prominent areas of necrosis such as fibrous histiocytoma or synovial sarcoma may also appear cystic [11].
  

  Fig 26: A patient with a slow-flow venous malformation of the posterior thigh and knee. PD FS (A) T2W (B), GRE (C), Time-resolved angiography (D) and post-Gd (E) images depicting the complex morphology of the lesion, invasion of multiple anatomical planes, intense enhancement and deliniation of multiple draining veins.
• Peripheral nerve sheath tumours are classified as schwannomas or neurofibromas. They present as fusiform lesions, oriented along the long axis of the nerve, giving a ''yo-yo on a string'' appearance, with a central low T2W signal (target sign) and rind of surrounding fat (split fat sign) [13]. Lesions affecting the peroneal nerve may cause foot drop and paraesthesia over the dorsum of the foot, while MRI may depict oedema of the affected muscles due to denervation [16].
  

  Fig 24: A patient with NF type II. Pre- (A, B) and post-GD images (C,D) showing a fusiform lesion oriented along the peroneal nerve appearing like a ''yo-yo on a string'' and demonstrating the ''target sign' - central fibrocollagenous core (low T2 signal) surrounded by myxoid periphery (high T2 signal). The nerve can be seen in continuation with the mass.
  

  Fig 25: Pre-contrast MR images of a lobulated mass with central necrosis invading the tibial nerve. The tumor was histologically proven to be a malignant nerve sheath tumor. 6% of the peripheral nerve sheath tumors may undergo malignant transformation [20].