Synovial folds or synovial plicae are remnant duplications of synovium formed during embryonic development.
They are of particular interest and commonly described in the knee, elbow and hip joint but can be also found in other joints 1-4.
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Terminology
The term ‘plica’ is commonly used if a synovial fold becomes symptomatic or a source of pain 1.
Gross anatomy
Normal synovial folds are thin and have a pliable consistency with a semitranslucent pinkish appearance 1. They are continuous with the joint capsule and project into the joint space at specific locations.
Synovial folds are described in the following joints and locations 1-4:
- synovial plicae of the knee
- synovial folds of the elbow
- synovial folds of the hip
- spine
- atlantoaxial joints
- cervical and lumbar spine facet joints
- shoulder joint
- posterior synovial fold: considered a normal variant 4
- septum between the subacromial and subdeltoid bursa: a possible rare cause of shoulder impingement
- ankle plica (rare)
- spanning anteriorly across the ankle joint
- originates in the anterolateral gutter from the osteocartilaginous fossa of the lateral malleolus
- blends with the soft tissue of the anteromedial gutter
- wrist and scaphotrapezial joint (rare)
- temporomandibular joint (uncertainty about their role in symptoms)
Function
Functions of synovial plicae include the following 2,3:
- production of synovial fluid and lubrication of articular surfaces
- transition and accommodation of neurovascular structures
- space-filling and joint stabilization
Histology
Histologically synovial folds are composed of the synovial membrane with an underlying layer of loose connective tissue containing capillaries and venules 1.
Variants
There are different morphologic variants including band-like, leaf-like or villous synovial folds 2-4.
Radiographic features
MRI
On MRI synovial folds appear as hypointense meniscoid or band-like structures surrounded by synovial fluid. They may be better visualized in the setting of a joint effusion.
Signal characteristics
- T1: isointense
- T2: low signal intensity
- PDFS/T2FS: low signal intensity
CT/MR arthrography
Synovial folds or plicae may be nicely visualized by CT/MR arthrography and usually appear as low signal intensity soft tissue bands or shelf-like structures within the brightly stained joint fluid. However, they might display normal morphological variations 4,5.
Clinical importance
Synovial folds or plicae can become a cause of pain as a result of impingement and/or chronic inflammation due to repetitive stress, direct trauma or chronic inflammatory conditions 2.
A diffusely thickened plica associated with synovitis and/or chondral injury in the absence of any other significant pathologies should make one think of plica syndrome 1.
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