This 29-year-old presents with knee pain, and you are shown one sagittal water-weighted fat-suppressed image of a high signal intensity mass (green arrow). You have three choices for the diagnosis.
- Capsulosynovial cyst
- Ganglion pseudocyst
- Parameniscal pseudocyst of meniscal origin
Which one do you choose?
Start by eliminating meniscal cyst of meniscal origin or pseudocyst of meniscal origin. You are not shown a meniscus tear, nor is there any horizontal signal in the meniscus. Meniscal cysts are misnamed, as they are diffusional events that occur from horizontal tears or weakening of the meniscus center that allows synovial fluid to dissect through the meniscus into the surrounding tissues that are walled off by fibrous tissue. In other words, they are not epithelial lined. True cysts are epithelial lined. Capsulosynovial cysts are epithelial lined.
A meniscal cyst typically has a tail or a small “nubbin” extending into the center of the meniscus. However, the abnormality in this case has a tail extending underneath the meniscus (pink arrow). Such a tail is typical of a ganglion pseudocyst, which is the answer in this case. While parameniscal cysts of meniscal origin and ganglion pseudocysts look identical histologically, the pseudocyst of meniscal origin has a tail extending below it or occasionally above it. This tail is below or along the undersurface of the meniscus, not into its center.
Finally, capsulosynovial cysts tend to be less septated than this lesion. The longitudinal elliptical or “cigar shape” is atypical for a capsulosynovial cyst. Capsulosynovial cysts tend to generate less mass effect or expansile characteristics. Capsulosynovial cysts are associated with an effusion, capsulitis and sometimes synovitis. On this single image, no effusion is shown. Ganglia result from degeneration of capsules or tendon sheaths. The material that diffuses through these structures is virtually identical to the material that diffuses through the meniscus in a parameniscal pseudocyst or so-called meniscal cyst of meniscal origin. It is the origin of the “tail” or “nubbin” that allows the differentiation between the two, not the pathologist.
This may be of clinical importance in the treatment plan because many parameniscal pseudocysts of meniscal origin are approached arthroscopically from the inside going outward while symptomatic ganglia are approached from the outside going inward using an external approach. Therefore the treatment implications are not inconsequential. This author has encountered many cases where a reader has interpreted a lesion of this type as a meniscal cyst. A surgeon enters a joint arthroscopically and finds nothing. This is a good way to incur the wrath of your orthopaedic surgeon.
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