This evidence-based study employed different staining techniques on the cyst samples collected from patients of rare and severely painful synovial cyst of knee cruciate ligament and found out that it is just a pseudo-cyst leading to inflammation and pain.

Synovial cyst of knee cruciate ligament (SCKCL) is not so common condition but when occur causes severe knee pain. The aetiology of SCKCL is also not well-known. The study intended to evaluate the SCKCL pathogenesis by cyto- and histopathological analysis.

A total of ten patients went through arthroscopy including five patients with a past knee injury. For cyst wall tissue sections, Hematoxylin & eosin staining and for cyst fluid smear, Papanicolaou staining was used. Prussian blue staining was applied to both the fluid smear and wall section. The probable involvement of several cell types in the SCKCL development was assessed using immunohistochemical staining for vascular endothelial cells (CD31), monocytes (CD68), hematogenous stem cells (CD117), mesothelial cells (MC), and epithelial cells (CK).

There is no erythrocyte present in the fluid. Although, all patients cyst wall and fluid present with Prussian blue stained hemosiderin particles, proposing past haemorrhage. Plasmocytes and lymphocytes were present in abundance in both cyst wall and fluid. Moreover, the  Abundant CD68(+) monocytes infiltrated the cyst lining; whereas the  MC(+) mesothelial cells among the four samples were present in inadequate amount. The cyst submucosa was also abundant with CD68(+) monocytes. The proliferated capillaries are known to be stained with CD31. Eight specimens observed with CD117-positive hematogenous stem cells sporadically.

As per the findings, SCKCL is not a mature synovial cyst but is an inflammatory pseudo-cyst, originated from past minor intra-ligament chronic inflammation and hemorrhage.