Neglected, Perineural, Chronic, Extra-articular, Lateral, Gangliomatous Synovial Knee Cyst With Peroneal Neuropathy: A Case Report.

Lateral knee cysts are relatively common synovial swelling. They are commonly benign swelling due to synovial fluid herniation from the joint. They are usually asymptomatic and in most cases are so small that only magnetic resonance imaging (MRI) can confirm the diagnosis. A synovial cyst of size more than 4 cm with nerve compression leading to peroneal neuropathy has grave clinical implications. We report an unusual case of a large, neglected, symptomatic, chronic, lateral synovial cyst with foot drop in a 67-year-old male. He presented after almost two months of onset of weakness in the right lower limb. A cystectomy with nerve exploration and decompression was done. The functional outcome for the knee was graded to be good based on the Lysholm knee scoring scale, with no instability or restricted movements of the knee. There was no recurrence of cysts at one year. The dorsiflexion of the foot improved only partially due to the delay in treatment, though it allowed him a bipedal gait with the support of a foot drop splint.  The lateral synovial cyst with extra-articular component approached with an "early" open cystectomy and nerve decompression may preserve the knee function with good functional outcomes. Any delay in treatment may impact the overall outcomes. An early intervention has favored a better recovery outcome for both knee function and neuropathy.