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As it is post arthritis
,so first possibility is bursitis
that is quite common around knee joint.there 4-5 bursa on the anterior aspect of knee joint ,similar number on medial & lateral aspect.they are named according to there location.They may be communicating to the knee joint or non communicating.Inflammation of single or group of bursae may occur causing sign & symptoms.So consult orthopedic & get it investigated & treated.
‘Hope I have answered your query, I will be available to answer your follow up queries, “Wish you Good Health and speedy recovery”