Hello and welcome to HCM,
Pain in the knee joint without any history of trauma needs to be investigated.
Pain along with swelling and blue-black discolouration of the knee joint is suggestive of spontaneous effusion or more likely bleeding in the knee joint.
Blue-black colour can occur if there is underlying bleeding in the knee joint.
An arthroscopic examination of the knee joint is required to look for evidence of effusion and in case fluid is present, examination of the fluid collected in the knee joint is required.
At the same time assessment of coagulation system is required which includes-
platelet count,
bleeding time, clotting time, PT, INR and
aPTT.
These investigations will determine if any abnormality is present in the coagulation system which may be responsible for spontaneous bleeding into the knee joint.
Consult your orthopedician for these investigations.
Thanks and take care
Dr Shailja Puri