Is this just a solitary case? Have you ruled out the possibility of Baker's Cysts? Potentially one could press on a nerve which may refer pain to the calf. Equally the cyst may burst causing a build up of fluid within the calf temporarily which can cause pain.
People with knee osteoarthritis could conceivably have osteoarthritis of the lumbar spine. Very common. Hence, calf pain would be related to the sciatic nerve, especially in its S1 branch, or radiating pain from the longitudinal spine ligaments.
perhaps due to the reason that these patients also develope effusion in the posterior capsule besides swessing of the whole joint. Calf pain is also diagnosed in many patients with a baker cyst so it seems reasonable that a posterior effusion with pressure und the posterior neurovascular structures are the reason for calf pain in patients wit knee arthritis
The majority of these are longterm probs. Antalgic gait being one with a loss of extn, therefore they (the patient) continually stress a phasic muscle along with adaptive shortening of their hamstrings.