Unilateral calf hypertrophy.

Hi guys, wonder if anyone can help me with this confusing case! i have a 43yr old male patient with downs syndrome. He has been referred to hydro for OA hip (right worse than left) but on examination his left calf is markedly larger that his right. he had on operation on his right knee for after reapeated patellar dislocations when he was younger ( 30 years ago).
All literature for this condtion points towards a neurogenic source, possibly L5/S1 radiculopathy. I thought this was a strong possibility since the patient has OA of the hip but on commencing a spinal examination i found no significant abnormalities. He was a little stiff in lumbar extension and lateral flexion. Testing neurological sign ie dermatomes was difficult as the patient has downs syndrome so not sure if patient feedback was totally reliable. Babinski was negative, ankle jerk not tested. Im not sure of the next step to take. Does anyone have any ideas or any tips to improve the accuracy of my assessment or should i just send him off for an xray now? Thanks! :o