This quote is from a necropsy instruction:
"If growing birds are lame, and there is no evidence of infection or rickets, cut into the proximal tibia and look for necrosis caused by osteomyelitis or dyschondroplasia and split the spine at T4 to look for spondylolisthesis (kinky-back), a plug of cartilage impinging on the cord or consider Marek's disease."
Is this a meat bird showing signs of a problem?
You might also consider mareks or some other condition which would cause "lameness:"
diagnosis charts at bottom
Sorry I don't know the cause of the problem...just trying to help.