Sciatica refers to back pain caused by a problem with the sciatic nerve. This is a large nerve that runs from the lower back down the back of each leg. When something injures or puts pressure on the sciatic nerve, it can cause pain in the lower back that spreads to the hip, buttocks, and leg.
Osteoarthritis of the shoulder is a gradual wearing of the articular cartilage that leads to pain and stiffness. As the joint surface degenerates, the subchondral bone remodels, losing its sphericity and congruity. The joint capsule also becomes thickened, leading to further loss of shoulder rotation.
The nerve can also get stuck to the bone by fibrous tissue so that it can't move as the arm is raised. This condition is called nerve entrapment. Other things that can cause suprascapular nerve entrapment include bone fracture, cysts, and enlarged veins. Cysts form most often when damage to the labrum (rim of fibrous cartilage around the shoulder joint) allows fluid from the joint to escape and pool inside the cyst.