Painful arc syndrome is caused by impingement of -

Rotator cuff impingement syndrome is a painful disorder which is thought to arise from repetitive compression or rubbing of the tendons (mainly supraspinatus) under the coracoacromial arch. Normally, when the arm is abducted, the conjoint tendon slides under the coracoacromial arch. As abduction approaches 90 degrees, there is a natural tendency to externally rotate the arm, thus allowing the rotator cuff to occupy the widest pa of the subacromial space. If the arm is held persistently in abduction and then moved to and fro in internal and external rotation (as in cleaning a window, painting a wall or polishing a flat surface) the rotator cuff may be compressed and irritated as it comes in contact with the anterior edge of the acromion process and the taut coracoacromial ligament. This attitude (abduction, slight flexion and internal rotation) has been called the 'impingement position'. Perhaps significantly, the site of impingement is also the 'critical area' of diminished vascularity in the supraspinatus tendon about 1 cm proximal to its inseion into the greater tuberosity. Reference Apley&;s system of ohopaedics-9th edn -pg no 341.