A 68-year-old man from rural Maharashtra presents with a 3-month history of progressive left hip pain, worse on weight-bearing and internal rotation. He denies trauma. On examination, the left hip is held in flexion, adduction, and internal rotation. Passive external rotation and abduction are restricted and painful. X-ray pelvis shows a crescent sign with flattening of the femoral head. Which anatomical structure is primarily affected, and what is the underlying pathophysiology?