In normal bone, bone formation and bone resorption are closely coupled. In osteoporosis, the net rate of bone resorption exceeds the rate of bone formation, resulting in a decrease in bone mass without a defect in bone mineralization. Men with prematurely decreased testosterone may have increased osteoclast activity. These changes result in further net loss of bone. The amount of bone available for mechanical support of the skeleton eventually falls below the fracture threshold, and the patient may sustain a fracture with little or no trauma.
Bone loss affects both cortical and trabecular bone.
Histologically, there is a reduction in cortical thickness and in the number and size of trabeculae of cancellous bone, with normal width of the osteoid seams. Trabecular bone loss predominates in typical postmenopausal osteoporosis. Peak bone mass in men and women occurs by the middle of the third decade of life and plateaus for about 10 years, during which turnover of bone is constant, with bone formation approximately equaling bone resorption. The major clinical manifestations of osteoporosis are bone fractures, which cause chronic pain. However, not everyone with low bone mass will experience fracture.