Bone Metabolism and Osteoporosis
The basis of bone metabolism depends on the activity of osteoblasts and osteoclasts, which is influenced by serum calcium and vitamin D levels. Disruption of bone homeostasis occurs when the activity of osteoclasts overrides that of osteoblasts. This can consequently lead to development of osteoporosis, which is characterized by loss of bone density. Osteoporosis can be categorized into three types: postmenopausal (I), senile (II), and secondary (III). Osteoporosis is typically diagnosed using dual-energy x-ray absorptiometry (DEXA). The DEXA scan generates a bone mineral density (BMD) score that is converted to the corresponding T-score by comparing the patient’s BMD to that of a healthy 30-year-old adult in a sex- and ethnicity-specific manner and calculating the standard deviation. Osteopenia (weakening of the bones of less severity than osteoporosis) has a T-score in the range of -1.0 > T > -2.5, whereas osteoporosis has a T-score of ≤ -2.5. Many therapies for prevention and treatment of osteoporosis exist including nonpharmacologic agents (calcium and vitamin D supplements) and pharmacologic agents (bisphosphonates, teriparatide, estrogen replacement/raloxifene, calcitonin, and denosumab). With an increasingly aging population and longer life expectancy, prevalence of osteoporosis among spine surgery patients will likely increase. Consequently, it will be important to screen for osteoporosis in patients undergoing spine surgery to effectively secure the most favorable surgical strategy and to ensure proper perioperative management for optimal patient outcomes as patients with osteoporotic bones are at higher risk for subsequent compression fractures as well as hip and distal radius fractures.
