While on your rounds, you see two individuals. First up is Jenna, a 70-year-old female who presents with left hip pain after falling while getting out of bed. She sustained a fracture of the right hip, and preoperative chest x-ray reveals that she had pre-existing asymptomatic vertebral fractures before her fall. She denies any other symptoms and physical examination was otherwise normal. Then you see Gerald, a 46-year-old male who presents with a mild but noticeable limp and hip pain on the right side after falling from a chair. Examination is unremarkable. In Jenna’s case, a DEXA scan was performed, revealing a T -2.8 score. In Gerald’s case, radiographs of the hip showed a right hip fracture and abnormally dense hip bones.
Both seem to have some type of bone disorder. But first, a bit of physiology. Bones have a hard-external layer of cortical bone and a softer internal layer of spongy bone composed of trabeculae. The trabeculae are like a framework of beams that give structural support to the spongy bone. Now, these are replaced every few years in a process called bone remodeling. The process has two steps: bone resorption, which is when osteoclasts break down bone by releasing hydrogen and collagenases, and bone formation, which is when osteoblasts form new bone by secreting osteoid seam. Osteoid seam is mainly made up of collagen and it acts like a scaffold upon which hydroxyapatite, a combination of calcium and phosphate, deposits. Bone formation requires an alkaline environment, which is why bone cells also produce alkaline phosphatase, an important marker of bone cell activity.
At a cellular level, remodeling begins when osteoblasts release receptor activator of nuclear factor κβ ligand, or RANKL for short, which binds to RANK receptors on the surface of osteoclast, activating them to begin bone matrix demineralization. Once there’s been sufficient bone demineralization, osteoblasts secrete osteoprotegerin, which inactivates RANKL. This causes the osteoclasts to stop demineralizing the bone, and osteoblasts to secrete osteoid seam.
Another high-yield concept is that osteoblasts are derived from mesenchymal stem cells in the periosteum, a membrane covering the surfaces of bones and consisting of an outer fibrous layer and an inner cellular layer. By contrast, osteoclasts originate from hematopoietic progenitor cells, more specifically from a fusion of monocyte and macrophage precursors. These differentiate once osteoblasts secrete RANK-L and macrophage colony-stimulating factor or M-CSF, which interact with their respective receptors on the osteoclast membrane. Now, keep in mind that parathyroid hormone also regulates osteoclast maturation, but it does so indirectly, by stimulating RANK-L and M-CSF secretion from osteoblasts. However, it’s action actually depends on it’s serum levels. At low, intermittent levels, the hormone exerts anabolic effects- meaning it promotes bone formation. Conversely, chronically increased parathyroid hormone levels, like, for example, in primary hyperparathyroidism, cause catabolic effects, meaning it promotes bone resorption. Additionally, the hormone also increases calcium levels and decrease phosphate levels by increasing its urinary excretion.