Alterations in Bone Function: Structure, Metabolism, and Disease

A hallmark of osteoporosis is the degradation of the internal architecture. Specifically, the trabeculae decrease in amount and width, leading to a loss of overall bone mass. It is important to note that cancellous bone is lost much faster than cortical bone during this process.

Clinical Note: Common etiologies for osteoporosis include estrogen deficiencies (particularly post-menopause), inadequate calcium intake, and physical disuse.

Metabolic Bone Disease: Rickets and Osteomalacia

While osteoporosis is characterized by a loss of total bone mass, other metabolic disorders involve the "softening" of the bone due to mineralization issues. These conditions are primarily driven by Vitamin D deficiency, as well as inadequate levels of calcium or phosphorus, or renal disease.

The clinical manifestation of these deficiencies depends heavily on the age of the patient:

• Rickets: Occurs in children.
• Osteomalacia: Occurs in adults.

Both conditions result in soft, weak bones that are prone to deformity and pain.

Structural Deformities

As the bones soften, they may bend under the weight of the body or fail to develop correctly, leading to several distinct spinal deformities and limb misalignments:

• Kyphosis: An exaggerated forward rounding of the upper back.
• Genu Valgum: Commonly known as "knock-knees," where the knees tilt inward toward each other.
• Genu Varum: Commonly known as "bowlegs," where the knees stay wide apart.

Neoplasms and Bone Tumors

Beyond metabolic and structural issues, bone function can be altered by the presence of tumors. Neoplasms in the bone can be categorized as either benign or malignant.

• Sarcomas: These are malignant neoplasms that originate directly within the bone tissue.
• Metastatic Lesions: These are not primary bone cancers but rather cancers that have spread to the bone from other organs (primary carcinomas). The most common primary sites that metastasize to bone include the breast, prostate, lung, and kidney.

Common sites for bone involvement include the vertebral bodies, the pelvis, the ribs, and the proximal ends of the femur and humerus.

Exam Tip: When differentiating between Rickets and Osteomalacia, remember the age factor: Rickets = Children; Osteomalacia = Adults. Both are linked to Vitamin D and mineralization issues.