Osteoporosis

Alexander Krawiecki

Joseph M. Lane

Joseph L. Barker

INTRODUCTION

Osteoporosis (OP) is a common metabolic bone disease, characterized by a decrease in bone density, deterioration of bony matrix microarchitecture, diminished material properties, and susceptibility to low-energy “fragility” fractures. OP is a silent disease often underrecognized and undertreated. Fracture may be the first and only manifestation, making OP a major contributor to morbidity and mortality worldwide.
- The World Health Organization (WHO) has defined OP on the basis of bone mineral density (BMD) measurements obtained on dual energy x-ray absorptiometry (DEXA) as a T-score equal to or greater than 2.5 standard deviations (SD) below ideal bone mass.
- Fracture is the result of an unfavorable combination of bone mass, bone geometry, material properties, and vector of trauma. A low BMD is associated with an increased risk of fracture.
- Low bone density may be secondary to failure to achieve optimal bone mass, bone loss caused by increased bone resorption, or inadequate replacement of lost bone as a result of decreased bone formation.
- Primary OP (80%)
  - Type I, idiopathic OP, is seen in postmenopausal women. Peak bone mass is reached at the age of 25 to 30 years in women with accelerated loss during the perimenopausal period. Estrogen maintains bone density, leaving postmenopausal women at increased risk of fracture.
  - Type II, senile OP, affects men and women older than 70 years.
  - Secondary OP (20%) is a result of some underlying disease state and may be corrected by treatment of the primary disease.
- Osteopenia refers to radiographic finding of decreased bone density, classically a T-score between 1.0 to 2.5 SD below ideal. Osteopenia may coexist with OP, osteomalacia (OM), or other conditions.
- OM is characterized by delayed or altered mineralization of normal matrix, independent of the amount of bone mass. The same disease, affecting growing bones in children, is called rickets.

ETIOPATHOGENESIS

I. Bone strength

is a result of many factors including mineralization, turnover, and microarchitecture. Low initial levels of adult bone mass or acceleration in bone loss can reduce bone mass and strength to below the fracture threshold (the bone mass below which the propensity to fracture is increased).

II. Bone density

is by far the greatest measurable predictor of fracture risk, accounting for 70% of bone strength. Genetics may account for 70% to 85% of the variation in peak bone mass.

III. Areas rich in trabecular bone

(e.g., the vertebral bodies, the intertrochanter of the hip, and the distal radius) have the greatest turnover and therefore the greatest propensity to fracture.

IV. GENERAL ETIOLOGY

Advanced age.
Female gender.
White race and/or northwest European extraction; first-generation Chinese.
Family history (one-third have strong inheritance).
Fall risk (impaired vision and dementia).
History of fragility fracture (a major risk factor). Prior fracture is the strongest predictor of future fracture.
Collagen disorders (e.g., osteogenesis imperfecta, Ehlers-Danlos syndrome, and Marfan’s syndrome) and unspecified hypermobility variants.
Rheumatologic diseases such as rheumatoid arthritis and ankylosing spondylitis.
Multiple myeloma.

V. NUTRITION AND SOCIAL PRACTICE

Inadequate calcium intake.
Malabsorption including celiac sprue (4.7% of patients with OP), inflammatory bowel disease (e.g., Crohn’s or ulcerative colitis), irritable bowel syndrome (IBS), lactose intolerance, and gastric bypass surgery.
Alcoholism/smoking.
Vitamin D deficiency (20% to 40% of the population).
Low body weight or body mass index (BMI).

VI. ENDOCRINE CONDITIONS

Estrogen deficiency
- Menopause. Inadequate calcium intake and postmenopausal estrogen deficiency account for 90% of OP cases.
- Amenorrhea/oligomenorrhea (e.g., the female athlete).
- Lactation. Contributes to negative calcium balance if calcium supplementation is inadequate.
Androgen deficiency.
Hyperthyroidism.
Hyperparathyroidism.
Cushing’s disease.
Type I diabetes mellitus.