Bone Remodeling Rate in Adults
In healthy adults, approximately 5-10% of total bone volume is replaced annually through the remodeling process, though this rate varies significantly by skeletal compartment and age. 1
Understanding the Remodeling Process
Bone remodeling occurs continuously throughout adult life in discrete anatomical units called basic multicellular units (BMUs), where osteoclasts and osteoblasts work sequentially to remove old bone and replace it with new tissue. 2, 3
Key Temporal Characteristics
The complete remodeling cycle at a single BMU site takes approximately 3-6 months in adults, with osteoclastic resorption lasting 2-4 weeks followed by osteoblastic bone formation over 3-4 months 3
Individual osteoclast nuclei turn over rapidly at approximately 8% per day, requiring continuous recruitment of new preosteoclasts from bone marrow through circulation 3
Osteoblast teams form bone most rapidly at their onset and progressively slow down over time, with some becoming buried as osteocytes, some dying, and the remainder transforming into flat lining cells 3
Age-Related Changes in Remodeling Rate
After peak bone mass is achieved (typically by age 30), bone remodeling remains balanced for 1-2 decades before age-related bone loss begins. 2
Postmenopausal Acceleration
During the first 5 years after menopause, bone density decreases by approximately 2% per year, followed by an annual loss of approximately 1% for the remainder of a woman's life 4
Trabecular bone (vertebral column, hip ends) experiences faster loss than cortical bone due to its larger surface area available for remodeling 4
Compartment-Specific Differences
The relative importance of cortical remodeling increases with age as cancellous bone is progressively lost and remodeling activity increases in both compartments 2
Age-related bone loss results from both increased resorptive activity and reduced bone formation capacity, creating an imbalance in the remodeling process 2
Clinical Implications for Monitoring
Serial BMD monitoring should be performed every 2 years in most patients, though 1-year intervals are appropriate for those on glucocorticoids or initiating new therapy. 5
Bone Turnover Markers
Biochemical markers of bone remodeling include formation markers (osteocalcin, bone-specific alkaline phosphatase, P1NP, P1CP) and resorption markers (NTX, CTX), though these show marked variability ranging from 15-40% due to physiologic factors 6
Morning fasting serum PINP and serum CTX are recommended by the IOF and IFCC for evaluating bone turnover, as overnight fasting significantly reduces variation 6
Important Caveats
Remodeling balance and remodeling rate are not co-regulated in adult human bone, meaning that changes in the amount of bone formed per remodeling unit do not necessarily correlate with changes in the number of active remodeling sites 7
Both wall width (amount of bone formed per BMU) and mineral apposition rate correlate negatively with age (r = -0.75, p < 0.0001 and r = -0.27, p = 0.05, respectively), but neither correlates with indices of remodeling rate such as mineralizing perimeter 7