Changes in cortical width with bone turnover in the three different endosteal envelopes of the ilium in postmenopausal osteoporosis.

Histological indicators of bone turnover were compared in the three endosteal envelopes (cancellous, endocortical, and intracortical) of iliac bone specimens obtained from 82 osteoporotic women, to assess the correlation between bone turnover and bone volume in different remodeling sites. Although there was a significant but weak correlation between the mineral apposition rate (MAR), a histological indicator of bone formation at the basic multicellular unit (BMU) level, and the three endosteal envelopes, the bone formation rate corrected for bone surface (BFR/BS) and mineralizing surface (MS/BS), indicators of the rate of bone formation reflecting activation frequency, in the cancellous and endocortical envelopes was more closely related to the rate in the intracortical envelope. The endocortical BFR/BS and MS/BS were higher than the rate in the cancellous envelope (1.6-2.1 times and 2.0-2.4 times, respectively), indicating a higher turnover rate in the endocortical envelope. According to stepwise regression analysis of the significant determinants contributing to bone mass, several histological determinants relating to bone turnover were identified: (1) trabecular thickness (Tb.Th) was a positive determinant, whereas age and cancellous bone volume referent BFR (BFR/BV) were negatively correlated determinants of the cancellous bone volume (BV/TV) (R2 = 0.50, p < 0.001); and (2) the endocortical wall thickness (W.Th) of the given side and the cortical width (Ct.Wi) of the opposite side were positive determinants, whereas the cancellous osteoid surface (OS/BS), cancellous MAR, and endocortical eroded surface (ES/BS) of the given side were the negatively correlated determinants of the Ct.Wi of the thicker cortex (R2 = 0.62, p < 0.001). In the thinner cortex, the endocortical W.Th of the given side and Ct.Wi of the opposite side were only used as the positive determinants of the Ct.Wi of the given side (R2 = 0.55, p < 0.001). In addition: (3) a significant but weak correlation was found using the intracortical BFR/BV as a positively correlated determinant of the cortical porosity (Ct.Po) in the thicker cortex (R2 = 0.17, p < 0.01). Although these histological determinants do not fully explain the mechanisms of bone loss, an increased rate of bone turnover contributes to bone loss not only in the cancellous and intracortical envelopes, but also in the endocortical envelope, indicating increased endocortical bone resorption in osteoporosis.