Decreases in cortical thickness, and not changes in trabecular microstructure, are associated with the pubertal increase in forearm fractures in girls

We previously demonstrated that forearm fractures in girls peak between age 8 and 11 years, during the time of maximal pubertal growth (JAMA 2003; 290:1479). The structural basis for this increase is unclear but may be related to the transient cortical thinning and increased porosity related to increased calcium demands during maximal growth. It is also possible that there may be alterations in trabecular microstructure during puberty that reduce bone strength. To address these possibilities, we used high-resolution 3D pQCT (XtremeCT, Scanco, voxel size ∼90 microns) to assess trabecular and cortical bone parameters in healthy girls aged 7-16 yrs (n=46) without a prior history of fracture. Subjects were classified into 4 groups based on bone age (BA) assessed using hand and wrist x-rays: Group I (BA, 6-8 yrs, n = 8), Group II (BA, 9-11 yrs, n = 15), Group III (BA, 12-14 yrs, n = 13), and Group IV (BA, 15-16 yrs, n = 10). There were no significant differences (P \u3e 0.2) in trabecular parameters (bone volume/total volume, BV7TV; trabecular number, Tb.N; or trabecular thickness, Tb.Th) through the course of puberty, expressed as a percentage of the mean value in Group I (Panel A of figure). By contrast, there were marked but transient decreases in cortical thickness (C.Th) which decreased by 50% during puberty (P \u3c 0.005 for group I vs. group III, P = 0.07 for group I vs. group II) before rising sharply by 89% at the end of puberty (P \u3c 0.005 for group I vs. group IV). This pattern was a mirror image of the previously described rise in distal forearm fractures in this population (Panel B), and there were similar changes in cortical vBMD (data not shown).Our findings, which represent the first direct assessment of trabecular and cortical structure during growth in children, thus demonstrate that: (1) perhaps surprisingly, puberty is associated with minimal changes in trabecular microstructure, suggesting that these parameters may be established very early in life; and (2) the transient increase in distal forearm fractures during growth in girls is largely explained by temporary cortical thinning (and perhaps increased cortical porosity). Further studies are needed to define the mechanisms underlying these alterations in cortical parameters as well as preventive measures that may attenuate these changes and, hence, distal forearm fractures during growth