Regional differences in cortical porosity in the fractured femoral neck

Although bone mass is a contributory risk factor for intracapsular hip frac ture, its distribution and porosity within the femoral neck is also, import ant for bone strength. III femoral neck biopsies from 13 women with intraca psular hip fracture (mean +/-SEM: 75.1 +/- 2.1 years, OP) and 19 cadaveric samples (9 men and 10 women [control] aged 79.4 +/- 1.7 years), a segmental analysis was used to quantify circumferential variations in the characteri stics of cortical bone haversian systems. In female control femoral necks, there was an increasing porosity gradient between the inferior (I) (7.7 +/- 0.6%) and superior regions (S) (16.05 +/- 1.8%, p < 0.005). In walking, th ese regions undergo compression and tension, respectively. In mean, a simil ar trend was observed, but the differences were not significant (I: 11.1 +/ - 1.2%; S: 14.1 +/- 1.7%; p = 0.133). This porosity gradient was not mainta ined in the fracture group (I: 10 +/- 1%; S: 12.65 +/- 1.2%). In contrast, porosity in the fracture group was greatest in the anterior cortex, being 4 1% higher in that quadrant than in controls (p = 0.06). The areal density o f haversian canals ranged from 16.7 to 21.3 canals/mm(2) with no significan t differences between fractures and controls. In the control women, mean ca nal diameter was highest in the superior region (60 +/- 2.8 mu m), and thes e canals were significantly larger than those in the inferior region (49.4 +/- 1.4 mu m, p < 0.05). This difference was less marked in the fracture ca ses (I: 53.21 +/- 2.5 mu m; S: 59.1 +/- 2.8 mu m; p = 0.0878). Although the mean canal diameter in the anterior quadrant of the fracture cases was hig her. than in the control women this did not reach significance (OP/F: 59.5 +/- 3 mu m; control/F: 52.7 +/- 2.6 mu m; p = 0.106). However, the proporti on of "giant" canals with diameters >385 mu m (defined as the top 0.5% in t he controls) was doubled in the anterior region of the fracture eases (OP/F : 1.28%; control/F: 0.69%; p < 0.005). Porosity is related to the square of the canal radius; therefore, such canals make a substantial contribution t o cortical porosity. Previous work has shown that the elastic modulus of bo ne decreases approximately as the square root of porosity. Therefore, the i ncreased porosity and the higher prevalence of "giant" canals have a marked ly negative influence on the ability of the cortical shell to withstand str esses associated with a fall, The mechanisms responsible for the localized generation of "giant" haversian canals, and ultimately the "trabecularizati on" of the cortex require further investigation. (C) 1999 by Elsevier Scien ce Inc. All rights reserved.