Abstract
Introduction: Femoral neck (FN) fragility has been attributed to age-related bone loss, with increased loss in women. It has been shown that the mechanical properties of a supporting structure will also change with any alteration to the structure’s dimensions. The purpose of this study was to identify the age-related changes that take place in the morphology of the mid cross-section of the FN, and the implications for its mechanical properties in the different regions around the mid FN cross-section.
Materials and Methods: Measurements were taken from peripheral quantitative computed tomogram (pQCT) images of 81 cadaveric femurs (36 F, 45 M). The mid FN cross-section was segmented radially into eight regions and the cortical bone thickness (CT) and change of the centroid position (CP) of the FN cross-section were measured. The age-related effects of the corresponding changes in the proportion of cortical bone and the “resistance to bending” (section modulus, (Z)) were also measured.
Results: Four femurs were excluded because there were clear signs of OA being present. The maximum difference in regional CT between men and women, was less than 7% (Female: 3.07 ± 0.108mm; Male: 3.28 ± 0.123 mm (mean ± SEM) p =0.21). However, there were regional differences in CT between the young under fifty, (Un50, n=26) and the old, (Abv50), (ANOVAs for young vs old: CT p = 0.001 t 0.01). These effects were attributable to differences in the inferior region, where there was an increase in thickness of the cortical bone of 27% with senior status (Abv50: 3.44 ± 0.09mm; Und50: 2.70 ± 0.12mm. p = 0001) counter balanced by anterior and posterior loss. There was a corresponding change in CP, the distance of the superior, posterior, and superoposterior regions to the FN cross-section’s centroid, 7.6% (Abv50: 20.88 ± 0.28mm; Und50: 19.40 ± 0.47mm; p = 0.005); 6.7% (Abv50: 14.67 ± 0.2mm; Und50: 13.74 ± 0.32mm; p = 0.01); and 8%(Abv50: 17.95 ± 0.24; Und50: 16.61 ± 0.37), respectively. When these two measurements were combined (CP divided by CT) to provide the Local Buckling Ratio (BLR), where the higher the ratio the more unstable the structure, there were significant differences in superoanterior, 30%(Abv50: 15.8 ± 0.52; Und50: 12.1 ± 0.59;p=0.0001); anterior, 20%(Abv50: 10.1 ± 0.32; Und50: 8.3 ± 0.4; p=0.001); inferior, 35%(Abv50: 4.37 ± 0.14; Und50: 5.8 ± 0.34; p=0.0001); inferoposterior 18%(Abv50 8.6 ± 0.27: Und50: 7.36 ± 0.41; p=0.008); posterior, 29%(Abv50: 14.0 ± 0.33; Und50: 10.8 ± 0.5; p=0.0001) and superoposterior, 14%(Abv50: 14.6 ± 0.3; Und50: 12.8 ± 0.4; p=0.001), regions. There was no significant difference in bending resistance nor in the proportion of cortical bone.
Conclusions: A more uniform cortical thickness, seen in the young, would optimise fracture resistance to overloading from unusually loaded directions. Ageing was associated with a thickening of the inferior cortex and thinning of the cortex elsewhere. This effects the location of the area that is least susceptible to the loading forces experienced in stance – that is of the FN mid cross-section’s neutral bending axis – as it will be nearer to the inferior region. Such a change in the morphology will produce deterioration in the FN’s capacity to take a load as shown by the detrimental change in the LBR. This change may indicate that the potential for femoral neck fracture increases with age when load is applied in a direction different to normal stance eg through the greater trochanter.
Correspondence should be addressed to Dr Carlos Wigderowitz, Honorary Secretary of BORS, Division of Surgery & Oncology, Section of Orthopaedic & Trauma Surgery, Ninewells Hospital & Medical School Tort Centre, Dundee, DD1 9SY.
