Abstract
Introduction: Previously, 3D finite element models of the midfemoral neck (MFN) predicted maximal compressive stress in the inferior cortex and trabeculae during the heel-strike of normal gait. We hypothesised that older women (compared with younger women) would have relative preservation of inferior trabecular bone, reflecting continued walking activity into old age. Recently, we identified preservation of the infero-anterior femoral neck cortex in old age despite women at 85 having 55–80% thinner superior cortices than women aged 25. Here, we examined regional trabecular bone mineral density in an extended region of interest (ROI) of the MFN, to establish age-associated differences and their likely contribution to hip fracture in a cross-sectional study of 100 women aged 20–90 years using 64-slice computed tomography (CT).
Methods: 100 healthy women were recruited by decade from age 20 to 90 (inter-quartile range 38–72). Participants consented to an extension of a routine clinical pelvic CT scan (Siemens 64-slice CT) using a fixed threshold of 450mg/cm3 and Mindways Software (BIT-2) to analyse both hips (1mm slice thickness, 0.59 mm voxel size). The starting position for cross sections was a 1mm thick MFN slice where the femoral neck height to width ratio was 1.4, since this location along the neck axis has been shown to be highly reproducible and unaffected by age (mean 51%, SE 0.016%). 5 parallel 1mm thick slices were evaluated towards the midline. Age effects on trabecular BMD (tbBMD mg/cm3.) were evaluated using linear regression (by anatomical quadrant).
Results: Trabecular BMD was significantly lower at 85 than at 25 years old amounting to over 50% difference in 3 out of 4 quadrants: Supero-posterior tbBMD difference between 25–85 years by modelling −146mg.cm3., (absolute %) −53%, r 2 (age) 0.61, p < 0.0001; Infero-anterior −25mg.cm3., −24%, r 2 0.094, p < 0.002. In the infero-anterior quadrants (that receive maximal stress at the heel strike in normal gait) there was relative preservation (r squared only 0.094) and only a 24% difference between 85 and 25. Body weight had no association with tbBMD in any quadrant (unlike cortical thickness).
Conclusion: Trabecular bone was better preserved in the infero-anterior quadrant in elderly women than in the other 3 quadrants. Since tbBMD also makes a small but significant contribution to the reduction of the risk of failure of femoral necks ex-vivo, increasing trabecular bone in the remaining quadrants should be included in strategies to prevent hip fracture, alongside efforts to strengthen the cortex.
Correspondence should be addressed to Miss B.E. Scammell at the Division of Orthopaedic & Accident Surgery, Queen’s Medical Centre, Nottingham, NG7 2UH, England
