STRUCTURAL PROPERTIES OF TRABECULAR CORES FROM FEMORAL HEADS. Sylvana García-Rodríguez. 1. , Meghan C.M. Crookshank. 2. , Norma J.
STRUCTURAL PROPERTIES OF TRABECULAR CORES FROM FEMORAL HEADS Sylvana García-Rodríguez1, Meghan C.M. Crookshank2, Norma J. MacIntyre2,3 Mark Harrison2, Everett L. Smith4, Rick Sellens2 and Heidi-Lynn Ploeg1 Department of Mechanical Engineering and 4Department of Population Health Sciences, University of Wisconsin, Madison, WI, USA, http://www.engr.wisc.edu/groups/BM/ 2 Human Mobility Research Centre, Kingston General Hospital, Queen’s University, Kingston, ON, Canada, http://me.queensu.ca/hmrc/ 3 School of Rehabilitation Science, McMaster University, Hamilton, ON, Canada 1
INTRODUCTION Bone density and fracture risk are clinically assessed by dual-energy x-ray absorptiometry (DXA). The advantages of DXA are its accessibility, low radiation dose and established ability to predict population-based fracture risk in postmenopausal women; however, it provides two-dimensional data, limiting its ability to accurately predict bone’s structural properties. Computed tomography (CT) has grown as a tool to evaluate the threedimensional (3D) mechanical properties of bone, which may serve to improve the ability to predict bone fracture risk for individual men and women. The purpose of this study was to establish a comprehensive dataset of structural properties (e.g. CT x-ray attenuation in Hounsfield units HU, bulk density ρblk, apparent density ρapp, mineral content MC, and apparent elastic modulus Eapp) of trabecular bone from human femoral heads. This dataset will provide valuable insights into the probability density functions of, and correlations between, these measured properties. METHODS AND PROCEDURES Seven femoral heads were donated by hip replacement recipients with Research Ethics Board approval (age 66 – 87 y; two male, five female). The hip heads were CT scanned (GE Lite Speed Plus) pre- and post-operatively.
Two to three, 7 mm thick slices were cut (Exact diamond-coated band saw) from each head. The slices were oriented approximately perpendicular to the hip joint load. From each slice, 10 mm diameter trabecular bone cores were extracted and milled to a 5 mm height. Bulk dimensions and wet weights were recorded. The femoral head with cores removed were “reassembled” and CT scanned with the same pre-machined scan parameters. A custommade registration algorithm aligned the preand post-machined segmented CT data before the data were subtracted from each other. The resulting data set corresponded to the prepared bone cores, therefore providing their x-ray attenuation data (HU). Compression testing was performed on each core using ZETOS (Jones et al., 2003; GarcíaRodríguez et al., 2008). With 10 N preload, displacement of 30 µm was applied quasistatically (0.7 µm/s) while measuring force (Kistler, type 9011A). Force and displacement were normalized to stress and strain, respectively, using bulk dimensions. Eapp was determined from the slopes of the stress-strain curves. Cores with Eapp outside the ZETOS operating range (58.3-1860 MPa) were excluded. The cores were dried in a 70°C oven (24 hrs) and defatted in a soxhlet extractor (24 hrs). Ashing was performed in a 700°C oven
(24 hrs). Dimensions and weights were recorded before and after the drying and defatting procedures. Dry, defatted weight (Wdd) over dry, defatted bulk volume (Vdd,blk) gave ρapp. Material mineral content (MMC) was determined by dividing ash weight by Wdd. The ratio of ash weight to Vdd,blk was defined as structural mineral content (SMC) (Cowin, 2001). Histograms of HU, ρapp, MMC, SMC and Eapp were analyzed for normality using the Anderson-Darling test.
however the Eapp distribution was skewed to the left and MMC distribution was skewed right. Although it seems contradictory that Eapp is skewed left, this result supports the hypothesis that MMC alone does not define structural properties. High MMC may indicate thicker trabecular struts; however the stiffness of the structure may be compromised by micro damage or reduced connectivity, as in osteoporotic bone. SUMMARY
RESULTS From seven femoral heads, 85 cores were prepared and 20 were excluded due to low compression stiffness (Eapp
