The relationship of whole human vertebral body creep to geometric, microstructural, and material properties

Abstract

Creep, the time dependent deformation of a structure under load, is an important viscoelastic property of bone and may play a role in the development of permanent deformity of the vertebrae in vivo leading to clinically observable spinal fractures. To date, creep properties and their relationship to geometric, microstructural, and material properties have not been described in isolated human vertebral bodies. In this study, a range of image-based measures of vertebral bone geometry, bone mass, microarchitecture and mineralization were examined in multiple regression models in an effort to understand their contribution to creep behavior. Several variables, such as measures of mineralization heterogeneity, average bone density, and connectivity density persistently appeared as significant effects in multiple regression models (adjusted r²: 0.17-0.56). Although further work is needed to identify additional tissue properties to fully describe the portion of variability not explained by these models, these data are expected to help understand mechanisms underlying creep and improve prediction of vertebral deformities that eventually progress to a clinically observable fracture.

Keywords: Creep; Geometry; Material properties; Microstructure; Vertebral body.

Figure 1

Cubic volumes of interest were extracted from the cancellous centrum of microcomputed tomography images to calculate stereological parameters (bottom; trabecular thickness map presented). Average, standard deviation, and coefficient of variation of the grey values were calculated using the entire cube (top; exploded 2mm axial view demonstrating trabecular bone grey value distribution).

Figure 2

(a) HRCT images were segmented to produce cancellous, shell, and cancellous+shell binary masks which were multiplied with source CT volumes to produce volumetric BMDs for each region (cBMD, shBMD, and iBMD, respectively). (b) The anterior-posterior and lateral-medial projections of the integral cancellous+cortical mask were used to calculate in each orientation area (Area.AP, Area.LM), height (Height.AP, Height.LM) and width (Width.AP, Width.LM).

Figure 3

A representative creep curve (74 year old male). Displacement parameters that define the creep behavior were calculated as: (A) elastic displacement before creep (D_e), (B) creep displacement (D_cr), (C) total displacement (D_tot), (D) elastic recovery (R_el), (E) creep recovery (R_cr), and (F) residual displacement (D_res). Residual from creep alone (D_res-cr) was calculated as D_cr-R_cr. Quasistatic loading and unloading occurred over 100 seconds. Curve fitting was performed on the region between A–C.

Figure 4

Measured vs. predicted creep displacements for D_cr. The multiple regression model for D_cr contained BMD.LM, GV.SD, and Conn.Dn (r²_adj=0.56).