Normal vertebral body size and compressive strength: Relations to age and to vertebral and iliac trabecular bone compressive strength

Abstract

Three thoracic (T 5−T 7) and three lumbar (L 1−L 3) vertebral bodies and the anterior parts of both iliac crests were removed from 44 normal individuals aged 15–87 years who had died suddenly. Small, cylindrical samples of trabecular bone (length 5 mm, diameter 7 mm) from T 6, L 1, and L 3 and from the standard site for iliac crest biopsies were compressed in an Alwetron-250 materials testing machine. Whole vertebral bodies from T 5, T 7, and L 2 with cut planoparallel end-plates were compressed in an Instron materials testing machine. The maximum compressive stress value σ max of the whole vertebral bodies and of the vertical vertebral trabecular bone decreased with age with almost parallel linear regression lines. At any age the σ max for whole vertebral bodies was about 1.6 MPa (1 MPa = 100 N cm 2 ) higher than for the trabecular bone. The average cross-sectional area of the vertebral bodies increased by 25–30% from the age of 20 to 80 years. The anisotropic properties of the vertebral trabecular bone (expressed as the ratio between the vertical and horizontal σ max) increased markedly with age. A highly significant positive correlation was observed between the vertical vertebral trabecular bone σ max (x) and the total vertebral body σ max (y = 0.90x + 1.75, r = 0.88, P < 0.01). The slope was not significantly different from 1, whereas the intercept was positive ( P < 0.01). The average total vertebral body σ max (range 1.5–7.8 MPa) could be predicted from mechanical tests on horizontal iliac crest bone biopsies with standard error of estimate (SEE) of 0.92 MPa. The study shows that the compressive strength of whole vertebral bodies depends mainly on the trabecular bone compressive strength, which decreases with age. The increase in vertebral trabecular bone anisotropy with age and the continuous periosteal growth will to some extent compensate for the obligatory age-related loss of vertebral trabecular bone mass. In normal individuals the average vertebral body compressive strength can be estimated from mechanical tests on horizontal iliac crest bone biopsies in spite of these age-related changes in internal and external architecture.