Introduction: Cranial endplates of human vertebrae are injured more often than caudal, in both young and elderly spines. We hypothesise that cranial endplates are inherently vulnerable to compressive loading because of structural asymmetries in the vertebrae.
Methods: Sixty-two "motion segments" (two vertebrae and the intervening disc and ligaments) were obtained post-mortem from thirty-five human spines (17F/18M, age 48-92 yrs, all spinal levels from T8-9 to L4-5). Specimens were compressed to failure while positioned in 2-6 degrees of flexion, and the resulting damage characterised from radiographs and at dissection. 2 mm-thick slices of 94 vertebral bodies (at least one from each motion segment) were cut in the mid-sagittal plane, and in a para-sagittal plane through the pedicles. Microradiographs of the slices were subjected to image analysis to determine the thickness of each endplate at 10 locations. Optical density of the endplates and adjacent trabecular bone was also measured. Measurements obtained in cranial and caudal regions, and in mid-sagittal and pedicle slices, were compared using repeated measures ANOVA with age, level and gender included as between-subject factors. Linear regression was used to determine significant predictors of compressive strength (failure stress).
Results: Fracture affected the cranial endplate in 55/62 specimens. Cranial endplates were thinner than caudal (p=0.003) by 14% and 11% on average, in mid-sagittal and pedicle slices respectively. Caudal but not cranial endplates were thicker at lower spinal levels (p=0.01). Optical density of trabecular bone adjacent to the endplates was 6% lower cranially than caudally (p=0.004), and the average optical density of trabecular bone in mid-sagittal slices was 10% lower in women than in men (p=0.025). Vertebral yield stress (mean 2.22 MPa, SD 0.77 MPa) was best predicted by the density of trabecular bone underlying the cranial endplate of the mid-sagittal slice of the fractured vertebra (r(2)=0.67, p=0.0006).
Conclusions: When vertebrae are compressed naturally by adjacent intervertebral discs, cranial endplates usually fail before caudal endplates because they are thinner and supported by less dense trabecular bone.