Porosity and Thickness of the Vertebral Endplate Depend on Local Mechanical Loading

Study Design. Mechanical and microcomputed tomography (micro-CT) study of cadaver spines. Objective. To compare porosity and thickness of vertebral endplates with (1) compressive stresses measured in adjacent intervertebral discs and (2) grade of disc degeneration. Summary of Background Data. Endplate porosity is important for disc metabolite transport, and yet porosity increases with age and disc degeneration. We hypothesize that porosity is largely determined by mechanical loading from adjacent discs. Methods. Forty motion segments (T8-9 to L4-5) were dissected from 23 cadavers aged 48 to 98 years. Each was subjected to 1 kN compression during which time intradiscal stresses were measured by pulling a pressure transducer along the disc's midsagittal diameter. "Stress profiles" revealed the average pressure in the nucleus, and the maximum stress in the anterior and posterior annulus. Specimens were further dissected to obtain discs with endplates (and 5 mm of bone) on either side. Microcomputed tomography scans (resolution 35 mu m) were analyzed to calculate thickness and porosity in the midsagittal regions of all 80 endplates. Average values for the anterior, central, and posterior regions of each endplate were obtained. Disc degeneration was assessed macroscopically and microscopically. Results. Endplate porosity was inversely related to its thickness, being greatest in the central region opposite the nucleus, and least near the periphery. Superior endplates (relative to the disc) were 14% thicker (P < 0.001) and 4% less porous (P = 0.008) than inferior. In each of the 3 endplate regions (anterior, central, and posterior), porosity was inversely and significantly related to mechanical loading (pressure or maximum stress) in the adjacent disc region (P < 0.01 in all cases). Disc degeneration was best predicted by (reduced) nucleus pressure (R-2 = 0.46, P < 0.001) and (reduced) maximum stress in the anterior annulus (R-2 = 0.31, P < 0.001). Conclusion. Mechanical loading is a major determinant of endplate thickness and porosity. Disc degeneration is more closely related to reduced disc stresses than to endplate thickness or porosity.