Seeing double: a comparison of microstructure, biomechanical function, and adjacent disc health between double- and single-layer vertebral endplates.
METHODS
Experimental and computational assessment of thickness, porosity, biomechanical behavior, and adjacent disc glycosaminoglycan content in double- and single-layer bony endplate samples harvested from human cadaver spines.


OBJECTIVE
To determine if the second layer of bone in double-layer vertebral endplates allows the superficial layer to achieve a more optimal balance between its biomechanical and nutritional functions.


BACKGROUND
Proper disc health requires the endplate to balance opposing biomechanical and nutritional functions. Previous studies investigating endplate function report seeing double: some endplates have a second layer of bone. However, it remains unclear whether the second layer of bone has any functional advantage. Such information could shed light on the factors that protect against disc degeneration.


METHODS
Six lumbar spines were obtained from human cadavers (32-84 yr) and scanned with magnetic resonance imaging. Cylindrical cores that included the endplate and underlying trabecular bone were harvested from the center of the superior vertebral endplates (6 double- and 12 single-layer endplates) and imaged using micro-computed tomography. The thickness and porosity of the bony endplate layers were measured for each core. High-resolution finite element analysis was performed to assess biomechanical behavior. Glycosaminoglycan content within the adjacent nucleus tissue was quantified using the dimethylmethylene blue technique.


RESULTS
The superficial layer of the double-layer endplates was 50% thinner (P = 0.009) and tended also to be more porous than single-layer endplates. Strains were higher in thinner endplates; however, the second layer of bone in the double-layer endplates had a stiffening effect so that despite being thinner than single-layer endplates, the superficial layer of the double-layer endplates had a similar risk of damage. After adjusting for age, glycosaminoglycan content was significantly higher in the nucleus tissue adjacent to the double-layer endplates (P = 0.01).


CONCLUSIONS
Compared with single-layer endplates, double-layer endplates seem to permit a more optimal balance between endplate biomechanical and nutritional functions, and may therefore offer a significant protective factor against disc degeneration.
Orthopaedic Bioengineering Laboratory, Department of Orthopaedic Surgery, University of California, San Francisco, CA 94143, USA.
Aaron J
AJ
Fields
Francisco
F
Sahli
Azucena G
AG
Rodriguez
Jeffrey C
JC
Lotz
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D002102:Cadaver; D005260:Female; D020342:Finite Element Analysis; D006025:Glycosaminoglycans / Q000378:metabolism*; D006801:Humans; D007403:Intervertebral Disc / Q000378:metabolism / Q000473:pathology* / Q000503:physiopathology; D055959:Intervertebral Disc Degeneration / Q000378:metabolism / Q000473:pathology* / Q000503:physiopathology; D016014:Linear Models; D008159:Lumbar Vertebrae / Q000378:metabolism / Q000473:pathology* / Q000503:physiopathology; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D055114:X-Ray Microtomography
D006025:Glycosaminoglycans