Effect of test environment on intervertebral disc hydration

Water content of fresh human lumbar intervertebral discs (with adjacent endplates) was assessed in three studies: 1) after each of seven specimen preparation steps. 2) during exposure to either saline spray or a saline bath, and 3) during exposure to a saline bath and 445 N axial compression, either without or with previous exposure to the bath and no compression ("free swelling").

To assess the effect on disc hydration of various aspects of specimen preparation and testing environments.

Water content is an important determinant of disc behavior. Specimen preparation method and testing environments may be important determinants of water content, yet no work appears to have been reported specifically on this topic.

Endplate-disc-endplate specimens were prepared from refrigerated cadavers within 24 hours of death by transverse sectioning of adjacent vertebral bodies. Water content change was determined by specimen weight change across each time interval of interest.

Specimen preparation (including multiple freeze-thaw cycles) produced no water content change. Saline spray and plastic film wrap resulted in no change, but saline bath exposure resulted in a 24% increase over 7 hours, 44% of which occurred in the first 0.5 hour. A subsequent 7 hours of 445 N compression reduced the overall increase to 10%. This was not significantly different from the 8% increase that resulted from initial exposure to saline bath and compression.

Specimen preparation as typically performed and specimen exposure to saline spray and plastic film wrap do not result in hydration change. Exposure to saline bath results in substantial swelling, which can either be reversed or prevented by axial compression in the physiologic range. Whether discs exposed to saline spray and wrap without compression and those exposed to saline bath with compression behave the same and which of these more closely mimics the in vivo condition are important issues for the experimentalist to test.