Introduction:  Degenerative disc disease is characterized by a loss in proteoglycan and water in the nucleus pulposus (NP), and a loss in disc height which leads to alterations in the biomechanical function of the intervertebral disc.  Injectable NP particulate allograft consists of cryomilled NP tissue and is intended to replace nucleus pulposus tissue loss due to degeneration by introducing additional extracellular matrix into the disc space.  The purpose of this study was to assess the characteristics of nucleus pulposus allograft that are responsible for its potential utility as a cushion between the vertebrae.  Therefore, GAG content, swelling capacity, total collagen, and GAG:hydroxyproline ratio were assessed.  The hypothesis is that nucleus pulposus particulate retains properties similar to that of native tissue.

Methods: NP particulate was generated from cadaveric intervertebral discs from lumbar and thoracic vertebrae.  For native tissue, an approximately 1 cm³ piece of tissue from the nucleus pulposus of a single disc from each donor was collected.  A total of 5 donors were used to directly compare native tissue to NP particulate.  Additionally, 40 cadaveric donors stratified by age (15-25, 26-35, 36-45, and 46-55 yr-old) were used to further evaluate GAG content and swelling capacity (n=10, 5 females and 5 males per age group).  GAG and total collagen (hydroxyproline) content were assessed using commercially available kits, and swelling capacity was determined using a centrifugation method.  For GAG content and swelling capacity, two-way ANOVA was used to compare differences between the main effects of age and sex.  Sidak’s multiple comparison test was used to show differences between age groups within male and female groups.  T-tests were used to determine the difference between native and particulate groups for collagen content and GAG:hydroxyproline ratio.

Results: NP particulate and native tissue were not significantly different in the outcomes measured.  Total collagen content was 85.11±30.13 in the native group and 118.8±85.87 µg hydroxyproline/tissue dry weight in the particulate group (p=0.38).  GAG:hydroxyproline ratio was ~12:1 in native and ~9:1 in particulate (p=0.36).  The average GAG content in NP particulate from 40 donors was 666.1 ± 156.9 µg/mg tissue and was not significantly different between males and females. When males and females were pooled GAG content was reduced in older groups compared to 15-25 yr-olds: 1.3-fold lower (p=0.0187) in 36-45 yr olds and 1.5-fold lower (p=0.0005) in 46-55 yr-olds.  The average swelling capacity in NP particulate from all donors was 608.4±113.1%.  When sexes were pooled, swelling capacity was ~1.2-fold higher in 46-55 yr-olds compared to 15-25 yr-olds.    

Discussion:  These results suggest that NP particulate has high GAG levels and retains the important original characteristic of swelling.  In the 40-donor analysis, the primary differences in levels of GAG and swelling capacity were in the male donors and is consistent with well-established age-associated decline in GAG content.  However, the decline in GAGs did not negatively alter the swelling capacity of the particulate.  These findings suggest that the NP particulate may be a viable therapeutic option to replace tissue loss in the degenerative disc.