The effect of injection volume on disc degeneration in a rat tail model. 
A prospective study to compare the effects of injection of various doses of phosphate-buffered saline (PBS) solution into the intervertebral disc.  To evaluate the effect of injection volume on disc degeneration in a rat tail-disc model.  There are many studies on disc degeneration models induced by needle injection.  Recently, novel treatments for disc degeneration have been developed that are administered by injection.  However, it is unknown whether injection volume affects disc degeneration.  A total of 180 Sprague Dawley rats were randomized into five groups and injected with 0(control), 1.0, 2.0, 2.5, or 3.0 μL of PBS solution.  Discs were harvested at weeks 1, 2, and 4 after injection and were evaluated using radiography, histology, and biochemistry (glycosaminoglycan, hydroxyproline, and water content).  No significant differences in radiography, biochemistry, or histology were observed at any of the three sampling times between the 1.0, 2.0 μL groups and the control.  The 2.5 and 3.0 μL groups exhibited significant decrease in radiographic disc height index and water content since week 2.  The glycosaminoglycan content of 2.5 μL group decreased significantly by week 4 and that of 3.0 μL group decreased at weeks 2 and 4.  Significant hydroxyproline content decrease was only observed for 3.0 μL group during week 4.  Significantly higher histologic score was observed in 3.0 μL group since week 1 and 2.5 μL group since week 2.  The three parameters of 3.0 μL group indicated more severe disc degeneration than those of 2.5 μL group, particularly during week 4.  When the volume of PBS injected into the rat tail-disc exceeded a threshold, it rapidly exhibited degenerative changes according to radiographic, biochemical, and histologic analysis.  The degenerative changes were dose-dependent and increased as the dose increased.