Experimentally induced low back pain from hypertonic saline injections into lumbar interspinous ligament and erector spinae muscle.

Injection of hypertonic saline into back muscles or ligaments can induce acute low back pain (LBP). However, no study has systematically investigated pain characteristics from these structures. Further, induced muscle pain can change with stretching and contraction, which is problematic for studies into the effect of pain on sensorimotor control. However, it is unclear whether this occurs with experimental ligament pain. In separate sessions, 10 healthy volunteers received a single bolus injection of hypertonic (0.2ml, 5% NaCl) or isotonic saline (0.3ml, 0.9% NaCl) into L4/5 interspinous ligament, or hypertonic saline into the left paraspinal muscle. Pain intensity, size and duration were recorded, and a body chart was completed for each injection. Changes in pain intensity and size with stretching or back muscle contractions were also assessed during muscle and ligament pain. Injection of hypertonic saline into the interspinous ligament produced central LBP that was longer in duration and greater in intensity and size compared to hypertonic saline injection into lumbar paraspinal muscles. Isotonic saline injection into the interspinous ligament yielded mild pain that was short-lasting (<2min). Intensity and size of muscle pain reduced with stretching and contraction, whereas these tasks did not affect ligament pain. Surprisingly, some participants pointed to a location of pain that was 1-2 segments above or below the injected level. The results highlight that injection into the interspinous ligament elicits consistent pain that is not influenced by trunk movements. These findings support the implementation of this experimental ligament pain model in research.