Ultrasound guidance for intervertebral disc location and paravertebral tissue swelling quantification in a mouse model of intervertebral disc herniation.

PURPOSE: This study aims to use ultrasound guidance to locate the intervertebral disc (IVD) of a mouse caudal spine, establish a mouse model of IVD herniation by needle puncture and cyclic pump pressure, and perform ultrasound to longitudinally detect paravertebral tissue swelling to help evaluate the pathological progression after IVD herniation.

METHODS: An animal ultrasound was used to scan the caudal spine of mice to locate the IVDs. IVD herniation was induced in the mice by puncturing the caudal IVDs (Co4/5, Co5/6, and Co6/7) with a 26-G needle and applying cyclic pump pressure using an improved microinjection pump. Magnetic resonance imaging (MRI) was used to detect IVD degeneration, and ultrasound was performed to assess swelling in the paravertebral tissue before and at one, two, and four weeks after surgery. Additionally, a parallel cross-sectional cohort of mice was euthanized at each time point. Alcian Blue/Orange staining was utilized to evaluate inflammation, and immunofluorescence staining detected the location of nucleus pulposus (NP) cells in the IVDs. Finally, the relationship between swelling volume and inflammation or IVD degeneration was evaluated using linear regression analysis.

RESULTS: The IVD was identified based on the pixel of ultrasound images, which aligns with the IVD location for AB/OG staining in the IVDs. In the mouse model of IVD herniation, a bilateral rupture of the annulus fibrosus and the herniation of the NP tissue into the paravertebral tissue were observed 24 h after surgery. The Pfirrmann grade and peak separation value from magnetic resonance T2 weighted (MRT2) images showed that the IVD degenerated one week after surgery compared to before surgery (grade 1 and 3, P < 0.0001; P = 0.001). There was no significant difference between one and two weeks post-surgery (grade 3 and 3, P = 0.851; P = 0.469). However, a difference was observed between two and four weeks post-surgery in the Pfirrmann grade and peak separation value (grade 3 and 4, P = 0.037; P = 0.011). Additionally, there was a strong negative correlation (r = - 0.875, P < 0.0001) between the Pfirrmann grade and the peak separation value. Meanwhile, paravertebral tissue swelling and inflammation peaked one week after surgery (P < 0.0001; P < 0.0001), decreased at two weeks (P < 0.0001; P = 0.004), and reduced further by four weeks (P = 0.001; P = 0.270). Furthermore, there was a stronger correlation between paravertebral tissue swelling and inflammation (r = 0.832, P < 0.0001) compared to the Pfirrmann grade (r = 0.364, P = 0.041) or the peak separation value (r = - 0.324, P = 0.007).

CONCLUSIONS: Ultrasound can guide IVD localization, which may assist in establishing a mouse model of IVD herniation. Additionally, ultrasound can provide longitudinal measurements of swelling in paravertebral tissue, which may contribute to the pathological investigation of IVD herniation.