Diminution of In Vivo Thrust Forces of a Spinal Manipulation Device in an Animal Model

Research Objectives: To measure applied peak forces delivered by a commercially available spinal manipulative device (Activator V) in an in vivo animal model. It is hypothesized that surrounding in vivo soft tissues will greatly dampen peak spinal manipulative forces in a non-linear fashion due to the viscoelastic and/or hydrostatic properties of in vivo tissues.

Design: Experimental Study

Setting: Laboratory

Participants: Nineteen anesthetized adult cats (11 male, 8 female; 4.0 - 5.8kg).

Interventions: A commercial Activator V spinal manipulation device with an attached dynamic load cell and a custom tip was used to deliver spinal manipulative thrusts at 4 progressive settings (1-4) to the L7 spinous process of deeply anesthetized cats. The Activator V delivers mean peak forces up to 220N when tested directly on a load cell.

Main Outcome Measures: Applied peak forces were measured in Newtons (N) using high speed data acquisition. Mean thrust forces between device settings were compared using ANOVA and post hoc Tukey’s HSD tests.

Results: Mean thrust forces (standard deviations) for Activator V device settings were: Setting 1= 25.7N (6.4), Setting 2= 41.4N (10.3), Setting 3= 37.5N (17.3) and Setting 4= 39.8N (9.4). Significant differences were found between Settings 1 vs 2 (p < 0.0001), 1 vs 3 (0.003), 1 vs 4 (p < 0.0002). Only a minor trend of increasing peak forces appeared to occur at device settings 2 through 4 that was not statistically significant. The peak force differences were more pronounced when tested in direct load cell contact.

Conclusions: The viscoelastic nature of surrounding in vivo tissues created a significant diminution of applied spinal manipulative forces which may have important clinical safety and/or mechanistic implications. These findings highlight the need to further investigation into tissue dampening to applied spinal manipulative forces and the actual forces reaching deep tissue structures such as the intervertebral disc and low back multifidi muscles.

Author(s) Disclosures: Authors declare no conflicts of interests. This work was supported by NIH/NCCIH R21 AT010517 to WRR.