Toward more useful pressure-controlled discography: in vitro evaluation of injection speed, sensor location, and tube length

Pressure-controlled manometric discography is used by clinicians to evaluate discogenic pain. However, some would improve diagnostic accuracy. The goal of this study was to investigate potential confounding factors that might affect discographic results. Pressure differences depending on different speed of injection, lengths of connecting tubing and locations of sensors were evaluated using an in vitro model system.

Two sets of automated discography devices were arranged to record post-syringeal pressure pressures (PSPs) and intradiscal pressures (IDPs) in an "air chamber disk model" representing intradiscal pressure. PSPs and IDPs were measured simultaneously while varying injection speeds, and using intrasyringeal and extrasyringeal pressure sensors and contrast medium-filled tubing of different lengths. All pressure/volume curves were collected and viewed dynamically, and stored for further analysis.

At injection speed of 0.1 cc/second, the mean pressure difference (mean ΔP) between PSP and IDP was 38.1 psi. As injection speed was reduced, mean ΔP was proportionally decreased. Mean ΔP was 5.3 psi at injection speed of 0.01 cc/second and 0.7 psi at 0.005 cc/second. Mean ΔP values were significantly higher when pressures were recorded using intrasyringeal sensor: at injection speed of 0.1 cc/second, PSP and IDP values were 82.9 and 30.1 psi, respectively, compared with 50.6 and 12.5 psi measured by extrasyringeal sensor. Mean ΔP due to increased length of tubing was not significant.

Discography can be better performed with low speed injection (≤0.01 cc/second), using an extrasyringeal sensor. Difference of length of connecting tubings did not cause significant pressure differences. These data suggest that automated discography is a helpful adjunct to improve diagnostic accuracy, due to extrasyringeal location of pressure sensor and greater control of injection speed.