Numerical investigation on the differential injection angle effects of intrathecal anesthetic drug delivery

In lumbar anesthesia surgery, the accurate determination of key parameters such as injection angle and dosage is facilitated by the real-time observation of intrathecal drug distribution. In this study, a non-invasive and visual computational fluid dynamics method was used to simulate drug injections in the “cloverleaf†lumbar thoracic segment with a multiphase mixture model, clarifying the intrathecal unsteady flow behaviors and the drug transport mechanisms. The transport properties of three clinically significant injection angles — 0∘, 45∘ and 60∘— were compared, determining the fastest and slowest block onset angles based on drug concentration levels in the thoracic vertebral plane. The calculation results are aligned with clinical practice trends, and it is found that significantly different drug amounts are required for different injection angles, with higher doses necessitated for ultrasound-assisted anesthesia. During anesthesia, a trend of increasing and then decreasing local anesthetic concentrations in each vertebral plane was observed, with the presence of a minimum effective dose of local anesthetic at the target block site. Injecting at the “concave†part of the cloverleaf (45∘ and 60∘) results in greater drug pulsation, faster diffusion and more efficient mixing than injecting at the positive “convex†part of the cloverleaf cross-section (0∘). At the L2 vertebral plane, the flow rate and mixing ratios during injection for angles of 0∘, 45∘ and 60∘ were approximately 1:4:2. Compared with orthostatic 0∘ injections, 45∘ and 60∘ oblique injections resulted in higher drug concentrations in the T12-10 vertebral plane during the pre-transport period, with 45∘ injections being the fastest. However, over a longer period of time, 45∘ injections resulted in the lowest drug concentrations in the target block plane (T10), necessitating a higher dose. A faster anesthetic response was produced by 0∘ injections at the same dose. Valuable insights for the application of appropriate drug dosages in clinical anesthesia practice are offered by the calculations.