Angle-Free Ultrasound-assisted Technique for Difficult Lumbar Puncture to Enhance Precision and Success in Complex Spinal Anatomy: A Case Series with Longitudinal Learning Curve Analysis

Background Lumbar puncture is a routinely performed procedure, yet it presents significant challenges in patients with abnormal spinal anatomy, such as severe scoliosis or obesity. Conventional ultrasound-assisted neuraxial techniques, while superior to landmark-based methods, are constrained by two fundamental limitations: inaccurate projection of the probe’s geometric center onto the skin when the probe is tilted or rocked, and the difficulty of consistently replicating the exact probe angle during needle insertion. These limitations contribute to multiple needle passes, prolonged procedure time, and increased patient discomfort and complication risk. To address these issues, we developed a novel angle-free ultrasound-assisted technique designed to eliminate the need for replicating probe angulation during needle insertion. Case presentation: This angle-free approach was utilized for serial lumbar punctures in two patients with spinal muscular atrophy requiring repeated intrathecal nusinersen administration. Both patients presented with extreme anatomical complexity, including high body mass index (30 and 35.1 kg/m²) and severe scoliosis (Cobb angles 90° and 83°). The technique follows a three-step protocol: (1) Preprocedural imaging to select the optimal interlaminar space; (2) Ultrasound-guided postural adjustment until the probe could be held perpendicular to the bed edge and parallel to the floor while maintaining target visualization; (3) Needle insertion along this orthogonal trajectory. All 22 procedures (11 per patient) were completed successfully without persistent neurological complications. Procedural outcomes demonstrated a first-attempt success rate of 81.8% (Case 1) and 100% (Case 2), with first-pass success rates of 45.5% and 54.5%, respectively. Longitudinal analysis across sequential procedures revealed a consistent reduction in both the number of needle passes and needling time, reflecting a favorable learning curve. Conclusions The angle-free ultrasound-assisted neuraxial technique offers a valuable alternative for patients with complex spinal anatomy. By using real-time postural adjustments to achieve a needle trajectory, it enhances procedural precision and eliminates the technical hurdle of replicating probe angulation. The structured "4P" framework (Planning, Positioning, Perpendicular, Parallel) offers a practical guide for clinical adoption. The observed learning curve suggests that clinicians can readily acquire and apply this technique to manage such challenging cases.