Background and objectives The ‘loss of resistance’ technique is used to determine entry into the epidural space, often by a midline needle in the interspinous ligament before the ligamentum flavum. Anatomical explanations for loss of resistance without entry into the epidural space are lacking. This investigation aimed to improve morphometric characterization of the lumbar interspinous ligament by observation and measurement at dissection and from MRI.
Methods Measurements were made on 14 embalmed donor lumbar spines (T12 to S1) imaged with MRI and then dissected along a tilted axial plane aligned with the lumbar interspace.
Results In 73 interspaces, median (IQR) lumbar interspinous plus supraspinous ligament length was 29.7 mm (25.5–33.4). Posterior width was 9.2 mm (7.7, 11.9), with narrowing in the middle (4.5 mm (3.0, 6.8)) and an anterior width of 7.3 mm (5.7, 9.8).
Fat-filled gaps were present within 55 (75%). Of 51 anterior gaps, 49 (67%) were related to the ligamenta flava junction. Median (IQR) gap length and width were 3.5 mm (2.5, 5.1) and 1.1 mm (0.9, 1.7).
Detection of gaps with MRI had 100% sensitivity (95% CI 93.5 to 100), 94.4% specificity (72.7, 99.9), 98.2% (90.4, 100) positive predictive value and 100% (80.5, 100) negative predictive value against dissection as the gold standard.
Conclusions The lumbar interspinous ligament plus supraspinous ligament are biconcave axially. It commonly has fat-filled gaps, particularly anteriorly. These anatomical features may form the anatomical basis for false or equivocal loss of resistance.
- regional anesthesia
- pain management
Data availability statement
Data are available upon reasonable request. Data access is subject to approval. Application can be made via the corresponding author.
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Contributors The following summarizes the authorship contributions made with regard to this scienfic work. SuL—research design, planning, and conduct data acquisition, analysis and interpretation manuscript drafting, revision and approval. Agreed to be accountable guarantor. StL—data analysis manuscript drafting, revision and approval. Agreed to be accountable. HH—data analysis manuscript drafting, revision and approval. Agreed to be accountable. GC—research design, planning and conduct data acquisition manuscript drafting, revision and approval. Agreed to be accountable. DJS—research design, data interpretation manuscript drafting, revision, and approval. Agreed to be accountable. DR—research design, planning, and conduct data acquisition and interpretation manuscript drafting, revision, and approval. Agreed to be accountable. The following summarizes the non-authorship contributions made with regard to this scienfic work. Jasmine Slater— artwork figures 1 and 2C.
Funding This research was funded via an Australian and New Zealand College of Anaesthetists grant N18/005.
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.