Effective epidural analgesia depends on accurate identification of the epidural space for delivery of analgesic mixtures. The technique of loss of resistance to either saline or air is the most commonly used method to locate the epidural space. Although this technique has a high success rate, it is associated with a significant incidence of accidental dural punctures. Accidental dural punctures might be related to the operator performance or to individual anatomical variations.

A normal ligamentum flavum is fundamental in the loss of resistance technique. However, many cadaver dissection and radiological studies in the literature have showed that the ligamentum flavum may in fact not fuse at the midline; thereby leaving a gap between its left and right portions. Ligamentum flavum midline gaps are thought to be a potential cause of failed recognition of loss of resistance during epidural needle placement and consequently accidental dural puncture. Therefore, if it were feasible to avoid inserting the epidural needle at the same level where a ligamentum flavum midline gap exists, then dural punctures may be less likely.

Lumbar spine ultrasound has been very useful in consistently identifying important anatomical landmarks for epidural needle placement including the ligamentum flavum. It is unknown at this time whether ultrasound is a useful modality to detect abnormal anatomy. Our study will evaluate by ultrasound scan the lumbar vertebral column of patients who have had a previously recognized accidental dural puncture. Our objective is to determine whether or not there is a correlation between accidental dural punctures and abnormalities of ligamentum flavum as seen by ultrasound imaging. Our hypothesis is that patients who have had accidental dural punctures have abnormal anatomy of ligamentum flavum that can be detected by lumbar spine ultrasound.

Lumbar ultrasound imaging from L1-S1 will be performed with the patient in the sitting position. The best possible image captured at the transverse approach for each of the interspaces from L5-S1 to L1-L2 will be saved for analysis of both ligamentum flavum (primary outcome) and symmetry (secondary outcome). A normal ligamentum flavum is described as a hyperechoic continuous midline band, which is aligned with the hyperechoic signal of the transverse processes. An abnormal ligamentum flavum will be described as a non-continuous or even absent signal. The presence of symmetric anatomy is defined as an equal distant between the left and right articular processes and the transverse processes to the midline.

Incidence of abnormal ligamentum flavum and asymmetry will be presented in a descriptive way and the level of anatomical abnormality detected by ultrasound will be correlated to the level of dural puncture reported on the anaesthesia record.