Defining a bony landmark-based target zone for superior cluneal nerve blockade: a cadaveric mapping study

Patients with entrapment of the superior cluneal nerve (SCN) experience pain and dysesthesia in the gluteal region. Ultrasound-guided injection is the preferred approach for the management of SCN entrapment. Mapping of the emerging sites of SCN branches is required to identify the optimal injection site of local anesthetics in the entrapment of the SCN.

Methods: a total of 10 lumbar-gluteal regions (obtained from six formalin-fixed cadavers) were dissected at ×16 magnification using a surgical microscope. The anatomical landmarks used in this study included the lumbar spinous processes, the posterior superior iliac spine (PSIS), the iliac crest and its highest point. A Cartesian coordinate system originating from the PSIS was established to label the emerging sites of each SCN branch. A reference line was identified connecting the highest point of the iliac crest with the PSIS.

The SCN branches pierced the thoracolumbar fascia at a mean lateral distance (X-axis) of 6.6±1.1 cm and a mean vertical distance (Y-axis) of 6.8±1.3 cm relative to the reference line. Their distribution was systematically mapped and illustrated in proportion to this reference line, which had a mean length of 9.2±1.4 cm. In particular, in nine specimens, vascular structures were observed traversing the triangle of Petit, but no SCN branches were found within this anatomical landmark.

Injection into the lateral half of the reference line over the iliac crest, over a 3 cm² area, can produce high success in the management of SCN entrapment.