Chapter 28

### INTRODUCTION

The Simplicity III procedure is a technique to denervate the sacroiliac (SI) joint by creating a thermal “strip” lesion along the sacrum lateral to the S1-S4 neural foramina and medial to SI jointline. The procedure is usually combined with radiofrequency ablation of the ipsilateral L5 primary dorsal ramus. Though multiple methods have been endorsed to denervate the SI joint, the Simplicity III procedure is a convenient and effective alternative for achieving maximal disruption of nociceptive input from the SI joint. The Simplicity III is a rigid tripolar probe with a sharp-tip that creates an adequate strip lesion that is technically easy to perform. Its pre-curved shape closely approximates the curvature of the sacrum, and it does not require an introducer.

### ANATOMY

• The sacroiliac (SI) joint is the largest axial joint in the body and functions to provide axial stability and to dissipate truncal loads to the lower extremities.

• It displays a great deal of inter- and intrapatient anatomical variability with respect to size, shape, and contour (Figure 28-1A, B).

• The SI joint is classified as a diarthrodial joint because it contains synovial fluid however, only the anterior third of the joint at the interface of the sacrum and ilium is truly synovial. The remainder of the joint is a fibrous articulation of the sacrum and ilium, comprised of ligamentous and cartilaginous connections.

• The joint is supported by muscles that also serve to deliver regional muscular forces to the pelvic bones. As the musculature of the SI joint is shared with the hip joint (eg, gluteus medius/minimus, biceps femoris, and piriformis), it cannot function independently.

• The joint is subject to shearing forces in a multitude of directions, but rotates only minimally about all 3 axes.

• From puberty onward, the joint begins to display degenerative changes, which are inevitable by the eighth decade of life.

• Though the exact mechanism of SI joint pain is likely multifactorial and not completely understood. The pathology of the joint has been implicated as a cause of chronic low back pain in up to 30% of all afflicted patients.

###### Figure 28-1.

(A) Side-to-side anatomical variation in CT scan of the same patient. (B) Side-to-side anatomical variation in CT scan of the same patient. (Used with permission from Harold Cordner, MD)

### NEUROANATOMY

• The innervation of the SI joint continues to be a topic of great debate.

• The posterior surface of the joint has been described to have sensory innervation originating as cephalad as the medial branches of the posterior primary ramus of L4 with contributions from L5 primary dorsal ramus.

• The lateral branches from sacral nerve roots from S1 to S4 supply most of posterior surface of the SI joint.

• The anterior joint ...

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