# Sacroiliac Joint Interventions

## Introduction

The sacroiliac (SI) joint is a significant source of low back and buttock pain, accounting for an estimated 15-30% of axial low back pain. Accurate diagnosis requires a combination of clinical examination and confirmatory diagnostic injections. Treatment options span a wide range, from intra-articular injections to lateral branch radiofrequency ablation and, in refractory cases, SI joint fusion.

## Anatomy and Biomechanics

The SI joint is a diarthrodial joint with two distinct components: the anterior one-third is synovial, and the posterior two-thirds is syndesmotic, consisting of the interosseous ligament. The joint surface is auricular (ear-shaped) and features irregular ridges and depressions that interlock to resist shear forces. Primary stabilizers include the anterior sacroiliac ligament, posterior sacroiliac ligament, interosseous sacroiliac ligament, sacrotuberous ligament, and sacrospinous ligament.

Innervation is complex and variable. The posterior joint is supplied by the lateral branches of the S1-S3 dorsal rami, with occasional contributions from the L5 dorsal ramus. The anterior joint receives innervation from the L2-S2 ventral rami, making it inaccessible to percutaneous denervation -- this is why lateral branch ablation targets only the posterior innervation. Joint motion is limited to 2-4 degrees of rotation and 1-2 mm of translation (nutation and counternutation).

<image>Detailed anatomical illustration of the sacroiliac joint in a posterior view showing the auricular surface with cartilaginous covering, the interosseous sacroiliac ligament, posterior sacroiliac ligaments (short and long), sacrotuberous ligament, and sacrospinous ligament. The lateral branches of S1, S2, and S3 dorsal rami are shown coursing over the posterior sacrum toward the joint, with their variable branching patterns highlighted.</image>

## Provocative Physical Examination Maneuvers

A cluster of three or more positive provocative tests has demonstrated good sensitivity and specificity for SI joint pain. The distraction test applies a posteriorly directed force to the bilateral anterior superior iliac spines. The compression test applies medially directed force to the bilateral iliac crests with the patient in lateral decubitus. The thigh thrust (posterior shear) test delivers axial force through the flexed hip with the patient supine, creating a posterior shearing force across the SI joint. Gaenslen's test maximally flexes one hip while extending the contralateral hip off the table edge, stressing both SI joints simultaneously. The FABER (Patrick) test places the hip in flexion, abduction, and external rotation; pain at the SI joint (rather than the groin) suggests SI joint pathology. The sacral thrust test applies anteriorly directed force over the midline sacrum with the patient prone.

| Provocative Test | Patient Position | Maneuver | Positive Finding |
|-----------------|-----------------|----------|-----------------|
| Distraction | Supine | Posterior force on bilateral ASIS | Reproduces SI joint pain |
| Compression | Lateral decubitus | Medial force on iliac crests | Reproduces SI joint pain |
| Thigh thrust (posterior shear) | Supine | Axial force through flexed hip | Posterior shearing pain at SI joint |
| Gaenslen's test | Supine (hip off table edge) | Flex one hip, extend contralateral | Reproduces SI joint pain |
| FABER (Patrick) | Supine | Flexion, abduction, external rotation | Pain at SI joint (not groin) |
| Sacral thrust | Prone | Anterior force over midline sacrum | Reproduces SI joint pain |

## Diagnostic SI Joint Injection

Fluoroscopically guided intra-articular injection is the reference standard for confirming SI joint pain. The patient is positioned prone, and fluoroscopy is directed to visualize the inferior aspect of the SI joint. A 22- or 25-gauge spinal needle is advanced into the inferior synovial recess. Arthrographic confirmation with 0.5-1 mL of contrast demonstrating intra-articular spread is mandatory before injecting anesthetic. A positive diagnostic block is defined as 75% or greater pain relief during the expected duration of the local anesthetic. Dual blocks reduce false-positive rates, which may reach 20% with single blocks. Some practitioners inject a combination of local anesthetic and corticosteroid for simultaneous diagnostic and therapeutic benefit.

<image>Fluoroscopic image illustration showing the technique for SI joint injection. An AP fluoroscopic view demonstrates a spinal needle entering the inferior one-third of the SI joint from a posterior approach, with contrast arthrogram showing dye spreading within the joint capsule outlining the auricular surface. Anatomic landmarks including the sacral foramina, iliac crest, and joint margins are labeled.</image>

## Lateral Branch Radiofrequency Ablation

Lateral branch RFA targets the lateral branches of the S1-S3 dorsal rami and, in some protocols, the L5 dorsal ramus. Conventional RF uses strip lesions created by placing the electrode along the lateral sacral crest at the S1, S2, and S3 levels. Cooled RF (such as the SInergy system) creates larger, spherical lesions that improve nerve capture given the variable anatomy of lateral branch nerves -- for this reason, cooled RF is generally preferred over conventional RF for lateral branch ablation at the sacrum.

The technique involves placing electrodes at predetermined positions along the sacral lateral crest and the L5-S1 junction. Sensory stimulation is less reliable in this region due to the small caliber and deep location of lateral branches. The MINT randomized controlled trials (Patel et al., 2012) using cooled RF showed significant improvement in pain and function at 3 and 6 months compared to sham. Expected duration of relief is 6-12 months, and repeat ablation is effective in most initial responders.

## SI Joint Fusion Indications

Minimally invasive SI joint fusion (using triangular titanium implants, for example) is considered for patients who have failed conservative and interventional management. Indications include confirmed SI joint pain by diagnostic injection, failure of at least 6 months of conservative care, and significant functional impairment. The INSITE and iMIA randomized controlled trials demonstrated superiority of SI joint fusion over conservative management at 6 and 24 months.

The surgical technique involves lateral transgluteal placement of implants across the SI joint under fluoroscopic guidance. Contraindications include active infection, severe osteoporosis, and significant lumbar pathology that may be contributing to symptoms. Complications include implant malposition, nerve injury, hematoma, and infection, with overall complication rates less than 5%.

<image>Lateral fluoroscopic view illustration showing minimally invasive SI joint fusion with three triangular titanium implants placed across the sacroiliac joint from a lateral transgluteal approach. The implants are shown traversing the ilium and anchoring into the sacral body. Labels indicate the ilium, sacrum, implant positions, and the trajectory of insertion.</image>

## Emerging Therapies

Several emerging therapies are under investigation for SI joint pain. Platelet-rich plasma (PRP) intra-articular injections have shown promise in early studies for SI joint pain refractory to corticosteroid injections. Prolotherapy (dextrose injection into SI joint ligaments) has limited but supportive evidence from small randomized controlled trials. Peripheral nerve stimulation of the lateral branches or dorsal rami is being explored as a neuromodulatory alternative to ablation.

## Clinical Pearls

A cluster of at least three provocative physical examination maneuvers should always be used before pursuing diagnostic injection, because a single test is insufficient for diagnosis. Intra-articular placement must be confirmed with contrast arthrography before injecting local anesthetic, as extra-articular injection is a common cause of false-negative blocks. Cooled RF is generally preferred over conventional RF for lateral branch ablation due to the variable anatomy and deeper nerve course at the sacrum. SI joint dysfunction should be considered in patients with post-lumbar fusion adjacent segment pain, since fusion increases biomechanical stress on the SI joint. Dual diagnostic blocks with strict pain relief thresholds significantly improve outcomes for both RF ablation and fusion.

## References

1. Patel N, Gross A, Brown L, Gekht G. A randomized, placebo-controlled study to assess the efficacy of lateral branch neurotomy for chronic sacroiliac joint pain. *Pain Med*. 2012;13(3):383-398.
2. Polly DW, Cher DJ, Wine KD, et al. Randomized controlled trial of minimally invasive sacroiliac joint fusion using triangular titanium implants vs nonsurgical management for sacroiliac joint dysfunction: 12-month outcomes. *Neurosurgery*. 2015;77(5):674-690.
3. Laslett M, Aprill CN, McDonald B, Young SB. Diagnosis of sacroiliac joint pain: validity of individual provocation tests and composites of tests. *Man Ther*. 2005;10(3):207-218.
4. Cohen SP, Chen Y, Neufeld NJ. Sacroiliac joint pain: a comprehensive review of epidemiology, diagnosis, and treatment. *Expert Rev Neurother*. 2013;13(1):99-116.