Definition and anatomy of the Posterior Sacral Network (PSN)
The sacroiliac joint complex is a diarthrodial, synovial joint and posterior ligamentous network that receives both anterior innervation from the lumbosacral plexus and posterior sensory innervation via the posterior sacral network (PSN). The PSN represents the principal posterior sensory supply to the sacroiliac joint complex and is therefore a key anatomical substrate for interventional procedures targeting sacroiliac region pain. This dual innervation underscores the complexity of nociceptive input from the sacroiliac joint complex and the need to distinguish posterior network–mediated pain from symptoms arising from anterior or extra-articular structures in clinical and interventional planning.
Anatomically, the PSN is described as a plexus of nerves that originates primarily from the S1 through S3 lateral branches, with variable contributions from the S4 lateral branch and the L5 dorsal ramus. These lateral branches arise from the posterior rami and then converge to form the PSN as they course toward the posterior sacroiliac joint region. At the level of the lateral sacral crest, the PSN typically extends from the second to just below the third transverse sacral tubercle, defining a reproducible corridor along which the posterior innervation of the sacroiliac joint travels. This anatomical configuration provides a rational target zone for both diagnostic blocks and radiofrequency ablation techniques that aim to denervate the posterior sacroiliac complex.
The segmental exit pattern of the lateral branches from the posterior sacral foramina is clinically relevant for both periforaminal and strip-lesion techniques. At S1, the lateral branches primarily exit at the inferolateral quadrant of the posterior sacral foramen; at S2, up to two lateral branches may exit from the superolateral and/or inferolateral quadrants; and at S3, the lateral branches predominantly exit from the superolateral quadrant. After exiting, these branches course laterally and posteriorly, converging into the PSN along the periosteum of the lateral sacral crest. The periosteal location of this network is a critical anatomical feature because it dictates that effective diagnostic blocks and ablative lesions must be delivered at the level of the periosteum to reliably capture the posterior sacroiliac innervation.
In addition to the main plexus, occasional accessory branches may innervate the superior portion of the sacroiliac joint, typically located between the first and second transverse sacral tubercles and, less commonly, just proximal to the first tubercle. The PSN also gives rise to the medial cluneal nerves, which supply cutaneous sensation over the medial buttock; consequently, sensory changes in this region can occur following interventions targeting the PSN. Collectively, these anatomical observations support the concept of the PSN as a discrete, periosteal neural network that can be systematically targeted along the lateral sacral crest or adjacent sacral surface for both diagnostic and therapeutic procedures in sacroiliac joint complex pain.
Role of PSN in sacroiliac pain generation
Low back pain is highly prevalent, and a substantial proportion of cases are attributed to nociception arising from the sacroiliac joint complex. The sacroiliac joint complex accounts for a notable fraction of mechanical low back pain presentations, and its posterior sensory input is mediated predominantly through the PSN. Because the PSN conveys afferent signals from the posterior joint and associated ligamentous structures, it represents a principal pathway by which sacroiliac complex pathology can manifest as localized or referred pain in the lumbosacral and buttock regions. This neuroanatomical relationship provides the rationale for targeting the PSN in interventional strategies for sacroiliac joint complex pain.
Pain signals originating from the sacroiliac joint complex can be interrupted by image-guided percutaneous radiofrequency ablation of the PSN, which has been associated with reductions in pain and disability in appropriately selected patients. The posterior sacral network specifically addresses the posterior sacroiliac structures, including the posterior joint and ligaments, which may not be fully addressed by intra-articular sacroiliac joint injections alone. Histological and anatomical work has demonstrated the presence of nerve fibers within the sacroiliac joint capsule and adjoining ligaments, and sacral lateral branch neurotomy is designed to modulate this posterior nociceptive input. Thus, the PSN is central to the pathophysiology of posterior sacroiliac complex pain and a logical target for neurotomy-based interventions.
The clinical relevance of the PSN is further supported by outcome data from sacroiliac joint radiofrequency ablation studies. Systematic reviews and meta-analyses of sacroiliac joint radiofrequency procedures, which predominantly target the sacral lateral branches and PSN, have reported that a proportion of patients achieve at least 50% pain relief for six months, with a subset achieving complete pain relief over the same period. These findings, although variable across techniques and patient selection protocols, indicate that modulation of PSN-mediated nociception can translate into clinically meaningful improvements in pain and function. The variability in outcomes has been attributed in part to differences in how completely the PSN is captured by different lesioning strategies, underscoring the importance of understanding PSN anatomy in optimizing therapeutic effect.
The PSN’s contribution to sacroiliac pain generation also has implications for diagnostic paradigms. Because sacral lateral branch neurotomy targets posterior innervation, the response to intra-articular sacroiliac joint injections does not necessarily correlate with the likelihood of success from PSN-directed radiofrequency procedures. Diagnostic blocks that anesthetize the innervation lateral to the sacral foramina at the level of the periosteum have been shown to be effective in capturing the posterior sacroiliac complex, supporting the concept that PSN-mediated nociception is a distinct and clinically relevant pain generator. Consequently, the PSN is increasingly recognized as a critical structure in both the diagnosis and interventional management of sacroiliac joint complex pain.
Diagnostic tools and imaging guidance
Diagnostic evaluation of sacroiliac joint complex pain in interventional practice commonly incorporates physical examination, imaging, and targeted blocks of the PSN or intra-articular sacroiliac joint injections. Prognostic PSN blocks and diagnostic intra-articular sacroiliac joint blocks are described as standard practice for confirming sacroiliac joint complex pain prior to radiofrequency ablation of the PSN. These diagnostic procedures aim to transiently interrupt nociceptive input from the sacroiliac complex, thereby helping to identify patients in whom PSN-targeted interventions are more likely to yield clinically meaningful benefit.
Fluoroscopy has traditionally been the primary imaging modality for both diagnostic blocks and radiofrequency ablation of the sacral lateral branches and PSN. Fluoroscopic guidance allows visualization of bony landmarks such as the posterior sacral foramina, sacral ala, and sacroiliac joint line, facilitating accurate placement of needles for periforaminal or strip-lesion techniques. In periforaminal approaches, needle positions are often described relative to a clock-face orientation around the lateral margin of the posterior sacral foramina, with lesion sites placed at defined distances from the foramen to encompass the exiting lateral branches that contribute to the PSN. These fluoroscopic techniques have been widely adopted and form the basis for many of the clinical studies evaluating sacroiliac joint radiofrequency ablation.
Ultrasound guidance has emerged as an alternative or adjunct imaging modality for procedures targeting the PSN. Cadaveric and clinical work has demonstrated that ultrasound can reliably identify key landmarks such as the sacral hiatus, posterior sacral foramina from S3 to S1, and the transverse sacral tubercles along the lateral sacral crest. Using these landmarks, ultrasound-guided techniques have been developed to place radiofrequency cannulae along the lateral sacral crest between the first and third transverse sacral tubercles, corresponding to the typical extent of the PSN. In a prospective study of ultrasound-guided sacroiliac joint radiofrequency ablation, needle placement along the lateral sacral crest was shown to be consistent and anatomically aligned with the PSN, supporting the feasibility of ultrasound-only guidance for PSN-targeted procedures.
Comparative data suggest that ultrasound-guided sacroiliac joint radiofrequency ablation can achieve clinical outcomes similar to fluoroscopy-guided techniques over short-term follow-up. In one study, no statistically significant differences were observed in pain intensity or disability scores between ultrasound- and fluoroscopy-guided sacroiliac joint radiofrequency ablation at two months post-procedure. Additionally, randomized work has shown that a single-depth ultrasound-guided block at the lateral sacral crest can provide anesthetic coverage equivalent to a fluoroscopy-guided multisite, multidepth sacral lateral branch block protocol, reinforcing the concept that targeting the PSN at the periosteal level along the lateral sacral crest is sufficient for diagnostic purposes. These findings support the use of both fluoroscopy and ultrasound as viable imaging modalities for PSN-focused diagnostic and therapeutic interventions, with technique selection guided by operator expertise, equipment availability, and procedural goals.
PSN as a target for radiofrequency ablation
The PSN is a principal target for radiofrequency ablation in patients with sacroiliac joint complex pain that is refractory to conservative measures. Sacroiliac joint radiofrequency ablation reduces pain through thermal coagulation of nerve fibers carrying afferent pain signals originating primarily from the sacroiliac joint complex, with the PSN representing the main posterior conduit for these signals. Image-guided percutaneous radiofrequency ablation of the PSN has been associated with reductions in pain and disability in carefully selected patients, and is considered a standard interventional option for recalcitrant sacroiliac joint pain.
Multiple radiofrequency techniques have been described to target the PSN, including conventional thermal, cooled, multipolar, multilesion probes, and multitined electrodes, with needle placements configured as linear strip lesions or periforaminal arrays. In linear strip lesion techniques, an extensive continuous lesion is created along the sacrum to capture the posterior innervation of the sacroiliac joint as it traverses from the posterior sacral foramina to the sacroiliac joint. Needle placements for strip lesions have been described along the lateral sacral crest, lateral to the posterior sacral foramina, and over the posterior sacroiliac joint, with the extent of the lesion typically spanning from the S1 superior articular process to as distal as the fourth posterior sacral foramen. These configurations are designed to encompass the typical distribution of the PSN between the second and third transverse sacral tubercles.
Periforaminal techniques focus on needle placement around the lateral margin of the posterior sacral foramina, often in a semicircumferential pattern defined relative to a clock-face orientation. Lesion sites are positioned at specified distances from the foramen to intersect the lateral branches as they exit and converge into the PSN. Although periforaminal approaches have been widely used, cadaveric modeling suggests that bipolar strip lesion techniques may achieve higher rates of lateral branch and PSN capture compared with conventional monopolar periforaminal methods. In particular, bipolar palisade configurations, which use multiple closely spaced electrodes to generate overlapping lesions, have been shown in anatomical models to capture a greater percentage of lateral branches than traditional monopolar techniques.
Estimated capture of lateral branches with linear strip lesion techniques in cadaveric models ranges from approximately 93% to nearly complete capture, with complete capture of all lateral branches in a majority of specimens when using ultrasound-guided lateral crest or fluoroscopy-guided palisade techniques. Despite this high predicted neural capture, clinical outcomes have been more modest, with reported proportions of patients achieving at least 50% pain relief at six months ranging from the upper 30% range to just under 70% in some series. Systematic reviews of sacroiliac joint radiofrequency ablation, encompassing a variety of PSN-targeted techniques, have similarly reported that 32–89% of patients may achieve at least 50% pain relief for six months, with 11–44% achieving complete pain relief over the same period. These data highlight both the therapeutic potential and the variability of outcomes associated with PSN-directed radiofrequency ablation, emphasizing the importance of technique selection and patient selection in clinical practice.
Benefits of complete neural ablation
The concept of complete PSN ablation has emerged from anatomical and cadaveric work demonstrating that more extensive lesioning strategies can achieve higher rates of lateral branch and PSN capture. Cadaveric modeling suggests that targeting the PSN with large bipolar strip lesions can result in substantially higher rates of neural capture compared with periforaminal radiofrequency ablation performed with conventional monopolar electrodes. In some models, the rate of complete neural capture with conventional monopolar periforaminal techniques has been estimated to be relatively low, whereas bipolar strip lesion configurations have achieved markedly higher capture rates. These findings provide a mechanistic rationale for pursuing lesion strategies that approximate complete PSN ablation in order to optimize clinical outcomes.
Multi-tined and multipolar technologies have been developed to facilitate larger and more contiguous lesions along the sacral surface. Nimbus, a commonly used multi-tined radiofrequency probe, generates a large bipolar lesion and has been identified as an option for achieving more complete PSN neural ablation when used in a strip-lesion configuration. In cadaveric models, linear strip lesion techniques using ultrasound-guided lateral crest or fluoroscopy-guided palisade approaches have been estimated to capture over 90% of lateral branches, with complete capture in a majority of specimens. These lesion geometries are designed to encompass the PSN between the second and third transverse sacral tubercles, where the network is most consistently located.
Clinical and observational data suggest that sacroiliac joint radiofrequency ablation can provide significant and durable relief for sacroiliac origin pain, particularly when lesion strategies encompass the L5 dorsal ramus and the lateral branches from S1 to S3 that form the PSN. A prospective observational study using a bipolar strip lesion technique spanning from the L5 dorsal ramus to the lateral branches of S3 reported that half of the treated patients achieved more than 50% pain relief at 12 months, indicating that extensive capture of the PSN and related branches can translate into sustained clinical benefit. Meta-analyses of sacroiliac joint radiofrequency ablation, which include techniques designed to maximize PSN capture, have also demonstrated significant improvements in pain and disability for up to 12 months post-procedure.
The potential benefits of more complete PSN ablation extend beyond pain intensity to functional outcomes and health care utilization. Sacroiliac joint radiofrequency ablation has been associated with improvements in function and reductions in health care use, including decreased reliance on opioid prescriptions in some cohorts. In the context of ongoing clinical trials, such as randomized comparisons of bipolar strip lesion techniques versus conventional monopolar periforaminal methods, investigators aim to determine whether more complete PSN ablation leads to higher proportions of patients achieving clinically meaningful reductions in pain, disability, and pain-related sleep disturbance, as well as favorable changes in patient-reported global impression of change and quality-of-life measures. These efforts reflect a growing emphasis on optimizing lesion completeness as a strategy to enhance the therapeutic impact of sacroiliac joint radiofrequency ablation.
Implications for clinical practice
The anatomical characterization of the PSN and its central role in posterior sacroiliac joint complex innervation have direct implications for clinical practice. Understanding that the PSN is a periosteal plexus extending primarily between the second and third transverse sacral tubercles along the lateral sacral crest informs both diagnostic and therapeutic targeting. Diagnostic blocks and radiofrequency lesions delivered at the level of the periosteum in this region are more likely to capture the posterior sacroiliac innervation than injections or lesions placed at other depths or locations. This anatomical precision supports the use of standardized landmark-based approaches under fluoroscopy or ultrasound to improve the consistency and effectiveness of PSN-directed interventions.
From a procedural standpoint, the choice between periforaminal and strip-lesion techniques, as well as between monopolar and bipolar or multipolar technologies, should be informed by the goal of maximizing PSN capture while maintaining procedural safety. Cadaveric and modeling data indicate that bipolar strip lesion configurations, including palisade and lateral crest techniques, achieve higher rates of lateral branch and PSN capture than conventional monopolar periforaminal methods. At the same time, technical factors such as tip spacing, tip offset, and the undulating contour of the dorsal sacrum can influence lesion morphology and completeness, suggesting that meticulous attention to needle placement and alignment is essential. Clinicians should be aware that moving strip lesions slightly medial to the lateral sacral crest, while remaining lateral to the posterior sacral foramina, may place needles on a smoother sacral surface and reduce tip offset, potentially improving lesion continuity.
Patient selection remains a critical determinant of outcomes in PSN-targeted radiofrequency ablation. The use of prognostic PSN blocks and diagnostic intra-articular sacroiliac joint injections as part of a structured diagnostic work-up is standard practice in many interventional protocols and clinical trials. These diagnostic procedures help identify patients with sacroiliac joint complex pain who are more likely to benefit from PSN ablation, and they provide a framework for evaluating the causal relationship between the intervention and subsequent changes in pain, function, and health care utilization. Given that sacral lateral branch neurotomy addresses posterior sacroiliac structures, clinicians should recognize that responses to intra-articular injections alone may not fully predict outcomes from PSN-directed radiofrequency procedures.
Ongoing and planned randomized controlled trials comparing novel bipolar strip lesion techniques with conventional monopolar periforaminal methods are expected to further clarify the clinical significance of more complete PSN ablation. These studies are designed to compare proportions of patients achieving predefined thresholds of pain relief, disability reduction, and quality-of-life improvement, as well as to assess procedural time, adverse effects, and subsequent interventional health care utilization, including repeat radiofrequency ablation and surgical fusion. The results may influence future treatment paradigms by encouraging more standardized approaches to PSN neurotomy and by informing evidence-based selection of radiofrequency technologies and lesioning strategies in the management of sacroiliac joint complex pain.
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