Sacroiliac Joint Pain: An Underdiagnosed Cause of Chronic Low Back and Buttock Pain
Sacroiliac Joint Pain: An Underdiagnosed and Often Overlooked Cause of Low Back Pain
Low back pain is one of the most common reasons for medical consultation worldwide. While disc pathology and lumbar facet joint pain are frequently considered, the sacroiliac joint (SIJ) remains an underappreciated but important pain generator.
Over the past two decades, improved anatomical understanding, more consistent diagnostic approaches, and modern interventional techniques have brought SIJ pain into sharper focus. At Pain Spa, we commonly assess patients whose persistent low back or buttock pain is ultimately traced to the SI joint complex and/or its surrounding ligamentous structures.
Introduction
The sacroiliac joint may account for a meaningful proportion of chronic non-radicular low back pain. It is frequently underdiagnosed because symptom patterns overlap with other spinal and pelvic conditions, imaging may be normal, and pain referral patterns can be variable.
A structured, mechanism-based assessment helps determine whether symptoms are driven by the SI joint itself, the posterior ligamentous complex, or a combination — while also considering coexisting contributors such as lumbar facet joints, piriformis-related pain, hip pathology, or inflammatory sacroiliitis.
Functional Anatomy of the Sacroiliac Joint
The sacroiliac joint is a true diarthrodial joint, but it is structurally unique. The anterior component is synovial with hyaline cartilage, while the posterior component is predominantly ligamentous. Motion is minimal (only small degrees of rotation and millimetres of translation), yet forces transmitted across the joint during walking, sitting, bending and single-leg loading are substantial.
Key ligamentous structures (often clinically important)
- Anterior sacroiliac ligament
- Interosseous sacroiliac ligament
- Long posterior sacroiliac ligament
- Short posterior sacroiliac ligaments
- Sacrotuberous ligament
- Sacrospinous ligament
The posterior ligamentous complex is richly innervated and may be the dominant pain generator in many patients.
Innervation of the SI Joint Complex (Why Technique Matters)
The nerve supply of the SI joint complex is variable. Posterior innervation is most relevant for denervation procedures and is typically supplied by the lateral branches of S1–S3, with variable contribution from the L5 dorsal ramus and rare contribution from S4. These branches form a fine plexus along the lateral sacral crest (often described as a posterior sacral network).
Because lateral branch exit points and trajectories vary significantly between individuals, lesion coverage strategy is critical when radiofrequency denervation is used.
What Causes SI Joint Pain?
Pain can arise from intra-articular synovial inflammation, degenerative change, posterior ligamentous strain, micro-instability, trauma, pregnancy-related laxity, altered biomechanics after lumbar fusion, or inflammatory disease. Importantly, pain frequently arises from the posterior ligamentous complex rather than cartilage degeneration alone.
Clinical Presentation (Pain Pattern Is Key)
Provocative tests may support suspicion, but no single test is diagnostic in isolation. The clinical history and distribution of pain are central.
Typical features
- Unilateral or bilateral pain typically below L5
- Deep buttock pain, often localised around the PSIS region
- Radiation into the posterior thigh (typically not below the knee)
- Occasional groin or lateral hip referral
- Worse with prolonged sitting, sitting-to-standing transitions, turning in bed, stairs, or single-leg loading
Symptoms may mimic lumbar facet pain, discogenic pain, piriformis-related pain, or hip pathology — which is why careful differentiation is essential.
Inflammatory Sacroiliitis and Spondyloarthropathy (Do Not Miss)
Not all SI-region pain is mechanical. Inflammatory sacroiliitis must be considered, especially in younger patients or those with prominent morning stiffness.
Features that raise suspicion
- Morning stiffness > 30–60 minutes
- Improvement with movement rather than rest
- Night pain (often second half of the night)
- Alternating buttock pain
- History of psoriasis, inflammatory bowel disease, uveitis, or relevant family history
In this context, MRI can be particularly helpful (e.g., bone marrow oedema), and rheumatology input may be appropriate. Management differs significantly from mechanical SI joint dysfunction.
Imaging — Helpful for Excluding Causes, Limited for Mechanical SI Pain
Imaging has limited value in diagnosing mechanical SI joint pain. X-rays and CT may show degenerative change that correlates poorly with symptoms, while MRI is often normal in mechanical SI pain.
Imaging is most valuable to rule out inflammatory sacroiliitis, fracture, infection, tumour, and other significant pathology. Mechanical SI joint pain is primarily a clinical diagnosis that is often confirmed by targeted image-guided injection.
Diagnostic Injections — Diagnostic and Therapeutic
Image-guided injection remains the most reliable method of confirming sacroiliac joint–mediated pain. It is equally important to recognise that these injections are often therapeutic as well as diagnostic.
What a “diagnostic” injection involves
A small volume of local anaesthetic is placed into the suspected pain-generating structure. Depending on clinical suspicion, this may be intra-articular within the SI joint, around the posterior ligamentous complex, or along the sacral lateral branches. Significant short-term relief while the local anaesthetic is active supports the SI joint complex as a dominant pain generator.
Why steroid is commonly added
In clinical practice, a corticosteroid is often added to local anaesthetic. The local anaesthetic helps confirm the diagnosis, while the steroid can reduce synovial and/or ligamentous inflammation and may prolong relief.
How long can benefit last?
Duration varies. Some patients experience relief for a few weeks, others for several months, and in selected cases benefit may extend up to approximately 6 months. Chronicity, biomechanics, coexisting pain generators, and rehabilitation engagement all influence duration.
Why injection response matters for next steps
If an injection produces excellent relief but benefit does not last long, this often indicates the correct pain generator has been identified and that a longer-lasting intervention such as radiofrequency denervation may be appropriate.
Image Guidance — Ultrasound and Fluoroscopy (Joint and Ligaments)
Fluoroscopy (X-ray guidance)
Fluoroscopy is traditionally considered the gold standard for intra-articular SI joint access. Contrast can confirm accurate needle placement and this is particularly useful when formal diagnostic confirmation is required.
Ultrasound
Ultrasound can be particularly helpful when ligamentous or periarticular pain is suspected. It allows real-time soft-tissue assessment, supports targeting of posterior ligamentous structures (including the long posterior ligament complex), and avoids radiation. Ultrasound may also be useful to guide periarticular or posterior joint injections in selected cases.
Key point: Ultrasound and fluoroscopy are complementary. At Pain Spa, the modality and target (joint vs ligaments) are selected based on clinical assessment rather than a one-size-fits-all approach.
SI Joint Denervation and Cooled Radiofrequency
When SI joint injections provide meaningful relief but the benefit is temporary, radiofrequency denervation is often the most appropriate next step. In practical terms: if SI injections are positive but do not last long, denervation is the treatment that aims to provide longer-lasting relief.
What is targeted?
SI denervation targets the posterior sacral network — most commonly the lateral branches of S1, S2 and S3, with variable contribution from the L5 dorsal ramus (and rare contribution from S4). These nerves form a variable plexus rather than a single consistent nerve trunk, which is why technique and lesion coverage matter.
Why small “spot” lesions can fail
Conventional monopolar RF produces relatively small focal lesions. Because lateral branch exit points and trajectories vary substantially, a small lesion may miss relevant branches in some patients.
Bipolar and Quadripolar (Quadruple Strip) Lesion Techniques
Modern SI denervation approaches aim to create a broader, more continuous lesion field along the posterior sacral network to improve consistency of nerve capture.
- Bipolar strip lesioning uses two electrodes so current flows between them, creating a wider, more continuous lesion than a single monopolar lesion.
- Quadripolar (quadruple strip) techniques extend this concept using multiple electrodes in a strip configuration to create an extended lesion field across S1–S3 levels, improving coverage where anatomy is variable.
The aim is not just “burning one spot,” but achieving consistent coverage of the relevant lateral branches that supply the posterior SI joint complex.
Cooled Radiofrequency (Coolief)
Cooled RF uses internal cooling to reduce charring at the tissue interface, allowing more energy delivery and producing a larger lesion volume. This can be particularly advantageous for SI denervation because the target nerves form a variable plexus. In appropriately selected patients, cooled RF may provide durable relief over many months.
Evidence and expected duration
Clinical outcomes depend heavily on patient selection, confirmation with diagnostic blocks, and technical execution. In appropriately selected patients, meaningful relief commonly lasts many months (often 6–12 months or longer), and repeat treatment may be considered if symptoms recur after a strong initial response.
SI Joint Pain After Lumbar Fusion
After lumbar fusion, load transfer across the pelvis changes and stress across the SI joint can increase. In patients with persistent low back pain after fusion, the SI joint is a common pain generator and should be considered early to avoid unnecessary repeat lumbar procedures.
Treatment Pathway Summary
- Clinical suspicion based on history and pain distribution
- Exclude inflammatory causes where indicated (especially with prolonged morning stiffness)
- Image-guided diagnostic/therapeutic injection (local anaesthetic ± steroid)
- If significant but temporary relief → consider SI joint denervation (bipolar/quadripolar and/or cooled RF)
- Rehabilitation to consolidate improved movement tolerance and pelvic stability
The Pain Spa Approach
At Pain Spa, sacroiliac joint pain is assessed using a mechanism-based framework that distinguishes intra-articular SI pain, posterior ligamentous pain, mixed presentations, and coexisting contributors such as lower lumbar facet joints, piriformis-related pain, hip pathology, and inflammatory sacroiliitis where indicated.
Dr Krishna is highly experienced in the full spectrum of SI joint complex interventions, including ultrasound-guided ligament and periarticular injections, fluoroscopy-guided intra-articular diagnostic and therapeutic injections, sacral lateral branch denervation, and advanced radiofrequency strategies including bipolar and quadripolar (quadruple strip) lesion techniques and cooled radiofrequency ablation. Given the anatomical variability of SI innervation and the importance of adequate lesion coverage, procedural experience and careful patient selection significantly influence outcomes.
Conclusion
Sacroiliac joint pain is common, frequently underdiagnosed, and often mistaken for lumbar spine pathology. The posterior ligamentous complex is an important pain generator in many patients. Imaging is most helpful for excluding inflammatory or serious pathology, while diagnostic injection remains a reliable confirmatory step.
Diagnostic injections are often therapeutic (local anaesthetic plus steroid) and can provide relief ranging from a few weeks to several months, occasionally up to around six months. When injections help but do not last long, SI joint denervation — particularly using anatomically informed bipolar/quadripolar strip strategies and/or cooled radiofrequency — offers a rational next step for longer-lasting relief in appropriately selected patients.
Frequently Asked Questions (FAQ)
Can SI joint pain cause sciatica? It can cause posterior thigh pain that mimics sciatica, but true nerve root compression symptoms are different and require separate assessment.
Can MRI miss SI joint pain? Yes. Mechanical SI pain often has normal imaging. MRI is most useful for excluding inflammatory sacroiliitis and other significant pathology.
How long do SI joint injections last? Some patients improve for a few weeks, others for several months, and in selected cases benefit may extend up to around six months (especially when steroid is included and rehabilitation consolidates improvement).
What if an SI joint injection helps but does not last long? This often confirms the diagnosis but indicates the need for a longer-lasting option. SI joint denervation (including bipolar/quadripolar strip techniques and/or cooled RF) is commonly considered in this situation.
How long does SI joint radiofrequency last? Relief commonly lasts many months in appropriately selected patients, often 6–12 months or longer, and repeat treatment may be considered if pain returns after a strong initial response.
Do I need SI joint fusion? Most patients do not. Fusion is reserved for carefully selected refractory cases after conservative and interventional options have been appropriately tried.
To book a consultation, please contact Pain Spa at clinic@painspa.co.uk or visit www.painspa.co.uk for further information.
Medical disclaimer: This article is for education only and does not replace personalised medical advice. If you have rapidly worsening symptoms, fever, unexplained weight loss, significant neurological symptoms, new bladder/bowel disturbance, or other red-flag features, seek urgent medical review.