Inclusion Criteria:
* To be eligible to participate in this study, a subject must meet ALL of the following criteria, as determined by the Investigator:
1. Age and Body Mass Index (BMI)
1. Adult subjects aged ≥ 21 years at screening.
2. BMI ≤ 40.
2. Chronic SIJ-Related Pain
1. Has experienced low back and/or buttock pain for at least 6 months that has been inadequately responsive to non-surgical management.
2. Has experienced nonradicular pain that is maximal below the L5 vertebrae, localized over the posterior SIJ, and consistent with SIJ pain.
3. Diagnosis of SIJ Dysfunction
a. Has a diagnosis of SIJ dysfunction (degenerative sacroiliitis or SIJ disruption) based on ALL of the following: i. A positive Fortin's finger test; and ii. Pain elicited at least three (3) of five (5) SIJ-specific physical examination maneuvers (FABER test, Gaenslen's test, Distraction, Thigh Thrust, Compression); and iii. At least 75% reduction in pain, with associated improvement in previously painful maneuvers, for the expected duration of action following two (2) image-guided, contrast-enhanced intra-articular diagnostic SIJ injections, performed on separate visits using anesthetics with different durations of action; and iv. Completion of at least one (1) therapeutic intra-articular SIJ injection (e.g., corticosteroid injection) within the past 12 months.
4. Baseline Disability and Pain Severity
a. Baseline Oswestry Disability Index (ODI) score ≥30%; and b. Baseline SI joint pain score of ≥60 mm on a 100 mm Visual Analog Scale (VAS).
5. Nicotine Abstinence
a. Is able and willing to completely avoid all forms of nicotine (including smoking, vaping, e-cigarettes, tobacco, or nicotine pouches) for the entire duration of the study.
6. Treatment Decision
a. The treating physician has independently determined that the LINQ procedure is an appropriate treatment option for the subject's SIJ dysfunction in the course of routine clinical care, and the subject has agreed to undergo this treatment.
7. Informed Consent and Compliance
1. Is willing and capable of providing written informed consent; and
2. Is willing and able to comply with all study-related requirements, procedures, and follow-up visits.
Exclusion Criteria:
* Exclusion Criteria Subjects must meet none of the following criteria to be eligible for participation in this study, as determined by the Investigator:
1. Pain Source Uncertainty a. Inability to confirm, in the Investigator's clinical judgment, that the subject's primary pain generator is the sacroiliac joint.
2. Alternative Primary Spine Pathology
a. Presence of severe low back pain attributable to other spinal pathology, including but not limited to lumbar disc degeneration, lumbar disc herniation, lumbar spondylolisthesis, lumbar spinal stenosis, lumbar facet degeneration, or lumbar vertebral body fracture.
3. Other Sacroiliac Pathology
a. Presence of known sacroiliac joint pathology other than degenerative SIJ dysfunction, including inflammatory sacroiliitis (e.g., ankylosing spondylitis or other HLA-associated spondyloarthropathies), tumor, infection, acute fracture, or crystal arthropathy.
4. Non-SIJ Pain Syndromes
a. History of coccydynia or prior coccygectomy, endometriosis, cluneal neuralgia, or pudendal neuralgia.
5. Recent Sacral Interventions
a. Prior sacral radiofrequency ablation performed within 6 months prior to enrollment.
6. Recent Pelvic Trauma
a. History of major pelvic trauma within 12 months prior to enrollment.
7. Severe Osteoporosis
a. Prior diagnosis or suspicion of severe osteoporosis, defined as a prior bone mineral density T-score of \< -2.5 or a history of osteoporotic fracture.
8. Medications Detrimental to Bone or Soft-Tissue Healing
a. Current use of medications known to adversely affect bone quality or soft-tissue healing, as determined by the Investigator.
9. Anatomic Constraints
a. Anatomic anomalies or defects that, in the Investigator's opinion, would preclude safe or biomechanically appropriate device placement.
10. Systemic Inflammatory or Rheumatologic Disease
a. Presence of a chronic inflammatory rheumatologic condition, such as rheumatoid arthritis.
11. Centralized Pain Syndromes
a. Current diagnosis of fibromyalgia.
12. Infection Risk
a. Evidence of current local or systemic infection that would increase surgical risk.
13. High-Dose Opioid Therapy (≥90 MME/day)
a. Current use of systemic opioid analgesics at Screening/Baseline with an average daily dose ≥90 morphine milligram equivalents (MME/day), based on medication reconciliation (prescription records and subject report) and standard MME conversion factors.
14. Current Nicotine Use (Self-Reported)
a. Self-reported use of any nicotine-containing products (including smoking, vaping/e-cigarettes, cigars, pipes, smokeless tobacco, nicotine pouches, and nicotine replacement products) within 6 weeks prior to Screening/Baseline.
15. Secondary Gain or Litigation
a. Currently receiving or actively seeking workers' compensation or disability benefits or involved in injury-related litigation.
16. Pregnancy
a. Currently pregnant or planning to become pregnant within two (2) years following enrollment, as self-reported.
17. Vulnerable Populations
a. Individuals who are prisoners or wards of the state.
18. Substance Abuse
a. Known or suspected active drug or alcohol abuse, including opioid misuse.
19. Psychiatric Conditions
a. Diagnosed psychiatric disorder (e.g., schizophrenia, major depressive disorder, personality disorder) that, in the Investigator's judgment, could interfere with study participation.
20. Concurrent Interventional Research
a. Current participation in another clinical trial.
21. Neurologic Conditions Affecting Rehabilitation
a. Presence of a significant neurologic disorder that would interfere with participation in physical therapy or post procedural rehabilitation.
22. Other Confounding Medical Conditions a. Any other medical condition or pain condition not intended to be treated in this study that, in the Investigator's judgment, could interfere with study procedures, accurate pain reporting, or interpretation of study endpoints.
