Question explored with the scientific record
What causes spasms in left rear section of pelvis?
The short version: the evidence points to the sacroiliac joint and the piriformis muscle as the most likely sources, but the diagnostic tests used to confirm them are surprisingly unreliable.
The evidence you retrieved focuses almost entirely on the sacroiliac (SI) joint and the piriformis muscle as causes of posterior pelvic pain. A 2005 review found SI joint dysfunction is implicated in 10–25% of low back pain cases [5]. A 2018 study of women with chronic pelvic pain found that 45% had piriformis tenderness, compared to only 12% of pain-free controls [17]. The anatomy supports this: the sciatic nerve can run through or around the piriformis in several variant patterns, and one case report found a 1 in 10,000 chance of having bilateral variations that could compress the nerve [2]. An MRI study showed that 75% of S2 nerve roots and 97% of S3 nerve roots actually traverse the piriformis muscle [3], meaning the muscle and nerve are physically intertwined in most people.
But here is the problem. The physical exam tests used to diagnose SI joint dysfunction have poor reliability. A 1999 study found that the standing flexion test had a kappa coefficient of only 0.052 between examiners, barely above chance [13]. A 2011 study of four common palpation tests found all of them had kappa values near zero [9]. This means a clinician's judgment that your SI joint is the problem is not very trustworthy. The gold standard for diagnosis is an intra-articular injection that provides more than 90% relief [5, 12], but that is an invasive procedure, not a first step.
The treatment evidence is mixed. A 2019 randomized trial found that both manipulation and stabilization exercises improved pain and disability, with no significant difference between them [6]. A 2016 trial found that combining acupuncture with physiotherapy gave the best results, reducing pain from a VAS of about 7.4 to 1.2 [10]. A 2020 study of platelet-rich plasma injections showed modest improvement, about a 2-point drop on a 10-point pain scale, with most of the benefit in the first 4 weeks [11]. The 2025 study of surgical fusion was funded by the device manufacturer and showed large pain reductions, but it had no control group and the funding source means the results must be treated as marketing, not independent evidence [15].
My call: the most likely causes are SI joint dysfunction or piriformis syndrome, but the diagnostic tests are too unreliable to be confident without an injection-based confirmation. Start with physical therapy and acupuncture, which have the best evidence and no serious risk. Confidence: moderate for the causes, low for the accuracy of any specific diagnosis without an injection test.
| Condition | Prevalence in pain patients | Diagnostic reliability | Best first treatment |
|---|---|---|---|
| SI joint dysfunction | 10–25% of low back pain [5] | Poor (kappa near 0) [9, 13] | Physical therapy + acupuncture [6, 10] |
| Piriformis syndrome | 45% in chronic pelvic pain [17] | Anatomically plausible [2, 3] | Stretching, manual therapy |
Sources examined 17
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Sciatic Nerve Morphometry in the Gluteal Region in a Nigerian Population: An Anatomical Study
Morphometric measurements of the sciatic nerve and piriformis muscle were quantified in Nigerian cadaveric lower limbs to describe typical dimensions and right–left and gender-related variations.
DOI: 10.23880/jhua-16000171 -
A unique case of bilateral sciatic nerve variation within the gluteal compartment and associated clinical ramifications
This case report documents bilateral variations in the emergence and course of the sciatic nerve within the gluteal compartments of a single elderly female cadaver, describing distinct peroneal/tibial trajectories on each side and highlighting clinical implications for diagnosis…
DOI: 10.4066/AMJ.2015.2266 -
Magnetic resonance imaging of the sacral plexus and piriformis muscles
This study evaluates the anatomical relationships between the piriformis muscle and sacral nerve roots using T1-weighted MRI in 100 patients without clinical suspicion of piriformis syndrome, revealing significant variations in nerve root positioning and common muscle size asymm…
DOI: 10.1007/s00256-008-0486-8 -
Stratificational relationship among the main nerves from the dorsal division of the sacral plexus and the innervation of the piriformis
An anatomical cadaveric study mapping the dorsal division nerves of the sacral plexus and their roots to the piriformis, revealing a stratified, multi-origin innervation pattern for the piriformis and challenging the notion of a single nerve supply.
DOI: 10.1002/ar.1092330417 -
Sacroiliac Joint Dysfunction
A systematic literature review of sacroiliac joint dysfunction as a source of low back and posterior pelvic pain, summarizing epidemiology, diagnostic testing, imaging, nonoperative and operative management, and presenting an evidence-based treatment algorithm emphasizing physic…
DOI: 10.1097/01.ajp.0000131413.07468.8e -
Comparison of manipulation and stabilization exercises in patients with sacroiliac joint dysfunction patients: A randomized clinical trial
This randomized clinical trial compares the effects of manipulation and stabilization exercises on pain and disability in patients with sacroiliac joint dysfunction, finding no significant difference in outcomes between the two treatments.
DOI: 10.1016/j.jbmt.2018.01.014 -
Safety and effectiveness of minimally invasive sacroiliac joint fusion in women with persistent post-partum posterior pelvic girdle pain: 12-month outcomes from a prospective, multi-center trial
This study evaluates the safety and effectiveness of minimally invasive sacroiliac joint fusion in women with persistent postpartum pelvic girdle pain, demonstrating significant improvements in pain, function, and quality of life at 12 months post-surgery.
DOI: 10.1186/s40064-015-1359-y -
Referred pain location depends on the affected section of the sacroiliac joint
This study investigates the relationship between the location of referred pain and specific sections of the sacroiliac joint in patients with sacroiliac joint dysfunction, revealing distinct pain patterns associated with each section.
DOI: 10.1007/s00586-014-3604-4 -
The assessment of pelvic landmarks using palpation: A reliability study of undergraduate students
This study investigates the inter-rater reliability of four traditional static palpation tests for diagnosing sacroiliac joint dysfunction among undergraduate osteopathy students, revealing poor reliability across all tests.
DOI: 10.1016/j.ijosm.2010.10.005 -
Comparison of the efficacy of Acupuncture and Conventional Physiotherapy treatment in the management of SIJ dysfunction- A Randomised controlled trial (RCT)
In a randomized controlled trial of 40 women with sacroiliac joint dysfunction, acupuncture, conventional physiotherapy, and their combination were compared, with acupuncture plus conventional physiotherapy yielding the greatest and most sustained reductions in pain and disabili…
DOI: 10.34058/njmr.v18i2.121 -
Effectiveness of Ultrasound-Guided Platelet-Rich Plasma Injections in Relieving Sacroiliac Joint Dysfunction
A prospective, single-arm study evaluating ultrasound-guided platelet-rich plasma injections for sacroiliac joint dysfunction shows significant short-term improvement in disability and pain, with effects largely in the first 4 weeks and sustained but diminishing benefits up to 6…
DOI: 10.1097/phm.0000000000001389 -
Sacroiliac Joint Dysfunction in the Athlete: Diagnosis and Management
A clinical review outlining sacroiliac joint (SIJ) dysfunction in athletes: its anatomy, presentation, diagnostic strategies (emphasizing multiple positive physical examination maneuvers and selective imaging), and a stepped management approach including physical therapy, inject…
DOI: 10.1249/JSR.0000000000000410 -
Inter-examiner and intra-examiner reliability of the standing flexion test
This study assesses how consistently multiple examiners and the same examiner perform the standing flexion test for sacroiliac joint dysfunction in asymptomatic young adults, finding negligible inter-examiner reliability and only moderate intra-examiner reliability, implying lim…
DOI: 10.1054/math.1999.0173 -
A Comparison of Two Different Methods to Treat Hip Pain in Runners
This study compares the effectiveness of two treatment methods for hip pain in runners, finding that a manipulative technique targeting sacroiliac joint dysfunction significantly reduces pain compared to hip mobilization.
DOI: 10.2519/jospt.1993.17.4.172 -
Minimally Invasive Posterior SI Joint Fusion with a Novel Cortical Allograft: Real-World, Long-Term, Outcomes from a Large, Multisite US Cohort
A multicenter, retrospective real‑world study of 258 patients across six US sites showing that posterior sacroiliac joint fusion with a cortical allograft (LinQ Fusion) provides durable, clinically meaningful pain relief with a favorable safety profile over a mean follow-up of a…
DOI: 10.2147/orr.s538286 -
The Functional Relationship Between the Craniomandibular System, Cervical Spine, and the Sacroiliac Joint: A Preliminary Investigation
This pilot study investigates the functional relationship between the craniomandibular system, cervical spine, and sacroiliac joint, revealing significant dysfunctions in both regions following induced occlusal interference in healthy subjects.
DOI: 10.1080/08869634.2003.11746252 -
Pelvic musculoskeletal dysfunctions in women with and without chronic pelvic pain
This cross-sectional study of 84 Iranian women compared pelvic musculoskeletal dysfunctions between those with chronic pelvic pain (CPP) and pain-free controls, finding higher rates of pelvic asymmetry, sacroiliac-provocation positivity, Carnett's sign, abdominal and pelvic floo…
DOI: 10.1016/j.jbmt.2017.05.001