The pain that appears in the dimples of the lower back
The sacroiliac joint (SI) connects the sacrum — the triangular bone at the base of the spine — to the ilia of the pelvis. It is a joint with minimal motion (only 2-4 degrees of rotation), but one that supports an extraordinary load: all the force of transmission between the trunk and the lower limbs passes through it. When dysfunctional, it generates a deep and uncomfortable pain exactly at the "dimples of Venus" (the small pits over the posterior superior iliac spines), which worsens with walking, climbing stairs, turning in bed, or standing on one leg.
It is estimated that 15 to 30% of all low back pain has the sacroiliac joint as the primary or significantly contributing source. However, it is one of the most underdiagnosed causes of low back pain: most patients receive the generic label of "mechanical low back pain" without the SI being specifically tested. Lumbar spine MRI — the most requested exam — does not adequately evaluate the sacroiliac joint, and the patient spends years treating the spine while the true source of pain remains overlooked.
Medical acupuncture with periarticular electroacupuncture has proved a valuable tool for sacroiliac dysfunction, acting on modulation of local inflammation, deactivation of trigger points in the pelvic stabilizing muscles (multifidus, gluteus medius, piriformis), and restoration of regional neuromuscular control. See also our guide on stabbing pain in the middle of the buttock, which frequently coexists with sacroiliac dysfunction.
How sacroiliac dysfunction develops
Load asymmetry and repetitive microtrauma
Lower-limb length difference, functional scoliosis, pregnancy, or asymmetric activities (such as always running on the same track) create chronic unilateral overload on the SI joint. The joint capsule and ligaments suffer repetitive microinjuries, generating low-grade periarticular inflammation.
Insufficiency of the stabilizer system
The deep multifidus and the gluteus medius are the main dynamic stabilizers of the pelvis. When inhibited by pain, sedentary lifestyle, or postpartum, the SI joint loses its "muscular locking" and becomes hypermobile — each step generates excessive micromovement and joint irritation.
Trigger points in the pelvic stabilizers
The overloaded muscles (multifidus, gluteus medius, piriformis, quadratus lumborum) develop trigger points that refer pain to the sacroiliac region, amplifying and perpetuating the pain. The piriformis, in particular, crosses directly over the SI joint and may compress the adjacent sciatic nerve.
Sensitization of the sacral dorsal branches
Persistent periarticular inflammation sensitizes the L4-S3 dorsal branches, lowering the pain threshold and creating a state of regional hyperalgesia. Previously painless movements — such as turning in bed or rising from a chair — begin to generate intense pain.
Antalgic gait pattern and vicious cycle
The patient adopts a protective gait with shortened stride and asymmetric load transfer, which overloads the contralateral side and perpetuates bilateral dysfunction. The resulting inactivity further weakens the stabilizer system, closing the cycle.
Clinical data on sacroiliac dysfunction
Recognizing sacroiliac pain
Clinical pattern of sacroiliac dysfunction
- 01
Pain localized at the "dimple" (pit) over the posterior superior iliac spine — the patient points with one finger
- 02
Pain when turning in bed, especially when rolling from one side to the other
- 03
Worsens with climbing stairs, getting in or out of the car
- 04
Pain when standing on one leg (Stork sign)
- 05
Pain that may radiate to the buttock, groin, or posterior thigh — but does not pass the knee
- 06
Short-lived morning stiffness (< 30 minutes) that improves with movement
- 07
Difficulty sitting for prolonged periods, especially on hard surfaces
- 08
Improvement with use of a pelvic belt or manual compression over the ilia
Myths about sacroiliac pain
Myth vs. Fact
Lumbar spine MRI investigates the sacroiliac joint
Conventional lumbar spine MRI focuses on the discs and vertebrae and generally does not include adequate cuts of the SI joint. A specific MRI of the sacroiliac joint or pelvis is necessary to evaluate joint inflammation (sacroiliitis). In addition, mechanical SI dysfunction — the most common cause — frequently does not show changes on imaging, with diagnosis being essentially clinical (by provocation tests).
Sacroiliac pain is the same as sciatic pain
Sacroiliac pain may radiate to the buttock and posterior thigh, mimicking sciatica. However, true sciatic pain (from lumbar nerve root compression) typically passes the knee, follows a specific dermatome, and may cause neurologic déficit (weakness, loss of reflex). SI pain is more diffuse, generally does not pass the knee, and does not cause neurologic déficit. The straight leg raise test is negative in pure sacroiliac pain.
Sacral pain in young adults is always muscular
Ankylosing spondylitis — an inflammatory rheumatic disease — classically manifests as bilateral sacroiliitis in young adults (18-35 years). The cardinal symptom is low back/sacral pain with prolonged morning stiffness (> 30 minutes) that improves with exercise and worsens with rest. When a young patient presents this pattern, the physician should request HLA-B27, CRP, and MRI of the sacroiliac joint to exclude inflammatory cause before attributing the pain to a mechanical cause.
The joint everyone forgets to examine
Treatment protocol
Assessment and specific diagnosis
1st visitBattery of sacroiliac provocation tests (FABER, Gaenslen, compression, distraction, thrust). Stork test for instability. Assessment of pelvic asymmetry, limb length, and gluteus medius strength. If inflammatory pattern (morning stiffness > 30 min, young, improvement with exercise): order HLA-B27, CRP, and SI MRI.
Periarticular and trigger-point deactivation
Sessions 1–4Periarticular needling of the SI joint at the dorsal branches of L5-S2 with electroacupuncture 2 Hz. Dry needling of the deep multifidus (L4-S1), piriformis, and gluteus medius — the three muscles that most contribute to sacroiliac pain. Guidance on temporary use of a pelvic belt during activities that provoke pain.
Active stabilization and progression
Sessions 4–8Progressive introduction of pelvic stabilization exercises: activation of the transversus abdominis, bridge with gluteus activation, single-leg support exercises. Acupuncture continues with focus on the quadratus lumborum and tensor fasciae latae when they have associated trigger points. Also assess the relationship with low back pain on rising from a chair, a pattern frequent in these patients.
Autonomy and prevention of recurrence
Sessions 8–10+Home program of pelvic stabilization exercises and gluteus medius strengthening. Progressive spacing of sessions. Correction of postural asymmetries (insole if limb difference, ergonomic adjustment). Monthly maintenance sessions in the first 3 months for consolidation.
Clinical pearl: the Fortin test
Frequently asked questions
Frequently Asked Questions
Sacroiliac pain is typically localized in the region of the lumbar "dimples" (pits over the PSIS), worsens with asymmetric load activities (climbing stairs, turning in bed, supporting on one leg), and generally does not pass the knee. Discogenic pain tends to be more central, worsens with trunk flexion, and may radiate below the knee with a radicular pattern. The physician differentiates through specific provocation tests — at least 3 positive tests for the SI are required to confirm the origin.
Sacroiliac pain is extremely common in pregnancy (up to 40% of pregnant women in the third trimester) due to ligamentous laxity from relaxin and the change in center of gravity. Medical acupuncture is considered safe in pregnancy when performed by an experienced physician, avoiding contraindicated points. Periarticular electroacupuncture and dry needling of the gluteus medius and piriformis can offer significant relief in a period when pharmacologic options are limited.
MRI of the SI is essential when there is suspicion of inflammatory sacroiliitis — especially in young adults with prolonged morning stiffness. For mechanical SI dysfunction (the most common cause), MRI is frequently normal, since the problem is functional and not structural. In these cases, diagnosis is clinical, made by provocation tests performed by the physician. Ordering lumbar spine MRI to investigate sacroiliac pain is a frequent error — the cuts do not adequately include the SI joint.
In the acute and inflammatory phase, high-impact exercises or asymmetric load can worsen the pain. However, strengthening of the pelvic stabilizers (gluteus medius, multifidus, transversus abdominis) is fundamental for medium- and long-term treatment. The key is gradual progression: begin with low-load isometric activation after pain control with acupuncture, and progress to functional exercises. Avoid high-impact unilateral exercises (running, jumping) until adequate pelvic stabilization.