Can the SI Joint Cause Knee Pain?

The sacroiliac (SI) joint can cause knee pain, though it does so through indirect pathways that often surprise both patients and clinicians. In a study of 50 patients with confirmed SI joint pain, half reported pain radiating into the lower extremity, and roughly 28% described pain traveling below the knee.¹ The connection runs through shared nerve roots, altered muscle activation, and changes in how the pelvis transfers load to the legs. Understanding these mechanisms matters because knee pain that actually originates at the SI joint will not improve with treatments aimed at the knee itself.

How SI Joint Pain Travels to the Knee

The SI joint sits at the base of the spine where the sacrum meets the ilium, and it shares nerve supply with structures that extend well into the leg. When the joint becomes inflamed or dysfunctional, pain signals can travel along these shared nerve pathways and show up in areas far from the joint itself. Clinicians call this “referred pain,” and the SI joint has an unusually wide referral map. The study mentioned above documented 18 distinct patterns of pain referral from the SI joint, with pain showing up in the buttock, groin, thigh, and areas below the knee including the foot.¹ Younger patients were significantly more likely to report pain reaching below the knee, which may relate to differences in nerve sensitivity or tissue compliance with age.

This referral pattern can closely mimic sciatica, which is leg pain caused by compression of a spinal nerve root. A study comparing patients with SI joint-related leg pain to those with lumbar radiculopathy found that the SI joint can produce symptoms clinically indistinguishable from a nerve-root problem.² The patients with SI joint-origin leg pain were more often female, had a shorter history of symptoms, and more frequently reported a fall onto the buttocks as the triggering event. Groin pain was also more common in the SI group, which can serve as a useful clue when trying to sort out where the pain is actually coming from.

At least one published case report describes a patient whose only complaint was isolated knee pain, with no buttock or back symptoms at all, and the cause turned out to be SI joint dysfunction. The case highlights a concept called regional interdependence: a problem in one part of the body producing symptoms entirely in another.³ This is an extreme example, but it illustrates why clinicians increasingly check the SI joint in patients whose knee pain does not respond to standard knee-focused treatments.

Quadriceps Inhibition and Anterior Knee Pain

Beyond referred pain, the SI joint affects the knee through a more mechanical pathway: it can shut down the muscles that stabilize the kneecap. Research has shown that patients with anterior knee pain frequently have measurable SI joint dysfunction, and that this dysfunction contributes to inhibition of the quadriceps muscles. In a study using a technique called the interpolated twitch method to measure muscle activation, patients with anterior knee pain and SI joint dysfunction showed substantial quadriceps inhibition in both legs, though worse on the affected side. After SI joint manipulation, muscle inhibition decreased and knee extensor strength increased, particularly in the involved leg.⁴

This finding has practical implications. Anterior knee pain, the dull ache around or behind the kneecap that gets worse with stairs and prolonged sitting, is one of the most common musculoskeletal complaints. Standard treatment focuses on strengthening the quadriceps. But if the quad weakness is being driven by a dysfunctional SI joint, strengthening exercises alone may not solve the problem because the nervous system is actively suppressing the muscle’s ability to fire fully. Addressing the SI joint first can remove that neurological brake and allow the quadriceps to respond to strengthening work.

The Biomechanical Chain from Pelvis to Knee

The pelvis is the foundation for the entire lower limb. The major muscles that control the knee, including the quadriceps, hamstrings, and the iliotibial band, all originate from or cross through the pelvis. When the SI joint is unstable or stuck, it changes how the pelvis sits and moves, and those changes cascade downward. Pelvic instability is strongly associated with a valgus shift of the knee under load, meaning the knee collapses inward during activities like running, squatting, or landing from a jump. This inward collapse places abnormal stress on the kneecap, the medial compartment, and the anterior cruciate ligament.⁵

A study on pelvic stabilization exercises demonstrated how directly the pelvis and knee are linked. After a program targeting the gluteus maximus, gluteus medius, and other hip stabilizers, dynamic knee valgus dropped significantly, and maximal isometric force increased in all tested muscles.⁵ The knee improved because the pelvis improved. This is one of the clearest illustrations of why treating the knee in isolation can miss the actual driver of symptoms.

Research on athletes reinforces this link. In a study of 204 athletes, those with SI joint pain were significantly more likely to report a history of lower limb overuse injuries compared to athletes without SI joint complaints.⁶ The association ran in both directions: SI joint dysfunction appeared to increase the risk of downstream injuries, and lower limb injuries may have contributed to SI joint problems through compensatory movement patterns. Athletes with SI joint pain accumulated a disproportionate share of the total injuries reported in the study.

Why SI Joint-Related Knee Pain Is Often Missed

One of the biggest obstacles to identifying the SI joint as a source of knee pain is the diagnostic overlap with lumbar spine problems and hip pathology. All three structures can refer pain to the knee region, and they frequently coexist in the same patient. A case report in the neurosurgical literature described a patient with both lumbar radiculopathy and SI joint dysfunction, noting the lack of clarity in distinguishing referred pain from true radicular pain. The underlying mechanisms of pain generation are still incompletely understood, making clinical differentiation genuinely difficult.⁷

The hip joint adds another layer of confusion. Hip pathology refers pain to the groin and thigh in the majority of cases, but it can also produce buttock pain in a large percentage of patients and pain below the knee in roughly a fifth.⁸ Direct knee pain from the hip was rare in that study, occurring in only about 2% of cases, but the overlap in buttock and thigh patterns makes it easy to confuse hip and SI joint problems. The practical takeaway is that when knee pain does not fit a clear local diagnosis, examining the lumbar spine, SI joints, and hips as a group is the most reliable approach. Researchers have specifically recommended a thorough physical examination of all three regions with additional imaging to sort out the source.²

Testing for SI Joint Dysfunction

There is no single test that definitively identifies the SI joint as a pain source, but a cluster of physical examination maneuvers performs reasonably well. Five provocative tests are commonly used: the FABER test (where the examiner flexes, abducts, and externally rotates your hip), the compression test, the distraction test, the thigh thrust, and Gaenslen’s test. When at least three of these five are positive, the likelihood that the SI joint is the problem is high. Studies have found that this three-out-of-five cluster has about 91% sensitivity and 78% specificity, and it predicts an 85% chance that a diagnostic injection into the joint will confirm the source.⁹

If you are experiencing knee pain that has not responded to standard treatments, and especially if you also have stiffness or discomfort in your low back or buttock (even mild), asking your clinician to run through these SI joint provocative tests is reasonable. The tests are quick, require no equipment, and can redirect treatment toward the actual source of pain. A diagnostic injection, where a numbing agent is placed directly into the SI joint under imaging guidance, remains the gold standard for confirmation. If the injection temporarily eliminates both the SI area discomfort and the knee symptoms, that is strong evidence the SI joint is the origin.

Treatment Approaches That Address Both Problems

When the SI joint is identified as the driver of knee symptoms, treatment targets the pelvis rather than the knee. Physical therapy is the first-line approach, and specific exercises have been studied for exactly this dual-pain presentation. A case study examining a single-leg lateral oblique decline squat exercise found that it reduced both SI joint pain and knee pain simultaneously, with pain scores dropping to 2 out of 10 for both the buttock and the knee.¹⁰ The exercise also increased anterior pelvic tilt, suggesting it helped restore more normal pelvic alignment.

For patients who do not get enough relief from physical therapy, injections into the SI joint with corticosteroids or local anesthetic can provide temporary but meaningful improvement. In cases where injections help but the relief does not last, radiofrequency denervation, a procedure that uses heat to disrupt the nerves supplying the joint, may offer longer-term relief lasting up to a year.¹¹ Surgical fusion of the SI joint is reserved for severe, refractory cases. In patients who underwent fusion and had radicular-type pain, resolution of leg pain and restoration of neurological function were reported.¹²

The progression from conservative to interventional is important here. Most people with SI joint-related knee pain will improve with targeted physical therapy, especially programs that strengthen the gluteal muscles and restore pelvic stability. The more invasive options exist for a minority whose symptoms persist despite consistent rehabilitation.

Leg Length Discrepancy as a Hidden Contributor

One factor that connects SI joint dysfunction and knee pain and often goes unrecognized is leg length discrepancy. When one leg is even slightly shorter than the other, it changes how weight is distributed through the pelvis with every step. The SI joint on one side absorbs more load, and the knee on the opposite side may compensate through altered mechanics. A study of male secondary school students found that SI joint dysfunction was present in about one in five participants, and its prevalence was significantly higher among those with a real leg length discrepancy of half a centimeter or more.¹³

Shoe inserts can address this from the bottom up. Research on patients with SI joint pain who used shoe inserts during gait training found decreased pain, and the authors attributed this partly to compensation for functional leg length differences, which reduced mechanical stress on the SI joint during walking.¹⁴ If a small leg length discrepancy is contributing to your SI joint dysfunction, and that dysfunction is producing knee symptoms, a simple heel lift or custom orthotic may be part of the solution. It is one of those interventions that costs almost nothing but requires someone to actually check for the problem, which does not always happen in a standard knee evaluation.

When the Direction Runs the Other Way

The relationship between the SI joint and the knee is not always one-directional. Knee injuries can also cause or worsen SI joint dysfunction. After a significant knee injury, people change how they walk. They limp, shift weight to the uninjured side, and alter their stride length. These compensations load the pelvis asymmetrically and can destabilize the SI joint over weeks or months. The athlete injury data described earlier showed that the association between SI joint problems and lower extremity injuries ran in both directions, with overuse injuries of the lower limb strongly associated with SI joint pain.⁶

This bidirectional relationship means that someone recovering from knee surgery or a significant knee injury should have their SI joint monitored as part of rehabilitation. Persistent low back or buttock ache during knee recovery is not always just from sitting around. It may indicate that compensatory gait patterns have created a new problem at the pelvis, which can then feed pain signals back down to the knee and slow recovery. Breaking the cycle usually requires addressing both regions rather than focusing exclusively on the joint that was originally injured.

Red Flags That Suggest the SI Joint Is Involved

Not every case of knee pain warrants an SI joint evaluation. But certain patterns should raise suspicion:

Buttock pain: Even mild or intermittent aching on one side of the buttock, especially if it showed up around the same time as the knee symptoms, points toward the SI joint.
Pain with transitions: If your knee hurts more when getting out of a car, rolling over in bed, or going from sitting to standing, those are loading patterns that stress the SI joint.
Failed knee treatments: Knee bracing, patellofemoral taping, quad strengthening, and even arthroscopy that did not improve symptoms should prompt a look upstream.
History of a fall or impact: Landing on your buttock, a slip on ice, or a hard landing from a height can shift the SI joint and produce symptoms that initially seem unrelated.
Postpartum onset: Hormonal changes during pregnancy loosen the ligaments around the SI joint, and this laxity sometimes persists, producing both pelvic and knee complaints.

None of these individually proves the SI joint is to blame, but two or three together make a strong case for including it in the diagnostic workup. The provocative tests discussed earlier are the fastest way to screen, and they can be performed in any clinical setting without imaging or specialized equipment. If the cluster comes back positive and your knee symptoms improve after an SI joint-directed treatment, you have your answer.

1
PubMed. Sacroiliac joint pain referral zones
2
PubMed Central. Sciatica-like symptoms and the sacroiliac joint: clinical features and differential diagnosis
3
PubMed. Isolated knee pain: a case report highlighting regional interdependence
4
PubMed. Decrease in quadriceps inhibition after sacroiliac joint manipulation in patients with anterior knee pain
5
PubMed Central. Effectiveness of pelvic stabilization exercises on knee valgus, muscle activity, and strength in individuals with dynamic knee valgus
6
PubMed Central. The sacroiliac dysfunction and pain is associated with history of lower extremity sport related injuries
7
PubMed Central. Radiculopathy with concomitant sacroiliac dysfunction and lumbosacral degenerative disease: illustrative case
8
Oxford Academic (Pain Medicine). Hip Joint Pain Referral Patterns: A Descriptive Study
9
PubMed Central. Successful Diagnosis of Sacroiliac Joint Dysfunction
10
PubMed Central. Effect of the single-leg, lateral oblique, decline squat exercise on sacroiliac joint pain with knee pain
11
PubMed. Sacroiliac joint pain: a comprehensive review of epidemiology, diagnosis and treatment
12
PubMed Central. Patient Reported Outcomes from Sacroiliac Joint Fusion
13
PubMed Central. Prevalence of asymptomatic sacroiliac joint dysfunction and its association with leg length discrepancies in male students in selected junior secondary schools in Ibadan
14
PubMed Central. The effect of gait training with shoe inserts on the improvement of pain and gait in sacroiliac joint patients