The sacroiliac (SI) joint connects the sacrum, the triangular bone at the base of the spine, to the ilium, the large bone of the pelvis. Reinforced by a network of powerful ligaments, its primary function is transferring the load and forces generated by the upper body to the legs. The SI joint allows only a minimal amount of movement, which helps absorb shock during movement. SI joint pain is a frequent source of discomfort in the lower back and buttock region. Because its symptoms mimic other conditions, a systematic approach is necessary to pinpoint the SI joint as the precise source of a patient’s pain.
Recognizing the Signs of SI Joint Dysfunction
The trademark symptom of SI joint dysfunction is pain concentrated in the buttock or gluteal region, typically felt on one side of the body. This unilateral pain pattern helps distinguish it from more generalized low back pain or discomfort related to the lumbar spine.
The pain frequently radiates beyond the immediate joint area, often following referral patterns into the groin, the back of the thigh, or occasionally extending down the leg below the knee. This radiating pain can sometimes be mistaken for sciatica, which is caused by nerve compression in the lumbar spine.
Certain movements consistently provoke the pain because they place mechanical stress directly on the joint. Patients commonly report increased discomfort when sitting for extended periods or when transitioning from a seated to a standing position. Activities that involve single-leg loading, such as climbing stairs or running, also tend to aggravate the joint, suggesting an issue with the joint’s load-bearing stability.
Physical Examination and Provocation Tests
After the patient’s history suggests SI joint involvement, a clinician performs a physical examination using specific maneuvers known as provocation tests. These tests are designed to stress the joint and reproduce the patient’s familiar pain. The goal is to determine if the SI joint capsule and its supporting ligaments are the source of the pain, and a positive diagnosis is supported when three or more tests successfully elicit the patient’s discomfort.
The following tests are commonly used to isolate the SI joint:
- Thigh Thrust: Applies a posterior shear force to the SI joint while the patient lies supine with the hip flexed.
- Compression Test: Performed with the patient lying on their side, applying downward pressure on the upper iliac crest to compress the joint.
- Distraction Test: Applies tensile force across the joint’s anterior ligaments by pushing the anterior superior iliac spines outward.
- FABER Test (Flexion, Abduction, and External Rotation): Stresses the anterior aspect of the joint by positioning the affected leg across the opposite knee.
Collectively, these tests provide high confidence in isolating the SI joint as the pain generator.
Role of Imaging and Differential Diagnosis
While the physical exam strongly suggests SI joint dysfunction, imaging studies play a supporting role in the diagnostic process. Standard imaging, such as X-rays and Computed Tomography (CT) scans, is primarily used to identify gross structural issues like fractures, advanced degenerative changes, or arthritis within the joint. X-rays can reveal joint space narrowing or bone spurs, while CT scans can show evidence of joint erosion.
In cases of SI joint dysfunction where the pain is functional rather than structural, these images often appear normal. Magnetic Resonance Imaging (MRI) is more sensitive for detecting soft tissue issues and early inflammatory signs, such as bone marrow edema. However, these findings are not specific to SI joint dysfunction and can be seen in healthy individuals or those with other conditions.
The main function of imaging is differential diagnosis, which means ruling out other conditions that mimic SI joint pain. These include lumbar disc herniation, spinal stenosis, hip joint pathology, tumors, or infections. By excluding these potential sources of pain, the clinician can focus with greater certainty on the SI joint.
Confirming the Diagnosis with Targeted Injections
The definitive step in diagnosing SI joint pain is the targeted diagnostic injection, often referred to as an SI joint block. This procedure functions as the “litmus test” because it directly addresses the suspected source of the pain by injecting local anesthetic into the joint space. To ensure the medication reaches the precise location, the procedure is performed under strict image guidance, typically using fluoroscopy. A contrast dye is often injected first to confirm correct needle placement before the anesthetic is administered.
A positive diagnostic injection requires a substantial reduction in the patient’s pain, commonly defined as a 50% to 75% decrease in discomfort immediately following the procedure. This relief must be temporary, lasting only for the duration of the anesthetic, which confirms that the injected joint was the source of the pain. This diagnostic injection, using anesthetic alone, is distinct from a therapeutic injection, which adds a corticosteroid for longer-term relief.