The experience of pain in the lower back that travels into the leg is common. Because symptoms overlap significantly in the region of the buttocks and upper leg, two distinct conditions—sacroiliac (SI) joint pain and sciatica—are frequently mistaken for one another. While both can cause substantial discomfort in the same general area, they arise from completely different structures and require targeted approaches for effective resolution. Understanding the specific origins and characteristics of each condition is the first step toward accurate diagnosis and appropriate care.
Defining Sacroiliac Joint Pain
Sacroiliac joint pain originates directly from the joint where the spine meets the pelvis. This joint, located on either side of the lower back, connects the sacrum, a triangular bone at the base of the spine, to the ilium, the large wing-shaped bones of the pelvis. The SI joints function to bear the weight of the upper body, distributing the load to the legs, and also provide stability and shock absorption during movement. Pain arises when the joint experiences inflammation (sacroiliitis) or dysfunction due to mechanical instability or stiffness. Causes include traumatic injury, hormonal changes during pregnancy, or degenerative arthritis. The resulting discomfort is typically a deep, aching pain localized to the lower back and buttock region, often on one side, which may refer downward into the groin, hip, or upper thigh, but usually does not extend past the knee.
Understanding Sciatica
Sciatica is not a diagnosis itself but rather a descriptive term for symptoms caused by the compression or irritation of the sciatic nerve. The sciatic nerve is the longest and thickest nerve in the body, formed by nerve roots emerging from the lower lumbar and sacral spine. It travels from the lower back through the hip and buttock, and then branches down the back of each leg, providing motor and sensory function. The pain associated with sciatica is nerve-related and follows the specific path of this nerve. The most common causes of compression are a herniated disc, which pushes on the nerve root, or spinal stenosis, a narrowing of the spinal canal. Other causes include bone spurs, vertebral misalignment, or piriformis syndrome. The characteristic sensation is a sharp, shooting, or electrical pain that radiates down the leg, often accompanied by numbness, tingling, or muscle weakness in the affected limb.
Differentiating Pain Patterns and Causes
SI joint pain is a mechanical or inflammatory issue within the joint itself, whereas sciatica is a neurological issue caused by nerve compression. SI joint pain is typically described as a dull ache or deep, localized discomfort centered in the buttock and lower back. While it can radiate, this referred pain usually remains above the knee and rarely involves the foot.
Sciatica, conversely, presents as a sharp, burning, or “electric shock” sensation that distinctly follows the nerve’s pathway down the back of the leg, frequently extending below the knee and into the foot or toes. The movements that aggravate the pain also differ significantly. SI joint pain is often triggered by transitional movements like standing up from a chair or putting weight onto one leg. Sciatica pain is more commonly aggravated by actions that increase pressure on the spinal nerve roots, such as prolonged sitting, bending forward, coughing, or sneezing. True sciatica often involves neurological signs like weakness or changes in reflexes, which are generally absent in isolated SI joint pain.
How Professionals Determine the Source
Clinicians rely on a methodical approach combining patient history, physical examination, and targeted testing to distinguish between the two conditions. The physical exam includes specific provocation tests designed to stress either the joint or the nerve. For suspected SI joint pain, tests like the Thigh Thrust, Distraction, or FABER test are used to apply pressure to the joint; a positive result reproduces the patient’s familiar pain. In contrast, the Straight Leg Raise test is a classic maneuver for sciatica; pain shooting down the leg below the knee suggests tension on the sciatic nerve. Imaging studies such as X-rays or MRI scans are often used to rule out other spinal causes, like a herniated disc or spinal stenosis. The most definitive diagnostic tool for SI joint pain is a fluoroscopically guided diagnostic injection: if injecting a local anesthetic directly into the SI joint provides a significant, temporary reduction in pain, the joint is confirmed as the pain source.