The sacroiliac (SI) joint connects the sacrum, the triangular bone at the base of the spine, to the pelvis’s iliac bones. Pain originating from this area, known as sacroiliac joint dysfunction or sacroiliitis, is a common cause of persistent low back and buttock pain following Total Hip Arthroplasty (THA). When a hip replacement resolves the original hip pain, the SI joint may emerge as a new source of discomfort due to altered body mechanics. Recognizing this potential complication is the first step in finding appropriate treatment.
How Hip Replacement Alters Biomechanics
Total Hip Arthroplasty fundamentally changes the mechanical forces distributed through the pelvis and lower spine. The surgery aims to restore hip alignment and function, but this alteration can shift stress to adjacent joints, primarily the SI joint. Since the hip, spine, and pelvis function as an integrated system, a change in one area necessitates compensation in the others.
A significant factor is the restoration or introduction of a Leg Length Discrepancy (LLD). While surgeons strive for equal leg lengths, even a minor change can force the pelvis to tilt or rotate, creating abnormal shear forces across the SI joint. This pelvic obliquity forces the ligaments and capsule of the SI joint to manage loads they were not designed to handle, leading to inflammation and pain.
Altered gait patterns and muscle function further contribute to mechanical stress. Before surgery, patients often develop compensatory walking patterns to avoid hip pain, and these habits may persist after THA. The new joint alignment and surgical trauma can also weaken the core and gluteal muscles responsible for stabilizing the pelvis. When these muscles fail to support the pelvis, the SI joint is subjected to excessive movement or strain, leading to dysfunction. Patients with SI joint pain post-THA often exhibit increased motion in the joint when transitioning between standing and sitting, indicating a loss of stability.
Differentiating SI Joint Pain from Other Post-THA Pain
Distinguishing SI joint pain from other sources of discomfort, such as residual hip pain, trochanteric bursitis, or lumbar spine issues, is challenging due to overlapping pain referral patterns. SI joint pain is typically described as a deep, aching sensation centered in the buttock, directly over the joint. This pain may radiate down the back of the thigh, but rarely extends below the knee.
The pain is often aggravated by specific movements that stress the joint, such as transitioning from sitting to standing, standing on one leg, climbing stairs, or sitting for prolonged periods. Hip joint pain is usually focused in the groin or front of the thigh, while lumbar spine pain may include neurological symptoms like numbness or weakness. Clinicians use specific physical examination maneuvers, known as pain provocation tests, to manually stress the SI joint and reproduce the patient’s symptoms.
The definitive method for confirming the SI joint as the source of pain is a diagnostic injection. This procedure involves injecting a local anesthetic directly into the joint under imaging guidance (fluoroscopy or CT). If the patient experiences a significant, temporary reduction in pain immediately following the injection, it confirms the SI joint is the primary pain generator. This objective confirmation is required before pursuing more advanced therapies.
Conservative Treatment Options
Initial management of SI joint pain focuses on non-surgical approaches aimed at reducing inflammation and restoring proper biomechanical function. Physical therapy (PT) is a first-line treatment central to long-term relief. Therapists prescribe exercises to stabilize the pelvis by strengthening the deep core and gluteal muscles. PT also employs manual techniques, such as joint mobilization and muscle energy techniques, to correct subtle positional faults or asymmetry within the pelvis.
Medications manage pain and inflammation in the short term. Nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen or naproxen, reduce inflammation within the joint capsule. Muscle relaxants may be prescribed temporarily to ease surrounding muscle spasms that often accompany joint dysfunction. These medications provide a window of reduced pain, allowing the patient to participate fully in physical therapy and strengthening exercises.
If oral medications and PT are insufficient, image-guided corticosteroid injections are often the next step. A therapeutic injection delivers a corticosteroid, a potent anti-inflammatory agent, directly into the SI joint. The steroid reduces inflammation, providing pain relief lasting several weeks to months, though duration varies widely. For patients whose pain is related to a measurable LLD, a customized shoe lift for the shorter leg can help level the pelvis and reduce abnormal stress on the SI joint.
Advanced Interventional Procedures
When conservative treatments fail to provide lasting relief, and the pain becomes chronic or debilitating, advanced interventional procedures may be considered. These treatments are reserved for patients whose pain is confirmed to originate from the SI joint and is refractory to multiple rounds of injections and physical therapy.
Radiofrequency Ablation (RFA) is a minimally invasive procedure that targets the small nerves transmitting pain signals from the SI joint. Under imaging guidance, a needle delivers radiofrequency energy, creating a small lesion to temporarily interrupt the nerve’s ability to send pain signals. While RFA does not treat the underlying joint problem, it can provide significant pain relief lasting six months to over a year, allowing the patient time to continue strengthening exercises.
Sacroiliac Joint Fusion (arthrodesis) is typically considered a last-resort surgical option for severe pain unresponsive to all other treatments. This procedure involves inserting specialized hardware, such as titanium implants or pins, across the joint to stabilize it permanently. Eliminating motion within the dysfunctional joint eliminates the source of mechanical pain. Fusion is generally performed using minimally invasive techniques and aims to provide a durable, long-term solution for chronic SI joint instability.