If you have symptoms of back and leg pain, which part of the body is the problem? I believe that the answer of many people is a herniated lumbar disc. It is true that lumbar disc herniation is a common cause of low back and leg pain, but it is not the only cause. There is growing evidence that alterations in the pathological features of the sacroiliac joint are an important factor in causing lower back pain. Pain from the sacroiliac joint occurs frequently, but if the features are not known, it is highly likely to be missed because the pain is similar in extent to that of lumbar spondylolisthesis. Therefore, identification of whether the pain originates from the sacroiliac joint is the key to successful treatment.
I. Anatomy and its function
1.Anatomy
The sacroiliac joint is located in the lower part of the central part of the body and belongs to the basal structure of the spine, which is the joint that bears the greatest gravity in the body. The sacroiliac joint is formed by connecting the two sides of the sacrum with the inner posterior edge of the upper part of the ilium, that is, the auricular joint surface.
The sacroiliac joint is an important structure that supports the load of the human trunk and completes mechanical transmission. Gravity falling on the lumbosacral junction is transmitted to the hip via the sacrum and sacroiliac joint. When standing, gravity is transmitted to the free lower limb bones through the acetabulum; when sitting, it is transmitted to the sciatic tuberosity.
2.Functional operation
The sacroiliac joint is both a micro-movement joint and a weight-bearing joint. Because there are several strong ligaments and many muscles around the sacroiliac joint, it strengthens the stability of the sacroiliac joint and pelvis to adapt to the needs of the functional activities of the spine, making it the central hub of the human body to bear gravity and resolve gravity and bear the rebound force of the lower limbs. In addition, the human body’s latissimus dorsi, posterior thoracolumbar fascia and the opposite gluteus maximus muscles cross to form the stabilization system of the pelvis, where the sacroiliac muscle plays the role of increasing the tension of the posterior thoracolumbar fascia, thus enhancing the stability of the pelvis.
The sacroiliac joint itself allows a slight misalignment, but its main function is still stabilization. In the physiological state it is mainly subjected to pressure in the vertical and anterior-posterior directions. Since the sacral vertebrae are wide at the top and narrow at the bottom, they are inserted in a wedge shape between the iliac bones on both sides, so the greater the weight bearing, the closer the sacroiliac joint contact, which is the “self-locking phenomenon” of the sacroiliac joint. This tight junction between the sacrum and the pelvis creates a solid base for the entire spine. However, when you sit down, it is no longer as stable, because the sacrum is no longer fully wedged into the pelvis. This is why patients with sacroiliac pain may feel more comfortable standing.
II. Clinical signs and diagnosis
1.Clinical manifestations
Patients with acute injury of sacroiliac joint have the pelvis tilted to the healthy side, scoliosis, protective “crooked hip limp” gait, and cannot hold the chest and straight back. Since trunk rotation mainly occurs in the thoracic spine, the trunk rotation function is less affected; the lower extremities on both sides can appear unequal (long and short legs), and asymmetric internal rotation or valgus (yin and yang feet); the posterior superior iliac spine, posterior inferior iliac spine and other bony signs on both sides can appear asymmetric, and there can be local pressure pain and percussion pain. Sometimes striae can be palpated. In patients with chronic injury, their protective gait may not be obvious. However, the above signs can still be detected on physical examination, and patients with their chronic injuries will often have complaints of joint rest pain.
2.Pathological mechanism
When the pelvis and the sacrum move in opposite directions, the sacroiliac joint, the link between the two, is subjected to considerable pressure and pain ensues. The cause may be an accident, a sudden movement, poor standing and sitting posture, and sleeping habits.
Women are 8-10 times more likely to experience sacroiliac pain than men, mainly due to the biological structure and hormonal differences between the two sexes. Another point is that the wide hips of women also increase the instability of the sacroiliac in daily activities. It can be said that the larger the hips, the greater the torque on the sacroiliac joint.
3.Difference from lumbar synostosis
Most of the hip pain in sacroiliac joint pain has a common conscious pain range, which is about 75px outward from the posterior superior iliac spine and spreads towards the head and tail. By paying attention to this pain range, it can be distinguished from low back and hip pain caused mainly by lumbar spine pathology. Moreover, the pain originates more from the posterior ligamentous area outside the joint cavity than from within the joint cavity.
Typical differences between lumbar disc herniation and sacroiliac joint strain
In sacroiliac joint strain, there is no significant radiating pain and no changes in sensation, muscle strength or reflexes compared to lumbar disc herniation.
In sacroiliac joint strain, the pressure pain is in the sacroiliac joint and not in the paraspinal process.
The pain symptoms of lumbar disc herniation could be relieved after rest, and the pain symptoms of sacroiliac joint strain were not significantly relieved after rest.
III. Conclusion
The sacroiliac joint is an important cause of lower back pain, but due to the structural complexity of the sacroiliac joint, there are still some difficulties in its diagnosis in the clinic. Detailed clinical history, careful physical examination, including various active or passive excitation tests, combined with imaging analysis, are the methods to improve the efficiency of sacroiliac joint pain diagnosis.