Sacroiliac Joint: Structure, Function & Diseases

The sacroiliac joint is the connecting link between the pelvis and the spine. Due to the everyday heavy use of this region of the body, it has an important function. However, the high load on the joint also promotes the easy development of painful complaints.

What is the sacroiliac joint?

With sacroiliac joint (also sacroiliac joint,

Sacroiliac joint – abbreviated as ISG, sacroiliac joint) refers to the bilateral articular junction of the sacrum (Os sacrum) and ilium (Os ilium). The joint is located between the 1st and 3rd sacral vertebrae. Since it is surrounded by tight ligaments that severely restrict the range of motion, it is a so-called “amphiarthrosis” (Greek : amphi=around, around and arthros = joint). Occasionally, the ISG is also referred to as a “fake” joint due to its minimal mobility. In addition to the sacroiliac joint, the joints between the tarsal and carpal bones are also considered amphiarthroses. Despite its internally limited flexibility, the sacroiliac joint plays an important role in the overall human musculoskeletal system. This is ultimately where the transmission of forces from the upper body and legs takes place.

Anatomy and structure

The sacrum consists of five intergrown vertebrae and forms the stable base of the spine. It is located above the coccyx, below the lumbar vertebrae. The ilium makes up a large portion of the pelvis, extending from the iliac crest to the hip joint. The L-shaped sacroiliac joint connects the two meeting bony surfaces (facies auricularis) of these bones. As with all joints, they are each covered with articular cartilage (ligamenta sacroiliaca interossea)and connective tissue fibers and are surrounded by strong, supportive ligaments. The sacroiliac joint lies in a narrow joint cavity and has only a small joint space. It is only designed to transmit small forces, and the surrounding ligaments must compensate for strong acting forces and pressure accordingly so as not to overload the joint. The nature of the sacroiliac joint changes throughout life depending on gender. In men, the flexibility of the joint decreases with advancing age – to the point of joint blockage. The reason for this is an increasing irregularity of the bony surfaces, which are still smooth in young people. In women, the sacroiliac joint remains comparatively more mobile, but here, too, signs of wear and tear due to age occur.

Function and tasks

The ilium and sacrum support the complete load of the upper body and are thus heavily used parts of the human musculoskeletal system. The sacroiliac joint, as their junction, thus also has an important function. It helps transfer the weight of the upper body to the lower limbs and is part of the elastic stabilization of the trunk. Supported by tight fibers and ligaments, the sacroiliac joint has almost no range of motion despite its articular function, since it mainly performs force-transmitting functions. The flexibility of the joint is limited to only 1-2° of rotational motion or 2-4 mm of motion in each direction (so-called nutation and counter-nutation). The heavy strain on the lower back requires this tight fixation. Particularly in a sitting position, the joint is subjected to heavy stress in everyday life. Under certain circumstances, a sacroiliac joint can be more mobile than its counterpart on the other side of the body. In addition to anatomical causes, this functional asymmetry can also be caused by excessive one-sided movement stress. In principle, female sacroiliac joints have a higher mobility, since the nature and position of the meeting joint surfaces are more functionally pronounced than in men. This circumstance plays an important role especially during childbirth. As a result of hormonal effects during the birth process, the ligaments that fix the joint soften and thus allow the joint zone to stretch. This increases the diameter of the pelvis and facilitates the passage of the child.

Diseases and complaints

According to research, pathological changes in the sacroiliac joint are responsible for up to 25 percent of lower back pain. People with a predisposition to weak ligaments are particularly at risk for developing disorders in the joint.Overstretched ligaments can lead to overmovement of the joint due to a lack of support function. This favors degenerative changes, such as bony hardening, which is a countermeasure taken by the body to stabilize the joint. Rheumatic complaints also frequently occur in the area of the sacroiliac joint and can sometimes trigger severe pain. In particular, complaints associated with ankylosing spondylitis often occur in this region of the body. Finally, abrupt movements during sports activities or in the context of accidents can lead to lesions or painful complaints in the sacroiliac joint. Often, lifting heavy loads incorrectly or “kicking into the void” also result in injuries to the joint and, under certain circumstances, lead to so-called ISG blockages. Long periods of sitting at work, lack of exercise and untreated foot, knee and hip malpositions can also damage the joint in the long term. Inflammations, strains and other diseases in the area of the sacroiliac joint are expressed by sometimes severe, often unilateral and localized pain in the buttock area. The pain can radiate to the groin and lumbar region. This can be accompanied by tingling and other sensations. The complaints often occur primarily in a sitting position, while walking and standing are more pain-free. Targeted training of the surrounding musculature can help to relieve the sacroiliac joint and thus alleviate the symptoms. In addition, pain-relieving injections and, if necessary, surgical intervention may be appropriate.