The sacroiliac (SI) joints are the link between the triangular shaped bone at the base of the pelvis (the sacrum) and the pelvic bone on either side (the ilium). The SI joints are meant to be stable unlike the shoulder and hip joints which are designed for mobility. The SI joints function to stabilize and support the pelvis, transmit the weight of the upper body to the legs and to act as a “shock absorber” during walking or other activities by preventing impact forces from reaching the spine.
The SI joint has two surfaces. The covering on both the sacral and the iliac joint surfaces is hyaline cartilage, the same as most all joints in the body. The cartilage on the sacral surface is much thicker than the cartilage on the iliac side. As we age, there is wear and tear on these surfaces and a joint which is smooth when we are young becomes more rough and degenerated with each subsequent decade.
There is a limited amount of motion in an SI joint which is functioning properly. The motion that does occur at the sacroiliac joint occurs in multiple planes. The sacrum translates (glides between the two ilea), the sacrum nutates (rocks forward and backward), and also pivots on a diagonal axis all at the same time. The motions are very small with the gliding motion being about 1.6 millimeters and with 1-2 degrees of sacral rocking motion in males, and 3-4 degrees of sacral rocking motion in females.1
When the SI joint is functioning properly there is effective transfer of force and load across the SI joint from the lower extremities (legs) to the spine.2
There are three components necessary for optimal SI joint function. Each of these components are described below:
- Form closure: passive forces
- Force closure: dynamic forces
- Motor control: the coordination between the nervous system and the muscles that stabilize the lower back and pelvic region3
Form Closure is the passive SI joint stabilization that is imparted by the shape of the bones comprising the joint and the capsule and ligaments supporting the joint.4, 5, 6 For example the irregular joint surface with its ridges and grooves and the wedged shape of the sacrum itself limit motion and provide passive stability to the joint. The other major contributors to form closure are the joint capsule and the multiple strong thick ligaments (dense fibrous tissue that joins two bones together). These ligaments support both the front and the back of the SI joint.
Sacroiliac Joint Surface:
Sacroiliac Joint Shape:
Sacroiliac Joint Ligaments (back in upper left and front in lower right):
Force Closure is the dynamic stabilizing pressure that actively compresses the SI joints. Force closure is generated by contraction of the stabilizing muscles and their attachments to the bones and ligaments of the SI joint. 4, 5, 6 The primary group of muscles that provide force closure to the SI joints are the core muscles or the local stabilizing muscles. Three of the four muscle groups making up the core muscles attach directly to the sacrum and/or ilium. These muscles are the transversus abdominus in the front, the multifidus or erector spinae in the back and the pelvic floor muscles at the bottom. The diaphragm is the fourth of the core muscles and functions essentially as the lid of the force barrel that stabilizes the pelvis and both SI joints. The core muscles contract before motion of the spine or extremities.
Core: Local Stabilizing Muscles
There is a secondary group of muscles that selectively stabilize each of the SI joints. These muscles and their attachments are described as slings. These muscle groups cross the midline and also cross the SI joint from above to below. An example of a muscular sling would be the left latissimus and the right gluteus maximus muscles stabilizing the right SI joint. Multiple muscular slings stabilize each SI joint. These muscle slings contract during motion of the spine or lower extremities. Proper tension, strength, mobility and balance in these muscles and integrity of their attachments are necessary for adequate force closure of the SI joints.
Motor Control is the process by which humans use their nervous system to activate and coordinate the muscle firing and limb movement involved in the performance of a motor skill.7, 8, 9 It is the integration of sensory information, both about the world and the current state of the body, to determine the appropriate set of muscle forces and joint activations to generate some desired movement or action. This process requires cooperative interaction between the nervous system and the musculoskeletal system.10, 11 In regard to the SI joint, the core muscles should prepare the system for the impending load or action by contracting before a load reaches the low back and pelvis. When someone catches a ball, if they have good motor control, the core muscles contract automatically to stabilize the low back and pelvis including the SI joint as the ball is caught. Before they throw the ball to another person, the core muscles will contract to stabilize the pelvis and SI joint automatically.
SI joint dysfunction may occur when any, or all of the components necessary for SI joint stability are not functioning properly. When SI joint dysfunction occurs, the patient may or may not have pain in the SI joint itself. SI joint dysfunction may result in pain in the soft tissues around the SI joint and/or in other areas of the body that may be secondarily affected. SI joint pain is the term that is used when there is pain from the joint itself. Patients with SI joint pain typically have discomfort during provocative testing. The provocative tests are physical examination tests that stress the SI joint(s) in various directions. Studies have shown that if a patient has three or more positive tests on a cluster of 5 specific provocative tests, it is highly likely that the patient has pain arising in the SI joint.12
Sacroiliac pain can occur in patients with widely different history, physiology, and physical examination findings. The three components of SI joint function may be affected by everyday wear and tear, degenerative arthritis, an injury, a loss of muscle length or strength, or the hormonal, muscular and postural changes that occur during and after pregnancy. Other surgeries such as spine surgery may limit motion above or below the SI joint making it more susceptible to wear and tear.
In our next three newsletters, we will describe the diagnosis and treatment of three different patients with SI joint dysfunction and pain. The first patient is a healthy 45 year old woman who slipped and fell on a patch of ice. The second patient is a 65 year old man with excessive wear and tear at the SI joint after a two level lumbar fusion surgery 10 years earlier. The third patient is a 35 year old woman with ongoing SI joint pain related to pregnancy and continuing after the delivery of each of her three children. We hope that these patient scenarios will help describe what a typical patient with SI joint dysfunction due to trauma or ligamentous disruption, adjacent segment degeneration after spine surgery, or due to persistent pain that developed in the peripartum period (last month of pregnancy and first few months after delivery) might experience during the workup and treatment of their condition.
- Sturesson B, Selvik G, Udén A. Movements of the sacroiliac joints. A roentgen stereophotogrammetric analysis. Spine. 1989 Feb;14(2):162-5.
- Snijders CJ, Vleeming A, Stoeckart R. Transfer of lumbosacral load to iliac bones and legs. Part 1: Biomechanics of self-bracing of the sacroiliac joints and its significance for treatment and exercise. Clin Biomech (Bristol, Avon). 1993 Nov;8(6):285-94.
- Panjabi MM. The stabilizing system of the spine. Part II. Neutral zone and instability hypothesis. J Spinal Disord. 1992; 5:390–6; discussion 397.
- Lee DG, Vleeming A. Impaired load transfer through the pelvic girdle- a new model of altered neutral zone function. In: Proceedings from the 3rd interdisciplinary world congress on low back and pelvic pain. Vienna, Austria. 1998.
- Vleeming A, Stoeckart R, Volkers ACW, Snijders CJ. Relation between form and function in the sacroiliac joint. Part 1: Clinical anatomical aspects. Spine. 1990 Feb; 15(2):130-2.
- Vleeming A, Stoeckart R, Volkers ACW, Snijders CJ. Relation between form and function in the sacroiliac joint. Part 2: Biomechanical aspects. Spine. 1990 Feb;15(2):133-6.
- Snijders CJ, Vleeming A, Stoeckart R. Transfer of lumbosacral load to iliac bones and legs. Part 1: Biomechanics of self-bracing of the sacroiliac joints Snijders CJ, Vleeming A, Stoeckart R. Transfer of lumbosacral load to iliac bones and legs. Part 1: Biomechanics of self-bracing of the sacroiliac joints and its significance for treatment and exercise. Clin Biomech (Bristol, Avon). 1993 Nov; 8(6):285-94.
- Hodges PW, Richardson CA. Inefficient muscular stabilization of the lumbar spine associated with low back pain. A motor control evaluation of transversus abdominis. Spine. 1996; 21(22):2640-50.
- Richardson CA, Snijders CJ, Hides JA, Damen L, Pas MS, Storm J. The relationship between the transversely oriented abdominal muscles, sacroiliac joint mechanics and low back pain. Spine. 2002; 27(4):399-405.
- Rosenbaum, David A. (1991). Human motor control. San Diego, CA: Academic Press. p. 411. ISBN 0-12-597300-4.
- Jump up. Wise, Stephen P.; Shadmehr, Reza (July 10, 2002). "Motor Control". Encyclopedia of the Human Brain. Academic Press. pp. 137–157. ISBN 978-0122272103.
- Laslett M. Evidence Based Diagnosis and treatment of the painful Sacroiliac Joint. J Man Manip Ther. 2008; 16(3):142-52.
- Ha KY, Lee JS, Kim KW. Degeneration of sacroiliac joint after instrumented lumbar or lumbosacral fusion: a prospective cohort study over five-year follow-up. Spine. 2008 May 15; 33(11):1192-8.
- Ivanov et al: Lumbar Fusion Leads to Increases in Angular Motion and Joint Stress across the Sacroiliac Joint: A finite element analysis. Spine. 2009; 34(5):E162-9.