Form & Function of SIJ
Since the sacroiliac joint is at the center of motion in the body, understanding its form and function is a necessity in developing proper diagnosis, treatment and rehabilitation methods for our musculoskeletal system. Yet, the sacroiliac joint has remained an enigma; it is no wonder that the rest of the musculoskeletal system appears disjointed. The question remains: how does the sacroiliac joint function in accordance with its form? The Serola Theory answers this question.
Until now, there was no central, unifying biomechanical model that we can use to understand and effectively treat the musculoskeletal system as an integrated structure. As importantly, when advancements are made in the field of biomechanics, it is difficult for them to be incorporated into the diagnostic and treatment paradigms when there is no central model. Therefore, misconceptions and dead-ends abound.
Part of the problem is our inclination to compartmentalize regions of the body, which leads us to view the musculoskeletal system as, basically, a collection of parts. Too often, each problem area is treated as a separate entity. Biomechanical relationships between the back, knee, foot, shoulder, TMJ, etc. are not well-thought-out; lower extremity problems are not considered to relate to upper extremity problems, etc.
The least understood joint in the body happens to be the body’s core, the sacroiliac joint. (See Structure). Various theories expound on whether it even moves and, if so, what induces movement, how it moves, how much it moves, how it is injured, how it is stabilized, how it is mobilized, what constitutes degeneration, how it coordinates with nearby structures, whether it is a keystone in a Roman arch or is suspended by ligaments, and even if it is, in fact, the core of our musculoskeletal system (current thought is centered on the lumbar spine).
The Serola Theory proposes a new model for integrating the entire musculoskeletal system into a single, continuous whole so that we can diagnose, treat, and prevent injuries more effectively. The sacroiliac joint functions as the core integrating structure of the musculoskeletal system in which the cranium, neck, back, arms, pelvis, legs, and feet move in synchronous motion in a pattern of nutation/counternutation. With a good understanding of the Serola Theory, most musculoskeletal pain will no longer be a mystery and treatment options may be more effective.
Regions of the SIJ
The sacroiliac joint is divided into two regions p390. (See SIJ Surfaces). Anteriorly, there is an L-shaped synovial region and posteriorly, a syndesmosis region that is 1.57 times larger than the synovial region . The central part of the syndesmosis is the interosseous ligament. While the articular part functions in guiding movement, the syndesmosis functions in weight-bearing during the transfer of forces from the trunk, through the pelvis and legs, inferiorly to the ground, and in the transmission of ground reaction forces superiorly to the spine.
2D vs 3D Perspectives
In the current literature, it is widely accepted that nutation and counternutation are the primary movement patterns of only the sacroiliac joint and are not thought to pertain to movement in the rest of the body. False assumptions such as this, based on a 2-dimensional perspective, have led researchers astray. On the other hand, the Serola Theory, based on a 3D perspective, demonstrates the integration of nutation and counternutation throughout our musculoskeletal system.
For simplicity, movement of the sacroiliac joint is described as bilateral motion, where the sacrum moves in opposition to the two ilia moving together. For example, we may say that while the sacrum goes forward, the ilia go backward. But, while bilateral movement occurs, it is rare; most of the time, the sacroiliac joint moves in a reciprocating unilateral pattern. (See Bilateral Nutation and Reciprocating Unilateral Nutation).
Developmentally, until the late teens, the sacroiliac joint is smooth and flat such that a gliding motion can occur in all directions . By the early twenties, the iliac surface develops a ridge running down the middle of the two wings. At the same time, the sacral surface develops a matching groove.
In a 2D perspective, the fact that the iliac ridge fits into a corresponding sacral groove gives the impression that the sacrum forms a “track” into which the iliac groove slides. This arrangement forms the basis for the keystone concept, which is misleading in its statement that this ridge and groove arrangement is a normal development in the adult to provide form closure and help maintain stability in the sacroiliac joint by increasing friction [4, 5].
According to a keystone proponent p49-51, the sacroiliac joint appears to move in a rotary fashion along this track, about a coronal axis within the interosseous ligament. Supposedly, in nutation, the sacrum appears to slide inferiorly (along the relatively vertical portion of the joint) and posteriorly (along the relatively horizontal portion). In counternutation, the sacrum appears to slide superiorly (along the relatively vertical portion of the joint) and anteriorly (along the relatively horizontal portion). However, I would like to emphasize that the above movement pattern is mistaken.
While this sliding may appear to occur at the articular region in a 2D perspective, the arrangement of the interosseous ligament fibers does not allow sliding in the syndesmosis region . However, a similar and extensive ridge and groove appear on the joint surfaces in the both the articular and syndesmosis region , indicating a pivoting movement around a longitudinal oblique axis, causing alternating compression of the articular and syndesmotic regions.
In a 3D perspective, in accordance with its surface orientation, bound tightly by the surrounding ligaments, the sacrum cannot slide on the ilia; it can only pivot in a swaying pattern. For this pivoting to occur, a vast array of ligaments goes from the ilium to the front and back of the sacrum, near its edges, to pull the sacrum through its range of motion. (See Sacroiliac Ligaments and Sacrum Functions as a Cone).
The sacrum, as a whole, remains relatively stationary in its position between the two ilia. Movement is present but normally limited to about 2 degrees. This model fits well with Levin’s  analogy of sacral hub motion being affected by “twisting” of the wheel formed by the two ilia. (See Sacral Movement Induced by Innominates.
Axial SI Joint
Soon after the ridge and groove appear, a bony tubercle, known as the axial sacroiliac joint, develops on the ilium, with a corresponding depression on the sacrum, just posterior to the articular region of the sacroiliac joint. The fact that the axial joint develops soon after the ridge and groove appear supports the idea it is interrelated with sacroiliac movement. It is situated about 15 mm posterior to the most concave point of the articular region, and anterior to the interosseous ligament. The axial joint functions as the center of sacroiliac motion p66    p619. (See Sacroiliac Ligaments and 3D Views of the Sacroiliac Joint.
To help explain how a suspensory model may function, an analogy may help. While each innominate rotates about the center of the femoral head, the sacrum pivots on an oblique axis  about the two axial sacroiliac joints simultaneously, in a swaying, or wobbling motion [3, 15]. As a pivot point, the axial joint would allow motion similar, in an indirect sense, to a fulcrum within a lever system but, because the axial joint is non-weight bearing, the motion cannot be generated by direct compression. Instead, the action can be compared to an inverse lever system where, instead of weight pressing downward on the ends of the lever, the lever (sacrum) is being pulled upward by surrounding ligaments, with the axial joint acting as a pivot point at the bottom central position of the lever (sacrum). In actuality, there is no lever; the concept was mentioned in this analogy solely to give an idea of where the center of motion would occur. No downward or upward stress would be placed on the axial ligament. In this manner, the axial ligament can function in a proprioceptive role but not directly act to limit range of motion.
Facets Curl Around the Sacrum
The movement pattern of the sacroiliac joint is complicated due to the shape of the sacrum and the orientation of the upper and lower articular surfaces, which form a modified saddle joint. When horizontal lines are drawn from both sides of the upper articular surfaces of the sacroiliac joint, they would converge posterior to the sacrum. When horizontal lines are drawn from both sides of the lower articular surfaces, they would converge anterior to the sacrum [15-18], demonstrating that the superior portions of the sacral surface face posterolaterally and the inferior portions face anterolaterally. In effect, they wrap around the lateral edges of the sacrum, rounding out the articular portions of the sacrum, thus contributing to the cone shape. . Thus, Instead of being a straight lever, the sacral surfaces are shaped like an airplane rotor, which impart a twisting motion, thereby creating an oblique axis. As a pivot point, the axial joint is at the area where the joint twists from a posterior orientation to an anterior orientation, around the level of S2. One unique thing about this arrangement is that it would allow the sacral base to move in the direction opposite to the ilium, in a scissors-like movement, which demonstrates its function as the structural core.
SIJ Surface: Flat vs. Ridge & Groove
From the side, as mentioned earlier, the sacroiliac articulation is shaped like an “L” with a vertical and horizontal portion. Each portion has an interlocking ridge on the ilium and corresponding groove on the sacrum. It is thought that the interlocking ridge and groove guide the actual movement pattern of the sacroiliac joint but it seems reasonable that the groove and ridge are more the result of the movement pattern imposed on the sacrum by the ilia during gait.
The ideal surface shape at the articular region is flat, as it is in youth, not to transfer weight during compression, but so the sacral and iliac surfaces can move very close together, yet not rub. The Serola Theory proposes that, after tearing of the interosseous ligament, the resulting aberrant motion produces alternating shearing and compressive forces during the anterior/posterior movement of the ilia, as they pivot around the sacral axial joints, leaving the ridge and groove impressions. (See Ridge and Groove).
Sacrum Functions as a Cone
The sacrum is not wedge shaped so that it can function as the headstone in an arch. Rather, the Serola Theory proposes that the sacrum is better compared to a cone, with the articular surfaces wrapping around its edges. This configuration allows the large anterior/posterior motions of gait to transfer to flexion/extension (counternutation/nutation) in the innominates, and then get geared down to the sacrum, from which it travels up the spine, where the components of counternutation/nutation are recognized as flexion/extension, rotation, and side-bending. Simultaneously, energy is transferred from the spine to the legs, providing a synchronized muscular force, assisted by gravity, which facilitates energy transfer and maximizes potential performance. In this manner, movement is integrated into one continuous pattern throughout the musculoskeletal system, based on the movement pattern of our core.
This concept appears to fit well with Gracovetsky’s  hypothesis on the role of the spine in locomotion, in which he discusses the function of the trunk muscles transferring rotary movement of the spine to the anterior/posterior movements of gait. However, he did not discuss the integral function of the sacrum or sacroiliac joint, which is proposed in the Serola Theory.
In a normally functioning sacroiliac joint, contact between the ilium and sacrum are made only by muscles and connective tissue, such as ligaments, as they pull the sacroiliac joint through its range of motion, or vice versus. The best way to visualize the movement pattern is to consider gait, in which sacral movement reciprocates with the ilia. In order to accommodate the two opposing iliac movements, during right nutation, the right side of the sacral base pivots anteriorly and inferiorly as the right ilium moves posteriorly and inferiorly while, simultaneously, during left counternutation, the left side of the sacral base pivots posteriorly and superiorly as the left ilium moves anteriorly and superiorly, and the sacral apex moves toward that side.
The sacrum remains relatively stationary, sandwiched between the two ilia. When they reach the end point of their range of motion, the sacrum and ilia are stopped by the restraining ligaments, and reverse their movement; the sacrum pivots accordingly, swaying around its two axial joints, while remaining in place within the pelvis.
The vast array of sacroiliac ligaments functions both to pull our core through its range of motion, and to prevent and limit injury. Receptors in each ligament coordinate with each other to provide smooth efficient motion by directly regulating all of the muscles that attach to the bones that make up the sacroiliac joint, specifically the sacrum and innominates.
When the ligaments sprain, they do so as a unit, and directly alter muscular programming throughout the trunk, pelvis, and upper legs to avoid further stress on the ligaments which, in turn, leads to dysfunctional movement patterns. In other words, the ligaments act to protect themselves, and use the muscles to do it (See Muscular Adaptations). As the core goes, so goes the rest of the structure. (See The Nutation Lesion).
Evidence has been presented that attention should be re-focused from the articular region of the sacroiliac joint to the syndesmosis region. With the correct understanding that the sacrum functions as a cone suspended by ligaments, we can better realize how the sacroiliac joint functions in movement and weight transfer, and how dysfunction occurs. With insight into the mechanisms of injury, one can understand why sacroiliac joint dysfunction is extremely common, especially in childhood and many sports, due to falls or other physical trauma.
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