ANATOMY OF THE PELVIS/SACROILIAC JOINT -...

Cook, Orthopedic Manual Therapy: An Evidence-Based Approach, 2/E © 2012 by Pearson Education, Inc., Upper Saddle River, NJ

ANATOMY OF THE PELVIS/SACROILIAC JOINT

Joint Anatomy

The pelvic girdle consists of the two innominate bones, the sacrum, and the bones of the

coccyx. Six joints lie within the pelvic girdle, which include the two sacroiliac joints (SIJ), the

sacrococcygeal, in many cases the intercoccygeal, and the pubic symphysis. The sacroiliac joint is a

synovial joint with the sacral surface covered with hyaline cartilage and the iliac surface covered with

a type of fibrocartilage (1)(Table 11.1). The anterior portion is synovial and the posterior-superior

portion is classified as a typical syndesmosis joint (2). These articular surfaces are different from any

other joint within the body (3). The surfaces are designed for stability versus mobility; the persistence

of selected furrows and ridges create an extremely rough integrated surface of congruency (4).

Table 11.1: General Information about the Sacroiliac Joint.

Topic Information Joint type Diarthrodial joint (synovial) surrounded anteriorly and posteriorly by

a joint capsule with inner synovial membranes and hyaline cartilage on both joint surfaces

Surface regions Only a superficial layer of hyaline cartilage covers the iliac surface, and its roughened surface is suggested to increase the friction coefficient of the joint, thus increasing joint stability

Shape of the joint The SIJ is roughly L-shaped with a short superiorly orientated joint and a longer A-P orientated arm, which intersects at approximately the S2 level

Figure 11.1: The Sacroiliac Joint


Wilder et al. (5) reported that the topography of the SIJ encourages the function of shock

incorporation through energy absorption of the ligamentous structures. However, the topography of

the SIJ includes flat joints that encumber very large forces, which is a recipe for shear-related

instability (6). Without some form of external passive or active stabilization, the SIJ would allow

considerable unorganized movement.

It has long been noted that women have a greater prevalence of SIJ pain than men, a

problem that is theoretically associated with joint architecture and functional needs. Ebraheim et al.

(7) reported that average females exhibit 12.8 percent less joint surface contact than males, a finding

that contrasted the report of Sgambati et al. (2). Other investigators have reported similar gross

movements between males and females (8) but notable differences in adolescent anatomy (9). Age-

related changes appear to be similar as well (8).

The pubic symphysis is a nonsynovial amphiarthrodial joint (10). The joint exhibits a thick

intrapubic fibrocartilaginous disc between the two hyaline covered pubis bones. Dysfunction to the

symphysis pubis generally results in groin pain and/or inflammatory involvement with adjacent

structures (11,12). As with the cervical spine, most fibrocartilaginous discs exhibit a cavity within the

structure, which is thought to be evidence of degeneration of the disc (1). Injury to the pubic

symphysis may be the result of direct injurious force or microtrauma related to repetitious activity

(12,13).

Figure 11.2: The Anterior Pelvis and the Symphysis Pubis


The sacrococcygeal joint is a synovial symphysis connection (1) and can be an origin site for

coccygeal pain (14). Similar to the pubic symphysis, a fibrocartilaginous disc between the sacrum and

the first coccyx bone develops and assists in allowing movement to the structures. When present, the

intercoccygeal joint also presents with a fibrocartilaginous disc.

Osseous Structures

The sacroiliac joint consists of the articulations of sacrum and the innominate bones. The

innominate bones are composed of the ilium, the pubis, and the ischium. The ilium is the superior-

most structure that forms the articulation with the sacrum and two-fifths of the surface of the

acetabulum (1). The pubis is the inferomedial aspect of the innominate and constitutes one-fifth of the

articulation with the acetabulum of the hip. The ischium is the inferolateral aspect of the innominate

that provides the floor of the acetabulum and the posterior two-fifths of the articular surface of the

acetabulum rim. The sacral-coccyx articulation (sacrococcygeal joint) includes the union of the coccyx

and the sacrum (1). The pubic symphysis includes the articulations of the two left and right pubic

bones.

Figure 11.3: Medial and Lateral View of the Innominates

The sacrum is a triangular-shaped bone that articulates with the ilia of the innominates. The

sacrum is formed by the fusion of five sacral vertebrae (15), a process that results in a very strong

and stable keystone. In addition to the articulation with the innominates, the sacrum articulates with

the fifth lumbar process at the base and at the two superior articular processes. The sacrum serves

as an attachment site for multiple ligaments such as the sacrotuberous ligament, the ventral sacroiliac


ligament, the sacrospinous ligament, and the iliolumbar ligament and muscles including the piriformis,

the multifidi, the erector spinae, and the gluteus maximus.

The triangular-shaped coccyx includes the complete or partial fusion of four bones (15). Of

the four segments, the first is the largest; and each vertebra progressively is smaller distally. The

coccyx provides an attachment for the sacrosciatic ligaments, the coccygeus muscles, and the tendon

of the external sphincter muscle.

Ligaments and Fascia

Several structures are involved in the control of movements and/or stabilization of the

sacroiliac and pelvic complex (Table 11.2). The primary ligaments associated with the sacroiliac joint

include the ventral sacroiliac ligament, the interosseous sacroiliac ligament, the long dorsal sacroiliac

ligament, the iliolumbar ligament, the sacrotuberous ligament, and the sacrospinous ligament (1). The

ligaments of the sacroiliac joint provide afferent output from the joint capsule and are innervated by

the dorsal rami of the sacral nerves S1-S4 (16).

Table 11.2: The Ligaments of the Sacroiliac Joint Region.

Ligament Information Interosseous ligament and posterior SI ligament

Lie directly posterior to the SIJ and form a strong link between the sacrum and ilium that limits nutation and inferior shear of the sacrum

Long dorsal SI ligament From posterior superior iliac spine to S2 and S3 sacral crests. This ligament limits sacral counternutation and is commonly a source of pain in patients with SIJ symptoms.

Sacrotuberous ligament Attaches from the ischeal tuberosity to the inferior lateral angle (ILA) of sacrum. The sacrotuberous ligament limits sacral nutation and will be tensioned by hamstrings contraction or stretch due to fascial linkage between biceps femoris and the lateral portion of the sacrotuberous ligament. The sacrotuberous ligament has also been shown to have fascial connections to the posterior layer of thoracolumbar fascia, gluteus maximus, and piriformis

Iliolumbar ligaments Runs from the TP of L4 and L5 to the anterior surface of the iliac crest

Sacrospinous ligament Runs from the ILA to the ischial spine

The ventral sacroiliac ligament and the interosseous sacroiliac ligament are less studied and

are poorly understood. Bowen and Cassidy (17) report that the ventral sacroiliac ligament is primarily


a thickening of the anterior aspect of the SIJ capsule. Although it is reported to be very strong, the

interosseous ligament is not suspected as a cause of clinically determined low back pain (17,18).

The sacrotuberous ligament has extensive connections posteriorly with the gluteus maximus,

the long head of the biceps femoris, and the sacrospinous ligament and anteriorly with the

iliococcygeus muscle (19). During loading or tension to the sacrotuberous ligament, the amount of

available movement (posterior rotation of the ilium to the sacrum or anterior rotation of the sacrum

with relation to the ilium) is restricted. Conversely, during counternutation (anterior rotation of the ilium

to the sacrum or posterior rotation of the sacrum with relation to the ilium), a slackening of the

sacrotuberous ligament occurs allowing freer movement and less stability contribution from this

ligament (19).

The long dorsal ligament can be palpated directly caudal to the posterior superior iliac spine.

The long dorsal ligament is closely integrated with the erector spinae, the posterior layer of the

thoracolumbar fascia, and the sacrotuberous ligament. The ligament contributes to the movement

stability of the SIJ, specifically counternutation (19). During counternutation, the tension is increased

in the long dorsal ligament and is slackened during nutation, a finding in opposition to the

sacrotuberous ligament (Table 11.3). Although both ligaments are physiologically connected, each

performs contradictory stabilization tasks for the SIJ.

Table 11.3: Nutation and Counternutation of the Sacroiliac Joint

Movement Plane of Motion Biomechanical Orientation Sacral flexion Sagittal Sacral nutation: sacral promontory tilts anteriorly

and inferiorly Sacral extension Sagittal Sacral counternutation: sacral promontory back

to the neutral position from an anterior-tilted position


Figure 11.4: Nutation and Counternutation of the Sacrum

The sacrospinous ligament originates on the sacrum and coccyx and inserts on the ischial

spine of the innominate (1). This ligament has been associated with numerous pelvic floor

dysfunctions including pudendal nerve entrapment (20), vaginal vault prolapse (21), and urinary

incontinence (22).

The iliolumbar ligament is one of a few structures that actually cross the sacroiliac joint.

Bogduk and Twomey (23) outlined five separate bands of the iliolumbar ligament, which traverse from

the transverse process of L5 to the quadratus lumborum, the iliac crest, and the posterior aspect of

the iliac tuberosity. The iliolumbar ligament ossifies by the fifth decade and is demarcated from the

quadratus lumborum muscle (24).

The iliolumbar ligament appears to restrict sagittal nutation and counternutation of the SIJ

(25,26). The ligament is able to perform a function of stabilization to the SIJ whenever the stability of

L5 is in tact. In situations where L5 is not stabilized, the SIJ may demonstrate corresponding

increased movement and decreased stability. This finding lends credence to the supposition that

lumbar and pelvic stability are inter-related. It is proposed that the ventral bands provide greater SIJ

stability in the sagittal plane versus the dorsal bands (26).

The ligaments of the pubic symphysis and the sacrococcygeal joint are less studied. The

ligaments associated with the pubic symphysis include the superior pubic ligament and the interior

arcuate ligament. The stabilization contributions of these ligaments and the associated symphysis


joint create a typically stable structure (1). The ligaments associated with the sacrococcygeal joint are

the ventral, dorsal, and lateral sacrococcygeal ligaments. The ventral sacrococcygeal ligament is a

continuation of the anterior longitudinal ligament, whereas the dorsal ligament is a continuation of the

posterior longitudinal ligament of the spine (1).

The thoracolumbar fascia envelopes the dorsal muscles of the trunk and forms an attachment

for several upper limb and trunk muscles, including the latissimus dorsi, the gluteus maximus, the

transversus abdominis, and internal oblique musculature (27). The thoracolumbar fascia transmits

forces between the spine, pelvis, and legs, and may play a role in trunk stabilization during rotation

and flexion-based activities (28). Vleeming et al. (29) purported that increased tension within the

thoracolumbar fascia leads to increased force closure and subsequent increased stability.

Summary

• The pelvic girdle consists of the two innominate bones, the sacrum, and the bones of the

coccyx. • The sacroiliac joint is a synovial joint with the sacral surface covered with hyaline cartilage

and the iliac surface covered with a type of fibrocartilage. • The primary ligaments associated with the sacroiliac joint include the ventral sacroiliac

ligament, the interosseous sacroiliac ligament, the long dorsal sacroiliac ligament, the iliolumbar ligament, the sacrotuberous ligament, and the sacrospinous ligament.

BIOMECHANICS OF THE PELVIS/SACROILIAC JOINT

Movement Analysis

The mobile sacroiliac joints function as shock absorbers (28) even during older ages (30–31).

Mooney (3) suggests that the principle function of the SIJ is to “dissipate or attenuate the loads of the

torso to the lower extremities and vice versa.” The amount of movement has been debated for

decades by clinicians and a number of assumptions have prevailed through clinical proviso.

A 2008 study by Goode et al. (32) analyzed actual three-dimensional (3-D) Cartesian

movements in studies that limited analysis to Roentgen stereophotogrammetric analysis (RSA).

Because SIJ movement is 3-D, the Cartesian system more accurately defines movement by

rotations and translations within the six degrees of freedom. RSA involves a 3-D mapping of

movements and is much more accurate than use of skin markers or other measures that have a


high degree of error. The review captured data from seven manuscripts, which were also evaluated

for study quality.

SIJ rotation ranged between –1.1 to 2.2 degrees along the X–axis, –0.8 to 4.0 degrees

along the Y-axis, and –0.5 to 8.0 degrees along the Z-axis during a number of functional

movements such as bending over, squatting, or sitting to standing. During the same functional

movements, the measures of SIJ translation ranged between –0.3 to 8.0 millimeters (mm) along the

X-axis, –0.2 to 7.0 mm along the Y-axis, and –0.3 to 6.0 mm along the Z-axis. The studies that

reported the largest degree of motion also presented with the largest degree of instrument

measurement error. The review summary suggested that the motion of the SIJ is limited to minute

amounts of rotation and of translation, suggesting that clinical methods using palpation for

diagnosing SIJ pathology may have limited clinical utility.

Movement of the pubic symphysis has also been investigated. Walheim and Selvik (33)

indicated the pubic symphysis rotates 3 degrees and translates 2 degrees. During single-legged

stance, the symphyseal can move vertically 2.6 mm and sagittally 1.3 mm on the weight-bearing side

(34). During walking, the pubic symphysis can piston (move up and down) up to 2.2 mm in the vertical

direction and 1.3 mm in the sagittal direction. Because hypermobility beyond this normal range could

lead to pubis instability, several investigators have examined the diagnostic utility of a standing x-ray

to determine vertical displacement (12,35,36). Berezin et al. (35) reported movements up to 5.9 mm

(s.d. 3.3 mm) in a population of patients with pelvic pain versus only 2 mm in an asymptomatic

population. Rieger and colleagues (37) reported that fail rates during externally applied loads

generally resulted in disruption in a vertical direction, typically at the pubic symphysis.

Because the movements of the pelvis, even when displaced, are so small, the most

discriminatory radiographic analysis appears to be the Chamberlain x-ray (38,39). The Chamberlain

x-ray involves an x-ray performed while the patient is standing on a box on the involved lower limb,

while the other lower limb is non–weight-bearing. This maneuver allows a vertical migration of the

weight-bearing side (pubic symphysis) and has been associated with pelvic instability (36).


Summary

• The mobile sacroiliac joints function as shock absorbers during younger and older ages (with

a principle function to dissipate or attenuate the loads of the torso to the lower extremities and vice versa.

• During single-legged stance, the symphyseal can move vertically 2.6 mm and sagittally 1.3 mm on the weight-bearing side.

Muscular Contribution

Thirty-five muscles attach to the pelvis and/or sacrum and have a direct or indirect role in

stabilizing this region (15). Muscular contribution is necessary for both form and force closure at the

SIJ/pelvis. Form closure includes the passive stabilization contributions of interlocking ridges and

grooves on the joint surfaces (28) and ligamentous stabilization. Force closure is a term that

corresponds to increased SIJ/pelvis stiffness by isolated contraction of selected muscle groups. Key

muscles of force closure are the gluteus maximus, biceps femoris, erector spinae, and latissimus

dorsi (4,38,40–43). Other muscles, commonly identified as local muscles, are responsible for the

preset tension on the pelvis prior to initiation of movement. These muscles include the transversus

abdominis, the multifidi, the piriformis, and the pelvic floor muscles.

The transversus abdominis is a key muscle for stabilization of the sacroiliac joint (44,45).

Others have found the feed-forward temporal contraction of the multifidus and obliquus internus

abdominis a necessity prior to pelvic stabilization during hip flexion in standing (46,47). Pain of the SIJ

was associated with delays of these muscles.

The multifidi originations of the lower lumbar spine insert onto the articular processes of the

sacrum, the posterior thoracodorsal fascia, and the sacrotuberous ligament (15). The deep fibers of

the muscle span from one segment to the adjacent segment, while the superficial fibers span three

segmental levels. The deep fibers of the multifidi are considered the most significant stabilizers of the

pelvis and lumbar spine.

The piriformis originates from the anterior aspect of the sacrum, the anterior aspect of the

posterior inferior iliac spine, the upper aspect of the sacrotuberous ligament, and the capsule of the

sacroiliac joint (15) and inserts on the greater trochanter of the femur. Snijders et al. (6) suggest that

the piriformis plays an important role during force closure of the SIJ. The muscle is implicated as a


contributor for chronic buttock pain (48) and can refer pain to the sacroiliac region (15). Often, imaging

of the pelvis and lumbar spine performed on patients with suspected piriformis syndrome is

unremarkable (49).

The pelvic floor muscles such as the levator ani and coccygeus muscles have shown to

contribute considerably to pelvic ring stability. In a study by Pool-Goudzwaard et al. (50), simulated

tension in the pelvic floor muscles increases SIJ stiffness by 8.5 percent. Selected authors have

outlined the contribution of the pelvic floor muscles to increasing intra-abdominal pressure and an

increase in lumbar spinal stiffness (51,52). The muscle functions to stabilize the SIJ through force

closure and the capacity to provide counterforce stiffness on the ligaments of the SIJ (29). O’Sullivan

et al. (53) outlined inappropriate pelvic floor descent (versus ascent) during altered motor control

recruitment of the pelvic muscles in patients with SIJ pain during the active straight leg raise.

Hungerford et al. (46) indirectly lent evidence to the importance of the contribution of these

muscles to stability through their report of delayed onsets of the multifidus, the internal obliques, and

the gluteus maximus in patients with sacroiliac joint pain. Multiple authors (47,51,54,55) have reported

that a feed- forward mechanism of selected pelvic stabilization exists in asymptomatic individuals and

is absent in patients with lumbopelvic pain. O’Sullivan et al. (53) suggested abnormalities in

stabilization of the diaphragm and the pelvic floor muscles during an active straight leg raise in

patients with known pelvic dysfunction. This finding demonstrates the critical role these local muscles

play in concert with ligamentous attachments for preset stability, a role very similar to the one played

in the lumbar spine.

The erector spinae is a complex muscle group that originates on multiple lumbar segments

and inserts into the medial aspect of the posterior superior iliac spine (PSIS) bilaterally and the

intermuscular aponeurosis (15). The muscle can provide lumbar extension–based movements or

unilateral side flexion when contracted on one side only.

The quadratus lumborum is thought to contribute to stability by increasing the tension of the

iliolumbar ligament. The transversus abdominis, psoas, quadratus lumborum and multifidus were

each noted to have segmental attachment patterns in the lumbar spine (56). As a group, these

muscles surround the lumbar motion segments from the anterolateral aspect of a vertebral body to


the spinous process. Generally, the quadratus lumborum is considered a strong ipsilateral flexor (57).

McGill et al. (58) suggest that electromyographic evidence, together with architectural features such

as attachment location and activity during selected movements make the quadratus lumborum a

better stabilizer of the spine than the psoas.

Using surface electromyographic (EMG) analysis, van Wingerden et al. (59) analyzed a

battery of muscle groups to determine which muscles most significantly contributed to force closure.

Their findings outlined the significant contribution of the erector spinae, the biceps femoris, and the

gluteus maximus to increased stiffness and decreased shear at the SIJ. To a lesser extent, the

latissimus dorsi also contributed. Notable were the contralateral and simultaneous contractions of the

erector spinae with the gluteus maximus and the contractions of the biceps femoris ispilaterally, with

the gluteus maximus. The aspect of the erector spinae that contributed most was the distal aspect,

specifically the multifidus, hypothetically contributing with the attachments to the sacral ligamentous

structures.

Summary

• Stabilization of the SIJ occurs through form and force closure. Form closure includes the

passive stabilization contributions of interlocking ridges and grooves on the joint surfaces and ligamentous stabilization. Force closure is a term that corresponds to increased SIJ/pelvis stiffness by isolated contraction of selected muscle groups.

• Thirty-five muscles attach to the pelvis and/or sacrum and have a direct or indirect role in stabilizing this region. Muscular contribution is necessary for both form and force closure at the SIJ/pelvis.

• Key muscles of force closure are the gluteus maximus, biceps femoris, erector spinae, and latissimus dorsi; however, contribution of the local muscles, such as the transversus abdominis, the multifidi, the piriformis, and the pelvic floor muscles are responsible for the preset tension on the pelvis prior to initiation of movement.

Dysfunction versus Pain

Clinicians often use a number of identifiers during implication of the SIJ region. The first

identifier used involves the term dysfunction, which proposes that the joint is malfunctioning in

some manner resulting in movement abnormalities and mal-positions. The second identifier

involves the term pain, which assumes the SIJ is the pain generator. Unfortunately, the terms SIJ

dysfunction and SIJ pain are commonly used interchangeably, as though each has the same

meaning, which they do not (60).


SIJ Dysfunction

In essence, a dysfunction of the pelvic girdle occurs when stabilization is lost or when

asymmetric stabilization is present between the two sides of the SIJ (61). Normally, a predictable

biomechanical sequence occurs in asymptomatic patients within the lumbopelvic region that stabilizes

the pelvis and prepares the structure for weight bearing. This process occurs through two different

methods—movement initiation and muscular contraction. Muscle contraction of the multifidi, the

gluteus maximus, and the piriformis creates initial tension in the sacrotuberous ligament and

thoracolumbar fascia prior to nutation, pre-engaging the ligaments for stability (28). During movement,

nutation (posterior rotation of the ilium with respect to the sacrum) tensions the majority of the largest

SIJ ligaments such as the sacrospinous and sacrotuberous ligaments (19). The contact area of the

sacroiliac joint is the lowest during posterosuperior displacement of ilium on the sacrum (7), thus

requires the greatest amount of force closure. Because of the muscle connections to the ligaments

and due to the nature of nutation’s increase in force closure during ligamentous tension, the SIJ

demonstrates increased stiffness. Anterior rotation of the innominate on the sacrum (counternutation)

appears to be a primary source of pain and instability in patients with chronic pelvic pain (37). Causal

displacement that occurs during counternutation (anterior rotation of the innominate on the sacrum)

places stress on the long dorsal ligament, which is normally taut during neutral positions (40).

Problems associated with feed-forward failures of ligament laxity discourage the process of

form and force closure and lead to SIJ/pelvic instability. This is the primary dysfunction associated

with SIJ and pubic symphysis pain. Instability is defined as “the inability of the joint and surrounding

structures to bear load without uncontrolled displacements” (25). Many patients report pain during

movements of the lower extremity and describe a feeling of paralysis during painful provocation (36).

Additionally, an instable pelvis has been associated with the inability to move the lower limbs (36).

Instability of the sacroiliac joints has long been suspected as a cause of low back pain and

lumbopelvic dysfunction (47). Stabilization of the SIJ is accomplished through increased friction

throughout the joint during tension of selected muscles and ligaments (62). More specifically, Pool-

Goudzwaard et al. (28) suggest that the SIJ is protected from traumatic shear forces in three ways.


First, because the joint is wedge shaped, the sacrum is stabilized during weight bearing by locking

into the innominates like a keystone. Second, the cartilage surfaces of the SIJ are not smooth as in

other synovial joints, thus this encourages stability, specifically in men. Men demonstrate more

cartilage abnormalities than women, thus providing greater passive stability based on structural

interface (17); however, in situations that do not involve pregnancy, ranges are similar between sexes

(8). Third, multiple furrows and ridges within the joint itself encourage a locking function during weight

bearing. This complementary system creates ‘form closure’ secondary to a closely congruent passive

locking system that is further enhanced during weight bearing (4).

Force closure involves the active element of controlled stability (28). Vleeming et al. (4)

identified this process as a ‘self-locking’ or ‘self-bracing’ mechanism that allows enough functional

movement but controlled stability to appropriately transfer large forces from the legs to the trunk. To

accomplish this challenging task, several ligaments, muscles, and fascial elements are critical. As

discussed previously, the force closure muscular contributions of the multifidi, transversus abdominis,

and other surrounding musculature are critical.

In 2002, Damen et al. (61) reported that instability is not necessarily a requisite for SIJ

dysfunction, rather that asymmetric stability is more likely the cause. Normal subjects demonstrate

large variations in laxity that is commonly stabilized by appropriate muscle contribution. Their study

used a doppler imaging (DIV) method of measuring stiffness, which analyzes pelvic girdle mobility in

vivo.

One notable consideration of the sacroiliac joint is the purported ability of the joint to assume

a ‘subluxed’ position. Because of the surface irregularities, Vleeming et al. (41) have proposed that it

is theoretically possible for the joint to move and assume a new position that is ‘locked’ into a position

of displacement. Furthermore, the amount of displacement may be so minute that radiographic

verification is unlikely (4,63). Consequently, a dysfunction may be present, which is typically

associated with stability loss, but will not likely have palpable or identifiable mal-positions (or such

subtle mal-positions that these cannot be identified) when involved. At present, the ability to palpate

variations in SIJ stability has not been confirmed.


SIJ Pain

SIJ pain reflects a situation in which the SIJ is the pain-generating component. Studies have

shown that the SIJ has more pain receptors than movement receptors embedded within the joint (64).

A number of systematic problems may cause pain and inflammation and pain from a dysfunction may

be present as well. What is worth noting is that pain can be present with a stability dysfunction;

however, the dysfunction may be below a clinical threshold in its presentation.

Summary

• Dysfunction of the pelvic girdle occurs when stabilization is lost or when asymmetric

stabilization is present between the two sides of the SIJ. • Anterior rotation of the innominate on the sacrum (counternutation) appears to be a primary

source of pain and instability in patients with chronic pelvic pain. • A posterior rotated innominate tends to be a more stable position as compared to anterior

rotation. • The SIJ is protected from traumatic shear forces in three ways: 1) through the wedge-shaped

anatomy, 2) through the interlocking furrows and ridges, and 3) through the shape of the surface cartilage.

• Because of the surface irregularities, it is theoretically possible for the joint to move and assume a new position that is ‘locked’ into a position of displacement, a position that is too small for radiographic verification.

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