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Sacroiliac Joint (SI Joint) Dysfunction is one of the common cause of under-recognized mechanical low back pain. 

» Causes of Sacroiliac Joint Dysfunction: 

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» Anatomy of Sacroiliac Joint:

• Sacroiliac joints are weight bearing joints between the articular surfaces of sacrum and ilium.
• Sacroiliac joints are made up of iliac, sacral auricular surfaces and tuberosities [1,2,3,4,5].
  • Auricular surfaces from a synovial joint, with a capsule and a cavity filled with fluid.
  • Tuberosities, connected by a interosseous ligament, constitute a fibrous form of a synarthrosis.
• SI joint is reinforced by some of the strongest and most massive ligaments of the body.

• Ventral Sacroiliac Ligament 
  • Assists the symphysis pubis in resisting separation or horizontal movement of the innominate bones at the SI joint.
  • Palpated at Baer's SI point (Point on a line from the umbilicus to the anterior superior iliac spine (ASIS) 5 cm from umbilicus).
  • Stressed using transverse anterior/posterior compression pain provocation test.
  • Weakest among the sacro iliac ligaments.
• Long Dorsal Sacroiliac Ligament
  • During incremental loading of the sacrum, it becomes tense during counternutation (base of the sacrum moves backward) and slackens with nutation (opposite movement of sacrum) [6].
  • Palpated in the area directly caudal to the posterior superior iliac spine (PSIS).
• Interosseous Sacroiliac Ligament
  • Largest syndesmosis in the body and functions as the major bond between the bones filling the irregular space posterior-superior to the joint.
  • Resist anterior and inferior movement of the sacrum.
  • Primary barrier to direct palpation of SIJ.
• Sacrotuberous Ligament
  • Plays significant role in stabilizing against nutation of the sacrum, and conteracting against the dorsal and cranial migration of the sacral apex during weight bearing.
  • Tension increaseswith contraction of Gluteus maximus.
• Sacrospinous Ligament
  • Along with sacrotuberous ligament, it opposes forward tilting of the sacrum on the hip bone during weight bearing of the trunk and vertebral column.
• Function of Sacroiliac Ligaments:
  • Works collectively as a force transfer for the hip and trunk muscles, producing innominate and/or sacral movements, in response to induced forces from the femur and/or vertebrae.
  • They also help to prevent the following:
    • Craniocaudal dislocation of sacrum
    • Anterior gapping (lateral innominate rotation)
    • Posterior gapping (medial innominate rotation)
    • Hyperflexion (posterior innominate rotation, or mutation)
    • Hyperextension (anterior innominate rotation, or counternutation)

» Facts regarding Sacroiliac Joint:

• Sacroiliac Joints are inherently stable [7,8,9].
• The joints are designed for load transfer [10,11] and can safely transfer enormous compressive forces under normal conditions [9].
• The sacroiliac joints has very little movement in non-weight bearing (average 2.5 degrees rotation) [1,12,13,14,15] and even less in weight-bearing (average 0.2 degrees) [16].
• Movements of the SI joint cannot be reliably assessed by manual palpation, particular in weight-bearing [16,17,18].
• Due to its anatomical makeup, intra-articular displacements within the SI joints are unlikely to occur.
• Distortions of the pelvis observed clinically are likely to occur secondary to changes in the pelvic and trunk muscle activity, resulting in direcitonal strain and not positional changes within the SI joints themselves [19].
• Pain relief from from manipulation is likely to result from nociceptive inhibition based on neuro-inhibitory factors and/or altered patterns of motor activity [20,21].
• When clinical signs of reduced force closure have been identified (positive Active SLR), the increased movement is identified at the symphysis pubis - not the sacroiliac joints [22].
• Pain from the sacroiliac joints is located primarily over the joint (inferior sulcus) and may refer distally, but not to the low back [23,24,25,26,27,28,29,30].
• Sacroiliac joint disorders can be diagnosed using clinical examination [29,31,32,33,34] which includes finding of pain primarily located to the inferior sulcus of the SI joint, positive pain provocation tests for SI joints and absence of painful lumbar spine impairment.
• The SI joint has many muscles that act to compress and control it (force closure), thereby enhancing pelvic stability allowing for effective load trasnfer via pelvis during a variety of functional tasks [7,8,9,35,36,37,38,39,40,41,42,43,44].

» Motions at Sacroiliac Joint:

• Three types of motion are available to the inominate bones:
  • Symmetrical Motion - Movement of both innominates as a unit relation to the sacrum [45,46,47,48].
    • During trunk flexion or bilateral hip flexion, the sacrum nutates (rotates anteriorly), so that the promontory moves ventrocaudally while the apex moves dorsocranially. 
    • This motion is resisted by following structures:
      • Wedge shape of the sacrum
      • Ridges & depressions of the articular surfaces
      • Friction coefficient of the joint surface
      • Integrity of the interosseous & sacrotuberous ligaments
    •  The sacrum counternutates, or moves in opposite direction, during trunk extension or bilateral hip extension.
    • This motion is resisted by the long dorsal sacroiliac ligament [6].
    • Combination of rotation and translation is angular movement of the sacrum, during which the iliac crests move closer together while the iliac tuberosities move further apart.
    • Greatest amount of movement, as much as 5.6+/-1.4 mm, occurs when going from recumbent to standing and reverses in direction when moving from standing to recumbent [47].
    • Rotation is accompanied by translation, which results in increased ligamentous tension and absorption of energy [49]. The SI joint thereby function as shock absorbers.
  • Asymmetrical Motion - Antagonistic movement of each innominate bone with relation to the sacrum, which includes movement at the symphysis pubis.
    • Pelvic torsion occurs at SI joints when asymmetrical forces are applied to the pelvis, as in static one-legged stance and the one-legged stance that occurs during gait and asymmetrical falls.

    

    • Clinicians can assess manually the end position that results from movement of one innominate bone relative to the other by palpating the relative prominence of the right and left ASISs & PSISs [2,50,51,52].
      • If left ASIS moves upward, the right ASIS and left PSIS  become more prominent while left ASIS and right PSIS become less prominent.
    • The proposed axis for pelvic torsion is transverse and passes through the symphysis pubis [53].

Lumbopelvic Motion - Rotation of the spine and both innominates as a unit around the femoral heads.

• Lumbopelvic Rhythm:

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» The Integrated Model of Function:

• Has four components
  • Form closure (structural)
  • Force closure (myofascial action)
  • Motor control (specific timing of muscle action/inaction during loading)
  • Emotions
• Form Closure refers to a state of stability within the pelvic mechanism, with the degree of stability dependent upon its antomy, with no need for extra forces to maintain the stable state of the system [54].
  • Following anatomic structures assist with form closure:
    • Wedge shaped sacrum and the friction coefficient of the articular cartilage - Both the coarseness of the cartilage and the complementary grooves and ridges increase the friction coefficient, and thus, contribute to form closure [55].
    • The integrity of the ligaments
    • The shape of the closely fitting joint surface
  •  Integrity of form closure is clinically evaluated with the long-arm and short-arm shear tests.
• Force Closure, the need for extra forces to keep an object in place, requires friction to be present [54].
  • The degree of friction depends on the compressive forces acting through the joint.
  • This dynamic force relies on intrinsic and extrinsic supports involving the osseous, articular, neurologic, and myofascial systems, and gravity. 
  • When sacrum moves towards nutation, the increase in ligament tension facilitates the force closure mechanism [54].
  • Force closure not only corresponds to muscles around the SI joint but all the core muscles - local and global muscle system.

» Local Muscle System:

• Pelvic floor muscles, diaphragm, transverse abdominis and deep multifidus.
• Other muscles under research to be included in local system - medial fibers of psoas, medial quadratus lumborum, lumbar part of iliocostalis and longissmus, posterior fibers of internal oblique.
• ROLE OF LOCAL MUSCLE SYSTEM:

• Stabilize the joints of the spine and pelvic girdle in preparation for (or in response to) the addition of loads.
• The mechanism involved
  • ↑ intra abdominal pressure [56,57,58,59,60]
  • ↑ng tension of thoracolumbar fascia [35,57,59,60,61] 
  • ↑ng articular stiffness [59,62,63]
• If the CNS can predict the timing of load the local muscle system's functions is anticipatory [58,59,64,65,66,67,68,69,70].

» Global muscle system:

1. There are 4 muscle slings

- Click on the image to enlarge

• Posterior Oblique - latissmus dorsi and gluteus maximus through thoracolumbar fascia
  • Significant contributor to load transference through the pelvic girdle during the rotational activities of gait.
• Anterior Oblique - exteranl oblique, anterior abdominal fascia, contralateral internal oblique and adductors of thigh
  • The oblique abdominals, acting as phasic muscles, initiate the movement [71] and are involved in all movements of the trunk and upper & lower extremities, except when the legs are crossed [72].
• Longitudinal - peronei, biceps femoris, sacrotuberous ligament, deep lamina of thoracolumbar fascia and erector spinae
  • This system counteracts any anteiror shear (sacral nutation), as well as facilitating the compression through the SI joints.
• Lateral Sling - Primary stabilizers of hip - gluteus medius and minimus, tensor fascia lata and lateral stabilizers of lumbopelvic region
  • Functions to stabilize the pelvic girdle on the femoral head during gait through a coordinated action. These muscles are reflexively inhibited with an instability of the SI joint.

 » Biomechanics of sacral motion:

• Sacrum is counternutated in supine [16] and nutated in sitting or standing.
• In optimal posture, sacrum is suspended between the hip bones in slight nutation but not complete [73].
• During the initial stages of forward bending, the sacrum completely nutates between the hip bones and should remain in nutation throughout the ROM.
• When an individual stands with excess kypholordosis or sway back, the sacrum is completely nutated and no further nutation will occur during forward or backward bending.

 » Biomechanics of Gait:

• An efficient gait requires a fully functioning lumar-pelvic-hip complex [74,75].
• Following describes the gait sequence:

- Click on the image to enlarge

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