unit supports abdomen and provides dynamic link between vertebral column and LL.

Closed osteoartciular ring composed of multiple bones and joints

1. sacral promontory

2. transverse processes: fuse with costal elements to form sacral ala

3. superior articular processes: articulate with L5 facets

on posterior sacral surface

• represents fused transverse processes of S1-S5 vertebrae
• 3 deep depressessions are located in lateral sacral crest at S1,S2,S3 vertebrae and serve as attachment sites for dorsal interossous SI ligament
• fuses with costal elements of sacral vertebrae to form lateral aspect of the sacrum which is wide superiorly while inferiorly the A-P dimension of the sacrum narrows to a thin border which curves medially to joint S5 vertebral body and forms inferior lateral angle of the sacrum (ILA)

lie between lateral and intermediate sacral crest

transmit dorsal sacral ramus of each sacral spinal nerve

1. contains articular surface of sacrum which is auricular (L shaped) and within the first 3 sacral vertebrae

2. short arm of the L-shaped articular surface lies in a vertical plan and is contained within the first sacral segment

3. long arm articular surcfe lies in an anteroposterior plane within the second and third sacral segments

4. contours of the articular surface have been reported to be highly variable dpending on age

5. articular surface is irregular and rough

• Coccyx Bone

• triangular shaped bone at bottom of sacrum
• results from fusion of 4 coccygeal segments
• base of coccyx has oval facet which articulates with inferior aspect of S5 vertebral body

1. ilium

2. pubis

3. ischium

• interosseous Sarcoiliac ligament

sacral surface is covered with hyaline cartilage while iliac portion is covered with fibrocartilage

depth of articular cartilage differs both within same articular surface and on opposing side

1. external fibrous layer: contains fibroblasts, blood vessels and collagen fibers

2. inner synovial layer

ligament of SI

1. weakest of group, basically thickening of capsule

2. can be palpated at Baer's point

SI ligament

• strongest of group
• fills the space between the lateral sacral crest and iliac tuberosity

SI ligament

• runs from lateral sacral crest at S3 & S4 to PSIS and inner lip of iliac crest
• lies posterior to interosseous ligament
• attaches to soft tissue
• deep lamina of posterior layer of thoracolumbar fascia
• aponeurosis of erector spinae
• mulitifidi
• superior band of ST ligament

• tension increases:
• contraction of muscles that join to the ligament
• counternutation of sacrum
• loading of ST ligament

tension decreases:

• nutation of sacrum
• contraction of LAT or GLUT

• composed of 3 fibrous bands
• 1. lateral band: connects ischial tub to PIIS
• 2. medial band: runs anteroinferolaterally from transverse tubercles of S3-S5 and lateral margin of lower sacrum and coccyx to ischial tuberosity
• 3. superior band: runs superficial to interosseous ligament and connects coccyx to PSIS
• muscular attachments:
• pirformis, glut max, biceps formis, multifidus

• SI ligament
• runs from lower lateral aspect of sacrum and coccyx to ischial spine of innominate
• closely connected to coccygeal muscle

• 5 bands
• anterior, superior, inferior, posterior, veterical
• individual bands of ligament have been described to be variablein number and form, but consistenyl arise from transverse processes of L4&L5 and blend inferiorly with SI ligaments and laterally with iliac crest

increases tension by:

1. motion: arms, legs, trunk

2. contraction: muscles that attach to it

3. stretching of muscles attached to it

increases tension allows inprovement of pelvic girdle and low back stability

fibrocartilagenous disc, but no synovial tissue or fluid, so symphysis

osseous surfaces are covered by a thin layer of hyaline cartilage and separated by the fibrocartilagenous disc

• forward motion of sacral promontary into pelvis about coronal axis in interrosseous ligament

occurs baterally when moving supine lying to standing or early stage of trunk forward bending

occurs unilaterally during flexion of LE

arthrokinematic motion occurs as sacrum glides inferior and posterior

glides are resisted by wedge shape of sacrum, irregular articular surface of SI joint, fricion of coefficient of the joint, interosseus and ST ligs, muscles that attach to the ligaments

• backward motion of sacral promontory about a coronal axis within the interossseous ligament
• occurs bilaterally whn lying supine and in some people towards end of forward flexion of the trunk, occurs unilaterally during exn of LE
• arthrokinematic motion: occurs as sacrum glides anterior and superior, this motion is resisted by long dorsal SI ligament and contraction of multifidus

• Occurs bilateraly during forward bending of the trunk and when rising to a seated position from supine lying

• Occurs unilaterally during exn of LE
• Produces motion with pelvic girdle and is same motion as sacral counternutation

• Athrokinematic motion: occurs as innominates glides inferior and posterior. Same motion as sacral counternutation

• Occurs bilaterally during backward bending of trunk

• Unilaterally with flexion of LE

• Produces motion within pelvic girdle and same motion as sacral nutation

• Arthrokinematic motion: occurs as innominate glides anteriorly and superiorly. This is same arthrokinematic motion as sacral nutation

• Stable situation with closely fitting joint surfaces where no extra forces are needed to maintain the state of the system. The contributions to form closure:

1. Shape of joint surface
2. Friction of coefficient of articular cartilage
3. Integrity of ligaments that approximate the joint

• Extra forces are needed to keep the object in place, and friction must be present

1. Factors that increase intra-articular compression will increase the friction coefficient and ability of the joint to resist translation

2. Numerous systems contribute to dynamic stability of the system and include the following:

○ Intrinsic factors

Osseous integrity  Articular/ligamentous integrity §Myofascial integrity 

 Neuron control ○

Extrinsic factors

• inner unit: pelvic floor muscles, transverse abdominus, multifidus, diaphragm
• outer unit: 4 systems

1. posterior oblique: LAT, glut MAX, TLF

2. deep longitudinal: erector spinae, deep lamina of TLF, ST ligament, biceps femoris

3. anterior oblique: oblique abdominals, contralateral adductor muscles, anterior ab fascia

4. lateral: gluteus medius and minimus and contralateral adduction of the thigh

• symtpms slow or fast onset
• history of trauma in past
• pain localized to region around joint, but can radiate through pelvis to anterior aspect of ipsilateral groin and or down posterolateral buttock and thigh to the knee
• hypomobility: pain can go to opposite side if there a long time secondary to overloading
• aggrevating activities: walking, ambulating up/down stairs, rolling in bed, sit to stand transfers, standing on one leg
• uncomfortable in any one prolonged position or activity and may be considered fidgity

1. hypermob of SI joint can occur secondary to repeated microtrauma, major trauma, or hormonal changes of prego
2. report seend onset of unilateral SI and/or pubic symph pain which may radiate into groin, buttock, posterior thigh and/or abdomen
3. mechanism of injury is often unexpected vertical load through a WB limb (or ischium), or a load applied to the trunk whil in flexed/rotated position
4. aggravating factors include: unilateral WB, forward bending, lifting, lying supring and rolling fromthis position, fast walking, and any prolonged activity

1. increase motion of SI joint coupled with soft end feel on stability testing
2. muscle gaurding/holding patterns
3. joint play test are blcoked and results of palpating for position assessment of innominate and sacrum differeniate the various dysfunctions. articular compression must be reduced to restore pelvic stability
4. gait: displacement of COG is excessive when SI joint is unstable or copmressed. Will see a componsated and uncomponsated trendelenburg gait
5. posture: adopt to resting posture than unlaods affected SI joint
6. functional test: asymmetries in motion of PSI with motion palpation similar to that of hypomobile SI joint, palpation for position will differentiate
7. articular mobility/stability tests: if hypermobile joint is compressed the gliding tests are completely blocked (differentiation between compressed hypomobile joint and hypomobile joint is quality of end feel with stability testing and resting position of the innominates/sacrum with palpation) if hypermobile joint is not compressed the stability tests reveal increased motion and a soft end feel

1. increased lumbosacral angle secondary to structure, overweight, poor abdominal tone, prego

2. posterior SI joint ligamentous weakness

3. lumbar spine hyperextension

4. labor and delivery complication

5. weak glut med and max

• diffuse lumbosacral pain
• gluteal pain, occasionally sciatic pain painful
• prolonged standing painful
• walking and stair climbing(especially down stairs or incline)

• deep sacral sulci and shallow ILA
• palpation tenderness over ILA, posterior SI joint ligament and baer's point
• increased piriformis and psoas tone
• sacral nutation hypermobility if not compressed, hypomobility if compressed
• swelling over posterior sacral area

• reduced lumbosacral angle secondary to structure
• flexed sitting/stand postures
• lumbar spine hyperflexion
• squatting, bending, and lifting
• coccygeal muscles spasm

• diffuse lumbosacral pain
• occasional gluteal pain
• pain with walking and stair climbing
• pain with sit <--> stand stransfer

in hypermobility derrangments, describe the signs of sacral counternutation

1. shallow sacral sulci and deep ILA
2. less prominent PSIS
3. spasm or tight pelvic diaphragm muscles
4. sacral flexion restriction

1. left psis inferior and posterior
2. left asis is superior and ant.
3. tenderness of left posterior SI ligament and left sacral sulcus
4. tenderness over left ischial tuberosity and left ILA

1. rt hip hyperexn
2. rt. hip flexor tightness
3. golfing
4. rt glut medius and max weakness & abdominal weakness
5. jarring landing on outstretched rt leg

• rt side LS pain & glut pain
• occassional rt. ant. hip pain
• pain walking and stairs
• occasional pain relieft with walking (due to decreasing muscle tone)

1. rt PSIS sup., and right ASIS inf.
2. rt ischial tub post
3. tenderness over right PSIS (ST lig) and ILA
4. long rt leg supine
5. rt iliac post. rotation restriction (compressed) or hypermobility (noncompressed)

• pts with localized pain in absence of mpositive mobility/stability findingchallenging to treat
• most likely cause of a biomechanical nature can be overuse of the articular and myofascial tissues secondary to altered function elsewhere (hip, knee, foot, etc). THe most effective treatment approach for this will look at the function of the trunk and lower kinetic chain as a biomechanical linkage and address and improve dysfunctions noted
• pt could have nonmechanical disorder, and a list various disease conditions that can affect the SI joint follows:
• inflammatory disorders, joint infection, metabolic disorder,

• insidious or acute
• activities which stress musculature that inserts in area of symphysis pubis (rectus ab and adductor muscle groups) can lead to osteitis pubis
• activities requiring forceful abduction strain the anterior portion of the pelvic ring
• single limb support activities place significant shear forces across the symphysis
• can occur secondary to avulsion fractures of muscle inserting in the area

• can occur in all populations and age ranges, but most common 20-30yrs who are athletically active
• 5x more in males
• sharp stabbing, and burning pain in area of symphysis which can radiate down medial aspect of leg and into scrotal area
• pain increased by running, pivoting, twisting, climbing stairs, kicking, sit-ups, leg raises, Valsalva maneuver
• laying on side and inducing compression through symphysis
• predisposing factors include limited hip IR, leg length discrepancy, excessive pronation, genu varum or valgum
• palpation: tender over symphysis
• stretching of any muscle inserting in area increases pain
• contraction of any muscle inserting in area increases pain
• compression across symphysis creates pain
• X-ray findings can include irregular bony margins, pubic bone sclerosis, and widening of the joint space