Pelvis and Sacrum Anatomy

• Lecture objectives involve attaining working knowledge of the basic anatomy of the pelvis and sacrum including neurovascular structures, pelvis and sacrum motion and biomechanics, identification of the superior, middle and inferior transverse sacral axes, the characteristics and diagnostic criteria of pelvic somatic dysfunctions, and pelvic and lumbar mechanics during gait

Functional Anatomy:

• The false pelvis is part of the lower abdomen and is walled laterally by the ilia
• The true pelvis is inferior and posterior to the abdomen
• The true pelvis includes the sacral promontory, arcuate line, pectinate line, pubic bones, and inferior fascia of the pelvic diaphragm

Bones of the Pelvis

• There are two innominate bones, each comprised of the ilium, ischium, and pubis
• The sacrum consists of 5 fused vertebrae
• The sacrum has a base, an apex, and it transfers weight

The Pelvis

• Embryologically, the innominate starts as a single cartilaginous element
• By the late teens to early 20s, complete fusion of the innominate occurs
• Each innominate bone consists of the ilium, ischium, and pubis
• Each have primary ossification centers

Sacrum

• The sacrum is a large inverted triangle formed by the fusion of five sacral vertebrae
• The superior aspect is the base and the inferior aspect the apex
• The sacrum articulates superiorly to L5, forming the LS junction (Ferguson's angle)
• It articulates bilaterally with the innominates to form the complete ring of the pelvis
• S1's anterior superior portion is called the sacral promontory
• The anterior surface is concave, while the posterior surface is convex with palpable spinous tubercles
• The medial row of tubercles is formed by the fusion of sacral articular processes
• The lateral row is formed by the fusion of sacral transverse processes, ending inferiorly in the ILA
• The sacral canal has four bilateral sacral foramina for the ventral and dorsal rami of the first four sacral spinal nerves
• The sacral hiatus is a defect near the apex where sacral epidural nerve blocks given
• The coccyx attaches to the sacral apex via the sacrococcygeal joint
• The Ganglion impar, where the R and L sympathetic chains join, rests on the anterior surface of the coccyx

Transverse Sacral Axes:

• Superior transverse sacral axis: Located above S2 and is the axis around which the cranial primary respiratory mechanism creates motion
• Middle transverse sacral axis: Located at S2 and is the axis for sacroiliac motion and forward and backward bending
• Inferior transverse sacral axis: Located below S2 and is the axis for iliosacral motion, which involves rotation of the innominates

Accessory Ligaments of the Pelvis:

• Three accessory ligaments include the iliolumbar, sacrotuberous, and sacrospinous ligaments
• The sacrospinous ligament forms lesser and greater sciatic foramen

Iliolumbar ligament

• Attaches from the anterior surface of the iliac crest and anterior surface of sacral base to L4 and L5 transverse processes
• The lower fibers integrate with the anterior SI ligament, integrating SI mechanics with the L-spine
• Irritation can be caused by LS instability
• Pain can be referred to the groin via the ilioinguinal nerve, which mimics the pain in an inguinal hernia
• Implicated in nerve entrapment syndrome

Sacrotuberous ligament:

• Runs from the inferior medial border of the sacrum and inserts on the ischial tuberosities and posterior margins of the sciatic notches

Sacrospinous ligaments:

• Anterior to the sacrotuberous ligaments and attaches to the ischial spines, dividing the space into a greater and lesser sciatic foramen
• The SS and ST ligaments restrain anterior movement of the sacrum within the pelvic bones
• In a weight-bearing position, without strong pelvic ligaments, the sacral base tends to rock anteriorly
• Stress on these ligaments can cause lumbosacral imbalance, chronic back pain, and joint degeneration

Neurovascular System:

• The arterial supply begins with the abdominal aorta, branching into the R & L common iliac arteries
• Aortic bifurcation occurs at the L3, umbilicus level
• The R and L common iliac arteries diverge and descend to the LS junction, then divide into the internal and external iliac arteries
• The external iliac artery supplies the lower extremity
• The internal iliac artery divides into anterior and posterior trunks, supplying the pelvis
• The posterior trunk contains the iliolumbar artery, lateral sacral arteries, and superior gluteal artery
• The anterior trunk contains the umbilical, superior vesical, inferior vesical/vaginal, middle rectal, obturator, internal pudendal, and inferior gluteal arteries

Somatic and Autonomic Nervous System:

• Somatic Plexus: Sacral (L4-S4), Sciatic n. (L4-S3), Pudendal n. (S2-S4), and Coccygeal (S4-C0)
• Autonomic NS: SANS (T12-L2) and PANS (S2-S4)

Somatic Plexuses:

• Sacral plexus (ventral rami of L4-S4): Gray rami communicants from the sympathetic trunk connect with anterior rami and carry postganglionic SANS fibers, while pelvic splanchnic nerves carry PANS fibers from S2-4.
• Sciatic Nerve (L4-S3): Forms the Common fibular (peroneal) n. (L4-S2) and Tibial n.(L4-S3); innervates muscles of thigh (post. compartment), leg, and foot and gives sensation from foot and lateral leg
• Pudental (S2-4): Goes through Lr. sciatic foramen, inferior to piriformis, and enters gluteal region; innervates skin and muscles of perineum
• Additional branches form the Superior gluteal (L4-S1), which targets the Glut med & min and TFL; Inferior gluteal (L5-S2) which targets the Glut max; and Obturator n. (L2-L4), containing cutaneous nerve which passes through ST ligament
• Coccygeal plexus: (S4-CO) – S5 & C0 penetrates coccygeus m. & ss & st ligaments, join s4, and forms anococcygeal nerves and innervates skin in anal triangle.

Autonomic Innervation of Pelvic Viscera

• Lower GI and GU: SANS (T12 – L2 via paravertebral trunk and sacral splanchnics) and PANS (S2- S4)

Muscles

• No specific muscles move the SI Joint
• Primary muscles: Pelvic Diaphragm
• Secondary muscles: Partial attachment to true pelvis: rectus abdominis, transverse abdominis, obliques, QL, piriformis, obturator internus

Muscles: Primary

• Intrinsic Pelvic Diaphragm: Levator Ani (iliococcygeus, pubococcygeus, puborectalis) and Coccygeus

• Muscles move synchronously with the abdominal diaphragm and aid in lymphatic return

• Primary muscles (muscles of the perineum) provide support during defecation, inhibit bladder activity, and assist in providing lumbosacral pelvic support, innervated by the pudental nerve

• Divided into urogential triangle and the anal triangle

• Intrinsic to the pelvic floor

• Urogenital diaphragm (urogenital triangle) spans the area between the ischiopubic rami and is filled by a thick fibrous sheet (the perineal membrane) containing a layer of skeletal muscle and neurovascular elements.

• Pelvic diaphragm is formed by the Levator ani (iliococygeus, pubococcygeus, puborectalis) and the Coccygeus muscles which overly the sacrospinous ligaments and attach to the ischial spine

• The pelvic diaphragm works synchronously with the abdominal diaphragm and aids in lymphatic return

Muscles: Secondary

• Rectus Abdominis
• Transverse Abdominis
• Obliques
• Quadratus Lumborum
• Lower Extremities - piriformis and obturator internus

Anterior Landmarks

• Iliac Crest
• ASIS
• Pubic Symphysis
• Pubic Tubercle
• Medial Malleoli

Posterior Landmarks

• PSIS
• Sacral Sulcus
• Sacral Base
• ILA
• Sacrotuberous Ligament
• Ischial Tuberosity
• Iliac Crest
• Piriformis Muscle
• Iliolumbar Ligament

Pelvic Diagnosis (Somatic Dysfunction)

• Innominate dysfunction: anterior rotation, posterior rotation, superior shear, inferior shear, outflare, inflare
• Pubic Bone: Superior, Inferior, Abduction, and Adduction

Standing Flexion Test

• Positive test occurs on the side that moves first and furthest
• Indicates an Iliosacral Somatic Dysfunction of the ipsilateral side
• Name based off landmarks on the side of a positive SFT

ASIS Compression Test

• Indicates Iliosacral Somatic Dysfunction of the ipsilateral side
• Positive test occurs on the side that moves the least/restricted

Superior/Inferior Innominate Shear

• Superior Innominate Shear (upslipped): Shear between innominate & sacrum with superior subluxation, often associated with pelvic or low back pain and a history of falling on one ischial tuberosity
• Inferior Innominate Shear (downslipped): Shear between the innominate and sacrum with inferior subluxation; rare, often reduced by walking, and may be associated with pelvic or low back pain

Left Supererior Innominate Shear (upslipped)

• Left ASIS superior
• Left medial malleolus superior
• Left PSIS superior
• Left ischial tuberosity superior
• Left sacrotuberous ligament lax
• Positive Left Standing Flexion Test

Right Innominate Inferior Shear (downslipped)

• Right ASIS inferior
• Right medial malleolus inferior
• Right PSIS inferior
• Right ischial tuberosity inferior
• Right sacrotuberous ligament tight
• Positive Right Standing Flexion Test

Innominate Rotations

• Posterior Innominate Rotations: Rotated posteriorly around a transverse axis compared to the contralateral innominate
• Inguinal/groin pain affecting the Rectus Femoris may occur
• Knee pain due to Sartorius muscle may also occur
• Resistant to anterior rotation of pelvis

Posterior Rotation of lilium

• Results in superior travel of the acetabulum and a decrease in leg length

Left Innominate Posteriorly Rotated

• Left + standing flexion test
• Left ASIS superior
• Left medial malleolus superior
• Left PSIS inferior
• Left sacral sulcus deep

Posterior Rotators

• Gluteus maximus muscle
• Semitendinosis muscle
• Biceps Femoris muscle
• Semimembranosus muscle
• Piriformis muscle (weak)
• Iliopsoas
• External abdominal oblique muscle

Innominate Rotations

• Anterior Innominate Rotations
• Travel inferiorly of the acetabulum leading to a increase in leg length

Right Innominate Anteriorly Rotated

• Right standing flexion test +
• Right ASIS inferior
• Right medial malleolus inferior
• Right PSIS superior
• Right sacral sulcus shallow

Anterior Rotators

• TFL muscle
• Quadraceps muscles
• Quadratus lumborum
• Iliocostal muscles
• Internal abdominal oblique muscles
• Latissimus dorsi muscle

Innominate Flares

• Innominate flares are apparent when the ASIS is more medial on one side than the other
• The Standing Flexion Test assists with diagnosis
• Measure from the umbilicus or xyphoid to each ASIS

Inflares:

• Positive standing forward bending test leads to closer ASIS to midline (medial)

Outflares:

• Positive standing forward bending test leads to ASIS being further from midline (lateral)

Right Innominate Medial (inflare)

• Right standing flexion test +
• Right ASIS medial
• Right PSIS lateral
• Right sacral sulcus wide

Left Innominate Lateral (outflare)

• Left standing flexion test +
• Left ASIS lateral
• Left PSIS medial
• Left sacral sulcus narrow

Pubic Symphysis

• Motion occurs around a transverse axis during normal gait
• Dysfunctions are common during pregnancy and childbirth
• Fibrocartilaginous joint with movement determined by its anatomic shape, ligaments, and muscular attachment
• Muscular forces acting on each pubic ramus result in rotation at the symphysis, about a transverse axis
• The Interpubic disc, a Synarthrosis, is covered in hyaline cartilage and has a surface that is not flat and resists shearing forces
• The area is marked by an Inguinal Ligament that serve as a palpatory surgical landmark
• Dysfunctions like Abduction are more common during pregnancy and childbirth
• Can move in three planes of motion: Inferior/superior, Anterior/posterior and Rotation

Pubic Shears

• Uneven tension on pelvic diaphragm
• Associated symptoms: Constipation, Urinary Symptoms, Dyspareunia, Supra-pubic pain, Low back pain, and Anterior thigh pain

Left Superior Pubic Shear

• Left standing flexion test +
• Left pubic tubercle superior
• Left inguinal ligament tense and tender
• Muscles involved with Superior Pubic Symphysis Rectus abdominus and Pyramidalis muscle

Right Inferior Pubic Shear

• Right standing flexion test +
• Right pubic tubercle inferior
• Right inguinal ligament tense and tender

Innominate somatic dysfunction diagnoses that can be determined by:

• Lateralization test with the Standing Flexion test or ASIS compression test (after a hip flop maneuver)
• Check levels of ASIS's (in supine position)
• Check levels of medial malleoli (in supine position)
• Check levels of PSIS's (in prone position)
• These include Anterior Rotation, Posterior rotation, Superior shear, and Inferior shear

Pelvic Mechanics During the Walking Cycle

• Walking induces (reversible) neutral oblique axis mechanics in the sacrum with every step, e.g., right rotation on right oblique axis, then left on left oblique axis
• It induces side bending of the lumbar spine towards the side of the weight bearing leg
• It also causes anterior rotation of the weight bearing ilium and posterior rotation of the opposite ilium

The Walking Cycle

• Moving the right foot shifts the body to the left
• An SLRR motion occurs in the lumbar spine
• A left oblique axis occurs in the sacrum
• The sacrum rotates left on its oblique axis as the sacrum rotates in an anterior/forward direction
• This explains why the sacrum moves opposite in relation to L5