Flashcards in Arthrology Of Hte Vertebral Column Deck (229):
The ligamentous sacro-iliac joints (or most of the ligamentous joints of the vertebral column) are examples of which joint classification?
Which classification of cartilage joint is the first to appear developmentally?
Which cartilage joint occurs between bones formed by endochondral ossification?
What are the four consistent features of synovial (death)joints?
Articular or fibrous capsule, synovial membrane, articular cartilage and synovial fluid
Type I articular receptors are located in what part of the joint?
Superficial layer of the fibrous capsule
What is the function of the type I articular receptor?
They monitor the joint "at rest"
What are the characteristics of the type I articular receptors?
Located in the superficial layer of the fibrous capsule, resemble Ruffini endings, most numerous in cervical zygapophyses and they monitor the joint "at rest"
Type II articular receptors are located in what part of the joint?
Deeper strata of the fibrous capsule
What is the function of type II articular receptors?
They monitor the joint during normal range of motion
What are the characteristics of type II articular receptors?
Located in deeper strata of the fibrous capsule, resemble pacinian corpuscles, most numerous in the cervical spine and monitor the joint during normal range of motion
What is the location of type III articular receptors?
Collateral and intrinsic ligaments of many joints, they were not thought to be present along the vertebral column
What is the function of the type III articular receptors?
They monitor the joint during extreme motion or beyond normal range of motion
What are the characteristics of type III articular receptors?
Present, in collaterals and intrinsic ligaments, resemble Golgi tendon organs, not initially observed along the vertebral column and monitor extreme joint motion
What is the function of type IV articular receptors?
Nociceptive, they monitor pain
Type IVa articular receptors would be present in what locations?
Fibrous capsule, articular fat pads or adventitia of blood vessels
Type IVb articular receptors would be present in what locations?
Accessory ligaments in general, dense in the posterior longitudinal ligament of the spine
What are the 3 modifications of articular synovial membrane?
1) synovial villi
2) articular fat pads or Haversian glands
3) synovial menisci and intra-articular discs
Articular fat pads are most numerous in what location along the vertebral column?
What are the specific functions of type A synovial cells?
What is the specific function of Type B synovial cells?
Secrete proteinaceous substances and hyaluronic acid
Which collagen fiber type predominates in articular cartilage?
What are proteoglycans composed of?
A core protein and glycosaminoglycans
Cartilage is able to change shape due to compression, a characteristic known as _____?
What is implied when cartilage is said to have elastic properties?
Cartilage can deform and returns to original volume rapidly, a time independent property
What is implied when cartilage is said to have viscoelastic properties?
Cartilage can deform but returns to original volume slowly, a time dependent property
Which theory of joint lubrication implies a loss of fluid form the cartilage into the joint space during compression results in increased viscosity of the synovial fluid?
Which theory of joint lubrication implies water loss form the synovial fluid increases viscosity of the remaining synovial fluid?
Which theory of joint lubrication implies an adsorption of lubricant onto cartilage surfaces is responsible for the low-friction observed during movement?
What are the properties of synovial fluid?
It is yellow-white, viscous, slightly alkaline and tastes salty
Synovial fluid consists of what specific chemical groups?
Fats, salts, albumins and hyaluronate
Which substance in synovial fluid was first thought to be responsible for its viscosity and lubricating behavior?
What is a complex synovial joint (diarthrosis)?
Within the simple joint or the compound joint, the articulating surfaces are separated by an articular disc (intra-articular disc) or meniscus
What morphological classification of synovial joints is classified as nonaxial?
Plane (diarthrosis arthrodia)
What morphological classifications of synovial joints would be classified as uniaxial?
Hinge (diarthrosis ginglymus) and pivot (diarthrosis trochoid)
What morphological classifications of synovial joints would be classified as biaxial?
(Diarthrosis) bicondylar, (diarthrosis) condylar, (diarthrosis) ellipsoidal and saddle (diarthrosis sellar)
Which vertebrae will have common ligaments?
Segments C2-L5 inclusive
What is the maximum number of common ligaments IDed with a vertebral couple?
What is the reason that nine common ligaments are IDed but only 8 will be attached at any specific vertebral couple?
The ligamentum nuchae and spurapsinous ligaments attach to the spinous tubercles, but only one of these will be IDed at a single vertebral couple
What common ligament(s) will attach to the vertebral arch?
The ligamentum flavum
In the fetus, which vertebral levels will demonstrate intervertebral discs?
These between C2 and Co1 inclusive
What is the number of true intervertebral discs IDed in the adult?
In the adult, which vertebral levels will demonstrate a true intervertebral disc?
What is the effect of aging on the intervertebral disc nucleus pulposus?
It becomes more fibrous as water and proteoglycans concentrations diminish
What is the cervical nucleus pulposus composed of?
What is the consequence of non-aggregated glycosaminoglycans in the lumbar nucleus pulposus?
The nucleus pulposus will lose water under deformation conditions
What cells is associated with the nucleus pulposus until about age 11?
Which type of collagen is dominant in the nucleus pulposus?
Collagen type II
What is unusual about the cervical annulus fibrosus?
It lacks any lamellar or layered organization
What is the appearance of the cervical annulus fibrosus?
A horse-shoe with the anterior margin thick and the lateral margins tapering to the uncinate processes; the posterior marking is thin
What is the organization of the lumbar annulus fibrosus?
It has 12-14 concentric cylindrical lamellae
What is the reason the anterior margin of the lumbar intervertebral disc is les susceptible to rupture, herniation, prolapse, or protrusion?
The anterior margin of each lamellae is thick and the anterior longitudinal ligament is talons better developed than; the posterior allele or posterolateral longitudinal ligament
What is the water concentration in the lumbar annulus fibrosus at birth and after thirty?
Brith - 78%; 30 - 70%
What is the consequence of aggregated glycosaminoglycans in the lumbar annulus fibrosus?
The annulus fibrosus will retain water under deformation conditions
Which type of collagen is dominant in the annulus fibrosus?
Collagen type I
What is the organizational pattern for collagen fibers in the annulus fibrosus?
They are parallel with one another in a single lamellus and angled
What is the organization of collagen fibers between lamellae?
Collagen fibers will be angled in the opposite direction such that a spiral - counter spinal organization is observed
What is the average angle of collagen fibers within the annulus fibrosus?
They average 50-60 degrees
What is the principal type of collage fiber within the cartilage end plate?
The type II collagen fiber
What is the direction of collagen fibers within the cartilage end plate?
Collagen fibers are aligned anterior to posterior
What cell type is associated with the cartilage end plate?
What is the earliest indicator of intervertebral disc pathology of degeneration?
Changes in the histology of the cartilage end plate
What part of the intervertebral dis is the "growth plate"?
The transition zone
What part of the intervertebral disc is innervated?
The outer lamellae of the annulus fibrosus
What are the types of receptor endings in the intervertebral disc?
Nociceptors and proprioceptors
What is the relationship between size of the intervertebral disc and receptor endings?
The larger the disc, the greater the variety of receptor endings
What is the proposed function of receptor ending density in the anterior part of the intervertebral disc?
They provide feedback during extension
What innervates the annulus fibrosus at the posterior part of the intervertebral disc?
The recurrent meningeal/sinu-vertebral/sinus vertebral nerve
What forms the posterior neural plexus of the vertebral column?
The sinu-vertebral nerve (sinus vertebral nerve, recurrent meningeal nerve)
What forms the anterior neural plexus of the vertebral column?
Fibers form the ventral primary ramus
What is the name given to the white ramus communicans which becomes embedded within the annulus fibrosus of the intervertebral disc?
The paradiscal ramus communicans
How does the intervertebral disc attach to the vertebral body?
Sharpey's fibers from the outer lamellae of the annulus fibrosus are firmly embedded into the epiphyseal rims of the adjacent vertebral bodies
In what regions of the vertebral column is the space formed between vertebral bodies by the intervertebral disc the greatest?
The cervical and lumbar curves
Which curve direction will be associated with the greatest space observed between vertebral bodies?
The lordotic or anterior curves
Wha structure is formed following migration of sclerotomes to surround the notochord?
The perichordal blastema
What is formed within the perichordal blastema between the sclerotomites?
The intrasclerotomal fissure or fissure of Von Ebner
What does the intrasclerotomal fissure or fissure of Von Ebner become?
The perichordal disc
What part of the intervertebral disc will the notochord form?
The nucleus pulposus
What part of the intervertebral disc will the perichordal disc form?
The annulus fibrosus
What is the earliest indicator of the position of the adult intervertebral disc?
The intrasclerotomal fissure or fissure of Von Ebner
What vertebral levels will the anterior longitudinal attach to?
Those between occiput and S3 inclusive are traditionally indicated
What is the lowest extent of the anterior longitudinal ligament based on recent studies?
What is the function of the anterior longitudinal ligament?
It brakes or limits dorsi-flexion or hyperextension of the vertebral column
What was ossification of the anterior longitudinal ligament in the lumbar region IDed as?
Anterior longitudinal ligament ossification in the lumbar region is now IDed as ______?
Diffuse Idiopathic Skeletal Hyperostosis or DISH
What vertebral levels will the posterior longitudinal ligament attach to?
Those between C2-S3 inclusive are traditionally indicated
Newer research suggests what regional variation in the posterior longitudinal ligaments?
Prominent in the cervicals occasional in the thoracics, rare in the lumbars
What is the relationship of the posterior longitudinal ligament to the vertebral Foramen?
It forms the anterior boundary of the spinal canal
What is the innermost layer of the posterior longitudinal ligament called?
The peri vertebral ligament
Where is ossification of the posterior longitudinal ligament most commonly IDed?
The cervical spine with an 80% incidence
What is the clinical sign of posterior longitudinal ligament ossification in the cervical spine?
A loss of hand and finger dexterity
What is the incidence of posterior longitudinal ligament ossification in the thoracic and lumbar spine?
About 10% at the thoracic and 10% at the lumbar spine
What is the clinical sign of posterior longitudinal ligament ossification in the lumbar spine?
What is the gender, age and ethnic bias associated with ossification of the posterior longitudinal ligament?
It is greater in males over 50 and has a higher incidence in the Japanese
What is the acronym for ossification of the posterior longitudinal ligament?
At one time ossification of the posterior longitudinal ligament was an example of what condition?
Diffuse Idiopathic Skeletal Hyperostosis or DISH
What vertebral levels will the ligamentum flavum be attached to ?
What are the specific body attaching sites for he ligamentum flavum?
In which region of the spine will the ligamentum flavum be paired?
The lumbar spine
What is the name given to the ligamentum flavum based on appearance and histology?
It is a yellow elastic ligament
What fibers are most ligaments made up of?
Collagen type I fibers which are whitish in appearance
What is the color of most ligaments in the body?
They are whitish
What is now thought to be a major function of the ligamentum flavum?
It is an early prime factor in extension of the vertebral column
Where is ossification of the ligamentum flavum commonly IDed?
The thoracic spine or thoracolumbar transition zone
What is the acronym for ossification of the ligamentum flavum?
What is the relationship between the capsular ligament and mobility?
The more lax/loose the capsular ligament is, the greater motion of he joint
What regions of the vertebral column demonstrate the greatest laxity of capsular ligaments?
The cervical and lumbar regions
What layers are present in the capsular ligament?
Superficial layer of collagen fibers and a deep layer of elastic fibers
What muscle will blend with the capsular ligament posteriorly?
What is the pattern of innervation for the zygapophysis capsular ligament?
It is typically innervated by 3 spinal nerves
If the zygapophyses leader capsular ligament is not significantly involved in restricting motion what is its function?
It is probably involved in proprioceptive feedback to the muscles stabilizing the vertebral couple during movement
What vertebral levels will the interspinous ligament be attached to?
Those between C2-S1 inclusive
What is the status of the human interspinous ligament compared with other common ligaments?
They are poorly developed as ligaments although collagen fibers are present
What is the status of the cervical interspinous ligament?
It is lacking, poorly developed, or simply considered the deep anterior part of the ligamentum nuchae
What is the status of the lumbar interspinous ligament?
It is broad and thick with 3 layers IDed form anterior to posterior
What is the classic function of the interspinous ligament?
It brakes or limits flexion of the vertebral column
What is now thought to be a major function of the interspinous ligament?
It is more likely a proprioceptive transducer for the spinal reflex
What will the ligamentum nuchae be attached to?
The external occipital protuberance, external occipital crest, the posterior tubercle of the posterior arch of C1, and spinous tubercles between C2-C7 inclusive
What is the name given to the superficial layer of the ligamentum nuchae?
The funicular layer or part
What is the name given to the deep layer of the ligamentum nuchae?
Lamellae layer or part
What are the attachment sites for the superficial layer of the ligamentum nuchae?
The external occipital protuberance, external occipital crest and spinous tubercle of C7
What is the histological make-up of the limgentum nuchae?
It is a yellow elastic ligament
What is the primary yellow elastic or elastic ligament of the spine?
The ligamentum flavum
What is the termination level inferiorly for the spurapsinous ligament according to current literature?
Primarily at L4 (73%), between L4 and L5 (5%)
Where is the spurapsinous ligament said to be best developed?
In the lumbar spine
What is now thought to be a major function of the spurapsinous ligament?
It is a proprioceptive transducer for the spinal reflex
Where in the vertebral column is the intertransverse ligament best developed?
In the lumbar spine
What is the classification of the atlanto-occipital joint?
Synovial (diarthrosis) ellipsoidal
What are the attachment sites of the anterior atlant-occipital ligament?
It is attached to the upper margin of the anterior arch of C1 and to the anterior margin of the Foramen magnum
What is the median thickening of the anterior atlanto-occipital ligament called?
The anterior longitudinal ligament
What is the classic function of the anterior atlanto-occipital ligament?
It brakes or limits "extension" of the skull over the cervical spine
Based on the amount of ossification o fate anterior free margin of the posterior atlanto-occipital ligament what structures will form?
An incomplete ponticulus posticus of a complete or a complete one
What names are given to the opening formed by the ponticulus posticus?
Arcuate Foramen or retro articular canal
What is the classic function of the posterior atlanto-occipital ligament?
It brakes or limits axial rotation, flexion, and perhaps lateral bending of the skull on atlas
What amount of flexion - extension is accommodated by the atlanto-occipital joint?
About 25 degrees
What amount of axial rotation is accommodated by the atlanto-occipital joint?
Abut 3-8 degrees one side axial rotation
What amount of lateral bending is accommodated by the atlanto-occipital joint?
About 5 degrees
Which motion is bets accommodated by the atlanto-occipital joint?
What are the joint surfaces of the median atlanto-axial joint at the anterior bursa?
The fovea dentis of C1 and the facet for fovea dentis of C2
What are the joint surfaces of the median atlanto-axial joint at the posterior bursa?
The groove fro the trades atlantal ligament of C2 and the transverse atlantal ligament
What ligament is formed by the transverse atlantal ligament and its perpendicular extension?
The cruciate ligament or cruciform ligament
What is the function of the transverse atlantal ligament?
It is the primary stabilizer of the atlanto-axial joint restriction the distance of C2 form the anterior arch of C1
What is the ADI?
The atlanto-dental interspace, a radiographic distance between the surfaces of the anterior bursa of the median atlanto-axial joint
What is the ADI of children compared to that of adults?
About 4.5 mm in children; 2-3mm in adults
What are the characteristics of the capsular ligament of the lateral atlanto-axial joint?
It is lax or loose and demonstrates a meniscoidal fold within the joint cavity
What are the attachment sites for the accessory atlanto-axial ligament?
The base of the odontoid process and vertebral bod of axis to the tubercle for the transverse atlantal ligament n the lateral mad of C1; a superior continuation may attach on theocipitla bone use behind that of the alar ligament
What ligament forms the posterior boundary of the spinal canal at C1/C2?
The posterior atlanto-axial ligament
What are the degrees of movement facilitated at the atlanto-axial joint?
About 20 degrees flexion-extension, 40 degrees on side axial rotation, and 5 degrees of lateral bending
The occiput-C1-C2 joint complex accounts for what percent of all cervical axial rotation?
What name is given to the occipital-C2 region of the spine?
The carniovertebral junction
What ligaments are present at the occipital-axial joints?
The apical ligament of the dens/apicodental ligemtn, alar ligament and membranes tectoria/tectorial membrane
What ligament attaches to the anterior margin of the Foramen magnum and the tip of the odontoid process of C2?
The apical ligament of the dens
Embryologically, what forms the apical ligament of the dens?
What is the function of he apical ligament of the dens?
It has no known function
What ligament attaches to the posterolateral part of hate odontoid process of C2 and to surface on the medial morder of the occipital condyle or as far anterior as the Anterolateral margin of the Foramen magnum?
The alar ligament
What is the function of the alar ligament?
Together they function to resist axial rotation
What forms the cranial continuation of the posterior longitudinal ligament?
The membrane tectoria or tectorial membrane
What are the attachment sites for the superficial layer of he membrane tectoria or tectorial membrane?
The posterior part of the inferior epiphyseal rim and vertebral body of C2 to the capsular ligament of the atlanto-occipital joint and cranial dura of the posterior cranial fossa
What ligament is anterior to the dura mater of the medulla oblongata - spinal cord junction?
The membrane tectoria or tectorial membrane
What is the function of the membrana tectoria or tectorial membrane?
It resists flexion and extension of the skull on the upper cervical spine
List, in order, the ligaments in a midsagittal plane form the dura mater at the level of the medulla oblongata to the anterior bursa of the medieval atlanto-axial joint.
The membrana tectoria, the cruciate ligament, the capsular ligament of the posterior bursa of the median atlanto-axial joint, the alar ligament
For the cervical spine below C2, what is the range of flexion - extension?
About 90 degrees or about 18 degrees per couple
For the cervical spine below C2, what is the range of one side lateral bending/
About 50 degrees or about 10 degrees per couple
For the cervical spine below C2, what is the range of one side axial rotation?
About 33 degrees or about 6 degrees per couple
For the cervical spine below C2, what is the range of motion allowed for each type of join motion IDed?
90 degrees flexion-extensions, 50 one side lateral bending, 33 one side axial rotation
What are the joint surfaces of the costocentral joint?
The costal facet or costal Demi-facet on the vertebral body and the articular surface of the head of the rib
What are the joint surfaces of the costotransverse joint?
The transverse costal facet on the vertebral transverse process and the articular surface of the tubercle of the rib
What are the joint surface so the costocentral joint of the 1st, 11th, and 12th rib?
The superior costal facet on the vertebral body of T1, T11 or T12 and the articular surface of the head of the corresponding rib.
An intra-articular ligament will be IDed with which ribs?
At what rib will the superior costotransverse ligament be absent?
The 1st rib
What ligaments will attach to the neck of the 12th rib?
The superior costotransverse ligament form T11 no the lumbosacral ligament form L1
Which ribs will have an attachment for the inferior costotransverse ligament?
Which ligament "fills" the costotransverse Foramen?
The inferior costotransverse ligament
Which vertebrae will have an attachment for the lateral costotransverse ligament?
At what rib will the lateral costotransverse ligament be absent?
The 12th rib
Which vertebral couples of the thoracic spine have the greatest motion?
T11/T12 and T12/L1
Which range of motion is greatest for lower thoracic vertebral couples?
Which range of motion is least for lower thoracic vertebral couples?
One side axial rotation
Which ligaments replace the intertransverse ligament at the lumbosacral joint?
The iliolumbar ligament and lumbosacral ligament
What muscle is intimately attaché to the superior iliolumbar ligament?
Which of the current ligaments form the iliolumbar ligament complex represents the iliolumbar ligament of classical descriptions?
The superior iliolumbar ligament
Which ligament form the iliolumbar ligament couples represents the lumbosacral ligament of classical descriptions?
The inferior iliolumbar ligament
What is the joint classification given to each ligament of the iliolumbar ligament complex?
Fibrous (amphiarthrosis) syndesmosis
Which vertebral couple of the lumbar spine has the greatest range of motion?
Which range of motion is greatest for all lumbar vertebral couples?
Flexion - extension
Which range of motion is least for L1-L5 vertebral couples?
One side axial ration
Which range of motion is least for the L5/S1 vertebral couple?
One side lateral bending
List, in order, from cranial to caudal the ligaments forming the anterior boundary of the spinal column.
The anterior atlanto-occipital ligament, anterior atlanto-axial ligament, anterior longitudinal ligament and the anterior sacrococcygeal ligament
List, in order, from cranial to caudal the ligaments forming the anterior boundary of the spinal canal.
The membrana tectoria, posterior longitudinal ligament and the deep posterior sacrococcygeal ligament
List, in order, from cranial to caudal the ligaments forming the posterior boundary of he spinal canal.
The posterior atlanto-occipital ligament, posterior atlanto-axial ligament, ligamentum flavum and the superficial posterior sacrococcygeal ligament
What forms the closure for the spinal canal inferiorly?
The fusion of the deep posterior sacrococcygeal ligament with the superficial posterior sacrococcygeal ligament
What is the homolog of the capsular ligament at the sacrococcygeal joint?
The intercornual ligament
What is the auricular surface of sacrum composed of?
True articular cartilage, a modification of hyaline cartilage
What is the auricular surface of the ilium composed of?
Articular cartilage, interspersed with Fibrocartilage
What is the superficial appearance of the auricular surfaces of the sacro-iliac joint by age 15?
The sacrum develops a sacral groove while the ilium develops an iliac ridge
Which gender has greater unevenness of the auricular surface of the sacro-iliac joint?
What forms the accessory sacro-iliac joint?
The sacral tuberosity and the iliac sulcus
What pathological or age-related modifications of the sacro-iliac joint may occur?
Degenerative arthrosis and ankylosis
What does degenerative arthrosis mean?
A condition off degeneration of the joint, sometimes referre to as degenerative joint disease (DJD)
What does ankylosis mean?
A condition of fibrous adhesion occurs within the joint
What is the age and surface bias associated with degenerative arthrosis of the sacro-iliac joint?
Age 40, the iliac auricular surface
What is the age and gender bias associated with ankylosis of the sacro-iliac joint?
Age 50 and male bias particularly in African American males
Wha rage and gender bias is associated with ossification of the anterior sacro-iliac ligament?
Age 40 and male bias
Which is the strongest of the sacro-iliac ligaments?
The interosseous sacro-iliac ligament
What passes between the layers of the interosseous sacro-iliac ligament?
Dorsal rami from the sacral spinal nerve
What ligament blends with the lateral border of the long posterior sacro-iliac ligament?
The sacrotuberous ligamnet
What is formed by the continuation of the sacrotuberous ligament along the ischial ramus?
The falciform process
What separates the greater sciatic and lesser sciatic foramina?
The sacrospinous ligament
What is the apparent function of synovial villi?
Increase the surface area of synovial membrane available for secretion - absorption phenomena
What is the primary function of bound glycosaminoglycans in articular cartilage?
Form a network fro water retention
What morphological classification of synovial joints would be classified as multiaxial?
Ball and socket (diarthrosis enarthrosis), (diarthrosis spheroidal), (diarthrosis cotyloid) are all classifications given to the same type of joint
Diarthrosis ginglymus joints are commonly called ____ joints based on action.
Diarthrosis trochoid joints are commonly called ______ joints based on actions.
What is the precent of IVD height contribution to the length of each region of the VC?
C- 22-25%, T-20%, L33%
What innervates the outer lamellae of the annulus fibrosus at the lateral part of the IVD?
Fibers from the ventral primary ramus, fibers from the white ramus communicans, fibers from the paradiscal ramus communicans, fibers from the gray ramus communicans
What is the appearance of the posterior longitudinal ligament in the lumbar spine?
It is hourglass shaped along its lateral margins
What will the ligamentum flavum attach to in addition to the lamina?
They may blend with the capsular ligament of the zygapophysis
The capsular ligament may blend with which other common ligament?
The ligamentum flavum
What will the spurapsinous ligament be attached to?
The spinous tubercles along the vertebral column form C7-sacrum
What will the intertransverse ligament be attached to?
The transverse tubercles and TVP of adjacent vertebrae along the VC from C1-L5
What is the status of the cervical intertransverse ligament?
It is said to be paired with an anterior and posterior intertransverse ligament present
What is the status of the lumbar intertransverse ligament?
It is well developed with two parts IDed, a ventral and dorsal slip
What part of the lumbar intertransverse ligament covers the intervertebral foramen?
The ventral slip
What part of the intertransverse ligament in the lumbar spine divides the body wall into an anterior and posterior muscular compartment?
The dorsal slip
Where is accessory atlanto-axial ligament observed?
Anterior to the membrana tectoria/tectorial membrane
The lumbosacral ligament will blend with which sacro-iliac joint ligament??
The anterior sacro-iliac ligament
What are the attachment sites for the anterior iliolumbar ligaments?
The vertebral body and TVP L5 is attached to the ilium
What ligament represents the continuation of the posterior longitudinal ligament at the sacrococcygeal joint?
The deep posterior sacrococcygeal ligament
What ligament represents the homolog of the ligamentum flavum at the sacrococcygeal joint?
The superficial posterior sacrococcygeal ligament