Articulations And The Axial Skeleton

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Articulations

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They occur wherever 2 bones meet

Movement occurs at articulations (but not all of them allow it)

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2 ways that articulations are classified:

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Structurally: what tissue joins the articulating bones (like cartilage, fibrous tissue ...etc.)

Functionally: the range of movement permitted

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3 types of articulations

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– synarthroses (no movement)

– amphiarthroses (little movement)

– diarthroses (free movement)

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Types of synarthroses

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Fibrous synarthroses

cartilagenous synarthroses

bony fusion

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Fibrous synarthroses

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bones joined by dense irregular CT (if joined by fibrous CT, it's fibrous synarthroses)

examples of fibr.syn are: suture and gomphosis

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Suture

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between bones of skull

these appear as bumpy lines

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Gomphosis

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between each tooth and its bony socket

this is basically a ligament connecting bone to the socket

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cartilagenous synarthroses

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bones joined by cartilage

an example is synchondrosis

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synchondrosis

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e.g. epiphyseal plate of a growing bone, first
costosternal joint

there's also a synchondrosis b/w rib number 1 and the sternum

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Bony fusion

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this occurs when 2 bones that were separate at one point in development fuse to become one

an example includes synostosis

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Synostosis

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e.g:

-skull (the 2 frontal bones come together and fuse)

-sacrum

-hip bones

-sternum (sometimes there isn't complete fusion and there are gaps)

-vertebrae (the vertebra consists of multiple bones that fuse together)
-mature long bones

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Types of amphiarthroses

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fibrous amphiarthroses

cartilaginous amphiarthroses

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fibrous amphiarthroses

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bones joined by a ligament or band of CT

an example is syndesmoses (e.g. distal tibiofibular joint)

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cartilaginous amphiarthroses

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bones are joined by a wedge of cartilage

an example of cart. amphi. is sympyses (e.g. intervertebral discs, symphysis pubis)

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Characteristics of diarthroses (synovial joints)

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fibrous joint capsule:

-this is a dense irreg. CT capsule that encloses the articulating surface of the 2 bones

-it helps to hold the bones together, but it also this allows containment of fluids in the space

articular cartilages (hyaline cartilage) cover articular surfaces

synovial membrane covers all internal,
nonarticular surfaces; secretes synovial fluid (acts as lubricant)

joint cavity contains synovial fluid

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Synovial fluid

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– lubricant
– shock absorber
– medium for solute transfer between blood & cartilages

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Synovial joints: accessory structures

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articular discs or menisci, such as medial
& lateral menisci of knee joint

fat pads

tendons

bursae

intrinsic ligaments

extrinsic ligaments

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articular discs or menisci

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these are wedges of fibrocartilage

these discs help to improve the congruity b/w the 2 articulating surfaces, so they help complete the match b/w the 2 surfaces to increase stability

fibrocartilage is tough and can endure the 'pounding' the bone experiences as we walk

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fat pads

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these fill the space, but they are malleable, so they can accommodate changes in joint space with movement

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bursae

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are sacks of synovial membrane containing synovial fluid

– facilitate relative mvt between structures by eliminating friction b/w them

– eg. may be subcutaneous or subtendinous

– when inflamed, result in bursitis

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Stability vs. Mobility

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stability is inversely proportional to mobility

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stability / mobility is influenced by:

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– the shape of articulating surfaces

– the capsule (baggy capsule = high mobility, tight capsule = high stability)

– ligaments

– tone of surrounding muscles (more muscle tone = more tension in tendon = more stability)

– other tissues around the joint

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Functional Classification of Synovial Joints: Types of movement

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linear motion (aka translation)

angular motion: the angle b/w the pencil and paper is being changed

circumduction: the pencil is swirling w/o the tip moving

rotation: there's rotation but the tip doesn't move and the angle w/ the plane stays the same

gliding

uniaxial: move along one axis

biaxial: along 2 axes

triaxial: along 3 axes

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Structural Classification of Synovial Joints

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Gliding joint (e.g. intercarpal joints): there's no movement around any axis, it's just gliding. the terms uni/bi/triaxial don't applythere's no movement around any axis, it's just gliding. the terms uni/bi/triaxial don't apply

Hinge Joint (e.g. elbow joint): moves like a hinge on a door (single axis)

Pivot joint (e.g. atlantoaxial joint):

atlantoaxial joint - atlas holds up the cranium, and axis gets its name b/c it is the axis around which the atlas rotates

Ellipsoidal joint (e.g. radiocarpal joint):

this is the wrist joint it allows movement along 2 axes

the 2 articulating surfaces are oval; one is concave and the other is convex

you can't rotate them b/c one dimension is greater, but you can roll them from side to side and front to back, b/c one's concave and the other convex

Saddle joint (e.g. 1st carpometacarpal):

allows movement along 2 axes

they are convex from side to side, but concave from front to back (like pringles)

Ball-and-Socket Joint (e.g. shoulder joint):

allow movement in 3 axes

the major difference is that these have round articulating surfaces, so rotation can occur, and this allows movement along the long axis

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Forms of uniaxial joints

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hinge joint

pivot joint

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Forms of biaxial joints

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ellipsoidal joint

saddle joint

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Forms of triaxial joints

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ball-and-socket joint

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The axial skeleton consists of:

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• The skull

• The vertebral column
– 24 vertebrae
– the sacrum
– the coccyx

• The thoracic cage
– 24 (12 paired) ribs
– 1 sternum

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The appendicular skeleton consists of:

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• The pectoral girdles & upper limbs

• The pelvic girdles & lower limbs

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The skull

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consists of cranium and facial bones

includes the 8 bones that form the cranium and the 14 facial bones

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The cranum

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• internally, the cranial cavity houses the brain

• externally, provides muscular attachment

• articulates inferiorly with the vertebral column