Joints: Types, Classifications, and Functions

Joint (articulation)

Any place where adjacent bones, or bone and cartilage, come together to form a connection

Structural classification of joints

Based on whether bones are connected by fibrous tissue or cartilage, or meet within a fluid-filled joint cavity (fibrous, cartilaginous, synovial)

Functional classification of joints

Based on the amount of movement between the bones (synarthrosis, amphiarthrosis, diarthrosis)

Fibrous joint

Bones united by fibrous connective tissue; no joint cavity

Cartilaginous joint

Bones united by hyaline cartilage or fibrocartilage; no joint cavity

Synovial joint

Articulating bones are not directly connected but contact each other within a lubricated joint cavity; the most common joint; allows free movement

Synarthrosis

Immobile or nearly immobile joint; strong union; important where bones protect internal organs

Amphiarthrosis

Slightly movable joint (e.g., intervertebral discs, pubic symphysis)

Diarthrosis

Freely movable joint; includes all synovial joints; found in the appendicular skeleton

Functional class of fibrous joints

Synarthrosis or amphiarthrosis, depending on location

Functional class of cartilaginous joints

Synarthrosis or amphiarthrosis

Functional class of synovial joints

All are diarthroses

Why do immobile and slightly movable joints exist?

They protect internal organs, give stability to the body, and allow limited movement

How can slight movements at intervertebral joints produce large body movements?

The small movements between adjacent vertebrae sum together along the length of the vertebral column

Uniaxial joint

Diarthrosis allowing movement in one plane/axis (e.g., elbow: bending and straightening)

Biaxial joint

Diarthrosis allowing movement in two planes/axes (e.g., knuckle/metacarpophalangeal joint)

Multiaxial joint

Diarthrosis allowing movement in all three anatomical planes (e.g., shoulder and hip)

Three anatomical planes used to describe axes of motion

Transverse, frontal, and sagittal

Pubic symphysis (functional role)

Amphiarthrosis with very little mobility; provides weight-bearing stability to the pelvis

Three types of fibrous joints

Suture, syndesmosis, gomphosis

Suture

Narrow fibrous joint between most bones of the skull; functionally a synarthrosis (some may allow slight movement)

Which skull bone does NOT join the others by a suture?

The mandible

Function of skull sutures

Strongly unite skull bones to protect the brain and form the face

Fontanelles

Wide areas of connective tissue between skull bones in newborns and infants (top and sides of skull)

Function of fontanelles at birth

Provide flexibility so skull bones can move closer or overlap slightly during passage through the birth canal

Function of fontanelles after birth

Allow rapid growth of the skull and enlargement of the brain; they narrow during the first year and become sutures

Synostosis

Fusion of adjacent bones when connective tissue or cartilage ossifies into bone

Syndesmosis

Fibrous joint in which two parallel bones are united by a ligament or a broad interosseous membrane; functionally an amphiarthrosis

Interosseous membrane

Wide sheet of connective tissue joining parallel bones at a syndesmosis (forearm and leg)

Where are syndesmoses found?

Between the shafts of the long bones of the forearm (radius/ulna) and the leg (tibia/fibula)

Tibiofibular syndesmosis

Interosseous membrane plus anterior and posterior ligaments at the distal tibiofibular joint; strongly unites bones, locks the talus in place, and gives stability for weight bearing

Antebrachial interosseous membrane

Flexible forearm membrane that allows rotation of the radius; gives greater mobility than the tibiofibular syndesmosis

Syndesmosis damage

Usually a bone fracture with a torn interosseous membrane; causes pain and instability; must be immobilized or muscles may misalign the fracture during healing

Gomphosis

Peg-and-socket fibrous joint anchoring a tooth root into its socket in the maxilla or mandible; a synarthrosis

Periodontal ligament

Short bands of dense connective tissue between the tooth root and the bony socket at a gomphosis

Two types of cartilaginous joints

Synchondrosis and symphysis

Synchondrosis

Cartilaginous joint where bones are joined by hyaline cartilage, or where bone joins a hyaline cartilage structure; a synarthrosis; may be temporary or permanent

Temporary synchondrosis

Hyaline cartilage that is later replaced by bone (e.g., epiphyseal plate/growth plate); ends as a synostosis

Permanent synchondrosis

Retains its hyaline cartilage and does not ossify with age (e.g., costochondral joints of the thoracic cage)

Epiphyseal plate

Growth plate of a growing long bone; a temporary synchondrosis; injury can stop bone lengthening

Temporary synchondroses of the hip bone

Cartilage joining the ilium, ischium, and pubis in childhood; replaced by bone (synostoses) to form the single adult hip bone

Sacrum formation

Sacral vertebrae are united by synostoses to form the adult sacrum

Costochondral joint

Joint between the anterior end of a rib and its costal cartilage; a permanent synchondrosis

Sternochondral joints

Between sternum and costal cartilage; the 1st (first rib to manubrium) is a synchondrosis; the 2nd-7th are synovial joints

Symphysis

Cartilaginous joint where bones are joined by fibrocartilage; strong but allows limited movement; an amphiarthrosis

Why is fibrocartilage strong?

Contains numerous thick bundles of collagen fibers, resisting pulling and bending forces better than hyaline cartilage

Narrow vs. wide symphysis

Narrow gap = pubic symphysis; wide gap = intervertebral symphysis (intervertebral disc)

Key structural feature unique to synovial joints

A joint cavity between the articulating bones

Articular capsule

Fibrous connective tissue sleeve attached to each bone just outside the articulating surface; forms the walls of the joint cavity

Articular cartilage

Thin layer of hyaline cartilage covering each articulating bone surface; not continuous between bones; acts like a Teflon coating to reduce wear

Synovial membrane

Thin membrane lining the inner surface of the articular capsule; secretes synovial fluid

Synovial fluid

Thick, slimy lubricant that reduces friction and nourishes the avascular articular cartilage

Ligament

Strong band of fibrous connective tissue connecting bones; supports and strengthens joints, allows normal movement but limits range of motion

Extrinsic ligament

Ligament located outside the articular capsule

Intrinsic ligament

Ligament fused to or incorporated into the wall of the articular capsule

Intracapsular ligament

Ligament located inside the articular capsule

Tendon

Dense connective tissue structure attaching muscle to bone

Dynamic ligament

A muscle and its tendon crossing a joint; contraction strength increases automatically with force to support the joint (very important at the shoulder)

Articular disc

Small, oval fibrocartilage structure between articulating bones (e.g., sternoclavicular, distal radioulnar, temporomandibular joints)

Meniscus

Larger C-shaped fibrocartilage structure between articulating bones; provides shock absorption (e.g., knee)

Fat pad

Cushion between bones found at some synovial joints

Bursa

Thin connective tissue sac filled with lubricating fluid; reduces friction between skin, ligaments, muscles, or tendons, usually near a joint

Subcutaneous bursa

Between skin and underlying bone (e.g., prepatellar bursa, olecranon bursa)

Submuscular bursa

Between a muscle and bone, or adjacent muscles (e.g., trochanteric bursa at the lateral hip)

Subtendinous bursa

Between a tendon and a bone (e.g., subacromial bursa, suprapatellar bursa)

Tendon sheath

Smaller, bursa-like connective tissue sac surrounding a tendon where it crosses a joint; contains lubricating fluid

Pivot joint

Rounded part of a bone rotates within a ring formed by another bone and a ligament; uniaxial (e.g., atlas/axis, proximal radioulnar joint)

Atlantoaxial joint as a pivot

Dens of C2 (axis) projects into the atlas (C1), held by a ligament; allows head to turn side to side ('no')

Proximal radioulnar joint

Pivot joint where the radial head is encircled by a ligament and rotates against the ulna's radial notch; allows forearm rotation

Hinge joint

Convex end of one bone fits the concave end of another; bending/straightening only; uniaxial (elbow, knee, ankle, interphalangeal joints)

Condyloid joint

Shallow depression of one bone with a rounded structure of another; oval shape; biaxial (knuckle/metacarpophalangeal joints, radiocarpal joint of wrist)

Saddle joint

Both surfaces are concave in one direction and convex in the other; biaxial; e.g., first carpometacarpal joint (thumb), sternoclavicular joint

First carpometacarpal joint

Saddle joint between trapezium and first metacarpal; gives the thumb its opposable movement

Plane joint

Flat or slightly curved surfaces of similar size slide on each other; small movements, limited by ligaments; classified as multiaxial

Examples of plane joints

Intercarpal, intertarsal, acromioclavicular, and zygapophysial (facet) joints

Ball-and-socket joint

Rounded head of one bone fits into the concave socket of another; multiaxial; only the hip and shoulder (glenohumeral)

Mobility vs. stability at ball-and-socket joints

Shoulder: little ligament support, large range of motion. Hip: strong ligaments, less motion but more stability for weight bearing

Zygapophysial (facet) joints

Plane joints between superior and inferior articular processes of adjacent vertebrae; limited motion

Regions of the vertebral column with the greatest range of motion

Cervical and lumbar

Cervical vertebral motion

Flattened articular processes facing up/down allow flexion, extension, lateral flexion, and rotation

Thoracic vertebral motion

Overlapping spinous processes and the thoracic cage greatly limit flexion, extension, and lateral flexion

Lumbar vertebral motion

Allows considerable flexion, extension, and lateral flexion; articular process orientation largely prevents rotation

Atlanto-occipital joint

Between atlas (C1) and occipital condyles of the skull; rocking motion for flexion/extension of the head (nodding 'yes')

Atlantoaxial joint

Between atlas (C1) and axis (C2); three articulations including the dens pivot; allows head rotation (shaking 'no')

Temporomandibular joint (TMJ)

Between mandibular fossa/articular tubercle of the temporal bone and the mandibular condyle; has an articular disc; allows opening/closing, side-to-side, and protraction/retraction of the jaw

TMJ movement when opening the mouth

Combined hinge and gliding: mandible hinges down while condyle and disc glide forward onto the articular tubercle

TMJ dislocation

Occurs when the mouth opens too wide or after a blow; condyle moves anterior to the articular tubercle and the person cannot close the mouth

TMJ disorder causes

Arthritis, cartilage wear, muscle fatigue/teeth grinding, articular disc damage, or jaw injury; treated with pain medication or bite guards

Shoulder (glenohumeral) joint

Ball-and-socket joint with the largest range of motion in the body; mobility comes at the cost of stability

Glenoid cavity

Shallow socket of the scapula, about one third the size of the humeral head

Glenoid labrum

Fibrocartilage lip around the glenoid cavity that slightly deepens the socket

Shoulder joint capsule

Thin and loose to allow large motions; thickenings form weak intrinsic ligaments (coracohumeral and three glenohumeral ligaments)

Rotator cuff

Thickening of the capsule formed by fused tendons of supraspinatus, infraspinatus, teres minor, and subscapularis; primary support of the shoulder

Rotator cuff muscles

Supraspinatus, infraspinatus, teres minor, subscapularis

Shoulder bursae

Subacromial bursa and subscapular bursa; reduce friction between rotator cuff tendons and scapula

Direction of most shoulder dislocations

Inferior, because muscles and ligaments strongly support the anterior, superior, and posterior humeral head

Adhesive capsulitis (frozen shoulder)

Scar tissue forms between the articular capsule and surrounding structures after shoulder injury, reducing mobility

Elbow joint

Uniaxial hinge joint (humeroulnar) plus humeroradial and proximal radioulnar joints, all in one articular capsule

Humeroulnar joint

Trochlea of the humerus with the trochlear notch of the ulna; the hinge of the elbow

Ulnar collateral ligament

Medial elbow ligament from the medial epicondyle of the humerus to the ulna; anterior part resists hyperextension; injured in baseball pitchers