Chapter 8: Joints-involves the sites where bones meet-weakest part of the skeleton -classified by structure and function

-structural classification focuses on the material binding the bones together and whether or not a joint cavity is present

1) Fibrous Joints-generally immovable

2) Cartilaginous Joints-has both rigid and slightly movable examples

3) Synovial Joints -freely movable

-functional classification is based on the amount of movement allowed at the joint

1) Synarthroses -immovable joints-found in axial skeleton

2) Amphiarthroses-slightly movable joints-found in axial skeleton

3) Diarthroses-freely movable joints -predominate in the limbs

Fibrous Joints -bones are joined by fibrous tissue (i.e.: dense fibrous connective tissue) -no joint cavity is present -movement allowed depends on the length of connective tissue fibres uniting the bones -are mostly movable -3 types of fibrous joints:

1) Sutures-seams that occur only between the bones of the skull-filled with minimal amounts of short connective tissue fibre-immovable

2) Syndesmoses-bones are connected by ligaments, cords, or bands of fibrous tissue -connecting fibres are longer than those in sutures

-length of connecting fibres vary-limited amount of movement and is dependant on the length of connecting fibres -i.e.: the interosseous membrane connecting the radius and ulna

3) Gomphoses -a "peg-in-socket" fibrous joint-the only example is the articulation of a tooth with its bony alveolar socket -"nail and bolt"-the fibrous connection is the short periodontal ligament between the tooth and the socket

Cartilaginous Joints -articulating bones are united by cartilage -lack joint cavity-not very movable-2 types:

1) Synchondroses-a bar or plate of hyaline cartilage that unites the bones at a synchondrosis (junction of cartilage)

-all synchondroses are synarthrotic -involve areas of growth -e.g.: epiphyseal plate in long bones of children, the immovable joint between the costal cartilage of the first rib and the manubrium of the sternum

2) Symphyses-"growing together" -the articular surfaces of the bones are covered with hyaline cartilage which fuse into a plate of fibrocartilage (the main connecting material)

-compressible and resilient -acts as a shock absorber-some degree of joint movement

-designed for strength and flexibility -e.g.: intervertebral joints and the pubic symphysis of the pelvis

Synovial Joints-articulating bones are separated by a fluid-containing joint cavity -allows for freedom of movement -include almost all limb joints and most joints in the body

-6 distinguishing features:

1) Articular Cartilage -hyaline cartilage covers the opposing bone surfaces as articular cartilage

-act as spongy cushions that absorb compression so the ends of the bones don't crush each other

2) Joint (Synovial Cavity) -has a joint cavity

-a potential space that contains a small amount of synovial fluid

3) Articular Capsule-the joint cavity is enclosed by a 2-layered articular capsule / joint capsule -the external layer is a tough fibrous capsule

-made up of dense irregular connective tissue -continuous with the periostea of the articulating bones -strengthens the joint so bones are not pulled apart

-the inner layer of the joint capsule is a synovial membrane composed of loose connective tissue

-lines the fibrous capsule internally -covers all internal joint surfaces that have no hyaline cartilage

4) Synovial Fluid -occupies free spaces within the joint capsule -viscous, egg-white consistency -provides a slipper, weight-bearing film that reduces friction between cartilages -"weeping lubrication"

-synovial fluid is forced out by pressure and seeps back in when there is less pressure

-nourishes cartilage cells -has phagocytic cells that rid the joint cavity of microbes and cellular debris -warms up during movement

5) Reinforcing Ligaments -synovial joints are reinforced and strengthened by ligaments

6) Nerves and Blood Vessels -synovial joints are richly supplied with sensory nerve fibres that innervate the capsule -richly supplied with blood vessels which supply the synovial membrane -some synovial joints have cushioning fatty pads between the fibrous capsule and the synovial membrane or bone (i.e.: hip and knee joints)

-some have articular discs or menisci that separate articular surfaces-they extend inward from the articular capsule and divide the synovial cavity into 2-they improve the fit between articulating bone ends -found in the knee, jaw, etc.

Bursa and Tendon Sheaths-closely associated with synovial joints-bags of lubricants or "ball bearings" that reduce friction between adjacent structures during joint activity

-bursae are flattened fibrous sacs lined with synovial membrane and containing a thin film of synovial fluid -found with ligaments, muscles, skin, tendons, articulations of bones

-a tendon sheath is an elongated bursa that wraps completely around a tendon subjected to friction -common where several tendons are crowded together in narrow spaces (i.e.: wrist)

Factors Influencing the Stability of Synovial Joints -joints must be stabilized so that they do not dislocate-the stability of synovial joints depends on 3 factors:

1) Articular Surfaces-the shapes of articular surfaces determines what movements are possible at a joint -many joints have a shallow socket or noncomplementary articulating surfaces -large articular surfaces or balls with deep sockets have improved stability (i.e.: hip joint)

2) Ligaments -capsules and ligaments of synovial joints unite the bones and prevent excessive or undesirable motion -the more ligaments a joint has, the stronger it is-if ligaments are the major means of bracing a joint, it is unstable

-ligaments can only stretch 6% of its length

3) Muscle Tone-the muscle tendons that cross the joint are the most important stabilizing factor -tendons are kept taut at all times by their muscle tone -muscle tone helps reinforce the shoulder, knee joints, and arches in the foot

Types of Movements Allowed by Synovial Joints

1) Gliding Movements -occurs when one flat bone surface glides or slips over another -occur at the intercarpal and intertarsal joints and the flat articular processes of the vertebrae

2) Angular Movements-increase or decrease the angle between 2 bones

a) Flexion-bending movement, usually along the sagittal plane, that decreases the angle of the joint and brings the articulating bones closer together -i.e.: bending the head forward on the chest, bending the knee from a straight to angled position

b) Extension -bending movement along the sagittal plane that increases the angle between the articulating bones and typically straightens a flexed limb or body part -hyperextension involves extending the joint beyond anatomical position

c) Abduction-movement of a limb away from the midline or median plane of the body, along the frontal plane -i.e.: raising the arm or thigh laterally, spreading the fingers apart-bending the trunk away from the body midline is lateral flexion

d) Adduction -movement of a limb toward the body midline -i.e.: putting your fingers closer together

e) Circumduction-moving a limb so that it describes a cone in space

-the distal end of the limb moves in a circle-consists of flexion, abduction, extension, and adduction performed in succession

3) Rotation-turning a bone around its own long axis -i.e.: medial rotation: the femur's anterior surface moves toward the median plane of the body-lateral rotation: the femur's anterior surface moves toward the lateral plane of the body

4) Supination -rotating the forearm laterally so that the palm faces anteriorly or superiorly

5) Pronation-rotating the forearm medially so the palm faces posteriorly or inferiorly

6) Dorsiflexion and Plantar Flexion of the Foot-up and down movement of the foot at the ankle

-dorsiflexion: lifting the foot so that its superior surface approaches the shin

-plantar flexion: depressing the foot and pointing the foot downward

7) Inversion and Eversion

-inversion: the sole of the foot turns medially

-eversion: the sole of the foot turns laterally

8) Protraction and Retraction-nonangular anterior and posterior movements in a transverse plane

9) Elevation and Depression

-elevation: lifting a body part superiorly

-depression: moving the elevated part inferiorly

10) Opposition-touching the thumb tips to the other fingers on the same hand

Types of Synovial Joints

1) Plane Joints-involves 2 flat opposing surfaces-allows for gliding movement-e.g.: intercarpal joints

2) Hinge Joints-a cylindrical end of one bone conforms to a trough-shaped surface on another -motion is along a single plane -e.g.: elbow

3) Pivot Joints -the rounded end of one bone conforms to a sleeve or ring composed of bone of another -e.g.: the joint between the atlas and dens of the axis; allows for the "no" head movement

4) Condyloid Joints -oval articular surface of one bone fits into a complementary depression in another -permits all angular motions (i.e.: flexion, extension, abduction adduction, circumduction) -e.g.: the wrist and knuckle joints

5) Saddle Joints-allow greater freedom of movement than condyloid joints-each articular surface is concave and convex areas; articular surfaces fit together (concave to convex surfaces) -shaped like a saddle -e.g.: carpmetacarpal joints of the thumbs (seen through the twiddling of thumbs)

6) Ball and Socket Joints-the spherical or hemi-spherical head of one bone articulates with the cuplike socket of another -universal movement -e.g.: shoulder and hip are the only examples

Common Joint Injuries

1) Sprains-partially torn ligaments -can repair themselves slowly (due to poor vascularization) -completely torn ligaments need surgery

2) Cartilage Injuries-usually in the knee-cannot repair itself because it is avascular -pieces can break off and interfere with joint function

3) Dislocations-bone is forced out of the normal position at a joint-easy to get dislocated again because ligaments get stretched

Knee Joint-has the 6 distinguishing characteristics of synovial joints-the largest and most complex joint in the body

-has a single joint cavity but consists of 3 joints in one:-femeorpatellar joint between the patella and the lower end of the femur -lateral tibiofemoral joint and medial tibiofemoral joint make up the lateral and medial joints between the femoral condyles above and the C-shaped menisci (semilunar cartilages) of the tibia below

-the menisci deepens the shallow tibial articular surfaces and helps to prevent side-to-side rocking of the femur on the tibia and absorb shock transmitted to the knee joint-menisci are attached only at their outer margins and are frequently torn free

-the femoralpatellar joint is a plane joint and the patella glides across the distal end of the femur during knee flexion

-the tibiofemoral joint acts primarily as a hinge and permits flexion and extension -structurally, it is a bicondylar joint-some rotation is possible when the knee is partly flexed and when the knee is extending -when the knee is fully extended, side-to-side movements and rotation are strongly resisted by ligaments and the menisci

-the knee joint is only partially enclosed by a capsule -it has a relatively thin articular capsule present only on the sides and posterior aspects of the knee where it covers the bulk of the femoral and tibial condyles

-anteriorly, where the capsule is absent, three broad ligaments run from the patella to the tibia below

-the patellar ligament is bordered by the medial and lateral patellar retinacula which merge into the articular capsule on each side

-the patellar ligament and retinacula are continuations of the tendon of quadriceps muscle of the anterior thigh

-there are many bursae associated with the knee joint-the subcutaneous prepatellar bursa is often injured when the knee is bumped anteriorly

-all 3 types of joint ligaments are used to stabilize and strengthen the capsule of the knee joint

-capsular and extracapsular ligaments act to prevent hyperextension of the knee and are stretched taut when the knee is extended

-the extracapsular fibular and tibial collateral ligaments are critical in preventing lateral or medial rotation when the knee is extended

-the tibial collateral ligament is broad and flat and runs from the medial epicondyle of the femur to the medial condyle of the tibial shaft

below and is fused to the medial meniscus

-the oblique popliteal ligament is part of the tendon of the semimembranosus muscle that fuses with the joint capsule and helps stabilize the posterior aspect of the knee joint

-the arcuate popliteal ligament arcs superiorly from the head of the fibula over the popliteus muscle and reinforces the joint capsule posteriorly

-the intracapsular ligaments are called cruciate ligaments because they cross each other and from an X in the notch between the femoral condyles

-they act as restraining straps to prevent anterior-posterior displacement of the articular surfaces and to secure the articulating bones when we stand

-these ligaments are in the joint capsule but are outside the synovial cavity-synovial membrane covers their surfaces

-the 2 cruciate ligaments run superiorly to the femur and are named for their tibial attachment site

-the anterior cruciate ligament attaches to the anterior intercondylar area of the tibia and passes posteriorly, laterally, and upward to attach to the femur on the medial side o its lateral condyle

-the ACL prevents forward sliding of the tibia on the femur and resists hyperextension of the knee-it is somewhat relaxed when the knee is flexed and taut when the knee is extended

-the posterior cruciate ligament is stronger and attached to the posterior intercondylar area of the tibia and passes anteriorly, medially, and superiorly to attach to the femur on the lateral side of the medial condyle

-the PCL prevents backward displacement of the tibia or the forward sliding of the femur

-the knee capsule is reinforced by muscle tendons -the most important are the strong tendons of the quadriceps muscles of the anterior thigh and the tendon of the semimembranosus muscle posteriorly -the greater the strength and tone of these muscles, the less the chance of knee injury

-the knee has a built-in locking device that provides support fro the body in the standing position

-as we begin the stand up, the wheel-shaped femoral condyles roll like ball bearings across the tibial condyles -the flexed leg begins to extend at the knee-the lateral femoral condyle stops rolling before the medial condyle stops, so the femur spins (rotates) medially on the tibia, until the cruciate and collateral ligaments of the knee are twisted and taut and the menisci are compressed-the tension in the ligaments locks the joint into a rigid structure that cannot be flexed until it is unlocked-it is unlocked when the popliteus muscle rotates the femur laterally on the tibia, causing the ligaments to become untwisted and slack

-of all body joints, the knees are the most susceptible to sports injuries, especially horizontal blows

-common knee injuries involve collateral ligaments, cruciate ligaments, and cartilages (menisci) -lateral blows to the extended knee are the most dangerous

-ACL injuries occur when you change direction quickly and this twists the hyperextended knee

Shoulder (Glenohumeral) Joint -most freely moving joint in the body -a ball-and-socket joint-the large hemispherical head of the humerus fits in the small, shallow glenoid cavity of the scapula -the glenoid cavity is slightly deepened by a rim of fibrocartilage known as the glenoid labrum; it is only about 1/3rd the size of the humeral head and contributes little to joint stability

-the articular capsule that encloses the joint cavity is thin and loose and contribute to the joint's freedom of movement

-the few ligaments reinforcing the shoulder joint are located mostly on the anterior aspect

-the coracohumeral ligament is superiorly located and provides the only strong thickening of the capsule and helps support the weight of the upper limb

-3 glenohumeral ligaments strengthen the front of the capsule but are weak and may even be absent

-muscle tendons that cross the shoulder joint contribute the most to the joints stability

-the tendon of the long head of the biceps brachii muscle of the arm is the "superstabilizer" and secure the head of the humerus against the glenoid cavity

-4 other tendons and their associated muscles make up the rotator cuff which encircles the shoulder joint and blends with the articular capsule

-the rotator cuff is made up of the scapularis, supraspinatus, infraspinatus, and teres minor

-the shoulder's reinforcements are weakest anteriorly and inferiorly and so the humerus tends to dislocate in the forward and downward direction

Elbow Joint -allows for flexion and extension only-in the joint, the radius and ulna articulate with the condyles of the humerus

-the close gripping of the tochlea by the ulna's trochlear notch forms the hinge and stabilizes the joint

-a lax articular capsule extends inferiorly from the humerus to the ulna and radius and to the anular ligament that surrounds the head of the radius

-allows for rotation of the head of the radius during pronation and supination of the forearm

-anteriorly and posteriorly, the articular capsule is thin and allows for substantial freedom for elbow flexion and extension

-side-to-side movements are restricted by the ulnar collateral ligament medially and the radial collateral ligament laterally

Hip (Coxal) Joint -a ball-and-socket joint-has a good range of motion but not as much as the shoulder-movement occurs in all possible planes but are limited by the joint's strong ligaments and its deep socket

-formed by the articulation of the spherical head of the femur with the deeply cupped acetabulum of the hip bone -the depth of the acetabulum is enhanced by a circular rim of fibrocartilage called the acetabular labrum

-the diameter of the labrum is less than that of the head of the femur and these articular surfaces fit snugly together-hip dislocations are rare

-the thick articular capsule extends from the rim of the acetabulum to the neck of the femur and completely encloses the joint

-several strong ligaments reinforce the capsule of the hip join; these are extracapsular ligaments:

-the anterior iliofemoral ligament is a strong V-shaped ligament

-the pubofemoral ligament is a triangular thickening of the inferior part of the capsule

-the ischiofemoral ligament is a spiraling posterior ligament

-the ligaments are arranged in such a way that they "screw" the femur into the acetabulum when a person stands up straight and this provides stability

-the ligamentum teres (ligament of the head of the femur) is a flat intracapsular band that runs from the femur head to the lower lip of the acetabulum

-it is slack during most hip movements and is not important in stabilizing the joint-mechanical function is unclear-contains an artery that helps supply the head of the femur; damage to this artery may lead to arthritis of the hip joint

-muscle tendons that cross the joint and the bulky hip and thigh muscles that surround it contribute to its stability and strength -stability mainly comes from the deep socket that securely encloses the femoral head and the strong capsular ligaments

Temporomandibular Joint-jaw joint-lies anterior to the ear-at this joint, the mandibular condyle articulates with the inferior surface of the squamous temporal bone

-posteriorly, it forms the concave mandibular fossa-anteriorly, it forms a knob called the articular tubercle

-the lateral aspect of the loose articular capsule that encloses the joint is thickened into a lateral ligament

-within the capsule, an articular disc divides the synovial cavity into superior and inferior compartments

-there are 2 distinct movements associated with the TMJ:

-the concave inferior disc receives the mandibular condyle and allows the hingelike movement of depressing and elevating the mandible while opening and closing the mouth

-the superior disc surface glides anteriorly along with the mandibular condyle when the mouth is opened wide

-the anterior movement braces the condyle against the articular tubercle so that the mandible is not forced through the thin roof of the mandibular

fossa when we bite hard foods

-the superior compartment allows this joint to glide from side to side -as the superior teeth are drawn into occlusion (contact between teeth), the mandible moves with a side-to-side movement called lateral excursion

-this lateral movement is unique to mammals and is apparent in horses and cows as they chew

-the TMJ is the most easily dislocated joint in the body due to its shallow socket -it dislocates anteriorly and the mandibular condyle ends up in a skull region called the infratemporal fossa and the mouth will remain wide open