Joints and Articulations

Joints: Articulations

• Joints, also known as articulations, signify where rigid skeletal elements meet
• The Greek root "arthro" means joint

Articulation Types

• Bone to bone articulating joints represent the majority
• Other articulations can be bone to cartilage
• The teeth in bony sockets are another example

Joint Function

• Joints resist forces like crushing and tearing
• Although elements strengthen them, joints are the weakest part of the skeleton

Joint Classification

• Functional classifications are based on the permitted movement amount
• Structural classifications are based on the material that binds the bones
• It also depends on the absence or presence of a joint cavity and capsule

Functional Classifications

• Synarthroses: Immobile, typical in the axial skeleton such as the skull
• Amphiarthroses: Slightly movable, found in the axial skeleton like the tibia/fibula
• Diarthroses: Freely movable, prevalent in the appendicular skeleton, for instance, the knee

Structural Classifications

• Fibrous joints are connected by fibrous connective tissue and lack a joint cavity
• Cartilaginous joints are connected by cartilage with no joint cavity
• Synovial joints have a joint cavity, a joint capsule, and synovial fluid for lubrication

Fibrous Joints

• Generally immobile or only slightly movable

Types of Fibrous Joints

• Sutures: Use very short, interconnecting fibers to hold the bones together and the bone edges interlock
• Sutures are found only in the skull
• Syndesmosis: Bones are held together by a ligament
• The fibrous tissue varies in length, and is longer than sutures
• An example is the tibia/fibula joint
• Gomphosis: Peg-in-socket fibrous joints
• They’re located between the teeth and either the maxilla or mandible
• Periodontal ligament holds the tooth in the socket

Cartilaginous Joints

• Synchondroses: Hyaline cartilage unites the bones
• They are found in epiphyseal plates and in the joint between the first rib and manubrium
• Symphyses: In symphyses, fibrocartilage unites the bones
• They resist tension and compression
• Symphyses are slightly movable joints that provide both strength and flexibility
• They are located in between vertebrae as well as in the pubic symphysis

Synovial Joints

• Most movable of the joints; are all diarthroses

General Structure of Synovial Joints

• Articular cartilage (hyaline) covers the ends of opposing bones
• Articular cartilage absorbs compression
• Contains synovial fluid, which is called weeping lubrication
• Synovial fluid acts like a sponge, soaking up, then squeezing it out when the bones come closer together
• Joint cavity contains synovial fluid
• Synovial cavity holds a small amount of fluid
• Articular capsule surrounds the joint cavity
• Bi-layered
• Fibrous capsule is the external layer
• Dense irregular connective tissue
• Strengthens joint and supports/provides stability
• Connects with the periosteum of bone
• Synovial membrane is the internal layer
• Synovial membrane contains loose connective tissue
• It produces synovial fluid
• Synovial fluid provides lubrication
• Viscous fluid, similar to raw egg white
• A blood filtrate
• Comes from the capillaries in the synovial membrane
• Glycoprotein molecules secreted by fibroblasts are in it
• Reinforcing ligament describes the thickened parts of the fibrous capsule
• Extracapsular Reinforcing ligaments are outside the capsule
• Intracapsular Reinforcing ligaments are inside the capsule
• Articular discs are only present in some synovial joints
• For example, menisci in the knee are needed because the condyles do not fit well together
• Without menisci, they would would be touching at very small areas, and forces would be transmitted only through these areas, which is harmful
• Another example is the temporomandibular joint (TMJ)
• They increase surface area to improve force transfer

Sensory and Blood Supply

• Richly supplied with sensory nerves
• Detect pain, which is a consequence of tissue damage
• Monitor capsule stretch to tell you where your body is in space and proprioception
• Rich blood supply
• Provides most of the synovial membrane so it can synthesize synovial fluid
• Branches from nerves of major blood vessels exist
• Redundancy results from multiple pathways offering an alternative should other pathways become blocked
• For instance, if your leg falls asleep, lost blood flow has affected the nerves

Synovial Fluid

• Without synovial fluid, friction would occur and cause overheating and tissue damage
• Heat damage is not very easily repaired since heating proteins denatures them
• Bursae and tendon sheaths serve to resist friction between body parts
• Closed bags of lubricant
• Essentially, they're be the same thing, but are simply found in different areas
• For instance, the shoulder has both
• The subacromial bursa is a bursa which prevents friction on the ligament above it during overhead movements
• Tendon sheath surrounds the biceps tendon, which travels up the intertubercular groove and attaches to the supraglenoid tubercle of the scapula
• Without the sheath, the tendon would experience a lot of friction since it's in so much contact with bone
• Joint stability and flexibility are inversely related

Factors Influencing Joint Stability

• Articular surfaces dictate how bones fit
• If they relate very well, that creates stability
• For example, think of a deep hip socket, or an elbow joint
• If they do not have much contact, that creates greater flexibility
• For example, think of the shallow shoulder joint
• Ligaments impact joint stability
• The more there are in a joint, the stronger it will be
• Restrict movement, helping stabilize a joint
• Hip has three very big and strong ligaments
• Those in the shoulder are loose
• They are not elastic; once they get stretched, they do not recoil - Once it is hurt once, the joint has permanently become more flexible
• If they stretch too much, they tear, and can cause greater instability
• Muscle tone has the most important factor!
• If muscle tone fails, the other two are likely to fail as well
• Sometimes strengthening muscle tone can compensate for stability loss from ligament tears
• Muscles cross joints, allowing action of the joint
• Muscles help promote stability because they attach to two different bones

Movements Allowed by Synovial Joints

• Gliding occurs when one bone moves across the surface of another
• This action is facilitated by flat surfaces on adjacent bones
• Gliding occurs in carpals, tarsals, and articular processes of vertebrae
• Angular movement refers to alterations in the angle between bones
• Angular movements occur in elbows, knees, the vertebral column, shoulder, the hip, etc
• Flexion (anterior movement) and Extension (posterior movement)
• The knee is the exception since flexion and extension are reversed
• Abduction (movement away from midline) and adduction (movement towards midline)
• Fingers use the midline of the hand as reference
• Circumduction combines flexion, extension, adduction, and abduction
• Rotation means movement around a bone's long axis
• For example, shaking head no, or rotating arms and legs at the shoulder/hip

Special Movements

• Supination: Forearm rotates laterally and palm faces anteriorly
• Pronation: Forearm rotates medially and palm faces posteriorly
• In the pronated position, the radius/ulna are crossed, making it vulnerable to fractures
• Dorsiflexion: Pulling the foot backwards
• Plantar Flexion: Pointing foot downwards
• Inversion: Turning the sole of the foot medially, which is how we usually sprain it
• Eversion: Turning the sole of the foot laterally and we're more stable in this direction
• Protraction: Jaw juts anteriorly
• Retraction: Jaw moves back posteriorly
• Elevation: Lifting a body part superiorly
• Depression: Moving the elevated part inferiorly
• Opposition: Thumb moves across palm to touch the tips of other fingers
• Reposition: Thumb moves away from other fingers

Synovial Joints Classified by Shape

• Plane joint: Features flat articular surfaces, allowing short gliding movements
• Movements are nonaxial, typical of intertarsal and intercarpal joints
• Hinge joint: Cylindrical end of one bone fits into a trough on another, enabling angular movements
• These movements are uniaxial, like flexion and extension, seen in the ankle, elbow, and joints between phalanges
• Pivot joints: Allow rotation movements
• Movement is uniaxial, where a rotating bone only turns around its long axis, as seen in the proximal radioulnar joint, or the joint between the atlas/axis
• Condyloid joints: Allow both side-to-side (adduction/abduction) and back-and-forth (flexion/extension) movements
• Biaxial, this movement occurs around two axes that are perpendicular to each other
• An example is at the metacarpal -phalangeal joints (knuckles)
• Saddle joints feature concave/convex surfaces on each articular surface
• Biaxial movement from axes that are perpendicular to each other
• Adduction/abduction and Flexion/extension can occur
• Thumb (between first metacarpal and trapezium)
• Ball-and-socket joints: These joints have a spherical head of one bone and a round socket of another
• Multiaxial: Movement in all axes (three, perpendicular to each other), in each plane can be combined
• The hip and shoulder joints are examples of this type

Ligaments

• Ligaments restrict movement in the direction opposite their location
• Excessive stretch in that direction will damage the ligament

Glenohumeral Joint

• The glenohumeral joint is the shoulder
• The joint exists between the glenoid cavity of the scapula and the head of the humerus
• Freely movable, but lacks stability
• A thin, loose articular capsule -> less stability
• Muscle tendons provide the joint stability through the rotator cuff (group of four muscles), which offers most of the shoulder's stability

Ligaments of the Shoulder

• Ligaments usually take on the names of the bones to which they attach
• Coracoacromial ligament: Attaches to the two processes of the scapula
• Coracohumeral ligament: Attaches the coracoid process to the humerus
• Capsule surrounds the entire joint!
• Thicker on the anterior side of the joint, creating the glenohumeral ligaments - However, all the ligaments are still relatively loose
• Transverse humoral ligament
• Holds the biceps tendon in place in the intertubercular groove - It is attached to both tubercles
• Glenoid labrum Forms a ring around the socket Deepens the glenoid cavity with soft tissue

Elbow joint

• A hinge joint allows flexion and extension
• Meeting point of the ulna and humerus
• Tendons of the biceps and triceps brachii add stability
• So, tight fitting articular surfaces stabilize it
• Ligaments stabilize the elbow

Elbow action

• When biceps and triceps contract, the two motions cancel each other out
• Ultimately, their contraction only causes stability. The joint capsule surrounds the entire joint
• Structures included in the joint
• Cavity with synovial fluid
• Other ligaments
• Annular: Wraps around the head of the radius, but it is not part of the elbow joint - Lies more so between the radio-ulnar joint
• Radial collateral: Side of the elbow, attaching to the humerus and radius
• Ulnar collateral: Medial side of the elbow, attaching to the humerus and ulna

Hip Joint

• Ball and socket joint
• Characterized by limited movement and high stability, due to these characteristics:
• Deep socketed acetabulum
• Three strong ligaments
• Muscles and tendons contribute a small amount in stability

Hip Ligaments

• Iliofemoral ligament: Located anterior and superior, resists adduction and internal rotation
• Pubofemoral ligament: Located anterior inferior, restricts extension, abduction, and external rotation
• Ischiofemoral ligament: Located posterior inferior, restricts flexion, abduction, and internal rotation
• Hip replacement involves replacing the proximal femur with titanium and constructing a new acetabulum

Knee Joint

• Largest and most complex joint
• Primarily a hinge joint for flexion and extension
• Exhibits slight internal rotation (of tibia) when knee is flexed, followed by slight external rotation of the tibia upon extension. This is not intentional, but results from how the condyles fit
• The knee is bicondyloid
• Two fibrocartilage menisci resist tension and compression.
• Quadriceps muscle feeds into the quadriceps tendon, which contains the kneecap and continues into the patellar ligament
• Several bursae, like the deep infrapatellar and suprapatellar, are present
• Anterior cruciate ligament attaches anteriorly to the tibia and posteriorly to the femur
• Posterior cruciate ligament attaches posteriorly to the tibia and anteriorly to the femur
• The joint capsule covers the posterior and lateral aspects of the knee, specifically tibial-femoral condyles, but not the anterior aspect

Knee Ligaments : Anterior

• Patellar, medial, and lateral retinacula cover the knee anteriorly

Knee Ligaments: Collateral

• Tibial collateral ligament, or medial collateral ligament: Medial tibia to medial femur, restricts abduction
• Fibular collateral ligament, or lateral collateral ligament: Lateral fibula to lateral femur, restricts adduction Ligaments of the knee joint become taut when the knee is extended
• Extracapsular ligaments
• Fibular and tibial collateral ligaments
• Oblique popliteal ligament resists extension on the back side of knee
• Arcuate popliteal ligament similarly resists extension on back side of the knee
• Several muscles crossing externally around the knee enable additional stability

Intracapsular Ligaments

• Cruciate ligaments (cross each other like an X)
• Each runs from proximal tibia to distal femur
• Positioned like crossing fingers, where the index is the posterior cruciate ligament (PCL) and middle the anterior cruciate ligament (ACL)
• ACL resists the tibia shifting forwards
• If doctor tugs the tibia forward and it isn't resisted, there is an ACL tear
• PCL resists the tibia shifting backwards, and this can be tested the same way
• Knee replacement involves:
• Plastic spacer replacing the menisci
• Metal tray on the proximal tibia
• Metal covering of the distal femur

Ankle Joint

• Hinge joint between the tibia/fibula and talus
• Only allows for dorsiflexion and plantar flexion
• Inversion and eversion occur between the tarsals
• Medial ligament, or deltoid ligament, restricts eversion
• Calcaneal fibular ligament: the lateral side, resists inversion
• Anterior talofibular ligament is commonly injured
• Resists anterior shifting of the foot, that can be used used in the tests for ACL tear