Human Anatomy and Physiology - Joints

Classification of Joints

• Joints, also called articulations, are sites where two or more bones meet.
• Functions:
  • Provide skeleton mobility.
  • Hold the skeleton together.
• Two classifications:
  • Structural: based on the material that binds the joints and whether a cavity is present.
    • Fibrous
    • Cartilaginous
    • Synovial
  • Functional: based on the movement the joint allows.
    • Synarthroses: immovable joints.
    • Amphiarthroses: slightly movable joints.
    • Diarthroses: freely movable joints.
• Structural classifications are more clear cut.

Fibrous Joints

• Bones are joined by dense fibrous connective tissue.
• No joint cavity is present.
• Most are immovable, depending on the length of connective tissue fibers.

Sutures

• Rigid, interlocking joints of the skull.
• Allow for growth during youth with short connective tissue fibers for expansion.
• In middle age, sutures ossify and fuse.

Syndesmoses

• Bones are connected by ligaments, bands of fibrous tissue.
• Fiber length varies, which affects movement.
  • Short fibers offer little to no movement (e.g., inferior tibiofibular joint).
  • Longer fibers offer a larger amount of movement (e.g., interosseous membrane connecting radius and ulna).

Gomphoses

• Peg-in-socket joints.
• The only examples are the teeth in alveolar sockets.
• The fibrous connection is the periodontal ligament which holds the tooth in the socket.

Cartilaginous Joints

• Bones are united by cartilage.
• Similar to fibrous joints, they have no joint cavity and are not highly movable.

Synchondroses

• A bar or plate of hyaline cartilage unites bones.
• Almost all are synarthrotic (immovable).
• Examples:
  • Temporary epiphyseal plate joints that become synostoses after plate closure.
  • Cartilage of the 1st rib with the manubrium of the sternum.

Symphyses

• Fibrocartilage unites bone in a symphysis joint.
• Hyaline cartilage is also present as articular cartilage on bony surfaces.
• Symphyses are strong, amphiarthrotic (slightly movable) joints.
• Examples: intervertebral joints, pubic symphysis.

Synovial Joints

• Bones are separated by a fluid-filled joint cavity.
• All are diarthrotic (freely movable).
• Includes almost all limb joints.
• Characteristics:
  • Six general features.
  • Bursae and tendon sheaths are associated with them.
  • Stability is influenced by three factors.
  • Allow several types of movements.
  • Classified into six different types.

General Structure

• Six general features:
  • Articular cartilage: Hyaline cartilage covering ends of bones that prevents crushing of bone ends.
  • Joint (synovial) cavity: Small, fluid-filled potential space unique to synovial joints.
  • Articular (joint) capsule: Two layers thick.
    • External fibrous layer: Dense irregular connective tissue.
    • Inner synovial membrane: Loose connective tissue that makes synovial fluid.
  • Synovial fluid: Viscous, slippery filtrate of plasma and hyaluronic acid.
    • Lubricates and nourishes articular cartilage.
    • Contains phagocytic cells to remove microbes and debris.
  • Different types of reinforcing ligaments.
    • Capsular: Thickened part of the fibrous layer.
    • Extracapsular: Outside the capsule.
    • Intracapsular: Deep to the capsule; covered by the synovial membrane.
  • Nerves and blood vessels.
    • Nerves detect pain and monitor joint position and stretch.
    • Capillary beds supply filtrate for synovial fluid.
• Other features:
  • Fatty pads: For cushioning between the fibrous layer of the capsule and the synovial membrane or bone.
  • Articular discs (menisci): Fibrocartilage separates articular surfaces to improve the “fit” of bone ends, stabilize the joint, and reduce wear and tear.

Bursae and Tendon Sheaths

• Bags of synovial fluid act as lubricating “ball bearings”.
• Not strictly part of synovial joints but closely associated.
• Bursae: Reduce friction where ligaments, muscles, skin, tendons, or bones rub together.
• Tendon sheaths: Elongated bursae wrapped completely around tendons subjected to friction.

Factors Influencing Stability of Synovial Joints

• Three factors determine the stability of joints to prevent dislocations:
  • Shape of articular surface (minor role): Shallow surfaces are less stable than ball-and-socket joints.
  • Ligament number and location (limited role): The more ligaments, the stronger the joint.
  • Muscle tone keeps tendons taut as they cross joints (most important): Extremely important in reinforcing shoulder and knee joints and arches of the foot.

Movements Allowed by Synovial Joints

• All muscles attach to bone or connective tissue at no fewer than two points:
  • Origin: Attachment to immovable bone.
  • Insertion: Attachment to movable bone.
• Muscle contraction causes insertion to move toward the origin.
• Movements occur along transverse, frontal, or sagittal planes.
• Range of motion:
  • Nonaxial: Slipping movements only.
  • Uniaxial: Movement in one plane.
  • Biaxial: Movement in two planes.
  • Multiaxial: Movement in or around all three planes.
• Three general types of movements:
  • Gliding
  • Angular movements
  • Rotation

Gliding Movements

• One flat bone surface glides or slips over another similar surface.
• Examples:
  • Intercarpal joints
  • Intertarsal joints
  • Between articular processes of vertebrae

Angular Movements

• Increase or decrease the angle between two bones.
• Movement along the sagittal plane.
• Include:
  • Flexion: Decreases the angle of the joint.
  • Extension: Increases the angle of the joint.
  • Hyperextension: Movement beyond the anatomical position.
  • Abduction: Movement along the frontal plane, away from the midline.
  • Adduction: Movement along the frontal plane, toward the midline.
  • Circumduction: Involves flexion, abduction, extension, and adduction of a limb, describing a cone in space.

Rotation

• Turning of a bone around its own long axis, toward the midline or away from it.
  • Medial: Rotation toward the midline.
  • Lateral: Rotation away from the midline.
  • Examples: Rotation between C1 and C2 vertebrae, rotation of humerus and femur.

Special Movements

• Supination and pronation: Rotation of the radius and ulna.
  • Supination: Palms face anteriorly; radius and ulna are parallel.
  • Pronation: Palms face posteriorly; radius rotates over ulna.
• Dorsiflexion and plantar flexion of foot.
  • Dorsiflexion: Bending foot toward shin.
  • Plantar flexion: Pointing toes.
• Inversion and eversion of foot.
  • Inversion: Sole of the foot faces medially.
  • Eversion: Sole of the foot faces laterally.
• Protraction and retraction: Movement in the lateral plane.
  • Protraction: Mandible juts out.
  • Retraction: Mandible is pulled toward the neck.
• Elevation and depression of the mandible.
  • Elevation: Lifting a body part superiorly (e.g., shrugging shoulders).
  • Depression: Lowering a body part (e.g., opening jaw).
• Opposition: Movement of the thumb (e.g., touching the thumb to the tips of other fingers on the same hand or any grasping movement).

Types of Synovial Joints

• Six different types based on the shape of the articular surface and the movement the joint is capable of:
  • Plane
  • Hinge
  • Pivot
  • Condylar
  • Saddle
  • Ball-and-socket

Selected Synovial Joints

• Synovial joints are diverse with general features but also have unique structural features, abilities, and weaknesses.
• Five main synovial joints:
  • Jaw (Temporomandibular Joint)
  • Shoulder (Glenohumeral)
  • Elbow
  • Hip (Coxal)
  • Knee

Temporomandibular Joint (TMJ)

• The jaw joint is a modified hinge joint.
• The mandibular condyle articulates with the temporal bone.
  • The posterior temporal bone forms the mandibular fossa, while the anterior portion forms the articular tubercle.
• The articular capsule thickens into a strong lateral ligament.
• Two types of movement:
  • Hinge: Depression and elevation of the mandible.
  • Gliding: Side-to-side (lateral excursion) grinding of teeth.
• The most easily dislocated joint in the body because of shallow socket of joint.
  • Almost always dislocates anteriorly, causing mouth to remain open
    • To realign, physician must push mandible back into place
  • Symptoms: ear and face pain, tender muscles, popping sounds when opening mouth, joint stiffness
  • Usually caused by grinding teeth, but can also be due to jaw trauma or poor occlusion of teeth
    • Treatment for grinding teeth includes bite plate
    • Relaxing jaw muscles helps

Shoulder (Glenohumeral) Joint

• The most freely moving joint in the body.
• Stability is sacrificed for freedom of movement.
• Ball-and-socket joint with a large, hemispherical head of the humerus fitting into the small, shallow glenoid cavity of the scapula.
• The articular capsule enclosing the cavity is thin and loose, contributing to freedom of movement.
• Reinforcing muscle tendons contribute most to joint stability.
  • The tendon of the long head of the biceps brachii muscle is the “superstabilizer”.
    • Travels through the intertubercular sulcus and secures the humerus to the glenoid cavity.
  • Four rotator cuff tendons encircle the shoulder joint:
    • Subscapularis
    • Supraspinatus
    • Infraspinatus
    • Teres minor
• Shoulder dislocations are common injuries due to mobility in the shoulder
  • Structures reinforcing this joint are weakest anteriorly and inferiorly, so the head of the humerus can easily dislocate forward and downward
  • The glenoid cavity provides poor support when the humerus is rotated laterally and abducted – ex: when a football player uses arm to tackle an opponent
  • Blows to the top and back of the shoulder can also cause dislocations.

Elbow Joint

• The humerus articulates with the radius and ulna.
• Hinge joint: Formed primarily from the trochlear notch of the ulna articulating with the trochlea of the humerus, allowing for flexion and extension only.
• The anular ligament surrounds the head of the radius.
• Two capsular ligaments restrict side-to-side movement:
  • Ulnar collateral ligament
  • Radial collateral ligament

Hip (Coxal) Joint

• Ball-and-socket joint with the large, spherical head of the femur articulating with the deep cup-shaped acetabulum.
• Good range of motion, but limited by the deep socket.
  • Acetabular labrum: Rim of fibrocartilage that enhances the depth of the socket (hip dislocations are rare).

Knee Joint

• The largest, most complex joint of the body.
• Consists of three joints surrounded by a single cavity:
  • Femoropatellar joint:
    • Plane joint that allows gliding motion during knee flexion.
  • Lateral joint and Medial joint:
    • Together called the tibiofemoral joint.
    • The joint between the femoral condyles and the lateral and medial menisci of the tibia.
    • Hinge joint that allows flexion, extension, and some rotation when the knee is partly flexed.
• The joint capsule is thin and absent anteriorly.
• Anteriorly, the quadriceps tendon gives rise to three broad ligaments that run from the patella to the tibia:
  • Medial and lateral patellar retinacula that flank the patellar ligament.
    • Doctors tap the patellar ligament to test the knee-jerk reflex.
• At least 12 bursae are associated with the knee joint.
• Intracapsular ligaments:
  • Reside within the capsule, but outside the synovial cavity.
  • Help to prevent anterior-posterior displacement.
    • Anterior cruciate ligament (ACL):
      • Attaches to the anterior tibia.
      • Prevents forward sliding of the tibia and stops hyperextension of the knee.
    • Posterior cruciate ligament:
      • Attaches to the posterior tibia.
      • Prevents backward sliding of the tibia and forward sliding of the femur.
• Knee injuries with the 3 C’s:
  • Collateral ligaments
  • Cruciate ligaments
  • Cartilages (menisci)
  • Lateral blows to an extended knee can result in tears in the tibial collateral ligament, medial meniscus, and anterior cruciate ligament.
  • Injuries affecting just the ACL are common in runners who change direction, twisting the ACL.
  • Surgery is usually needed for repairs.

Disorders of Joints

Common Joint Injuries

• Cartilage tears:
  • Due to compression and shear stress.
  • Fragments may cause the joint to lock or bind.
  • Cartilage rarely repairs itself.
  • Repaired with arthroscopic surgery.
    • Partial menisci removal renders joint less stable but mobile; complete removal leads to osteoarthritis.
    • Meniscal transplant possible in younger patients; perhaps the meniscus can be grown from own stem cells in the future.
• Sprains:
  • Reinforcing ligaments are stretched or torn.
  • Common sites are the ankle, knee, and lumbar region of the back.
  • Partial tears repair very slowly because of poor vascularization.
  • Three options if torn completely:
    • Ends of ligaments can be sewn together.
    • Replaced with grafts.
    • Just allow time and immobilization for healing.
• Dislocations (luxations):
  • Bones forced out of alignment.
  • Accompanied by sprains, inflammation, and difficulty moving the joint.
  • Caused by serious falls or contact sports.
  • Must be reduced to treat.
  • Subluxation: Partial dislocation of a joint.

Inflammatory and Degenerative Conditions

• Bursitis: Inflammation of a bursa, usually caused by a blow or friction; treated with rest and ice and, if severe, anti-inflammatory drugs.
• Tendonitis: Inflammation of tendon sheaths, typically caused by overuse; symptoms and treatment are similar to those of bursitis.
• Arthritis: >100 different types of inflammatory or degenerative diseases that damage joints; the most widespread crippling disease in the U.S.
  • Symptoms: Pain, stiffness, and swelling of the joint.
  • Acute forms: Caused by bacteria, treated with antibiotics.
  • Chronic forms: Osteoarthritis, rheumatoid arthritis, and gouty arthritis.
    • Osteoarthritis (OA):
      • The most common type of arthritis.
      • Irreversible, degenerative (“wear-and-tear”) arthritis; usually part of the normal aging process.
    • Rheumatoid arthritis (RA):
      • Chronic, inflammatory, autoimmune disease of unknown cause (immune system attacks own cells).
      • Signs and symptoms include joint pain and swelling (usually bilateral), anemia, osteoporosis, muscle weakness, and cardiovascular problems.
      • Inflammatory blood cells migrate to the joint, releasing inflammatory chemicals that destroy tissues.
    • Gouty arthritis: Deposition of uric acid crystals in joints and soft tissues, followed by inflammation; more common in men; typically affects the joint at the base of the great toe.
    • Lyme disease: Caused by bacteria transmitted by tick bites; symptoms include skin rash, flu-like symptoms, and foggy thinking; may lead to joint pain and arthritis; treatment involves a long course of antibiotics.

Developmental Aspects of Joints

• By embryonic week 8, synovial joints resemble adult joints.
• A joint’s size, shape, and flexibility are modified by use.
  • Active joints have thicker capsules and ligaments.
• Advancing years take a toll on joints.
  • Ligaments and tendons shorten and weaken.
  • Intervertebral discs are more likely to herniate.
  • Most people in their 70s have some degree of OA.
• Full-range-of-motion exercise is key to postponing joint problems.