Joints: Structure, Classification, and Disorders

Classification of Joints

• Joints, also called articulations, are sites where two or more bones meet.

• Joints give the skeleton mobility and hold it together.

• Two classifications of joints:

  • Structural: Based on what material 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 joined by dense fibrous connective tissue.

• No joint cavity.

• Most are immovable, depending on the length of connective tissue fibers.

• Three types of fibrous joints:

  • Sutures

  • Syndesmoses

  • Gomphoses

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, becoming immovable joints that protect the brain.

• Closed, immovable sutures are referred to as synostoses.

Syndesmoses

• Bones connected by ligaments, bands of fibrous tissue.

• Fiber length varies, so movement varies.

  • 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.

• Only examples are the teeth in alveolar sockets.

• Fibrous connection is the periodontal ligament, which holds the tooth in the socket.

Cartilaginous Joints

• Bones united by cartilage.

• Like fibrous joints, have no joint cavity.

• Not highly movable.

• Two types:

  • Synchondroses

  • Symphyses

Synchondroses

• Bar or plate of hyaline cartilage unites bones.

• Almost all are synarthrotic (immovable).

• Examples:

  • Temporary epiphyseal plate joints, which become synostoses after plate closure.

  • Cartilage of the first rib with the manubrium of the sternum.

Symphyses

• Fibrocartilage unites bone in symphysis joint.

• Hyaline cartilage also present as articular cartilage on bony surfaces.

• Symphyses are strong, amphiarthrotic (slightly movable) joints.

• Examples:

  • Intervertebral joints

  • Pubic symphysis

Synovial Joints

• Bones separated by a fluid-filled joint cavity.

• All are diarthrotic (freely movable).

• Include almost all limb joints.

• Characteristics of synovial joints:

  • Have six general features

  • Have bursae and tendon sheaths associated with them

  • Stability is influenced by three factors

  • Allow several types of movements

  • Classified into six different types

General Structure of Synovial Joints

• Six general features:

  • Articular cartilage: Hyaline cartilage covering the ends of bones, preventing 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 of some synovial joints:

  • Fatty pads: 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 that 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 the articular surface (minor role).

    • Shallow surfaces are less stable than ball-and-socket.

  • 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 the immovable bone.

  • Insertion: Attachment to the movable bone.

• Muscle contraction causes insertion to move toward origin.

• Movements occur along transverse, frontal, or sagittal planes.

• Range of motion allowed by synovial joints:

  • 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.

• Angular movements 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. The limb describes a cone in space.

Rotation

• Turning of a bone around its own long axis, toward or away from the midline.

  • Medial: Rotation toward the midline.

  • Lateral: Rotation away from the midline.

  • Examples:

    • Rotation between C1 and C2 vertebrae

    • Rotation of the 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 the foot.

  • Dorsiflexion: Bending the foot toward the shin.

  • Plantar flexion: Pointing the toes.

• Inversion and eversion of the 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 the 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 of synovial joints, categorized 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; all have general features, but some also have unique structural features, abilities, and weaknesses.

• Five main synovial joints:

  • Jaw (Temporomandibular Joint)

  • Shoulder

  • Elbow

  • Hip

  • 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.

Clinical - Homeostatic Imbalance 8.1

• Dislocation of the TMJ is most common because of the shallow socket of the joint.

• Almost always dislocates anteriorly, causing the mouth to remain open.

  • To realign, a physician must push the mandible back into place.

• Symptoms: Ear and face pain, tender muscles, popping sounds when opening the 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 a 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:

  • The large, hemispherical head of the humerus fits in the small, shallow glenoid cavity of the scapula.

  • Like a golf ball on a tee.

• The articular capsule enclosing the cavity is thin and loose, contributing to the freedom of movement.

• Glenoid labrum: Fibrocartilaginous rim around the glenoid cavity that helps to add depth to the shallow cavity.

  • The cavity still only holds one-third of the head of the humerus.

• Reinforcing ligaments:

  • Primarily on the anterior aspect.

  • Coracohumeral ligament: Helps support the weight of the upper limb.

  • Three glenohumeral ligaments: Strengthen the anterior capsule but are weak support.

• Reinforcing muscle tendons contribute most to joint stability.

  • The tendon of the long head of the biceps brachii muscle is a “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

Clinical - Homeostatic Imbalance 8.2

• 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 (e.g., when a football player uses an 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.

• A hinge joint is formed primarily from the trochlear notch of the ulna articulating with the trochlea of the humerus.

  • Allows 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.

• The large, spherical head of the femur articulates 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).

• Reinforcing ligaments include:

  • Iliofemoral ligament

  • Pubofemoral ligament

  • Ischiofemoral ligament

  • Ligament of the head of the femur (ligamentum teres):

    • Slack during most hip movements, so not important in stabilizing.

    • Does contain an artery that supplies the head of the femur.

• Greatest stability comes from the deep ball-and-socket joint.

Knee Joint

• The largest, most complex joint of the body.

• Consists of three joints surrounded by a single cavity:

  • Femoropatellar joint:

    • Plane joint.

    • Allows a gliding motion during knee flexion.

  • Lateral joint and medial joint:

    • Together, they are called the tibiofemoral joint.

    • Joint between the femoral condyles and the lateral and medial menisci of the tibia.

    • A hinge joint 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.

• Capsular, extracapsular, or intracapsular ligaments act to stabilize the knee joint.

• Capsular and extracapsular ligaments help prevent hyperextension of the knee.

  • Fibular and tibial collateral ligaments: Prevent rotation when the knee is extended.

  • Oblique popliteal ligament: Stabilizes the posterior knee joint.

  • Arcuate popliteal ligament: Reinforces the joint capsule posteriorly.

• 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 (PCL):

    • Attaches to the posterior tibia.

    • Prevents backward sliding of the tibia and forward sliding of the femur.

Clinical - Homeostatic Imbalance 8.3

• The knee absorbs a great amount of vertical force; however, it is vulnerable to horizontal blows.

• Common knee injuries involved the 3 C’s:

  • Collateral ligaments

  • Cruciate ligaments

  • Cartilages (menisci)

• Lateral blows to the 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.

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 the joint less stable but mobile; complete removal leads to osteoarthritis.

• Meniscal transplant possible in younger patients.

• Perhaps a meniscus grown from one's 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 the 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

• 
  More than 100 different types of inflammatory or degenerative diseases that damage joints.

• 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)

• Most common type of arthritis.

• Irreversible, degenerative (“wear-and-tear”) arthritis.

• May reflect the excessive release of enzymes that break down articular cartilage.

  • Cartilage is broken down faster than it is replaced.

  • Bone spurs (osteophytes) may form from thickened ends of bones.

• By age 85, half of Americans develop OA, more women than men.

• OA is usually part of the normal aging process.

• Joints may be stiff and make a crunching noise referred to as crepitus, especially upon rising.

• Treatment: Moderate activity, mild pain relievers, capsaicin creams.

• Glucosamine, chondroitin sulfate, and nutritional supplements are not effective.

Rheumatoid Arthritis (RA)

• Chronic, inflammatory, autoimmune disease of unknown cause.

  • The immune system attacks its own cells.

• Usually arises between ages 40 and 50 but may occur at any age; affects three times as many women as men.

• Signs and symptoms include joint pain and swelling (usually bilateral), anemia, osteoporosis, muscle weakness, and cardiovascular problems.

• RA begins with inflammation of the synovial membrane (synovitis) of the affected joint.

• Inflammatory blood cells migrate to the joint and release inflammatory chemicals that destroy tissues.

• Synovial fluid accumulates, causing joint swelling.

• The inflamed synovial membrane thickens into abnormal pannus tissue that clings to articular cartilage.

• Pannus erodes cartilage, scar tissue forms, and connects articulating bone ends (ankylosis).

• Treatment includes steroidal and nonsteroidal anti-inflammatory drugs to decrease pain and inflammation.

• Disruption of the destruction of joints by the immune system.

  • Immune suppressants slow the autoimmune reaction.

  • Some agents target tumor necrosis factor to block the action of inflammatory chemicals.

• Can replace the joint with a prosthesis.

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.

• In untreated gouty arthritis, bone ends fuse and immobilize the joint.

• Treatment: Drugs, plenty of water, avoidance of alcohol and foods high in purines, such as liver, kidneys, and sardines.

Lyme Disease

• Caused by bacteria transmitted by tick bites.

• Symptoms: Skin rash, flu-like symptoms, and foggy thinking.

• May lead to joint pain and arthritis.

• Treatment: 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.