Human Anatomy & Physiology - Articulations

Functions of Joints

Joints connect two bones and perform three key functions:

• Enable Movement: Joints serve as the link between bones, allowing movement when muscles and tendons exert force across the joint.

• Provide Stability: Some joints allow limited or no movement, offering stability critical for structures like the skull, which protects underlying structures.

• Allow Long Bones to Lengthen: The epiphyseal plate is where long bones grow in length during skeletal development; it functions as a temporary joint.

Functional Classification of Joints

Joints are classified by the amount of motion they permit:

• Synarthrosis: No movement between articulating bones.

• Amphiarthrosis: Small amount of movement between articulating bones.

• Diarthrosis: Freely moveable, allowing a wide variety of specific movements.

Structural Classification of Joints

Joints can also be classified by their structural features:

• Fibrous Joints: Fastened together by dense regular collagenous connective tissue without a joint space between articulating bones; can be synarthroses or amphiarthroses.

• Cartilaginous Joints: Fastened together with cartilage without a joint space; can be synarthroses or amphiarthroses.

• Synovial Joints: Diarthrosis joints that have a layer of hyaline cartilage on the articulating surface of each bone; the joint space is a fluid-filled cavity between articulating bones.

Fibrous Joints

These joints have dense regular collagenous connective tissue, which provides stability but allows little to no motion.

• Types: sutures, gomphoses, and or

Sutures

• Immoveable joints between the edges of bones that make up the cranium.

• Fully fused sutures are very stable, protecting the brain.

Gomphoses

• Immoveable joint (synarthrosis) between each tooth and the bony socket in the jaw.

• The periodontal ligament, a strong fibrous membrane, firmly links the tooth to the jaw bone.

Syndesmoses

• Joints between the tibia, fibula, ulna, and radius.

• Bones are joined by an interosseous membrane or ligament composed of dense regular collagenous connective tissue.

• Allows for a small amount of movement (amphiarthroses).

Cartilaginous Joints

In these joints, articulating bones are held together by cartilage. There is no joint cavity, and they allow for little to no motion.

• Types: synchondroses and symphyses.

Synchondroses

• Bones are linked together by hyaline cartilage; synarthroses.

  • Epiphyseal plates: Hyaline cartilage connecting the diaphyses and epiphyses of developing long bones; replaced with bone during maturation. Motion occurring at the epiphyseal plate could disrupt structure, disturb function, and possibly development

  • First sternocostal and costochondral joints: Synchondroses that stabilize the rib cage and persist into adulthood.

Symphyses

• Bones are united by a fibrocartilaginous pad or plug; functionally amphiarthroses.

  • Best suited for regions that must resist compression and tension with slight motion (Structure-Function Core Principle).

    • Intervertebral joints: Located between adjacent vertebral bodies of the spinal column.

    • Pubic symphysis: Located between the pubic bones of the pelvic girdle.

Epiphyseal Plate Fractures

• The epiphyseal plate in a child’s long bone is one of the weakest parts of the developing skeleton.

• Even a minor injury can fracture this delicate structure, possibly with lifelong consequences such as differences in limb length, limb deformities, and early-onset arthritis (joint inflammation).

• Most common causes: recreational activities, accidents, and competitive athletics; injuries may affect any joint but are most often at epiphyseal plates of the forearm.

• Symptoms: swelling, pain, and redness over the injured joint.

• Treatment: Depends on the severity of the fracture and must be determined by careful diagnosis.

  • Minor epiphyseal plate fractures can generally be managed by immobilizing the joint with a cast.

  • Severe fractures usually require surgery.

• Many patients benefit from rehabilitation exercises to strengthen bones and muscles surrounding the joint and regain full function. Fractures do not usually impair bone development if appropriately managed.

Structural Elements of Synovial Joints

• Joint cavity (synovial cavity): Space between articulating bones.

• Articular capsule: Double-layered structure.

  • Outer fibrous layer keeps articulating bones from being pulled apart and isolates the joint from the rest of the body.

  • Inner layer (synovial membrane) lines the entire inner surface except where hyaline cartilage is present; cells secrete synovial fluid (Structure-Function Core Principle: inner layer provides means for obtaining life-sustaining substances and eliminating waste products.)

• Synovial fluid: Thick liquid with three main functions:

  • Lubrication: Reduces friction between articulating surfaces.

  • Transportation: Moves nutrients and waste products in the absence of blood vessels within the joint.

  • Shock absorption: Helps to evenly distribute stress and force on articular surfaces during movement.

• Articular cartilage: Thin layer of hyaline cartilage covers all exposed articulating bones within the joint.

  • Provides a smooth surface for articulating bones to interact and reduces wear and tear created by friction (Structure-Function Core Principle).

  • Avascular because it is isolated within the capsule; relies on synovial fluid for oxygen, nutrients, and waste removal.

• Other components: Adipose tissue, nerves, and blood vessels.

Stabilizing and Supporting Factors of Synovial Joints

Synovial joints allow more mobility but less stability than other joint types. The following structures provide additional stabilization:

• Ligament: Strand of dense, regular, collagenous connective tissue; links one bone to another, providing additional strength and reinforcement to the joint.

• Tendon: Structural component of skeletal muscle; dense regular collagenous connective tissue; connects muscle to bone.

  • Tendons cross associated joints and provide stabilization when muscles are contracted.

  • Muscle tone: Continuous level of muscle contraction that provides stabilizing force.

• Bursae and tendon sheaths: Provide stabilization forces in high-stress regions.

  • Bursa: Synovial fluid-filled fibrous structure that minimizes friction between moving parts of joints.

  • Tendon sheath: Long bursa that surrounds tendons and protects them as they slide across a joint.

Bursitis

• Inflammation of a bursa; can result from a single traumatic event (fall), repetitive movements (pitching a baseball), or inflammatory disease (rheumatoid arthritis).

• Most common sites: shoulder, elbow, hip, and knee.

• Clinical features: pain both at rest and with motion; the joint may feel tender, swollen, and warm.

• Treatment: Aimed primarily at reducing pain and swelling.

  • Rest, ice, compression of the injured area, and medications are beneficial in the early stages of the injury.

  • Anti-inflammatory steroid medications may be injected directly into the bursa itself; fluid can also be removed from the bursa (relieves swelling).

  • Left untreated, can become chronically painful and increasingly difficult to cure.

Arthritis

• Inflammation of one or more joints; results in pain and limitations of joint movement.

  • Osteoarthritis: Most common form; results from wear and tear, injuries, and advanced age; causes pain, joint stiffness, and lost mobility.

  • Rheumatoid arthritis: Joint destruction mediated by an individual’s own immune system.

  • Gouty arthritis: Joint damage due to an inflammatory reaction to uric acid crystal deposits.

Functional Classes of Synovial Joints

Bones in a synovial joint move in different planes around an axis or axes. Possible joint configurations include:

• Nonaxial joints: Allow motion in one or more planes without moving around an axis.

• Uniaxial joints: Allow motion around only one axis.

• Biaxial joints: Allow motion around two axes.

• Multiaxial (triaxial) joints: Allow motion around three axes.

Understanding Axes of Motion

• Elbow joint: Only one axis; acts like a hinge and allows motion in one plane perpendicular to the axis; allows the forearm and hand to move upward toward the shoulder or make the opposite movement away from the shoulder.

• Metacarpophalangeal joints: Biaxial; located between the proximal phalanges and metacarpals.

  • Axis 1: Allows proximal phalanges to move toward and away from the palm of the hand (same as the elbow joint).

  • Axis 2: Allows fingers to be squeezed together or fanned out.

• Shoulder joint:

  • Humerus can move forward and backward around axis 1 (when arms swing back and forth while walking).

  • Humerus can also move away from and toward the body around axis 2 (jumping jacks).

  • Humerus can rotate (move in a circular fashion) around axis 3 (throwing a Frisbee).

Movements at Synovial Joints

Four general types of movement can take place at a synovial joint:

• Gliding movements: Sliding motion between articulating surfaces; nonaxial.

• Angular movements: Increase or decrease the angle between articulating bones.

Specific Types of Angular Motion

• Flexion: Decreases the angle between bones by bringing bones closer to one another.

• Extension: Increases the angle between bones (opposite of flexion); bones move away from one another.

• Hyperextension: Extension beyond the anatomical position of the joint.

• Abduction: Motion of a body part away from the midline of the body or another reference point.

• Adduction: Motion of a body part toward the midline of the body or another reference point; opposite of abduction.

• Circumduction: Only unpaired angular movement; a freely moveable distal bone moves on a fixed proximal bone in a cone-shaped motion; a combination of flexion-extension and abduction-adduction.

• Rotation: Nonangular motion; one bone rotates on an imaginary line running down its middle longitudinal axis.

Special Movements

Types of movements that are not defined by the previous categories:

• Opposition and reposition:

  • Opposition: (first carpometacarpal joint) allows the thumb to move across the palmar surface of the hand.

  • Reposition: The opposite movement returns the thumb to the anatomical position.

• Depression and elevation:

  • Depression: Moves a body part in an inferior direction.

  • Elevation: Moves a body part in a superior direction.

• Protraction and retraction:

  • Protraction: Moves a body part in an anterior direction.

  • Retraction: Moves a body part in a posterior direction.

• Inversion and eversion:

  • Inversion: Rotational motion; the plantar surface of the foot rotates medially toward the midline of the body.

  • Eversion: Rotates the foot laterally away from the midline.

• Dorsiflexion and plantarflexion:

  • Dorsiflexion: The angle between the foot and leg decreases.

  • Plantarflexion: The angle between the foot and leg increases.

• Supination and pronation: Rotational movements of the wrist and ankle regions.

Tips to remember supination and pronation:

• Hold a cup of soup; your hand is supinated.

• Pour it out when your hand pronates.

Tips to remember abduction and adduction:

• Abduction – abduct (take away) part from body.

• Adduction – add part back to body.

Range of Motion

• Range of motion: amount of movement a joint is capable of under normal circumstances.

• Example: Range of motion of knee – joint is moved from a relaxed state to full flexion and then returned to a fully extended state.

• Uniaxial joints (knee) tend to have the smallest range of motion; multiaxial joints (shoulder) tend to have the greatest.

Types of Synovial Joints

• Plane joint (gliding joint): The most simple and least mobile articulation between flat surfaces of two bones.

• Hinge joint: The convex articular surface of one bone interacts with the concave depression of a second bone; allows for uniaxial movement.

• Pivot joint: A rounded end surface of one bone fits into a groove on the surface of a second bone; uniaxial movement; one bone pivots or rotates around the other.

• Condylar or ellipsoid joint: A biaxial joint; the oval, convex surface of one bone fits into the shallow, concave articular surface of a second bone.

• Saddle joint: Each bone’s articulating surface has both a concave and convex region; a great deal of motion for a biaxial joint.

• Ball-and-socket joint: Multiaxial articulation; the articulating surface of one bone is spherical and fits into a cup-shaped depression in a second bone; a wide range of motion around all three available axes.

Specific Hinge Joints

Elbow

Very stable hinge joint; two articulations and three strong ligaments support the articular capsule:

• Humeroulnar joint: The larger of the two joints; the articulation between the trochlea of the humerus and the trochlear notch of the ulna.

• Humeroradial joint: The articulation between the capitulum of the humerus and the head of the radius.

• Radial collateral ligament (lateral collateral ligament): Supports the lateral side of the joint.

• Ulnar collateral ligament (medial collateral ligament): Supports the medial side of the joint.

• Anular ligament: Binds the head of the radius to the neck of the ulna; stabilizes the radial head.

Knee

• Patellar ligament: Distal continuation of the quadriceps tendon; connects the distal patella to the anterior tibia.

• Tibiofemoral joint: Articulation between the femoral and tibial condyles.

• Patellofemoral joint: Articulation between the posterior surface of the patella and the anterior patellar surface of the femur.

• Medial and lateral meniscus: C-shaped fibrocartilaginous pads between the femoral and tibial condyles; provide shock absorption and stability.

• Tibial collateral ligament (medial collateral): Connects the femur, medial meniscus, and tibia to one another; provides medial joint stabilization and prevents the tibia from shifting too far laterally on the femur.

Knee Injuries and the Unhappy Triad

• Despite supportive structures, the knee joint is susceptible to injury by quick changes in direction; contact sports athletes (football or soccer) are at risk, especially if the knee is struck from the side or from behind.

• A lateral blow (illegal block below the knees) ruptures the tibial collateral ligament; because it is attached to the medial meniscus, the force is transmitted to the menisci, often resulting in a tearing of the lateral meniscus; occasionally also the medial meniscus and the anterior cruciate ligament, creating the “unhappy triad.”
  *Fibular collateral and posterior collateral ligaments can also be damaged; less common.

• Treatment: Depends on severity.

  • Initial interventions: Rest, ice, compression, and anti-inflammatory medications to reduce swelling and minimize pain.

  • More severe injuries: May require surgical repair of damaged ligaments.

  • Physical therapy and rehabilitation: Strengthen surrounding muscles; also helpful.

Shoulder (glenohumeral joint)

One of the articulations of the pectoral girdle; connects the upper extremity with the axial skeleton; the ball-shaped head of the humerus articulates with the glenoid cavity on the lateral scapula.

• Glenoid labrum: Fibrocartilaginous ring that increases the depth of the glenoid cavity and provides more stability to the multiaxial joint.

• Biceps brachii tendon: Provides a stabilizing force as it passes over the joint; helps keep the head of the humerus within the glenoid cavity.

• Tendons of four muscles form the rotator cuff: provide most of the joint’s structural stabilization and strength: supraspinatus, infraspinatus, subscapularis, and teres minor.

Shoulder Dislocations

• The mobility of the shoulder joint comes at the expense of stability; shoulder injuries are very common, accounting for more than half of all joint dislocations.

• Dislocated shoulder: Specific to the glenohumeral joint; the head of the humerus is traumatically displaced from the glenoid cavity.

• Separated shoulder: Another common injury specific to the acromioclavicular joint; is not actually a component of the shoulder.

• Contact sport athletes are especially susceptible to shoulder injuries; anyone can suffer them under the right circumstances.

• Any fall with the hand and forearm outstretched can result in dislocation injury; the impact may force the head of the humerus through the inferior wall of the articular capsule (weakest relative to the rest of the capsule).

• Chest wall muscles pull the dislocated humeral head superiorly and medially; the head comes to rest inferior to the coracoid process of the scapula.

• Makes the shoulder look flattened or “squared off”; the injured person often holds the wrist of the affected shoulder against the abdomen (least painful position).

• Some minor shoulder dislocations can “pop” back into place with limited effort; more severe injuries may need surgical repair.

Hip (coxal joint)

Very stable, multiaxial articulation between the acetabulum and the ball-shaped head of the femur; the anatomy makes it stable enough for weight-bearing responsibilities (Structure-Function Core Principle).

• Acetabular labrum: fibrocartilaginous ring; helps to stabilize the head of the femur within the acetabulum.

• Retinacular fibers: intracapsular ligaments that surround the neck of the femur and reinforce the joint capsule.

• Iliofemoral ligament: a Y-shaped ligament that reinforces the anterior aspect of the external joint capsule.

• Ischiofemoral ligament: a spiral-shaped ligament that supports the posterior joint capsule.

• Pubofemoral ligament: a triangular-shaped ligament that supports the inferior aspect of the joint capsule.

• Ligament of the head of the femur: a small ligament that connects the head of the femur with the acetabulum and provides a pathway for small blood vessels to the femoral head.

Hip Joint Replacement Surgery

• Hip replacement: A surgical procedure that replaces a painful, damaged joint with an artificial prosthetic device; an individual may elect to have a hip replaced due to debilitating pain and subsequent loss of joint function.

• Severe arthritis, trauma, fractures, and bone tumors can all progress to the point where hip joint replacement is an option.

  • Total replacement: Removes and replaces the head of the femur; reconstructs the acetabulum.

  • Partial replacement: Removes only the head of the femur; replaces with a prosthetic device, leaving the acetabulum intact.

• The choice of replacement depends on many factors, including the type of injury, the patient’s age, and general health.

• Surgical complications are rare and further minimized with good post-procedure follow-up care.

• A rigorous rehabilitation program usually follows surgery to restore normal function; it may take 3–6 weeks for the patient to completely return to normal daily activities.