Osseous system

OSSEOUS SYSTEM

INTRODUCTION

Osteology is the branch of anatomy that studies the structure and function of bones and the skeleton. The osseous system (skeletal system) is made up of bones, cartilage and joints. An average adult human skeleton contains about 206 bones which together:

• Give shape to the body
• Protect internal organs (for example the skull protects the brain; the ribcage protects the heart and lungs)
• Provide a framework for muscle attachment so movement is possible
• Store essential minerals (especially calcium and phosphate)
• House marrow where blood cells are formed (hematopoiesis)

DIVISION

The skeleton is grouped into two major parts:

• The Axial Skeleton — forms the central axis: skull, vertebral column and thoracic cage.
• The Appendicular Skeleton — includes the shoulder girdles, upper limbs, pelvic girdle and lower limbs.

FUNCTIONS OF OSSEOUS SYSTEM

• Protection: Shields delicate organs (brain, heart, lungs).
• Support: Maintains body shape and posture.
• Movement: Bones act as levers pulled by muscles.
• Mineral storage: Reservoir for calcium and phosphate.
• Hematopoiesis: Red bone marrow produces red blood cells, white blood cells and platelets.
• Metabolic roles: Bone cells participate in mineral homeostasis and some hormone-related functions.

CLASSIFICATION OF OSSEOUS SYSTEM (BONES BY SHAPE & FUNCTION)

Bone types, features and examples

BONE CELLS

Bone is living tissue made up of several specialized cell types that build, maintain and resorb bone.

• Osteoblasts — bone-building cells. They synthesize bone matrix (collagen + organic components) and initiate mineralization. Found where bone is forming (growth plates, periosteum, fracture sites).
• Osteocytes — mature bone cells derived from osteoblasts that have become embedded in the mineralized matrix (in lacunae). They monitor bone health and help regulate mineral content. If needed, they can revert to a less mature form to assist repair.
• Osteoclasts — bone-resorbing cells. Large, multinucleated cells that break down bone matrix, releasing calcium and phosphate into the blood. Important for bone remodeling and calcium homeostasis.

STRUCTURE OF BONE

Bone is the hardest connective tissue and exists in two structural forms:

• Compact (cortical) bone — dense, forms the outer shell of most bones and the shafts of long bones; about 80% of adult skeleton mass. Provides strength for weight-bearing.
• Cancellous (trabecular or spongy) bone — porous, made of a lattice of trabeculae; lighter and more flexible than compact bone; found at bone ends (epiphyses), inside flat bones and vertebrae.

Bones also contain marrow spaces: red marrow (hematopoietic) and yellow marrow (fatty).

Microscopic Structure of Bone (Compact bone)

Compact bone is organized into repeating structural units called osteons (Haversian systems):

• Haversian (central) canal — runs longitudinally through each osteon and houses blood vessels, nerves and lymphatics.
• Lamellae — concentric rings of mineralized matrix around the Haversian canal.
• Lacunae — small cavities between lamellae that contain osteocytes.
• Canaliculi — tiny channels radiating from lacunae that connect osteocytes to each other and to blood supply; permit nutrient and waste exchange.
• Volkmann’s canals — transverse channels that interconnect Haversian canals.

This organized microstructure gives compact bone its mechanical strength and ability to remodel.

I. THE AXIAL SKELETON

The axial skeleton forms the central bony axis and includes:

• Skull (cranium + facial bones)
• Vertebral column (spine)
• Thoracic cage (ribs and sternum)

It supports and protects the central nervous system, thoracic organs, and forms the general framework of the trunk.

1) The Skull

The skull has two main parts:

• Cranium — encloses and protects the brain; formed by eight bones.
• Facial skeleton — fourteen bones forming the face, nasal cavity and mouth structures.

The Cranium (8 bones) — brief overview

• Frontal (1) — forms forehead and superior orbit; contains frontal sinuses (help reduce skull weight and add resonance).
• Parietal (2) — form sides and roof of cranium; meet at sagittal suture.
• Temporal (2) — form lower sides of skull; contain structures of the ear (petrous part) and mastoid process. Articulate with mandible (temporomandibular joint).
• Occipital (1) — forms posterior skull; contains foramen magnum for spinal cord passage.
• Sphenoid (1) — “keystone” that joins cranial and facial bones; contains the sella turcica that houses pituitary gland.
• Ethmoid (1) — light, spongy bone at anterior skull base; contributes to medial orbital walls and nasal septum; cribriform plate allows olfactory nerve fibers to pass.

The Face (14 bones) — brief overview

• Maxillae (2) — form upper jaw, floor of orbit and parts of nasal cavity; contain maxillary sinuses.
• Zygomatic bones (2) — cheekbones; part of lateral orbit.
• Mandible (1) — lower jaw; only movable skull bone (via temporomandibular joint); bears lower teeth.
• Nasal bones (2) — bridge of the nose.
• Lacrimal bones (2) — small bones forming medial orbit walls; contain nasolacrimal canal for tears.
• Palatine bones (2) — form posterior hard palate and part of nasal walls.
• Inferior nasal conchae (2) — scroll-shaped bones that increase nasal cavity surface (warming & humidifying air).
• Vomer (1) — forms lower part of nasal septum.
• Hyoid bone (1, not facial) — U-shaped bone in neck (above larynx), not directly articulated to other bones; provides attachment for tongue and neck muscles.

2) The Vertebral Column

The vertebral column (spine) is a flexible, strong column that supports the trunk and protects the spinal cord. It is made of 26 vertebrae in adults (some fused):

• Cervical (7) — neck region (C1–C7). First two are specialized:
• Atlas (C1) — supports the skull, has no body; permits nodding.
• Axis (C2) — has the odontoid (dens) process allowing rotation (shaking head «no»).
• Thoracic (12) — each articulates with a pair of ribs; heart-shaped bodies, long downward spinous processes.
• Lumbar (5) — large, robust bodies to bear weight; short thick spinous processes.
• Sacrum (1 — 5 fused) — formed by fusion of five sacral vertebrae; connects spine to pelvic girdle. Has sacral foramina for nerve passage and a sacral promontory.
• Coccyx (1 — 4 fused) — tailbone; small fused vertebrae at the base.

Functions of the Vertebral Column

• Protects the spinal cord inside the vertebral canal.
• Supports the skull and trunk; transfers weight to the pelvis.
• Provides attachment points for ribs, muscles and ligaments.
• Intervertebral discs between vertebrae absorb shock and allow flexibility.
• Vertebral foramina and intervertebral foramina allow passage of spinal cord and spinal nerves.

II. THE APPENDICULAR SKELETON

The appendicular skeleton includes the bones of the shoulder girdle, upper limbs, pelvic girdle and lower limbs. It is specialised for movement and interaction with the environment.

1) Shoulder Girdle

The shoulder girdle connects each upper limb to the trunk. It consists of two clavicles (collarbones) and two scapulae (shoulder blades).

Clavicle (Collar Bone)

• An S-shaped bone that links the upper limb to the axial skeleton at the sternoclavicular joint.
• Medially articulates with the manubrium of the sternum and laterally with the acromion of the scapula (acromioclavicular joint).
• Acts as a strut to keep the shoulder away from the thorax, allowing a wide range of movement.

Scapula (Shoulder Blade)

• A flat triangular bone on the posterior chest wall.
• The glenoid cavity at its lateral angle forms the socket of the shoulder joint (glenohumeral joint).
• Important landmarks: spine of scapula, acromion, coracoid process — muscle attachments and joint formation.

2) The Upper Limb

Humerus

• Long bone of the upper arm.
• Proximal end: head articulates with glenoid cavity to form the shoulder joint. Greater and lesser tubercles serve as muscle attachment points.
• Distal end: articulates with radius and ulna to form the elbow joint (trochlea and capitulum visible on distal humerus).

Radius and Ulna (Forearm)

• Two parallel bones.
• Ulna: medial in anatomical position (toward little finger), longer, forms major part of the elbow joint (trochlear notch).
• Radius: lateral (thumb side); its head articulates with the capitulum and participates in wrist joint.
• Interosseous membrane between them stabilizes the forearm and transmits forces.

Carpal (Wrist) Bones

• Eight small bones arranged in two rows (proximal: scaphoid, lunate, triquetrum, pisiform; distal: trapezium, trapezoid, capitate, hamate).
• Form flexible wrist and small gliding joints; ligaments and retinacula hold tendons in place.

Metacarpals and Phalanges

• Five metacarpals form the palm (numbered I–V from thumb to little finger).
• Phalanges: 14 finger bones — three in each finger (proximal, middle, distal) and two in the thumb.

3) Pelvic Girdle

The pelvic girdle is formed by two innominate (hip) bones, each made by fusion of ilium, ischium and pubis. Together with the sacrum they make the pelvis.

Innominate Bone

• Ilium: large, wing-like superior portion (iliac crest and anterior superior iliac spine are palpable landmarks).
• Ischium: posterior inferior part — ischial tuberosity bears weight when sitting.
• Pubis: anterior part; the two pubic bones meet at the pubic symphysis.

The Pelvis

• The pelvis is divided by the pelvic brim into the greater (false) pelvis above and the lesser (true) pelvis below.
• Male and female pelvises differ: female pelvis is generally broader with a wider pelvic inlet/outlet to facilitate childbirth.

4) Lower Limb

Femur

• The longest and strongest bone in the body (thigh bone).
• Proximal end: head (articulates with acetabulum), neck, greater and lesser trochanters (muscle attachments).
• Distal end: two condyles form part of the knee joint.

Tibia (Shin Bone)

• Larger medial bone of lower leg; proximal end articulates with femur (knee), distal with talus (ankle).
• Tibial tuberosity is an important landmark for quadriceps tendon attachment.

Fibula

• Thin lateral bone; does not bear major weight but provides muscle attachments and forms the lateral malleolus which stabilises the ankle.

Patella (Kneecap)

• Sesamoid bone embedded in the quadriceps tendon; improves mechanical advantage of knee extension and protects the knee joint.

Tarsals, Metatarsals, Phalanges

• Tarsals: seven bones form the ankle and posterior foot (talus, calcaneus (heel bone), navicular, cuboid, and three cuneiforms).
• Metatarsals: five long bones forming the foot arch and dorsum.
• Phalanges: 14 toe bones (two in big toe, three in each other toe).

JOINTS

INTRODUCTION

A joint (articulation) is any connection between bones. Joints allow varying degrees of movement and are composed of bone ends plus connective tissues (cartilage, ligaments, tendons).

• Tendons attach muscle to bone.
• Ligaments connect bone to bone.
• Cartilage cushions and reduces friction at joint surfaces.

The study of joints is called arthrology.

CLASSIFICATION OF JOINTS

Joints are classified structurally and functionally. Structurally:

• Fibrous joints (synarthroses) - bones joined by fibrous tissue; little or no movement.
• Cartilaginous joints (amphiarthroses) - bones joined by cartilage; limited movement.
• Synovial joints (diarthroses) - freely movable joints with a synovial cavity.

FIBROUS JOINTS (SYNARTHROSES)

• Immovable or very limited movement.
• Examples: sutures of the skull, gomphosis (tooth in socket).

CARTILAGINOUS JOINTS (AMPHIARTHROSES)

• Bones united by hyaline cartilage or fibrocartilage.
• Examples: intervertebral discs (fibrocartilage), pubic symphysis; allow limited movement and absorb shock.

SYNOVIAL JOINTS (DIARTHROSES)

Freely movable joints with common features:

• Articular surfaces covered by hyaline (articular) cartilage.
• Bones held together by ligaments.
• Enclosed by a fibrous capsule lined internally by a synovial membrane.
• Joint cavity contains synovial fluid for lubrication.
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Types of Synovial Joints and Examples

MOVEMENTS OF JOINTS

Joints permit various movements described by directional terms:

Gliding Movements

• Two relatively flat surfaces slide past one another (e.g., between carpal bones).

Angular Movements

• Flexion: Decreases the angle between bones (bending elbow).
• Extension: Increases the angle (straightening elbow).
• Abduction: Movement away from midline (raising arm laterally).
• Adduction: Movement toward midline (lowering arm toward body).
• Circumduction: Circular movement combining flexion, extension, abduction and adduction.

Rotational Movements

• Medial (internal) rotation: Turning a limb toward the midline.
• Lateral (external) rotation: Turning a limb away from midline.
• Pronation / Supination (forearm): Pronation turns palm posteriorly/downward; supination turns palm anteriorly/upward.

JOINTS OF THE UPPER LIMB

Sternoclavicular Joint

• A gliding joint between sternum and clavicle; includes a cartilage pad for shock absorption. Supports wide shoulder movement.

Acromioclavicular Joint

• Between acromion of scapula and lateral clavicle; has limited motion and helps scapular rotation.

Shoulder (Glenohumeral) Joint

• A ball-and-socket joint: head of humerus in glenoid cavity.
• Very mobile, stabilized by rotator cuff muscles, ligaments and capsule.
• Permit flexion, extension, abduction, adduction, rotation, circumduction.

Elbow Joint

• Primarily a hinge joint between humerus, radius and ulna.
• Components: humero-ulnar, humero-radial and superior radioulnar articulations.
• Movements: flexion and extension.

Radio-ulnar Joints

• Proximal and distal radioulnar joints allow pronation and supination of the forearm via pivot-like movement of radius around ulna.

Wrist Joint (Radiocarpal)

• A condyloid joint between distal radius and proximal carpal bones (mainly scaphoid and lunate).
• Movements: flexion, extension, abduction (radial deviation), adduction (ulnar deviation).

Metacarpophalangeal & Interphalangeal Joints

• MCP joints are condyloid allowing flexion/extension, abduction/adduction.
• IP joints are hinge joints allowing flexion and extension.

JOINTS OF THE LOWER LIMB

Hip Joint

• A deep ball-and-socket joint: head of femur in acetabulum; deeper socket than shoulder, making it more stable but less mobile.
• Reinforced by strong ligaments (iliofemoral, pubofemoral, ischiofemoral).
• Movements: flexion, extension, abduction, adduction, rotation, circumduction.

Knee Joint

• The largest, most complex hinge joint; consists of femorotibial and femoropatellar articulations.
• Important structures:
• Menisci (medial & lateral): semilunar cartilages that deepen the tibial surface and absorb shock.
• Cruciate ligaments (ACL & PCL): control anterior-posterior stability and rotational movement.
• Collateral ligaments: stabilize medial and lateral sides.
• Largest synovial membrane with bursae around the joint.

Movements: Flexion and extension (with slight rotation when flexed).

Ankle Joint (Talocrural)

• A hinge joint formed by tibia, fibula and talus.
• Stabilised by medial (deltoid) and lateral ligament complexes.
• Movements: dorsiflexion (toes toward shin) and plantarflexion (pointing foot downward).

JOINTS OF THE FOOT

• Tarsal joints: Gliding joints among tarsals (talus and calcaneus bear most weight).
• Tarsometatarsal joints: Slight gliding movements that help foot stiffness or flexibility.
• Metatarsophalangeal joints: Allow flexion/extension and some abduction/adduction.
• Interphalangeal joints: Hinge joints allowing flexion/extension of toes.

DISORDERS OF JOINTS

ARTHRITIS

General term for inflammation of joints. Common in middle and older age but may occur at any age.

• Rheumatoid arthritis (RA)
• An autoimmune polyarthritis often affecting small joints (hands, feet).
• Characterised by inflammation of the synovial membrane, pain, stiffness, progressive joint deformity, and systemic symptoms (e.g., fever).
• Management commonly includes anti-inflammatory drugs, disease-modifying agents and physiotherapy.
• Osteoarthritis (OA)
• Degenerative “wear-and-tear” disease of articular cartilage.
• Cartilage thins, bone surfaces may rub together causing pain, stiffness and reduced movement.
• Conservative management: physiotherapy, weight reduction, analgesics; severe cases may require joint replacement.

GOUT

• Caused by deposition of uric acid (sodium urate) crystals in joints and soft tissues.
• Presents as acute painful inflammatory attacks (commonly first metatarsophalangeal joint, ankle, knee).
• Related to high uric acid—due to overproduction or reduced excretion. Long-term management aims to lower uric acid levels.

DISLOCATIONS

• Occur when joint surfaces are completely displaced, often tearing the joint capsule and ligaments.
• Common sites: shoulder, hip (posterior most common), elbow.
• Require prompt reduction (medical) and rehabilitation.

KNEE SPECIFIC DISORDERS

• Slipped (torn) meniscus: Injury or detachment of semilunar cartilage causing pain, locking or restricted movement.
• Acute synovitis: Inflammation of synovial membrane after trauma, causing swelling (notably around the knee).
• Bursitis: Inflammation of bursae (fluid-filled sacs), commonly prepatellar bursitis (in front of the kneecap) causing swelling and pain.

BASIC PRINCIPLES OF JOINT CARE AND MANAGEMENT (GENERAL GUIDANCE)

• Maintain healthy body weight to reduce joint load.
• Regular exercise (strengthening, flexibility) to maintain joint function.
• Early medical evaluation for persistent joint pain, swelling, or deformity.
• Use of supportive aids (splints, braces) and physiotherapy for rehabilitation.
• Referral to specialists (orthopaedics, rheumatology) when needed.