Difficulty: Intermediate | Prerequisites: Tissue types, especially connective tissue (Study Notes 03).
The skeletal system provides the rigid framework that supports the body, protects internal organs, stores minerals, produces blood cells, and works with the muscular system to produce movement. This section covers bone structure and development, the two major skeletal divisions, the vertebral column, joint types and movements, common injuries, and bone healing. You will need this material to make sense of the muscular system (Study Notes 05), because muscles can only produce movement by pulling on bones across joints.
Bone is living connective tissue that remodels throughout life. The skeleton is divided into axial (skull, vertebral column, rib cage) and appendicular (limbs, girdles) divisions. Joints are classified by movement (immovable, slightly movable, freely movable), and injuries range from sprains and strains to fractures and dislocations. Bone healing follows a four-stage sequence.
Compact bone
Dense, solid bone tissue that forms the outer shell of all bones and the shaft of long bones. Organised into osteons (Haversian systems). In simple terms, this is the hard, heavy layer that gives bones their strength.
Spongy bone (cancellous bone)
Lighter, porous bone tissue found inside bones, especially at the ends of long bones and inside flat bones. Contains trabeculae (thin, bony struts) and houses red bone marrow. Think of it as a honeycomb structure that reduces weight while still providing support.
Ossification
The process of bone formation. Intramembranous ossification forms flat bones (skull) directly from mesenchymal tissue. Endochondral ossification replaces a cartilage model with bone (most of the skeleton).
Diaphysis
The shaft (middle section) of a long bone. Composed mainly of compact bone surrounding a medullary cavity.
Epiphysis
The expanded ends of a long bone. Composed of spongy bone covered by a thin layer of compact bone. The articular surface is covered by hyaline cartilage.
Epiphyseal plate (growth plate)
A layer of hyaline cartilage between the diaphysis and epiphysis in growing bones. This is where longitudinal bone growth occurs. It ossifies into the epiphyseal line once growth is complete (typically late adolescence).
Axial skeleton
The central axis of the body: skull, vertebral column (spine), and rib cage. Primarily protective.
Appendicular skeleton
The limbs and the girdles that attach them to the axial skeleton: pectoral (shoulder) girdle, upper limbs, pelvic girdle, lower limbs. Primarily for movement.
Fibrous joint
A joint held together by dense fibrous connective tissue with little or no movement. Example: sutures of the skull.
Cartilaginous joint
A joint connected by cartilage, allowing slight movement. Examples: intervertebral discs, the pubic symphysis.
Synovial joint
A freely movable joint enclosed by a joint capsule lined with synovial membrane, which secretes synovial fluid for lubrication. Examples: knee, elbow, shoulder, hip.
Ligament
A band of dense connective tissue connecting bone to bone across a joint. Provides stability.
Tendon
A band of dense connective tissue connecting muscle to bone. Transmits the force of muscle contraction to the skeleton.
Sprain
An injury to a ligament, caused by excessive stretching or tearing. Commonly occurs at the ankle, knee, or wrist.
Strain
An injury to a muscle or its tendon, caused by overstretching or tearing. Often occurs in the lower back or hamstring.
Dislocation
A displacement of a bone from its normal position at a joint. Requires medical reduction (repositioning).
Subluxation
A partial dislocation where the bone is partially displaced but not completely out of the joint.
Fracture
A break in a bone. Types include simple (closed), compound (open, bone pierces skin), comminuted (bone shattered into fragments), greenstick (incomplete, common in children), and stress (hairline crack from repetitive force).
Compact bone: dense, organised in concentric rings (lamellae) around central canals (Haversian canals) that carry blood vessels and nerves. The functional unit is the osteon.
Spongy bone: lighter, made of trabeculae arranged along lines of stress. Spaces between trabeculae contain red marrow (blood cell production) or yellow marrow (fat storage).
Long bones (femur, humerus) have a diaphysis and two epiphyses. Short bones (carpals, tarsals) are roughly cube-shaped. Flat bones (skull, sternum, ribs) are thin and broad. Irregular bones (vertebrae, some facial bones) have complex shapes.
Ossification begins in the embryo and continues into early adulthood.
Intramembranous ossification: flat bones form directly from connective tissue membranes (e.g., skull bones).
Endochondral ossification: a hyaline cartilage model is gradually replaced by bone (e.g., long bones, vertebrae).
The epiphyseal plate is the site of longitudinal growth. Cartilage cells divide on the epiphyseal side and are replaced by bone on the diaphyseal side.
Once the epiphyseal plate ossifies (closes), longitudinal growth stops. This typically occurs between ages 18 and 25.
Bones continue to remodel throughout life in response to mechanical stress (Wolff's Law) and hormonal signals.
Axial skeleton (80 bones): skull (cranial and facial bones), hyoid bone, auditory ossicles, vertebral column, and thoracic cage (sternum and ribs).
Appendicular skeleton (126 bones): pectoral girdle (clavicle, scapula), upper limbs, pelvic girdle (hip bones), lower limbs.
Tarsals (ankle) and carpals (wrist) are short bones that provide stability and a range of small, precise movements.
33 vertebrae in children; 26 in adults (because the sacral and coccygeal vertebrae fuse during development).
7 cervical, 12 thoracic, 5 lumbar, 1 sacrum (5 fused), 1 coccyx (4 fused).
Primary curves (thoracic and sacral): present at birth, concave anteriorly.
Secondary curves (cervical and lumbar): develop after birth as the infant learns to hold its head up (cervical) and walk (lumbar), convex anteriorly.
The vertebral column protects the spinal cord, supports the head, and provides attachment points for muscles.
Fibrous joints: no joint cavity, connected by fibrous tissue. Minimal or no movement. Examples: skull sutures, distal tibiofibular joint.
Cartilaginous joints: no joint cavity, connected by cartilage. Slight movement. Examples: intervertebral discs (symphysis), epiphyseal plates (synchondrosis).
Synovial joints: joint cavity present, lined with synovial membrane, filled with synovial fluid. Freely movable. Subtypes include hinge (elbow, knee), ball-and-socket (shoulder, hip), pivot (atlas-axis), saddle (thumb), condyloid (wrist), and gliding (intercarpal joints).
Flexion: decreasing the angle between bones (bending).
Extension: increasing the angle between bones (straightening).
Rotation: turning a bone around its own axis.
Abduction: moving a limb away from the body's midline.
Adduction: moving a limb toward the body's midline.
Additional movements: circumduction, pronation, supination, dorsiflexion, plantarflexion, inversion, eversion.
Ligaments connect bone to bone. They stabilise joints and limit excessive movement.
Tendons connect muscle to bone. They transmit the pulling force of a contracting muscle to produce movement.
Both are composed of dense regular connective tissue (primarily collagen fibres).
Sprain: ligament damage. Graded I (mild stretch) to III (complete tear).
Strain: muscle or tendon damage.
Dislocation: bone fully out of joint. Common at the shoulder (most mobile joint, least stable).
Subluxation: partial dislocation.
Fracture: bone break. Treatment depends on type: simple fractures may need only immobilisation; compound or comminuted fractures may require surgical fixation.
Haematoma formation: blood vessels in and around the fracture tear, forming a blood clot (haematoma) at the fracture site.
Soft callus (fibrocartilaginous callus) formation: fibroblasts and chondrocytes produce a collar of cartilage and fibrous tissue that bridges the break.
Hard callus (bony callus) formation: osteoblasts replace the soft callus with spongy bone (woven bone).
Remodelling: osteoclasts and osteoblasts reshape the bony callus into compact and spongy bone that matches the original structure. This phase can take months to years.
Orthopaedic surgeons rely on knowledge of bone structure and healing stages when deciding between conservative treatment (casting) and surgical intervention (plates, screws, intramedullary nails). The epiphyseal plate is clinically significant in paediatrics: fractures through the growth plate in children can disrupt growth and lead to limb length discrepancies if not managed correctly. Joint classification matters in physiotherapy and sports medicine, where treatment and rehabilitation programmes are tailored to the specific joint type and its normal range of motion.
Students often think bones are dead, static structures. Bone is living tissue that is continuously remodelled by osteoblasts (building) and osteoclasts (breaking down).
Ligaments and tendons are frequently confused. A simple mnemonic: Ligaments link bone to bone; Tendons tie muscle to bone.
The number of vertebrae is a common stumbling point. Adults have 26 vertebrae (not 33), because the sacral and coccygeal vertebrae have fused. Children have 33 individual vertebrae.
Students sometimes assume all joints are freely movable. Fibrous joints (sutures) are essentially immovable, and cartilaginous joints permit only slight movement.
⚠️ Know the difference between compact and spongy bone, including where each is found.
⚠️ The epiphyseal plate and its role in growth is a high-yield topic.
⚠️ Be able to classify joints by structure (fibrous, cartilaginous, synovial) and give examples of each.
⚠️ Axial vs appendicular skeleton: know what belongs to each division.
⚠️ Joint movements (flexion, extension, abduction, adduction, rotation) are tested frequently, often with scenario-based questions.
⚠️ Know the four stages of bone healing in order.
⚠️ Sprain vs strain: expect this distinction on exams.
True or false: Spongy bone is found mainly in the shaft of long bones.
The growth plate responsible for bone lengthening is called the __________.
True or false: A ligament connects muscle to bone.
Name the four stages of bone healing in order.
Fill in the blank: The __________ skeleton includes the skull, vertebral column, and rib cage; the __________ skeleton includes the limbs and girdles.
(Answers: 1. False, spongy bone is found at the ends (epiphyses) of long bones and inside flat bones. 2. Epiphyseal plate. 3. False, a ligament connects bone to bone; a tendon connects muscle to bone. 4. Haematoma formation, soft callus formation, hard callus formation, remodelling. 5. Axial; appendicular.)
Q: Explain the difference between compact bone and spongy bone in terms of structure, location, and function.
A: Compact bone is dense and solid, organised into osteons, and forms the outer layer of all bones and the shaft of long bones. It provides strength and resistance to bending. Spongy bone is lighter and porous, made of trabeculae arranged along stress lines, found at the ends of long bones and inside flat bones. It reduces overall bone weight and houses red marrow for blood cell production.
Q: A 12-year-old sustains a fracture through the epiphyseal plate of the femur. Why is this more concerning than a fracture through the diaphysis?
A: The epiphyseal plate is the site of longitudinal bone growth. Damage to it can disrupt the normal process of cartilage proliferation and ossification, potentially leading to premature growth plate closure, uneven growth, or a shorter limb on the affected side.
Q: Distinguish between a sprain and a strain.
A: A sprain is an injury to a ligament (connects bone to bone), typically caused by excessive stretching or tearing at a joint. A strain is an injury to a muscle or its tendon (connects muscle to bone), usually from overstretching or overloading.
Q: What is Wolff's Law, and how does it apply to bone remodelling?
A: Wolff's Law states that bone remodels in response to the mechanical stresses placed upon it. Bone subjected to regular stress (e.g., weight-bearing exercise) becomes thicker and stronger, while bone that is not loaded (e.g., during prolonged bed rest) loses density. This principle underlies recommendations for weight-bearing exercise to prevent osteoporosis.
Bone tissue connects directly to the muscular system (Study Notes 05), because muscles attach to bones via tendons and produce movement across joints. The vertebral column protects the spinal cord (nervous system, Study Notes 06). Bone marrow's role in blood cell production links to the cardiovascular and lymphatic systems. Calcium storage in bone ties into endocrine regulation (parathyroid hormone, calcitonin) and muscle contraction (calcium is essential for the sliding filament mechanism).
bone tissue, compact bone, spongy bone, cancellous bone, osteon, Haversian system, trabeculae, ossification, intramembranous ossification, endochondral ossification, diaphysis, epiphysis, epiphyseal plate, growth plate, osteoblast, osteoclast, Wolff's Law, axial skeleton, appendicular skeleton, vertebral column, cervical, thoracic, lumbar, sacrum, coccyx, primary curves, secondary curves, fibrous joint, cartilaginous joint, synovial joint, hinge joint, ball and socket, pivot joint, flexion, extension, rotation, abduction, adduction, ligament, tendon, sprain, strain, dislocation, subluxation, fracture, bone healing, haematoma, callus, remodelling, tarsals, carpals, anatomy and physiology, A&P