Anatomy chapters
Types of bones and joints
How bones are classified by shape and development, and how joints are classified by tissue and movement, with the examples examiners use.
- Bones are grouped by shape (long, short, flat, irregular, sesamoid, pneumatic) and by development (membrane or cartilage).
- Joints are grouped by the tissue joining them: fibrous, cartilaginous or synovial.
- Exams love the examples: primary vs secondary cartilaginous, and the type of each synovial joint.
On this page
Overview
The skeleton gives the body shape, protects organs, stores calcium and makes blood cells in red marrow.
Bones are classified in two ways: by their shape and by how they develop. Joints are classified by the tissue that joins the bones and by how much they move.
This is a short topic, but it produces easy marks because the questions are usually "which type is this example?".
Classification of bones by shape
| Type | Features | Examples |
|---|---|---|
| Long | Shaft with two ends; longer than wide | Humerus, femur, phalanges, metacarpals |
| Short | Roughly cube-shaped | Carpal and tarsal bones |
| Flat | Two thin plates of compact bone with spongy bone between | Skull vault, sternum, ribs, scapula |
| Irregular | Mixed shapes | Vertebrae, hip bone, many skull base bones |
| Sesamoid | Develop inside a tendon | Patella, pisiform, sesamoids of the thumb and big toe |
| Pneumatic | Contain air-filled spaces | Maxilla, frontal, ethmoid, sphenoid |
The patella is the largest sesamoid bone. Sesamoid bones protect a tendon and improve its leverage.
The phalanges are short in size but are classed as long bones, because each has a shaft and two ends.
Parts of a growing long bone
- Diaphysis: the shaft, which ossifies from the primary centre.
- Epiphysis: each end, which ossifies from a secondary centre.
- Epiphyseal (growth) plate: hyaline cartilage between them, where growth in length happens.
- Metaphysis: the flared part of the shaft next to the growth plate.
The metaphysis has a rich blood supply with hairpin-shaped vessels where blood flow slows down.
In children, acute osteomyelitis usually starts in the metaphysis, because bacteria settle in its slow-flowing looped vessels.
Blood supply of a long bone
The nutrient artery enters the shaft through the nutrient foramen and supplies the inner part of the bone and the marrow. Periosteal arteries supply the outer cortex. Metaphyseal and epiphyseal arteries supply the ends.
The nutrient canal points away from the growing end. A widely used memory aid for the limbs is "towards the elbow, away from the knee".
"To the elbow I go, from the knee I flee": nutrient foramina point towards the elbow and away from the knee.
Classification of bones by development
Membrane (intramembranous) ossification: bone forms directly in a sheet of mesenchyme. Examples are the bones of the skull vault, most of the face and most of the clavicle.
Cartilaginous (endochondral) ossification: a hyaline cartilage model forms first and is then replaced by bone. Most bones of the limbs, vertebral column and skull base form this way.
The clavicle is the first bone to start ossifying (around the fifth to sixth week of embryonic life) and ossifies mainly in membrane.
Classification of joints
Fibrous joints
The bones are joined by dense fibrous tissue. Very little or no movement occurs.
- Sutures: between the bones of the skull vault.
- Syndesmosis: bones joined by a ligament or membrane, such as the inferior tibiofibular joint and the interosseous membranes.
- Gomphosis: a peg in a socket, such as a tooth in its socket.
Cartilaginous joints
| Type | Tissue | Features | Examples |
|---|---|---|---|
| Primary (synchondrosis) | Hyaline cartilage | Usually temporary; becomes bone with age | Epiphyseal plate, first sternocostal joint, spheno-occipital joint |
| Secondary (symphysis) | Fibrocartilage | Permanent, in the midline, slight movement | Pubic symphysis, intervertebral discs, manubriosternal joint |
The pubic symphysis is a secondary cartilaginous joint, not primary. "Symphysis" means fibrocartilage in the midline.
Synovial joints
These are the freely movable joints. All synovial joints share the same basic features:
- Bone ends covered with articular (hyaline) cartilage.
- A joint cavity with a thin film of synovial fluid.
- A fibrous capsule lined by synovial membrane, which does not cover the articular cartilage.
- Ligaments that hold the bones together.
Some also have an articular disc or meniscus (knee, temporomandibular, sternoclavicular joints).
The temporomandibular and sternoclavicular joints are covered with fibrocartilage, not hyaline cartilage, because these bones ossify in membrane.
Types of synovial joint
| Type | Movement | Examples |
|---|---|---|
| Plane | Gliding | Acromioclavicular, intercarpal |
| Hinge | Flexion and extension | Elbow (humero-ulnar), interphalangeal, ankle |
| Pivot | Rotation around one axis | Median atlanto-axial, proximal radio-ulnar |
| Condyloid | Flexion, extension, abduction, adduction | Metacarpophalangeal joints |
| Ellipsoid | Same as condyloid, oval surfaces | Wrist (radiocarpal) |
| Saddle | As above plus some rotation | First carpometacarpal joint of the thumb |
| Ball and socket | Movement in all directions | Shoulder, hip |
The first carpometacarpal joint is the classic saddle joint and lets the thumb oppose.
Nerve supply of joints
Hilton's law: the nerve supplying the muscles that move a joint also supplies the joint itself and the skin over it.
This explains why joint pain is often felt along the course of a nerve. Hip disease, for example, can cause pain referred to the knee via the obturator nerve.
Clinical relevance
Acute osteomyelitis
How it happens: blood-borne bacteria lodge in the metaphysis of a growing long bone.
What you see: a child with fever, a painful limb and refusal to bear weight. Pus can lift the periosteum and kill the bone below it (a sequestrum).
Growth plate injuries
How it happens: the epiphyseal plate is weaker than the bone and ligaments around it in a child.
What you see: a fracture through the plate, which can stop or distort growth if the germinal layer is damaged.
Osteoarthritis of synovial joints
How it happens: wear of the articular cartilage, mainly in weight-bearing joints.
What you see: pain, stiffness, narrowing of the joint space and bony spurs (osteophytes).
Most questions give an example and ask for the type: a joint (primary or secondary cartilaginous, type of synovial joint) or a bone (sesamoid, pneumatic, membrane). Learn the examples in the tables, and the metaphysis link with osteomyelitis.
Quick revision
- Bone shapes: long, short, flat, irregular, sesamoid, pneumatic.
- Largest sesamoid: patella.
- Metaphysis: most vascular, commonest site of osteomyelitis in children.
- Membrane bones: skull vault, most of the face, clavicle.
- Primary cartilaginous: hyaline, temporary. Secondary: fibrocartilage, midline.
- Saddle joint: first carpometacarpal. Pivot: median atlanto-axial and proximal radio-ulnar.
- Hilton's law: the same nerve supplies the joint, its muscles and the overlying skin.
Written by The House Job from standard anatomy references (Snell's Clinical Anatomy by Regions is the book named in the UHS PGET syllabus).
FAQ
What is the difference between primary and secondary cartilaginous joints?
Primary joints are joined by hyaline cartilage and are usually temporary, like the growth plate. Secondary joints are joined by fibrocartilage, are permanent and lie in the midline, like the pubic symphysis.
Which is the largest sesamoid bone?
The patella, which lies in the tendon of quadriceps femoris.
Why does osteomyelitis start in the metaphysis?
Its blood vessels form slow-flowing loops near the growth plate, where bacteria can settle.
What type of joint is the first carpometacarpal joint?
A saddle type of synovial joint.