Page 2 of 11
AN2.1-6 | General features of bones & Joints — Part 1
CLINICAL SCENARIO
A 14-year-old fast bowler presents to the orthopaedic clinic with pain near the tibial tuberosity that worsens after practice. An X-ray shows fragmentation at the apophysis. Understanding how bones grow — and where growth plates sit — is the key to diagnosing Osgood-Schlatter disease and dozens of similar conditions you will encounter in clinical practice.
WHY THIS MATTERS
Every fracture you will reduce, every joint you will examine, and every X-ray you will interpret requires you to think in the language of bones and joints. This module gives you that vocabulary — the parts of a bone, how it grows, how joints move, and why they are supplied by specific nerves. Without this foundation, clinical orthopaedics, radiology, and even paediatrics become a guessing game.
RECALL
From school biology, you already know that the skeleton supports the body and protects organs. You may recall that bones contain calcium and that joints allow movement. Build on that foundation now — we move from what a bone does to how a bone is built, how it grows, and how its joints are engineered.
Parts of a Typical Long Bone (AN2.1)
A long bone such as the femur has three main regions:
Figure: Parts of a Typical Long Bone (AN2.1)
- Diaphysis (shaft) — the tubular middle portion composed of dense compact bone surrounding the medullary cavity. The medullary cavity contains yellow marrow in adults (haematopoietic red marrow in children).
- Epiphysis — the expanded ends, composed mostly of spongy (cancellous) bone covered by a thin shell of compact bone. The articular surface is covered by hyaline cartilage, not periosteum.
- Metaphysis — the transitional zone between diaphysis and epiphysis. In growing bones, this contains the epiphyseal plate (growth plate); in adults, only the epiphyseal line remains.
Blood supply of bones comes from three sources:
1. Nutrient artery — enters the diaphysis through the nutrient foramen, directed away from the growing end (mnemonic: "To the elbow I go, from the knee I flee").
2. Periosteal arteries — supply the outer compact bone.
3. Epiphyseal and metaphyseal arteries — supply the bone ends.
Nerve supply: Periosteum is richly innervated (explaining why fractures are painful); bone marrow and compact bone receive vasomotor sympathetic fibres.
Classification of Bones
Classification of Bones — Types and Features
| Type | Shape | Internal Structure | Examples |
|---|---|---|---|
| Long | Longer than wide | Medullary cavity in diaphysis, spongy epiphyses | Femur, humerus, phalanges |
| Short | Roughly cuboidal | Spongy bone throughout, thin compact shell | Carpal bones, tarsal bones |
| Flat | Two plates with diploe | Outer and inner tables of compact bone, spongy bone between | Skull vault, scapula, sternum |
| Irregular | Complex shape | Mixed cortical and cancellous | Vertebrae, hip bone, sphenoid |
| Sesamoid | Within tendons | Variable | Patella, pisiform, fabella |
Bones are classified by shape into five types:
Figure: Classification of Bones
- Long bones — longer than they are wide (femur, humerus, phalanges). Have a medullary cavity.
- Short bones — roughly cuboidal (carpal bones, tarsal bones). Spongy bone throughout with a thin compact shell.
- Flat bones — two plates of compact bone with spongy bone (diploë) between them (skull vault, scapula, sternum). The spongy bone of flat skull bones contains red marrow throughout life.
- Irregular bones — complex shapes that do not fit other categories (vertebrae, hip bone, sphenoid).
- Sesamoid bones — develop within tendons (see below).
Peculiarities worth noting:
- The clavicle is the only long bone that ossifies in membrane (not cartilage) and has no medullary cavity.
- The patella is the largest sesamoid bone in the body.
- Wormian (sutural) bones are accessory irregular bones found along skull sutures.
SELF-CHECK
A bone has a diaphysis with a medullary cavity, two expanded epiphyses, and a nutrient artery that enters its shaft. Which classification does it belong to?
A. Short bone
B. Flat bone
C. Long bone
D. Sesamoid bone
Reveal Answer
Answer: C. Long bone
The presence of a diaphysis, medullary cavity, and expanded epiphyses are defining features of a long bone. Short bones lack a medullary cavity; flat bones have diploë instead.
Ossification and Epiphyses (AN2.2)
Ossification is the process by which mesenchymal tissue is converted into bone. Two types exist:
Figure: Ossification and Epiphyses (AN2.2)
- Intramembranous ossification — bone forms directly from mesenchyme without a cartilage model. Examples: flat bones of the skull vault, clavicle, mandible (partially). An ossification centre appears as a cluster of osteoblasts that lay down osteoid.
- Endochondral ossification — bone replaces a pre-existing hyaline cartilage model. This is how most bones form. A primary centre appears in the diaphysis (usually during fetal life), and secondary centres appear in the epiphyses (usually after birth).
Laws of ossification:
• Law of Ossification of Long Bones: The primary centre appears first in the diaphysis; secondary centres appear later in the epiphyses.
• The growing end of a long bone is the end where the secondary centre appears first and fuses last.
• The nutrient artery is directed AWAY from the growing end.
Types of epiphyses:
• Pressure epiphysis — forms the articular surface, transmits weight (e.g., head of femur).
• Traction epiphysis (apophysis) — site of muscle/tendon attachment, does NOT form a joint surface (e.g., greater trochanter, tibial tuberosity).
• Atavistic epiphysis — phylogenetic remnant (e.g., coracoid process of scapula).
• Aberrant epiphysis — appears where one is not normally expected.
Clinical note: The epiphyseal plate is the weakest link in a child's bone. Injuries here (Salter-Harris fractures) can disrupt growth if they damage the germinal zone.
SELF-CHECK
The tibial tuberosity in an adolescent is a classic site for Osgood-Schlatter disease. What type of epiphysis is the tibial tuberosity?
A. Pressure epiphysis
B. Traction epiphysis (apophysis)
C. Atavistic epiphysis
D. Aberrant epiphysis
Reveal Answer
Answer: B. Traction epiphysis (apophysis)
The tibial tuberosity is a traction epiphysis (apophysis) because it serves as the attachment point for the patellar tendon. It does not form an articular surface. Repeated traction on an unfused apophysis causes the pain and fragmentation seen in Osgood-Schlatter disease — linking back to our opening clinical scenario.