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AN2.1-6 | General features of bones & Joints — SDL Guide

Your skeleton is not inert scaffolding — it is a living, dynamic organ system. In this module, you will explore what bones are made of, how they grow, and how they connect to form the joints that give you mobility. By the end, you will be able to examine a joint systematically, explain why hip problems can cause knee pain, and understand why a fractured growth plate in a child is a medical emergency.

Learning Outcomes

• Describe the macroscopic structure, types, blood supply and nerve supply of a typical long bone (AN2.1)
• Explain the process of ossification, the roles of primary and secondary centres, and the significance of the epiphyseal plate (AN2.2)
• Define sesamoid bones, give examples, and explain their clinical importance (AN2.3)
• Compare the three types of cartilage by structure and function (AN2.4)
• Classify joints, describe the movements at synovial joints, and outline a systematic joint examination (AN2.5)
• State Hilton's law and apply it to explain patterns of joint pain referral (AN2.6)

PREREQUISITES

You should be comfortable with the terms: skeleton, bone, cartilage, joint, muscle, calcium. No prior anatomy course is assumed.

References

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| ~45 min

What a Bone Is Made Of

Look at the femur (thigh bone) in the anatomy lab. It does not look like the chalk-white structure from school diagrams — it has a slightly yellow tinge, and when freshly sectioned, it feels moist. That is because bone is a living organ, not a dead mineral store.

Cut a long bone lengthwise and you see four distinct layers:

1. Periosteum (peri = around, osteon = bone) — a tough, fibrous sleeve covering the outer surface. It has two layers: an outer fibrous layer for attachment of tendons and ligaments, and an inner osteogenic (bone-forming) layer with cells that repair fractures. The periosteum is densely innervated — this is why breaking a bone hurts so intensely.

1. Compact (cortical) bone — the hard, dense outer shell. Arranged in concentric rings called osteons (Haversian systems). Provides strength and rigidity.

1. Spongy (cancellous or trabecular) bone — a honeycomb of bony struts (trabeculae) filled with red or yellow bone marrow. The spaces reduce weight while maintaining strength. Red marrow makes blood cells; yellow marrow is mostly fat.

1. Endosteum — a thin membrane lining the inner surface of compact bone and the trabeculae of spongy bone. Contains osteoblasts (bone-forming) and osteoclasts (bone-resorbing).

Types of Bones: Shape Follows Function

Bones are classified by shape, because shape reflects mechanical function:

Type	Examples	Function
Long bones	Femur, humerus, radius, phalanges	Leverage, movement
Short bones	Carpals (wrist), tarsals (ankle)	Compact strength, limited movement
Flat bones	Skull vault, sternum, scapula, ribs	Protection, large surface for muscle attachment
Irregular bones	Vertebrae, hip bone (os coxa), facial bones	Complex shapes for complex functions
Pneumatic bones	Maxilla, frontal, ethmoid, sphenoid	Air sinuses reduce skull weight
Sesamoid bones	Patella, pisiform, hallucal sesamoids	Described below

Sesamoid bones (from the Arabic simsim, sesame seed — they resemble sesame seeds in shape) are bones embedded within tendons. They develop where a tendon runs across a joint under high friction or pressure.

• Patella — the largest sesamoid, embedded in the quadriceps tendon (now called the patellar ligament below it). It increases the mechanical advantage of the quadriceps by moving the tendon away from the knee joint axis.
• Pisiform — a sesamoid in the tendon of flexor carpi ulnaris, forming part of the wrist (proximal row of carpal bones).
• Two sesamoids under the head of the 1st metatarsal — protect the flexor hallucis longus tendon.

Clinical note: Sesamoids can fracture (rare) or develop avascular necrosis. The patella is the classic example — a patellar fracture separates the quadriceps mechanism and the patient cannot extend the knee against gravity.

Blood and Nerve Supply of Bone

Bone needs blood supply for growth, repair, and metabolic activity. The vascular pattern of a long bone has four components:

1. Nutrient artery — the main supply. Enters the diaphysis (shaft) through the nutrient foramen, a small hole visible on dried bone. Branches into ascending and descending arteries supplying the diaphyseal marrow and inner two-thirds of cortical bone.

1. Metaphyseal vessels — multiple small arteries entering the flared ends (metaphyses). Supply the growth plate region.

1. Epiphyseal vessels — enter the epiphyses (bone ends) through the articular margin. Before epiphyseal fusion, these vessels are separate from the metaphyseal vessels — this means the epiphysis has an end-arterial blood supply, making it vulnerable to avascular necrosis (e.g., Perthes disease in children — avascular necrosis of the femoral head epiphysis).

1. Periosteal vessels — supply the outer one-third of cortical bone.

Nerve supply:
• The periosteum has a rich sensory nerve supply (pain fibres — this is why fractures and periostitis are exquisitely painful).
• The bone tissue itself has vasomotor fibres that regulate blood flow.
• The endosteum and marrow have sensory fibres — this explains the aching pain felt in bone metastases.