Classification of bone

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Classification of Bone

Bones can be classified by three major criteria: shape, tissue structure (histology), and developmental origin (ossification).

1. Classification by Shape

This is the primary anatomical classification, dividing bones into five types:

A. Long Bones

Tubular in shape - longer than they are wide
Consist of a shaft (diaphysis) and two expanded ends (epiphyses); the flared junction is the metaphysis
The diaphysis has a central medullary (marrow) cavity; the shaft wall is almost entirely compact bone, while the epiphyses are mostly spongy bone with only a thin outer compact shell
Articular surfaces are covered by hyaline (articular) cartilage
Examples: Femur, humerus, tibia, fibula, radius, ulna, metacarpals, phalanges

B. Short Bones

Nearly equal in length and diameter - cuboidal in shape
Possess a shell of compact bone with spongy bone and marrow space inside
Articular surfaces covered by hyaline cartilage; rest covered by periosteum
Examples: Carpal bones (wrist), tarsal bones (ankle)

C. Flat Bones

Thin and plate-like
Consist of two layers of relatively thick compact bone (inner and outer tables) separated by an intervening layer of spongy bone (called the diploe in the skull)
Examples: Scapula, sternum, ilium, bones of the calvaria (skull vault), ribs

D. Irregular Bones

Complex shapes that do not fit into the other categories
May have intricate geometry or contain air spaces (sinuses)
Examples: Vertebrae, bones of the face, ethmoid, hyoid, sacrum

E. Sesamoid Bones

Round or oval bones that develop within tendons, protecting them from wear
The largest is the patella (kneecap); others occur in tendons of the hands and feet (especially flexor tendons of the thumb and big toe)
Note: Accessory (supernumerary) bones such as the os naviculare are a related variant - extra bones that arise from failure of fusion of ossification centers, commonly found in the wrist, ankle, and calvaria. These should not be mistaken for fractures on imaging.

Some atlases (e.g., THIEME Atlas of Anatomy) also include a sixth type:

F. Pneumatic Bones

Contain air-filled spaces (paranasal sinuses)
Examples: Frontal bone, maxilla, sphenoid, ethmoid

2. Classification by Tissue Structure (Histological)

All bones consist of two types of bone tissue:

Feature	Compact (Cortical) Bone	Spongy (Cancellous / Trabecular) Bone
Structure	Dense, solid outer shell	Meshwork of interconnecting trabeculae
Structural unit	Osteons (Haversian systems)	Trabeculae (~200-300 µm thick)
Vascular supply	Haversian + Volkmann canals	Nourished by diffusion from marrow cavity
Location	Outer shell of all bones; full thickness of diaphysis	Interior of epiphyses, metaphyses, flat bones (diploe)
Marrow spaces	Surrounds medullary cavity	Trabeculae enclose marrow spaces

Compact bone is organized into osteons (Haversian systems) - concentric lamellar cylinders ~1 cm long and 250-350 µm in diameter, each centered on a Haversian canal containing vessels and nerves. Volkmann canals run perpendicularly and connect Haversian canals to each other and to the periosteum.

Between lamellae, osteocytes sit in lacunae and communicate via fine cytoplasmic processes in canaliculi.

Immature (woven) bone vs. mature (lamellar) bone is another histological distinction:

Woven bone: formed during development and fracture repair; collagen fibers arranged irregularly
Lamellar bone: normal adult bone; collagen fibers in organized parallel sheets

3. Classification by Developmental Origin (Ossification)

Type	Process	Bones
Intramembranous ossification	Bone forms directly from mesenchyme without a cartilage template	Flat bones of skull, mandible, clavicle
Endochondral ossification	A hyaline cartilage model is replaced by bone	All other bones (limb bones, vertebrae, ribs, base of skull)

Structure of a Typical Long Bone

Structure of a typical tubular bone showing compact and cancellous bone, periosteum, Haversian canals, Volkmann canals, osteocytes, and osteons

The diagram above (from THIEME Atlas of Anatomy) shows: (a) coronal section of the femur with cancellous and compact regions, (b) bone cells in cancellous trabeculae, (c) 3D microstructure of compact bone with osteons, Haversian canals, and Volkmann canals, (d) cross-section of a single osteon, and (e) the periosteum layers with Sharpey fibers.

Sources:

Histology: A Text and Atlas (Pawlina), 9th Ed. - General Structure of Bones
Gray's Anatomy for Students, 4th Ed. - Bone
THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System - Types of Bone
Histology: A Text and Atlas (Ross & Pawlina) - Bone as an Organ
Imaging Anatomy: Bones, Joints, Vessels and Nerves - Bones
Forensic Anthropology: A Comprehensive Introduction - Bone Types

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