Structure of a Long Bone: Parts & Anatomy
What Is a Long Bone?
A long bone is a bone whose length is greater than its width and which typically has a central shaft and expanded ends. The femur is the classic example. Other examples include the humerus, radius, ulna, tibia and fibula.
The basic regions of a long bone are the diaphysis, epiphyses and metaphyses. Its surface is covered by specialized tissues, while its internal structure contains compact bone, cancellous bone and marrow spaces.
Parts of a Long Bone
A typical long bone consists of a shaft, expanded ends and transitional regions between them.
1. Epiphysis
The epiphyses are the expanded ends of a long bone. Each long bone usually has a proximal and a distal epiphysis.
The epiphysis commonly contains a substantial amount of cancellous (trabecular) bone surrounded by a layer of compact bone.
The articular surface of an epiphysis is usually covered by articular cartilage where it participates in a synovial joint.
2. Diaphysis
The diaphysis is the shaft or central portion of a long bone. It is predominantly composed of compact bone.
The compact bone forms a strong outer wall around the medullary cavity, giving the shaft considerable resistance to bending and other mechanical forces.
3. Metaphysis
The metaphysis is the region between the diaphysis and epiphysis. In a growing bone, it lies adjacent to the epiphyseal plate.
The metaphyseal region has an important role in longitudinal growth during childhood and adolescence.
4. Periosteum
The periosteum is a vascular connective-tissue membrane covering the external surface of most bones.
It contains an outer fibrous layer and an inner cellular layer with osteogenic potential. The periosteum is firmly attached to bone by perforating Sharpey fibers.
The periosteum contributes to bone growth in width, repair and the attachment of tendons and ligaments.
It is also richly innervated, which contributes to the pain associated with many bone injuries.
5. Compact Bone
Compact bone, also called cortical bone, forms the dense outer portion of most bones.
In the shaft of a long bone it forms a thick wall surrounding the medullary cavity. Its organized microscopic structure provides strength while keeping the bone relatively lightweight.
6. Cancellous Bone
Cancellous bone, also called spongy or trabecular bone, consists of a network of bony trabeculae separated by spaces.
It is particularly prominent in the epiphyses of long bones and within many flat and irregular bones.
The spaces between trabeculae may contain bone marrow and blood vessels.
7. Medullary Cavity
The medullary cavity is the central cavity within the shaft of a long bone.
In adults, the medullary cavity of many long bones contains predominantly yellow marrow. Red marrow remains important in the axial skeleton and in selected regions of the proximal ends of certain long bones.
The cavity provides space for marrow and contributes to the efficient structural design of the bone.
8. Endosteum
The endosteum is a thin cellular membrane associated with internal bone surfaces.
It lines the medullary cavity and covers internal surfaces associated with trabecular bone and certain canals within bone.
Cells associated with the endosteal surface participate in bone remodeling.
9. Articular Cartilage
Where the epiphysis participates in a synovial joint, the articular surface is covered by hyaline cartilage.
Articular cartilage provides a smooth, low-friction surface and helps distribute loads across the joint.
Internal Structure of a Long Bone
A long bone is organized into several layers and compartments. The outer surface is associated with the periosteum, while compact bone provides the main cortical shell. Internally, the shaft contains the medullary cavity and the epiphyses contain considerable trabecular bone.
The cross-section demonstrates the relationship between compact bone, cancellous bone, bone marrow and the periosteal covering.
Long Bone Structure: Quick Comparison
| Structure | Location | Main Feature / Function |
|---|---|---|
| Epiphysis | Expanded end | Participates in joints and contains substantial cancellous bone. |
| Diaphysis | Shaft | Predominantly compact bone surrounding the medullary cavity. |
| Metaphysis | Between epiphysis and diaphysis | Associated with longitudinal growth in immature bones. |
| Periosteum | External bone surface | Vascular and innervated connective-tissue covering. |
| Compact bone | Outer bone | Provides strength and resistance to mechanical forces. |
| Cancellous bone | Internal trabecular regions | Lightweight supporting network containing marrow spaces. |
| Medullary cavity | Central shaft | Contains bone marrow. |
| Endosteum | Internal bone surfaces | Cellular lining involved in bone remodeling. |
| Articular cartilage | Joint surface | Provides smooth, low-friction articulation and load distribution. |
Why Is the Structure of a Long Bone Important?
The arrangement of compact and cancellous bone allows a long bone to withstand mechanical forces while remaining relatively lightweight.
- The diaphysis provides a strong shaft for transmitting forces.
- The epiphyses participate in joint formation.
- The metaphysis is important for longitudinal growth in immature bones.
- The periosteum contributes to bone nourishment, repair and growth in thickness.
- The medullary cavity provides space for bone marrow.
- Articular cartilage reduces friction at synovial joints.
High-Yield MBBS Points
- Diaphysis = shaft of a long bone.
- Epiphysis = expanded end of a long bone.
- Metaphysis = region between diaphysis and epiphysis.
- The shaft contains a thick layer of compact bone.
- The epiphyses contain substantial cancellous bone.
- The medullary cavity is located mainly within the shaft.
- The periosteum covers most external bone surfaces.
- The endosteum lines internal bone surfaces.
- Articular cartilage covers joint surfaces and lacks a periosteum.
- In adults, many long-bone shafts contain predominantly yellow marrow.
Clinical Relevance
Knowledge of long-bone anatomy is essential for understanding fractures, bone growth, orthopedic procedures and radiographic interpretation.
Fractures may involve the shaft, metaphysis or epiphysis, and their consequences depend on the structures involved. Injuries near a growth plate in children and adolescents may affect normal bone growth.
The periosteum is highly sensitive to pain, while damage to the blood supply of bone can interfere with healing and, in certain circumstances, contribute to bone necrosis.
Summary
A typical long bone consists of a diaphysis, two epiphyses and metaphyseal regions. The shaft is mainly compact bone surrounding a medullary cavity, while the expanded ends contain substantial cancellous bone.
The bone is covered externally by periosteum except at specialized surfaces such as articular cartilage. Internally, the endosteum lines the medullary and other internal surfaces. Together, these structures provide the mechanical strength, support, movement and biological functions of a long bone.
The anatomical diagrams used in this article are sourced from Wikimedia Commons and are used under their respective Creative Commons licenses.
- 603 Anatomy of Long Bone.jpg — OpenStax College, CC BY 3.0. Wikimedia Commons
- Simple bone cs.svg — Nefronus, CC BY-SA 4.0. Wikimedia Commons
Medical Disclaimer: The information provided in this article is strictly for educational, study, and exam-preparation purposes. It does not constitute professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for clinical decisions.