The shaft of a long bone is called the diaphysis. Also known as the bone shaft, the diaphysis is the long, cylindrical middle section between the rounded ends (epiphyses). It provides structural support, leverages for muscles, and protection for the marrow cavity. In this article, you will learn how the diaphysis differs from the epiphysis and metaphysis, its microscopic composition, its role in movement and hematopoiesis, and common clinical considerations related to this part of long bones such as the femur, tibia, and humerus.
Anatomy of a Long Bone
Long bones have three main anatomical parts: the diaphysis, the metaphyses, and the epiphyses.
The Diaphysis (Bone Shaft)
The diaphysis forms the primary shaft of long bones. It is composed of compact bone surrounding the medullary cavity, which contains yellow marrow in adults. The diaphysis grows in length through the activity of the epiphyseal plate (growth plate) located at the metaphysis.
The Metaphysis
The metaphysis is the transition zone between the diaphysis and the epiphysis. It contains the epiphyseal plate during growth, which gradually ossifies in adulthood.
The Epiphysis
The epiphysis is the rounded end of a long bone, initially growing separately from the shaft. It fuses with the diaphysis in late adolescence, marking skeletal maturity.
Together, these regions define the shape and function of long bones, enabling locomotion, posture, and hematopoiesis.
Microscopic Structure and Composition
Compact bone is organized into structural units called osteons (Haversian systems). Central canals (Haversian canals) run longitudinally through the diaphysis, housing blood vessels and nerves, while perforating canals (Volkmann’s canals) connect them.
Bone tissue consists of a mineralized matrix (largely hydroxyapatite) and collagen fibers, providing strength and flexibility. The medullary cavity within the diaphysis stores energy as yellow marrow, while red marrow is primarily located in flat and irregular bones in adults.
Functions of the Diaphysis
- Mechanical support for the skeleton
- Lever arm for skeletal muscles to generate movement
- Protection of the medullary cavity and marrow
- Mineral storage (e.g., calcium and phosphate)
- Site of hematopoiesis in certain flat and irregular bones (red marrow), though the diaphysis mainly contains yellow marrow in adults
Long Bones That Contain a Diaphysis
Examples include the femur, tibia, fibula, humerus, radius, ulna, and metacarpals/metatarsals when they are in a long bone configuration.
Clinical Considerations
Fractures of the diaphysis are typically treated with immobilization or surgical fixation. In children, injuries near the growth plate (metaphysis) can affect future bone length and alignment. Accurate diagnosis and treatment plans should be determined by healthcare professionals.
Key Takeaways
- The shaft of a long bone is called the diaphysis (bone shaft).
- The diaphysis is the central, cylindrical region that provides support and leverage.
- It is surrounded by compact bone and contains the medullary cavity.
- Growth in length occurs at the epiphyseal plate located at the metaphysis.
- Clinical conditions involving the diaphysis include fractures and infections, which require medical evaluation.