In bone growth, what is the specific term for the process by which bones increase in thickness or width rather than length?

Introduction / Context:
Bones grow not only in length but also in thickness to accommodate increased mechanical loads and maintain structural strength. Different terms describe these aspects of bone growth. This question focuses on identifying the correct term for the increase in bone width or thickness, rather than elongation at the growth plates.

Given Data / Assumptions:

• Long bones grow in length at the epiphyseal plates during development.
• Bones also grow in diameter to support body weight and muscle attachment.
• Options include appositional growth, concentric growth, closing of the epiphyseal plate, and a descriptive statement.
• The question asks specifically about width or thickness, not length.

Concept / Approach:
Appositional growth refers to the addition of new bone tissue at the outer surface, usually under the periosteum, which leads to an increase in bone diameter. Osteoblasts in the periosteum lay down new bone on the outer surface, while osteoclasts may resorb bone from the inner surface to remodel the medullary cavity. Closing of the epiphyseal plate marks the end of longitudinal growth. Concentric growth and the descriptive option about long bones reaching adult length and width are not the standard anatomical term for this process. Therefore, the correct approach is to recall that appositional growth is the technical term for growth in thickness.

Step-by-Step Solution:
Step 1: Distinguish between longitudinal growth (length) and radial or thickness growth of bones. Step 2: Recall that longitudinal growth occurs at the epiphyseal plates in children and adolescents. Step 3: Recognize that appositional growth describes the addition of new bone layers on the outer surface, increasing the diameter or thickness. Step 4: Compare this with closing of the epiphyseal plate, which actually signals the end of lengthwise growth rather than describing growth itself. Step 5: Conclude that appositional growth best matches the process by which bones increase in width.

Verification / Alternative check:
Anatomy and physiology textbooks commonly describe appositional growth as the mechanism for thickening of bone. Diagrams show osteoblasts adding bone to the outer surface and osteoclasts removing bone from the inner surface. This explanation appears under headings for bone remodeling and growth in thickness, consistently using the term appositional growth. This confirms that appositional growth is the correct term for the process in question.

Why Other Options Are Wrong:
Option B (Concentric growth): This is not a standard term for bone thickening in anatomy. Concentric lamellae refer to layers in osteons, but this option does not name the growth process itself.
Option C (Closing of the epiphyseal plate): This event marks the end of longitudinal growth and does not describe growth in width. It actually stops future bone lengthening rather than increasing size.
Option D (Long bones reaching adult length and width): This is a description of a final state, not a technical term for the process by which bones thicken.

Common Pitfalls:
Students may confuse epiphyseal plate activity with all forms of bone growth and therefore misattribute width increase to the same mechanism. Others may focus on descriptive phrases instead of the precise anatomical term. It is important to memorize appositional growth as the term specifically reserved for increases in bone thickness and diameter.

Final Answer:
The process by which bones increase in thickness or width is called appositional growth.