What is the primary mechanism of tissue expansion when using tissue expanders (tissue expansion devices) to cover a bald area?

Mechanism of Tissue Expansion for Covering Bald Areas

The primary mechanism of tissue expansion when using tissue expanders to cover bald areas is mitosis (increased mitotic activity in the epidermis), not connective tissue disruption.

Biological Process of Tissue Expansion

Tissue expansion works through a complex process that primarily involves:

1. Epidermal Hypertrophy and Mitosis: When a tissue expander is placed beneath the scalp and gradually inflated, it creates mechanical stress on the overlying tissue. This stress triggers increased mitotic activity in the epidermis, leading to actual new tissue formation 1.

2. Biological Tissue Creep: The ability of skin to increase in surface area during conventional tissue expansion is primarily due to biological tissue creep - a process where cells multiply in response to mechanical stress 1.

3. Vascular Changes: The expansion process stimulates angiogenesis (formation of new blood vessels), creating a highly vascular flap that improves viability of the expanded tissue 1.

Tissue Changes During Expansion

During the expansion process, several histological changes occur:

• Epidermal Layer: Becomes hypertrophic with increased mitotic activity
• Dermal Layer: May show temporary thinning during expansion
• Subcutaneous Tissue: Adipose thickness decreases temporarily
• Vascular Network: A vascular capsule forms around the expander, enhancing blood supply

Clinical Application in Treating Alopecia

For bald area coverage, the process typically involves:

1. Placement of a tissue expander in the subgaleal plane beneath hair-bearing scalp 2
2. Gradual inflation over 6-8 weeks
3. Removal of the expander and excision of the bald area
4. Advancement of the expanded hair-bearing scalp to cover the defect 2

Expansion Results and Tissue Gain

Research shows that tissue expansion provides both temporary "loan" and permanent "dividend" of tissue:

• While expansion can create up to 63% increase in tissue surface area during the expansion phase, approximately 30% permanent gain remains after the expander is removed and the flap is inset 3
• This permanent gain results from a combination of tissue stretch and actual increase in cellular and extracellular components 3

Common Pitfalls and Complications

When performing tissue expansion for scalp reconstruction:

• Extrusion of the expander (reported in about 14.5% of cases) 4
• Risk of pressure necrosis if expansion is too rapid
• Technical placement issues can lead to complications
• Most soft tissue changes return to pre-expansion state over time following discontinuation of the expansion process 1

The use of self-inflating tissue expanders has made the process less painful and more convenient for patients, particularly children, as they don't require repeated clinic visits for injections 4.