# Excisional Surgery and Flap Design

## Introduction

Excisional surgery remains the cornerstone of dermatologic surgical practice. A thorough understanding of skin biomechanics, wound tension vectors, and reconstructive options is essential for achieving optimal oncologic and cosmetic outcomes. This lecture covers principles of excision, closure planning, and the major categories of local flap reconstruction.

## Principles of Excisional Surgery

### Preoperative Planning

The long axis of the excision should be aligned parallel to the **relaxed skin tension lines (RSTLs)** to minimize scar visibility. **Langer lines** are cadaveric in origin, while RSTLs are the in vivo equivalent, identified by having the patient animate facial muscles. Closures should be planned to place incision lines at boundaries between **cosmetic subunits** whenever possible. **Functional considerations** are paramount: distortion of free margins including the eyelid, lip, alar rim, and eyebrow must be avoided.

### Surgical Margins by Tumor Type

For **low-risk BCC**, 3 to 4 mm clinical margins achieve 95% clearance. **Standard-risk SCC** requires 4 to 6 mm margins. **Melanoma in situ** requires 5 to 10 mm margins. **Invasive melanoma** margins are based on Breslow depth: 1 cm for tumors 1 mm or less, 1 to 2 cm for tumors 1 to 2 mm, and 2 cm for tumors greater than 2 mm. **DFSP** requires 2 to 3 cm margins with wide local excision (or Mohs surgery as an alternative).

| Tumor | Recommended Margins | Notes |
|-------|-------------------|-------|
| Low-risk BCC | 3–4 mm | 95% clearance |
| Standard-risk SCC | 4–6 mm | Higher margins for high-risk features |
| Melanoma in situ | 5–10 mm | Or Mohs/staged excision |
| Melanoma ≤1 mm | 1 cm | Breslow-based |
| Melanoma 1–2 mm | 1–2 cm | Breslow-based |
| Melanoma >2 mm | 2 cm | Breslow-based |
| DFSP | 2–3 cm (or Mohs) | Extensive subclinical extension |

### Elliptical Excision Design

The elliptical excision should have a **length-to-width ratio** of 3:1 to 4:1 to prevent standing cones (dog ears), with the angle at each apex at **30 degrees or less**. The long axis should be oriented along RSTLs. **Dog ear repair** can be accomplished by excising a Burow triangle, performing an M-plasty (which shortens the scar), or using a hockey stick modification.

<image>Diagram showing elliptical excision design with 3:1 length-to-width ratio, 30-degree tip angles, alignment along relaxed skin tension lines, and three methods of dog-ear correction including Burow triangle, M-plasty, and hockey stick modification</image>

## Undermining and Wound Closure

### Undermining Principles

Undermining reduces wound tension by recruiting adjacent tissue. The **plane of undermining** varies by location: on the scalp, undermining is performed in the subgaleal loose areolar tissue; on the forehead, sub-frontalis or subcutaneous; on the nose, immediately above perichondrium or periosteum; and on the cheek, within subcutaneous fat. Undermining should extend **at least equal to the wound diameter** in all directions.

### Layered Closure

**Deep dermal sutures** using buried interrupted technique (typically 4-0 or 5-0 polyglactin/poliglecaprone) reduce dead space and tension. **Epidermal sutures** with simple interrupted, running, or running subcuticular technique provide eversion and fine approximation. **Wound eversion** is critical for optimal scar outcome and is achieved with deep suture placement and bottle-shaped bite geometry.

## Local Flap Classification

### By Blood Supply

**Random pattern flaps** are perfused by the subdermal plexus with no named vessel and ideally have a length-to-width ratio of 3:1 or less. **Axial pattern flaps** are based on a named artery (for example, the paramedian forehead flap on the supratrochlear artery), allowing greater length.

### By Movement

**Advancement flaps** move tissue in a straight line toward the defect. **Rotation flaps** pivot tissue around a point adjacent to the defect. **Transposition flaps** move tissue laterally over intervening skin to reach the defect.

## Key Flap Designs

### Advancement Flaps

The **unilateral advancement** flap involves simple forward movement, with a Burow triangle excised at the base to facilitate advancement. The **bilateral advancement (H-plasty)** uses two opposing advancement flaps and is useful on the forehead and scalp. The **V-to-Y advancement (island pedicle flap)** advances a V-shaped incision into the defect with the donor site closed as a Y, providing excellent results for the nasal ala and lip.

### Rotation Flaps

The **classic rotation flap** rotates a semicircular arc of tissue into a triangular defect. The arc length should be **4 to 8 times the width** of the defect for adequate movement. A **back-cut** at the pivot point increases mobility but compromises pedicle blood supply. The **O-to-Z (bilateral rotation)** design uses two opposing rotation flaps and is ideal for scalp defects.

### Transposition Flaps

The **rhombic (Limberg) flap** transposes tissue into a rhomboid (60/120 degree) defect. There are four possible orientations, and the surgeon should choose the one that borrows tissue from the area of greatest laxity and places scars in favorable lines. The **Dufourmentel flap** is a modification allowing coverage of defects with angles other than 60/120 degrees. The **bilobed flap** uses two transposition flaps in sequence: the primary lobe covers the defect, and the secondary lobe, which is smaller, closes the primary donor site. This is the classic choice for **nasal tip and alar defects**. **Z-plasty** transposes two triangular flaps to change scar direction and lengthen contracted scars; a 60-degree Z-plasty yields **75% gain in length**. The **note flap** is based on the nasolabial fold for alar and perinasal reconstruction.

<image>Illustration comparing four classic local flap designs: rhombic (Limberg) flap showing the 60/120-degree geometry, bilobed flap with primary and secondary lobes, Z-plasty with 60-degree angles showing scar reorientation, and V-to-Y island pedicle advancement</image>

## Interpolated Flaps

The **paramedian forehead flap** is an axial flap based on the **supratrochlear artery** and represents the gold standard for large nasal defects. It is a two-stage procedure with pedicle division at 3 weeks. The **melolabial (nasolabial) interpolation flap** is used for alar and lateral nasal wall defects, with the pedicle passing over or under intervening tissue. The **retroauricular flap** covers anterior ear and conchal bowl defects. All interpolated flaps require a **second stage** for pedicle division and inset.

## Complications and Management

**Hematoma** is most common within the first 24 hours and is reduced by meticulous hemostasis and pressure dressings. **Infection** occurs at a low rate (1 to 5%) in clean dermatologic surgery, with prophylactic antibiotics indicated for high-risk sites (below the knee, wedge excision of lip or ear, skin grafts). **Flap necrosis**, usually affecting the distal tip, is increased by excessive tension, smoking, and a narrow pedicle. **Free margin distortion** including ectropion, eclabium, and alar notching is prevented by proper flap design and appropriate vector of tension. **Trapdoor deformity (pincushioning)** results from circumferential scar contraction elevating the flap and is treated with intralesional triamcinolone or scar revision.

<image>Anatomical diagram of the face showing cosmetic subunits, relaxed skin tension lines, and key neurovascular structures at risk during dermatologic surgery including temporal branch of facial nerve, marginal mandibular nerve, and spinal accessory nerve in the posterior triangle</image>

## Key Clinical Pearls

Excisions should always be oriented along RSTLs, and closures should respect cosmetic subunit boundaries. The rhombic flap has four possible orientations, and the choice should be based on tissue laxity and scar line placement. The bilobed flap is the workhorse for nasal tip defects, with the secondary lobe designed smaller than the primary. Z-plasty at 60-degree angles provides 75% theoretical gain in scar length and reorients the central limb by 90 degrees. The paramedian forehead flap based on the supratrochlear artery is the gold standard for large nasal reconstructions.

## References

1. Rohrer TE, Cook JL, Kaufman AJ. *Flaps and Grafts in Dermatologic Surgery*. 2nd ed. Elsevier; 2018.
2. Zitelli JA, Moy RL, Abell E. The bilobed flap for nasal reconstruction. *Arch Dermatol*. 1989;125(7):957-959.
3. Krishnan R, Garman M, Nunez-Gussman J, Orengo I. Advancement flaps: a basic theme with many variations. *Dermatol Surg*. 2005;31(8 Pt 2):986-994.
4. Robinson JK, Hanke CW, Siegel DM, Fratila A, eds. *Surgery of the Skin*. 3rd ed. Elsevier; 2015.