Numerous reconstructive options have been described throughout the long history of dermatologic surgery, which dates back at least 2600 years.1 These reconstructive options fall into two categories: flaps and grafts. Whereas grafts use nonvascularized tissues (such as skin, fat, or cartilage) from a donor area to fill the wound (also known as the defect), flaps employ geometric principles to move and rearrange vascularized tissues around the wound to achieve closure, at least in the case of local flaps. Some flaps follow a linear geometry, whereas others are designed in a curvilinear fashion. This distinction depends on the studies conducted by the original author of the flap technique. However, every flap, regardless of its original design, can theoretically be performed with either a linear or curvilinear geometry. In other words, a curvilinear flap can be reshaped to follow a linear path, and conversely, a linear flap can be modified to follow a curvilinear path. To the best of our knowledge, no previous studies have directly addressed whether differences exist between linear and curvilinear flaps. Therefore, this article aims to provide readers with insights into the advantages and disadvantages of performing linear or curvilinear flaps through a review of the scattered literature on the subject.

Skin surgical wounds, typically resulting from excision of skin cancers, are usually circular because of the radial growth pattern of tumors. Mohs micrographic surgery exemplifies this process, in which only the tumor and a thin margin of surrounding tissue are removed, resulting in a circular wound. However, surgeons may reshape these circular wounds into a linear form by performing an elliptical excision. This technique is analogous to directly closing a circular wound while simultaneously correcting the resulting standing cone deformities, commonly referred to as dog ears. Dermatologic surgeons routinely anticipate the formation of these deformities and excise them, together with the tumor and its surgical margin, by drawing two triangles at opposite poles of the anticipated wound. Notably, removal of these healthy tissue triangles is not essential from an oncologic perspective. In essence, elliptical excision represents the most basic form of a linear flap (advancement flap).2,3

Analysis of elliptical excisions highlights the key advantage of curvilinear flaps over linear flaps: preservation of healthy tissue. Flap movement typically induces formation of dog ears, which is biomechanically expected. Although excision of these deformities may improve aesthetic outcomes, it can also facilitate flap movement by “pushing” tissue into the primary wound when the secondary wound (originating from the dog ear) is directly closed. However, whereas curvilinear flaps require excision only of dog ears, linear flaps necessitate removal of additional healthy tissue to reshape the wound from a curvilinear to a linear geometry (Figs. 1 and 2). Moreover, the curvilinear flap that would originally match the curvilinear wound must be enlarged because the wound per se has been expanded during reshaping from curvilinear to linear geometry. This results in mobilization and undermining of additional tissue, producing longer scars. This phenomenon is more evident with transposition flaps (Fig. 1) than with advancement (Fig. 2) or rotation flaps.

Fig. 1.

Two monolobed transposition flaps used to reconstruct surgical defects: one with linear geometry (Limberg flap) (A and B) and the other with curvilinear geometry (banner flap) (C and D). The reconstructive concept is similar in both techniques, with the principal difference being the greater amount of healthy tissue sacrificed with the linear flap.

Fig. 2.

(A and B) A Mohs micrographic surgery wound reshaped from a circular to a square configuration to accommodate the linear geometry of a classic island advancement flap. (C and D) An H-flap modified into an SH-flap to minimize sacrifice of healthy tissue, resulting in a final vertical scar with an S-shaped appearance.

Finally, wrinkles, also referred to as skin-folding lines or relaxed skin tension lines, are typically curvilinear. Therefore, a curvilinear flap may be better suited to align with these natural wrinkle patterns than a linear flap.4 Paraphrasing a quote from the famous Spanish architect Antoni Gaudí: “There are no straight lines or sharp corners in nature. Therefore, flaps should have no straight lines or sharp corners.”

Linear flaps have two distinct advantages. The first and most important advantage is that linear flaps may be easier to reproduce.5 Indeed, calculating lengths and angles is generally simpler than calculating radii and curvatures. This characteristic makes linear flaps more accessible to less experienced dermatologic surgeons. The 2nd advantage is that linear flaps may be subjected to lower tension than curvilinear flaps. This finding has been demonstrated in pig-skin models and may be related to the distribution of tension forces over a longer scar (quality of evidence C).6 As mentioned previously, linear flaps tend to be larger than their curvilinear counterparts.

Finally, the 3rd and perhaps most interesting advantage is that the trapdoor effect is less likely to occur in linear flaps than in curvilinear flaps. The trapdoor effect, also referred to as pincushioning, is the upward movement of tissue within the borders of a depressed scar.7,8 This represents a relatively frequent complication of transposition flaps, particularly on the nasal ala9 (Fig. 3). However, it may also complicate island advancement flaps, rotation flaps, or any curvilinear scar resulting from surgery or trauma.

Fig. 3.

Demonstration of the possibility of designing a bilobed flap with either linear or curvilinear geometry. (A and D) Bilobed flap used to reconstruct a Mohs micrographic surgery wound on the nasal tip. The classic curvilinear design was squared off, requiring sacrifice of healthy tissue to adapt the rounded Mohs defect to the squared flap. Favorable aesthetic results were observed at 1 month, without trapdoor deformity. (E–H) Mohs micrographic surgery wound located on the alar sulcus repaired with a bilobed flap. In this case, the defect and the first lobe were triangularized to align with the Burow triangle of the second lobe. At 1-month follow-up, the aesthetic outcome was favorable, with mild hypertrophic scarring on the first lobe secondary to tip epidermal necrosis. No trapdoor deformity developed. (I–L) A curvilinear bilobed flap selected to reconstruct a Mohs micrographic surgery wound between the nasal tip and ala. At 1-month follow-up, a mild trapdoor deformity was evident and required two intralesional triamcinolone acetonide injections for resolution.

Clinically, the trapdoor effect typically appears between 3 weeks and 6 months after surgery. Although its pathogenesis remains incompletely understood, several contributing factors have been proposed.7,8 Among these, lymphatic or venous obstruction and contractile tension vectors converging toward the center of the flap are considered the most common causes.

The insightful book on quality of evidence in procedural dermatology by Alam et al. discusses the evidence supporting preventive and corrective interventions for the trapdoor effect.7 Whereas corrective interventions, such as intralesional corticosteroid injections, Z-plasties at flap borders, and flap debulking, are well recognized (quality of evidence C),7 preventive strategies are less widely accepted.

One proposed preventive measure is contact inhibition of scar myofibroblasts by ensuring that the flap base remains in contact with the wound bed, which may reduce trapdoor formation by minimizing scar contraction (quality of evidence D).7 Nevertheless, the most established preventive measure against trapdoor deformity (quality of evidence C) is wide undermining of the recipient site.7 Its effectiveness has been demonstrated in a controlled animal study.8

Similarly to Z-plasties performed at the flap borders, undermining of the surrounding skin alters the horizontal forces of scar contracture around the flap. Theoretically, if these horizontal contractile forces are oriented circumferentially around the scar, as occurs with curvilinear flaps, the trapdoor effect becomes more likely. Consequently, linear flaps may be less prone to developing this deformity because they disperse contractile forces away from the center of the flap. This concept has been discussed in several renowned textbooks of dermatologic surgery10–13 and in scientific articles,14,15 although it has not yet been confirmed through controlled trials (quality of evidence D).7

Curvilinear flaps have three main advantages: (1) They sacrifice less healthy tissue; (2) They are smaller, thereby mobilizing less healthy tissue and producing shorter scars; (3) Their scars are more easily concealed within the natural curvilinear lines of the skin. Linear flaps also have three principal advantages: (1) They are easier to reproduce; (2) They generate less tension at flap borders; (3) They are less prone to development of the trapdoor effect (Table 1). However, the latter two advantages require confirmation through further studies.

To provide guidance regarding selection between linear and curvilinear flaps, the following practical recommendation may be proposed (quality of evidence D): When the flap used to reconstruct the surgical wound is a small transposition flap or island advancement flap, it may be preferable to design it with linear geometry. In these situations, sacrifice of healthy tissue has minimal impact on the patient, and the resulting scars are only slightly longer. In addition, the flap may experience lower tension and may be less susceptible to trapdoor deformity. Conversely, when a larger flap is required, a curvilinear design may be preferable because it minimizes morbidity by reducing sacrifice of healthy tissue, shortening scars, and decreasing the amount of tissue that must be mobilized or undermined.

None declared.