A Modified Type III Keystone Flap for Digital Mucous Cyst of the Eponychium

Abstract

Digital mucous cysts (DMCs), also known as myxoid cysts, periungual ganglion cysts, and myxomatous cutaneous cysts, commonly occurr at the distal interphalangeal joint (DIPJ) of the fingers and toes. Due to the dense and inflexible skin at the dorsal fingertip, wound dehiscence and necrosis may sometimes be caused by tension sutures. The keystone flap (KF), designed as a curvilinear-shaped trapezoidal keystone with two V-Y advancements at the exterior peripheral corners, has been gaining popularity as a local flap that can close defects with a lower tension. In the reported case, while facing the DMC at the eponychial fold, we applied a modified type III KF with minimal elevation of the eponychium and internal rotation of the opposite flaps to cover the triangular defect. Postoperative outcomes showed that the flap was viable with sufficient perfusion and no wound dehiscence or infection. During follow-up, the grooving deformity of the nail was corrected, and no tumor recurrence was noted. Moreover, there were no restrictive scars or limited range of motion on the DIPJ.

INTRODUCTION

Digital mucous cyst (DMC) is a common benign myxoid cystic tumor typically located at the dorsal side of the distal interphalangeal joint (DIPJ); it is associated with osteoarthritis.^1,2 DMCs predominantly occur in women and persons 40–60 years of age; they are prevalent in the index and middle fingers.^1,3,4 DMC usually presents as a nontender, solitary, dome-shaped lesion containing mucous discharge.² About half of the patients experience discharge and nail dystrophy or deformity before a medical consultation.³ Conservative treatments include compression, aspiration, cortisone injection, cyst unroofing, liquid nitrogen, and carbon dioxide laser therapy; however, they have a lower cure rate and less satisfaction than surgical excision.^1,3 Thus, the defect left after tumor removal is an issue in treating DMC, and numerous local flaps have been proposed for reconstruction.

There are four original types of keystone flap (KF) classified by Behan.⁴ Type I KF is designed as a curvilinear-shaped trapezoidal keystone with skin-only incision and two V-Y advancements at the exterior peripheral corner. Based on the design of type I KF, type II KF is formed with division of the deep fascia along the outer curvilinear edge, with (IIb) or without (IIa) skin grafting on the secondary defect. Different from type I and type II KF, type III KF is composed of double opposing KFs. Last, the type IV KF is raised with up to 50% subfascial undermining to advance and rotate for covering the defect with careful protection of perforators.⁴ (See figure, Supplemental Digital Content 1, which shows the original classification of keystone design perforator island flap, http://links.lww.com/PRSGO/C311.) We herein provide a novel option with type III KFs for reconstructing the defect at the nail matrix after DMC removal for the advantages of simple technique and reduced wound tension.

CASE PRESENTATION

A 66-year-old man with more than 30 years of experience as a Chinese chef with no systemic disease presented to our clinic with a painless nodular lesion at the nail matrix of the left middle finger found 6 months prior (Fig. 1). He had undergone conservative treatment; however, the lesion recurred within 2 months. A 5-mm eponychial fold mucus cyst was impressed during the physical examination with an obvious longitudinal grooving deformity, possibly due to the mass effect. Although there was no limit to the range of motion of the DIPJ, the skin lacked laxity.

Fig. 1.

Digital mucous cyst of eponychium of left middle finger with longitudinal nail grooving deformity.

Unlike the traditional elliptical or round incision, an isosceles triangle incision was created for the resection in this case. Type III KFs were designed with one side of each trapezoidal KF, along with a part of the eponychium (Fig. 2). The entire procedure was performed under a local anesthetic ring block with 1% lidocaine. Once anesthetized, a digital tourniquet was placed. The mucous cyst was removed alongside the overlying skin, and the germinal center was carefully protected. The stalk of the cyst was subsequently ligated. The opposite KFs were raised as preoperative planning with skin incision only as traditional type I KF with V-Y advancements at the peripheral corners. The wounds were closed using 5-0 nylon in simple interrupted sutures after the flaps were advanced and rotated internally (Fig. 3). [See figure, Supplemental Digital Content 2, which shows a preoperative design of type III keystone flap for the triangular defect (blue) after digital mucous cyst excision and the two opposite keystone flaps that were approximated after internal rotation, http://links.lww.com/PRSGO/C312.]

Fig. 2.

Preoperative design of tumor excision in the triangular incision and the modified double-opposite, type III, keystone flap.

Fig. 3.

The two opposite flaps, sizing 8 mm in width and 10 mm in length, with skin incision only and mild elevation of the eponychium, were rotated internally for approximation. Wounds were then closed using 5-0 nylon. Exterior peripheral corners formed the V-Y advancements as keystone flaps used to be.

Postoperative care was as usual, with antibiotic ointment and splint fixation for 1 week. The recovery was uneventful; the grooving deformity was corrected, and there was no limit to DIPJ movement or DMC recurrence in the follow-up (Fig. 4). (See Video [online], which shows there was no limited range of motion of the distal interphalangeal joint.)

Fig. 4.

Three-month postoperative follow-up. The defect was restored by acceptable scars, and the nail deformity was recovered.

Video 1. demonstrates that there was no limited range of motion of the distal interphalangeal joint.

Download video file ^{(13.2MB, mov)}

DISCUSSION

While managing DMC with surgical intervention, the necessity of removing the overlying skin remains controversial.^1,3 With careful preservation of the epidermis, reconstruction with a proximally based skin flap or so-called proximal nail fold flap raised from the eponychium has been described and applied with satisfactory results.¹ However, once the resection includes the attenuated skin, reconstruction of the defect is challenging for surgeons as the skin over the dorsal fingertip is inflexible.^5,6 The recurrence varied from 0% to 8% in previous studies with different surgical methods, including direct longitudinal excision and T, U, or H incisions on the DIPJ.³ Therefore, direct closure may be suitable and indicated for small lesions. Nevertheless, a tense suture might cause wound dehiscence, edge necrosis, infection, and hypertrophic scar for large lesions.⁷ Covering the defect after DMC removal with vascularized soft tissue is recommended for better restoration and a lower recurrence rate.

Rotational flaps and full-thickness rotational advancement flaps extending from the proximal interphalangeal joint are commonly applied due to their easy technique and the advantage of nontension closure.³ But both flaps had the disadvantage of remarkable long scars even for a small defect.⁵ Bipedicle advancement flap (length/width ratio = 2) created by a new incision performed parallell to the defect enables another option for small defects.⁷ Besides, rhomboid flaps have the benefit of simple planning but are limited by size as they are elevated from the skin fold at the DIPJ.⁸

Furthermore, the digital artery perforator flap and reverse island flap from the lateral finger showed satisfactory results; however, the dissection technique takes time and microsurgery training.⁹ Distal-based lateral finger flaps ranging from digital artery perforator flaps may offer an easier flap with no vessel dissection needed.⁵ Although the scar could be behind the lateral aspect of the finger, the elevation of the flap puts the neurovascular bundle at risk of iatrogenic injury. In most cases reconstructed with the local flap mentioned above, the lesions are generally located on the DIPJ, which is quite different from lesions on the eponychium, as in our case, which is dense and lacks skin laxity.

In 2003, Behan first presented a keystone-designed island perforator flap based on an angiotome from a dynamic vascular network.⁴ Since then, the KF has been widely applied because of its reliable versatility, simple technique, shorter operation time, great functional and esthetic results, and most importantly, low complication rate.^4,6 It has been used for restoring lateral aspect lesions of the finger and has shown favorable outcomes.⁶ Furthermore, Zadeh et al. described a “mouth fish” KF design for the defect left after DMC excision.¹⁰ In the current case, according to previous non-surgical treatments, the overlying skin is thin and fused with the cyst wall, necessitating its resection. Despite the lack of visible evidence of perforators from the branch of the digital artery, based on the ladder pattern formed by longitudinal digital vessels with multiple transverse interconnections, skin over digits has sufficient vascular supply by multiple cutaneous perforators for the KF to be raised in this region.^7,10 To prevent compromising the blood supply, the incisions of the flap edges were skin only to preserve as many perforators from the digital artery as possible. Also, minimal detachment of the eponychium from the nail plate, which we performed as one of the borders of the KF, showed no harm to nail growth in previous studies.¹

CONCLUSION

Type III KF might be an ideal local flap that provides similar tissues and texture for the DMC located at the nail matrix, as it provides a reliable flap with a simple surgical technique, shorter operation time, great esthetic outcomes, and less restrictive scar on the DIPJ.