The Superficial Ulnar Artery “Trap” Flap: Pearls and Pitfalls to Guide Aberrant Flap Harvest
28 June 2019
18 July 2019
26 August 2019 (online)
The superficial ulnar artery (SUA) is a known variant of the ulnar pedicle that manifests in 0.7 to 9.38% of the population.  When present, the SUA lies more radial along the forearm flexor muscles—coursing just below the investing fascia—before establishing the superficial palmar arch in the hand. Unfamiliarity with this variant can result in iatrogenic vascular injury and/or failure to capture the ulnar pedicle when using standard anatomic landmarks to guide perforator flap dissection. Herein, we describe a case of SUA encountered during ulnar artery perforator flap (UAPF) elevation and discuss relevant pearls/pitfalls to aid the safe and efficient harvest of flaps from this territory. We have seen four such cases among approximately 160 ulnar artery perforator flaps performed in our institution.
A 68-year-old female patient was planned to undergo hemiglossectomy reconstruction with an UAPF from the nondominant extremity. Using the senior author's standard approach, the flap was elevated from radial-to-ulnar in the suprafascial plane toward the flexor carpi ulnaris (FCU) tendon. Typically, dissection proceeds subfascial, beyond this point, to protect perforators within the septum between FCU and flexor digitorum superficialis (FDS). In this case; however, a superficial pedicle was encountered—approximately 1 cm ulnar to our radial incision—running subfascially along the surface of the FDS tendons ([Fig. 1]). Completion of the distal incision confirmed the absence of a vascular pedicle accompanying the ulnar nerve. The SUA and venae comitantes (VC) were ligated and divided distally, and the ulnar/proximal incisions were made. Rapid elevation of the flap ensued from distal-to-proximal and was aided by the paucity of muscular branches within the forearm, as well as the remote pedicle course relative to the ulnar nerve. Superficial dissection proceeded proximal to the antecubital fossa until the VC reached satisfactory diameter for microvascular anastomosis. In all four cases, the vascular pedicle ran immediately below the investing fascia but superficial to the FDS muscle belly, and slightly ulnar to the forearm midline ([Fig. 2]). The SUA appeared to be slightly smaller in caliber (1.5–2.0 mm) than the usual ulnar artery.
The SUA is the second most common arterial anomaly of the upper limb and knowledge of its existence and defining characteristics is mandatory to ensure safe/reliable harvest of forearm-based flaps. Generally speaking, the radial/superficial lay of the SUA within the mid-distal forearm increases its susceptibility to injury when elevation begins with the radial incision. In case a small UAPF is designed and centered over the typical landmarks, making the radial incision first may lead to complete omission of the radially displaced pedicle. As such, we recommend incising the distal border of flap first to confirm the location/depth of the ulnar artery. The position of the skin paddle may then be adjusted as necessary to ensure capture the pedicle supplying the flap. Although we have not had a chance to verify it, but we assume that, because of the superficial location of the pedicle, its path in the mid-upper forearm might be traced with a hand-held Doppler device. Thus, a quick Doppler examination in the mid-upper forearm may confirm the presence of ulnar artery trap.
If the superficial ulnar variant is encountered, intraoperatively, and the vessel/perforators are adequately protected, flap elevation may proceed rapidly without concern for ulnar nerve injury deep to the FCU. Few muscular branches and more convenient exposure further simplify pedicle dissection through the forearm. If needed; a long pedicle length may be harvested, as the SUA originates from either the brachial or axillary artery in the upper arm ([Supplementary Fig. 1], available in online version only).  
Most ulnar forearm flaps at our institution are used for oncologic oral and pharyngeal reconstruction. For patients with prior SUA flaps, the possibility of a similar anomaly in the opposing extremity should be anticipated and its presence evaluated for via direct palpation/Doppler ultrasound examination.  This is particularly important in cases of cancer recurrence, where the contralateral forearm is often targeted as the secondary donor-site. Finally, the astute microvascular surgeon should be aware of this anatomic variation while approaching each radial forearm flap, as misinterpretation and/or inadvertent division of the SUA can critically jeopardize perfusion to the hand.
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