Innervated First Web Space Flap
The first web space flap from the foot was first used by May in 19761 for durable, pliable, and sensate soft-tissue coverage of hand wounds. This neurovascular free flap represents a technical achievement in hand surgery that builds upon the principles of innervated tissue transfer laid out in classic flaps by Moberg2 and Littler,3 but it spares a hand donor site and allows more flexibility in flap design and inset. The first web space flap from the foot offers highly sensate tissue for coverage of the critically important parts of the hand.
In the algorithm for sensate free flaps to the hand, it is first important to distinguish between anatomic areas requiring protective sensation versus critical sensation.4 Areas that need only protective sensation include dorsal and palmar skin and can be reconstructed with a variety of sensate flaps, including dorsalis pedis, radial forearm, medial or lateral upper arm, tensor fasciae latae (TFL), anterolateral thigh (ALT), saphenous artery, and several other flaps. Areas that require critical sensation, on the other hand, include the digital pulp, finger stumps, and the first web space and are reconstructed with the glabrous skin–bearing flaps. As compared with other flaps,5 first web space flap from the foot is the gold standard neurosensory free flap for locations where the highest level of two-point discrimination (2PD) is required.
Though the ability of the first web space flap to accomplish wound coverage with highly sensate tissue makes it a gold standard, the complexity of the dissections limits the indications.
The first web space flap is used to restore sensate soft tissue to areas destroyed by infection, trauma, or prior surgery. Also, the flap will protect exposed tendons, joints, and neurovascular structures while providing padding for exposed bone. However, the patient must have a stable wound, clear of any infection, in an area where optimal 2PD is required. Furthermore, the remainder of the hand must otherwise have good function (including fine motor movements, pinch, and grasp) so as to make use of the restored sensation. We feel that the indicated locations include those with critical sensory function, namely the digits, especially the tips and pinching surfaces, and the first web space. Given the limited surface area of the first web space of the foot, this flap can also be used in a key location of a larger reconstruction or in combination with another flap.6 Finally, the patient must be compliant with a rigorous postoperative rehabilitation to realize the desired functional reeducation and outcome.
The first web space flap from the foot is technically challenging. First, the dissection of the vascular and neural pedicles is tedious. Second, the vascular anatomy is variable, as will be discussed. Finally, as the donor site is on the foot, there may be delays in healing or pain with ambulation. Patients have also reported painful neuromas and mild cold intolerance of the foot.7
The first web space flap is elevated from the foot to include the soft tissue overlying the lateral great toe and medial second toe. As a result, more than 50% of the flap surface area is glabrous skin. From a sensory perspective, the in situ 2PD here is 11 mm,1 which is significantly better than with other flaps, such as the dorsalis pedis flap (33 mm).8 The arterial supply to, and venous outflow from, the flap are dual, allowing for multiple configurations of inset. The dorsalis pedis artery branches into the dorsal metatarsal and digital arteries as well as into the deep plantar and plantar digital arteries, all of which are joined by distal perforating arteries in the toes ( Fig. 5.1a,b ). The flap′s inflow comes from the first dorsal metatarsal artery (FDMA), which is a branch of the dorsalis pedis artery (DPA) and from the common plantar artery. The FDMA origin can be variable, however. In the majority of instances (78%), the FDMA branches off of the DPA in the superficial tissues and subsequently courses deep to the extensor hallucis brevis tendon ( Fig. 5.2 ). In the remaining 22% of instances, the FDMA branches off the DPA after it dives to join the plantar arterial tree, in which case the FDMA is plantar to the metatarsal axis until it reaches the first web space.1
Venous outflow sources are also dual, including the venae comitantes accompanying the FDMA arterial inflow as well as the superficial dorsal venous system that drains into the great saphenous vein. The exact locations of superficial veins is marked after application of venous tourniquet ( Fig. 5.3 ).
The first web space flap has a dual sensory input originating from the deep peroneal nerve (dorsally) and the plantar nerves ( Fig. 5.4a,b ). The deep peroneal nerve branch is most commonly used due to its ease of dissection. However, the addition of a plantar digital nerve or common digital nerve provides even greater sensory input. Interestingly, although the in situ 2PD of the first web space is 11 mm, an even greater sensitivity of 6–8 mm is achieved after transfer to the hand.9–12 This is thought to be due to the greater cortical representation of the hand, fingers, and thumb dermatomes in the brain.
Preparation for the procedure should include a thorough examination of the defect and subsequent flap design. Woo et al described four variations in flap design based on amount and configuration of tissue needed, ranging from a small web skin flap to an adjuvant web flap with a larger foot flap ( Fig. 5.5 ).7 Tinel sign should be tested to approximate the length of needed neural pedicle and to help monitor postoperative axonal growth. We also recommend imaging of the vascular anatomy of the feet and hand by angiography ( Fig. 5.6 ). This angiography must include a lateral view of the foot, not typical for these studies routinely, to assess the branching pattern of the FDMA. Not only will this help inform the surgeon of anatomic variations, it will also allow the surgeon to choose the more favorable donor site. The superficial venous anatomy should also be outlined on the donor and recipient sites with a venous tourniquet just prior to the procedure.
Preoperative imaging of both feet is essential to delineate the FDMA anatomy and guide the course of the dissection.
The creation of a template of the defect on Esmarch bandage or paper is essential to plan the appropriate flap on the foot.
Since the first web space flap has dual innervation, the recipient nerve can be chosen based on recipient site anatomy.
During the flap harvest, dissect the pedicle from proximal to distal but raise the web space flap in a distal to proximal fashion.
The vascular pedicle must be tunneled and protected in the hand; further, the pedicle must be checked throughout the range of motion of the digit.