Anatomic Basis of Flexor Digitorum Longus Flap for Small Skin Defect Coverage in the Distal Third of the Leg

Providing soft-tissue coverage for small defects of the distal leg has been a challenge. Rotation flaps are of limited use due to the poor blood supply of the skin and the tendinous nature of the muscles in this area. Free muscle transfer is often necessary to enlarge the defect in order to accommodate the transferred muscle, thus leaving an undesirably larger defect. The flexor digitorum longus (FDL) muscle has a long, flat belly that extends distally. Its blood supply and distal extent make it an ideal choice for providing coverage for small defects in the leg.

The FDL muscle arises from the posterior medial side of the middle third of the tibia and the intermuscular septum. The muscle obtains a width of about 5 cm. Muscle fibers converge at the medial malleolus into a tendon. From five fresh-frozen legs, the authors measured the distance between the medial mallelolus and the distal end of the transposed flap. Most of the distal third of the leg could be covered by the FDL muscle flap. The posterior tibial artery gives off four or five branches proximal to the FDL at its origin. Ligating the distal one or two artery branches and separating the muscle belly from its tendon permit local rotation without compromising the blood supply to this muscle.

The FDP is exposed through a zigzag incision on the medial side of the lower leg. The saphenous nerve and vein are protected. Distal muscle belly release and mobilization from the surrounding fascia provide a flat muscle that can be rotated around much of the distal third of the leg. When used to cover the lateral side of the leg, the authors prefer to pass the muscle flap behind the tibia to avoid pressure on the flap from the sharp anterior tibia edge.

Small soft-tissue defects of the distal third of the leg can be covered by rotation of the flexor digitorum longus muscle flap. The authors have successfully used the FDL muscle flap in three consecutive patients with open tibial and fibular fractures. The use of this muscle has decreased the need for free muscle transfer for these defects.