19 Rationale for Lower Limb Sensate Flaps

Joon Pio Hong

Summary

The ideal reconstruction of sole should lead to optimal functional and aesthetic results, achieving durable coverage for weight bearing, adequate contour for footwear, protective sensation, and solid anchorage to deep tissue to resist shearing. Flaps with thin thickness accommodating the defect can be the best chance for prolong stability. A sensate flap with thin skin component may be ideal in providing plantar surface reconstruction. However, if a sensate flap cannot be used, a thin flap without including deep fat and deep fascia may regain some protective sensation within a 12-month period.

sensate flaps – plantar reconstruction – microsurgery – nerve flap – thin skin flap

19 Rationale for Lower Limb Sensate Flaps

19.1 Introduction

Sensate flaps are flaps that provide sensation on the recipient site and innervated flaps are flaps that have direct nerve coaptation. It has been known that skin flaps and muscle flaps do have the ability to regain sensation to a certain level with or without direct nerve reinnervation. ¹ ^, ² ^, ³ ^, ⁴ Thus, theoretically any flap can be potentially sensate flaps, but the nomenclature of sensate flap is frequently used synonymously with innervated flaps. In this chapter, the same traditional definition will be used. The weight-bearing plantar foot remains the most important recipient site for sensate flaps. The importance of sensation during gait cannot be overemphasized. This section will elaborate the approach in using sensate flap mostly in regard to plantar reconstruction.

In flaps with skin components, specific sensory nerves are identified above the deep fascia. When a cutaneous nerve is identified, flaps using the skin component can be designed to incorporate the sensory nerve. The recipient cutaneous nerve is located around the defect and is used as a recipient nerve.

Muscle flaps are frequently needed to cover large defects of the extremity and foot. Free muscle flaps with skin grafts have been traditionally popular, considered to be easier in tailoring into the soft-tissue defects, providing good cushioning and solid anchorage to the deep structure and thus can best replace the complex subcutaneous architectural structure of the plantar surface. ³ ^, ⁵ ^, ⁶ But questions have been raised with regard to their durability against long-standing pressure. ⁷ ^, ⁸ In muscle flaps that were used to reconstruct the foot without reinnervation, gait and stance analysis with nonsensory innervated muscle flaps revealed the ability to bear weight, but with decreased pressure and duration. ¹ ^, ² ^, ³ Other reports speculate that functional reconstruction outcome may be more related to underlying bony deformities. ⁹ Nevertheless, muscle flaps with skin graft to reconstruct the foot may be bulky and require frequent secondary debulking proceudres. ³ Furthermore, frequent ulceration has been reported with muscle flaps with skin grafts that may need high maintenance (Fig. 19‑1). ³ This is most likely due to the lack of normal skin anatomy and sensory recovery. Although there have been multiple reports that muscle flaps regain deep pressure sensation even without cutaneous innervation, they will never regain complete sensory recovery. ¹ ^, ² ^, ³ ^, ¹⁰ Muscle flaps with intact motor nerves or with reanastomosis of the motor nerve to suitable motor or sensory nerves at the recipient site may have higher chance to retain protective sensibility. ¹ ^, ² ^, ³ Nevertheless, with good maintenance involving visual inspection, proper footwear, and daily management, muscle flaps do provide a good solution especially for extensive defects of the foot.

**Fig. 19.1** Minor ulceration after muscle free flap with skin graft is noted during follow-up. Although the overall contour looks fine, minor complications continues to occur.

Second choice for plantar reconstruction will be fasciocutaneous flaps. The 101 in plastic surgery starts with the concept of “like-with-like” reconstruction. Reconstruction of the weight-bearing sole remains difficult and challenging due to the specialized dermal–epidermal histology, and fibrous septa of the subcutaneous layer provide a unique property to withstand pressure and absorb shock of the gait. ¹¹ ^, ¹² ^, ¹³ ^, ¹⁴ The only skin that is identical to the plantar skin is the palmer skin, which is limited in source. Conventional local flaps from the plantar regions can be used to achieve ideal reconstruction but is limited to small defects. For larger and extensive defects, microvascular flap reconstruction is often required. But whatever flap is chosen, it is nearly impossible to replace the unique property of the weight-bearing sole. The ideal reconstruction of the sole should lead to optimal functional and aesthetic results, performing successful function in everyday life and occupation, wearing usual footwear with minimal donor site morbidity. To achieve these goals, characteristics such as durable coverage for weight bearing, adequate contour, protective sensation, and solid anchoring to deep tissue to resist shearing should be provided. The fasciocutaneous flaps may be able to provide some of the characteristics of an ideal flap for plantar reconstruction. However, the problem of sliding phenomenon has made it a controversial option. ⁸ ^, ¹⁵ The anatomical structure of multiple fat layers in addition to the superficial fascia between the deep fascia and the skin makes the skin surface slide resilient to external environment, but this property of the fasciocutaneous flap increases the gliding of the skin and makes it less ideal to resist shearing. The thicker the flap and the more the fat structure, the greater the gliding of the flap. Rautio et al reported that the optimal thickness of the flap should be less than 6 mm for adequate tightening and to resist shearing. ¹⁶ The third choice for reconstruction can be the perforator flap. Perforator flaps can be elevated as a fasciocutaneous flap, but when raised above the deep fascia it becomes a skin flap with subcutaneous fat. When using the perforator flaps such as the anterolateral thigh flap, which is one of the most frequently used perforator flaps, with the fat component present despite elevating it suprafascially, it can still be very bulky making it a less ideal choice for foot reconstruction. Since the introduction of thin perforator flaps elevating it above the superficial fascia and including only superficial fat and the skin, it may become thin enough to mimic the ideal thickness of the flap for plantar reconstruction. ¹⁷ ^, ¹⁸ Further report in using perforator flaps after thinning removing the deep fat and the deep fascia show that it may reduce the shearing in plantar surface reconstruction. ⁴ Another interesting finding from this study showed that perforator flaps with adequate subcutaneous fat after removing the deep fat may provide a cushioning effect over the plantar surface unlike that seen with forearm free flap. ⁴ Within the superficial fat layer, multiple small lobules surrounded by fibrous network may attach firmly to the surface of the defect and limit the sliding of the skin surface. One must ask, due to the anatomical position of the sensory nerves in the skin lying just above the deep fascia, whether it is better to include the nerve as a sensate flap despite the flap being thick. Although there is no clear study to answer this question, our experience shows that there is a difference in sensory recovery between sensory perforator flaps and insensate perforator flaps. The difference was statistically significant only at the third month, suggesting that earlier sensory recovery is expected by sensory nerve coaptation. ⁴ However in nonsensate flaps, it ultimately regained protective sensation in 83.3% at 1 year without sensory nerve coaptation. This result is similar with the reports in which forearm fasciocutaneous flaps were used. Santanelli et al presented that flaps gained protective sensibility in 12 months regardless of surgical nerve repair. ¹⁶ It is most likely, as seen in the case of forearm and anterolateral thigh perforator flap, that thinness of the flap may play an important role in reestablishing sensibility. Thus, in this chapter we will talk about the application of thin flaps for plantar surface reconstruction. The thinning technique of perforator flaps will be covered in another chapter.

Maintenance of protective sensation is essential especially for the feet and other weight-bearing areas. Without protective sensation, one can be at risk of ulcers and injuries from shoes and objects stepped on without shoes. Thus, the controversy on which flap can best accommodate the sensation after reconstruction has been under continuous debate. The ideal reconstruction of the foot, especially the weight-bearing areas of the plantar surface, should result in adequate contour for normal footwear, thick durable skin, protective sensation, and solid anchorage to the deep structures to resist shearing forces. ¹⁹ Studies have shown benefits of protective sensation for ankle and heel reconstruction both with rotational flaps and by microvascular tissue transfer. ¹⁶ ^, ²⁰ ^, ²¹ Sensory reinnervation for free flaps showed faster sensory recovery and flaps without sensory reinnervation did recover protective sensation to a certain degree. ⁴ ^, ¹⁶

19.1.1 Nerve Flaps

The sensory nerves are supplied by an extrinsic and intrinsic blood supply. The extrinsic blood supply to the peripheral nerves consists of arteriae nervorum that rises directly from the perforating vessels. These perforating vessels may originate from various deeper vessels as seen from Nakajima et al’s perforator classification. ²² These small arteriae nervorum enters the nerve and terminates intraneurally. The intrinsic blood supplies are the longitudinally oriented arterioles located on the epineurium, perineurium, and endoneurium. The intrinsic vessels communicate and receive vascular supply from the extrinsic arteries (vasa nervosum) and the end branches. ²³ ^, ²⁴ Thus, the nerve flap can be harvested from superficially located sensory nerves based on perforating vessels originating from a proximal source vessel. Based on this clear distinction of tissue being supplied by a vascular source, the term “vascularized nerve graft” would be a misnomer and “nerve flap” should be used as the right term to minimize confusion. There has not been an extensive report in using nerve flaps for nerve defects. However, sporadic reports are showing that the regeneration of nerve may be faster and thus functionally beneficial.

Well-known examples of nerve flaps would be superficial radial nerve flap based on the radial artery and accompanying veins, saphenous nerve flap proximally based on the branches from femoral artery and distally based on the saphenous artery, and sural nerve flap based on the superficial sural artery or medial sural artery. ²⁵ ^, ²⁶ ^, ²⁷ ^, ²⁸ ^, ²⁹

In the cases where perforators are not found leading to the superficial nerve, one can use the accompanying vein along the superficial nerve in an arterialized fashion. Typical example would be the sural nerve flap where an arterial supply such as superficial sural artery and medial or lateral sural artery would be missing and the lesser saphenous vein would be arterialized ³⁰ ^, ³¹ ^, ³²

19.2 Attributes and Detriments

19.2.1 Attributes

Sensate flaps may benefit plantar reconstruction.
Sensate flaps will regain faster sensory recovery.
Muscle flaps and nonsensate skin flaps will regain some form of sensation when the flap is thin enough.

19.2.2 Detriments

Sensate flaps may be too thick as the sensory nerve is located just above the deep fascia.
One must decide the benefits of sensate flaps versus thin flaps.

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