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The anterolateral thigh (ALT) flap is becoming a popular option for reconstructing a variety of soft-tissue defects, especially in the head and neck. Thinning of the flap may extend its usefulness to situations requiring less bulk, and the successful use of this technique has previously been described in the Far East. However, similar results have not yet been produced in the West. To investigate this, it is proposed that ‘one-stage thinning of the ALT flap does not disrupt the blood supply to any area of the flap skin’. A series of 10 ALT flaps were raised from Western European cadavers. The arteries of the flaps were injected with Indian ink and latex rubber, and six of the flaps were cleared by the Spalteholz technique. Patterns of dye filling were compared in full-thickness and thinned specimens, and the arterial organisation within the subcutaneous fat was studied. We saw 14 perforators in 10 ALT flap dissections. These arose from the descending branch of the lateral circumflex femoral artery in eight cases and from the transverse branch in two cases. Large branches from the perforator were seen to form an arterial plexus at the level of the deep fascia, which communicates with the subdermal plexus supplying the skin. Further branches arose from the perforator and travelled obliquely through the fat to reach the subdermal plexus. In the thinned cadaver ALT flaps, dye perfusion did not reach the distal portions of the subdermal plexus. There was reduced dye filling in comparison to the full-thickness specimens. Thinning of the ALT flap reduces arterial perfusion in cadaver specimens. This allows rejection of the null hypothesis. The fascial plexus and the oblique vessels supplying the subdermal plexus are likely to be damaged or removed during thinning. This may explain the observed reduction in subdermal-plexus filling in the thinned specimens. In the clinical setting, disruption of the arterial supply in this manner could lead to ischaemia and skin necrosis in thinned flaps. One-stage thinning of the ALT flap may not be advisable in the Western population.
The anterolateral thigh (ALT) flap has enjoyed a surge in popularity over the last decade, finding a wide range of reconstructive applications in the abdomen, the extremities and the head and neck.
This is, in part, because of the various practical advantages of the ALT flap, which offers a large skin paddle, a consistent pedicle length, good quality of skin and minimal donor-site morbidity.
A further feature of the ALT flap is the relatively thick layer of subcutaneous fat, which allows the flap to be used, for example, in post-mastectomy breast reconstruction.
However, each of these series of thinned flaps contains a small number of cases of partial or complete necrosis of the flap skin. Necrosis of thinned flaps may occur more frequently in the Western population, owing to the generally greater volume of subcutaneous fat present, requiring more extensive fat dissection to achieve the desired thickness.
Ross, GL, Dunn, R, Kirkpatrick, J, et al. To thin or not to thin: the use of the anterolateral thigh flap in the reconstruction of intraoral defects. Br J Plast Surg 2003;56.
The observed patterns of necrosis may be suggestive of vascular damage during thinning in the Western population.
Consideration of this problem led to formulation of the null hypothesis: ‘one-stage thinning of the ALT flap will not affect the blood supply to any area of the flap skin’.
To test this hypothesis, the arteries of a series of cadaver ALT flaps were injected before and after thinning, to see whether thinning had affected the degree of perfusion.
1. Materials and methods
1.1 Flap raising
Ten ALT flaps were dissected from cadavers of Western European descent. A line was drawn from the anterior superior iliac spine to the midpoint of the lateral border of the patella. An incision was made medial to this in the middle third of the line, down to the plane immediately below the deep fascia. The flap was then undermined and dissected laterally toward the intermuscular septum between the rectus femoris and vastus lateralis muscles. Medial displacement of the rectus femoris muscle allowed exploration of the intermuscular space, to expose either the septocutaneous perforator (if present) or the beginning of the myocutaneous perforator. All branches from the perforators were clipped, using an Ethicon Ligaclip MCA staple gun, and divided. Terminal branches of the descending or transverse branch of the lateral circumflex femoral artery (LCFA) were divided by the same method, distal to the most distal perforating branch. The main pedicle was traced to the origin of the descending or transverse branch of the LCFA and divided as close as possible to the origin from the main vessel.
1.2 Normal arterial supply to the ALT
The thighs of five cadavers were prepared by femoral-artery injection of embalming fluid followed by a three-part dilution of ammonia and, finally, a mixture of Indian ink and latex rubber. Once set, the mixture produced a firm flexible blue–black cast of the arterial tree.
ALT flaps were dissected in each of the specimens, using the above technique. The vasculature of the ALT was fully exposed, showing the branching pattern of the lateral circumflex femoral system in each case.
The first flap raised was subsequently cleared using the modified Spalteholz clearing technique. This technique involves dehydration of the specimen in graded alcohol solutions before immersion in methyl salicylate. The ground substance is rendered transparent, allowing the blue–black cast of the vascular tree within the flap tissue to be visualised in three dimensions.
1.3 Arterial supply to the ALT flap from its perforator
In an unfixed cadaver, the perforator to the ALT flap was located via a single 15 cm incision on the anterior thigh. The descending branch of the LCFA was ligated and divided proximal and distal to the origin of the perforator. The perforator was cannulated and injected with embalming fluid, dilute ammonia and dye. The dye injection produced a region of discolouration on the ALT, and a large (20 cm×15 cm) ALT flap was raised, encompassing the entire discoloured area. This flap was cleared using the modified Spalteholz technique.
Two further ALT flaps, measuring 15 cm×5 cm and 4 cm×5 cm, respectively, were raised from two unfixed cadaver thighs. These were also separated from all vascular connections in the thigh before injection through their respective perforators. The flaps were then cleared as described above.
1.4 Arterial supply to the thinned ALT flap
Two ALT flaps were raised using the technique described above. Both flaps were thinned using dissecting scissors, prior to separation of the pedicle. The superficial-to-deep thickness of each flap was reduced to 3–4 mm, leaving only the skin and a thin layer of subcutaneous fat. A cuff of fat and deep fascia was left around each perforator, with a radius of 2 cm in the first specimen and 1 cm in the second. The perforator arteries were then divided and cannulated to allow injection of embalming fluid, dilute ammonia and dye, before Spalteholz clearing of the flaps.
1.5 Microscopic and histologic examination
We stained 7 μm sections of ALT tissue with haematoxylin and eosin for histological examination. After gross examination, all cleared flaps were also sectioned at 1 cm intervals with a microtome blade in the mediolateral plane. The sections were subsequently examined under a stereoscopic microscope, allowing fine study of the vascular supply to the fat and skin.
2. Results
2.1 Normal blood supply to the ALT
In the 10 cadaver thighs dissected, five different branching patterns in the LCFA system were observed. In one novel variation, the LCFA was found to arise as a common trunk directly from the femoral artery, before dividing into one ascending and two descending branches. The first of these descending branches supplied the ALT flap in this case.
We found 14 perforators in 10 ALT flap dissections; 10 were myocutaneous perforators and four were septocutaneous perforators. Perforators arose from the descending branch of the LCFA in eight cases and from the transverse branch in two cases.
2.2 Features of the microvasculature
Examination of cleared ALT flaps showed two distinct arterial plexuses within the subcutaneous fat. Several large branches arise from the perforator soon after it pierces the deep fascia and travel across the superficial surface of the deep fascia to form a plexus at that level (Fig. 1, Fig. 2) . This fascial plexus sends small communicating arteries up through the fat toward the skin (Fig. 3) , which transforms perpendicularly to form a second arterial plexus, the subdermal plexus (Fig. 4) . The ALT flap thus appears to be supplied by two arterial plexuses: the deep fascial plexus and the subdermal plexus.
Fig. 1A cadaver ALT flap, cleared in methyl salicylate and viewed from the deep surface. The flap is partially translucent, showing the arteries to a depth of approximately 2–3 mm from the deep surface. The perforator that supplies the flap can be seen passing through the deep fascia at X. Several large branches (1–4) are seen to arise from the perforator soon after it pierces the fascia, and these travel in all directions away from the main vessel.
Fig. 2Close-up of the deep surface of a completely cleared flap, showing the fascial plexus. The perforator (running from top centre) is seen to pierce the deep fascia (arrowed). Several relatively large vessels branch off and travel in all directions through the fat.
Fig. 3Branch from the deep fascial plexus. The deep fascia (top) is being dissected away from the fat. An artery is seen arising from the fascial plexus 3 cm from the perforator. It pierces the fat (arrow) and ascends toward the skin, which is out of picture shot at the bottom.
Fig. 4Skin surface of flap 1, showing complete filling of the subdermal plexus from multiple arterial sources. The truncated ends of several relatively large vessels from adjacent vascular territories are arrowed.
Fig. 4 shows the subdermal plexus of flap 1, with complete filling of the plexus following femoral-artery injection of dye. Filling of the plexus is aided by contributions from several relatively large arteries in adjacent vascular territories, the truncated ends of which can be seen at the margins of the flap.
In addition to the two plexuses, several large branches arise from the perforator and travel obliquely through the fat to reach the skin (Fig. 1). These branches do not contribute to the fascial plexus and, instead, travel directly toward the skin to form part of the subdermal plexus.
Fig. 5(b) also shows a vessel running in the subdermal plexus, giving rise to a branch that travels back down into the fat (arrowed). This is one example of several vessels seen to follow a recurrent course, supplying the skin before the subcutaneous fat.
Fig. 5(a) Haematoxylin-and-eosin stained 7 μm section. (b) Cleared specimen, 1 cm section. A thick band of connective tissue (arrowed) is seen dividing the fat into superficial and deep layers. Also arrowed is a recurrent artery, branching from the subdermal plexus back down into the subcutaneous fat.
On histological examination, the subcutaneous fat was seen to be divided into superficial and deep layers by a denser band of connective tissue (Fig. 5(a)). This was also observed in 1 cm sections of cleared tissue as a relatively dense brown layer separating the deep and superficial layers of fat (Fig. 5(b)).
2.3 Supply to the ALT flap from its perforator
A region of discolouration was observed on the ALT following injection through the perforator vessel (Fig. 6) . This corresponds to the area of skin supplied by the perforator, and measures 17 cm×14 cm.
Fig. 6Region of skin supplied by a single ALT flap perforator. Injection of dye through the descending branch of the LCFA produced a discoloured region on the ALT, the margins of which are denoted by the solid line. The dashed line indicates the margins of the ALT flap that was subsequently raised.
After Spalteholz clearing, the three full-thickness ALT flaps showed filling of the subdermal plexus in all regions following injection of dye through the perforator alone (Fig. 7) . However, the subdermal plexus was seen to be less densely filled in these specimens than in the first flap (Fig. 4), which was allowed to fill from all arterial sources in the thigh.
Fig. 7Skin surface of flap 2. The subdermal plexus is filled in all areas after injecting dye via the perforator alone.
Thinning of the ALT flap to leave a 2 cm fascial cuff had an adverse effect on the arterial supply to the skin. The subdermal plexus showed complete dye filling in the full-thickness portion of the flap, but substantially reduced dye filling in the more distal thinned portions (Fig. 8) . The most distal portions, marked X and Y, showed little or no dye in the subdermal plexus.
Fig. 8Skin surface of an ALT flap thinned to leave a 2 cm cuff. Very little dye is seen in the distal thinned portions of the flap, marked x and y.
Thinning of the ALT flap to leave a 1 cm fascial cuff had a greater adverse effect on the arterial supply, with minimal dye perfusion into the substance of the flap (Fig. 9) .
Fig. 9Skin surface of an ALT flap thinned to leave a 1 cm cuff. No dye has perfused the substance of the flap.
As with any cadaver dye-injection study, the extent to which the arteries have filled with dye is uncertain. In this study, injections were carried out via the femoral artery or the flap perforators in order to minimise the problem of low perfusion pressure. Despite adequate perfusion pressure, there is still the question of whether the arterial networks within the flap have entirely filled with dye. In vivo, choke vessels between adjacent angiosomes may be open or closed, depending on pressure gradients and various other factors.
It is not known exactly how these choke vessels respond to pressure changes post-mortem, and this could potentially create an artificial result. In effect, the dynamic vascular territory of a perforator in vivo may differ from the anatomical vascular territory defined by cadaver studies.
Another potential problem arises in the interpretation of results from three-dimensional specimens. The perception of a vascular communication where none exists is possible, since vessels on different depth planes in the specimen may partially or completely overlie one another, creating the illusion of continuity. To overcome this problem it was necessary to rotate the specimen in all three planes under close scrutiny. Specimens were examined in every orientation, under direct vision and using a stereoscopic microscope.
The small number of specimens in this series is also a potential limitation. Spalteholz clearing of large blocks of tissue is a time-intensive process, and, as a result, only six flaps were cleared in this study. Results from this small series, while not sufficient for statistical analysis, may provide an idea of what might be expected in the clinical setting.
3.1 Normal blood supply
The LCFA system is highly variable in its supply of the ALT skin. This has been reported extensively in the past, and a classification of the branching of the LCFA was outlined by Kimata et al.
The numbers of septocutaneous versus myocutaneous perforators, and the frequency of their origin from the transverse or descending branches of LCFA, are in keeping with those reported previously.
Results indicate that the skin of the ALT is supplied by two arterial plexuses: the subdermal plexus deep to the skin and the deep fascial plexus. The fascial plexus communicates with the subdermal plexus by sending small ascending branches through the fat. Additional large branches were identified arising from the perforator and travelling obliquely through the fat to the skin. The deep fascial plexus and the oblique branches from the perforator appear to contribute to the supply of the skin and would be at risk of disruption during the thinning process.
This pattern of arterial branching correlates most closely with types 2 and 3 in the classification outlined by Kimura et al.
However, these branching patterns do not appear to be mutually exclusive, as stated by Kimura et al. The perforator or its large branches may extend sideways across the deep fascia, giving off large branches that gradually ascend into the fat toward the skin (Fig. 1). Although branches from the perforator may travel directly to the skin, as in Kimura's type 1, there are additional contributions to the subdermal plexus from the fascial plexus and the oblique branches, as described above.
A further interesting feature of the microvasculature was the presence of a number of recurrent arteries, which appear to supply the skin before the subcutaneous fat. In Moon and Taylor's description of the arterial supply to the TRAM flap following transfer,
they proposed that recurrent flow may be artificially produced in large vessels that were divided when the flap was raised. However, we believe that the vessels seen in our series (Fig. 5) may represent true physiological recurrent arteries. As they descend into the fat from the subdermal plexus, the arteries are seen to branch and decrease in calibre. It seems unlikely therefore, that blood would flow toward the skin in these vessels. The clinical significance of these vessels is, at present, unknown.
3.3 Division of subcutaneous fat
A relatively dense band of connective tissue was seen to divide the subcutaneous fat into superficial and deep layers. This may be analogous to the subcutaneous fascia previously described in the subcutaneous fat of other regions, including the abdomen and buttocks.
It has been suggested that, when thinning the ALT flap, the surgeon may safely remove the deeper large lobular layer of fat to leave the more superficial layer of smaller fat lobules.
However, it remains to be seen whether this is true in the Western patient population and whether the surgeon may differentiate between the two layers clinically.
3.4 Filling of the subdermal plexus
The skin of the ALT is normally supplied by several arteries, with partially overlapping vascular territories.
Fig. 4 shows flap 1, which was raised after injecting dye through the femoral artery. As seen here, the subdermal plexus supplying the skin is densely filled with black dye. However, it is unlikely to have been supplied solely by the perforator that was subsequently identified and dissected. Several relatively large arteries are seen at the margins of the flap, which were divided during the dissection. The large size of these vessels indicates that they are unlikely to have been supplied by the perforator and may have been filled from perforators in adjacent vascular territories.
In a clinical free-flap transfer, the perforator vessel must take over the arterial supply to the entire flap to ensure survival, as there is no contribution from adjacent vascular territories after dissection of the flap margins.
Clearing of full-thickness ALT flaps following injection through the perforator alone demonstrates the reduction in the arterial supply to the skin as a result of dissecting the flap margins. Fig. 7 shows dye filling the subdermal plexus in all regions of one of these flaps. However, it should be noted that the plexus appears to be less densely filled than with the normal blood supply to the ALT (Fig. 4). These findings indicate that following clinical free-flap transfer the full-thickness ALT flap may have a reduced, though still viable, arterial supply. Numerous clinical studies have demonstrated the viability of the full-thickness ALT flap,
and our results appear to complement these. Fig. 6 shows the region of skin supplied by the perforator in a cadaver specimen. The discoloured area measures 17 cm×14 cm and was produced by injecting dye through the perforator alone. The potentially large skin paddle is one of the many advantages of the ALT flap, and the discoloured area in Fig. 6 may give an idea of the full extent of the flap.
3.5 Effect of thinning
Dye filling in the thinned ALT flaps is markedly reduced in comparison with the full-thickness specimens (
Ross, GL, Dunn, R, Kirkpatrick, J, et al. To thin or not to thin: the use of the anterolateral thigh flap in the reconstruction of intraoral defects. Br J Plast Surg 2003;56.
). Reports from groups based in Asia currently recommend leaving a fascial cuff of 2 cm around the perforator when thinning in order to protect the arterial supply to the skin.
In our series, thinning of the ALT flap to leave a 2 cm cuff resulted in significant disruption of the vasculature. The full-thickness portion around the perforator showed normal subdermal-plexus filling, whereas the thinned portion of the flap showed markedly reduced dye filling in the more distal regions.
Thinning of the ALT flap to leave a smaller 1 cm cuff resulted in even greater vascular damage. A minimal amount of dye perfused the substance of the flap, indicating substantial arterial disruption. The observed effects of thinning on the arterial supply to the skin allow the rejection of the null hypothesis. Thinning the flap can damage the arterial supply to the skin in cadaver specimens.
Diminished supply to parts of the skin in the thinned specimen may be explained in terms of the microcirculation. Removal of the deep fascia also removes the fascial plexus, which sends small ascending vessels toward the skin. Furthermore, large branches from the perforator that travel obliquely through the fat may also be divided, depending on the extent of thinning. The relative contributions of these vessels to filling of the subdermal plexus (and therefore to the supply of the skin) are difficult to ascertain from this study. However, their significance cannot be ignored, since removing them (as in one-stage thinning) has been shown to disrupt the vascular supply to areas of the skin of the ALT flap in an experimental setting.
The success previously reported with thinning of the ALT flap may be explained in two ways. First, a generally smaller volume of subcutaneous fat may be present in patients from the Far East. This could mean that a lesser degree of fat dissection is required to achieve the desired thickness than in Western patients, resulting in less vascular damage. Second, the distal thinned portions of flaps may not actually receive a blood supply from the perforator. Thinning the flap tissue to 3–4 mm may allow that portion of the flap to survive as a graft, deriving its nutrient supply by diffusion alone.
Thinning of the ALT flap is shown here to disrupt the blood supply to the skin in cadaver specimens. This allows us to reject the null hypothesis that ‘one-stage thinning of the ALT flap will not affect the blood supply to any area of the flap skin’. The observed damage is probably due to removal of the fascial plexus and damage to large vessels that run obliquely through the fat to supply the skin. These results may help to explain the patterns of skin necrosis seen in clinical cases.
Ross, GL, Dunn, R, Kirkpatrick, J, et al. To thin or not to thin: the use of the anterolateral thigh flap in the reconstruction of intraoral defects. Br J Plast Surg 2003;56.
The presence of a subcutaneous fascia dividing the subcutaneous fat may have clinical implications for thinning of the ALT flap, although this will require further clinical studies.
The findings of this study, combined with a high reported morbidity rate, suggest that thinning of the ALT flap using previously described techniques may be inadvisable in the Western European population.
References
Wei F
Jain V
Celik N
et al.
Have we found an ideal soft-tissue flap? An experience with 672 anterolateral thigh flaps.
Ross, GL, Dunn, R, Kirkpatrick, J, et al. To thin or not to thin: the use of the anterolateral thigh flap in the reconstruction of intraoral defects. Br J Plast Surg 2003;56.
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