SCALP SURGICAL ANATOMY

The scalp extends from the superior nuchal line posteriorly to the supraorbital rims anteriorly and is delineated by the pinnae bilaterally. The scalp is comprised of five layers: skin, subcutaneous connective tissue, galea, subaponeurotic loose areolar tissue, and pericranium. The skin is tethered to the underlying subcutaneous connective tissue and galea by muscle and fibrous bands. Local flaps commonly incorporate the outer three layers of the scalp, and flap undermining is easily performed through the subgaleal space, a relatively avascular plane.¹

The galea consists of a thick membranous tendon (galea aponeurotica) that completely encases both the frontalis muscle anteriorly and the occipitalis muscle posteriorly. These muscles are joined in the midline by posterior and anterior extensions of the galea. The galea tightly wraps over the skull because of the antagonistic actions of the frontalis and occipitalis muscles. The temporoparietal muscles originate from the superficial temporal fascia and insert into the lateral border of the galea aponeurotica. Whereas the frontalis/galea/occipitalis are at the depth of the muscles of facial expression, the temporoparietalis is at the deeper level of the muscles of mastication. The subgaleal space lies over the pericranium on the crown of the scalp but extends over the temporoparietalis muscle laterally. This is important, as undermining should be performed in the relatively avascular subgaleal plane above the temporoparietalis muscle and under the frontalis and occipitalis muscles. The scalp areas associated with muscle and fascia exhibit greater mobility than the central regions that consist only of dense underlying galea. As such, more distensible flaps arise from the peripheral scalp, where the skin overlies the frontalis, occipitalis, and temporalis muscles (Figure 10.1).²

Figure 10.1 In this wound in the temporoparietal area, note that beneath the galea and subgaleal space lies the temporalis fascia and the temporoparietalis muscle, which inserts into the cranium. Pericranium is seen superiorly.

Blood vessels and sensory nerves traverse the scalp in a centripetal fashion through the subcutaneous connective tissue layer. The scalp’s abundant arterial supply is derived from the internal carotid system anteriorly (supraorbital and supratrochlear arteries) and from the external carotid system posteriorly (occipital, postauricular, and superficial temporal arteries). The extensive vascular anastomoses allow a single superficial temporal artery to provide sufficient blood supply to virtually the entire scalp; however, the midline scalp possesses a relative watershed zone where perfusion may be less predictable. The scalp lymphatic filtration system interlaces with medium-sized blood vessels in the subdermal and subcutaneous layers. Since the scalp lacks lymph nodes, there are no barriers to lymph flow. Lymph flows in a centrifugal pattern toward the parotid, occipital, preauricular, and postauricular areas.³

The ophthalmic branch of the fifth cranial (trigeminal) nerve provides sensation to the anterior scalp. The occipital nerve, a second cervical nerve derivative, supplies the posterior scalp. The auriculotemporal and maxillary nerves are responsible for sensation to the temple area while the great auricular nerve supplies the postauricular area.¹ Infiltrating local anesthetic around the scalp’s periphery (through which all the sensory nerves traverse) can produce total scalp anesthesia.

EVALUATION OF THE SCALP DEFECT

When presented with a scalp defect or a lesion to be excised, the surgeon must evaluate the various reconstructive options that will result in the optimal surgical outcome.

The following questions need to be considered:

What is the depth of the wound? Is the wound through the skin, galea, or periosteum?

Some superficial wounds can appropriately heal by second intention and may even have hair regrowth if the depth of the wound does not extend below the level of the terminal hair bulbs. Most full-thickness wounds will require undermining in the subgaleal plane for tissue movement. Without an intact periosteum, a skin graft will not likely survive unless the underlying bone is burred to induce pinpoint bleeding.⁴

Is the defect surrounded by hair-bearing or glabrous skin?

On a bald scalp, second intention healing or skin grafting may be reasonable options, as the resulting scars will be less conspicuous on hairless skin. If the defect is surrounded by dense terminal hair growth, either primary closure or flap alternatives should be considered in order to avoid focal alopecia.

How much tissue laxity is present?

Scalp laxity can significantly vary by age and location, and from person to person. Scalps in infants, for example, are quite loose and mobile. As one ages, scalp mobility can change dramatically. The crown of the scalp usually is the least mobile area, whereas the peripheral scalp is more distensible. When evaluating for scalp laxity, one can be deceived by the amount of movement obtained by simply squeezing the sides of the defect together. This movement is partly due to the laxity of the dermis, and the actual movement of the surrounding scalp will be dictated by the galea, which is far stiffer and more immobile.

Can adequate anesthesia be obtained with local infiltration and/or nerve blocks for the type of reconstruction planned?

The degree of undermining required for a scalp closure is far greater than that in many other facial regions. These undermining requirements need to be evaluated before a reconstruction under local anesthesia is initiated. Even with adequate local anesthesia, scalp reconstruction can be quite difficult when patients hear and feel the pressure sensation of the undermining conducted through the underlying bone. General anesthesia may be more suitable for some anxious patients.

BASIC RECONSTRUCTION CONCEPTS

The scalp’s anatomy affords unique reconstructive advantages and disadvantages. The galea aponeurotica is the basis for this uniqueness. This stiff fascial layer is located beneath the subcutaneous connective tissue. Beneath it is the loose subaponeurotic connective tissue, also called the subgaleal space. This plane is relatively avascular, making it ideal for atraumatic undermining (Figure 10.2). Blunt dissection is all that is needed, and this can be achieved by scissor dissection or by simply using one’s gloved finger. This relative ease of undermining can be very deceptive, however, as the degree of additional movement afforded by wide undermining can be surprisingly small. For the best reconstructive outcome, the galea must be fully reapproximated. It is common for less experienced surgeons, particularly when confronted with very tense wounds, to not fully reapproximate the galea. In these cases, the final reapproximation of the wound’s edges involves stretching the overlying skin. The undesirable tension placed on the epidermal edges results in spread and inverted scars (Figure 10.3). It is therefore important that the galea be reapproximated by reaching underneath the leading edge of the defect and grabbing a large bite of galea with the suture needle (Figure 10.4). The strength of a properly realigned galea allows for minimal tension on the overlying skin, which leads to a more aesthetically appropriate scar.¹ If proper wound eversion is not also present, the final scar will likely be less than ideal.

Figure 10.2 Subgaleal space. The loose aponeurotic connective tissue layer is bluntly dissected with wide scissor undermining. Note the avascularity of this tissue plane.

Figure 10.3 There are two linear scars on this scalp. The inferior scar is inverted and spread, a likely result of insufficient undermining and failure to reapproximate the galea at the time of wound closure. The superior scar is well approximated and more aesthetically appropriate.

Figure 10.4 The galea typically retracts back into the wound. For proper suture technique, a larger bite needs to be taken in order to reapproximate the galea and reduce the overlying skin tension. Note that only the galea (without the skin) has been sutured.