Essentials of Cutaneous Anatomy for Dermatological Surgeons
Head and Neck
Knowledge of the superficial anatomy of the head and neck is important, as most skin tumours occur at these sites.
Incision lines should be placed along relaxed skin tension lines (wrinkle lines) or follow anatomic boundaries where possible, because scars are less noticeable at these sites. With the patient seated, mark the important anatomical boundaries and any wrinkle lines, before local anaesthetic is injected, using Bonnies Blue or other suitable skin marker.
Sites with potential for poor functional or cosmetic results.
- In hair bearing areas incisions should, where possible, be made parallel to the obliquely orientated hair follicles to reduce damage done to adjacent hair follicles.
- Above the eyebrows horizontal closure of large excisions may produce a quizzical looking elevation of the eyebrow. At this site defects may have to be closed vertically or obliquely.
- Around the lips mark the vermilion margin, if this could be involved in the incision, before injection of local anaesthetic. If this precaution is not taken there is a risk that when the vermilion is re-sutured there will be a notched defect on the vermilion line. Such a blemish on the upper lip is particularly difficult to disguise in females
- On the lower lid, particularly in individuals with lax lower eyelids, closure lines should be made vertically or obliquely rather than transversely. This avoids wound tension or scar contraction causing the lower lid to avert producing an ectropion. The strength of the lower lid tension should be assessed pre-operatively by gentle manipulation to assess just how vulnerable the patient is to this problem. Patients with lax skin lid tension may require a lid tightening procedure to be done at the same time as the tumour excision to prevent poor postoperative lid positioning.
Blood supply to the head and neck
The blood supply to the face comes principally from the external carotid arteries. These freely anastamose across the midline and with the internal carotid arteries. The enormous number of anastomoses means that there is no risk of local ischaemia in a healthy individual as a result of tying off named arteries on the face.
The arteries encountered during superficial skin surgery include branches of the facial artery and superficial temporal artery. The following branches are frequently seen:
- Superior and inferior labial arteries run through the muscles around the mouth and anastamose across the midline. The artery is palpable on the lip just inside the mouth at the vermilion/mucosal junction.
- The facial artery crosses the face via the naso-labial fold and the medial canthus. The artery is frequently divided during tumour excision at these sites.
- Branches of the superficial temporal artery run a superficial course across the temple and in thin people are commonly visible. To avoid cutting these vessels during tumour surgery the distance between the base of the tumour and the underlying vessel can be increased by hydro-dissection.This involves injection of 10-20mls of sterile normal saline or water immediately before incision. The injected fluid provides a buffer zone through which dissection can be done without the risk of cutting the underlying vessel.
There are 2 important veins in the head and neck that may present problems during superficial skin surgery.
- Emissary veins carry blood out from the intracranial venous system through the bony skull across the subgaleal space and into the extracranial venous plexus. They are commonly present in the midline on the back of the scalp (parietal emissary vein). As they cross the subgaleal space, they may be severed during undermining. If the vein is torn the bleeding will be impossible to stop by diathermy or suturing. Bleeding may stop after prolonged direct pressure but bone wax may be required. This comes in suture pack sized sterile pieces and is simply pressed into the foramen from which blood issues using the round end of a forceps rather like a palette knife.
- The external jugular vein lies on top of the sternocleidomastoid muscle but beneath the platysma muscle. If divided the ends will need to be carefully sutured. If the patient’s head and shoulders are elevated the potential negative pressures in the venous system at this point may cause air to enter a cut vein risking an air embolus. Placing the patient’s head down increases the venous pressure in the neck vessels preventing this.
Nerve supply to the head and neck
Branches of the trigeminal nerve provide facial sensation. Knowledge of the position of the 3 main branches is important in nerve block anaesthesia. Transection of small cutaneous sensory nerves on the head and neck will produce localised areas of numbness that may take 1-2 years to improve.
Horizontal incisions on the forehead involving the full thickness of the frontalis muscle will result in division of these sensory branches and numbness of the scalp. Patients do not like this and need to be warned that it may happen. If the incisions or undermining are done above the frontalis muscle the sensory nerves are less likely to be damaged. Removal of complete branches of the nerve, e.g. the maxillary branch, may be necessary, if this is involved by tumour, and will result in wide areas of anaesthesia on the lower face and side of nose. Patients must be warned of these risks before surgery is attempted.
Damage to motor nerves is potentially disabling and can result in a significant functional deficit. The motor nerve supply to the facial muscles comes from the 7th or facial nerve. The potential effect of division of motor nerves can be observed after local anaesthetic injection when motor nerve function can be blocked temporarily. There are two branches to be aware of.
- The temporal branch of the facial nerve supplies the orbicularis oculi and the frontalis muscles. On the temple, just lateral to the frontalis muscle and particularly in thin people, excision of a tumour often results in removal of all the subcutaneous tissue down to periosteum. In some individuals this may involve removal of the branch that innervates the frontalis muscle. Division of the nerve results in paralysis of the frontalis muscle with an inability to raise the eyebrow and in some individuals a droopy eyebrow. In most patients this deficit is only noticeable during attempts to raise the eyebrow and causes little inconvenience. In others the droopy eyebrow becomes a problem and obscures vision. As a result the eyebrow and upper lid may have to be elevated using a brow lift operation. Re-innervation may occur spontaneously after division of the nerve so that muscle movement may be regained up to 1 year later. There is no need to consider suturing of the nerve if this is divided, as the consequences are not severe. Patients with tumours on the temple and overlying the zygomatic arch should be warned of the risk of paralysis to the frontalis muscle.
- The marginal mandibular branch of the facial nerve innervates the muscles of the lip. Loss of this nerve results in loss of oral continence so that food and drink leakage occurs during eating and drinking. Furthermore, speech is distorted because of inability to move the lips properly. Division of this nerve would thus be potentially disastrous and if this occurs the nerve should be re-sutured as soon as possible by a suitably qualified expert. The nerve follows a relatively superficial course after leaving the lower pole of the parotid gland. It runs beneath the mandible towards the front of the jaw but in older people it is often lower in position. It crosses the mandible at the anterior border of the masseter muscle. The operator should be aware of the risk of damage to the marginal mandibular nerve during surgery below the angle of the jaw from the front of the masseter muscle to the side of the chin.
The accessory or 11th cranial nerve is the only other nerve providing a significant motor supply on the head and neck that may be damaged during skin surgery. This nerve supplies sternocleidomastoid and the trapezius muscles. Division of this nerve results in weakness in raising or shrugging the shoulder on the affected side, winging of the scapula, and weakness of abduction of the arm. The deficit is therefore considerable and damage to the accessory nerve should be avoided.
A working knowledge of the superficial anatomy of the neck is important. The sternocleidomastoid muscle conveniently divides the neck into the anterior triangle (potentially hazardous) and posterior triangle (few hazards). In the anterior triangle of the neck there are many important structures. Provided the operator stays above the platysma muscle at this site these structures will be avoided. Operations beneath the platysma muscle in the anterior triangle of the neck should be avoided unless the operator has a detailed knowledge of the anatomy and is experienced in working in this area. However, sometimes the platysma muscle is readily identified, in other patients it is barely visible. Beware of the potential for deep incisions to be created during the removal of cysts from the neck.
In the posterior triangle of the neck the accessory nerve is the only potentially hazardous structure. Its position in the neck is indicated by Erb’s Point; which is defined as the point where a plumb line dropped vertically from the midpoint between the angle of the jaw and the mastoid process crosses the posterior border of the sternocleidomastoid muscle. At Erb’s point the accessory nerve emerges from under the sternocleidomastoid muscle and runs downwards across the floor of the post triangle to insert into the trapezius muscle.
Trunk and Limb
Important structures are much more likely to become easily visible in very thin individuals during skin surgery. For example, on the inner upper arm and lower leg, major nerves, veins and arteries may be visible in thin individuals immediately after skin excision.
Keloids and Hypertrophic Scars
Keloids are common on the upper chest and back. Young people, Afro-Caribbeans and individuals who already have keloids are particularly vulnerable. Operations at these site should be avoided unless absolutely essential.
Common (Lateral Popliteal) Peroneal Nerve
On the lateral aspect of the knee the common peroneal nerve (lateral popliteal) can be damaged as it winds round the neck of the fibula. At this site the nerve can be palpated against the bone. This motor nerve supplies foot dorsiflexors and elevators so that injury will result in foot drop.
Undermining levels at different body sites
Undermining increases skin mobility and this plays an essential part in the closure of skin defects. The level of undermining varies with body site and these have been listed below.
- Head and neck – mid-fat.
- Nose – beneath the facial musculature ie just above perichondrium and Periosteum
- Scalp – in the subgaleal plane.
- Forehead – beneath the frontalis muscle or in the deep fat above the frontalis muscle.
- Trunk and limbs – wide excisions are best undermined just above the deep fascia or in the deep fat.