- William PD Chen
Classically, the normal eyelid anatomy can be conceptualized as consisting of two layers: the anterior skin–orbicularis oculi muscle layer, and the posterior layers of the levator muscle and aponeurosis, Muller’s muscle and tarsal plate.
In general, the Caucasians upper eyelids that shows a lid crease are thinner than the Asian counterpart. This arises from a combination of factors which include a higher point of fusion of the orbital septum onto the levator aponeurosis, the relatively higher position of preaponeurotic fat pads and resultant thinner lower segment of eyelid, less preseptal fat and thinner orbicularis. Comparatively, Caucasians with an eyelid crease possess a greater number of distal fibers of the levator aponeurosis that terminates towards the skin along the superior tarsal border as well as the area above it to form the eyelid crease. When the levator contracts, the tarsal plate vectors upward and the eyelid crease invaginates easily. In addition, Caucasians may often have a deep-set supratarsal sulcus ( Fig. 16.1 ).
Asians’ upper eyelid anatomy is further divided into two groups: those with an upper eyelid crease and those without any crease. In Asians with a crease, the upper eyelid may still be thicker than a Caucasian’s with a crease, yet there is the presence of distal fibers of the levator aponeurosis terminating towards the skin along the superior tarsal border. Despite the lower point of fusion of the orbital septum, when this levator contracts, there is an invagination of skin along the superior tarsal margin to form a clinically apparent upper eyelid crease. When the lids are open and the subject is looking ahead, there is a greater degree of fullness in the preseptal region compared to a Caucasian with a crease, but less than that typically seen in Asians who do not possess an upper lid crease ( Fig. 16.2 ). Asians with absent eyelid crease typically have thicker eyelids due to the presence of hypertrophied orbicularis as well as the presence of fat in the pretarsal, preseptal, and preaponeurotic areas. The orbital septum fuses with the levator aponeurosis at a lower point compared to those Caucasians with an upper eyelid crease. There are relatively few fibers or no attachment at all from the levator aponeurosis towards the skin along the superior tarsal border. Their pretarsal and preseptal zones are thicker compared to Caucasians or Asians who have an eyelid crease ( Fig. 16.3 ).
In aesthetic Asian blepharoplasty by way of the external incisional approach, a skin incision is made along the designed crease and varying amounts of skin, muscle, and soft tissues may be removed; this is then coupled with various ways of crease construction via fixation or attachment of skin to the levator aponeurosis, skin to the tarsus, or orbicularis-to-aponeurosis fixation.
The surgical results often depends on a complex interaction between the degree of excessive tissues overlying the pretarsal and preseptal areas, the presence of fat, the thickness of skin over each of the two areas mentioned above, the position of the globe, the brow position, levator function, as well as whether there is a firm adhesion of skin to orbicularis in the pretarsal area. Normally, when the eyes are looking straight ahead and the lids are open, the anterior layer is in passive relaxation, allowing the posterior levator–Muller’s–tarsus to actively contract and pull the lid margin upward into an open position. The posterior layer only has to retract (glide) up and inward for 2–4mm relative to the anterior layer for a reasonable crease to be observed. The vertical width (in millimeters) of the eyelid fold overhanging the crease can be measured simply by subtracting the ‘observed crease width’ (from eyelash to inferior border of eyelid fold) from the anatomic crease width (measured with eyelid fold moved away to expose the true crease line). This varies between 2 and 4mm. The normal anterior layer offers little resistance and is not a ‘resisting platform’ against the levator muscle–tarsus when the posterior layer contracts; exception to this will include heavy eyelid, those with abundant preaponeurotic or suborbicularis fat and loose subcutaneous areolar tissues, and those with poor levator function, including true ptosis.
The role of the preaponeurotic space and fat is often mentioned as hindering any attempt at surgical construction of a crease, and the surgical dictum requires that at least a portion of the fat be excised. There is nothing inherently wrong with this concept; in fact, when the patient presents with excessive soft tissues along the preaponeurotic platform, this author has advocated a beveled approach towards the preaponeurotic space along the upper line of incision and performing a trapezoidal debulking of the skin, orbicularis, small amount of septum as well as inferiorly migrated fat. The problem arises when there is an overly aggressive excision of the preaponeurotic fat, or the procedure is accompanied by excessive hemorrhage within the preaponeurotic space, which is surrounded by orbicularis oculi in front as well as vertical communicating arterial branches of the marginal arcade, the peripheral arcade as well as the deep orbital arcade. The patient often develops a sunken supratarsal sulcus with total loss of fullness to the preseptal zone, and may have poor crease invagination, a stiffened eyelid skin with underlying cicatrix involving the pretarsal and preseptal area ( Fig. 16.4 ). During revision, one sees a collapse or obliteration of the preaponeurotic space and absence of preaponeurotic fat. The anterior and posterior layers appear fused into a single layered complex. One can visualize this as if the levator muscle now has to carry or lift the upper tarsal plate against the weight of a double load of eyelid layers, as opposed to the usual scenario where the tarsal plate glides up and under the anterior layers of skin and orbicularis, with preaponeurotic fat acting normally as a glide layer. In the scarred scenario, the skin–orbicularis is now acting as a ‘resisting layer’ towards the posterior layer of levator muscle. The absence or presence of this ‘glide zone’ (with non-adhering preaponeurotic fat within its space) in the middle between the two layers can therefore hinder or facilitate the formation of the crease. The author has observed the presence of tightly bound preaponeurotic fat in some ‘single-eyelid’ (creaseless) individuals coming in for primary surgery where the amorphous infiltrated fat in the zone between the skin–orbicularis and posterior levator muscle may have contributed to the lack of a crease. Careful repositioning of this fibrosed fat to a higher level seems to facilitate the up-vectoring of the lid and crease formation, perhaps by eliminating its ‘Velcro’ effect on the two lamellae.
