Lower eyelid malposition is one the most common and devastating complications of lower eyelid blepharoplasty; it can be temporary or permanent. Surgeons who perform blepharoplasty should feel comfortable identifying those patients at risk for postoperative scleral show and ectropion.
Surgeons performing a blepharoplasty can choose from many available surgical options. Incisions can be placed on the skin or on the conjunctival surface; fat can be removed, repositioned, or added; and canthal support can be established by tightening, anchoring, or reconstructing. To choose the appropriate procedure to provide lower eyelid canthal support, the surgeon must understand the etiology of lower eyelid malposition and have a thorough knowledge of periorbital anatomy (see Relevant Anatomy).
For detailed descriptions of various blepharoplasty procedures, see the Eyelids section of the Medscape Reference Plastic Surgery journal. For information on many kinds of aesthetic procedures, visit Medscape’s Aesthetic Medicine Resource Center.
In the presence of excess scleral show, ectropion, or both, the lower eyelid appears unnatural and distorted. Some clinicians use these terms interchangeably, but distinguishing between scleral show and ectropion is important. Ectropion requires retraction of the lid with lid margin distraction and lowering from the globe, whereas scleral show alludes to only retraction of the lid. [1, 2, 3]
According to the American Society of Plastic Surgeons, approximately 221,000 blepharoplasties were performed in the United States in 2008.  Lower eyelid malposition is the most common complication of blepharoplasty. 
The pathophysiology of lower eyelid malposition is multifactorial. One can think of this process as an imbalance between gravity and the lower lid elasticity and canthal support. Involutional or senile ectropion is due to laxity of the lateral and medial canthal tendons. Gravitational forces acting upon excessive eyelid skin and prolapsed orbital fat may distort the lower eyelid. Cicatricial forces can produce an ectropion after prior surgical dissection. Excessive skin resection from the eyelid can retract the lower lid.
A study by Griffin et al indicates that although eyelid laxity, middle lamellar scarring, and anterior lamellar shortage have been associated with postblepharoplasty lower eyelid retraction (PBLER), certain other factors play a more significant role in PBLER than has been thought. The study involved 46 patients, all of whom experienced eyelid retraction following primary transcutaneous surgery. The investigators determined that although eyelid laxity and anterior lamellar shortage were common in these patients, so were orbicularis weakness, negative-vector eyelid, and lower eyelid/inferior orbit volume deficit. Only 17% of eyelids in the study had a significant middle lamellar scar. 
First, assess the visual acuity of the patient and document any associated aggravating symptoms. Next, proceed with the assessment of skin and muscle. Excess skin and muscle resection with blepharoplasty may result in scleral show and lid malposition. The following items should be considered and noted during the physical examination:
Margin reflex distance-2 (MRD2): The distance from the corneal light reflex to the lower lid margin should be no greater than 5.5 mm.
Positive vector relationship: The most anterior portion of the globe lies posterior to the lower eyelid margin, which lies posterior to the anterior malar eminence projection.
Neutral vector relationship: The anterior globe, lower eyelid margin, and anterior malar eminence all lie in the same vertical plane.
Negative vector relationship: The anterior globe lies anterior to the lower eyelid margin, which lies anterior to the malar eminence. This situation puts the patient at increased risk for lower lid malposition.
Lateral canthus to orbital rim distance: A patient with deep-set eyes usually presents with a distance less than 1 cm. A patient with prominent eyes usually presents with a distance greater than 1 cm.
Canthal tilt: Assess which canthus (medial vs lateral) lies in a higher horizontal plane. This is important for correction of horizontal lid laxity.
Lower lid eversion: This can be divided into 4 subtypes.
- Type I – Minimal lid margin eversion
- Type II – Moderate lid margin eversion with scleral show
- Type III – Lash rotation
- Type IV – Frank ectropion
Assessment of the malar fat pad: Lower lid retraction may present with descent of the malar fat pad. Midface lift procedures maybe combined with lower lid surgery.
Midlamellar cicatricial retraction: Manual displacement of the lower lid in the superior direction should normally bring the lid margin to or above the level of mid pupil. Limited movement in this direction may indicate midlamellar scarring.
Medial canthal laxity: Distract the lower lid laterally. Normal displacement should be 0-1 mm.
Lateral canthal laxity: Distract the lower lid medially. Normal displacement should be 0-2 mm.
Horizontal lid laxity: Perform the distraction test or snap test to assess horizontal lid laxity. See the image below.A: The lower eyelid is evaluated for the presence or absence of adequate tone. The snap test is shown. This involves pulling the lower eyelid skin away from the globe with the thumb and index fingers. B: This photograph demonstrates the lid retraction test, which involves displacing the lower eyelid inferiorly in order to evaluate lower eyelid tone. C: This is a preoperative oblique view. Note the brow position in relation to the orbital rim. Note also the excessive eyelid skin and crow's feet in this particular patient. D: Preoperative frontal view of the same patient. Note the lower position of the left brow, the redundancy and asymmetry of the upper eyelid skin, and the crow's feet in the lateral orbital areas.See the list below:
- Snap test: The examiner pulls the lower lid inferiorly and releases. The speed at which the lid returns to normal position is assessed. Slow movement indicates canthal laxity.
- Distraction test: The examiner uses his or her thumb and index finger to displace the lower lid anteriorly. Abnormal laxity is present if the lower lid can be displaced more than 8 mm.
Patients may present with lower eyelid malposition after blepharoplasty. Surgical correction can improve cosmesis and prevent dry eye symptoms. Additionally, patients requesting blepharoplasty who are at high risk for developing postoperative eyelid malposition should undergo an adjuvant procedure for lower lid support in addition to primary blepharoplasty. 
The lower eyelid is divided into 3 separate layers. The anterior lamella consists of skin and orbicularis oculi muscle. The middle lamella consists of the tarsus, superior part of capsulopalpebral fascia, and orbital septum. The posterior lamella is made of conjunctiva and capsulopalpebral fascia.
The orbicularis oculi muscle has 3 components, consisting of the pretarsal, preseptal, and orbital portion. This muscle can be divided into an extracanthal region and a canthal region. The muscle in the canthal region is critical for eye closure with blinking, lower lid tone, and lacrimal pump mechanics.
The tarsus is attached to the medial and lateral orbital rims by the medial and lateral canthal tendons. The lateral canthal tendon is also termed the lateral retinaculum. The lateral retinaculum is made of the lateral horn of the levator palpebrae superioris muscle, pretarsal and preseptal orbicularis oculi muscles, the inferior suspensory ligament of the globe (Lockwood ligament), and the check ligaments of the lateral rectus muscle. The lateral retinaculum attaches to Whitnall tubercle on the lateral orbital rim. [8, 9]
For a more detailed discussion, see Medscape Reference article Eyelid Anatomy.
See the image below.