Why does ectropion after or during lower blepharoplasty happen?

Lower blepharoplasty can cause a variety of lower eyelid malpositions.  The biggest reason for these post surgical issues is not lower eyelid laxity.  It is direct damage to the motor nerves that supply the lower eyelid and serve the muscles that hold the lower eyelid against the eye.  The number one culprit in this in a skin approach lower blepharoplasty performed primarily by board certified general plastic surgeons.  This incision is made below the lower eyelid lashes and cuts the skin and muscle to access fat behind the eyelid.  Unfortunately this incision cuts the motor nerve to the muscle at the eyelid margin.  Sure, the surgeon may try to compenstate for this with a canthal tightening procedure but this also leads to a post-surgical eyelid that does not look quite natural.  Oculoplastic surgeons typically approach the lower eyelid at from behind the eyelid and avoid this injury.  The challenge is that many folks having work also need some skin removed.  A skin pinch can be used to augment the transconjunctival lower blepharoplasty.  There are also individual anatomic factors that make lower eyelid complications after blepharoplasty more likely.  This problems are common even in the hands of very experienced surgeons.

 Having done blepharoplasty extensively for more than 35 years I've never had a patient have an ectropion. This should be very rare  if performed by a very experienced facial plastic surgeon or eye plastic surgeon.  look at the surgeons results. The most common cause would primarily be too much skin removed during lower blepharoplasty. 

Ectropion is a severe contracture of the lower eyelid that can result after surgery or trauma, where the lower eyelid is scarred down exposing more of the sclera than normal, and even everted.  This is quite rare after a lower lid bleph.  More commonly lower lid lag is seen.  This results from too aggressive a skin resection.  Anterior bleph approaches do not lead to this if enough pretarsal muscle is kept intact in the procedure.   Oculoplastic surgeons are not immune to this issue.  Choose an experienced plastic or oculaplastic surgeon.  

There are a couple of reasons for the ectropion.  A major reason for this is unrecognized lower eyelid laxity. As we age the muscles of the lid become more lax, and this will vary tremendously from person to person.  If someone has a loose lower lid, they will be much more likely to get an ectropion if any skin is removed from the lid.  For this reason the lid has to evaluated properly both before and during the surgery.  If an eyelid is loose, there are ways to tighten it in the corner to help maintain stability.  Anytime too much skin is excised an ectropion can occur - even if you have a tight lower lid. An external skin-muscle surgery also has a higher incidence of ectropion compared to a transconjunctival approach. Occasionally some temporary ectropion can occur if you have significant swelling.  As the edema resolves the lid may move back to its normal position.Oculoplastic surgeons have extensive training in eyelid surgery but an ectropion can occur with any surgeon if care is not taken to avoid excessive skin excision, or if a loose lid is not tightened.

Ectropion after lower lid blepharoplasty can be caused by a variety of factors, including excessive lower lid skin excision, pre-existing lower lid laxity, and injury to the nerves that innervate the orbicularis muscle.  There are techniques to prevent postoperative ectropion due to these factors.  To minimize the risk of ectropion, my recommendation would be to seek out a surgeon who has been well-trained in lower lid blepharoplasty techniques.