Cheek Implants for Under Eye Circles and Overall Facial Flatness?

Cheek implants are very effective for cheek and midface enhancement.
For the under eye region, specifically designed implants are available (a.k.a. tear trough implants), but, personally, I am not a big fan of them. Rather, I believe that fat grafting is preferable if you are looking for long-lasting results.

Fillers are great for temporary solutions, as you stated. It sounds like you have a deficiency of your cheekbones. What that does is create a shadow under your eyes. If you lay back, I would bet that the dark circles go away. This is because the skin isn't pigmented, but your eyes are casting a shadow. This is what we call a "negative vector."
There are a number of implants that might be applicable. The come in many sizes and shapes. Some augment just the cheeks, some just the rim of your eye sockets. Others will do both. Seek out a qualified facial plastic surgeon who has experience with cheek and orbital rim implants for specific recommendations for you.

It is possible that cheek implants will help this problem. It really depends on whether there are hollows betweeen the lower eyelid and the midface or cheek. When there is poor bony support to the cheek fat, a separation between the lower eyelid and the cheek can develop, creating the appearance of puffy eyelids, which often look darker due to the shadowing. Those patients also tend to have deep nasolabial folds, another consequence of midface or malar fat descent.
If you have none of these things and really only have discoloration, there are other therapies such as topical ointments, allergy treatments, and laser which can be helpful with hyperpigmentation of the lower eyelids.
I might suggest getting a filler to see if the volume correction helps the color problem. If not, then implants will clearly not do a better job.
Good luck

With each passing year I find myself performing less 'invasive' lower lid surgery (i.e a formal lower blepharoplasty) and more lower lid rejuvenation by means of structural fat grafting.  The reality of aggressive lower lid surgery - which involves some combination of skin excision, fat pad reduction and/or fat pad transposition, orbicularis (muscle) redraping and excision, and canthal support (to hold up the lateral aspect of the lower lid while everything heals) - can easily create an 'operated' appearance.  Every experienced blepharoplasty surgeon understands quite well that the lower lid can be very unforgiving, and that an 'operated' appearance can be produced despite the best efforts of even the most skilled hands.

Some patients absolutely require an aggressive approach to the lower lids: those with bulging fat pads, marked skin and muscle redundancy, and obvious lower lid laxity.  Many patients with mild to moderate aging changes, on the other hand, can obtain youthful and natural-appearing results by means of structural fat grafting - to fill in the tear trough, and to blend the contour at the lower lid / cheek junction.  This can be especially useful for patients with a hollow or gaunt appearance at the junction of the lower lid and cheek, and for those with poorly projecting cheekbones that provide minimal support for the lower lid.  If this area is inherently hollow, then the last thing a surgeon should do is remove fat from behind the lower lid, as doing so will only serve to aggravate the hollow appearance and will actually accelerate the apparent aging of the eye area.

Patients with full cheekbones and excellent lower lid support usually do very well with a traditional lower blepharoplasty, although if I feel that I can produce an equivalent result by means of fat grafting and a modest amount of skin excision I will certainly choose the less invasive (and quicker recovery) fat grafting option.  Patients with poorly projecting, flat cheekbones in which lower lid support is lacking, on the other hand, are at significant risk for post-blepharoplasty complications and that obvious 'operated' lower eyelid appearance.  These patients are often best served by structural fat grafting to camouflage the hollowness and create some lower lid support.

Adding fat definitely improves lower lid support, and it often reduces the amount of lower lid skin excision that is required to produce an improved lower lid appearance and a blended lower lid / cheek junction.  I frequently see patients who have previously undergone a blepharoplasty procedure in which an excessive amount of upper and/or lower lid fat has been removed, and they invariably report that since that surgery they feel that they look tired, older, and even ill.  For patients with post-blepharoplasty hollowness, the only way to restore a healthy and more youthful appearance is to restore the missing soft tissue volume, and this can reliably be accomplished by means of structural fat grafting.

Structural fat grafting is a procedure in which small amounts (less than 0.1 cc at a time) of fat are carefully introduced in a series of discrete layers to gradually 'build' new soft tissue structure. As there is space between each micrograft, new blood vessels are able to grow into the grafted fat, allowing it to persist indefinitely. If this process of blood vessel ingrowth (neovascularization) does not occur, then the transferred fat cannot truly be considered a 'graft' and is instead just another temporary 'soft tissue filler' that is broken down and reabsorbed over several weeks.  Fat grafting requires specialized training and specialized surgical instruments, as well as patience and meticulous attention to detail on the part of the surgeon. When performed properly, permanent and natural-appearing aesthetic enhancements can be achieved.

Because the lower lid tissues tend to be quite thin, a conservative approach is an absolute necessity for fat grafting of this area.  If one overfills the lower lid  /cheek junction, and most of the fat persists, then the patient ends up with unnatural fullness that needs to be corrected.  I tell patients that our plan will be to add fat until an ideal contour is achieved and then we have to let it heal and see how much persists.  Because the amount of fat that survives is variable, some patients require a second and rarely even a third fat grafting procedure to reach the desired endpoint of improvement.  As fat grafting can be performed as an office procedure under mild oral sedation, and fat grafting recoveries are brief compared to traditional lower blepharoplasty surgery, supplemental fat grafting procedures are easily accomplished.

be sure to view many, many before and after photos from every angle and speak to previous patients before selecting your plastic surgeon

Cheek implants are placed through the mouth, in the gingival sulcus and are used for malar hypoplasia, which is when the cheeks are very flat. The cheeks themselves will not help the dark circles around the eyes. Makeup is the best treatment for dark circles in the lower lids. Make sure that any allergies are controlled with antihistamines and nasal sprays.