Special cases: secondary piggy-back lenses
Surgical options for the correction of pseudophakic refractive errors include intraocular lens (IOL) exchange, refractive laser correction (Laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK)) or the implantation of a secondary IOL by the piggyback technique.  The piggyback technique classically consists on the implantation of both the primary and secondary IOL in the capsular bag. The piggyback technique with an Add-On IOL consists on the implantation of the primary IOL in the capsular bag and the secondary IOL in the ciliary sulcus. The main indications for secondary piggyback IOL implantation are: postoperative ametropia and pseudophakic presbyopia.     
The piggyback technique was first described in 1993 by Gayton and Sanders in a case of cataract and microphthalmos, in which the calculated IOL power was +46 diopters (D). Subsequently, it began to be used to correct postoperative refractive errors. The same authors published in 1999 a case series of 8 eyes of 8 normal pseudophakes and 7 eyes of 7 postpenetrating keratoplasty pseudophakes in which a secondary IOL was implanted by the piggyback technique.
Types and Designs
Secondary piggyback IOLs are available as monofocal, multifocal, toric and multifocal toric models. There are three IOLs specifically designed for secondary implantation in the ciliary sulcus to correct pseudophakic ametropias or pseudophakic presbyopia: the Sulcoflex (Rayner Intraocular Lenses Ldt, East Sussex, United Kingdom), which was the first to be commercially available, the Add-On (HumanOptics, Erlangen, Germany) and the 1st Add-On (1st Q GmbH, Mannheim, Germany).
The Sulcoflex is a one-piece hydrophilic acrylic IOL with undulating haptics and posterior haptic angulation (10º). Optic diameter is 6.50 mm and overall length is 14.0 mm. Its optic has convex-concave shape. There are three types available: the Sulcoflex Aspheric (653L), in which the sphere range varies in 0.50 increments between -5.0 D and -0.50 D and between +0.50 D and +5.0 D, the Sulcoflex Toric (653T), in which the spherical equivalent range varies in 0.50 increments between -3.0 D and +3.0 D and the cylinder range varies in 1.0 D increments between +1.0 D and +3.0 D, and the Sulcoflex Multifocal (653F), which has an addition of +3.50 D (equivalent to +3.0 D at the spectacle plane) and in which the sphere range varies in 0.50 steps between -3.0 D and +3.0 D.
The Add-On is a three-piece foldable IOL. Optic diameter is 7.0 mm and overall length is 14.0 mm. The optic is made of silicone elastomer with UV absorber. It has a convex-concave shape and round anterior edge to prevent iris irritation. The modified C-loop haptics have zero-degree angulation and are made in high molecular weight polymethylmethacrylate (PMMA). There are 4 types: the SECURA-sPB (monofocal), in which sphere range varies in 0.50 D increments between -6.0 D and +6.0 D, the DIFFRACTIVA-sPB (multifocal), with an addition of +3.50 (equivalent to +3.0 D at the spectacle plane) and in which the sphere range varies in 0.50 D steps between -6.0 D and +6.0 D, the TORICA-sPB, in which the sphere range varies in 0.50 D steps between -6.0 D and +3.0 D and in 1.0 D steps between -30.0 D and -7.0 D and between +4.0 D and +6.0 D and the cylinder range varies in 1.0 D steps between +1.0 D and +3.0 D, and at last the TORICA-DIFF-sPB in which the sphere range varies in 0.50 D steps between -3.0 D and +3.0 D and the cylinder range varies in 0.50 D increments between +1.0 D and +4.0 D.
The 1st Add-On is a one-piece, foldable, hydrophilic acrylic IOL. Optic diameter is 6.0 mm and overall length is 13.0 mm. It has convex-concave shape, square design and 4 flex-haptics. There are three types: AddOn refractive (A4SW00), in which the sphere range varies from -10.0 D to +10.0 D in 0.25 D steps, the AddOn toric (6 different models), in which the sphere range varies in 0.25 D steps between -10.0 D and +10.0 D and the cylinder range varies in 0.75 D steps between +1.5 D and +9.0 D and in 1.0 D steps between +9.0 D and +11.0 D, and the AddOn progressive (three models) which has an addition of +3.0 D and in which the sphere range varies in 0.25 D steps between -3.0 D and -0.50 D and between +0.50 D and +3.0 D and in 0.50 D steps between -0.50 D and +0.50 D.
Preoperatively the pupils are dilated with eye drops (phenylephrine 10% and tropicamide 1%). Clear corneal incision is performed (1st Add-On IOL can be inserted through a 2.2 mm incision, Sulcoflex IOL through a 3.0 mm incision using the Rayner Single Use Soft-Tipped Injector and the Add-On through a 3.2 mm incision). Viscosurgical device is used to deepen the anterior chamber and the area between the anterior capsule and the posterior surface of the iris. The secondary IOL is implanted with an injector (Sulcoflex and 1st Add-On) or with a forceps (Add-On) into the sulcus. After implantation of the IOL, viscosurgical device is aspirated, including between the IOLs. The wound is hydrated with balanced salt solution and intracameral cefuroxime is administered.
Surgical follow up
Postoperatively, patients are prescribed topical antibiotic, steroids and NSAIDs.
There are some complications associated with the piggyback technique namely: interlenticular opacification,  postoperative elevation of intraocular pressure, pupillary optic capture after mydriasis, iris chafing, pigment dispersion syndrome and secondary pigmentary glaucoma. The implantation of the secondary IOL in the ciliary sulcus has an important advantage, over the classic implantation of both IOLs in the capsular bag, that is to increase the distance between the two IOLs, thus reducing the possibility of interlenticular opacification. The three IOLs (Sulcoflex, Add-On and 1st Add-On) were designed to avoid the complications previously described:
- The convex-concave configuration of the optic helps to increase the space between the two IOLs and contributes to avoid the interlenticular opacification as well as induced refractive errors and optical aberrations.
- The diameter of the optic of the Sulcoflex and Add-On IOL as well as the square design of the 1st Add-On IOL contribute to avoid the capture of the IOL during pupil dilation.
- The anterior round edge of the Add-On IOL helps to diminish iris irritation and consequently pigmentary dispersion and secondary pigmentary glaucoma.
- The terminal undulation of both Add-On torica and Sulcoflex contributes to a good rotational stability.
- The implantation of a secondary IOL with the piggyback technique is fairly easy for an experienced anterior segment surgeon and may be safer than explanting an IOL that was implanted many years beforehand. The explantation of an IOL, in which there is marked fibrosis of the capsular bag, may have serious implications, such as capsular rupture with vitreous loss, which can result in retinal tears/detachment or macular edema, zonular damage and ciclodialysis, as well as an increased difficulty in the implantation of an IOL with the correct power. In eyes submitted to penetrating keratoplasty and cataract surgery, in which the sutures have to be maintained for at least one year, the surgical correction of the induced astigmatism can only be managed later on, and so the piggyback IOL implantation may be the first choice.    
- The IOL power calculation of a secondary piggyback IOL is more predictable than that of an IOL exchange because it is calculated purely based on patient's subjective refraction and so it is not necessary to know the power of the primary IOL.
- The implantation of a secondary IOL is a reversible procedure, enables the correction of a wider range of refractive errors and has stable long-term refractive results in comparison to laser ablation procedures.  
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